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South Western CFRAM Study
Final Hydrology Report, Unit of Management 18
June 2016
The Office of Public Works
296235 IWE CCW R014 D
C:\Users\pig44561\AppData\Roaming\OpenText\OTEdit\EC_EUNAPiMS\c1500321418\296235-IWE-CCW-R014-D-
Hydrology_Report_UoM18.docx June 2016
South Western CFRAM Study
Final Hydrology Report,Unit of Management 18
South Western CFRAM Study
Final Hydrology Report, Unit of Management 18
June 2016
The Office of Public Works
Mott MacDonald, 5 Eastgate Avenue, Eastgate, Little Island, Cork, Ireland
T +353 (0)21 4809 800 F +353 (0)21 4809 801 W www.mottmac.com
Jonathan Swift Street, Trim Co. Meath
USER NOTICE
Please read carefully the following statements and conditions of use of the data, contained in this report. Accessing the information and data denotes agreement to, and unconditional acceptance of, all of the statements and conditions.
I have read in full, understand and accept all of the above notes and warnings concerning the source, reliability and use of the data available in this report.
I agree that the Commissioners of Public Works in Ireland have the absolute right to reprocess, revise, add to, or remove any data made available in this report as they deem necessary, and that I will in no way hold the Commissioners of Public Works in Ireland liable for any damage or cost incurred as a result of such acts.
I will use any such data made available in an appropriate and responsible manner and in accordance with the above notes, warnings and conditions.
I understand that the Commissioners of Public Works in Ireland do not guarantee the accuracy of any data made available, or any site to which these pages connect and it is my responsibility to independently verify and quality control any of the data used and ensure that it is fit for use.
I further understand that the Commissioners of Public Works in Ireland shall have no liability to me for any loss or damage arising as a result of my use of or reliance on this data.
I will not pass on any data used to any third party without ensuring that said party is fully aware of the notes, warnings and conditions of use.
I accept all responsibility for the use of any data made available that is downloaded, read or interpreted or used in any way by myself, or that is passed to a third party by myself, and will in no way hold the Commissioners of Public Works in Ireland liable for any damage or loss howsoever arising out of the use or interpretation of this data.
South Western CFRAM Study Final Hydrology Report,Unit of Management 18
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Revision Date Originator Checker Approver Description Standard
A August 2013 M Piggott C Jones S Pipe
R Gamble R Gamble Draft
B February 2014 M Piggott R Gamble R Gamble Draft Final
C October 2015 M Piggott B O’Connor B O’Connor Draft Final Minor amendments
D June 2016 M Piggott B O’Connor B O’Connor Revised to Final Status
Issue and revision record
This document is issued for the party which commissioned it and for specific purposes connected with the above-captioned project only. It should not be relied upon by any other party or used for any other purpose.
We accept no responsibility for the consequences of this document being relied upon by any other party, or being used for any other purpose, or containing any error or omission which is due to an error or omission in data supplied to us by other parties.
This document contains confidential information and proprietary intellectual property. It should not be shown to other parties without consent from us and from the party which commissioned it..
South Western CFRAM Study Final Hydrology Report,Unit of Management 18
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Chapter Title Page
Executive Summary i
1 Introduction 1
1.1 Context of the CFRAM Study __________________________________________________________ 1 1.2 SW CFRAM Study Process ___________________________________________________________ 1 1.3 Report Structure ____________________________________________________________________ 2 1.4 Flood Probabilities __________________________________________________________________ 4
2 Description of the Study Area 5
2.1 Extent ____________________________________________________________________________ 5 2.2 Characteristics of Rivers ______________________________________________________________ 7 2.3 Coastal Features ___________________________________________________________________ 8 2.4 Topography _______________________________________________________________________ 9 2.5 Rainfall ___________________________________________________________________________ 9 2.6 Geology _________________________________________________________________________ 13 2.7 Land Use ________________________________________________________________________ 13
3 Data Collection and Review 16
3.1 Data Register _____________________________________________________________________ 16 3.2 River Gauge Data __________________________________________________________________ 16 3.3 Rainfall Data ______________________________________________________________________ 22 3.4 Coastal Data ______________________________________________________________________ 24
4 Historical Flood Review 26
4.1 Historical Flood Events ______________________________________________________________ 26 4.2 Historical Flood Mechanisms _________________________________________________________ 31 4.3 Historical Flood Frequency Estimates __________________________________________________ 32
5 Rating Reviews 35
5.1 Gauge Review Selection ____________________________________________________________ 35 5.2 River Bride at Mogeely (Gauge 18001) High Flows Rating Review ____________________________ 35 5.3 River Blackwater at Ballyduff (Gauge 18002) High Flows Rating Review _______________________ 41 5.4 River Allow at Riverview (Gauge 18009) High Flows Rating Review ___________________________ 48 5.5 River Dalua at Allen’s Bridge (Gauge 18010) High Flows Rating Review _______________________ 54 5.6 River Blackwater Rating Checks ______________________________________________________ 59
6 Design Flows 68
6.1 Overview ________________________________________________________________________ 68 6.2 Definition of Sub-Catchments _________________________________________________________ 68 6.3 Flood Frequency Analysis ___________________________________________________________ 72 6.4 Hydrograph Generation _____________________________________________________________ 81
Contents
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6.5 Coastal Conditions _________________________________________________________________ 83
7 Hydrological Calibration, Sensitivity Testing and Uncertainty 89
7.1 Calibration Events _________________________________________________________________ 89 7.2 Uncertainty and Sensitivity Testing____________________________________________________ 101
8 Summary of Design Flows 103
9 Considerations for Hydrological and Hydraulic Model Integration 107
9.1 Inflows _________________________________________________________________________ 107 9.2 Downstream Conditions ____________________________________________________________ 108
10 Hydrogeomorphology 110
10.1 Approach _______________________________________________________________________ 110 10.2 Assessment _____________________________________________________________________ 110 10.3 Impact on Flood Risk ______________________________________________________________ 116
11 Joint Probability 117
11.1 Overview _______________________________________________________________________ 117 11.2 Fluvial-Fluvial Dependence _________________________________________________________ 117 11.3 Fluvial-Coastal Dependence ________________________________________________________ 119
12 Future Scenarios 121
12.1 Potential Climate Changes __________________________________________________________ 121 12.2 Potential Catchment Changes _______________________________________________________ 121 12.3 Design Future Scenario Conditions ___________________________________________________ 124
13 Conclusions, Key Findings and Recommendations 125
13.1 Conclusions and Key Findings _______________________________________________________ 125 13.2 Recommendations ________________________________________________________________ 127
Glossary 129
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The Office of Public Works (OPW) is undertaking six catchment-based flood risk assessment and
management (CFRAM) studies to identify and map areas across Ireland which are at existing and potential
future risk of flooding. Mott MacDonald Ireland Ltd. has been appointed by the OPW to assess flood risk
and develop flood risk management options in the South Western River Basin District. This hydrology
report is one of a series of reports being produced as part of the South Western Catchment Flood Risk
Assessment and Management Study (SW CFRAM Study). This report details the assessment of the
hydrological conditions across Unit of Management 18 (the Munster Blackwater catchment) which will form
the inputs into the subsequent hydraulic modelling and mapping of the key areas at risk.
A review and analysis of historical flood events, hydrometric data and hydrogeomorphological processes
has highlighted flooding issues to urban areas including Freemount, Kanturk, Mallow, Fermoy, Ballyduff,
Youghal, Rathcormac Tallow and Aglish. The Flood Studies Update methodologies have been used to
determine the design peak flows and characteristic flood hydrographs for eight specified flood probabilities
across the sub-catchments. Corresponding coastal conditions have been developed for Youghal.
Calibration events were identified across the catchment where there was sufficient historical flood data.
Potential future catchment changes relevant to the Blackwater catchment have been assessed including
changes in urban development, land use and hydrology related to global climate change. Two future
scenarios have been developed from this analysis, a Mid Range Future Scenario and High End Future
Scenario, which have been used to develop potential future flows and extreme sea levels.
The resultant design flood hydrographs and coastal conditions will form the inflows for the hydraulic
models. The knowledge of the hydrological processes and the historical flooding issues in the Blackwater
catchment established in this report will support the development of sustainable and appropriate flood risk
management options in those areas at greatest flood risk.
Executive Summary
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1.1 Context of the CFRAM Study
Flooding is a natural process that occurs throughout Ireland as a result of extreme rainfall, river flows,
storm surges, waves, and high groundwater. Flooding can become an issue where the flood waters
interact with people, property, farmland and protected habitats.
Flood risk in Ireland has historically been addressed through the use of structural or engineered solutions
(arterial drainage schemes and / or flood relief schemes). In line with internationally changing perspectives,
the Government adopted a new policy in 2004 that shifted the emphasis in addressing flood risk towards:
A catchment-based context for managing risk;
More pro-active flood hazard and risk assessment and management, with a view to avoiding or
minimising future increases in risk, such as that which might arise from development in floodplains;
Increased use of non-structural and flood impact mitigation measures.
A further influence on the management of flood risk in Ireland is the 'Floods' Directive [2007/60/EC]. The
aim of this Directive is to reduce the adverse consequences of flooding on human health, the environment,
cultural heritage and economic activity.
The Office of Public Works (OPW) is the lead agency in implementing flood management policy in Ireland.
The OPW have commissioned a number of Catchment Flood Risk Assessment and Management Studies
in order to assess and develop Flood Risk Management Plans (FRMPs) to manage the existing flood risk
and also the potential for significant increases in this risk due to climate change, ongoing development and
other pressures that may arise in the future.
Mott MacDonald Ireland Ltd. has been appointed by the OPW to undertake the Catchment-Based Flood
Risk Assessment and Management Study (CFRAM Study) for the South Western River Basin District,
henceforth referred to as the SW CFRAM Study. Under the project, Mott MacDonald will produce FRMPs
which will set out recommendations for the management of existing flood risk in the Study Area, and also
assess the potential for significant increases in this risk due to climate change, ongoing development and
other pressures that may arise in the future.
1.2 SW CFRAM Study Process
The overarching aims of the SW CFRAM Study are as follows:
Identify and map the existing and potential future flood hazard;
Assess and map the existing and potential future flood risk; and,
Identify viable structural and non-structural options and measures for the effective and sustainable
management of flood risk in the South Western River Basin District.
1 Introduction
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In order to achieve the overarching aims, the study is being undertaken in the following stages:
Data collection;
Hydrological analysis;
Hydraulic analysis;
Development of flood maps;
Strategic Environmental Assessment and a Habitats Directive Appropriate Assessment;
Flood risk assessment of people, economy and environment;
Development and assessment of flood risk mitigation options; and,
Development of the Flood Risk Management Plan (FRMP).
The resultant FRMP will set out recommendations for the management of existing flood risk and the
potential for significant increases in this risk due to climate change, ongoing development and other
pressures that may arise in the future.
The South Western River Basin District is split into five Units of Management (UoM). These Units follow
watershed catchment boundaries and do not relate to political boundaries. The Units are as follows;
The Blackwater catchment (UoM18)
The Lee / Cork Harbour Catchment (UoM19)
The Bandon / Skibbereen Catchment (UoM20)
The Dunmanus / Bantry / Kenmare Bay Catchment (UoM21)
The Laune / Maine / Dingle Bay Catchment (UoM22)
1.3 Report Structure
This report aims to assess the hydrological conditions across the Munster Blackwater catchment and
derive design peak flows, levels and hydrographs to be used in subsequent hydraulic modelling and
mapping of key areas at risk.
Table 1.1 outlines the report structure and scope of work with a description of the key contents.
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Table 1.1: Report Structure
Chapter Key Contents of Chapter
1. Introduction Context of the Study The SW CFRAM process and aims Scope of Work Flood Probabilities
2. Description of Study Area Description of study area Description of hydrological characteristics of study area
3. Data Collection and Review Overview of data used in the hydrological analysis Review and quality assessment of river level and flow data Review and quality assessment of rainfall data
Review and quality assessment of coastal data
4. Historical Flood Review Review of historical flood events Review of significant sources, pathways and receptors of
flooding
Estimation of flood probability for key historical events
5. Rating Reviews Analysis of spot gaugings at review gauge locations Hydraulic modelling used to extend rating curve Modelled rating curve extension
Application of revised rating curve
6. Design Flows Definition of sub-catchments Derivation of the index flood, design peak flows and flow
hydrographs Derivation of extreme sea levels and tidal curves
7. Hydrological Calibration, Sensitivity and Uncertainty
Review of historical data and selection of calibration events Derivation of calibration conditions Hydrological sensitivity and uncertainty in design hydrology
8. Summary of Design Flows Principal outputs and findings of design hydrology Preliminary design flows and hydrographs for hydraulic
modelling
9. Consideration for Hydrological and Hydraulic Model Integration
Full methodological approach to integrate hydrological outputs and hydraulic models
10. Hydrogeomorphology Assessment of existing hydrogeomorphological processes Consideration of flood risk impacts
11. Joint Probability Analysis Joint probability of fluvial events Joint probability of coastal events
12. Future Scenarios Potential impacts of climate change to rainfall, river flows, sea level and land movement
Potential catchment changes to land use and urbanisation Derivation of hydrology under future scenarios
13. Conclusions, Key Findings and Recommendations
Conclusions and key findings from the hydrological analysis and assessment
Summary of Design Existing and Future Hydrology Recommendations for hydraulic modelling and the FRMP Recommendations for future improvements in the
hydrological analysis
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1.4 Flood Probabilities
The SW CFRAM Study refers to flood probabilities in terms of annual exceedance probability in preference
to the use of “return periods” as used in previous reports. The probability or chance of a flood event
occurring in any given year can be a useful tool to better understand the rarity of specific magnitude events
for flood risk management. Due to popular descriptors of floods involving terms like the “1 in 100 year
flood” there can be a public misunderstanding that a location will be safe from a repeat event of the same
magnitude, extent and volume for the duration of the term (100 years in the above example). In reality,
flood events of a similar or greater magnitude can occur again at any time.
Annual Exceedance Probability, henceforth referred to as AEP, is a term used throughout this report and
the wider CFRAM studies to refer to the rarity of a flood event. The probability of a flood relates to the
likelihood of an event of that size or larger occurring within any one year period. For example, a one in
hundred year flood has a one chance in a hundred of occurring in any given year; 1:100 odds of occurring
in any given year; or a 1% likelihood of occurring. This is described as a 1% annual exceedance probability
(AEP) flood event.
Table 1.2 converts the ‘return periods’ to %AEP for key flood events as a reference to previous studies.
Table 1.2: Flood Probabilities
% Annual Exceedance Probability (%AEP)
Odds of a Flood Event in Any Given Year
Chance of a Flood Event in Any Given Year or
Previous ‘Return Period’
50% 1:2 1 in 2
20% 1:5 1 in 5
10% 1:10 1 in 10
5% 1:20 1 in 20
2% 1:50 1 in 50
1% 1:100 1 in 100
0.5% 1:200 1 in 200
0.1% 1:1000 1 in 1000
The hydrological analysis uses a number of other acronyms and technical terminology which are defined in
the glossary of this report.
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2.1 Extent
The South Western River Basin District covers an area of approximately 11,160 km2. The Study Area
includes most of County Cork, large parts of counties Kerry and Waterford along with small parts of the
counties of Tipperary and Limerick. The Study Area contains over 1,800 km of coastline along the Atlantic
Ocean and the Celtic Sea. There are five Units of Management within the South Western River Basin
District, which are listed below:
The River Blackwater catchment (UoM18)
The Lee / Cork Harbour Catchment (UoM19)
The Bandon / Skibbereen Catchment (UoM20)
The Dunmanus / Bantry / Kenmare Bay Catchment (UoM21)
The Laune / Maine / Dingle Bay Catchment (UoM22)
This report covers the Munster Blackwater in Unit of Management 18. It includes the Munster Blackwater
(henceforth referred to as Blackwater) downstream of Banteer to its outfall at Youghal, the River Allow,
River Bride and a number of smaller tributaries (Map 2.1). Unit of Management 18 contains nine Areas for
Further Assessment (AFAs) and over 238 km of high and medium priority watercourse associated with
these AFAs (Table 2.1).
Table 2.1: Areas for Further Assessment
Name Unique
ID Fluvial
Flood Risk Coastal
Flood Risk County Easting Northing
Contributing Catchment Area (km2)
Aglish 180247 Yes No Waterford 212250 91500 2.4
Ballyduff 180248 Yes No Waterford 196500 99500 2333.7
Fermoy 180252 Yes No Cork 182750 99500 1753.7
Freemount 180253 Yes No Cork 139500 114250 4.3
Kanturk 180254 Yes No Cork 138250 102750 307.5
Mallow 180262 Yes No Cork 155250 98500 1207.6
Rathcormac 180265 Yes No Cork 181750 91000 21.6
Tallow 180266 Yes No Waterford 199750 93750 18.1
Youghal 180267 Yes Yes Cork 210250 78750 > 3000
2 Description of the Study Area
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South Western CFRAM Study Final Hydrology Report,Unit of Management 18
Map 2.1: Unit of Management 18 Study Area
Allow Catchment
Blackwater Catchment
Bride Catchment
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2.2 Characteristics of Rivers
The Study considers 80km of High Priority Watercourse (HPW) in AFAs and 158km of Medium Priority
Watercourse (MPW) (Map 2.1). All grid references are to Irish National Grid (ING) and levels are to
Ordnance Datum Malin Head (mODM).
Allow Sub-Catchment, including Freemount and Kanturk AFAs
The Study considers the River Allow from Freemount to its confluence with the River Blackwater near
Banteer. The River Allow/Glashawee River rises near the Mullaghereirk Mountains (126480, 118570) and
flows in a south-easterly direction towards Freemount where it is joined by the Freemount Stream at Allow
Bridge. The River Allow then flows southwards towards Kanturk where it is joined by the similarly sized
River Dalua immediately downstream of Greenane Bridge in the town. The Allow continues to flow
southwards where it is joined by minor tributaries such as the Brogeen Stream before it flows through
Riverview gauge to its confluence with the Blackwater at Leaders Bridge (138500, 098760). The major
tributary of the River Dalua has been considered from Riverview Gauge (133745, 104485) to its confluence
with the River Allow downstream of Greenane Bridge, Kanturk (138255, 103065).
Smaller tributaries such as the Greenane Stream in Kanturk AFA and Kilknockane Stream along the Allow
MPW have not been considered separately in the hydrology as their contributing area is less than 1km2
and/or their contributing flow is less than 10% to the downstream reach. The recurring flooding at
Greenane was attributed to the urban drainage system which is not considered under the CFRAM brief.
Blackwater Sub-Catchment, including Mallow, Fermoy, Ballyduff and Youghal AFAs
The River Blackwater rises near Glenatripple (112420, 109040) flowing southwards to Rathmore, before
flowing eastwards to Banteer where it is joined by the River Allow from the north (138500, 098750). This
upstream reach of the Blackwater will not be modelled or mapped as part of this study because there were
no AFAs identified upstream. However, the flow contribution from the upper Blackwater is considered.
The River Blackwater continues eastwards where it is joined by the Glen River from the south and Awbeg
Minor from the north before flowing into Mallow. Within Mallow, there is a flood defence scheme that
comprises of a number of walls, embankments, penstocks and the extension of the culvert through Tipp
O’Neill Park. There are a number of smaller urbanised tributaries that join with the River Blackwater in
Mallow. The most significant of these are the Clyda River which joins from the south upstream of
Quartertown and Spa Glen which joins downstream of Mallow Town Bridge.
The River Blackwater continues to flow east downstream of Mallow towards the gauge at Killavullen some
10km downstream before being joined by the Awbeg Major from the north downstream of Castletownroche
(169340, 099930) and flowing eastwards into Fermoy. A series of flood embankments and demountable
defences form the Fermoy flood defence scheme to mitigate flood risk from the high water levels in the
River Blackwater during floods. The left bank (north) was completed in 2008. The walls and embankments
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along the right bank (south) are under construction at the time of this study. The River Blackwater is then
joined by the River Funshion and Araglin River from the north, 2km and 2.7km downstream of Fermoy.
The gradient of the River Blackwater continues to reduce as it flows eastwards through Ballyduff to
Lismore Weir (202250, 099080). Downstream of Lismore, the River Blackwater is considered fully tidal.
The river channel continues eastwards for another 6km before turning to flow southwards at the confluence
with the Glenshealan River. The River Blackwater flows southwards through Villerstown Gap before being
joined by the River Bride from the west and River Licky from the east. The River Blackwater then continues
to flow southwards to outfall into the Celtic Sea at Youghal (211500, 076250).
Bride Sub-Catchment including Rathcormac and Tallow AFAs
The River Bride is the second largest river within UoM18. The Bride rises from the Nagles Mountains
(164950,094090) and flows eastwards under the N8 road to Rathcormac, where it is joined by the
Shanowen River from Rathcormac ( 182060,091110) and the River Flesk from the south (182450,
091040). The River Bride continues to meander eastwards where it is joined by a number of smaller
tributaries before reaching Mogeely gauge (195640, 094130). Downstream of Mogeely, the Bride
continues east towards Tallow Bridge ( 199910,094330) where it becomes fully tidal, before joining the
River Blackwater some 12km downstream (209020, 091110). The much steeper and smaller Glenaboy
River flows from the south through Tallow before joining the River Bride upstream of Tallow Bridge.
Aglish AFA
The town of Aglish is situated in a minor sub-catchment of the lower Blackwater on the Ballynaparka River.
The small, steep Ballynaparka River rises 2km upstream of the town (213880,09090) flowing north-west
along the main street through Aglish before joining with the tributary immediately downstream of
Ballynaparka Bridge (212110,091508). Downstream of the confluence, the river flows west through Bleach
to join the tidal Goish River (210160,091750) and the River Blackwater 1km further downstream (209654,
092027).
2.3 Coastal Features
The River Blackwater can be considered tidal as far as Lismore, some 33km inland and the River Bride
can be consider tidal as far as Tallow Bridge, 30km inland. However, the relatively narrow channel and
floodplain between the steep valley sides limits the presence of in-channel bars, tidal loop channels and
wide estuarine flats until Youghal. Ferry Point, opposite Youghal (211110, 078060) constrains the incoming
tide, and protects Newtown and the eastern bank from extreme wave action. However, the western bank
from Youghal Mudlands (210140,080020) to Claycastle in Youghal (209115,75050) is vulnerable to wave
action and storm surges as identified by the Ireland Coastal Water level and Wave Study 2013 (ICWWS).
There is 1.7km of open coastline frontage at Claycastle, Youghal which goes across the fluvial watershed
between UoM18 and UoM19. This coastline has been considered as part of UoM18 since any coastal
flooding arising from this reach would affect the Youghal AFA.
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2.4 Topography
Map 2.2 displays the variation in elevation and topography of UoM18. The River Blackwater catchment
ranges from less than 5mODM at Youghal Mudlands to 440mODM at the source of the Blackwater.
Elevations can reach over 900mODM at Galtymore Mountain the headwaters of the Funshion. The areas
of high relief and steepest slopes are associated with the more resistant geology in the west and north as
described in Section 2.6 below. The River Blackwater and lower reaches of the Awbeg (Major) and
Funshion have much shallower gradients ranging from 1 in 840 at the confluence with the Allow to 1 in
3000 downstream of Lismore. This very low gradient for the lower 30km of the Blackwater results in
significant attenuation of flood discharges. The floodplain is typically 1km wide upstream of Cappoquin
bounded by the more resistant valley sides. The land to the east of Cappoquin is low-lying and forms a
small area of floodplain when the Blackwater overtops its banks. Downstream of Cappoquin, the tidal
floodplain is constrained by the more resistant geology on either side of the river.
The River Bride catchment ranges from 2mODM up to 400mODM in its headwaters with the bed slope
typically ranging from 1 in 600 in the upper reaches to 1 in 1700 in the lower tidal reaches. The southern
tributaries to the Bride tend to be steeper and have higher relief than those that drain areas to the north of
the Bride. The floodplain is relatively wide, ranging from 2 to 3km along its length.
The River Allow catchment typically has higher relief as it forms the Blackwater headwaters. Elevations
range from 70mODM at its outfall to 400mODM at the source (Mullaghareirk). The River Allow has a
typical gradient of 1 in 320 whilst the River Dalua tributary has a steeper gradient of 1 in 260. The steeper
gradient of the Dalua results in a slightly faster response to rainfall than the Allow.
2.5 Rainfall
Map 2.3 shows the variation in Standard Average Annual Rainfall across UoM18. Rainfall tends to be
greater in the west and decreases towards the east. This corresponds with the dominant wind direction in
the South West where storms tend to track west to east. Areas of high relief east of Mitchelstown also
experience higher SAAR. Map 2.4 shows the distribution of rainfall during the extreme event on the 19th
November 2009. The higher relief in the west combined with the dominant storm track leads to greater
rainfall in the headwaters of the Blackwater, Bride and over the higher relief at Cappoquin. The northern
catchments of the Awbeg Major and Minor both have lower rainfall totals. Their lower topography and
karstic geology leads to much lower flows than would be expected for a catchment of their size.
Given the large size of the Blackwater catchment (> 3000km2), rainfall and high flows in the upper
catchments is unlikely to occur at the same time as rainfall and high flows in the lower catchments as the
storm tracks across the catchment. A single storm tracking from west to east can exacerbate flooding as all
the flows from the upper catchment converge with the peak flow from tributaries in the lower catchment ,
raising water levels. However, gauge records indicate that the Bride tends to peak before the Blackwater at
Ballyduff.
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Map 2.2: Topography
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Map 2.3: Standard Average Annual Rainfall
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Map 2.4: Rainfall Distribution Across UoM18 on 19th
November 2009 Event
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2.6 Geology
Map 2.5 provides the underlying geology of UoM18. The geology can be summarised into relatively
resistant Devonian sandstone and volcanic geology and the less resistant Dinantian Limestones that run
west to east in bands. The resistant geology forms the major watershed boundaries between the
Blackwater and Bride catchments. The small tributaries underlain by the sandstone geology are more
prone to ‘flashy’ or a faster response to rainfall compared with the more permeable catchment in the north
of the Blackwater sub-catchment.
Conversely, the less resistant limestones form the river valleys along the Blackwater and Bride. The
majority of the River Funshion, River Awbeg (major), the River Bride and parts of the Blackwater at Mallow
to Killavullen are underlain by highly permeable karst geology which forms regionally important aquifers.
The permeable nature of these reaches is likely to mitigate flood peaks when unsaturated, but could
exacerbate and prolong flooding when the groundwater system is saturated.
2.7 Land Use
The Blackwater, Allow and Bride catchments are predominately rural in regards to land use, with the major
urbanised areas located around Mallow, Fermoy, Kanturk and Youghal. The smaller catchments of
Gooldshill Stream, Bearforest Stream and Hospital Stream in Mallow are more urban with between 9% and
19% covered by impermeable surfaces such as paving and tarmac. These surfaces increase surface water
runoff which reduces the time to peak giving a flashy response to any rainfall.
The rural land use comprises of largely pasture, with mixed agriculture interspersed with smaller wooded
areas on the valley sides. The significant wooded areas tend to be located on higher grounds near the
Araglin and Knockanore areas in the Lower Blackwater catchment. It was observed that polythene
coverings and polytunnels are used throughout the Allow, Blackwater and Bride catchments during
spring/summer which could significantly increase runoff. However, site visits indicate that this practice was
only applied to the minority of fields and was of short duration during the growing season. Therefore, the
impact of increased runoff due to polythene coverings was not considered in the design hydrology but may
be of local significance for specific events.
The headwaters of the Funshion, Araglin and Bride are dominated by the presence of peat bog and
moorland on the Knockmealdown and Nagles Mountains which could attenuate runoff reaching the river
channels. However, the impact of the peat bog and moorland reduced to less than 5% by the time these
rivers reach the study area on the River Blackwater and Bride respectively.
European Union agro-forestry policies since 1973 have led towards more intensive agriculture and
commercial forestry with associated land drainage. In particular, the removal of field boundary ditches as
natural drainage barriers and the raised embankments for the M8 motorway which have altered flow paths.
There is some evidence to support increased runoff and flows at the long-term gauge stations within the
Allow and Bride catchments. However, similar increases in flows have been observed in other catchments
in Ireland which have not experienced agricultural intensification. Therefore, climatic changes as well as
land use changes are involved in the change in flood response. Chapter 12 of this report assesses the
potential future changes in the catchment.
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Map 2.5: Geology
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The major urban areas are located at Mallow (population 11605), Youghal (population 8200) Fermoy
(population 7000) and Kanturk (population 1900). Of these Mallow is the most densely populated and
includes industrial areas along the River Blackwater valley upstream of the town. The increased presence
of tarmac and other impermeable materials increase the runoff and flashy response of the smaller urban
catchments. However, these urban areas form a relatively small proportion (<1%) of the larger Allow,
Blackwater and Bride catchment. Thus the urban land use is unlikely to significantly affect flows. The
remaining smaller settlements tend be located at the edge of the floodplains of the major rivers such as
Freemount and Aglish, or at crossing points such as Ballyduff and Lismore.
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3.1 Data Register
A range of different data sources has been used to undertake the hydrological data analysis for UoM18
(Table 3.1). The use of local hydrometric data can greatly improve and validate flood flows for historic
events and design flood events and has been reviewed in the following sections of this Chapter.
Table 3.1: Summary of Available Data
Type Details Owner Period of Available Data
River Flows 15 minute interval data series at 12 gauges with flow converted from water
level
The OPW
EPA (operated by Cork and Waterford County Council)
Various up to 2012
River Levels 15 minute interval data series at 21 gauges
The OPW
EPA (operated by Kerry County Council)
Various up to 2012
Rainfall Gauges Daily rainfall values at 53 gauges
Hourly rainfall at 32 gauges in the Mallow Flood Forecasting System
Hourly rainfall series at Valentia Observatory
Met Eireann
The OPW
Various up to 2012
Extreme Sea Level
Irish Costal Protection Strategy Study Total tide +surge design levels in
Youghal Bay.
The OPW Calculated for 2012
Wave Conditions Water levels, wave heights and wave periods at Youghal Harbour.
The OPW
Calculated for 2013
Sea level Ballycotton Tidal Gauge The OPW tidal network 2007 - 2012
A full register of hydrometric data used in this study can be found in Appendix A.
3.2 River Gauge Data
Map 3.1 shows the locations of river gauges in UoM18 with available water level and flow data. Chapter 6
discusses the analysis of those gauges selected for the modelled HPW and MPW reaches. The existing
hydrometric data from the wider area has been assessed for the following common issues:
Anomalous spikes or dips in water level and/or flow from the continuous data records;
Capping of water level and/or flow;
Trends in water level or flow over time that might be caused by systematic error of gauging equipment
or erosion/sedimentation;
Sudden shifts in level of the gauging datum;
Comparison of AMAX flows and levels from digital gauged data with manually extracted AMAX series;
Anomalous AMAX flood peaks in the AMAX series at each gauge;
Consistency of concurrent high flows downstream for AMAX events;
Length of data record to enable hydrological analysis; and,
Any significant data gaps.
Dromcummer and other gauges with shorter records have been adjusted for the missing periods based
on longer term gauges within the catchment using a pooled approach to extend records.
3 Data Collection and Review
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Map 3.1: Available Hydrometric Data
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Stations 18050, 18048, 18016
Long term flow and level records are available at Duncannon, Duarrigle and Dromcummer on the River
Blackwater. Of these only Dromcummer gauge is located within the modelled reach of the Blackwater. The
Dromcummer gauge has 22 years of reliable flow data up to 2004 available for this study. This can still be
used for statistically analysis but any long term estimates of flows should be adjusted to account for the
wetter years since 2002 based on neighbouring gauges with longer records. Duncannon and Duarrigle
should be used with caution as they are underlain by permeable geology which may make them unsuitable
for use as pivotal sites in impermeable areas.
Stations 18006, 18055, 18003, 18106 and 18107
There are long records available on the Blackwater between Mallow and Fermoy at the Comhlucht Siúcre
Eireann Teoranta (CSET) Mallow gauge (18006) and Killavullen gauge (18003). Approximately 10 years’
of flow data is available at Mallow Rail Bridge (18055) and 10 years’ of level data at Fermoy Bridge gauges
(18106 and 18107). These gauges has a complete and consistent record with the exception of the CSET
Mallow gauge during the 2009 floods where the original record indicated the flood peak occurring several
days after the peak at neighbouring upstream and downstream gauges. The level record has been
reviewed by EPA and corrected during this study such that the peak occurs on 19th November 2009 in line
with neighbouring gauges.
The initial hydrometric review of the flow series along the Blackwater indicated that the gauges in Mallow
consistently experience higher peak flows than Killavullen despite additional inflows from tributaries and a
30% increase in contributing area. Therefore a check of the high flow rating curves was undertaken (see
Chapter 5 of this report). The high flows ratings were subsequently updated at CSET Mallow, Killavullen
and a full rating curve derived for the downstream Fermoy Bridge gauge.
The progression and any attenuation of the flood flows were then reviewed using the 1D-2D hydraulic
model of Mallow as a routing model for a range of in-bank to out-of bank events. Figure 3.1 compares peak
flow at Mallow and Killavullen. Typically flow increases with increasing contributing area. Therefore the
flows at Killavullen should be above the 1:1 ratio dashed line. However, the modelled peak flows and
concurrent spot gaugings are at or below this 1:1 ratio between bankfull and QMED. This apparent loss
can be explained by greater storage on the floodplain as water spills out-of-bank but does not return to the
channel, thus is not recorded at the downstream gauge. Furthermore, karstic caves and depressions on
the floodplain may cause additional losses to groundwater when the ground is not saturated but were not
considered in this surface water analysis. However, above QMED the floodplain flow is fully connected to
the river channel and flow increases at Killavullen again. Figure 3.2 demonstrates these mechanisms from
the preliminary hydraulic model results.
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Hence, we can conclude that all the flow gauges between Mallow and Fermoy can be relied upon for
statistical analysis of extreme flood events above QMED flows as required for the CFRAM Study.
However, recorded peak flows at Killavullen between bankfull and QMED should be carefully considered in
combination with the floodplain conditions between Mallow and Killavullen before use.
Figure 3.1: Comparison of Flood Peaks between Mallow and Killavullen Gauges
0
100
200
300
400
500
600
700
0 100 200 300 400 500 600
Flo
w a
t K
illav
ulle
n 1
80
03
(m
3/s
)
Flow at Mallow Rail Bridge 18055 (m3/s)
Spot Gaugings 1:1 Ratio Modelled Flood Peaks Bankfull d/s of Mallow QMED
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Figure 3.2: Floodplain Snapshots of the Blackwater Mallow-Killavullen Reach
A: Below Bankfull Conditions < 130m3/s
B: Partial or Disconnected Floodplain Flooding 130-300 m3/s
C; Fully Connected Floodplain Flooding > 300m3/s
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Station 18002
Ballyduff on the Blackwater has a long record of 15 minute level data over 56 years with no significant data
gaps or datum shifts in the data set provided. These levels have been converted to flow using rating curves
over the years based on in-bank spot gaugings. These in-bank rating curves have been extrapolated to
estimate extreme out-of-bank flows which may underestimate floodplain and any by-passing flow over the
road at this site. Therefore, the rating curves will be reviewed at Ballyduff as well as, as number of other
gauges identified with potential by-passing flow in Chapter 5. The resultant flows at these gauges will be
used to inform the calibration of the hydraulic models in the relevant AFAs (Ballyduff, Kanturk, Mallow and
Fermoy).
Stations 18010, 18009, 18005, 18004 and 18001
Long terms records over 10 years are also available on the Blackwater tributaries including: Allen's Bridge
on the Dalua; Riverview on the Allow; Downing Bridge on the Funshion; Ballynomona on the Awbeg
(Major); and, Mogeely on the Bride respectively. The majority of the flow records have been edited by
OPW but are consistent with level records and are suitable for use following the rating review for the
Allen's Bridge and Riverview gauges.
Stations 18110, 18111 18019, 18109, 18056, 18105 and 18117
Level records were available for Kilbrin Road on the Allow, Church Road on the Dalua, Murphy's Bridge on
the Glen; Lombardstown, Mallow Town Bridge, Castlelands and Fermoy Mill on the Blackwater
respectively. The preliminary data review has highlighted several missing periods around 2006 and 2007
for Kilbrin Road and Murphy’s Bridge. The level data on the Blackwater gauges listed was of good quality
but typically less than 10 years in length and limited or no spot gaugings to develop a rating curve to
estimate flows. Therefore the level-only gauges across the catchment will be used to inform the calibration
of the hydraulic models local to these gauges but not in the hydrological analysis. Appendix A contains a
full list of the selected gauges and plots data quality.
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3.3 Rainfall Data
Available meteorological data from rain gauges and synoptic stations in and near to the catchment are
shown in Map 3.2.
Spatial distribution of intensity loggers and respective storage gauges (event based);
Identification of gaps or erroneous data which have been cross-referenced with the Met Eireann climate
stations to assess if significant events have been omitted;
Identification of shifts in rainfall records using temporal and cumulative plots; and,
Analysis of cumulative rainfall for key historic events.
The 32 rainfall gauges operated by OPW as part of the Mallow Flood Forecasting Scheme provide good
coverage across the Blackwater catchment to Lismore and Bride catchment to Tallow. The lower reaches
from Lismore to Youghal are not represented in this rainfall gauge network. However, the flood risk in the
lower reaches is dominated by coastal sources rather than fluvial or pluvial so rainfall data can be
interpolated from nearby sites. The data record available for these gauges is relatively short (< 3 years)
making them unsuitable for long term statistical analysis and rainfall-runoff routing of design events.
However, the rainfall gauge data provides details for recent calibration events since 2007, such as the
November 2009 event.
Based on the available data from Met Éireann and OPW, there are 33 hourly rainfall gauges within UoM18.
Additional detailed hourly rainfall data at Cork Airport (3904) and Roches Point (1004) will be used to
supplement and validate the rainfall data in UoM18, in conjunction with the daily rainfall gauges.
The preliminary meteorological analysis found a number of gaps in the data records at Fermoy,
Freemount, Kanturk, Mallow and Youghal gauges (3606, 5806, 1406, 6606 and 4106) particularly during
summer months. However, it is not expected that this will impact the hydrological analysis significantly as
most flood events occur in the winter months (October to March). Appendix A provides a summary of the
key rainfall gauges in the catchment.
Radar analysis is not necessarily appropriate because the accuracy of radar will be limited by the rain-
shadow effect in the mountainous areas and the distance from the Shannon radar station. It was agreed
with OPW that the daily storage gauges and river gauges within the catchments would be representative of
conditions on the ground. Therefore, radar data has not been considered further in the hydrological
analysis.
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Map 3.2: Available Meteorological Data
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3.4 Coastal Data
The locations of tidal gauges, extreme water sea level points and extreme wave condition points with
available coastal data in and near to the catchment are shown in Map 3.3.
Sea level data is also available at Ballycotton gauge since 2007. The data record was checked for
erroneous or poor quality data such as shifts in the datum, anomalous spikes and capping. There was
minor variation in the peak tide level and low tide levels, probably as a result of the gauging equipment and
variable atmospheric influences. The oscillation was within a 0.1 m tolerance and deemed suitable for
analysis. Therefore the Ballycotton gauge was deemed suitable to inform the total tide plus surge levels for
historic events since 2007. The Admiralty predicted tide level will be used to inform the astronomic tide
level for events prior to 2007.
The Irish Coastal Protection Strategy Study (ICPSS) data at point S_31 has been approved by OPW for
use directly as the coastal boundary to the Lower Blackwater and Youghal models. The extreme sea levels
will be used to define the magnitude of the tidal events along the coast for all AFAs. The hydraulic model of
the lower Blackwater will account for tidal influence when modelling fluvial events.
The Irish Coastal Water Level and Wave Study (ICWWS) also provide extreme wave heights, wave
periods and mean wave direction for those areas highlighted red in Map 3.3. The ICWWS data for Youghal
Harbour has been available for this report and covers the open coastline from Claycastle to Youghal
Mudlands.
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Map 3.3: Available Coastal Data
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4.1 Historical Flood Events
Table 4.1 summarises the source, extent and impact of flooding for the historic events identified where
sufficient evidence was available. Historic flood events in UoM22 were identified from the floods database
(www.floods.ie), previous reports, and interviews with Local Authority personnel and residents during the
Flood Risk Review. There were limited details available for historic flood events as detailed records of
impacts for events more than 20 years ago were scarce.
Flood Event of 17th of October 2012 affecting Youghal
The most recent flooding to take place in UoM18 was an extreme storm surge along the south west coast
of Ireland which inundated parts of Youghal town centre including areas along the quayside and adjacent
streets. An elevated water level of 2.6mODM due to high tide and 0.82m storm surge (as recorded at
Ballycotton gauge) spilled over the quayside walls and flowed down the roads. Approximately 30 to 40
properties would have been flooded if the property owners had not deployed individual flood gates or
sandbags1. Interviews with the town council indicated that flooding also occurs on Graffan Street, Market
Street and Catherine Street every one to two years.
Flood Event of 4th of August 2012 affecting Rathcormac
Flash-flooding in Rathcormac was caused by intense rainfall on 24th August 2012 between 2:00 am and
4:00 am. Intense rainfall fell, exceeding the capacity of the Shanowen River and a tributary through the
town which overtopped its banks at the junction to the north west of the School and flowed down the
surrounding roads. Subsequent investigations by Cork County Council found a major blockage on the
buried culvert between the Garda station and the graveyard outfall. This was subsequently removed.
However, the local engineer indicated that there is still a capacity issue with the Shanowen Stream that
causes the banks to overtop at a number of locations affecting the north of the village.
Flood Event 19th November 2009 affecting Kanturk, Mallow, Fermoy and Ballyduff
Widespread flooding occurred across the River Blackwater and River Bride catchments in the November
2009 event as a result of prolonged rainfall on already saturated catchments. This was the first significant
flood since the completion of phase 1 of the Mallow flood defence works. The Town Park and Mallow
Racecourse areas were flooded which closed the major road N72 for several days. A total of 8 properties
(7 residential and 1 commercial) were affected by flooding2. At Fermoy, flooding affected both banks of the
river, flooding a total of 22 residential properties, 16 commercial premises along Ashe Quay and O’Neill-
Crowley Quay.3 Access to the Hospital was also affected by flooding. There was also flooding of the N72 at
Mallow and Killavullen. Figure 4.1 shows the progression of the 2009 flood along the catchment which
generally increases down the catchment.
1 Mott MacDonald ( 2012) Flood Event Data Collection Youghal 17.10.12 on behalf of the OPW
2 Arup (2003) Munster Blackwater (Mallow) Drainage Scheme, Hydrology and Hydraulic Modelling Report.
3 Jacobs Babtie (2003) Munster Blackwater River (Fermoy) Drainage Scheme, Hydrology Report.
4 Historical Flood Review
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Table 4.1: Key Historical Flood Events
Date Flooding Mechanisms Areas Affected Properties Flooded Reported Duration of Flooding (Hours)
02/11/1980 Fluvial flooding along the Blackwater overtopping river banks
Mallow: Navigation Road, Bridge Street, Ward Terrace and Town Park
Not reported, Estimated to be over 100
48 hours
06/08/1986 Fluvial flooding along the Blackwater overtopping river banks
Mallow: Bridge Street and Town Park Over 70 properties flooded 32 hours
22/10/1988 No flooding reported but high flows recorded at gauge
Mallow: Bridge Street and Town Park Over 70 properties flooded 32 hours
26/08/1997 Flash flooding along the Freemount Stream combined with blockage at key culverts.
Freemount: R578 and Main Street
Estimated 20 properties flooded < 2 hours
30/12/1998 Fluvial flooding along the Blackwater overtopping river banks
Mallow: Bridge Street and Town Park Over 70 properties flooded 48 hours
06/11/2000 Fluvial flooding on an already saturated catchment
Mallow: Racecourse, Bridge Street and Town Park
Over 70 properties flooded 48 hours
27/10/2004 Tidal and fluvial flooding. The high tide overtopped quay walls in Youghal and high
river flow from the heavy rain flooded the Allow and Blackwater.
Youghal: Quayside, The Mall, Market Place and Catherine Street
Kanturk: Brogeen
40 residential properties
None reported
14 hours
10/01/2008 Fluvial flooding along the Blackwater overtopping river banks
Mallow: Bridge Street, Park Street and Meadowlands
Fermoy
Over 70 properties flooded
30/01/2009 Fluvial event due to intense rainfall overtopping banks by the National Primary School
Rathcormac: Main Street 1 residential property and 1 commercial
< 2 hours
19/11/2009 Fluvial event exacerbated by saturated catchment conditions led to prolong flooding
across the catchment.
Limited properties were affected due to the completion of the Mallow and Fermoy schemes
Mallow: Town Park area
Fermoy: Town Bridge and right bank
Killavullen: Fields flooded
Ballyduff: fields flooded
7 residential , 1 commercial
22 residential 16 commercial
None reported
None reported
17 hours +
04/08/2012 Fluvial event due to intense rainfall overtopping banks by the National Primary School
Rathcormac: Main Street 1 residential property and 1 commercial
< 2 hours
17/10/2012 Tidal flooding from storm surge overtopped quay walls.
Youghal: Quayside, The Mall, Market Place and Catherine Street
1 commercial, no residential because individual flood protection
measures in place (up to 40 if protection not in place)
14 hours
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Figure 4.1: Progression of the November 2009 Flood
0
100
200
300
400
500
600
700
18/11/2009
19/11/2009
20/11/2009
21/11/2009
22/11/2009
23/11/2009
24/11/2009
25/11/2009Fl
ow (m
3/s)
18006 CSET MALLOW FLOW Revised Rating 18055 MALLOW RAILWAY BR FLOW OPW Rating
18003 KILLAVULLEN FLOW Revised Rating 18107 FERMOY DS FLOW MM Applied Rating
18002 BALLYDUFF FLOW Revised Rating
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Flood Event of 10th January 2008
The Mallow, Fermoy and Ballyduff gauges record a high flow on the 10th January 2008 along the
Blackwater. Aerial photographs indicated flooding across Mallow Town Park to Lidl and flooding along
Meadowlands prior to the completion of the Mallow Drainage scheme. The flood risk to these areas has
since been reduced due to the installation of the embankments and flood walls as part of the scheme.
Flood Event of 27th October 2004 affecting Youghal
Youghal was flooded on 27th October 2004 by a combination of extreme high tides and an extreme storm
surge (1.5m over the predicted astronomical high tide level). Extreme waves damaged and overtopped the
sea defences at Youghal, which were known to be in a poor condition at the time. The beach at Front
Strand and Claycastle to the rear flooded with much of Youghal’s main harbour area inundated with flood
waters, flooding Catherine Street, Market Square and reaching up to North Main Street. Water levels were
at their highest at Barry’s Lane and Youghal Fire Station. Property flooding was reported, but the final
damage estimates are unknown.
The heavy rainfall also resulted in high flows along the Blackwater and Allow. However no property
flooding was reported in Mallow or Fermoy. The Kanturk area was affected when flooding occurred
downstream of the main town, due to the overtopping of the Brogeen River.
Flooding of 6th November 2000 affecting Dromcummer, Mallow, Fermoy and Ballyduff
Two large flood events occurred on the 5th and 6
th November 2000. The second event caused flooded as
the catchment was already saturated and river levels were high from the day before. Water depths rose to
2.8m early on the 6th of November in Mallow Town Park. Significant flooding was also reported at the Race
Course, Bridge Street and along Park Road. Extensive areas of Blackwater valley were flooded from
Dromcummer to Fermoy and Ballyduff as shown in the aerial videos taken just after the flood event. The
video extents have been visually inspected and used to verify with the hydraulic model.
Flood Event of 26th August 1997 affecting Freemount
Freemount was flooded due to intense rainfall falling over a relatively short period. Between the hours of
6:00PM and 11:00PM over 90mm of rain fell, exceeding the capacity of a tributary to the River Allow,
known locally as ‘Freemount Stream’. A considerable amount of debris was moved by the high flows which
blocked the four main culverts towards the east of the village, resulting in a flood depth of up to 1 m at the
right bank. Excess flood waters flowed down Main Street and an estimated IR£210,000 worth of damage to
private property (houses, cars, gardens) and a further IR£15,000 cost of cleanup (Cork County Council,
1997).4
4 Cork County Council (1997) Freemount Flood Report [online] www.floodmaps.ie
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Flood Event of 22nd
of October 1988 affecting Mallow and Fermoy
The floods that hit Fermoy and Mallow during October 1988 were due to heavy rainfall after a period of
consistently high rainfall events in the weeks preceding. With over 40mm of rain falling over the 21st
October, Mallow Town Park alongside the River Blackwater was flooded by 11:30 AM and Bridge Street
began to flood by 5:45 PM. Later in the day, flooding began at Ballyhadeen (07:15 PM) and Broom Lane
(07:30 PM). The maximum flood depth occurred at Bridge Street, with a depth of 1.6 m. The flood water
receded by 06:00 PM the next day. At Fermoy, the left bank at Fermoy Bridge to Brian Boru Square,
Frances Street and Rathealy Road flooded. Flooding also occurred to the south at Ashe Quay and O’Neil
Crowley Quay. The flood is ranked the 3rd
largest on record according to the available data at both Mallow
and Fermoy.
Flood Event of 6th August 1986 affecting Kanturk
On this date, flash flooding occurred throughout County Cork, Kanturk suffered a large flood in which the
River Allow and the River Dalua overtopped the river banks, causing flooding to properties around Market
Square. Mallow town was also affected with the areas around Mallow Town Park flooding.
Flood Event of the 2nd
of November 1980 affecting Kanturk, Mallow and Fermoy
The 1980 flood is the largest flood recorded at Mallow. At 11:30 AM the water level rapidly rose to flood
Mallow town centre at around 12:00 Noon. Peak level occurred at 03:00PM and at this point it was not
possible to pass Mallow Town Bridge. Bridge Street was flooded with water up to 2.5m deep and the
flooding had subsided within 24 hours (ARUP, 2002).5 Fermoy was also flooded along the north and south
banks at Ashe and O’Neill Quays, although more detailed information on the properties flooded was not
available.
Further flooding at Kanturk was caused by the overtopping of the River Allow and Dalua, a short distance
to the south of the confluence, and the western areas of town flooded to a maximum depth of 2m. An
estimated IR£370,000 of damage was caused with 178 houses affected. R579 Strand Street was also
flooded.
Other Recurring Events
The floodmaps.ie website also detailed a number of other recurring flood events without specific dates:
Kanturk – Recurring flooding from the Dalua at Town Park and the R578 twice a year.
Kanturk – Recurring flooding from the Allow at Strand Street.
Tallow – Recurring flooding from the River Bride due to a combination of high tides and heavy rainfall.
5 ARUP (2002) River Blackwater (Mallow) Drainage Scheme [online] www.floodmaps.ie
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4.2 Historical Flood Mechanisms
Following the review of the historic reports and other data, the key flood mechanisms identified in UoM18
include:
Fluvial or river flooding: Fluvial flooding can occur when the capacity of the river channel is exceeded
due to excess flow from heavy rainfall or releases from reservoirs upstream. Flood waters typically
overtop river banks at low sections or where water is constricted by bridges or culverts forcing water
levels to rise upstream and flood surrounding areas. Most of the flooding reported in UoM18 is
attributed to fluvial flooding mechanisms.
Pluvial or surface water flooding: Pluvial flooding can occur when overland flow from intense rainfall
or prolonged heavy rainfall is unable to enter the urban drainage network or river channel either
because they are already full or there is a blockage. Pluvial flooding is exacerbated by the increase of
impermeable areas (such as concrete or tarmac) associated with urbanisation which increases the
amount of overland flow. The most recent flooding in Rathcormac was partly attributed to pluvial
flooding. It should be noted that the study of pluvial flooding is not included in the scope of the CFRAM
Study.
Coastal or tidal flooding: Extreme sea levels, waves and storm surges overtop coastal defences and
river banks in tidally influenced reaches, particularly when combined with high river flows for tidal rivers.
The October 2004 event in Youghal was attributed to wave overtopping and the tide-locking of the
urban tributaries. According to anecdotal evidence Tallow is also at risk from tidal flooding when
combined with high flows on the River Bride.
In addition to the mechanisms listed above, flooding in Ireland can also occur from groundwater.
Groundwater flooding can occur when water levels rise above the ground to flood low-lying fields and
property basements, typically when the catchment is saturated. The onset of flooding is very slow and
therefore hazard to people is limited. The River Funshion and parts of the Blackwater between
Dromcummer and Ballyduff are susceptible to this form of flooding as they are underlain by highly
permeable karstic systems. However, there are no records of groundwater flooding. Hence, groundwater
flooding has been discounted from further analysis. It should be noted that the study of groundwater
flooding is not included in the scope of the CFRAM Study.
Based on the historical flood evidence, the key mechanisms for each of the AFAs are as follows:
Aglish: Flooding occurs at the Ballynaparka Bridge on the Ballnaparka Stream at Aglish, affecting
several nearby properties. The local engineers indicated that such flooding occurs annually although no
specific dates were given. Anecdotal evidence also suggests flooding from the Goish River to
Ballycullane when the River Blackwater is in flood. The key mechanisms will be verified during the
hydraulic modelling analysis.
Ballyduff: Flooding is caused by the overtopping of the River Blackwater flooding and inundates the
surrounding fields. However, it was reported to be contained within the masonry wall on the left bank in
the November 2000 event.
Fermoy: Flooding is caused the overtopping of the River Blackwater along the left bank at Thomas
Street and along right bank, flooding roads near the hospital. Frequent flooding along both the right
and left banks has occurred over recent decades. This has resulted in the development of flood
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embankments along the left bank and flood walls along the right bank. Previous studies have shown
that the weir and town bridge do not significantly affect flood levels.
Freemount: Flooding is caused by the overtopping of the River Allow and the tributary known as the
River Keen that runs through Freemount affecting low-lying properties. Under-capacity culverts under
Main Street used to result in water flowing down the road and inundating properties. The road culvert
has subsequently been improved and no flooding has been reported since.
Kanturk: Flooding is caused by the overtopping of the rivers particularly at Dalua Footbridge when
flooding on the River Dalua interacts with high flows on the River Allow flooding the town park area.
Mallow: Recurring flooding is caused by the overtopping of the River Blackwater and Spa Glen. The
constriction of flow at the bridges combined with the inflows from Spa Glen causes flood levels to
increase and flood Bridge Street and the park area on the left bank. Rapid runoff and under capacity of
the urban drainage systems can also cause flooding on the various urban tributaries that flow through
Mallow. The frequent flooding of Mallow led to the development of flood defence walls, embankments
and pumping stations at Bridge Street to protect vulnerable properties.
Rathcormac: Flooding occurs when the Shanowen River overtops the river banks at culverted and
bridged sections flooding Main Street. The more recent flood events have been caused by intense
rainfall events and under-capacity urban drainage network.
Tallow: Overtopping of the River Bride at Tallow Bridge is caused by a combination of high tide and
high flows in the River Bride and the Glenaboy River that flows through the town. The hydraulic model
will extend along the Bride and lower Blackwater to fully consider the interaction between the high river
flows and high tidal conditions at Youghal.
Youghal: Primarily coastal flooding from extreme storm surges causes flooding in the Town although
the Claycastle area is at risk from wave overtopping as well. High levels in the Blackwater Estuary can
also prevent discharge from the smaller tributaries causing flooding as water “backs up” behind the tidal
sluices at Youghal Mudlands.
4.3 Historical Flood Frequency Estimates
An estimate has been made of the frequency for the historical flood events where there were recorded
river flows for the AFAs in UoM18. The number of river flow gauges provides sufficient coverage for
estimate of historical floods for the AFAs .The recorded peak flow at the various gauges was compared to
their annual maximum series, and the relative frequency derived using the Gringorten formula:
Where i is the relative rank in the annual maximum flow series (AMAX) and n is the number of values in
the AMAX series. The Gringorten plotting position is the most appropriate plotting formula when
considering the EV1 and GLO distributions. The Gringorten estimate was then reviewed against the
design flows detailed in Chapter 6 of this report to establish the final %AEP estimate (Table 4.2).
12.0
44.0
n
iFi
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Table 4.2: Estimation of Flood Frequency for Historical Flood Events with Records of Flooding
AFA/MPW
Nearest Gauging Station Historical Flood Event
Station No. Location Date Peak Flow (m3/s) Rank AEP (%) Comments
Freemount/Allow 18009 Riverview factor to Freemount*
26/08/1997 < 23 1 >50% Localised event
Rainfall indicates a 2%AEP storm event
Rainfall Estimate 41 2%
Kanturk/Allow 18009 Riverview* 06/08/1986 284 2 2%
21/10/1988 187 5 20%
30/12/1998 167 7 20-50%
30/11/2000 165 8 20-50%
01/08/2009 313 1 1.3%
19/11/2009 160 10 20-50%
Mallow/Blackwater 18006 CSET Mallow* 02/11/1980 695 1 0.5-1%
06/08/1986 520 2 5%
21/10/1988 517 3 5%
30/12/1998 444 5 10-20%
06/11/2000 421 9 20%
10/01/2008 436 7 10-20%
19/11/2009 443 6 10-20%
Fermoy/Blackwater 18003 Killavullen factored to Fermoy Bridge*
02/11/1980 946 1 0.5-0.1%
06/08/1986 537 5 10-20%
22/10/1988 644 2 5%
30/12/1998 555 4 10-20%
18107 Fermoy Bridge* 10/01/2008 510 6 20%
19/11/2009 570 3 10%
Ballyduff/ Blackwater 18002 Ballyduff* 03/11/1980 700 6 5-10%
22/10/1988 814 1 2-5%
30/12/1998 571 13 10-20%
06/11/2000 669 8 10%
20/11/2009 734 2 5%
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AFA/MPW
Nearest Gauging Station Historical Flood Event
Station No. Location Date Peak Flow (m3/s) Rank AEP (%) Comments
Youghal/Blackwater 19068 Ballycotton 27/10/2004 2.80mODM 1 0.1 Coastal
17/10/2012 2.60mODM 2 1 Coastal
Rathcormac/Shanowen 18001 Mogeely factored to Shanowen Stream in Rathcormac*
30/01/2009 5 2 10%
19/11/2009 6 1 5%
04/08/2012 No Data 1 - Localised rainfall event of similar magnitude to 19/11/2009
Aglish/ Ballynaparka‡ N/A N/A N/A No Data - -
‡ No flood events found on internet, literature and official sources.
* Riverview, CSET Mallow, Killavullen, Ballyduff and Mogeely use the revised rating curves from Chapter 5
N.B. The ranking of events is relative to the record length. However the estimate of %AEP has been adjusted to consider longer records for those gauges with short periods of data such as Fermoy.
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5.1 Gauge Review Selection
Extreme flood flows can be estimated from recorded water levels at gauging stations where a stage-
discharge relationship is known. Historically, rating curves have been derived from in-bank gaugings and
extrapolated to estimate extreme flood flows. If the gauge is by-passed, the use of an in-bank rating curve
may significantly underestimate flood flows. However, it is not always safe or practical to observe level and
flow during out-of-bank conditions. Therefore, hydraulic modelling has been used in the CFRAM study to
simulate out-of-bank conditions and extend the rating curve for high flows.
A number of key gauges were identified as having significant bypass flow or only low-flow gaugings in
UoM18. A high flows rating review was undertaken at the sites listed in Table 5.1 to improve the estimates
of out-of-bank flows and to update the AMAX series for subsequent use in the derivation of the design
flows.
Table 5.1: Gauges Requiring Rating Reviews
Gauge Name Gauge Number Watercourse Nearest AFA
Mogeely 18001 River Bride Tallow
Ballyduff 18002 River Blackwater Ballyduff
Riverview 18009 River Blackwater Kanturk
Allen’s Bridge 18010 River Dalua Kanturk
The previous flood relief schemes at Mallow and Fermoy had identified atypical progression of flows
between the two towns where flows were recorded as decreasing downstream. Therefore, the high flows
rating curves at four additional gauging stations were also checked to assess the relatively of peak flows in
the AMAX series. These included:
18006 CSET Mallow Gauge on the River Blackwater
18055 Mallow Railway Bridge Gauge on the River Blackwater
18003 Killavullen Gauge on the River Blackwater
18107 Fermoy Bridge Gauge(s) on the River Blackwater
5.2 River Bride at Mogeely (Gauge 18001) High Flows Rating Review
Gauge Description
The gauge at Mogeely is located on the River Bride immediately upstream of Mogeely Bridge on the right-
bank. Water levels are recorded via a stilling well and converted to flow using the existing OPW rating
equation (valid since 01 April 2007). In-bank levels and flows are principally controlled by the bridge
structure and constriction of the channel 15 m downstream of the bridge. Out-of bank flows are constrained
by the valley sides and the road until water levels rise and overtop the road through the gaps, bypassing
the gauge (Figure 5.1). Although the wall along the road has gaps and will therefore not fully prevent flow
over the road, they are taken account of in the model by incorporating a lower spill coefficient to represent
the obstruction they provide.
5 Rating Reviews
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The OPW has provided 76 spot gaugings of level and flow at the gauge between 1965 and 2009. 44 of
these have been measured to the current gauge datum, and only 5 have been measured since the
application of the latest rating curves. The spot gaugings to the previous gauge datum have not been
considered in this review as the corresponding cross-section elevations were not available. The highest
observed level is 9.7mODM which is below bankfull and approximately 50% of QMED. This warrants a
review of the high flow rating. Figure 5.2, 5.3 and 5.4 analyse the relevant spot gaugings for hysteresis,
seasonality and periodicity. The spot gauging at 16 m3/s and 0.9m was identified as an outlier from the
other spot gaugings although there were no comments in the spot gaugings logs on the reliability of this
measurement. All the other spot gaugings since 1983 are relevant for the current datum of 8.43 mODM
and were used to calibrate the rating curve.
Figure 5.1: Flood Flow Paths During High Flow Conditions at Mogeely Gauge
2.0m Flood Depth
1.0m
0.5m
0.2m
0.0m
Gauge
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Figure 5.2: Spot Gauging Hysterisis At Mogeely Figure 5.3: Spot Gauging Seasonality At Mogeely
Figure 5.4: Spot Gauging Over Time Periods At Mogeely
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 20 30 40 50
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)Rising Falling
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 20 30 40 50
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)Summer Winter
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 20 30 40 50
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)1983-1989 1989-1998 1998-1999 1999-2007 2007-2009
Outlier
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A 1D-2D hydraulic model was developed of the River Allow extending over 1km upstream to consider all
incoming flows and 3km downstream so that the downstream boundary assumptions did not affect the
gauge. Please refer to the map provided in Appendix B showing the model extent. A digital terrain model
of the floodplain was developed using the national SAR DTM adjusted to match the recent river channel
survey in this area. Appendix B provides a geoschematic of the model extent, details of the model
development and assumptions made.
Revised High Flow Rating
The hydraulic parameters, such as the Manning’s ‘n’ values and downstream boundary, were adjusted until
the model matched the spot gaugings thus forming the “design scenario”. The following variations from the
design hydraulic parameters were then used to assess the sensitivity of the rating curve at the gauge:
Increased Manning’s ‘n’ to upper recommended limit
Reduced Manning’s ‘n’ to lower recommended limit
Raised downstream boundary stage-discharge relationship
The model results were converted to relative stage based on the surveyed gauge datum and compared
with the spot gaugings (Figure 5.5). The modelled stage-discharge matched well with the spot gaugings up
to 0.8m stage and was within the scatter of the higher spot gaugings. Therefore, the model was deemed
suitable to estimate out-of-banks flows subject to refinement in the future with high flows gaugings.
Figure 5.5: Revised Rating Curve at Mogeely
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 50 100 150 200
Sta
ge
(m
)
Flow (m3/s)
Spot Gaugings Maximum Recorded Stage Bankfull Level
Existing Rating Design Scenario Increased Manning's N
Raised Downstream Boundary Reduced Manning's N
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Based on the spot gaugings and model results, it is recommended that the original rating curve be used for
low flows up to 1.84m but the modelled stage-discharge is used to revise the rating curve from 1.84m to
3.27m stage. The modelled stage-discharge was spilt up into 2 segments to represent the following
changes in gradient for out-of-bank flow:
1.844m to 2.985m – out-of-bank flow;
2.986m to 3.271m – significant bypass flow on right bank.
Regression analysis was then carried out for each section to derive the rating curve equation with the best
fit where the correlation coefficients (R2) for all the segments were all greater than 0.999. The resultant
regression curves were then interpolated to find the upper transition stage and rating curve parameters
derived. The revised high flow rating is presented in Table 5.2 as the power law format Q=C(h-e)β where;
Q is discharge;
h is the gauge height of the water surface;
e is the gauge height of zero flow for a control of regular shape, or of effective zero flow control for a
control of irregular shape;
C (constant) is the discharge when the head (h-e) equals 1.0;
β (constant) is the slope of the rating curve when plotted on a log scale (ratio of the horizontal distance
to the vertical distance).
Table 5.2: Mogeely 18001 Recommended Revised Rating Curve Parameters
Segment Lower Limit
(m stage)
Upper Limit
(m stage) C e β
1 (Original Rating 7.1) 0.000 0.562 70.000 0.100 2.360
2 (Original Rating 7.2) 0.563 1.756 17.500 -0.200 1.600
3 1.757 2.985 15.361 0.019 2.180
4 2.986 3.271 35.820 0.890 2.060
Bold denotes recommended changes to the existing rating
The resultant rating curve is provided in Table 5.3. Further details on the modelling decisions and rating
development can be found in Appendix B.
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Table 5.3: Mogeely 18001 Recommended Revised Rating Curve
Stage (m above Gauge Datum) Flow (m3/s)
0.15 0.06
0.30 1.57
0.40 4.08
0.50 8.05
0.60 12.25
0.70 14.79
0.80 17.50
0.90 20.38
1.00 23.43
1.10 26.63
1.20 29.98
1.30 33.48
1.40 37.12
1.50 40.90
1.60 44.82
1.70 48.87
1.80 54.06
1.90 60.90
2.00 68.18
2.10 75.90
2.20 84.08
2.30 92.71
2.40 101.80
2.50 111.35
2.60 121.37
2.70 131.86
2.80 142.82
2.90 154.25
3.00 166.78
3.10 183.47
3.20 200.99
3.27 213.73
Note: Revised rating curve shown for modelled range 0.15m to 3.27m only
Figure 5.6 displays the suggested updated AMAX series based on the revised rating curve above.
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Figure 5.6: Mogeely 18001 Recommended Updated AMAX Series
5.3 River Blackwater at Ballyduff (Gauge 18002) High Flows Rating Review
Gauge Description
The gauge at Ballyduff is located on the River Blackwater immediately downstream of Ballyduff Bridge on
the left-bank. Water levels are recorded via a stilling well and converted to flow using the existing OPW
rating equation (valid since 01 March 1999). The Blackwater at Ballyduff has a very flat water level profile
and levels are controlled by local bed levels at low flows and Lismore Weir, 8 km downstream, during flood
conditions. Interviews with the local County Waterford engineers indicated that the site could also become
tide locked and flooding is likely with high tide combined with high flows from Killavullen. Out-of bank flows
are constrained within the narrow valley and the raised road to the north of the Bridge acts as a barrier to
flows above 11mODM. The flat gradient along this reach means that any flooding downstream towards
Lismore limits the volume available at Ballyduff and can further constrain flows.
The OPW has provided 121 spot gaugings of level and flow at the gauge between 1948 and 2010.
However, only 25 of these have been measured to the current gauge datum and during the period of the
latest rating equation. The highest observed flow of 303m3/s is approximately at bankfull. However, the
maximum recorded level in 2009 is 0.9m above this, warranting a review of the high flow rating. Figure
5.7, 5.6 and 5.9 analyse the relevant spot gaugings for hysteresis, seasonality and periodicity.
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
110.0
120.0
130.0
140.0
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
AM
AX
Flo
w (m
3/s)
Year
Original Updated
Previous QMED (76.1 m3/s)
Updated QMED (84.9 m3/s)
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Figure 5.7: Spot Gauging Hysterisis At Ballyduff Figure 5.8: Spot Gauging Seasonality At Ballyduff
Figure 5.9: Spot Gauging Over Time Periods at Ballyduff
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 50 100 150 200 250 300 350
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)Faling Rising
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 50 100 150 200 250 300 350
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)Summer Winter
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 50 100 150 200 250 300 350
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)1971-1994 1948-1971 1994-1999 1999 - present
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There was no hysteresis observed in-bank, although reports by local engineers indicated that out-of-bank
flows were dependent on downstream conditions. There were no obvious seasonality trends in the spot
gaugings. The latest spot gaugings have a slightly higher stage due to the changes in gauge datum and
the reprofiling of the nearby channel sections over time. There is only one high flows spot gauging near
bankfull. However, the OPW hydrometrics team confirm this observation was reliable and no issues were
recorded at the time of the observation. Therefore, the highest spot gauging of 303 m3/s has been used to
calibrate the model.
A 1D-2D hydraulic model was developed of the River Blackwater at Ballyduff extending over 2.5km
upstream to consider all incoming flows and 8km downstream to Lismore Weir which is the controlling
hydraulic structure during high flows. A digital terrain model of the floodplain was developed using latest
LiDAR and the recent river channel survey was used to develop the in-bank model. The watercourses
crossing the floodplain have been represented using 2D breaklines based on survey. Dense vegetation at
field boundaries have been explicitly represented on the floodplain based on OSi mapping and site
observations. Appendix B provides a geoschematic of the model extent, details of the model development
and assumptions made.
Revised High Flow Rating
The hydraulic parameters, such as the Manning’s ‘n’ values and downstream boundary, were adjusted until
the model matched the spot gaugings thus forming the “design scenario”. The following variations from the
design hydraulic parameters were then used to assess the sensitivity of the rating curve at the gauge:
Increased Manning’s ‘n’ to upper recommended limit
Decreased Manning’s ‘n’ to lower recommended limit
Increased downstream boundary stage-discharge relationship (i.e. greater backwater)
The increase in the downstream boundary provides an assessment of any tidal locking at Lismore as
anecdotally reported by the County Engineer.
The model results were converted to relative stage based on the surveyed gauge datum and compared
with the spot gaugings (Figure 5.10). The Manning’s ‘n values were calibrated so that modelled stage-
discharge matched well with the spot gaugings up to bankfull. The model predicted out-of-bank flow above
3.1m stage (10.38mODM) and bypass flow over the road above 3.5m (10.78mODM) stage which
significantly increases the high flow estimates compared to the existing rating.
The decreased Manning’s ‘n’ test significantly increased flow both in-bank and out-of-bank. Therefore the
rating is relatively sensitive to the Manning’s ‘n’ assumed. The increased downstream boundary test did
not differ significantly to the design scenario on the rising limb. However, the raised downstream boundary
caused greater backwater on the falling limb.
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Figure 5.10: Revised Rating Curve at Ballyduff
8
8.5
9
9.5
10
10.5
11
11.5
12
0 100 200 300 400 500 600 700
Lev
el
(mO
DM
)
Flow (m3/s)
Design Scenario Decreased Manning's 'n' Increased Manning's 'n'
Increased Downstream Boundary Spot Gaugings Bankfull
Existing Rating
Gauge
11.5mODN Water Level at Gauge
10.9mODN Water Level at Gauge
10.7mODN Water Level at Gauge
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Based on the spot gaugings and model results, it is recommended that the original rating curve be used for
low flows up to bankfull, but the modelled stage-discharge is used to revise the rating curve for out-of-bank
flows. The modelled stage-discharge was spilt up into 2 segments:
0.590 to 3.095m – in-bank flow;
3.096m to 6.010m – out-of-bank flow with backwater effect on floodplain.
Regression analysis was then carried out for each section to derive the rating curve equation with the best
fit where the correlation coefficients (R2) for all the segments were all greater than 0.999. The resultant
regression curves were then interpolated to find the upper transition stage and rating curve parameters
derived. The revised high flow rating is presented in Table 5.4 as the power law format Q=C(h-e)β where;
Q is discharge;
h is the gauge height of the water surface;
e is the gauge height of zero flow for a control of regular shape, or of effective zero flow control for a
control of irregular shape;
C (constant) is the discharge when the head (h-e) equals 1.0;
β (constant) is the slope of the rating curve when plotted on a log scale (ratio of the horizontal distance
to the vertical distance).
Table 5.4: Ballyduff 18002 Recommended Revised Rating Curve Parameters
Segment
Lower Limit
(m stage) Upper Limit (m
stage) C e β
1 (Existing Rating) 0.590 3.095 40.700 -0.473 1.560
2 3.096 6.010 64.802 1.100 2.199
The resultant rating curve is provided in Table 5.5. Further details on the modelling decisions and rating
development can be found in Appendix B.
Table 5.5: Ballyduff 18002 Recommended Revised Rating Curve
Stage (m above Gauge Datum) Flow (m3/s)
0.59 44.84
0.70 52.20
0.80 59.31
0.90 66.74
1.00 74.47
1.10 82.51
1.20 90.83
1.30 99.44
1.40 108.33
1.50 117.49
1.60 126.91
1.70 136.59
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Stage (m above Gauge Datum) Flow (m3/s)
1.80 146.52
1.90 156.70
2.00 167.12
2.10 177.78
2.20 188.67
2.30 199.80
2.40 211.15
2.50 222.73
2.60 234.53
2.70 246.54
2.80 258.77
2.90 271.21
3.00 283.85
3.10 297.74
3.20 331.48
3.30 367.20
3.40 404.93
3.50 444.67
3.60 486.46
3.70 530.29
3.80 576.20
3.90 624.20
4.00 674.30
4.10 726.51
4.20 780.86
4.30 837.35
4.40 896.00
4.50 956.82
4.60 1019.82
4.70 1085.03
4.80 1152.44
4.90 1222.08
5.00 1293.95
5.10 1368.07
5.20 1444.44
5.30 1523.08
5.40 1604.00
5.50 1687.22
5.60 1772.73
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Stage (m above Gauge Datum) Flow (m3/s)
5.70 1860.55
5.80 1950.70
5.90 2043.18
6.00 2137.99
Note: Revised rating curve shown for modelled range 0.59m to 6.00m only
Figure 5.11 displays the suggested updated AMAX series based on the revised rating curve above.
Figure 5.11: 18002 Ballyduff Updated AMAX Series
0
100
200
300
400
500
600
700
800
900
19
55
19
57
19
59
19
61
19
63
19
65
19
67
19
69
19
71
19
73
19
75
19
77
19
79
19
81
19
83
19
85
19
87
19
89
19
91
19
93
19
95
19
97
19
99
20
01
20
03
20
05
20
07
20
09
AM
AX
Flo
w (
m3
/s)
Original Revised Rating
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5.4 River Allow at Riverview (Gauge 18009) High Flows Rating Review
Gauge Description
The gauge at Riverview is located 180m upstream of the ford on the River Allow on the right-bank. The
Water levels are recorded via a stilling well and converted to flow using the existing EPA rating equation
C6 (valid since 12 April 2010). In-bank levels and flows are principally controlled by the ford acting as a
weir downstream (see Appendix B for photographs). Out-of bank flows are constrained by the valley sides
and slope. The bridge downstream and River Blackwater downstream do not cause backwater at this site.
The EPA and Cork County Council have provided spot gaugings of level and flow at the gauge between
1977 and 2012. Over 50 measurements have been observed to the current gauge datum since 2000
permitting good calibration of the rating curve within the observed range. The highest observed flow of
39.3 m3/s is less than 50% of QMED, warranting a review of the high flows rating. Figures 5.12, 5.13 and
5.14 analyse the relevant spot gaugings for hysteresis, seasonality and periodicity.
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Figure 5.12: Key Spot Gauging Hysterisis At Riverview Figure 5.13: Key Spot Gauging Seasonality At Riverview
Figure 5.14: Spot Gauging Over Time Periods At Riverview
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 20 30 40
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)
Rising Falling
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 20 30 40
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)
Summer Winter
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 20 30 40
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)
2001-present 1994-2001 1986-1994 1977-1986
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There was no hysteresis observed in the spot gaugings and no obvious indications of differences in the
stage-discharge relationship between summer and winter months. The latest spot gaugings have a higher
stage due to the changes in gauge datum over time. No information has been provided by the EPA on
datum changes. Site visit observations would suggest this datum change is caused by recalibration of
gauging equipment and minor silt build up behind weir/ford downstream of Riverview gauge. Only those
spot gaugings measured since 2001 were used to calibrate the model as these were consistent with the
current gauge datum and cross-section.
A 1D hydraulic model was developed of the River Allow from the confluence of the Allow and Brogeen to
the confluence of the Allow with the Blackwater at Leaders Bridge. This allows the model to account for all
incoming flows from upstream and any backwater influence downstream. A 1D approach was deemed
appropriate because the floodplain was at or above the bank levels so the velocities across the valley
section can be assumed to be similar. The small ditch on the left bank at 138534, 100845 can be assumed
to become flooded as water level rises because water can enter this feature further upstream. A digital
terrain model of the floodplain was developed using latest LiDAR, and the recent river channel survey was
used to develop the in-bank model. Appendix B provides a geoschematic of the model extent, details of the
model development and assumptions made.
Revised High Flow Rating
The hydraulic parameters, such as the Manning’s ‘n’ values and downstream boundary, were adjusted until
the model matched the spot gaugings thus forming the “design scenario”. The following variations from the
design hydraulic parameters were then used to assess the sensitivity of the rating curve at the gauge:
Increased Manning’s ‘n’ to upper recommended limit
Reduced Manning’s ‘n’ to lower recommended limit
Raised downstream boundary stage-discharge relationship
The model results were converted to relative stage based on the surveyed gauge datum and compared
with the spot gaugings (Figure 5.15). The modelled stage-discharge was within 0.02m of the higher spot
gaugings up to 73.1mODM or 1.32m stage (the highest gauged flow).
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Figure 5.15: Revised Rating Curve at Riverview
Flow paths estimated from 1D model of
floodplain and DTM
Based on the spot gaugings and model results, it is recommended that the original rating curve be used for
low flows up to 1.91m but the modelled stage-discharge is used to revise the rating curve from 1.91m to
3.73m stage. The modelled stage-discharge was spilt up into 3 segments to represent the following
changes in gradient:
1.991 to 2.812m – High flows in-bank
2.813 to 3.728m – Bypass flow across the floodplain.
Regression analysis was then carried out for each section to derive the rating curve equation with the best
fit, where the correlation coefficients (R2) for all the segments were all greater than 0.999. The resultant
regression curves were then interpolated to find the upper transition stage and rating curve parameters
derived. The revised high flow rating is presented in Table 5.6 as the power law format Q=C(h-e)β where;
Q is discharge;
h is the gauge height of the water surface;
e is the gauge height of zero flow for a control of regular shape, or of effective zero flow control for a
control of irregular shape;
C (constant) is the discharge when the head (h-e) equals 1.0;
β (constant) is the slope of the rating curve when plotted on a log scale (ratio of the horizontal distance
to the vertical distance).
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Table 5.6: Riverview 18009 Recommended Revised Rating Curve Parameters
Segment
Lower Limit
(m stage) Upper Limit (m
stage) C e β
1 (Existing Rating) 0.158 1.910 24.3027 0.000 1.926
2 1.911 2.812 20.844 0.000 2.221
3 2.813 3.728 17.702 0.500 2.832
The resultant rating curve is provided in Table 5.7. Further details on the modelling decisions and rating
development can be found in Appendix B.
Table 5.7: Riverview 18009 Recommended Revised Rating Curve
Stage (m above Gauge Datum) Flow (m3/s)
0.16 0.70
0.30 2.39
0.40 4.16
0.50 6.40
0.60 9.09
0.70 12.23
0.80 15.81
0.90 19.84
1.00 24.30
1.10 29.20
1.20 34.52
1.30 40.28
1.40 46.45
1.50 53.06
1.60 60.08
1.70 67.51
1.80 75.37
1.90 83.64
2.00 92.32
2.10 101.42
2.20 110.92
2.30 120.83
2.40 131.15
2.50 141.88
2.60 156.07
2.70 175.97
2.80 197.45
2.90 220.58
3.00 245.41
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Stage (m above Gauge Datum) Flow (m3/s)
3.10 272.00
3.20 300.41
3.30 330.68
3.40 362.88
3.50 397.07
3.60 433.29
3.70 471.61
3.72 480.72
Note: Revised rating curve shown for modelled range 0.16m to 3.72m only
Figure 5.16 displays the revised rating AMAX flows and suggested updated AMAX series for statistical
analysis.
Figure 5.16: Riverview 18009 Updated AMAX Series
0
50
100
150
200
250
300
350
19
82
19
83
19
84
19
85
19
86
19
87
19
88
19
89
19
90
19
91
19
92
19
93
19
94
19
95
19
96
19
97
19
98
19
99
20
00
20
01
20
02
20
03
20
04
20
05
20
06
20
07
20
08
20
09
20
10
20
11
20
12
AM
AX
Flo
w (
m3/s
)
Previous AMAX (EPA rating C6) Revised AMAX Series
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5.5 River Dalua at Allen’s Bridge (Gauge 18010) High Flows Rating Review
Gauge Description
The gauge at Allen’s Bridge is located on the River Dalua, 12m upstream of the Bridge on the right-bank.
A smaller tributary, the Rampart Stream, joins the River Dalua 900m downstream of the bridge. However,
any backwater at this confluence is unlikely to affect the gauge as the bed level and water levels are
significantly below the gauge channel section. Moreover, no interaction was observed between the two
channels upstream of the bridge due to higher ground levels on the floodplain separating the two
catchments. Water levels are recorded via a stilling well and converted to flow using the existing EPA
rating equation (valid since 13 September 1997). In-bank levels and flows are principally controlled by the
weir downstream of the bridge, with the bridge openings influencing only the most extreme flows. Flows
are liable to by-pass the bridge before water levels reach the soffit, as the flood waters are able to flow
over the road on the left bank below soffit level.
The OPW has provided 127 spot gaugings of level and flow at the gauge between 1977 and 2009.
However, only 21 of these have been measured to the current gauge datum since 2001. The highest
measured flow of 14 m3/s is below bankfull and the bridge soffit and approximately 30% of QMED. This
warrants a review of the high flow rating. Figures 5.17, 5.18 and 5.19 analyse the relevant spot gaugings
for hysteresis, seasonality and periodicity.
The highest spot gaugings tended to be observed on the rising limb rather than the falling limb of an event
but the spot gaugings did not suggest any hysteresis effect in-bank. There was no discernible difference
between spot gaugings observed in the summer and winter months for in-bank flows. The gauge datum
and channel section has changes less than 0.01m over the past few decades. Therefore, the stage-
discharge relationship from all spot gaugings is fairly consistent over the entire period.
A 1D hydraulic model was developed of the River Dalua from the opening of the floodplain 800m upstream
of the gauge and was extended 950m downstream of the bridge to ensure the model downstream
boundary does not affect the gauge. A 1D approach was deemed appropriate because the floodplain was
at or above the bank levels so the velocities across the valley section can be assumed to be similar. A
digital terrain model of the floodplain was developed using the national SAR DTM. This SAR data was
adjusted to meet the recent river channel survey and then was used to develop extended sections for the
floodplain. Appendix B provides a geoschematic of the model extent, details of the model development and
assumptions made.
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Figure 5.17: Spot Gauging Hysteresis At Allen’s Bridge Figure 5.18: Spot Gauging Seasonality At Allen’s Bridge
Figure 5.19: Spot Gauging Over Time Periods At Allen’s Bridge
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14 16
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)
Rising Falling
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14 16
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)
Summer Winter
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14 16
Stag
e (m
abo
ve G
auge
Dat
um)
Flow (m3/s)
1977-1982 1982-1994 1994-1997 1997-2001 2001- present
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Revised High Flow Rating
The hydraulic parameters, such as the Manning’s ‘n’ values and downstream boundary, were adjusted until
the model matched the spot gaugings thus forming the “design scenario”. The following variations from the
design hydraulic parameters were then used to assess the sensitivity of the rating curve at the gauge
(Figure 5.20):
Increased Manning’s ‘n’ to upper recommended limit
Reduced Manning’s ‘n’ to lower recommended limit
Raised spill coefficient over the road
Reduced spill coefficient over the road
Raised Weir coefficient
Raised downstream boundary gradient
Figure 5.20: Revised Rating Curve at Allen’s Bridge
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 50 100 150 200 250 300 350
Stag
e (
m a
bo
ve G
auge
Dat
um
)
Flow (m3/s)
Design Scenario Spot GaugingsRaised Downstream Boundary Decreased Manning's NIncreased Weir Coefficient Decreased Spill Coefficient Over RoadIncreased Spill Coefficient Over Road Existing RatingIncreased Manning's N Bankfull
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The existing rating curve has been derived from the spot gaugings measured at low flows when there is
negligible backwater influences from the weir and bridge structure. The hydraulic modelling results fit well
with the low flow spot gaugings, but there is significant backwater above 1m stage as the constriction of
the bridge and weir becomes more significant. The modelled backwater effect was verified during the
survey and results in lower flows than the previous EPA rating which does not consider backwater. Above
2.75m stage, the gauge is bypassed with flood waters spilling over the road increasing flow significantly
compared to the increase in water level. Figure 5.20 shows limited difference between the design scenario
and the Manning’s ‘n ’and raised downstream boundary sensitivity test which have less than 1m3/s
difference. The spill coefficient over the road sensitivity tests differ from the design above 2.7m stage by
less 15m3/s. However, the model was sensitive to the weir coefficient assumed under the bridge but the
raised weir coefficient did not match the spot gaugings at the low levels.
Based on the spot gaugings and design scenario model results, it is recommended that the original rating
equations be used for low flows up to 0.8m but the modelled stage-discharge is used to revise the rating
curve from 0.81m to 3.55m stage. The modelled stage-discharge was spilt up into segments to represent
the following changes in gradient:
0.806 to 2.754m – Flow influenced by backwater from the bridge and weir structure.
2.755 to 3.553m – Bypass flow spilling over the road.
Regression analysis was then carried out for each section to derive the rating curve equation with the best
fit where the correlation coefficients (R2) for all the segments were all greater than 0.999. The resultant
regression curves were then interpolated to find the upper transition stage and rating curve parameters
derived. The revised high flow rating is presented in Table 5.8 as the power law format Q=C(h-e)β where;
Q is discharge;
h is the gauge height of the water surface;
e is the gauge height of zero flow for a control of regular shape, or of effective zero flow control for a
control of irregular shape;
C (constant) is the discharge when the head (h-e) equals 1.0;
β (constant) is the slope of the rating curve when plotted on a log scale (ratio of the horizontal distance
to the vertical distance).
Table 5.8: Allen’s Bridge 18010 Recommended Revised Rating Curve Parameters
Segment Lower Limit (m
stage) Upper Limit (m
stage) C e β
1 (Existing equation 1) 0.21 0.373 106.017 0.000 4.111
2 (Existing equation 2) 0.374 0.805 22.586 0.000 2.544
3 0.806 2.754 19.369 0.000 1.835
4 2.755 3.553 18.245 0.851 2.983
The resultant rating curve is provided in Table 5.9. Further details on the modelling decisions and rating
development can be found in Appendix B.
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Table 5.9: Allen’s Bridge 18010 Recommended Revised Rating Curve
Stage (m above Gauge Datum) Flow (m3/s)
0.21 0.17
0.30 0.75
0.40 2.19
0.50 3.87
0.60 6.16
0.70 9.11
0.80 12.80
0.90 15.96
1.00 19.37
1.10 23.07
1.20 27.07
1.30 31.35
1.40 35.92
1.50 40.77
1.60 45.89
1.70 51.29
1.80 56.97
1.90 62.91
2.00 69.12
2.10 75.60
2.20 82.33
2.30 89.33
2.40 96.59
2.50 104.11
2.60 111.88
2.70 119.90
2.80 133.51
2.90 155.00
3.00 178.67
3.10 204.63
3.20 232.98
3.30 263.83
3.40 297.27
3.50 333.43
3.54 349.44
Note: Revised rating curve shown for modelled range 0.21m to 3.54m only
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Figure 5.21 displays the updated AMAX flows which have reduced by up to 50% due to the backwater
effect from the bridge downstream.
Figure 5.21: Allen’s Bridge 18010 Updated AMAX Series
5.6 River Blackwater Rating Checks
5.6.1 CSET Mallow 18006
The CSET Mallow gauge is situated by the old sugar factory on the right hand side of the floodplain at the
race course. The previous Mallow Drainage Scheme identified underestimation of high flows above
bankfull. The 1D-2D hydraulic model developed for the flood mapping of Mallow AFA supports this
underestimation. Figure 5.22 compares the existing rating with the Appendix B Mallow Drainage Scheme
rating and the CFRAM study results. Screenshots of the model results and aerial footage of the 2009 event
show flows exiting upstream of the gauge, flowing across the racecourse and re-entering at the
downstream meander. Therefore, the Appendix B Mallow Drainage Scheme rating curve was used to
update the AMAX series and flow series at CSET Mallow (Table 5.10). The resultant AMAX series is
shown in Figure 5.23.
0
20
40
60
80
100
120
140
160
19
81
19
82
19
83
19
84
19
85
19
86
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87
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88
19
89
19
90
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91
19
92
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93
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94
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95
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96
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97
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98
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99
20
00
20
01
20
02
20
03
20
04
20
05
20
06
20
07
20
08
20
09
AM
AX
Flo
w (
m3/s
)
Previous AMAX (EPA Rating C4) Updated AMAX (Revised rating)
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Figure 5.22: Check of CSET Mallow High Flows Rating
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Table 5.10: CSET Mallow 18006 Applied Rating Curve Parameters
Segment Lower Limit (m
stage) Upper Limit (m
stage) C e β
1 (Existing equation) 0.30 4.508 24.446 0.000 1.686
2 (Mallow Drainage Scheme/CFRAM Study) 4.509 6.000 38.430 2.070 2.341
Figure 5.23: Updated AMAX Series for CSET Mallow 18006
0
100
200
300
400
500
600
700
800
19
78
19
79
19
80
19
81
19
82
19
83
19
84
19
85
19
86
19
87
19
88
19
89
19
90
19
91
19
92
19
93
19
94
19
95
19
96
19
97
19
98
19
99
20
00
20
01
20
02
20
03
20
04
20
05
20
06
20
07
20
08
20
09
20
10
20
11
20
12
Flo
w (
m3
/s)
Updated AMAX Series Existing AMAX Series (EPA)
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5.6.2 Mallow Railway Bridge 18055
The Mallow Railway Bridge gauge is located on the right bank upstream of the Railway Bridge. The
majority of flows are gauged as the railway embankment prevents significant bypassing of the gauge.
Figure 5.24 compares the CFRAM model results with the spot gaugings and existing rating curve. Although
there some scatter in spot gaugings around 47mODM (road level on the right bank), the model results
agree well with the existing rating. Therefore the AMAX series has not been revised at this gauge.
Figure 5.24: Check of Mallow Rail Bridge High Flows Rating
5.6.3 Killavullen 18003
The Killavullen gauge is situated on the upstream face of Killavullen Bridge. The gauge is bypassed when
water overtops the raised road embankment on the left bank. The previous Fermoy Drainage Scheme
(2006) identified underestimation of high flows above bankfull based on section data. The 1D-2D hydraulic
model of Mallow AFA was extended to Killavullen using the latest detailed LIDAR to fully consider
floodplain flows. Figure 5.25 compares the existing rating with the Fermoy Drainage Scheme 2006 rating
and the CFRAM study results. Screenshots of the model results and aerial footage of the 2009 event show
considerable bypass flow on the left bank.
43
43.5
44
44.5
45
45.5
46
46.5
47
47.5
48
0 100 200 300 400 500 600
Leve
l (m
OD
M)
Flow (m3/s)
Spot gaugings Nov 2009 Event Peak Water Level Model Results (1D-2D) Existing Rating
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Figure 5.25: Check of Killavullen High Flows Rating
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The CFRAM model results agree well with the spot gaugings once the unreliable 2008 spot gauging has
been discounted due to issues whilst recording the high flow. The model results support the previous
Fermoy Drainage Scheme high flows above 38mODM. Therefore the Fermoy Drainage Scheme 2006 high
flows rating (Table 5.11) was used to update the AMAX series (Figure 5.26).
Table 5.11: CSET Mallow 18006 Applied Rating Curve Parameters
Segment Lower Limit (m
stage) Upper Limit (m
stage) C e β
1 (Existing equation) 1.381 4.069 36.2 0.54 1.528
2 (Fermoy Drainage Scheme/CFRAM Study) 4.070 - 3.617 0.0036 3.016
Figure 5.26: Updated AMAX Series for Killavullen 18003
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
900.0
1955 1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011
Existing AMAX Updated AMAX
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5.6.4 Fermoy Bridge 18106/18107
The 18106 and 18107 gauges at Fermoy Bridge are situated on the right bank upstream of the skew weir
and on the left bank downstream of the skew weir and bridge respectively (Figure 5.27a). The existing
Fermoy Drainage Scheme SOBEK hydraulic model has been converted to a full 1D-2D ISIS-TUFLOW
model to fully consider any floodplain flow for flood mapping. The combination of the skew weir and
arched bridge leads to complex interaction of flows which are simplified in the 1D approach. There is some
uncertainty in the threshold at which the weir drowns out both in the modelling and in the limited spot
gaugings available. Therefore, the analysis has been undertaken at the downstream gauge for the
hydrological analysis.
Figure 5.27 compares the CFRAM study model results with the OPW’s draft rating curve derived from spot
gaugings only. Screenshots of the 2009 event shows no bypassing on the right bank and only limited
bypass flow on the left bank. Based on the spot gaugings and model results, it is recommended that the
modelled stage-discharge was spilt up into segments to represent the following changes in gradient:
0.150m to 4.193m – In-bank flow at the downstream gauge
4.194m to 6.150m – Bypass flow spilling over the road on left bank.
Regression analysis was then carried out for each section to derive the rating curve equation with the best
fit where the correlation coefficients (R2) for all the segments were all greater than 0.999. The resultant
regression curves were then interpolated to find the upper transition stage and rating curve parameters
derived. The rating equations applied for the CFRAM Study are presented in Table 5.12.
Table 5.12: Fermoy Bridge 18107 Applied Rating Curve Parameters
Segment Lower Limit (m
stage) Upper Limit (m
stage) C e β
1 0.150 4.193 45.391 -0.300 1.637
2 4.194 6.150 17.812 -0.461 2.208
Figure 5.28 displays the calculated AMAX flows based on the rating equations above.
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Figure 5.27: Fermoy Bridge Rating Curve
A: Model Configuration B: Rating Curve
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Figure 5.28: Fermoy Bridge 18107 Calculated AMAX Series
0
100
200
300
400
500
600
700
2001 2002 2003 2004 2005 2006 2007 2008 2009
Flo
w (
m3/s
)
AMAX Year
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6.1 Overview
The hydrological approach draws on the data review described in Chapters 3, 4 and 5 of this report and
the latest Flood Studies Update (FSU) guidance. The hydrological analysis to derive design fluvial
hydrographs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1% AEP has been undertaken as follows:
Define the sub-catchments and locations at which to calculate design flows (Section 6.2);
Estimate the index flood flow for the 50% AEP flood (Section 6.3);
Estimate the flood growth curve to derive more extreme flood events (Section 6.3); and
Estimate the typical flood hydrograph shape (Section 6.4).
The hydrological analysis to derive design coastal conditions for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5%
and 0.1% AEP has been undertaken as follows:
Transformation of total tide plus surge levels along the coast to the model downstream extent (Section
6.5.1);
Estimate the typical tide plus surge profile (Section 6.5.1);
Estimate wave overtopping discharges at vulnerable locations (Section 6.5.2).
6.2 Definition of Sub-Catchments
6.2.1 Hydrological Estimation Points
Hydrological estimation points (HEPs) have been chosen at key locations in the River Blackwater
catchment to form the hydraulic model inflows, intermediate target flows for the model to achieve, and
downstream conditions for the model.
The HEPs were identified through a GIS analysis based on the following principles from Section 6.5.3 of
the Generic CFRAM Specification:
A central location within the AFA;
Flow gauging stations used in the hydrological analysis;
Upstream and downstream limits of each hydraulic model reach;
Major confluences which contribute significant flow to the modelled reach; and,
Locations where the physical catchment descriptors (PCD) significantly change from the upstream
catchment i.e. catchment centroid more than 25km away, ±0.15 change in BFI and ±0.07 change in
FARL.
Table 6.1 summarises the selected HEPs prior to hydraulic modelling. Individual maps and catchment
descriptors for each AFA and MPW reach are given in Appendix C.
6 Design Flows
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Table 6.1: Selected HEPs
HEP Type Number in UoM18
Gauged 9
Model Inflow 48
Downstream target/inflow 6
Target 98
Downstream tidal 1
TOTAL 155
6.2.2 Sub-Catchment Delineation
The River Blackwater catchments were conceptualised into sub-catchments based on the latest Flood
Studies Update (FSU) database (supplied 2011). Map 6.1 displays the three key sub-catchments.
GIS spatial analysis was undertaken on the national digital elevation model to determine slope aspect and
subsequently identify the watersheds for each catchment. The output from this GIS analysis was compared
with the automated FSU catchment boundaries and verified against manual interpretation from Ordnance
Survey mapping at 1:50,000 scale, previous hydrological reports, and observations from site visits. The
other physical catchment descriptors were also reviewed including; average slope (S1085); average
rainfall (SAAR); runoff indicators (SPR); permeability indicators (BFI); and attenuation (FARL). Information
from the Geological Survey of Ireland (GSI) was also used to assess the impact of underlying geology and
aquifers on permeability and groundwater dominance, as well as to inform those catchments influenced by
karstic systems.
Overall, the automated FSU catchment boundaries were found to match the Ordnance Survey mapping
well in areas of steep relief. However, where the terrain is flatter and the watershed less distinct, there
were some discrepancies between the FSU catchments, those derived from OSI mapping and the more
detailed 5m resolution national DTM (see Map 6.2). Therefore, the boundaries were modified and the
revisions adopted. However, these modifications were minor, were less than 1km2 in area and did not
significantly change the parameters for the HPW and MPWs reaches assessed as part of this CFRAM
study, nor the area draining into the neighbouring river basin district.
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Map 6.1: Sub-Catchments
Allow Catchment
Blackwater Catchment
Bride Catchment
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Map 6.2: Example Catchment Boundary Modification, Sruhaneballiv Stream
The other physical catchment descriptors were also reviewed including; average slope (S1805); average
rainfall (SAAR); runoff indicators (SPR); permeability indicators (BFI); and attenuation (FARL). Information
from the Geological Survey of Ireland (GSI) was also used to assess the impact of underlying geology and
aquifers on permeability and groundwater dominance, as well as inform those catchments influenced by
karstic systems.
Analysis of the catchment parameters for UoM18 indicates that:
The upper catchments of the Upper Blackwater, River Allow, River Dalua all have low BFI indicating
lower permeability and a faster hydrograph response to rainfall in the North West of UoM18.
Catchments to the north of the River Blackwater have a higher BFI value indicating much higher
permeability and a slower hydrograph response to rainfall.
The River Awbeg Minor, Awbeg Major and Funshion to the north of Mallow and Fermoy are underlain
by karst, and these rivers are spring fed in their upper reaches indicating groundwater dominance for
low flows.
The highest standard average rainfall is in the west and north east of the Blackwater catchment but the
Awbeg catchment has the lowest rainfall as it partly falls in a rainshadow effect from the western
mountainous areas.
All the modifications made to the original FSU database are provided in Appendix C.
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6.3 Flood Frequency Analysis
6.3.1 Methodology
Flood frequency analysis was undertaken at gauged and ungauged sites to derive the design fluvial
hydrographs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1% AEP events as boundary conditions for
the hydraulic modelling.
Gauged Sites
The index flood flow was derived from the median value of the Annual Maximum Flood Series (AMAX) at
gauged sites within or linked to the AFAs, and compared with the FSU 7-variable QMED rural estimate
(FSU WP2.3). Previous research by the FSR indicated that the index flood is proportional to AREA0.77
.
This relationship was applied as a check to identify atypical QMED flows for catchment size.
The Extreme Values (EV1), logistic (LO), generalised logistic (GLO) and generalised extreme value (GEV)
distributions were then fitted to the AMAX series to establish the most appropriate flood growth curve for
%AEP up to twice the record length at the gauged location (FSU WP2.2). For rarer, more extreme events,
hydrologically similar gauge sites were selected to form a pooling group based on the Euclidian distance
measure (dij) between catchment characteristics at the gauged site. Descriptors considered include AREA,
SAAR, BFISOILS, the ratio of the highest gauged flow to QMED, the presence of underlying karstic
features and any issues highlighted by the OPW hydrometric team.
It was not always possible to find sufficient pooling sites of a similar size, BFI and SAAR, and the selection
criteria had to be relaxed in order to achieve the target record length of 500 years ( 5 times the target
1%AEP). The selection of the pooling group was a balance between selecting hydraulically similar sites,
maintaining homogeneity across the group and achieving the required record length. The pooled L-
Moment average for each pooling group was then compared with the various distributions to guide the
selection of the most appropriate flood growth curve.
Ungauged Sites
At ungauged locations, the QMEDrural values were estimated using the 7 variable equation (FSU WP 2.3)
based on gauged data from 190 sites across Ireland:
408.0
185.0341.0217.2306.1922.0937.05
)21(
108510237.1
ARTDRAIN
SDRAINDFARLSAARBFISOILSAREAQMEDrural
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Where:
AREA is the total contributing area of the catchment
BFISOILS is an index of permeability
SAAR is the Standard Annual Average Rainfall between 1961 and 1990
FARL is an index of floodplain attenuation
S1085 is the typical slope between 10% and 85% along the river reach
ARTDRAIN2 is a proportion of the catchment which is artificially drained.
Pivotal gauged sites were then used to adjust the QMEDrural as recommended by FSU WP 2.3. The pivotal
gauged sites were selected from hydrologically similar gauges across Ireland with a preference for
geographically close locations to better represent rainfall characteristics in the South West area.
Hydrological similarity was guided by the similarity of physical catchment descriptors based on FSU
hydrological guidelines:
Area of pivotal site within a factor of 5 of the target ungauged HEP;
BFI soils index within 0.18 of the target ungauged HEP;
SAAR within a factor of 1.25 of the target ungauged HEP;
FARL within 0.05 of the target ungauged HEP.
Grade A gauges were assumed to be of reliable quality unless otherwise stated by the FSU report. Grade
B gauges were further assessed for the presence of lakes/reservoirs, significant karstified features and
FSU quality of the gauge, to ensure the gauge was suitable to inform the adjustment of QMED at the
ungauged target HEP.
The pooled analysis was used to derive appropriate flood growth curves for all ungauged sites. The
pooling group AMAX data was collated to create a combined record length of 500 years, which is in
accordance with the 5T rule of five times the record length of the target design event, i.e. the 1 in 100 year
or 1%AEP event. The criteria were lowered for selection of pooling group sites in the smaller tributary
catchments of Rathcormac, Fermoy, Aglish and Ballyduff in order to achieve a balance between finding
hydrologically similar sites and achieving the 500 years pooled record length from the target 1%AEP.
The pooling group was reviewed for gauges influenced by karstic geology based on the Geological Survey
of Ireland data and compared with the BFIsoils parameter. Sites influenced by karst were not necessarily
rejected as HEPs on the Blackwater, Awbeg, Funshion and Araglin are also karstic. However, gauges
19001, 19031, 21004 and 22009 were also rejected from pooling analysis due to the OPW’s hydrometric
team’s concerns with the estimation of high flows at these sites. The pooled L-Moment average for each
pooling group was used to identify discordant sites and select the most appropriate statistical distribution.
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Alternative approaches considered for ungauged and smaller catchments
It should be noted that the FSU 7 variable equation was not developed for catchments less than 5km2 in
size due to the lack of reliable gauge records for such small catchments in Ireland. Alternative methods,
including the rational method, were found to better represent small catchments on average but tended to
over predict peak flows for small lowland catchments (Institute of Hydrology 1978).The modified rational
method (1981) is also not suitable to estimate greenfield runoff as it was developed specifically for sewer
design. The consensus from an exhaustive literature review was that it was not possible to verify the most
appropriate methodology without gauged records.
6.3.2 Estimation of the Index Flood
The index flood flow was derived from the median value of the Annual Maximum Flood Series (AMAX) as
provided by OPW at gauged locations (Table 6.2). The revised AMAX series at Mogeely, Ballyduff,
Fermoy, Killavullen, CSET Mallow, Riverview and Allen’s Bridge were used to derive the revised QMED
flow.
Table 6.2: Gauged QMED Values
Gauge ID Name Watercourse
AMAX Series Length (Years)
QMED Since 2000
(Wet Period)
QMED for Full Record
Adjustment Factor to
Full Record Selected
QMED
18001 Mogeely Bride 41 96.8 84.9 0.88 84
18002 Ballyduff Blackwater 57 461.0 404.8 0.88 404.8
18107 Fermoy Blackwater 11 397.5 N/A 0.93Killavullen 369.6
18003 Killavullen Blackwater 28 362.2 337.4 0.93 337.4
18055 Mallow Rail Bridge
Blackwater 11 355.7 N/A 0.93Killavullen 330.7
18006 CSET Mallow Blackwater 35 305.2 302.2 0.99 302.2
18048 Dromcummer Blackwater 24 N/A 220.0up to 2002 1.18Riverview
Factor Reversed 259.6
18009 Riverview Allow 30 136.5 115.6 0.84 115.6
18010 Allen’s Bridge Dalua 27 46.7 46.9 1.01
46.9
The AMAX series at Fermoy and Mallow Rail Bridge gauges were only available since 2000. This
represents a wetter period in the flow and rainfall records at the longer term gauges, i.e. CSET Mallow,
Killavullen and Ballyduff. Therefore, the QMED at Fermoy and Mallow Rail Bridge were adjusted based on
the difference recorded at the longer term gauges (Table 6.2). Conversely the AMAX series for
Dromcummer was not available since 2002 thus underestimating QMED as it misses the wetter years and
significant 2009 event. Therefore, Dromcummer was adjusted to account for the wetter period based on
the reversed factor at the nearby Riverview gauge.
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For ungauged sites, the QMED was calculated using the FSU 7 variable approach and adjusted using
pivotal sites. The aforementioned gauges within the catchment were typically used as pivotal sites in the
Blackwater catchment as they were hydrologically similar. Different pivotal sites were used to adjust QMED
for the smaller ungauged tributaries in Mallow and Fermoy based on more hydrologically similar pivotal
sites and the previous Mallow Drainage Scheme flows.
Figure 6.1 provides a summary of the progression of QMED through the Blackwater catchment. The details
of the selected pivotal sites, QMED estimate and schematics for all HEPs are provided in Appendix D
along with the 95th percentile upper limit. The upper confidence limit of QMED (95
th percentile) has been
calculated for each HEP based on the factorial standard error of 1.37 (see WP 2.3).
QMED was checked to ensure the index flow value increased downstream with contributing area. The
increase in QMED flow between Mallow and Fermoy is relatively small compared to the increase in area.
This is the result of floodplain attenuation as water spill out-of-bank for flows around QMED. The recorded
QMED values at gauges were indexed to A0.77
/10 and factors were typically found to be between 10 and
14 at gauges across UoM18 and between 8 and 22 across the South West region. The ground-water
dominated tributaries of the Awbeg, Funshion and Araglin tended to have lower ratios due to the karstic
influence. The smaller tributaries in all the AFAs had lower ratios because the trends at the gauges with
much larger catchments do not necessarily reflect the hydrology in the smaller flashy catchments.
However, the FSU estimate was used to maintain consistency of approach across the study and
progression of flows along the catchment.
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Figure 6.1: Summary Schematic of QMED for the Blackwater Catchment
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6.3.3 Derivation of Flood Growth Curves
Flood frequency analysis was undertaken on the revised AMAX series at the gauges along the modelled
reach and used to derive appropriate flood growth curves as set out in the FSU methodology (Section
6.3.1). Pooling groups were derived and used to extend beyond the twice the record length for ungauged
catchments. The full pooling group details used to derive the flood growth curves are provided in
Appendix D.
Table 6.3 summarises the key flood growth curves selected at gauges along the modelled reach. The
single site EV1, LO and LN3 distributions were found to best fit the gauged data within the gauged record
and the pooled GLO distribution was typically found to be the best fit to extend the flood growth curve to
the 0.5%AEP and 0.1%AEP events.
Table 6.3: Selected Flood Growth Curves at Gauges in UoM18
Selected Flood Growth Curve
Flood Growth Factor for %AEP
ID Name 50% 20% 10% 5% 2% 1% 0.5% 0.1%
18001 Mogeely LO Single/GLO Pooled 1.00 1.22 1.35 1.46 1.61 2.05 2.30 3.03
18002 Ballyduff EV1 Single 1.00 1.35 1.58 1.79 2.08 2.29 2.50 3.00
18107 Fermoy EV1 Single 1.00 1.35 1.58 1.8 2.09 2.31 2.52 3.02
18003 Killavullen EV1 Single 1.00 1.35 1.58 1.80 2.09 2.31 2.52 3.01
18055 Mallow Rail Bridge
LN3 Single Site/Mallow Drainage Scheme 1.00 1.37 1.56 1.75 2.02 2.23 2.44 3.00
18006 CSET Mallow 1.00 1.37 1.56 1.75 2.02 2.23 2.44 3.00
18048 Dromcummer Mallow Drainage Scheme 1.00 1.37 1.56 1.75 2.02 2.23 2.44 3.00
18009 Riverview EV1 Single / GLO Pooled 1.00 1.32 1.54 1.76 2.05 2.28 2.87 3.15
18010 Allen’s Bridge EV1 Single / GLO Pooled 1.00 1.28 1.47 1.66 1.92 2.08 2.34 3.08
There have been a number of recent flood defence schemes in the Blackwater catchment. Therefore, the
design flows at CSET Mallow and Killavullen gauges were derived from the FSU statistical analysis
(Section 6.3.1) and compared with the recent flood defence scheme flows at Mallow and Fermoy to ensure
consistency between the different studies.
Figure 6.2 provides the resultant flood growth curves at CSET Mallow. Figure 6.3 provides the
corresponding L Moment plot of the pooled average. The FSU LN3 single site flood growth curve and
Mallow Drainage Scheme flood growth curve are similar and provide a good fit to the gauged data up to
the 1%AEP. The Mallow Drainage Scheme flow growth curve was found to be appropriate because it was
more consistent with the flood curves at Killavullen, Fermoy and Ballyduff. The Mallow Drainage flood
growth curve was applied as the design flood curve along the Blackwater for the hydrologically similar
reach between Dromcummer and Mallow Rail Bridge to ensure consistency of design flows along the
catchment.
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Figure 6.4 provides the resultant flood growth curves at Killavullen and Figure 6.5 provides the
corresponding L Moment plot. The EV1 single site flood growth curve was found to best fit the revised
gauge data. The record length at Killavullen is relatively long giving confidence in the estimate for more
extreme %AEP events. The Fermoy Drainage Scheme flood growth curve was very similar to the FSU
single sites analysis only diverging for the most extreme %AEP. The single site EV1 flood growth curve
was applied to the hydrologically similar reach from Killavullen to Fermoy to ensure consistency with the
EV1 flood growth curves selected at Fermoy and Ballyduff.
Table 6.4 summarises the changes in design flows between the CFRAM Study and previous drainage
schemes in Mallow in Fermoy accounting for changes in QMED and flood growth curves.
Table 6.4: Comparison of FSU Design Flows with Scheme Flows in Mallow and Fermoy
%AEP Peak Flows (m3/s)
CSET Mallow
Mallow Drainage Scheme Hydrology Table A11(2003)
CSET Mallow
FSU single site estimate
Killavullen
Fermoy Drainage Scheme Revised Hydrology (2006)
Killavullen
FSU single site estimate
50% 302 302 285 337
20% 413 376 399 420
10% 471 435 480 478
5% 529 500 562 538
2% 609 596 679 627
1% 674 676 771 701
0.5% 736 765 872 784
0.1% 904 1005 1131 1015
Selected Selected
Italics denote interpolated or extrapolated flow not modelled in the previous study.
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Figure 6.2: Flood Growth Curves at CSET Mallow Gauge (18006)
Figure 6.3: L-Moment Plot for CSET Mallow Gauge (18006)
50% 20% 10% 5% 2% 1% 0.5% 0.1%1
1.5
2
2.5
3
3.5
Flo
od
Gro
wth
Fa
cto
r
%AEP
EV1
LO
LN2
GEV
GLO
LN3
Mallow Scheme FGC
Single Site
AMAX
0.0
0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
L-K
urt
osi
s
L-Skewness
Series1 Pooled L-Moments LO LN2 EV1 GEV GLO LN3 polynomial Fitted Trendline
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Figure 6.4: Flood Growth Curves at Killavullen Gauge (18003)
Figure 6.5: L-Moment Plot for Killavullen Gauge (18003)
50% 20% 10% 5% 2% 1% 0.5% 0.1%1
1.5
2
2.5
3
3.5
4
Flo
od
Gro
wth
Fa
cto
r
%AEP
EV1
LO
LN2
GEV
GLO
LN3
Single
Fermoy Scheme FGC
AMAX
0.0
0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
L-Ku
rtosis
L-Skewness
Series1 Pooled L-Moments LO LN2 EV1 GEV GLO LN3 polynomial Fitted Trendline
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6.4 Hydrograph Generation
Flood extent, depth, velocity and hazard are governed by the shape and duration of a flood flow
hydrograph as well as the magnitude of the peak flow. Therefore, design inflow hydrographs were derived
at each HEP as follows.
At gauged locations, the hydrograph width analysis approach was used to derive the median flood
hydrograph as the characteristic flood hydrograph for subsequent use in the hydraulic modelling and
development of flood risk management options. The 15 minute flow data was extracted for each of the
AMAX events at each fluvial gauge, standardised by the peak flow, and the width exceedance for each
event derived at specified percentiles of the peak flow. The median of the width exceedance was then
used to compile the design flood hydrograph.
Figure 6.6 shows the progression of the standardised design flood hydrograph shape between CSET
Mallow and Ballyduff on the Blackwater. The flood hydrograph duration increases from 46 hours (< 2 days)
at Mallow to over 100 hours (> 4 days) at Ballyduff. The rising and falling limb also become more
prolonged and less flashy as the flood progresses down the catchment and the flood flows are attenuated
on the floodplain.
The FSU hydrograph pivotal site that best matched the gauged hydrographs was then used to derive the
design flows. Hydrograph pivotal site 25001, 15003 and 16001 were used for Mallow, Killavullen and
Ballyduff respectively as presented in Appendix D.
Figure 6.6: Progression of the Median Flood Hydrograph in the River Blackwater Catchment
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
-50 -40 -30 -20 -10 0 10 20 30 40 50
% o
f Pea
k Fl
ow
Time to Peak Flow (Hours)
18002 Ballyduff Median 18003 Killavullen Median 18006 CSET Mallow Median
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For the ungauged HEPs, the regression-based UPO-ERR-gamma curve was calculated from the physical
catchment descriptors as detailed in 6.3.1.
The three components of the hydrograph are:
Gamma Curve (Rising Limb) - n
𝑦 = (x + Tr
Tr
)n−1
[𝐸𝑥𝑝 (−x(n − 1)
Tr
)]
Inflection Point (Starting point of Recession Limb) - Tr
𝑥𝑜 =Tr
√𝑛 − 1 𝑦
𝑜= (
𝑥𝑜 + Tr
Tr
)𝑛−1
𝐸𝑥𝑝 (𝑥𝑜(𝑛−1)
𝑇𝑟
)
Exponential Decay Curve (Recession Limb) - C
𝑦 = 𝑦𝑜 𝐸𝑥𝑝 (–𝑥 − 𝑥𝑜
𝐶)
The n, Tr and C parameters were estimated from the physical catchment descriptors for the study area and
were used to derive an initial estimate of the flow hydrograph. The Tr and C values were subsequently
adjusted based on hydrograph pivotal sites from the FSU database. Hydrologically similar sites were
selected based on slope, attenuation and permeability and compared to the target sites catchment area,
SAAR and critical duration to ensure similar responses to rainfall. In some cases, the hydrograph pivotal
site differs to the pivotal site used to adjust QMED because not all gauges were available in the
hydrograph. The recession limb was adjusted where the UPO-ERR estimate was excessive in relation to
catchment area and the FSSR16 time to peak estimate.
Pivotal sites 15003, 35002, 25001, 15006 and 16001 were typically used for the Dalua, Allow, Blackwater
Mallow, Blackwater Killavullen and Blackwater Ballyduff reaches respectively based on the best match to
the gauged median hydrographs. The recession parameter “C” was manually adjusted to 10 for the River
Allow to better match the gauged hydrographs. The C parameter was also adjusted from 5 to1.6 for the
tributaries in Youghal to better match the time to peak suggested by FSSR16.The smaller tributaries, such
as the Knockawillin Stream, tended to better match pivotal site 36021 although more permeable larger
catchments, such as the Funshion, better matched pivotal sites 15005 or 16005.
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The details of the selected pivotal sites and typical design flood hydrographs for each reach are provided in
Appendix D.
6.5 Coastal Conditions
6.5.1 Total Tide plus Surge Levels
Extreme sea levels around the Irish coastline incorporate both the astronomic tide (caused by planetary
forcing) and storm surge elements (caused by atmospheric pressure), henceforth referred to as “total tide
plus surge levels”. The flood frequency analysis for extreme sea levels has already been undertaken as
part of ICPSS (2012) for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1% AEP events.
Total tide plus surge levels have been derived at Youghal as the tidal outfall of the River Blackwater. There
is no other AFA or MPW affected by coastal conditions in UoM18. In the absence of gauged data, the
CFRAM Study has assumed the same total tide plus surge levels at the tidal outfall of the Blackwater
(211780, 076350) as provided at ICPSS point S31. The resultant total tide plus surge level are
summarised in Chapter 8. The hydraulic model of the Lower Blackwater will be used to transform the tide
plus surge inland.
6.5.2 Design Tidal Curve
The shape of the astronomic curve defines the duration of the rising (flood) and falling (ebb) tide. In deep
water the astronomic curve can be assumed to be largely symmetrical depending on the relative phasing
of the various harmonic components. However, the shoaling of the tide in shallow estuarine areas can
modify the shape.
The admiralty tide tables6 were used to inform time differences in mean high water and low water between
the primary port (Cobh) and the local prediction points at Youghal to modify the astronomic tidal curve.
Storm surges caused by Atlantic storms can often cause elevated sea levels over several diurnal tidal
cycles. Surge residuals were calculated from the tidal gauge data along the south west coast for the most
extreme events (Figure 6.2). It is apparent that larger events tend to have a shorter duration than the
smaller event. The 48 hour duration has been assumed as a credible duration for an extreme surge event
and a symmetrical surge profile assumed in the absence of detailed gauge data at Youghal Harbour itself.
6 United Kingdom Hydrographic Office (2013) Admiralty Tidal Tables Volume 1, 2013.
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Figure 6.7: Typical Surge Duration in South West Ireland
The design surge profile was then standardised by the peak surge residual and scaled on top of the
astronomic curve to achieve the design extreme sea levels (Figure 6.3). It was assumed that the peak of
the surge and the peak of the spring astronomical high tide coincide. This provided a conservative estimate
of the combined tidal curve. It is recognised that the peak of the astronomic tide does not necessarily
correspond with the peak surge as they are governed by different mechanisms. However, without long
term tidal and surge residual data along the South West coast it is not possible to assess the joint
probability between these two elements
Figure 6.6 displays the combined tidal curves for the design 50%AEP event at Youghal.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 20 40 60 80 100 120 140 160
Surg
e R
esi
du
al (
m)
Duration above Predicted Tide (Hours)
Clonakilty Temporary Gauge Ballycotton Tidal Gauge
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Figure 6.8: Example Tide Plus Surge Curve Generation at Youghal
6.5.3 Wave Overtopping
The ICWWS identified three sections of coastline in Youghal that were potentially vulnerable to wave
overtopping from Claycastle to Youghal mudlands (Map 6.3). Reach B and C were further split as part of
this study to account for changes in defence type and orientation to wave attack. There are no other
reaches of coastline in UoM18.
The source-pathway-receptor model can be readily applied to wave overtopping:
Source – wave overtopping volumes based on wave run-up spilling over the coastal frontage
Pathway – flow path of the wave overtopping discharge from the coastal defence to the receptors
considering topography behind the defence.
Receptors – roads, properties, environmental designations etc. affected by the wave overtopping and
their relative location to the wave overtopping.
A screening process was undertaken for the vulnerable reaches and three approaches to assessing wave
overtopping were developed for the CFRAM study:
Wave overtopping unit discharges – the calculation of unit discharge is sufficient to inform flood risk
where wave overtopping volume is insufficient to flow down the backslope of coastal defences or the
water would immediately drain back to the sea due to high relief inland.
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
0 6 12 18 24 30 36 42 48
Wate
r Leve
l (m
OD
M)
Time (Hours)
Surge Profile scaled Astronomic curve Design Combined Tidal Curve 50% AEP
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Mapping of wave overtopping volumes – the mapping of total wave overtopping volumes is required
where wave overtopping discharges are able to flow down the backslope of coastal defences to affect
receptors, often in locations where the defences are above the coastal floodplain.
No consideration of wave overtopping – wave overtopping calculations are not required where still
water overtopping (Mechanism1) dominates as the additional volume from wave overtopping can be
considered negligible in comparison with the volume of the incoming tide.
Table 6.4 outlines the approach for each section based on the criteria above.
Table 6.5: Wave Overtopping Approach
Reach Source Pathway Receptors Approach
A Wave overtopping of grassed embankment
Flows away from grass embankment to low-lying areas inland.
Agricultural areas inland.
Mapping of wave overtopping volume for relevant scenarios
B1 Still water overtopping of a vertical concrete wall level with the quayside
Flows across quay and down roads towards Catherine Street.
Road and a few properties adjacent.
Wave volume is negligible so reach not considered in wave overtopping scenarios.
B2 Wave overtopping of a vertical concrete wall above the quayside
Flows across quay and down roads towards Market Square.
Road and a few properties adjacent.
Mapping of wave overtopping volume for relevant scenarios
C1 Wave overtopping of a vertical concrete wall above the quayside – similar to reach B2
Flows across down roads and round properties towards Strand Street.
Road and properties adjacent.
Mapping of wave overtopping volume for relevant scenarios
C2 Wave overtopping of vertical wall at top of riprap and shingle slope
Flows away from embankment to low-lying areas inland.
Roads, properties and caravans adjacent to Front Strand as well as agricultural areas inland.
Mapping of wave overtopping volume for relevant scenarios
The wave overtopping discharges were calculated for the Youghal sections above using empirical
equations of wave run up for simple slopes, composite slopes with walls and vertical walls and general
hydraulic principles to fully account for the transition from the valid limit of the empirical equations
(mechanism 2) to full still water overtopping (mechanism 1).
The six different combinations of total tide plus surge levels and wave heights from the ICWWS were
assessed to find the critical scenario for wave overtopping for each AEP. It should be noted that the
ICWWS uses the total tide plus surge levels from point ICPSS point S35 further along the coast for
computation efficiency. These levels have been combined with different extreme wave heights to represent
the target joint probability at Youghal.
Table 6.5 summarises the critical discharges for the target %AEP events. Reach C2 along Front Strand
was found to be most at risk from wave overtopping. The quayside (reach B10 was at risk from still water
overtopping in all scenarios therefore, further consideration of wave overtopping is not required. Full details
of the analysis for all scenarios can be found in Appendix D.
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Map 6.3: Wave Overtopping Sections at Youghal AFA
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Table 6.6: Critical Wave Overtopping Unit Discharges for Key %AEP
Critical Unit Discharge (l/s/m)
Reach Defence Type Effective Crest Level (mODM)
10%AEP 0.5%AEP 0.1%AEP
A Grassed Embankment
3.93 0.00 0.04 0.29
B1
Concrete vertical walls with no or bypassed walls above quay level
2.59 Still water
overtopping Still water overtopping
Still water overtopping
B2 Concrete vertical walls with walls above quay level
5.16 0.00 0.00 0.00
C1 Concrete vertical walls with walls above quay level
5.23 0.00 0.00 0.00
C2 Shingle and riprap leading to wall
4.73 62.59 185.09 278.76
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7.1 Calibration Events
7.1.1 Selection of Events
During the Flood Risk Review, historical flood evidence was collated for those events listed in Chapter 4.
Information was gathered from post-flood surveys, aerial footage and anecdotal evidence from local
residents. Table 7.1 scores each of these events based on a number of criteria related to the location,
hydrology and data availability on a scale of 0 to 3 where:
0 is not available
1 is poor or unlikely
2 is fair or possible
3 is good or likely
These scores are then combined to create an indicative calibration confidence score for the available
historical flood evidence in accordance with Guidance Note 237. The following events have been
considered for the calibration of the entire Allow and Blackwater sub-catchments based on the indicative
calibration score:
30th December 1998 – extreme fluvial event along the Blackwater and Allow;
5th/6
th December 2000 – extreme fluvial event along the Blackwater and Allow;
19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow.
The following events have been considered for the calibration of the Bride sub-catchments based on the
indicative calibration score:
30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride;
19th November 2009 – extreme fluvial event along the Bride.
Youghal will also be calibrated for the 17th October 2012 coastal event to calibrate coastal flood
mechanisms in the town.
There was no recorded flood history at Aglish therefore the verification of model parameters will rely on
sensitivity tests. There was insufficient evidence to support full calibration at Freemount as the hydraulic
structures under Main Street have changed since the flooding in 1997 and the current culverts do not
represent the historic conditions. Therefore the modelled outline will be verified taking account of the
historical flood frequency from recurring flooding reports, and sensitivity analysis on the key hydraulic
parameters used in accordance with GN23.
There were insufficient gauge levels, wrack marks or photographs to undertake a full calibration in Tallow.
However, key flow paths will be verified based on the existing flood reports and flood mechanisms
identified by the local engineer in Chapter 4.
7Jacobs, (January 2013) Guidance Note 23 Model Calibration. Version 1.
7 Hydrological Calibration, Sensitivity Testing and Uncertainty
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Table 7.1: Selection of Calibration Events
Event AFA/ Watercourse Lik
ely
Ac
cu
racy
of
Flo
w
Es
tim
ate
1
Lik
ely
Ac
cu
racy
of
Ga
ug
ed
L
eve
l
Es
tim
ate
Kn
ow
n
Hy
dra
ulic
Co
nd
itio
ns
2
Lik
ely
Ac
cu
racy
o
f S
po
t
Le
ve
ls3
Re
lia
ble
Flo
od
His
tory
4
Indicative Calibration
Score Calibration Approach
02/11/1980 Mallow/Blackwater
Kanturk/ Allow & Dalua
2 2 1 0 2 7
Significant catchment changes since event makes calibration difficult. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.
06/08/1986 Mallow/Blackwater
Kanturk/ Allow & Dalua
2 2 1 0 2 7
Significant catchment changes since event makes calibration difficult. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.
22/10/1988 Mallow/Blackwater
2 2 1 0 2 7
Significant catchment changes since event makes calibration difficult. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.
26/08/1997 Freemount/Keen (Freemount Stream)
1 0 1 0 2 4
Culvert changed significantly after this event. Previous culvert dimensions not available. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.
30/12/1998 Mallow/Blackwater
Fermoy /Blackwater 3 3 2 2 3 13
Entire catchment calibration available. Significant catchment changes since event makes calibration difficult. Calibrate main channel to large event data considering spot levels are accurate to within +/- 0.25m. Smaller tributaries should take note of uncertainties due to blockage.
06/11/2000 Mallow/Blackwater
Fermoy
3 3 3 1 3 12
Significant catchment changes since event makes calibration difficult. Extensive outline and photo information but no spot levels. Calibrate main channel to large event data considering that spot levels are derived from extent. Smaller tributaries in Mallow should take note of uncertainties due to blockage.
27/10/2004 Youghal/Coastal
Kanturk/Brogeen N/A for coastal
2 at Kanturk
0 1 1 3 7
Spot levels inferred from flood outline from Council. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.
10/01/2008 Mallow/Blackwater
Fermoy /Blackwater 1 1 1 2 3 8 Gauge data is incomplete for the full event at a number of sites between Mallow and Fermoy and the event is of a similar magnitude as the other selected events.
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Event AFA/ Watercourse Lik
ely
Ac
cu
racy
of
Flo
w
Es
tim
ate
1
Lik
ely
Ac
cu
racy
of
Ga
ug
ed
L
eve
l
Es
tim
ate
Kn
ow
n
Hy
dra
ulic
Co
nd
itio
ns
2
Lik
ely
Ac
cu
racy
o
f S
po
t
Le
ve
ls3
Re
lia
ble
Flo
od
His
tory
4
Indicative Calibration
Score Calibration Approach
30/01/2009 Rathcormac/Dromore Stream and School
Stream 2 1 2 1 2 10
Spot levels inferred from flood extent from Council. Calibrate main channel and Rathcormac tributaries to event data and compare with 2012 event spot levels.
19/11/2009 Mallow/Blackwater
Fermoy/Blackwater
Ballyduff/Blackwater
3 3 2 1 3 12
Entire catchment calibration available. Spot level provided but seems to be estimated from flood outlines. Calibrate main channel to large event data. Smaller tributaries in Mallow should take note of uncertainties due to blockage.
04/08/2012 Rathcormac/Dromore Stream
0 0 2 3 3 8
Spot levels and outline surveyed after event but gauge data not available. Modelled outline to reflect reasonable historic flood frequency, otherwise use sensitivity tests to assess hydraulic parameters.
17/10/2012 Youghal/coastal N/A for coastal
2 3 3 3 11 Level transferred from Ballycotton and Council observations. Calibrate main channel and coastal flood risk to large event data.
Note 1: 3 = gauged flows are available in the catchment, 2 = gauged flows used from pivotal gauges nearby, 1 = rainfall data used to estimate flows and 0= no flow estimate available
Note 2: Hydraulic conditions relate to controls on water levels during a flood e.g. level of blockage, wall collapse etc.
Note 3 Levels during a known flood event NOT at a gauged location that represents a true flood level rather than a localised issue.
Note 4 Any information that includes date/time, precise location and mechanism of flooding.
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7.1.2 Calibration Hydrology Approach
The prevalence of reliable river gauge data in AFAs means the river gauge data has been used for the
calibration events rather than detailed analysis of the rainfall data for calibration events prior to 2007.
Therefore, the inflows for ungauged tributaries have been transferred from these gauges based on the
relative flood frequency of each event. The phasing of the inflows from the ungauged HEPs will be
adjusted to ensure flood levels are met at the gauged locations.
More detailed rainfall data was available for the 2009 event. However, the prevalence of permeable
catchments across UoM18 made the FSSR16 approach unsuitable for many of the tributaries. The river
flow gauges were deemed to provide a better estimate of catchment hydrology as they account for any
permeable influences within the flow data.
30th December 1998
Figure 7.1 presents the gauged hydrographs throughout the Blackwater sub-catchment for the flood event
on 30th December 1998.
Figure 7.1: Progression of the December 1998 Flood Event
0
100
200
300
400
500
600
700
29
/12
/19
98
00
:00
29
/12
/19
98
12
:00
30
/12
/19
98
00
:00
30
/12
/19
98
12
:00
31
/12
/19
98
00
:00
31
/12
/19
98
12
:00
01
/01
/19
99
00
:00
01
/01
/19
99
12
:00
02
/01
/19
99
00
:00
Flo
w (
m3 /
s)
18003 KILLAVULLEN FLOW Revised Rating 18002 BALLYDUFF FLOW Revised Rating 18006 CSET MALLOW FLOW Revised Rating
18010 ALLENS BRIDGE FLOW Revised Rating 18009 RIVERVIEW FLOW Revised Rating
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Table 7.2 details the relative flood frequency applied to each model reach for 1998 calibration event. The
Ballycotton tidal gauge was not in operation for this event. Therefore, the tidal conditions have been
derived from the Admiralty tide table for 1998. The resultant total tide plus surge curve is shown in Figure
7.2 and will be applied to the lower Blackwater model at Youghal.
Table 7.2: Calibration Inflows for 30th
December 1998
Sub-catchment Reach Gauge %AEP
Blackwater Blackwater from Allow to Mallow and tributaries
CSET Mallow
(Dromcummer inactive)
15
Blackwater and tributaries through Mallow
CSET Mallow 15
Blackwater and tributaries from Killavullen to Fermoy
Killavullen 10
Awbeg, Funshion and Araglin tributaries
Downing Bridge 15
Blackwater downstream of Araglin to Ballyduff and Lismore
Ballyduff 15
Blackwater downstream of Lismore and tributaries
Ballyduff 15
Blackwater Outfall, Youghal Admiralty Tide Table MHWS
Allow Allow inflow Riverview 10
Figure 7.2: Total Tide Plus Surge at Youghal 30th
December 1998
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 6 12 18 24 30 36 42 48
Wat
er Le
vel (
mO
DM)
Time (Hours)
MHWS Curve
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5th – 6
th November 2000
Figure 7.3 presents the gauged hydrographs along the Blackwater sub-catchment for the flood event that
occurred from 5th to 6
th November 2000.
Figure 7.3: Progression of the November 2000 Flood Event
Table 7.3 details the relative flood frequency applied to each model reach for the 2000 calibration event.
The Ballycotton tidal gauge was not in operation for this event. Therefore, the tidal conditions have been
derived from the Admiralty tide table for 2000. The resultant total tide plus surge curve is shown in Figure
7.4 and will be applied to the lower Blackwater model at Youghal.
0
100
200
300
400
500
600
05
/11
/20
00
00
:00
05
/11
/20
00
12
:00
06
/11
/20
00
00
:00
06
/11
/20
00
12
:00
07
/11
/20
00
00
:00
07
/11
/20
00
12
:00
08
/11
/20
00
00
:00
08
/11
/20
00
12
:00
09
/11
/20
00
00
:00
Flo
w (
m3/s
)
18006 CSET MALLOW FLOW Revised Rating 18003 KILLAVULLEN FLOW Revised Rating 18002 BALLYDUFF FLOW Revised Rating
18001 MOGEELY FLOW Revised Rating 18010 ALLENS BRIDGE FLOW Revised Rating 18009 RIVERVIEW FLOW Revised Rating
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Table 7.3: Calibration Inflows for 5th-6
th November 2000
Sub-catchment Reach Gauge %AEP
Blackwater Blackwater from Allow to Mallow and tributaries
CSET Mallow
(Dromcummer inactive)
20
Blackwater and tributaries through Mallow
CSET Mallow 20
Blackwater and tributaries from Killavullen to Fermoy
Killavullen 20
Awbeg, Funshion and Araglin tributaries
Downing Bridge 1
Blackwater downstream of Araglin to Ballyduff and Lismore
Ballyduff 20
Blackwater downstream of Lismore and tributaries
Ballyduff 20
Blackwater Outfall, Youghal Admiralty Tide Table Less than MHWS. MHWS assumed.
Allow Allow inflow Riverview 7
Figure 7.4: Total Tide Plus Surge at Youghal 5th – 6
th November 2000
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 6 12 18 24 30 36 42 48
Wat
er
Leve
l (m
OD
M)
Time (Hours)
MHWS Curve
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30th January 2009
The 30th January 2009 event was similar in magnitude and duration to the more recent 24
th August 2012
event. Therefore the modelled flood outline, mechanisms and levels will be compared with the spot levels
from the 24th August 2012 to verify the model. Figure 7.5 presents the gauged hydrograph at Mogeely for
the flood event that occurred on 30th January 2009.
Figure 7.5: Recorded January 2009 Flood Event at Mogeely
Table 7.4 details the relative flood frequency applied to the Bride reach for the January 2009 catchment
calibration event. The Ballycotton tidal gauge has been used to inform tidal conditions at the Blackwater
outfall (Figure 7.6). The total tide plus surge curve will then be transferred to the Bride downstream based
on the modelled water level profile for the lower Blackwater.
0
20
40
60
80
100
120
140
29
/01
/20
09
12
:00
30
/01
/20
09
00
:00
30
/01
/20
09
12
:00
31
/01
/20
09
00
:00
31
/01
/20
09
12
:00
01
/02
/20
09
00
:00
01
/02
/20
09
12
:00
02
/02
/20
09
00
:00
02
/02
/20
09
12
:00
03
/02
/20
09
00
:00
Flo
w (
m3
/s)
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Table 7.4: Calibration Inflows for 30th
January 2009
Sub-catchment Reach Gauge %AEP
Bride Bride, Rathcormac and Tallow tributaries
Mogeely 10
Blackwater Outfall, Youghal Ballycotton >50
(1.935mODM)
Figure 7.6: Total Tide Plus Surge at Youghal 29th
January 2009 – 2nd
February 2009
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
-24 -18 -12 -6 0 6 12 18 24 30 36 42 48 54 60 66 72
Tota
l Tid
e P
lus
Surg
e L
eve
l (m
OD
M)
Time from Peak Flow at Mogeely(Hours from 30/01/2009 16:00)
Ballycotton Tide + Surge Level
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19th
– 21st November 2009
Figure 7.7 presents the gauged hydrographs throughout the Allow, Bride and Blackwater sub-catchments
for the flood event that occurred from 19th to 21
st November 2009.
Figure 7.7: Progression of the November 2009 Flood Event
Table 7.5 details the relative flood frequency applied to each model reach for the November 2009
catchment calibration event. The Ballycotton tidal gauge has been used to inform the tidal conditions at the
Blackwater outfall. The resultant total tide plus surge curve is shown in Figure 7.8 and will be applied to the
lower Blackwater model at Youghal.
0
100
200
300
400
500
600
700
19
/11
/20
09
19
/11
/20
09
20
/11
/20
09
20
/11
/20
09
21
/11
/20
09
21
/11
/20
09
Flo
w (
m3
/s)
18006 CSET MALLOW FLOW Revised Rating 18055 MALLOW RAILWAY BR FLOW OPW Rating
18003 KILLAVULLEN FLOW Revised Rating 18107 FERMOY DS FLOW MM Applied Rating
18002 BALLYDUFF FLOW Revised Rating
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Table 7.5: Calibration Inflows for 19th
- 21st November 2009
Sub-catchment Reach Gauge %AEP
Allow Freemount and Allow upstream of Dalua confluence
Riverview 30
Dalua and Brogeen Allen’s Bridge 20
Allow downstream of Dalua confluence
Riverview 30
Bride Bride, Rathcormac and Tallow tributaries
Mogeely 20
Blackwater Blackwater from Allow to Mallow and tributaries
CSET Mallow
(Dromcummer inactive)
10
Blackwater and tributaries through Mallow
CSET Mallow 10
Blackwater and tributaries from Killavullen to Fermoy
Killavullen 5
Awbeg, Funshion and Araglin tributaries
Downing Bridge 2
Blackwater downstream of Araglin to Ballyduff and Lismore
Ballyduff 10
Blackwater downstream of Lismore and tributaries
Ballyduff 10
Blackwater Outfall, Youghal Ballycotton >50
( 1.909mODM)
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Figure 7.8: Total Tide Plus Surge at Youghal 18th
-21st November 2009
17th October 2012
The October 2012 event was caused by extreme coastal conditions at Youghal. Figure 7.9 displays the
recorded total tide plus surge hydrograph at Ballycotton gauge in Youghal Bay. This will be applied directly
to the Youghal model.
Table 7.6 details the relative flood frequency of this coastal event for the Lower Blackwater reach. River
flows were observed to be within-bank. Therefore flow less than the 50%AEP design flow will be applied to
the minor tributaries through Youghal.
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
-54 -48 -42 -36 -30 -24 -18 -12 -6 0 6 12 18 24 30 36
Tota
l Tid
e P
lus
Surg
e L
eve
l (m
OD
M)
Time from Peak Flow at Mallow (Hours from 20/11/2009 03:00)
Ballycotton Tide + Surge Level
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Figure 7.9: Total Tide Plus Surge at Youghal 17th
October 2012
Table 7.6: Calibration Inflows for 17th
October 2012
Sub-catchment Reach Gauge %AEP
Blackwater Youghal Tributaries Ballyduff >50%
Blackwater Outfall, Youghal Ballycotton
Council observations
0.93%
7.2 Uncertainty and Sensitivity Testing
The SW CFRAM study requires an understanding of sensitivity in hydrological and hydraulic parameters in
order to inform the uncertainty analysis in the flood mapping process. The key areas of uncertainty in the
hydrological analysis of UoM18 are:
Uncertainty in the QMEDrural regression equation;
Uncertainty in the pooling group and statistical distribution used to estimate the flood growth curve;
Uncertainty in the transformation of water levels to Youghal.
All sensitivity analysis has been assessed at the 1%AEP as this is the target fluvial AEP for the CFRAM
study and the AEP event used in planning decisions and in agreement with Guidance Note 26. Uncertainty
in flow and level for more frequent events are considered within the error bounds for the 1%AEP.
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
-36 -30 -24 -18 -12 -6 0 6 12 18 24 30 36 42 48 Surg
e R
esi
du
al (
m)
Tota
l Tid
e +
Su
rge
Lev
el (
mO
DM
)
Hours to Peak Water level @ 17/12/2012 18:15
Ballycotton Total Tide + Surge Level surge residual
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Sensitivity in Flows
The FSU WP 2.3 states a factorial standard error (FSE) of 1.37 in the QMED rural regression equation
based on the 190 gauges across Ireland used to derive the equation coefficients. Approximate 95% upper
confidence limits for QMED were then calculated as follows:
95% 𝑐𝑜𝑛𝑓𝑖𝑑𝑒𝑛𝑐𝑒 𝑙𝑖𝑚𝑖𝑡 = 𝑄𝑀𝐸𝐷 ∗ 𝐹𝑆𝐸2
The uncertainty in the flood growth curves and pooling groups selected for a sample of 85 gaugings
stations across Ireland was investigated as part of the FSU WP 2.2. The percentage standard error in
design peak flow varied from 4.0 to 9.0 at the target fluvial 1%AEP.
The upper confidence limits from each source of peak flow uncertainty were combined to estimate overall
uncertainty in design peak flow at the target 1%AEP for ungauged HEPs. This resultant upper limit of the
1%AEP flow was typically within 10% to 30% of the design 1%AEP peak flow (see Appendix D). Therefore,
it was deemed that a sensitivity test of a 30% increase in peak flow at the target 1%AEP should be
considered in the subsequent hydraulic modelling of all HEPs in UoM18.
Sensitivity in Total Tide plus Surge Level
The Total Tide plus Surge Levels have been extracted from the RPS coastal model at offshore points
along the coast based on Extreme Value Analysis. There is some uncertainty in the transformation of the
total tide plus surge level to the near shore at Youghal as the frictional effects of the near shore bathymetry
has not necessarily been modelled. There is also inherent uncertainty in the derivation of the extreme
values for the rare %AEP events.
It was not possible to quantify the uncertainty at Youghal without long term tidal gauges records. The flood
history of coastal events at Youghal indicates two 1%AEP events in the past decade suggesting that the
ICPSS levels are underestimating the frequency of extreme events. The nearby Ballycotton tidal gauge
records are less than 5 years in length making an assessment of extreme total tide plus surge levels
unsuitable. Therefore, the GN 22 guidance was applied to consider a 0.5 m increase in water levels for
the design events which is broadly equivalent to the mid-range future scenario.
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The design flows from this hydrology report inform the inflows to the hydraulic model to assess flood risk
from the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP fluvial and tidal flood events. The key
hydrological findings for design flows in UoM18 are as follows:
Historic flood events
Major flood events were identified in UoM18 since 1980 from extreme river flows along the
Blackwater, Bride and Allow.
Extreme storm surges at Youghal were also identified as a source of coastal flood risk with major
events in 2004 and 2012.
The largest gauged events were on 19th November 2009 and 2nd November 1980 along the
Blackwater.
There have been a number of catchment changes since the 1980s and more recently the Mallow
and Fermoy Schemes have been completed changing the impact of flooding in these AFAs.
There was no flood history identified in Aglish.
The calibration in Blackwater and Allow sub-catchments will be based on the following events where
there is sufficient information:
30th December 1998 – extreme fluvial event along the Blackwater and Allow
5th/6
th December 2000 – extreme fluvial event along the Blackwater and Allow
19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow
The calibration in Bride sub-catchment will be based on the following events where there is sufficient
information:
30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride
19th November 2009 – extreme fluvial event along the Bride
Rating Reviews
The high flows rating equations were revised for four gauges within UoM18 at Allen’s Bridge,
Riverview, Ballyduff and Mogeely.
High flows rating equations were checks for a further four gauges at CSET Mallow, Mallow Rail
Bridge, Killavullen and Fermoy.
The revised rating curves increased out-of-bank flow estimates for all gauges except Allen’s Bridge,
where flows were reduced to account for the backwater effect from the bridge and weir downstream.
The revised high flows rating equations were used to update the AMAX series at these gauged
HEPs and update the QMEDrural adjustment factor for hydrologically similar ungauged HEPS
Design flood flows
Peak flood flows were derived along the Allow, Dalua, Blackwater, Bride and Ballynaparka Stream
and various tributaries within the AFAs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP
events using the recommended FSU methodology outlined in Work Package 2.2 and 2.3.
The design flood hydrograph at gauged HEPs was derived from the median flood width exceedance
above 50% of the peak flow.
The design flood hydrograph for ungauged HEPs was based on the hydrograph pivotal site fitted to
the observed median hydrograph at the gauges within the catchment.
Design coastal conditions
The design extreme sea levels were extracted from the ICPSS for the 50%, 20%, 10%, 5%, 2%, 1%,
0.5% and 0.1%AEP tidal events.
ICPSS point S_31 was used to derive the total tide plus surge levels at Youghal.
8 Summary of Design Flows
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The astronomic curve and surge profile were derived from the admiralty predicted astronomic tide
and typical duration of surge events in the South West.
The final design tidal curve was derived from the combined astronomic tide and design surge profile
scaled to meet the design extreme sea levels.
The ICWWS water level and wind conditions were used to derive wave overtopping at Youghal.
Youghal quayside was found to be at risk from still water overtopping (mechanism 1) in all
scenarios. The wave overtopping volume would be negligible in comparison to the still water
overtopping volumes. Therefore, wave overtopping has not been considered further for this reach.
Youghal at Claycastle was found to be at risk from wave overtopping in the design scenario and the
wave overtopping volume will be mapped in the subsequent hydraulic modelling and mapping stage.
Uncertainty and Sensitivity
The uncertainty of the 1%AEP target peak flow was estimated to range up to +30% in UoM18
ungauged HEPs which will inform the sensitivity tests in the hydraulic modelling.
The total tide plus surge levels could underestimate coastal risk based on the historical flood
frequency of storm surges at Youghal. A sensitivity test which raises the total tide plus surge level by
0.5m has been proposed in accordance with GN22
Tables 8.1 and 8.2 provide the design peak flows and total tide plus surge levels at key locations
respectively. These flows and levels are subject to change following the subsequent integration into the
hydraulic model and calibration processes.
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Table 8.1: UoM18 Design Peak Flood Flows at Key Locations
HEP ID Gauge/ Ungauged Location 50%AEP (m3/s) 20%AEP 10%AEP 5%AEP 2%AEP 1%AEP 0.5%AEP 0.1%AEP
Freemount AFA
18_2681_3 Knockeen/Freemount Stream 1.9 2.4 2.8 3.1 3.7 4.1 4.7 6.2
18_2734_2 River Allow downstream of Freemount Bridge 23 28 32 35 41 46 52 68
Allow MPW
18_2672_1 Allow d/s of Glannycumman confluence 27 33 37 41 48 54 60 79
18_546_1 Allow d/s of Knockawillin confluence 47 62 72 82 96 107 118 148
18_548_1 Allow d/s of Ballynoe confluence (Kanturk upstream) 48 64 75 85 99 110 122 152
Kanturk AFA
18_548_5 Kilbrin Road Level Gauge 18110 49 64 75 86 100 111 123 154
18_552_3 Riverview (18009) 116 153 178 203 237 264 292 364
18_1762_9 Allen's Bridge Gauge 18010 47 60 69 78 90 97 110 144
18_1756_3 Church Street 18111 65 83 95 107 125 135 151 200
18_2121_10 Brogeen d/s 17 21 23 26 30 34 37 49
Blackwater Reach 1 MPW
18_394_3 Allow at Leaders Bridge 119 157 183 209 244 271 341 374
18_393_4 Blackwater upstream of Allow confluence 137 188 214 241 277 306 334 411
18_802_3 DROMCUMMER 18048 (downstream of Glen) 260 355 405 455 523 579 633 778
18_2747_1 Blackwater downstream of Awbeg 273 373 425 478 550 608 664 817
18_2382_2 CSET MALLOW 18006 302 413 471 530 609 674 736 905
Mallow AFA
18_1638_2 Blackwater (Mallow Railway Bridge) 331 452 516 580 667 738 806 991
18_1632_3 Blackwater Town Bridge (upstream of Bearforest confluence) 334 454 522 586 674 746 815 1002
18_1630_1 Blackwater d/s of Mallow 337 455 526 591 680 752 821 1010
18_2541_9 Clyda d/s at confluence 33 48 55 64 75 83 91 111
18_1104_5 Gooldshill d/s at confluence 1.4 1.9 2.2 2.6 3.1 3.6 4.1 5.7
18_2474_4 Hospital Stream d/s at confluence 4.9 6.4 7.5 8.6 10.3 11.8 13.4 18.1
18_1631_3 Bearforest Stream d/s at confluence 1.8 2.6 3.1 3.6 4.4 5.0 5.6 7.1
18_2594_13 Spa Glen d/s at confluence 5.9 8.6 10.3 12.0 14.5 16.4 18.3 23.5
Blackwater Reach 2 MPW
18_1616_5 Killavullen Gauge 18003 337 456 533 607 705 779 850 1016
18_352_1 Blackwater d/s of Awbeg confluence 359 484 567 647 750 828 905 1083
18_351_2 Blackwater Castlehyde 365 492 577 658 763 842 920 1102
Fermoy AFA
18_1158_5 Fermoy Bridger Downstream 18107 370 499 585 667 773 853 933 1117
18_1158_8 Blackwater Fermoy d/s ( M8 bridge) 371 500 586 669 776 856 935 1120
Blackwater Reach 3 MPW
18_2286_1 Blackwater d/s of Funshion 389 524 613 698 809 892 974 1166
18_2462_1 Blackwater d/s of Araglin 400 538 630 718 832 917 1002 1199
Ballyduff AFA
18_2297_6 Ballyduff Gauge 18002 405 545 638 726 842 928 1014 1213
Blackwater Reach 4 MPW
18_2307_2 Blackwater at Lismore 416 560 655 747 865 954 1042 1247
18_2822_7+ Blackwater Youghal 535 720 842 960 1112 1226 1339 1602
Youghal AFA
18_2824_5 Tourig at confluence with Blackwater 12.1 15.4 17.7 20.2 23.9 27.1 30.7 41.0
18_967_9 Kilnatoora at confluence with Blackwater 1.5 1.8 2.1 2.4 2.8 3.1 3.5 4.7
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HEP ID Gauge/ Ungauged Location 50%AEP (m3/s) 20%AEP 10%AEP 5%AEP 2%AEP 1%AEP 0.5%AEP 0.1%AEP
Rathcormac AFA
18_1964_4 Shanowen d/s at confluence with Bride 3.7 4.6 5.2 5.8 6.8 7.7 8.6 11.4
18_1605_12 Bride u/s at M8 29 35 38 42 46 58 66 86
18_1600_1 Bride d/s of Shanowen confluence 38 47 51 56 62 78 88 116
Bride Reach 1 MPW
18_344_1 Bride downstream at Flesk 55 67 74 80 88 112 126 166
18_350_1 Bride downstream at Douglas 74 90 100 108 119 152 170 224
Mogeely Gauge
18_341_3 Mogeely Gauge 18001 85 103 114 124 137 174 196 257
Bride Reach 2 MPW
18_343_1 Bride bear Limekilnclose 87 106 117 128 141 179 201 264
18_2778_1 Bride at tidal limit (near Tallow Bridge) 91 111 123 134 147 187 210 276
18_2798_3+ Bride downstream 109 133 146 159 176 223 251 330
Tallow AFA
18_2186_4 Carrigroe d/s at Glenaboy confluence 0.7 0.9 1.1 1.2 1.5 1.6 1.9 2.5
18_962_6 Glenaboy u/s of Carrigroe confluence 4.9 6.2 7.1 8.1 9.5 10.7 12.1 16.1
18_910_5 Glenaboy d/s at Bride confluence 5.8 7.3 8.4 9.5 11.2 12.6 14.3 19.0
Aglish AFA
18_2805_2 Ballynaparka (lower) Downstream 4.0 5.0 5.8 6.5 7.7 8.7 9.8 13.1
18_2808_2 Goish River (MPW) Downstream 8.6 10.6 12.1 13.6 15.9 17.9 20.2 26.7
Table 8.2: UoM18 Design Total Tide Plus Surge Levels
Location Location 50%AEP (mODM) 20%AEP 10%AEP 5%AEP 2%AEP 1%AEP 0.5%AEP 0.1%AEP
Youghal ICPSS Point S31 2.19 2.28 2.36 2.42 2.52 2.58 2.65 2.81
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9.1 Inflows
Design hydrographs have been derived at HEPs to represent the hydrological processes across the
Blackwater catchment as discussed in Chapter 6 of this report. The HEPs will be integrated with the
subsequent hydraulic models as follows:
Point inflows at the upstream model extents;
Point inflows at key tributary inflows;
Lateral inflows representing the inflow from the intervening areas between target HEPs.
The point inflows representing the upstream model extents and tributary inflow will be integrated with the
relevant cross-section in the hydraulic model accounting for a significant displacement from the HEP
calculated location. The lateral inflows will be integrated with the relevant cross-sections at locations which
fit the following criteria:
Natural inflows from minor watercourses which are not considered explicitly within the hydrology;
Overland flow paths identified from surveyed low points in the river bank and site walkover.
The lateral inflows will be calculated from the difference between the design flow hydrographs from the
upstream and downstream HEPs for a reach and scaled accordingly to meet the design peak flows on the
main reach. The resultant hydrograph will be distributed evenly across those locations where the
contributing area increases linearly downstream or area weighted where the contributing area increases
disproportionally downstream. Table 9.1 outlines the total number of inflows based on the criteria above for
each model. These will be further refined and discussed in the hydraulics report.
Table 9.1: Model Inflows
Model Number of Inflows
Freemount 3
Allow 6
Kanturk 5
Blackwater Reach 1 8
Mallow 8
Blackwater Reach 2 7
Fermoy 3
Blackwater Reach 3 6
Ballyduff 3
Blackwater Reach 4 12
Youghal 3
Rathcormac 6
Bride Reach 1 5
Mogeely 1
Bride Reach 2 4
Tallow 4
Aglish 4
9 Considerations for Hydrological and Hydraulic Model Integration
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In order to enhance the modelling outputs and ensure hydrological continuity along the larger catchments,
the hydraulic models will be calibrated to the design peak flows derived at the target HEPs. The hydraulic
parameters will be adjusted and hydrological inflows scaled such that the hydraulic model maintains the
design peak flows along the reach. However, it should be noted that the design fluvial flows do not
consider the following hydraulic processes:
Backwater effect at confluences;
Exchange of flows between tributaries at confluences; and,
Significant modification to the hydrograph shape due to floodplain attenuation.
Therefore, it is not appropriate to calibrate the hydraulic model to HEPs upstream of confluences where
there are significant out-of-bank flows.
In UoM18, the median width hydrographs have been derived at the gauged locations to establish the
design storm duration at target HEPs across each catchment. The duration of the tributary inflows are
based on the FSR time peak equation (function of SAAR, S1085 and MSL) but will be iteratively refined to
achieve the flow at the gauges as part of the hydraulic modelling. The intermediate inflows account for the
difference in duration between the target HEPs within the same hydrological catchment. Table 9.2 outlines
preliminary design storm durations for UoM18.
Table 9.2: Preliminary Design Storm Duration
AFA/MPW Gauge Design Duration (Hours)
Freemount and Allow Tributaries FSR estimate 8
Kanturk/Allow 18009 20
18010 17
Blackwater Dromcummer to Mallow 18048 27
Mallow/ Blackwater 18006 30
Mallow Tributaries FSR estimate 6
Killavullen 18003 38
Fermoy/Blackwater 18107 40
Fermoy Tributaries FSR estimate 5
Ballyduff/Blackwater 18002 59
Ballyduff Tributaries FSR estimate 7
Youghal Tributaries FSR estimate 11
Bride 18001 46
Rathcormac and Tallow Tributaries FSR estimate 11
9.2 Downstream Conditions
The downstream conditions will be defined for each model as outlined in Table 9.3 to fully account for the
relevant fluvial and tidal backwater effects as appropriate. An iterative approach will be used to phase the
design tidal curves so that the peak tide coincides with the peak flow as a conservative estimate of flood
risk.
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Table 9.3: Downstream Boundary Conditions
Model Downstream Condition
Freemount Stage-discharge relationship based on the downstream water slope in the Allow model.
Allow Stage-discharge relationship based on the downstream water slope in the Kanturk model.
Kanturk Stage-discharge relationship based on the downstream water slope in the Blackwater Reach 1 model.
Blackwater Reach 1 Stage-discharge relationship based on the downstream water slope in the Mallow model.
Mallow Stage-discharge relationship based on the downstream water slope in the Blackwater Reach 2 model.
Blackwater Reach 2 Stage-discharge relationship based on the downstream water slope in the Fermoy model.
Fermoy Stage-discharge relationship based on the downstream water slope in the Blackwater Reach 3 model.
Blackwater Reach 3 Stage-discharge relationship based on the downstream water slope in the Ballyduff model.
Ballyduff Stage-discharge relationship based on the downstream water slope or water level over time in the Blackwater Reach 4 model depending on the relative tidal influence.
Blackwater Reach 4 Full tidal boundary using the results from the design tidal curves set out in Chapter 6
Youghal Full tidal boundary using the results from the design tidal curves set out in Chapter 6
Rathcormac Stage-discharge relationship based on the downstream water slope in the Bride Reach 1 model.
Bride Reach 1 Stage-discharge relationship based on the downstream water slope in the Mogeely model.
Mogeely Stage-discharge relationship based on the downstream water slope in the Bride Reach 2 model.
Bride Reach 2 Stage-discharge relationship based on the downstream water slope or water level over time in the Blackwater Reach 4 model depending on the relative tidal influence.
Tallow Stage-discharge relationship based on the downstream water slope or water level over time in the Bride Reach 2 model depending on the relative tidal influence.
Aglish Stage-discharge relationship based on the downstream water slope or water level over time in the Blackwater Reach 4 model depending on the relative tidal influence.
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10.1 Approach
The hydrogeomorphological processes ongoing in the river channels can have a significant impact on flood
flows and the resultant flood risk. The assessment of hydrogeomorphological features focuses on whether
the processes appear to be in equilibrium and whether there are any processes taking place at present
which are likely to affect the flood risk indicators. This may include:
Recent interventions to the channel/hydrology to control flood risk which have accelerated erosion or
deposition;
The use of inappropriate bank protection which may transfer erosion downstream; or
Straightening or reprofiling the channel which may cause the watercourse to attempt to revert back to a
more natural state.
This has included an assessment of:
Typical land use, soils and geology as provided in Chapter 2;
Channel gradient based on the river channel survey;
Bank and bed material and condition based on site visits, aerial photographs and survey photographs;
Channel planform based on Ordnance Survey maps and aerial photography; and
The presence of structures (bridges, weirs, culverts) /channel modifications (e.g. straightening, bank
protection, bank reprofiling).
The survey data and photographs are provided in a separate survey report. Key photographs have been
included in this report to inform the analysis.
10.2 Assessment
The HPW and MPW were split into broad reaches of similar hydrogeomorphological characteristics based
on the approach above, and an assessment made on the current erosion and deposition features (Map
10.1).
10 Hydrogeomorphology
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Map 10.1: Hydrogeomorphological Reaches
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River Allow Catchment
The River Allow and the River Dalua have transitory meandering planforms which are actively migrating
across the valley floodplain as evidenced by the changes over time in the river profiles from the pre-1900
boundary lines and aerial photography. The rate of erosion is greatest on the outside of the various
tortuous meander bends where in-channel velocities and stream power are high, undermining the non-
cohesive alluvial soils. Erosion is further exacerbated by trampling of cattle along open river banks in some
reaches. The Allow and Dalua’s meandering planforms are a natural response to the following factors:
The natural tendency of the river to adjust its channel gradient (bed slope) and planform to transport
the sediment load;
The trend of land drainage due to European Union agro-forestry policies potentially increasing
sediment load and runoff; and,
The increase in flows due to a climatic “wetter” period since 2000 as evident in the Riverview gauge
record.
There were a number of localised informal bank protection measures
observed during site visits along the Allow and Dalua limiting the sources
of sediment. However, it is likely that banks will continue to naturally
erode due to the reasons stated above. Riffles and pools have formed
within Kanturk as the rivers naturally adjust to the straightened planforms
between the walls, embankments and changes in velocities through the
various bridges and weirs.
There are a number of cut-off remnants of previous meander bends
(known as oxbow lakes where flooded) along the Dalua and to a lesser
extent on the Allow. These are likely to provide additional storage below
the general floodplain level but may also form flood flow routes during
flood. Woody debris was observed in channel throughout the more rural
reaches of the Allow and Dalua due to the presence of heavily vegetated
banks and wooded areas upstream. Such debris may raise upstream
water levels locally, increasing flood risk. However, these temporary
debris blockages would be quickly bypassed during a flood and may
become mobilised presenting a blockage risk to structures downstream.
River Blackwater Catchment Upstream of Mallow
The River Blackwater downstream of the Allow confluence has low sinuosity, gently meandering across the
valley floodplain. Bank erosion was observed on the outside of some meander bends with associated
gravel bars deposited on the inside of bends. Bank erosion and slumping was particularly evident where
cattle have unrestricted access to the river. However, the rate of erosion is less than observed on the
Allow because the Blackwater is less sinuous and therefore the near-bank velocities are not as severe.
Site and survey observations indicated there was some woody debris in the rivers but this was not likely to
block bridges given the relative size of the bridge openings.
Photo 10.1: Bank Erosion on the
Allow
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The Blackwater has been constrained by raised embankments and walls
through Mallow AFA. Scour protection and bank protection was
observed at a number of bridge structures indicating active erosion
upstream as the velocities change through these structures. Recent
reprofiling works downstream of Mallow Town Bridge associated with the
flood defence scheme have developed a two-stage channel, altering the
in-channel capacity to store flood waters for more extreme events.
Gravel bars tend to be deposited in areas of shallow flow near the
Mallow Bridge and weir where the channel has been over-widened in
the past to support the bridge structure.
The River Clyda, Spa Glen, Hospital, Bearforest and Gooldshill tributaries
are all steeper smaller catchments all with woody debris present in their
upper reaches. This woody debris combined with urban debris in the
urbanised reaches can modify water levels and flow paths locally but are
likely to be washed away during floods. However, this debris presents a
blockage risk to structures downstream which is managed by a number of
trash screens at key structures. Siltation was observed on the approach
to and under a number of bridge structures where the channel has been
over-widened, such as Lower Beecher Street. However, the rate of
deposition was not deemed to be significant in comparison to the
capacity of the structures. Sediment load and rate of deposition in the
flood storage areas is monitored as part of the scheme maintenance.
Only the lower reaches of the River Clyda were observed to be actively eroding banks where cattle had
unrestricted access to the riverside. The excess sediment is deposited in channel or in bars at the
confluence with the Blackwater and is not deemed to be out of balance.
Photo 10.2: Reprofiling at Mallow
Photo 10.3: Spa Glen Trash
Screen
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River Blackwater Catchment Downstream of Mallow
Between Mallow and Fermoy, the Blackwater develops a distinct riffle-
pool sequence in response to the constrained floodplain between the
more resistant valley sides. For example upstream of Ballyhooly, the river
meets the resistant geology forming riverside cliffs. There are a number
of in-channel gravel bars at riffle features, but these are unlikely to
present additional flood risk as they are likely to be bypassed on the
floodplain and the sediment will become mobile during extreme floods.
The larger in-channel islands are vegetated, such as downstream of
Killavullen gauge, and will have a greater impact on low flows. However,
these islands are still likely to modify flow paths during floods. The River
Awbeg (Major) joins the Blackwater at an acute angle causing flow
inefficiencies and deposition of sediment sands and gravels. The
watercourse has been artificially straightened and is trapezoidal in cross
section through Fermoy as part of the flood defence scheme. The banks
generally consist of a variety of engineered walls or embankments. Such
heavily engineered walls reduce sources of sediment within this reach. Scour of the bed was observed
upstream of the Fermoy labyrinth weir with the change in velocities and flow paths over this structure.
Downstream the river has been over-widened historically leading to deposition of gravel bars in the shallow
channel downstream of the bridge.
Downstream of Fermoy, the Blackwater is joined by the Funshion and Araglin and widens by 10m with the
additional flows. The river banks in this reach were less vegetated than upstream, reducing the presence
of woody debris. No active bank erosion was observed in the lower Blackwater. However, the bank profile
was evidently maintained, and access for cattle restricted, limiting the sources of sediment for this reach.
Lismore Weir and the associated in-channel islands control water level and velocities upstream for in-bank
flows. Downstream of Lismore weir, the Blackwater becomes tidal and silts and fine sediments are
naturally deposited between low and high tide during periods of slack water (low velocities). Despite being
tidal, there are no wide tidal flats or floodplain until Youghal as the river is constrained by the resistant
geology of the valley sides. Downstream of the N25 Bridge at Youghal, the Blackwater widens to over
800m resulting in wider intertidal flats on the left bank. Ferry Point constrains the tide further, trapping
sediment on the left bank and protecting this reach from extreme coastal conditions and associated
erosion.
Photo 10.4: Fermoy Bridge
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River Bride Catchment
The Rive Bride downstream of the N8 is typically meandering in planform
with typical sediment size ranging from gravels to small boulders at
Rathcormac, to silts and fines in the tidal reaches downstream of Tallow
Bridge. Woody debris was observed in the upstream reaches originated
from the vegetated banks. The accumulation of woody debris in the
upstream reach modifies local flow paths and scour but is unlikely to
impact on flood risk as these features are washed away during flood.
However, the woody debris may present a blockage risk to the smaller
bridges at Ballinterry Cross Roads and along the smaller tributaries
through Rathcormac. There are well developed pool and riffles
sequences along the Bride as the bed slope adjusts to the changes in
velocities around the meander bends. The river channel has been over-
widened at Mogeely Bridge causing deposition under the bridge which
may change the gauge datum overtime. Bank protection at bridge
structures near Conna limit the sources of sediment in these reaches. Downstream of Tallow Bridge, the
watercourse becomes more sinuous, widens and has tidal flows. Silts and fines are naturally deposited
between low and high tide during periods of slack water (low velocities). However, bank erosion was
observed where cattle had unrestricted access to the river from the adjoining pasture lands. In the lower
reaches, the river banks are raised above the floodplain such that any out-of –bank flow will be unable to
return to the river, thus increasing deposition on the floodplain during times of flood. The River Bride joins
the Blackwater at a more acute angle with deposition of silts and fines observed on the upstream/left bank
of the confluence where velocities between the two rivers are lowest.
Other Small Catchments
The Ballynaparka Stream flows through Aglish town and is relatively steep. The river channel is constricted
by a number of masonry walls and maintained embankments which limit bank erosion and sources of
sediment. However, deposition of sediment from upstream was observed where flows were constricted
through the various bridges and culverts as the velocities change through these structures. The channel is
heavily vegetated during the summer months, which further encourages deposition. Over time, the
deposition may limit the capacity of structures and the channel to convey flows. Further downstream the
Ballynaparka Stream is joined by a tributary from the east at Ballynaparka Bridge. This tributary is actively
eroding its banks in the upper reaches where cattle have unrestricted access to the river. The river channel
becomes embanked where it joins with the Goish River and develops a meandering planform constrained
by the flood defences. The River Goish has a sinuous meander planform until it is embanked at the tidal
outfall. Some bank erosion was observed further upstream. However, the historic mapping does not
indicate active meander migration since 1900. The excess sediment from the bank erosion upstream on
the Goish and Ballynaparka tributary is deposited in the flatter tidal reaches during periods of slack water
when velocities are low. Over time this deposition may reduce the tidal channel capacity as it is embanked
above the floodplain.
Photo 10.5: Lower Bridge Bank
Erosion
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10.3 Impact on Flood Risk
In summary, the River Allow and Dalua were observed to have the faster rate of erosion and
geomorphological change due to land use and climatic changes in the catchment over the previous
decades. This effect is largely located in rural agricultural reaches and is unlikely to significantly increase
risk of flooding downstream as the sediment load is unlikely to reduce capacity at structures in Kanturk
during floods. However, flood risk management options should consider the rate of erosion, bank stability
and sediment load if applied in these more rural reaches upstream of Kanturk.
Woody and urban debris was deemed to be a significant issue on the Mallow tributaries as this could
reduce the capacity and/or block the smaller bridges and culverts through the town. Trash screens have
been installed as part of the flood defence scheme at key structures to limit the blockage risk during floods.
However, the level and frequency of screen maintenance should be considered for the operation of the
flood relief scheme going forward.
The Lower Blackwater, River Bride and Ballynaparka Stream were all assessed to be largely in equilibrium
with no significant change in channel shape or planform over time. However, deposition in the tidal
embanked reaches of the River Bride and Ballynaparka Stream may reduce the channel capacity over time
thus increasing flood risk to the adjacent low-lying areas.
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11.1 Overview
The design flows on each river reach and total tide plus surge levels provided in Chapter 8 have been
derived independently of each other. In reality, there can be dependency between sources of flooding
which can be described by the joint probability to achieve a target %AEP event. The CFRAM study
considers the following joint probabilities:
Fluvial-fluvial – Where a range of combinations of flow on a main river combines with flow on a tributary
to generate a specific %AEP flood downstream.
Fluvial-coastal – Where an approaching depression generates a storm surge which combines with a
river flood to generate a specific %AEP at the coast.
The joint probability between total tide plus surge levels and extreme waves has been considered
separately under the ICWWS study. The resultant combinations have been assessed in Chapter 6 to
establish the critical scenario for wave overtopping for each target %AEP. Therefore, this will not be re-
examined in the following sections.
11.2 Fluvial-Fluvial Dependence
The joint probability between fluvial flows on the main watercourse and its tributaries was guided by the
methodology set out in Flood Studies Update Work Package 3.4. The FSU methodology assessed the
dependence between fluvial inflows based on the distance between catchment centroids; the ratio in
catchment area; and, the difference in FARL, a measure of flood attenuation due to reservoirs and lakes.
Table 11.1 sets out the different combinations in UoM18 for tributary inflows to achieve the target %AEP
on the main watercourse.
In UoM18, the joint probability of tributaries was found to be largely dictated by the size of the incoming
catchment relative to the main watercourse. The joint probability %AEP on the smaller tributary inflows
tended to be the more frequent smaller events in order to achieve the target flow on the main watercourse.
The flows on the smaller tributaries upstream of Fermoy and tributaries on the Bride were more correlated
with high flows on the main rivers because the same storm produced the high flows. However, smaller
tributaries downstream of Fermoy were less correlated with flows on the main river because the catchment
is remote from the Blackwater centroid and the same storm is less likely to produce high flows on both
catchments.
The exception was the River Dalua-River Allow and River Allow –River Blackwater combinations. These
have similar probabilities to the main river as the tributaries contribute approximately half of the flow to the
downstream reach.
11 Joint Probability
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Table 11.1: Key Joint Probabilities of Inflows
Target %AEP at downstream HEP on main
watercourse
50%
20% 10% 5 %
2 %
1% 0.5%
0.1%
Reach inflow WP 3.4 Table 13.1 Scenario Associated %AEP of Tributary Inflow
River Bride-Blackwater
Allow tributaries (including Glannycumman, Knockcloon, Garragort, Knockawillin, Ballynoe and Brogeen)
Blackwater reach 1 tributaries (including Glen River and Awbeg Minor)
Mallow tributaries ( including Clyda, Hospital, Spa Glen, Gooldshill and Bearforest)
Blackwater reach 2 tributaries (including Awbeg Major)
Fermoy tributaries (including Strawhill and Glenabo)
Bride tributaries (including Shanowen,Flesk,Shanowennadrimina and Douglas)
Catchment centroid within 25km
Significantly smaller catchment (Ratio of area greater than 2.7)
Difference in FARL less than 0.07
71 46 35 23 10 6.1 3.8 1.2
Blackwater reach 3 tributaries (including Funshion and Araglin)
Blackwater reach 4 tributaries (including Glenshelan, Goish, Finnisk and Licky River)
Youghal tributaries (including Tourig and Kilnatoora)
Catchment centroid beyond 25km from target
Significantly smaller catchment (Ratio of area greater than 2.7)
Difference in FARL less than 0.07
93 79 65 51 34 25 17 7.5
Dalua-Allow
Blackwater-Allow
Catchment centroid within 25km
Similar sized catchment (Ratio of area within 2.7)
Difference in FARL less than 0.07
57 30 17 9.4 4.3 2.3 1.2 0.3
%AEP values to 2 significant figures
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11.3 Fluvial-Coastal Dependence
It is not possible to statistically assess the joint probability between fluvial and tidal events along the South
West coast as there is limited concurrent river flow and with relatively short 3 year record of tidal gauge
data at Ballycotton gauge (ID 19068). A short record of less than 3 years is not deemed suitable to
undertake long term frequency and dependency analysis at this location. Therefore, the DEFRA
FD2308_TR1 desk-based assessment was used to estimate the fluvial-tidal joint probability combinations
in accordance with Guidance Note 20, Joint Probability Analysis ( RPS November 2012).
It was assumed that Youghal Harbour was similar to estuaries along the south coast of England in terms of
orientation to the dominant storm track as described in DEFRA FD2308_TR1. Based on the FD2308
research, the dependence of river flow and storm surge in these estuaries tended to be “well” to “strongly”
correlated. The strongly correlated CF (ratio of the actual frequency of occurrence of a particular joint
exceedence event to its probability of occurrence if the two variables were independent) was applied to
Youghal Harbour as a conservative estimate in the absence of detailed concurrent gauge data. Figure 11.1
outlines the resultant joint probabilities.
Figure 11.1: Joint Probability Curves of Tidal and Fluvial Events for Strongly Correlated Estuaries
0.1
1
10
100
1000
0 1 10 100 1000
Tidal Surge (Return Period)
Riv
er
Flo
w (
Re
turn
Pe
rio
d)
2
5
10
20
50
100
200
1000
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Fluvial-coastal dependence between river gauges on the Lower Lee and tidal gauges in Cork Harbour
were also assessed as part of the Lee CFRAM pilot study. This analysis concluded there was some
correlation between high flows and higher storm surges as the storm events that caused the surge also
caused high rainfall in the Lower Lee catchment. Extensive sensitivity analysis was undertaken on the
0.5% AEP event as part of the pilot study and found the two main critical scenarios to be as follows:
Target flow and the MHWS tide; and
50%AEP Flow and the target Total tide plus surge level.
Based on this analysis, the design scenarios will combine the design fluvial %AEP with the appropriate
tidal surge to achieve the target fluvial dominated %AEP and vice versa for the coastal-dominated
scenario. For example, the 0.5% tidal event combined with the >50% fluvial event will produce the design
0.5% extent. This approach ensures easy interpretation of the maximum fluvial dominant flood and
maximum coastal dominant flood for the design scenario.
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12.1 Potential Climate Changes
The range of potential impacts of climate change varies as there are significant uncertainties associated
with global climate predictions and local hydrological variation for periods more than 20 years in the future.
Therefore, two scenarios have been assessed to quantify the sensitivity of flood risk to potential climate
change namely, the Mid-Range future scenario (MRFS) and the High-Range future scenario (HRFS) as
detailed in Table 12.1.
Table 12.1: Allowance for Climate Change in Catchment Parameters
Catchment Parameter MRFS HRFS
Extreme Rainfall Depth +20% +30%
Flood Flows +20% +30%
Mean Sea Level Rise +0.5m +1.0m
Land Movement -0.5mm/year
i.e. +0.05m relative sea level rise over 100 years
-0.5mm/year
i.e. +0.05m relative sea level rise over 100 years
Source: Reproduced from Appendix F of National Flood Risk Assessment and Management Programme, Catchment-Based Flood
Risk Assessment and Management (CFRAM) Studies, Stage I Tender Documents: Project Brief.
The land movements quoted in Table 6.1 refer to postglacial readjustment of the underlying tectonic plate
since the last glacial period in Southern Ireland. This readjustment is not a climatic change but it does alter
the effective rate of sea level rise predicted with climate change.
It is important to note that the increase in sea level and flood flows applies to the entire tidal curve and
flood hydrograph, not just the peak.
12.2 Potential Catchment Changes
12.2.1 Urban Development
The way in which the land is used can significantly impact the flow routes across the catchment, how much
rainfall is stored, how much infiltrates into the ground, and how much evaporates. Future urban
development is likely to influence hydrology and flood risk in the following ways:
Increase the surface runoff from the catchment by increasing the area covered by impermeable
surfaces on previously undeveloped (“Greenfield”) sites;
Increase the proportion of surface runoff draining to urban drainage networks; and,
Increase the proportion of the population, properties and infrastructure within areas of flood risk.
All of these changes cause more water to reach the river channels quicker and affect more people,
property and environments.
12 Future Scenarios
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The greatest concentration of urban development and urban change is located at Mallow at Youghal.
However, there has been significant growth in smaller towns such as Tallow and Kilworth over the past
decade. Furthermore, the regional plans identify the Greater cork area and Mallow as a “hub” of growth
over the next 20 years.
Table 12.2 outlines the urban growth in housing units according to the South West Regional Authority
(SWRA) Planning Guidelines and linear extrapolation to estimate urban growth for the MRFS and HEFS.
The SWRA data is based on a 2010 baseline and accounts for the economic downturn in forecasts beyond
2010. The MRFS growth rate has been estimated on the projected increase in housing units between 2016
and 2022 accounting for the economic downturn. The HEFS growth rate has been estimated on the
average projected increase from the entire regional plan with a lesser impact from the economic downturn.
Table 12.2: Future Urban Growth
SWRA Plan Area
Housing Units Required MRFS% Growth
HEFS % Growth
2006 2010 2016 2022
Cork Gateway 111,581 127,749 153,000 182,044 3.16% 3.54%
Mallow Hub 4,191 5,341 7,500 10,498 6.66% 8.05%
Ring towns and Rural areas
42,951 46,472 50,317 54,160 1.27% 1.38%
Greater Cork area
154,532 174,221 203,317 236,203 2.70% 2.96%
Tralee Killarney Hub area
15,284 17,099 20,318 23,573 2.67% 3.16%
Kerry linked hub
29,565 33,541 39,855 46,239 2.67% 3.15%
Northern Area 33,497 37,993 43,885 46,186 0.87% 1.80%
Western area 36,606 41,745 47,989 50,729 0.95% 1.79%
Source: South West Regional Plan. BOLD text signifies relevant areas to the UoM.
In agreement with OPW, the forecast growth in housing units was assumed to be on previously
undeveloped land as a conservative estimate of urbanisation. The MRFS and HEFS do not account for any
beneficial impacts of Sustainable Drainage Systems in the future.
12.2.2 Land Use Change
The majority of the Blackwater catchment is currently rural and dedicated to agricultural or pastoral use.
The type of crops that are grown, the way the land is prepared and changes in land drainage practice all
affect how quickly rainfall reaches the watercourses. Land management practices also affect the amount of
silt that gets washed from the fields into the rivers during rainfall events. Given that these processes can
influence flood risk, both in a positive and negative way, we need to consider how land use and land
management may change in the future.
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There are many uncertainties surrounding the future of agriculture within the catchment. Land use will
depend upon society’s aspirations and needs, and will be driven by policies being implemented by both the
Irish government and the EU. The pressures and drivers that will affect how land is used in UoM22 include:
change to agricultural policy and land management subsidies in the EU;
opening of world markets making agriculture and pastoral activity less economically viable;
growth in world population increasing demand for food production;
change in typical annual temperatures with climate change resulting in changes in crop types grown;
diversification to other land uses, particularly for tourist related attractions;
drive to enhance and restore environmental habitats and landscapes;
drive to reduce carbon dioxide emissions through the use of carbon sinks and biofuels; and,
increasing energy prices could lead to increased biofuel use or make importing of produce
uneconomic.
All of these changes can either lead to intensification of activities and associated increased land drainage
and runoff or reduction in activities with associated increased infiltration and reduced runoff. There is very
limited information on most of these land cover changes as they are often driven by economic factors
which are rarely predicted more than 5 years into the future.
Deforestation to increase productivity of agricultural land can be a significant impact on rural land use in
Europe under the EU Common Agricultural Policies. Forested areas intercept rainfall, increase storage and
infiltration and slow surface water runoff into the river channels. The removal of natural forests can
encourage greater runoff. There is only limited evidence to suggest the extent of forest cover is a
significant controlling parameter on the regression equations used to estimate peak flood flows8. However,
the OPW guidelines identify commercial afforestation to increase productivity as the significant pressure on
rural land use in Ireland. Increased irrigation and drainage for the commercial forests can route more water
to the rivers thus reducing the time to peak. The OPW future scenarios guidelines recommend that
changes in forest cover can be reflected in a reduced time to peak due to these associated drainage
works.
Between 11% and 14% of the Allow, Blackwater and Bride sub-catchments are covered by forest as
defined by the Floods Studies Update. Therefore, any decrease in forested area is unlikely to impact future
flood hydrographs as forest covers such a small proportion of the catchment at present. Forest cover
increases to 31% on the Glen River catchment and to 38% on the Araglin catchment which flow into the
River Blackwater. The projected decrease in forest cover could reduce the time to peak by 17% and 33%
for the MRFS and HEFS respectively. The increased flashy response in these catchments would change
flood risk locally but would be unlikely to change the flood response of the larger rivers given their relatively
small catchment area.
8 Institute of Hydrology (1991). Plynlimon research: The first two decades. Report No. 109, Institute of Hydrology.
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12.3 Design Future Scenario Conditions
The present day design hydrology (derived in Chapter 5 of this report) was modified to consider the
relevant catchment and climate changes discussed in the previous sections. Table 12.3 summarises the
final Mid-Range and High-End Future Scenarios.
Table 12.3: Allowance for Future Condition in Catchment Parameters
Catchment Parameter MRFS HEFS
Flood Flows +20% +30%
Mean Sea Level Rise +0.5m +1.0m
Land Movement -0.5mm/year
i.e. -0.05m over 100 years
-0.5mm/year
i.e. -0.05m over 100 years
Urbanisation 0.87 to 6.66%/year 1.8 to 8.05%/ year
Forestation -1/6 Tp -1/3Tp
+ 10% PR
The design hydrology under future conditions has been adjusted for the predicted decrease in forest cover
in the relevant Glen and Araglin catchments only. The resultant future peak flood flows and future extreme
sea levels based on the Mid-Range and High End Future Scenarios are provided in Appendix E.
The predicted increase in river flows and sea level rise attributed to predicted climate change is the most
significant factor that influences design peak flows and levels in UoM18. Urbanisation has a relatively small
impact on design peak flows as the catchment remains predominately rural in both the MRFS and HEFS.
The degree to which the increased river flows and sea levels change flood risk to the AFAs will be
assessed as part of the subsequent hydraulic modelling and mapping. The relative increase in flows and
period of any tide-locking associated with the impacts of climate change should be considered in the sizing
of any floodplain storage options and frequency of maintenance activities.
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13.1 Conclusions and Key Findings
The design flows from this hydrology report inform the inflows to the hydraulic model to assess flood risk
from the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP fluvial and tidal flood events. The key
hydrological findings in UoM18 are as follows:
Historic flood events
Major flood events were identified in UoM18 since 1980 from extreme river flows along the
Blackwater, Bride and Allow.
Extreme storm surges at Youghal were also identified as a source of coastal flood risk with major
events in 2004 and 2012.
The largest gauged fluvial events were on 19th November 2009 and 2nd November 1980 along the
Blackwater.
The calibration in Blackwater and Allow sub-catchments will be based on the following events where
there is sufficient information:
30th December 1998 – extreme fluvial event along the Blackwater and Allow
5th/6
th December 2000 – extreme fluvial event along the Blackwater and Allow
19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow
The calibration in the Bride sub-catchment will be based on the following events where there is sufficient
information:
30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride
19th November 2009 – extreme fluvial event along the Bride
The Freemount and Tallow AFA model results will be verified against the historical flood frequency
(where available) and sensitivity analysis on the key hydraulic parameters as there is insufficient
evidence to undertake full calibration on the floodplain.
The Aglish model results will be checked based on sensitivity testing of hydraulic parameters.
Rating Reviews
The high flows rating equations were revised for four gauges within UoM18 at Allen’s Bridge,
Riverview, Ballyduff and Mogeely.
High flows rating equations were checks for a further four gauges at CSET Mallow, Mallow Rail
Bridge, Killavullen and Fermoy.
The revised rating curves increased out-of-bank flow estimates for all gauges except Allen’s Bridge,
where flows were reduced to account for the backwater effect from the bridge and weir downstream.
The revised high flows rating equations were used to update the AMAX series at these gauged
HEPs and update the QMEDrural adjustment factor for hydrologically similar ungauged HEPS
13 Conclusions, Key Findings and Recommendations
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Design flood flows
Peak flood flows were derived along the Allow, Dalua, Blackwater, Bride and Ballynaparka Stream
and various tributaries within the AFAs for the 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%AEP
events using the recommended FSU methodology outlined in Work Package 2.2 and 2.3.
The design flood hydrograph at gauged HEPs was derived using the median flood width
exceedance above 50% of the peak flow.
The design flood hydrograph for ungauged HEPs was based on the hydrograph pivotal site and
informed by the FSR calculated duration but will be iteratively refined to achieve the flow at the
gauges as part of the hydraulic modelling.
The design flood hydrographs will be applied to the hydraulic models as inflows to the upstream of
each river reach, tributary inflows and intermediate inflows for the catchment in between.
The outflow for the upstream models will form the inflow to the downstream models iteratively down
the Blackwater and Bride catchments except at Mallow where the CSET Mallow gauge will form the
inflow.
The joint probability between tributary inflows and the main watercourse was informed by FSU
WP3.4. The joint probability was found to be largely dictated by the size of the incoming catchment
in UoM18.
The exception was the River Dalua and River Allow as these tributaries join the River Allow and
River Blackwater respectively. These tributaries are of a similar size to the downstream reach and
therefore have broadly equal probabilities as they contribute approximately half of the flow to the
downstream reach.
Design coastal conditions
The design extreme sea levels were extracted from the ICPSS for the 50%, 20%, 10%, 5%, 2%, 1%,
0.5% and 0.1%AEP tidal events.
ICPSS point S_31 was used to derive the total tide plus surge levels at Youghal.
The astronomic curve and surge profile were derived from the admiralty predicted astronomic tide
and typical duration of surge events in the South West.
The final design tidal curve was derived from the combined astronomic tide and design surge profile
scaled to meet the design extreme sea levels.
The ICWWS water level and wind conditions were used to derive wave overtopping at Youghal.
Youghal quayside was found to be at risk from still water overtopping (mechanism 1) in all
scenarios. The wave overtopping volume would be negligible in comparison to the still water
overtopping volumes. Therefore, wave overtopping has not been considered further for this reach.
Youghal at Claycastle was found to be at risk from wave overtopping in the design scenario and the
wave overtopping volume will be mapped in the subsequent hydraulic modelling and mapping stage.
Storm surge events in Youghal Harbour were assessed to be strongly correlated to rainfall-river
flood events due to their location on the west coast and orientation to incoming storms.
Joint probability between the storm surge and river flood was calculated using the DEFRA FD2308
desk-based approach as per GN 22.
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Uncertainty and Sensitivity
The uncertainty of the 1%AEP target peak flow was estimated to range up to +30% in UoM18
ungauged HEPs which will inform the sensitivity tests in the hydraulic modelling.
The total tide plus surge levels could underestimate coastal risk based on the historical flood
frequency of storm surges at Youghal. A sensitivity test which raises the total tide plus surge level by
0.5m has been proposed in accordance with GN22
Hydrogeomorphology
The current erosion and deposition processes were assessed for all AFAs and intervening MPWs.
The fastest rate of erosion was observed in the upper Allow associated with natural meander
migration due to climate and catchment changes since the 1960s.
The largest deposition features were associated with the tidal reaches of the Blackwater, Bride and
Goish where siltation reduces the channel capacity. The deposition is likely to be washed away
during floods. These reaches are thus deemed to be in equilibrium.
Localised depositions was observed along the Ballynaparka Stream in Aglish and the Kilnatoora in
Youghal where the sediment load from the steep upstream reaches was greater than typical flows
creation siltation at structures.
Structures on the tributaries in Mallow were found to be at risk from blockage from urban debris and
the wooded section upstream. This is currently being managed by trash screens at key structures.
Future conditions
Two future scenarios were developed to assess potential future changes namely, the Mid-Range
future scenario (MRFS) and the High-End future scenario (HEFS).
River flows were predicted to increase by 20% and 30% due to climatic changes under MRFS and
HEFS respectively.
Sea levels were predicted to rise by 0.55m and 1.05m for the MRFS and HEFS respectively,
including allowance for 0.5mm/year post-glacial rebound land movements.
Urban extent was predicted to increase between 1% and 6% per year, and 2% and 8% year for the
MRFS and HEFS respectively, based on the forecasted rates in the South West Regional Authority
planning guidelines.
Time to peak was predicted to reduce by 17% and 33% for the MRFS and HEFS respectively along
the Glen and Araglin tributaries due to change in forest cover in their upper reaches.
The design peak flood flows and total tide plus surge levels were adjusted to represent the climatic
and catchment changes above for the MRFS and HEFS future scenarios accordingly.
13.2 Recommendations
The following recommendations can be drawn from the key findings above for the subsequent hydraulic
modelling, flood risk assessment, preliminary option development and FRMP:
The design peak flows and design total tidal levels presented in Table 8.1 and 8.2 should be used to
inform the subsequent hydraulic modelling in UoM18.
Inflows for intervening catchments should be distributed across minor watercourses and overland flow
paths identified from the survey based on the proportional increase in contributing area.
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The joint probability approach and analysis in Chapter 11 should be used to inform the combinations of
inflows and coastal conditions for the model boundaries.
The relevant hydraulic models should be calibrated as far as possible to these historic flood events:
– 30th December 1998 – extreme fluvial event along the Blackwater and Allow
– 5th/6
th December 2000 – extreme fluvial event along the Blackwater and Allow
– 19th November 2009 – extreme fluvial event along the Blackwater and high flows on the Allow
– 30th January 2009 - extreme fluvial event in Rathcormac and high flows along the Bride
– 19th November 2009 – extreme fluvial event along the Bride
– 17th October 2012 – extreme coastal event at Youghal
The remaining models which do not have sufficient historic information or gauge information should
use reasonable hydraulic parameters and the modelled flood outline should be compared with the
relative historical flood frequency.
The following sensitivity tests should be considered to assess the impact of hydrological assumptions
on flood extent and levels in the subsequent hydraulic modelling:
– Peak flow
– Downstream tide plus surge levels
The mapping of wave overtopping volumes should be considered for Claycastle in Youghal (Reach C2)
where wave overtopping discharges were found to be significant and the floodplain was located below
the defence.
Extreme wave scenarios have not been considered along Youghal quayside because the quayside
was flooded via mechanism 1, still water overtopping. Wave overtopping is negligible in comparison to
mechanism 1.
The following recommendations can be drawn from the hydrological analysis for future analysis in the
catchment:
Continued effort to obtain concurrent spot gaugings (where safe) for bankfull and out-of-bank
conditions at Allen’s Bridge, Riverview, CSET Mallow, Mallow Rail Bridge, Killavullen and Ballyduff.
Spot gaugings at Fermoy Mill Gauge 18117 to verify flow estimates from the Fermoy Bridge gauges.
The Fermoy Mill gauge is located away from the complex weir-bridge structure thus avoids any
uncertainties associated assumptions taken for these structures during the rating development.
Concurrent monitoring of sea levels at Ballycotton and Youghal Harbour quayside during storm surge
events would help verify the transformation of extreme coastal events.
The %AEP estimates for total tide plus surge levels should be reviewed periodically at Ballycotton as a
longer period of data becomes available.
The rate of meander migration (erosion) in the River Allow and Dalua should be taken into
consideration in the design of any flood mitigation options and sediment yields. It should be noted that
the meander migration is part of the rivers natural hydrogeomorphological response to climatic
changes in the past few decades.
The level and frequency of screen maintenance in Mallow should be considered for the operation of the
flood relief scheme going forward.
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AEP Annual Exceedance Probability; this represents the probability of an event being exceeded in any one year and is an alternative method of defining flood probability to ‘return periods’. The 10%, 1% and 0.1% AEP events are equivalent to 10-year, 100-year and 1000-year return period events respectively.
AFA Area for Further Assessment – Areas where, based on the Preliminary Flood Risk Assessment and the CFRAM Study Flood Risk Review, the risks associated with flooding are potentially significant, and where further, more detailed assessment is required to determine the degree of flood risk, and develop measures to manage and reduce the flood risk.
AMAX Annual Maximum Flood
BFISOILS Baseflow index from Irish Geological Soils dataset. Often used as a permeability indicator.
CFRAM Catchment Flood Risk Assessment and Management – The ‘CFRAM’ Studies will develop more detailed flood mapping and measures to manage and reduce the flood risk for the AFAs.
DAD Defence Asset Database
DAS Defence Asset Survey
EU European Union
EPA Environmental Protection Agency
FARL Index of flood attenuation due to reservoirs and lakes
FRMP Flood Risk Management Plan. This is the final output of the CFRAM study. It will contain measures to mitigate flood risk in the AFAs.
FRR Flood Risk Review – an appraisal of the output from the PFRA involving on site verification of the predictive flood extent mapping, the receptors and historic information.
FSU (WP) Flood Studies Update (Work Package) (2008 to 2011)
FSR Flood Studies Report (HR Wallingford, 1975)
GIS Geographical Information Systems
HA Hydrometric Area. Ireland is divided up into 40 Hydrometric Areas.
HEFS High-End Future Scenario to assess climate and catchment changes over the next 100 years assuming high emission predictions from the International Panel on Climate Change.
HEP Hydrological Estimation Point
HPW High Priority Watercourse. A watercourse within an AFA.
ICPSS Irish Coastal Protection Strategy Study (2012)
ICWWS Irish Coastal Water Level and Wave Study (2013)
ING Irish National Grid system, Ordnance Survey of Ireland
Glossary
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MPW Medium Priority Watercourse. A watercourse between AFAs, and between an AFA and the sea.
MRFS Mid-Range Future Scenario to assess climate and catchment changes over the next 100 years assuming medium emission predictions from the International Panel on Climate Change.
ODM Ordnance Datum Malin.
The current geodetic datum of Irish National Grid which references the mean sea level at Malin Head between 1960 and 1969.
OPW Office of Public Works, Ireland
OSi Ordnance Survey Ireland
PFRA Preliminary Flood Risk Assessment – A national screening exercise, based on available and readily-derivable information, to identify areas where there may be a significant risk associated with flooding.
QMED Median annual flood used as the index flood in the Flood Studies Update. The QMED flood has an approximate 50%AEP.
QMEDamax QMED derived from the annual maximum series at a gauged location
QMEDrural QMED derived from physical catchment descriptors according to the Flood Studies Update methodology.
QMEDadj QMED adjusted by the ratio of QMEDamax:QMEDrural at a hydrologically similar Pivotal site.
QMEDurban QMED adjusted to account for the impacts of urban areas according to the Flood Studies Update methodology.
S1085 Typical slope of the river reach between 10%ile and 85%ile along its length.
SAAR Standard average annual rainfall 1961 to 1990
SEA Strategic Environmental Assessment. A high level assessment of the potential of the FRMPs to have an impact on the Environment within a UoM.
SW CFRAM South Western Catchment Flood Risk Assessment and Management study
Total tide plus surge level Total tidal level formed of the astronomic tide and storm surge elements.
UoM Unit of Management. The divisions into which the RBD is split in order to study flood risk. In this case a HA.
WFD Water Framework Directive. A European Directive for the protection of water bodies that aims to, prevent further deterioration of our waters, to enhance the quality of our waters, to promote sustainable water use, and to reduce chemical pollution of our waters.