Engineers Ireland Cork Region

Design and Implementation of the Mallow Flood Relief Scheme KEN LEAHY, BE DipConLaw CEng MIEI Associate, Arup Consulting Engineers LUKE BALLANTYNE, MEng CEng MCIWEM MIEI Senior Hydrologist, Arup Consulting Engineers Paper presented to Engineers Ireland Cork Region on 10 February 2009

Aerial view of Mallow in Flood - January 2008 Photograph courtesy of Gaelic Helicopters Synopsis The town of Mallow has a long history of flooding from the River Blackwater, with the town bridge being destroyed by flooding as long ago as 1628. Flooding is a common occurrence in the town and the town park typically floods at least once a year. In 2002, Arup were commissioned by the Office of Public Works to develop a Flood Relief Scheme for Mallow, under the powers of the Arterial Drainage Act, with the aim of providing protection against a 1 in 100 year flood. This paper outlines the design options considered to defend the town from flooding from both the Blackwater and its tributaries, including a synopsis of the hydrology and hydraulic modelling carried out and provides an overview of the phased implementation of the recommended Schemes.

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Introduction Mallow, Co. Cork, is located 35km north of Cork City on the N20 Cork to Limerick Road and the N72 Killarney to Fermoy Road in the beautiful Blackwater Valley. The town is divided by the Blackwater with the Main Street and the commercial centre located to the north. There are approximately 80 recorded monuments within Mallow and its surrounds. The Blackwater is a cSAC and is also one of the countries most important game fisheries. Catchment Description The River Blackwater rises in County Kerry in the Mullaghereirk Mountains and initially flows south towards Rathmore before turning east, joining the Irish Sea at Youghal. (See Fig. 1) It has a number of significant tributaries upstream of Mallow including the Allow and the Dalua to the north and the Clyda to the south. The catchment area to Mallow is approximately 1178 km2 The upper (western) part of the catchment is a wet mountainous region with low permeability soils whereas the eastern part of the catchment is more rolling countryside around and downstream of Mallow with more permeable soils and limestone outcrops. The catchment upstream of Mallow is therefore very responsive to rainfall with high runoff rates. The floodplain is wide upstream of Mallow but narrows considerably downstream of Mallow Bridge.

Fig. 2 Historic Flood Events in Mallow

Fig. 1 – Blackwater Catchment to Mallow History of Flooding in Mallow Mallow has a long history of flooding with the town bridge being destroyed by flooding as long ago as 1628. (See Fig. 2 for historic flood events) Severe flooding has been experienced on a significant number of occasions during the last 100 years. Flooding is a common occurrence in the town and the town park typically floods at least once a year. In 2008 alone, Bridge Street was flooded twice. (See Fig 3 for aerial photo of 2008 flood)

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Fig. 3 Aerial Photograph of Mallow in Flood There has therefore been a need to assess possible alleviation or management measures to limit the flood risk to the town. UCC had previously prepared a preliminary flood study in 2000 to assess the risks and develop outline measures. In 2002, Arup were commissioned by the Office of Public Works to develop a Flood Relief Scheme for Mallow under the powers of the Arterial Drainage Act 1945. Principles of the Schemes The proposed schemes would provide

protection against a 1 in 100 year flood event. A flood warning system for the people of Mallow

would also be developed by OPW and would be implemented in parallel with the Schemes.

The design of the schemes would try to mitigate

any environmental impacts. The Schemes would include demountable

defences, defence walls and embankments, culverts, storm drains and pumping stations

All defence walls would be designed and

constructed to make allowance for future climate change but with the wall section only being constructed to the ‘excluding climate change)’ level

Park Road would be protected against some of

the smaller flood events. Benefits of the Schemes The benefits of the Schemes are reductions in the following: Damage to residential and commercial property

Stress and anxiety experienced by residents

resulting from the threat of future flooding Economic loss to vulnerable traders

Social disruption to the local community

Disruption and delays to traffic in the town

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Fig. 4 Timeline for implementation of the Scheme Timeline for Implementation of the Schemes • In 2002 following a public competition, Arup

were appointed by OPW to carry out a Feasibility Study to assess flood risk in Mallow and develop a Flood Relief Scheme

• In 2003 Arup produced a Final Engineering Report, which presented our initial findings and outlined details of the proposed Scheme.

• In 2004, at the request of OPW, Arup produced a Phasing Report outlining how the various works might be phased to ensure a speedy and beneficial implementation whilst complimenting annual budgetary constraints.

• Detailed design then commenced and in June 2005, 3 distinct Flood Relief Schemes were publicly exhibited, namely: o Munster Blackwater (Mallow North)

Drainage Scheme o Munster Blackwater (Mallow South)

Drainage Scheme o Munster Blackwater (Mallow West)

Drainage Scheme Each phase when complete will increase the level of flood protection within the phase area.

• In 2005, following a Section 8 Planning process, an Advance Works contract was awarded by Cork County for the elements of the ‘Mallow North’ Scheme which were located on public lands. This contract was funded by OPW and was completed in early 2006.

• In 2007, the ‘Mallow North’ Scheme was tendered under EU procurement rules and in Spring 2008, the Minister for Finance confirmed the construction of this Scheme which is now currently under construction and is due to be completed mid 2009.

• In August 2008, Arup were appointed to commence detailed design of the ‘Mallow South’ and ‘Mallow West’ Schemes. It is envisaged that one or both of these schemes will go to tender later in the year, with construction commencing in late 2009 or early 2010.

Preliminary Design Stage Overview The design process commenced with a desk study of all available data including hydrometric data collected over the years, anecdotal evidence of flooding, extracts from old journals etc. A Topographical and Hydrographic Survey was then commissioned to provide information on the existing channel. A Geotechnical Site Investigation was also carried out to determine the ground conditions with respect to both bearing capacity for structures and also for use in analyzing potential seepage under any proposed defence walls or embankments. Extensive Hydraulic Modelling was carried out of both the Blackwater and the various tributaries and this is described in more detail later in this paper. A geomorphologic assessment was also carried out on possible alterations to the existing channels. Once this work was completed, possible solutions were identified. Any options which appeared viable were assessed further both from a technical and commercial viewpoint. An Environmental Impact Statement was also completed (by others). The following sections describe the hydrology of the scheme including hydraulic modeling carried out and provides an overview of the various design options considered in relation to protection from both Blackwater flooding and combined Blackwater/tributary flooding. Hydrology Adopted approach. The approach adopted to the derivation of the design flows is summarized in brief below. For greater detail, the reader is referred to Lancaster & Marshall, 20031

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The approach adopted to the analysis of design flows for the River Blackwater drew upon the Flood Studies Report (NERC, 1975), which is still the standard for Ireland, but where possible used the new techniques available from the Flood Estimation Handbook (“FEH”; Institute of Hydrology, 1999), which has largely superseded the FSR in the UK. The importance of local data to the reliability of the design flows analysis is however a common theme to both texts, and as such the approach to any flow analysis problem is greatly influenced by the available data. There are two key river flow gauging stations on the Blackwater: Mallow CSET (“CSET”) and Killavullen. See Table 1 and Figure 5 below.

Station name

Station no.

Catchment area (km2)

Period of record

Location

Mallow CSET

18006 1058 1977- present

5.5km upstream of Town Bridge

Killavullen 18003 1258 1955 – present

12km downstream of Town Bridge

Table 1 Gauging stations data A statistical approach making use of local gauge data is the most accurate approach to the estimation of peak flow values. Such approaches are often based around the derivation of an easily measured (and compared) index flood, relating to a low return period, multiplied by a factor from a growth curve to scale the index flood up to a higher magnitude event. Use of the CSET gauge The long period of record available for both gauges meant that best results would be obtained from direct use of the gauges for derivation of the index flood. The CSET gauge would be the most appropriate for derivation of the index flood at Mallow, being the closest gauge, both in terms of river length and catchment area. This said, it was recognised that the CSET gauge could be bypassed once flows rose above bank level, by flow passing over the left bank floodplain. The existing rating for the gauge was extrapolated linearly from in-channel readings. Arup used hydraulic modelling (described later in this paper) to improve the rating at the gauge, finding that the curve changed dramatically for out-of-bank flows and that flows would have been underestimated at high water levels. Derivation of the Index Flood The improved rating at CSET was found to produce good matches in the hydraulic model against validation events. An average annual maximum flow

(Qbar) of 305m3/s was derived for CSET from its flow record. The short period of record for this gauge (24 years at the time of calculation) was modified for climatic variation against the longer record from the Kilavullen gauge, resulting in a reduction of flows to 288m3/s. This adjustment is not to be confused with allowances for forecast climate change, which was incorporated to design flow estimates as a 15% increase on flows, based on a review of published papers available for Ireland at the time. The Qbar value calculated at the CSET gauge was transferred to Mallow on the basis of the ratio of the theoretical Qbar values calculated from catchment parameters for the two sites. Derivation of the Growth Curve The growth curve was assessed by three methods: a single site analysis; a pooling group analysis; and use of the Regional Growth Curve for Ireland. Single site analysis of the CSET gauge showed that the local flow record best matched the Generalised Extreme Variable (GEV) Distribution. It was however noted that this record was sensitive to events in excess of the measured portion of the gauge’s rating curve, and that the length of record was short in comparison to the desired design event. The Pooling Group Analysis made innovative use of the FEH’s WinFAP statistical package to increase the number of years of record used to analyse the growth curve. The method ranked sites (from the UK hydrological record) to the subject site in terms of hydrological similarity, using an equation based on the three most important hydrological parameters: Catchment Area; Rainfall and Soils. It was considered valid to use UK gauged data, since much of the pooling group approach is based around the virtue of hydrological homogeneity over geographical similarity. The approach did need modification to allow for different representation of Soil parameters in the UK and Ireland: BFIHOST was calculated for the site from the Standard Percentage Runoff as shown in Equation 1, below: BFIHOST = -(SPR–72)/66.5 (Eqn. 1) The growth curve produced by the Pooling Group analysis produced remarkably similar results to the Irish Regional Growth Curve from the Flood Studies Report, supporting use of the Regional Growth Curve as the design basis for this site. The selected design flow for the Blackwater was 751 m3/s for the 1 in 100 year event, excluding climate change.

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Location 100Yr FDL (Excl. CC)

mAOD (Malin)Upstream of Blackrock Bridge

49.28

Downstream of Blackrock Bridge

48.61

Hospital Stream Confluence

48.5

Upstream of Mallow Bridge

48.46

Downstream of Mallow Bridge

47.92

Spa Glen Confluence

47.65

Table 2: 100yr flood defence level (excluding climate change)

Table 3: Flood frequency values (m3/s) for tributaries of the River Blackwater (excluding climate change)

0

100

200

300

400

500

600

700

800

1 10 100Return period (years)

Flow

(m3/

s)

Fig 5 Mallow Flood Frequency Curve

Historic Analysis A comparison of the statistical flood frequency analysis with the historic flood record for the town made use of the methods of Bayliss & Reed (2001)2 to apply Gringorten plotting positions to the flood record of significant peaks from the past 150 years. Testing this against the 1980 event (the highest flow event whose magnitude could be reliably derived from the flow record), the return period from the growth curve was just above 40 years, comparing favourably with estimated return periods from the historic record of between 30 and 40 years for the same event.

Tributary flows Tributaries entering the hydraulic model downstream of the CSET gauge all, with the exception of the Clyda, have small catchment areas, and do not have suitable gauge information for the extraction of index flood information. Water level gauges were installed on the Spa Glen that captured enough storm events to allow an analysis of the time to peak, baseflow index and standard percentage run-off in conjunction with rainfall data. The results of this were extended to the other minor tributaries in the town, the Spa Glen being considered a good hydrological analogue.

A comparison of the regional growth curve with growth curves derived for these catchments by the rainfall-runoff method showed that the rainfall-runoff method produced a much steeper growth curve.

This is to be expected, given that the regional growth curve was derived predominantly from records from large catchments, and as such, reflects greater routing effects due to floodplains and flatter river gradients. The growth curve from the rainfall-runoff analysis was therefore used for the tributaries, with the exception of the Clyda, for which the regional growth curve was used. Design flows for the tributaries were in summary based on the rainfall-runoff analysis, with some modification to the time to peak. Joint Probability The interaction of the Blackwater and its tributaries requires an understanding of the combinations of flow on each that may be expected to occur within the overall design standard of the scheme. The methods of Worth & Gamble (2001)3 were used to derive a set of combinations that could be considered. The most appropriate combinations for design combined a 100 year event on the Blackwater with a 25 year event on the tributaries. Comparison of these combinations with observed flow differences between CSET and Mallow Bridge appeared to validate these combinations.

Return period (years):

River Clyda Spa Glen Upper Spa Glen

Lower Spa Glen

Hospital Stream Bearforest Stream

Gooldshill Stream

2 44.7 - - - - - - 2.3 47.0 5.9 4.4 2.9 5.9 1.9 1.1 5 56.4 8.1 6.1 4.0 8.1 2.6 1.5 10 64.4 9.7 7.2 4.8 9.7 3.2 1.8 25 75.2 11.9 8.9 5.9 11.8 3.9 2.2 50 83.2 13.6 10.3 6.8 13.6 4.5 2.6

100 92.1 15.3 11.6 7.6 15.3 5.1 2.9

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Fig. 6 – Floodplain - channel interaction downstream of Mallow Bridge These combinations were however based on free flow and had to be modified where design proposals made use of flood storage on the tributaries: the use of storage would delay the peak of the tributary flow and increase the duration of the hydrograph, making it more appropriate to assume that two 100 year flood peaks could coincide. Hydraulic modeling A hydraulic model of the river Blackwater was built in HEC-RAS v3.1, extending from upstream of the CSET gauge, to 4km downstream of Mallow. Model calibration was not a straightforward task, largely due to uncertainty in flow values, caused by the poor rating at CSET. Fortunately there was a large range of flood levels recorded during a range of flood events, as well as a very useful helicopter video from the 2000 event. This allowed greater understanding, not just of roughness values, but of ineffective flow areas and energy losses due to channel-floodplain interaction.

The model was first calibrated against in-channel events, using level recorders installed at 5 key locations along the river, which had managed to capture events of up to 255m³/s. In-channel events have reduced uncertainty in flow values since they correspond to the accurately rated portion of the CSET rating curve, and have no floodplain flow. These calibrations produced reasonable fits to most rating curves, with the exception of the level recorder immediately upstream of Mallow Bridge. This was felt to be due to necessary simplification of the assumptions regarding ineffective flows at the bridge, which in reality allowed somewhat more flow through at lower flow events. The model was then calibrated against the 1998 event, using peak flow values derived from the Kilavullen gauge to provide inflows to the model. The calibration achieved was in the range +/- 0.20m.

The roughness values implied by the in-channel calibration were unexpectedly low (n = 0.025). Initial calibration of the model in the 1998 event while retaining these in-channel roughness values suggested unusually high floodplain roughness values. From inspection of the helicopter video footage, the velocities of flow on the floodplain areas appeared to be very low. There were a couple of reasons for this. The floodplain of the Blackwater alternates sides of the river channel as the river meanders. This is not especially unusual behaviour, but the presence of a number of limestone outcrops did mean that in some locations, the floodplain on one bank would disappear entirely, leading to the entire floodplain flow having to cross the main river channel to pass to the opposite bank (illustrated in Figure 6). The interaction between the two flow paths would lead to very high energy losses, and was exacerbated by heavy vegetation and raised ground levels on the river banks. It was concluded that the floodplain areas were therefore largely operating in ineffective flow, and they were represented as such in the model. These were later remodelled as a very high roughness value in order that options to improve floodplain conveyance could be modelled. An alternative calibration was identified, in which in-channel roughness increased at high flow events, due to the same energy losses from channel-floodplain flow interaction. This led to roughness values that corresponded closer with expected values. The model was intended for use in high flow events, so this calibration was deemed more appropriate, however the initial calibration was retained, with the intention of revisiting the calibration if a large flood event occurred during the project. A major event occurred in 2004, allowing improved understanding of the channel calibration. Comparing the two calibrations, it was found that the calibration with lower in-channel roughness

Steep banks Floodplainextent

Floodplain-Channel Interaction downstream of Mallow Bridge

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provided a closer match to the observed flood levels. The model’s final calibration was validated against the 1980, 2000 and 2004 flood events, generally achieving a match of 0.10 - 0.15m. This in turn fed into the hydrological analysis as the as the calibration of the model was used to readjust the rating curve (and flow record) at CSET. The revised design flow was 751m3/s Freeboard Minimum freeboard allowances were calculated following the methods of the Fluvial Freeboard Guidance Note (Environment Agency, 2001)4. This method takes account of hydrological and hydraulic uncertainty, as well as the consequences of overtopping, and physical effects such as settlement and superelevation. The defence levels produced by the addition of these freeboard values to the design water levels were subject to a final rationalisation that ensured a pragmatic level of variation in the design defences. Blackwater Options Considered Overview Initially, a number of Blackwater options were investigated. These included: • Flood Proof Properties • Use of a Flood Warning System • Upstream Catchment Storage • Soft Engineering Measures and River

Restoration • Change Storage Profile in Flood Plains • Improve Channel capacity through Maintenance • Change the cross sectional area of the channel • Allow more flow in the flood plain • Improve hydraulics at local key constraints • ‘Direct’ Flood Defences It could be seen at an early stage that a number of these options were not viable solutions and so were not considered in any further detail. It also became evident that regardless of what other combination of solutions might be used, ‘direct’ flood defences would be required. Therefore ‘Direct’ Flood Defences, plus the following option to reduce river levels and thus defence heights were considered in detail: • Floodplain Storage upstream of Mallow • Removal of island upstream of Mallow Bridge • Realignment of the Mallow Town Bridge

Approaches • River Straightening downstream of Mallow

Bridge • Flood Relief Channel downstream of Mallow

Bridge These options are discussed further below:

Floodplain Storage upstream of Mallow Floodplain storage options can have significant benefit in that the protection they provide (in the form of reduced flood flows) extends not just to the site under consideration, but to anywhere within a significant distance downstream of the storage site. However, storage options to protect towns on major rivers are commonly constrained by the volume of storage required to effect a significant change in flood flows. Two options were considered as follows: 1. Provision of New Storage (Reservoir) 2. Washland Structures to hold back filling of

existing floodplain as long as possible. Recognising that the threshold of flood damage at Mallow is a 1 in 2 year event, it was considered unrealistic to be able to prevent flooding altogether in the town through use of storage. Instead, a target was considered of reducing the 100year flood levels by 1m (equivalent to reducing the 100 year event to a 10 year event.) The volume of storage required to achieve such a reduction was 10 million m3. No suitable location was identified for a reservoir of sufficient volume, which means that the redeployment of existing floodplain storage was the only viable option for upstream attenuation of the flow. Off-line storage was considered through the modification of existing floodplains to create washlands. Analysis of changes in the rate of rise of flood hydrographs allows identification of the influence of the existing floodplain storage, following a method developed in McCartney & Naden (1995)5. By comparing the volume stored on the floodplain with the volume of flow above a given threshold of the 100 year hydrograph, it is possible to identify a flow level at which the floodplain capacity is sufficient to store any flows of a greater magnitude. This threshold, shown in Fig. 7, was identified at about 445m3/s. In theory therefore, it is possible to hold back floodplain storage until the 445m3/s level is reached so that flows are not greatly increased above this level. Inserting this revised design flow level in to the hydraulic model, it was found that this led to a reduction in water level at Town Bridge of up to 0.80m. However, in practice, it was felt the achievable level of reduction would have been less, due to efficiencies and landowner issues, so reductions of 0.5-0.6m were considered more realistic. To achieve this, the required depth of controlled floodplain storage would have been approx.1.4m.

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Fig. 7 Design Hydrograph (for Washland Analysis) Such washlands would be created by constructing 1.4m high embankments along the banks of the available floodplain from above Mallow Racecourse to the confluence with the Allow, to deny the flood plain to smaller events. Inflow to the washlands would be over the embankments, but outflow would be via penstock-controlled culverts through the embankments Such washlands could result in reducing the peak flow at CSET from 550m3/s to 445m3/s which would give a reduction in peak water level in the town of approx. 0.5 to 0.6m. However, such reduction in levels would not eliminate the need for defences in the town. A costing of this option revealed that the reduced cost of constructing lower defences in the town as a result of the lower predicted flood levels was far exceeded by the substantial costs of constructing the upstream embankments and control structures and was therefore not recommended. Removal of Island upstream of Mallow Bridge The reasons for the formation of the island are unknown but may have been related to the construction of the weir under the bridge in the 1960s, reducing low flow velocities upstream of the bridge, leading to deposition. The impact of removing the island has been modelled and the change in levels is negligible. (less than 100mm)

The initial environmental study confirmed that consideration of the removal of this island would involve significant further environmental survey work due to presence of Lamprey and freshwater pearl mussels in the Blackwater. Due to the negligible difference in level achieved by its removal, the possibility of it growing back and the various environmental constraints, this option was not recommended. Realignment of Mallow Town Bridge Approaches This option was ultimately chosen and is dealt with in more detail later in this paper. River Straightening downstream of Mallow Bridge The option of completely re-aligning the river downstream of Mallow Bridge was reviewed. In theory by straightening the river, downstream velocities would increase moving more water from the pinchpoint at the Bridge where serious flooding occurs. The optimum route for straightening the river was identified within the corridor of the extents of the 1 in 100year floodplain. This straightened route would reduce the length of the channel by approx. 280m.

River Blackwater - Design hydrograph - Volume of flow above thresholds,

and estimated volume stored on flood plain at a range of thresholds

0

5

10

15

20

25

200 250 300 350 400 450 500 550 Threshold flow (m3/s)

Volu

mes

(mill

ion

cu.n

)

3

Volume of peak of design hydrograph abovethreshold

Volume stored on flood plain - direct calc.

Volume stored on flood plain - macro calc.

10

Downstream Reach Length (m)

Left Hand Floodplain Channel

Right Hand Floodplain

Original River 3910 3976 3966

Straight River 3669 3684 3685 Difference -241 -292 -281 Table 4 – Revised Reach Lengths for Straightened River When this option was modelled, the result showed that water levels downstream of the ridge reduced by only approx. 200mm and there was no decrease in levels upstream of the bridge. The small benefit in level reduction versus the likely environmental impact, the risk of the river reverting to its former course and the significant cost involved all meant that this option was not considered to be viable. Flood Relief Channel downstream of Mallow Bridge As an alternative to the options of increasing the size of the existing channel, the option of a flood relief channel downstream of Mallow Bridge was considered. This would have less disturbance on bank full flow, maintaining the dominant discharge and minimising the risk of changes to the geomorphology. The flood plain corridor was plotted. A 10m strip was drawn adjacent to the existing channel. This strip was required to be maintained to restrict seepage and allow for recreational use. Because the existing channel meanders across the

Fig. 8 Downstream Flood Relief Channel

floodplain downstream of the bridge, the flood relief channel would need to start on the left bank just after the bridge and traverse the river twice. Control structures would be required at the upstream (and possibly downstream) ends of the flood relief channel to prevent flow of water along these channels until Blackwater levels exceeded bank full level. Options for control structures included sluice gates, weirs and radial gates. The use of fixed weirs at the upstream ends with no control on downstream ends was selected as the most cost effective solution and was therefore the recommended option. A flow of 75m3/s was used as the threshold for waters to spill into the flood relief channel. Such flow would occur 9% of the time and the average duration of each such flood event would be less than a day. Leaving the downstream ends open meant that the relief channel could be designed as a backwater for ecological benefit as it would flood from the downstream ends even at low flows. The proposed bed elevation and gradient was broken into the 3 different sections of the flood relief channel with individual slopes for each section of 1 in 1136, 1 in 1923, and 1 in 1265 designed to suit the flow and avoid low spots in the existing topography. With the bed elevations fixed, the maximum base width at each cross section was then calculated. Side slopes of 1 in 5 were assumed to allow for livestock access. The minimum base width available at the tightest point was 31m and so a base width of 30m was adopted to maintain a uniform width.

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The final alignment of the relief channels was designed on the basis of the following: • Straight as possible • As far away from main channel as possible • At cross over locations, the downstream flood

relief channel should start before the previous channel re-enters the main channel (to ensure water re-entering the main channel enters in line with the flow so that hydraulic efficiency is maximised and the returning flow does not cause excessive erosion of the bank.

• This solution also allowed for the re-grading around Mallow Bridge, which was required to facilitate flow into the upper relief channel.

The total length of the flood relief channel proposed was 2932m including overlapping sections where the channel crosses the river to the opposite floodplain. The original model assumed that the floodplain downstream of Mallow was ineffective as this gave the most reasonable values of roughness coefficients and also matched observed behaviour of the Blackwater. However, in order to introduce the flood relief channel, the flood plain must be effective. The baseline model therefore had to be recalibrated to allow for effective flood plains by amending the roughness coefficients until the levels matched the original figures. Initial indications showed that this work could reduce levels upstream of the bridge by as much as 0.7m extra over the reduction given by re-grading at the Bridge alone. However, given the many variables involved in assessing this option, several assumptions had to be made in the initial assessment and further modelling would have been required if this option was considered further. However, even assuming an upper bound in the level reduction, this option was not considered viable on commercial grounds. ‘Direct’ Flood Defences This option consists of constructing defences along the affected lengths of the channel to prevent floodwaters from reaching potentially affected properties. Such defences include; defence walls, embankments, pumping Stations, and demountable defences. ‘Direct’ Defences were required as part of the Scheme and are described in more detail later in this paper. Spa Glen Options Considered Overview Spa Glen flows through Mallow Town via a combination of open channel and a complex system of culverts joining the Blackwater immediately downstream of Mallow Bridge. However, Blackwater Flooding can flow up Spa Glen causing flooding via Tipp O’ Neill Park if flap valves or defences are not provided. However if flap

valves were to be provided, the Spa Glen would become ‘tide locked’ during periods of Blackwater flooding and if Spa Glen flows were sufficiently high to exceed the storage capacity of the Spa Glen floodplain, then flooding would again occur The Spa Glen options can therefore be broken into two main categories, namely: 1. Solutions isolating the Spa Glen from the

Blackwater 2. Solutions defending against a combination of

high Blackwater levels and high Spa Glen flows. A number of Spa Glen options were initially investigated including upstream catchment management, increasing channel conveyance, flood warning systems etc. but these were not considered viable options. Solutions isolating the Spa Glen from the Blackwater The following options were considered as possible solutions when isolating Spa Glen from the Blackwater: • Upstream Diversion of Spa Glen (known locally

as Hunter’s Diversion) • Pumping • Upstream Storage • Combinations of the above options Upstream Diversion of Spa Glen (known locally as Hunter’s Diversion) It has been established for some time that Spa Glen has contributed to flooding in Mallow. Local engineers had identified a route for a possible diversion of the Spa Glen as far back as the 70s. This option was reviewed in detail. The proposed diversion takes flow from where the Spa Glen crosses the old railway line to rejoin the Blackwater downstream of Mallow. 47% of the catchment is located upstream of this point which sets the maximum proportion of flow that can be diverted, although a small flow must be maintained in the existing channel at this point to ensure that the upper reaches retain their habitat. A control structure would be required at the diversion to maintain the minimum pass through flow whilst ensuring it was not exceeded during a flood event. The first part of the diversion would consist of a deep culvert some 600m long between the point of divergence and the N73. The depth to invert of this culvert would be some 10m at its deepest point and some excavation of rock would be required. An initial sizing of the culvert suggested a circular culvert of 2.4m diameter would be required.

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The depth of the culvert and the ground conditions allow for 3 possible construction methods; Micro-tunnelling, cut and cover or open channel. The advantage of tunnelling over the other two options is that it would minimise disruption to the various landowners. Downstream of the N73, it is proposed to continue the diversion by enlarging a small existing channel. The enlarged channel would be approx. 1m deep and 5 to 6 m wide with 1 in 2 side slopes until it reaches the floodplain of the Blackwater where it would widen to approx. 10m. This channel would be quite steep and would require some scour protection. Whilst investigating this option an alternative route was also identified some 800m further downstream. The potential benefits of this alternative route include the interception of a greater percentage of the catchment, and the shorter and potentially shallower route. However, further investigation of this route would have been required in terms of topography and ground conditions. This route was not considered further as the benefits were marginal in comparison with the Hunter’s diversion. Fig 9 Hunter’s Diversion Spa Glen Pumping If the area of the Spa was to be protected from flooding from the Blackwater by the use of a flap valve on the Spa Glen (instead of defences), the Spa Glen would effectively become ‘tide locked’ and flooding from high flows in Spa Glen could occur.

A possible solution was to position a large Pumping Station near the junction of Park Road and Bridge Street. The size of the pump station is clearly a function of the flows that need to be pumped. Outline design of a 2m3/s pumping station suggested that any larger pumping station would be difficult to accommodate in the available space. The results of the hydraulic modelling suggested that there is little or no existing attenuation of the peak flood at present meaning that the required pump capacity would need to be approx. equal to the peak flow. Given the space restrictions it is clear that this option is only feasible if the peak flows can be significantly reduced by a combination of an upstream diversion and/or upstream storage. Spa Glen Upstream Storage This proposal aims to attenuate the flows passing through the town centre by utilising the points where the Spa Glen crosses the N72 to act as throttles of a series of 3 potential storage areas identified upstream of the town. Volumes of storage were estimated based on surveyed channel sections and OS mapping of the area. A depth volume relationship was then identified for each storage area and these were then combined to generate a composite depth volume relationship The analysis carried out looked at various combinations of the following options: With and without Hunter’s diversion 100 year return period with and without climate

change Varying attenuated flows of 1, 2, and 3 cu.m/s

For each option, the maximum water level and hence maximum wall heights along the N72 were calculated. The wall heights would also need to include a freeboard height of 0.5m. The Initial analysis showed that an upstream storage solution was not viable for the entire Spa Glen catchment as the height of walls required would be approx. 5m. It should be noted that the flow in Spa Glen will be higher than the design attenuated flow due to the likely inefficiencies of reservoirs in series as opposed to a single storage reservoir and also due to inflow downstream of the storage areas. It is estimated that the pass on flow to the town would be approx. 1.5 times the calculated attenuated flow. Therefore this option would only be feasible with Hunter’s diversion in place. To achieve an attenuated flow of 2m3/s, a maximum wall height of approx. 3.3m would be required. This could be reduced to approx. 2.4m however if an attenuated flow of 3cu.m/s were used. The cumulative storage volumes were significant in terms of reservoir flood safety (eg, 120,000m3 for the storage option in conjunction with a 3m3/s pumping capacity) and would have imposed a different order of health and safety risk to Mallow, with associated stringent maintenance requirements.

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It is clear from the above that the attenuated flow would still be sufficiently large as to require pumping downstream and that the required capacity of such a pumping station would need to be in the order of 3 to 4.5cu.m/s. If the upstream storage were only to be used when the Blackwater is in flood, an automatic control system would be required to distinguish between high Spa Glen flows in parallel or not with high Blackwater levels. Such control would therefore be actuated penstocks linked to water level monitors in each of the storage area, Spa Glen in town and the Blackwater. Combined Option of Upstream Diversion, Upstream storage and Pumping The implementation of the Hunter’s diversion and upstream storage would only reduce defence heights by approx. 600mm, which could not be justified with respect to cost. If defences were not acceptable, and Spa Glen is to be isolated from the Blackwater when in flood, space limitations on the size of the pumping station meant that the peak flow from Spa Glen had to be reduced. It has been determined that upstream storage cannot reasonably reduce the flow from the entire catchment to a reasonable flow and therefore any solution of this nature must incorporate the Hunter’s diversion.

Fig. 10 Combined Option – Hunter’s Diversion, Upstream Storage and Pumping

Therefore, the combined option considered for isolating Spa Glen from the Blackwater incorporated the Hunter’s Diversion, upstream storage and a 3m3/s pumping Station. It was found that this option was considerably more expensive than defending against the combined Blackwater/ Spa Glen event.

Pass on Flow (m³/s)

Maximum Depth of Water Stored (m)

1 4.4 2 2.7 3 1.85

Table 5 – Summary of the Combined Storage and Pumping Results – 1 in 100 Year Return Period (with Hunter’s Diversion in place) Solutions defending against a combination of high Blackwater levels and high Spa Glen flows. The options considered for defending against the combined Blackwater/ Spa Glen were: • Defences Only • Defences and Extension of culvert through Tipp

O’ Neill (Energy) Park

14

Defences Only To consider the required defence height along the tributary it was first necessary to carry out a joint probability assessment of the Blackwater and Spa Glen flows. This analysis took account of the fact that the Blackwater flows from West to East, in the same general direction of most frontal rainfall systems, increasing the likelihood that rainfall would move down the Blackwater valley at a similar rate as the flood wave such that the timing of flooding on the Blackwater and its tributaries could coincide. It was established that a prudent approach was to allow for the worst case of the following: 1 in 25 year Spa Glen event in parallel with a 1

in 100 year Blackwater event 1 in 100 year Spa Glen event and bankfull in the

Blackwater

Table 6 Spa Glen flood levels, whole catchment Hydraulic modelling was then carried out to determine the required defence level. The modelling suggested that walls approx. 2.5m to 3m high would be required on the road side of Tipp O’ Neill park which would significantly detract from the appearance of the Park. A 2m high wall would also be required to defend Energy House and a new access road would need to be provided on higher ground to the northeast of Energy House. With this approach it was also necessary to consider the capacity of the upstream culverts on the N72, which are currently undersized, and result in floodwaters backing up and flowing along the road into the town. This option would also require all lengths of the culverts within the town to be assessed to ensure that they could withstand the surcharge loading and would not allow floodwaters escape into the town. The above points are discussed in more detail later in this paper. The option of demountable or partial demountable defences were also considered in this area, but due to the ‘flashy’ nature of the Spa Glen, it was unlikely

that sufficient warning could be given to allow for the demountable elements to be erected in time. Defences and Extension of culvert through Tipp O’ Neill (Energy Park) This option was ultimately chosen and is dealt with in more detail later in this paper. Other Tributaries – Options Considered Hospital Stream Defences including realignment of existing channel/culvert As the 1 in 25 year flow in the Hospital Stream is 13.6 m3/s, it was not feasible to isolate the stream from the Blackwater and pump the flow during a Blackwater event due to the scale of pumping station that would have been required. The only option considered therefore was to defend against the combined Blackwater/Hospital stream event. Originally it was intended to utilise a section of existing culvert through the Dairygold factory site by sealing any penetrations and carrying out local repairs. However, when a detailed inspection of this culvert was carried out it became evident that the condition of this culvert would not allow its reuse. An alternative route therefore had to be identified as it was not feasible to construct a new culvert on the existing line, as this was located underneath a working factory. A suitable new route was subsequently identified following discussions with Dairygold’s owners. The details of this route and the other defence work required are outlined later in this paper. Bearforest Stream Defences and Flood Relief Culvert As the 1 in 25 year flow in Bearforest Stream is 4.5 m3/s, it was not feasible to isolate the stream from the Blackwater and pump the flow during a Blackwater event due to the scale of pumping station that would have been required. The only option considered therefore was to defend against the combined Blackwater/ Bearforest stream event. Originally it was intended to defend along the full affected length of the stream. However, following a detail investigation of the stream it emerged that a number of old houses had been constructed over the existing stream and their floor levels were significantly below the proposed defence level. The existing structure over the culverts in some cases was merely domestic timber floor construction. To line these ‘quasi-culverts’ to a standard that would resist the surcharge load would have been very difficult to construct and would have lead to a decrease in the capacity of the channel under the properties and thus may have increased levels upstream. It was therefore decided that a flood relief culvert should be constructed, diverging from Bearforest stream just upstream of these

Spa Glen Discharge (m3s-1)

Blackwater level (mOD)

Water level at entrance to double box culvert (mOD)

Water level at bridge in park (mOD)

Water level down stream of Culvert 1 (mOD)

Qbar (5.9) Bankfull (45.43)

45.63 46.37 46.72

Qbar (5.9) 1 in 100 (47.52)

47.68 47.74 47.74

25 Year (11.9)

Bankfull (45.43)

46.10 46.76 47.01

25 Year (11.9)

1 in 100 (47.52)

48.16 48.29 48.29

100 Year (15.3)

Bankfull (45.43)

46.49 46.91 47.12

100 Year (15.3)

1 in 100 (47.52)

48.58 48.74 48.74

15

properties and outfalling to the Blackwater to the east of the existing channel. This relief culvert would only be used in flood events and would be controlled by a number of penstocks. The existing open channel would retain the flow in normal conditions. Flood defence walls would be required upstream of the point of divergence to defend the north of Linehan’s Lane. Gooldshill Stream Options Pumping The 1 in 25 year flow in Gooldshill Stream is 2.6m3/s (3m3/s including an allowance for climate change) Therefore the option of isolating the stream from the Blackwater during a flood event was considered feasible. An outline design for a pumping station with a capacity of 3m3/s was carried out and was costed. The downside to this option is the need for ongoing maintenance of a very sizeable Storm Water Pumping Station particularly given that it would be the only one used in the Mallow Scheme. Defences The alternative to pumping this flow was to defend against the combined Blackwater/Gooldshill Stream event. The extent of these defences would be upstream of the Ashbrook and Eglantine Housing estates which back onto the east and west bank of the stream respectively. It would also involve upgrading the culvert under the Quartertown Road. This option was also costed and was comparable to the pumped option. The downside to this option was the difficulty in constructing suitable defences along the narrow channel between the two housing estates. It would involve significant disruption to a number of household gardens and would also probably result in the lost of the tree line between the two estates. Defences and Flood relief culvert During detailed design, possible other alternatives to both the above options were reviewed. A route for a possible flood relief culvert has been identified which would avoid having to carry out work in the channel between the two estates. This option is however still only at feasibility stage at the time of writing. Mill Race Options Defences and Flood Relief Channel The Mill Race as the name suggests is a man-made headrace which takes flow from the Clyda to the site of an old Mill which is now no longer in use. The last part of the Mill Race goes through Millbrook housing estate and also flows to the rear of Wilton House. Both Millbrook Estate and Wilton House are susceptible to flooding from the Blackwater. There is a very steep cascade on the current route of the headrace in the vicinity of the old Mill. This route ran as a bypass to the Millwheel, which is now blocked off by a penstock. This cascade would prevent any passage of fish upstream. It was therefore decided to defend these properties from the Blackwater and isolate the Mill Race within the estate during a

Blackwater event by diverting it via a flood relief channel, west of the estate defence at a point where ground levels are above flood defence levels. This channel would be created by widening an already existing overflow channel which discharges to the floodplain. The isolated section of the Mill Race would be controlled by the use of an upstream and downstream penstock structure. Cost Benefit Analysis Costs The costs of the scheme have been based on whole life costings for a 50 year design life, considering capital costs and a maintenance regime. The basic capital costs were estimated from information from Arup’s involvement on the National Capital Cost Database for the Environment Agency of England and Wales, as well as Arup’s local knowledge and the OPW’s records from a long history of flood alleviation projects in Ireland. The basic bill of quantities was augmented by additional costs, generally expressed as a percentage of the bill of quantities to cover the following: • Temporary Works • General items / preliminaries • Overheads • Profit • Design & Site supervision • Archaeological works • Land purchases and compensation • Environmental works Maintenance regimes were worked out for the scheme, including mowing, replacement or refurbishment of components and inspections, including CCTV surveys of drainage. It was assumed that the capital costs were all incurred in the first 2 years of the project. The costs associated with each element of the maintenance regime were allotted against the point in the scheme’s lifetime that they would be expected to occur. Both were then discounted against time to give a Net Present Value. A risk schedule was maintained throughout the scheme, identifying issues that might affect its outcome. Risks that might affect the cost benefit assessment were subjected to a quantitative analysis to provide an upper and lower bound on the potential costs of the scheme. Some of the risks included were • Rates • Inflation • Ground conditions • Services • Condition & protection of existing structures • Access • Adverse weather conditions The risk assessment evaluated the upper bound of the capital costs to be 10% higher than the baseline case.

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Benefits The calculation of the benefits was based on the flood damages avoided by each option. Flood Damages were calculated by the application of the methods of FHRC (2003)6. The damage values suggested by the FHRC were modified for application in Ireland following the methods of Goodbody (2001)7, which use the purchasing price parity and government price indices to convert the underlying costs of FHRC 2003 to euros and update them to the present day. The benefits comprise 3 categories: • Direct Tangible damages (ie damages by the

floodwater to assets) • Indirect tangible damages (eg emergency

services costs; traffic disruption) • Intangible damages (eg. Stress and ill health) The main source of direct tangible damages is floodwater damage to property. Property types were identified in a walkover survey and property thresholds were surveyed. Water levels from the hydraulic model were used to calculate the flood depth above threshold at each property in a given event, from which a flood damage is looked up in a database. Emergency services costs were calculated based on 11% of the direct tangible damages. Traffic disruption was disregarded as the distance between the Town Bridge and Blackrock bridge is small enough that the costs of diversions are not significant compared to the direct damages. Intangible damages were taken as being equal to the tangible damages for residential properties on the advice of the OPW. Fig 11 Damage frequency curve at Mallow.

This process was repeated for a range of return periods and options, and used to build up a flood frequency curve such as the one in Fig. 11. The area under the curve is the average annual flood damage. Notice the influence that the threshold of damages has on the area under the curve; in this case, damages start to be incurred in the 1 in 2 year flood. The annual average damage was then converted to a Net Present value using a discount factor of 5%. The cost benefit ratio is, unsurprisingly, the ratio of the net present costs to the net present damages. A wide range of sensitivity scenarios were applied to the analysis; these covered: • Reduction of the discount factor • Reductions of benefits by 5% or 20%, • Use of the Upper bound and Lower bound cost

estimates, and combinations thereof. The total upper bound present value cost of the scheme was €27.5M. The present value of the benefit was €95.8, giving a scheme cost benefit ratio of approx. 4.5. This overall ratio proved robust against all of the sensitivity scenarios, with the lowest cost benefit ratio achieved being 3.5.

Table 7 – Summary of Scheme Capital Costs (excluding VAT)

Description

Capital Cost

(€ m)

Upper Bound Capital Cost (€m)

Lower Bound Capital Cost (€m)

Mallow North

10.7 11.8 10.7

Mallow West

7.6 8.4 7.5

Mallow South

5.7 6.3 5.6

0.000 0.500

0.2000.100

0.040

0.020

0.010

0.000

0

10

20

30

40

50

60

70

80

0.000 0.100 0.200 0.300 0.400 0.500

Frequency

Dam

age €m

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Implementation of the Schemes Overview As described earlier, following the preparation of a phasing report in 2004 it was decided that the proposed scheme for Mallow would be broken into 3 distinct scheme and these are dealt with separately below:

Fig 12. Mallow North Scheme Works Overview of Mallow North The Mallow North Scheme protects all of the affected areas to the north of the Blackwater, which includes the main street, and the main commercial centre of the town. Mallow North Advance Works Contract An advance phase of the Mallow North scheme was delivered in 2005/06 under a Section 8 Planning. The Office of Public Works funded this work with Cork County Council acting as the Employer. It consisted predominantly of the construction of a new road ramp and flood defence wall at the western end of Park Road together with the replacement of some of the culverts in the Spa. This work was all carried out on lands in public ownership and had an approximate capital value of circa €2.5M

Mallow North Main Contract The main contract for the Mallow North Scheme was let in early 2008 following a EU procurement competition. Works commenced on site in Spring 2008 and are currently on schedule to finish slightly ahead of programme in Summer 2009. This contract has an approximate capital cost of circa €9.3M

The elements of the Mallow North Scheme can be separated into the following categories: Re-grading Works at Mallow Bridge to reduce

flood levels in the town. Blackwater Only Defences (including

Demountable Defences) Combined Blackwater and Spa Glen Defences

and Extension of culvert through Tipp O’ Neill Park.

Combined Blackwater and Hospital Stream Defences (including realignment of a section of Hospital stream)

Foul Drainage Pumping Stations. These are described in more detail below. Re-grading Works at Mallow Bridge Mallow Bridge is a Multi Span Arch Bridge with 4 main arches and 4 small flood relief arches located on the left hand floodplain. The 4 smaller arches form part of the pre-1853 bridge that avoided damage during the flood of that year.

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Recorded level information at Mallow Town Bridge identified that the bridge caused an afflux of ~ 0.45m. Potential improvements to the bridge were constrained by the structural condition of the bridge, services running along the bed of the channel immediately downstream of the bridge and the architectural significance of the structure in terms of local character. These constraints very much reduced the viability of options based on regrading of the river channel bed. Options considered therefore focused on the restoration of flow capacity to the smaller arches, which had become obscured by local high ground levels upstream and downstream of these arches. Re-grading these banks to smooth slopes matching the ground level under the arches will reduce ineffective areas and thus allow more flow to pass, subsequently reducing upstream levels. Hydraulic modelling of this option suggested a significant reduction of peak levels upstream of the Bridge of. A sensitivity analysis was carried out on this model by assuming both effective and ineffective downstream flood levels. Both models gave similar answers. This works produced a reduction in water level as far upstream as the Blackrock Bridge which offset the marginal increases which would otherwise have resulted from construction of the defences, thus ensuring that predicted flood levels post scheme would not be any greater than the existing situation. The velocities through the arches are such that geotextile based scour protection will be required. Some local re-pointing of the Bridge Arches will also be required. Fig 13 – Re-grading Works at Mallow Bridge Blackwater Only Defences (including Demountable Defences) Defences against flooding from the Blackwater consist generally of flood defence walls and a short section of flood defence embankment located in the grounds of Mallow Castle.

The defence walls are located at: • Small carpark to the east of Bridge Street • Rear of the Bridge Street Properties in the

Council Carpark • Around the east and north perimeter of the Lidl

carpark • South West Corner of Tesco Carpark and south

and west of St. Patrick’s Place Whilst it was not cost effective to protect Park Road from large flood events, it was decided to strengthen the existing masonry boundary wall to the Park and fill in any gaps so as to provide protection to Park Road to a standard of approx. 1 in 2 year event. Currently Park road generally floods at least once a year. Generally the flood defence walls were designed to sit on grade with the toe of the foundation on the ‘wet’ side. However, due to ground conditions, the presence of services and geometric constraints, some walls had to be designed with the toe on the ‘dry’ side whilst others were designed using a piled foundation. All defence walls and embankments were checked for seepage and altered to suit. In some instances this involved the construction of a downstand beam, which doubled up as a shear key. In other instances, ‘clay stank’ or sheet pile ‘cut-off’ was used. Immediately to the east of Bridge Street, due to the proximity of the ‘New’ culvert to the existing property, an independent flood defence wall could not be constructed. The solution adopted here was to reconstruct the culvert with a flood defence wall ‘springing’ off one side of the culvert and the culvert design altered to take account of this additional loadcase. Demountable Defences There are a number of locations where permanent defence walls cannot be constructed. The most critical of these is across Bridge Street, which is the main artery through the town. To ensure the continuity of the flood defence at these locations, temporary demountable barrier systems will be used. These demountable barrier systems consist of extruded aluminium posts and backprops, which are bolted on to baseplates cast into the ground and aluminium dam beams, which slot in between these vertical posts. Special rubber seals complete the system. Demountable Barriers are only successful when used in conjunction with a reliable flood warning system and such a system is being developed by OPW and will be rolled out in parallel with the Flood Defence Schemes. The demountable barriers will only be erected when the flood warning system indicates that water levels are likely to approach flood condition and will be removed when levels have receded.

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An operational protocol document has been drafted for use by Cork County Council staff when responding to a flood warning. This outlines the order in which demountable defences have to be erected etc. for varying flood events. This document also deals with the works required to foul pumping stations, penstocks etc. during the flood event. The opes for the demountable defences have been strategically designed so that all locations can use a standardised dam beam length of just under 2 metres. Similarly the threshold levels have been standardised by modifying ground levels locally such that all demountable locations have one of three standard post heights with approximately one metre difference between post heights. This standardisation will help to keep the erection process as streamlined as possible thus reducing any potential for error or delay. Combined Blackwater and Spa Glen Defences (Defences and Extension of culvert through Tipp O’ Neill (Energy Park)) Modelling of the Spa Glen showed that the existing town culverts were not a restriction to the 100 year flow from the Spa Glen. However, high events in the Blackwater were capable of backing up water levels such that they caused flooding in Tipp O’ Neill Park. This could be prevented by extending the culverts further back through Tipp O’ Neill Park, using the gradient of the Spa Glen to contain water levels up to a location where sufficient head could be generated to pass the design flows through the culverts, and be contained without causing flooding or requiring tall structures adjacent to housing. The new culvert extended beyond the end of Spa Terrace on the N72 and upstream of Energy House, where a new flood defence embankment was constructed. The new flood defence embankment was constructed east-west at the northern end of Tipp O’ Neill Park and tied into high ground at either end.

Fig 15 Photomontage of Bridge St. Demountable Barriers in place

Fig 14 Installation of Demountable Barriers This included a local raising of the N72. A large control structure is also being constructed to divert flow from the open channel into the culvert. Flow monitors and level gauges linked via a suitable telemetry system will be used to send a warning to Cork County Council in high Spa Glen events only. The existing stream though the Tipp O’ Neill Park was maintained for its visual amenity, but the creation of penstock structures up and downstream of the Park allowed the stream’s isolation in the instance of high flows on the stream, or high levels in the Blackwater. The benefit of this option is that the views of the Park and Energy House are unaffected. Extending the culvert through the Park does not increase flood levels in the area. This option did rely on being able to pass higher flows through the existing and proposed culverts, with increased pressures and velocities, so the structural capacity of the culverts had to be verified, and where necessary, improved.

20

Fig 16 Aerial Photograph of Energy Park Culvert Construction

21

The following summarises the work required to the relevant culverts: The New double box culvert through Tipp O’

Neill Park was explicitly designed for this type of load case.

One side of the existing double box culvert

between Tipp O’ Neill Park and the clockhouse was also replaced with a larger culvert (3500mm x 1800mm) for capacity reasons.

The ‘New’ culvert which was constructed by

Cork County Council in 2000 was also deemed satisfactory subject to appropriate sealing of its precast joint.

The existing masonry ‘Castlelands’ culvert was

replaced along most of its length by a new precast concrete culvert save for a section at the castle end, where there was a sufficient depth of fill over the culvert to withstand the upward pressure.

The existing masonry ‘Clockhouse’ culvert is

being isolated from Blackwater flood events by the use of penstocks and flap valves.

As discussed earlier, the issue of a lack of capacity of the existing N72 culverts upstream of Tipp O’ Neill Park resulting in floodwaters being conveyed along the N72 also had to be addressed. The option of replacing all of these culverts with culverts of adequate capacity was reviewed but found to be extremely costly. The option ultimately decided upon was to strategically re-grade sections of the N72 to allow floodwaters travelling down the N72 to discharge back into the stream thus preventing such floodwaters reaching the town Combined Blackwater and Hospital Stream Defences The junction of Park Road and West End is low lying and is susceptible to flooding from the Blackwater and/or the Hospital Stream. As mentioned earlier, it was not possible to isolate the Hospital stream from the Blackwater and therefore defences were required to defend against the combined event. South of West End, these defences consisted of: • Defence walls to the east of Hospital Stream

(south and west of Westbrook Court) • Defence Wall to the west of Hospital Stream • New Road Ramp across Park Road tying into

high ground to the west of Park Road. Replacement of the existing double masonry arch bridge at West End with a new RC culvert was also required.

As described earlier, Hospital stream needed to be diverted around existing buildings once it entered the Dairygold site. The southern section of this diversion is a new 5200mm x 2300mm box culvert affectively replacing the existing culvert at the southern end of the site. The northern part of the diversion consists of a new open channel, which ties in to the existing stream to the north of the Dairygold site. A ‘sweetener’ flow will be maintained through the existing culvert in Dairygold but this will be isolated during a flood event by penstocks at both the upstream and downstream ends.

Foul Drainage Pumping Stations A number of foul or combined sewers traverse the proposed defence walls. As the drainage system in Mallow is quite old, it is possible that floodwaters could breach the sewers on the ‘wet’ side of the defence and use the sewers as a conduit to convey floodwaters to the ‘dry’ side. To overcome this problem, foul pumping stations have been constructed on these sewers. In normal conditions, the sewers will flow by gravity as normal. During a flood event, a flap valve in a specially constructed chamber on the wet side will prevent the floodwaters from propagating upstream. This will however prevent the foul/combined sewage from discharge resulting in a backing up of the system. To negate this, when the system backs up to a non critical level, the flow will spill over a side weir in a specially constructed overflow chamber on the dry side, where it will flow by gravity to a pumping station which will in turn pump the flow against the head of the flood into the downstream discharge manhole. A penstock will also be provided in the overflow manhole, which is to be closed in the event of a flood. This penstock acts as a backup in the event of failure of the flapvalve. To minimise the number of pump stations, a number of sewers were connected on the dry side where levels allowed. The design of the combined pumping stations allows for pumping the 1 in 30 year flow. Fig 17 Location of Mallow North Foul Drainage Pumping Stations

22

Mallow South Scheme (Currently at detailed design stage) Overview The extent of these works stretches from approx 50m west of Mallow Bridge to the intersection of Mill Street and Linehans Lane. The main areas protected in this phase include properties on Ballydaheen Road, Mill Street and Linehan’s Lane. The works can be split into: • Blackwater Only Defences • Combined Blackwater and Bearforest Stream

Defences including flood relief culvert

Fig 18 – Mallow South Proposed Works Blackwater Only Defences The northern end of Ballydaheen Road at its junction with Mill Street is susceptible to flooding as is this section of Mill Street. However, Mill Street rises up quite steeply either side of Mallow Bridge. Defences are required therefore for a short length both to the east and west of the Bridge. In both locations these defences will take the form of flood defence walls. The exact nature of these walls are currently being reviewed. A short section of demountable defence will be required to the east of Mallow Bridge at the location of the existing boat ramp in order to maintain a suitable ramp for launching. A short section of demountable defence may also be required to the west of Mallow Bridge

to facilitate a possible future boardwalk on the upstream side of the Bridge. A raised footpath will be constructed on the east side of the bridge so as to maintain views of both Mallow Castle and the river in this location. It is also intended to strengthen part of the existing bridge parapet to be used as a flood defence. The top level of the parapet at the southern end of the bridge is above flood level and remains so until past the point where the road level on the bridge also exceeds the flood defence level.

Combined Blackwater and Bearforest Stream Defences including flood relief culvert Defences and a flood relief culvert will be required to defend against the combined Blackwater/ Bearforest stream event. These works are as described earlier in this paper. Fig 19 Mallow South Existing Bridge Parapet

23

Mallow West Scheme (Currently at detailed design stage) Fig. 20 Mallow West Proposed Works Overview The extent of the works stretches from Blackrock Bridge (N20 Bridge and Railway Bridge) to the west of Millbrook housing estate. The main areas protected in this phase include properties in Ashbrook Estate, Eglantine Estate, Millbrook Estate, Wilton House and Quartertown Road. Blackwater Only Defences The Blackwater Defences in this location will consist of a combination of defence walls and embankments as per Fig 20 above. All of these defences are located on the riverside of the properties to the north of Quartertown Road. Embankments are currently proposed to be constructed to the north of Millbrook Estate and Wilton and most likely also to the west of Millbrook Estate although the later may take the form of a wall subject to further investigation. A wall is also proposed to the north and east of Spring Cottage where space restrictions do not allow the construction of an embankment. These defences will be augmented by the Mill Race and Gooldshill Stream solutions as described earlier in the paper and as summarised in the following paragraphs. Blackwater and Gooldshill Stream Defences The options for protecting against the combined Blackwater/ Gooldshill Stream event are still under review and will be a combination of defences and/or a flood relief culvert as described earlier in this paper.

Blackwater and Mill Race Defences including Flood Relief Channel Defences and a flood relief channel will be required to defend against the combined Blackwater/Mill Race event. These works are as described earlier in this paper. Conclusion

The Mallow North Flood Relief Scheme is currently under construction site and due to be completed by June 2009. The Mallow South and West Schemes are currently in detailed design and it is hoped that they will go to site in late 2009 and be completed by mid 2011. The design of the Schemes brought together a number of specialist engineering disciplines within Arup who were ably assisted by OPW, Cork County Council and Mallow Town Council. When completed, the Schemes, in association with an automated early warning system, will protect affected properties in Mallow against a 1 in 100 year Blackwater flood, thus transforming the lives of the many householders and businesses that currently live with the threat of frequent flooding and the associated damage caused. This Scheme is therefore another great example of the delivery of a sustainable engineering solution working in harmony with nature for the benefit of society.

24

Acknowledgements The authors wish to thank the Office of Public Works for their kind permission to publish this paper. In particular we wish to thank their representatives: Michael Collins, Cian O’ Domhnaill, Ezra MacManamon, Joe Pat O’ Donnell, John Kelly and Yvonne Jackson for the direction given throughout the project. We also wish to thank the other members of the Mallow Flooding Steering Group, Cork County Council and Mallow Town Council, most notably Pat Walsh, Frank Cronin, Roy Cunningham, Keith Jones, and Brian Quinn for their input, support and assistance throughout the project. A special word of thanks also to Mr. Edmond Flynn, former County Engineer, for his energy and commitment to the project. We also wish to acknowledge the contribution of the many Arup employees who have been involved at various stages of the project over the last seven years. In particular, we wish to thank Mr. Seamus Mulherin, Project Director for the wealth of background knowledge and expertise, which he brought to the project. Special thanks also to Mr. Michael Wilton for his leadership particularly at the early stages of the project. We also wish to acknowledge those people who have historically examined the issue of flooding in Mallow and whose work was an invaluable source of information for the project. These include Mr. Brendan Devlin, former County Engineer and a number of academics at University College Cork. We also acknowledge the many contributions received from local residents of Mallow who provided invaluable anecdotal information regarding historic flooding in Mallow. Finally, we wish to thank our colleagues in both McGinty & O’ Shea Ltd. and Bam Civil Ltd. without whose help the implementation of the project to date could not have been achieved in such a successful manner. This paper is dedicated to the memory of Mr. Jack O’ Keeffe, formally of Cork County Council who sadly passed away in 2007. Jack was an employee of Cork County Council for approx. 30 years and was a native of Mallow, living most of his life in lower Bridge Street in an area regularly affected by flooding. Jack sat on the Steering Group and the local knowledge he brought to the table during the early design stages of the project was invaluable.

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Assessment of Flood Flows on the Munster Blackwater River: A summary of techniques used to estimate flood flows at Mallow,” Paper to the Irish National Hydrology Seminar.

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7. Goodbody Economic Consultants (2001) “A review of cost benefit procedures for flood defence schemes,”

8. Arup Consulting Engineers, (2003) Final Engineering Report, Office of Public Works

9. Arup Consulting Engineers, (2003) Hydrology and Hydraulic Modelling Report, Office of Public Works

All relevant drgs contained in this paper are reproduced under Ordnance Survey Ireland License No. EN 0002809 Contact Details Mr. Ken Leahy Arup Consulting Engineers 15 Oliver Plunkett Street, Cork +353 21 4277670 Ken.Leahy@arup.com Mr. Luke Ballantyne Arup Consulting Engineers 50 Ringsend Road, Dublin 4 +353 1 233 4455 Luke.Ballantyne@arup.com Useful Websites www.arup.ie www.opw.ie www.flooding.ie www.floodmaps.ie www.corkcoco.ie