Shoreline variability in the vicinity of the Great …...Figure 1.1: Shoreline trends along the Norfolk – Suffolk coastline for 1991 – 2007 (adapted from Environment Agency Shoreline

Cefas contract report 00AB2D

Shoreline variability in the vicinity of the

Great Yarmouth Outer Harbour

Authors: Tony Dolphin, Jon Rees

Issue date: 08 July 2011

Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page i

Cefas Document Control

Title:

Submitted to: Marine Management Organisation

Date submitted: 08 July, 2011

Project Manager: Jon Rees

Report compiled by: Dr Tony Dolphin

Quality control by: Dr John Bacon

Approved by & date: Dave Carlin

Version: 1.0

Version Control History

Author Date Comment Version

Tony Dolphin 06 July, 2011 Draft for review 0.2

Tony Dolphin 08 July, 2011 Review and QA comments

incorporated

1.0

Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page ii

Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page iii

Shoreline variability in the vicinity of the

Great Yarmouth Outer Harbour

Authors:

Tony Dolphin, Jon Rees

Issue date: 8 July, 2011

Head office

Centre for Environment, Fisheries & Aquaculture Science

Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK

Tel +44 (0) 1502 56 2244 Fax +44 (0) 1502 51 3865

www.cefas.co.uk

Cefas is an executive agency of Defra

Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page iv

Executive Summary

1.1. The Great Yarmouth Outer Harbour (GYOH) was constructed in 2007/8. This short report

examines readily available data on the shorelines neighbouring the GYOH to give an initial

assessment of their natural variability and their response to the development to date.

1.2. The GYOH lies toward the centre of the Caister Ness to Lowestoft Ness coastal cell. Beaches in

this area are sheltered from North Sea waves by a 30‐km long sand bank complex known as

the Great Yarmouth Banks. Despite the reduction in inshore wave energy caused by wave

breaking and bottom friction on the banks, the shorelines there exhibit greater variability

(higher rates of change and high spatial variability in rates of change) than adjacent beaches

which are not in the lee of coastal banks (e.g., north Norfolk).

1.3. Shoreline change was investigated by analysing beach positions extracted from the

Environment Agency (EA) beach profiles and aerial photographs spanning almost 20 years. The

sea floor bathymetry surrounding the GYOH has also been analysed using soundings gathered

as part of the GYOH monitoring program. These bathymetry data do not extend out to the

Great Yarmouth Banks. At the time of writing no data on changes in the banks (which

influences inshore wave climate and the direction and rate of longshore sediment transport)

had been received.

1.4. Beaches to the north and south of the mouth of the Yare and the GYOH show the same spatial

pattern; in each area beaches accrete in the north and erode in the south. This pattern is a

reasonably persistent feature of the c. 20 year record and indicates shoreline rotation.

1.5. It was considered by HR Wallingford (2010) that if the GYOH had a detectable impact on

sediment transport and shoreline position, it would manifest as a blockage to the net

southerly directed longshore sediment transport. In this simplistic scenario the blockage

would cause a build‐up of sediment to the north of the GYOH and, as a result of reduced

sediment supply, erosion to the south. At the time of writing there was no evidence showing

build‐up (accretion) to the north and erosion to the south. Instead, our analysis shows that the

opposite occurs with accretion against the southern harbour wall at Gorleston and slight

erosion immediately north of the GYOH. Potential causes include natural coastline rotation

(presumably driven by inshore wave climate), an assumed recent phase of northerly longshore

Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page v

sediment transport acting counter to the long‐term transport direction (investigation

required) and complex circulation patterns including eddies.

1.6. On the assumption that the GYOH could result in detectable impacts other than that proposed

by HR Wallingford (2010), selected shorelines to the north and south were analysed for

changes in position before and after the development. These results show that:

Persistent erosion has been occurring in the Hopton area for the last 20 years and possibly

for longer. There is no evidence at the time of writing this report that the shoreline

behaviour has changed following construction of the GYOH.

The shoreline at the Gorleston Golf Course experiences no persistent trend and had short

phases (2‐5 years) of erosion and accretion. On average the beach is slowly accreting. Recent

phases of erosion are well within the natural envelope of variability on this beach, have

occurred previously and are not likely to be impacts of the GYOH construction. Future

analysis (in 2 – 3 years time) will help to clarify the situation.

At Gorleston, there is a strong trend of shoreline advance averaging 4.4 – 4.9 m/yr over the

20 year period. Following construction of the GYOH there was step‐change in accretion rate

from 3.4 – 15.1 m/yr . Although the timing of the step‐change in accretion rate coincides

with the GYOH development, cause and effect must be determined before impact can be

attributed.

At one kilometre north of the GYOH there are no persistent erosion or accretion trends, and

no strong evidence of the GYOH trapping longshore drift sediments moving south. The

pre/post‐construction rate shows a very minor change from 0.78 m/yr to 1.13 m/yr.

1.7. Changes in the elevation of the sea floor bathymetry following the GYOH construction are

localised with no detectable impacts offshore. Following construction the sea floor accreted to

the north, indicating that there has been a minor blockage to longshore sediment transport

for around 500 m. The build up of sediment is slowing with time suggesting bypassing

continues. A similar deposit is found at Gorleston. Further analysis of the bathymetry time‐

series is required to distinguish if these deposits occur at different times (indicating longshore

transport blockages) or at the same time (indicating cross‐shore transport with no inference

for GYOH impacts).

Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page vi

Table of contents

1 Introduction ................................................................................................................................... 1

2 Data and methods ......................................................................................................................... 2

2.1 Data ......................................................................................................................................... 2

2.2 Methods .................................................................................................................................. 2

2.2.1 Shoreline position from beach profiles – Excursion Distance Analysis (EDA) ................. 2

2.2.2 Shoreline position from aerial photographs ................................................................... 4

2.2.3 Shoreline position statistics from EDA and aerial photographs ..................................... 5

2.2.4 GYOH Bathymetry ........................................................................................................... 7

2.2.5 MCA/UKHO bathymetry .................................................................................................. 8

3 Results and discussion .............................................................................................................. 10

3.1 Overview ............................................................................................................................... 10

3.2 Shoreline behaviour .............................................................................................................. 11

3.2.1 Northern beaches: Great Yarmouth to Caister ............................................................. 11

3.2.2 Southern beaches: Gorleston ‐ Corton ......................................................................... 13

3.3 Pre and post GYOH construction shoreline behaviour ......................................................... 16

3.4 Bathymetry ........................................................................................................................... 21

4 Conclusions and recommendations ......................................................................................... 23

4.1 Summary of conclusions ....................................................................................................... 23

4.2 Conclusions ........................................................................................................................... 23

4.3 Recommendations for further investigation ........................................................................ 25

5 References ................................................................................................................................... 27

6 Acknowledgements .................................................................................................................... 28

Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 1 of 29

1 Introduction

This report has been commissioned by the Marine Management Organisation. Its purpose is to

provide an independent assessment of the shoreline variability in the Great Yarmouth area and to

investigate the detectable impacts of the Great Yarmouth Outer Harbour (GYOH) which was

constructed in 2007/8. The expected spatial and temporal scales over which GYOH impacts to the

coastal system occur are not known. This report has been produced in a very short time scale and

therefore some analyses/results could not be included. Therefore this report should be considered

as an interim document – see Section 4.3 for recommendations on future work.

The study area is the coastal cell bounded by the sedimentary headlands (locally called nesses) in the

north at Caister Ness and the south at Lowestoft Ness (Figure 1.2). The shorelines in this coastal cell

are sheltered by the Great Yarmouth Banks, a complex of sand banks that are 1 – 9 km offshore,

shelter 30 km of coastline and run approximately parallel to the coast. In the lee of the Great

Yarmouth banks, despite having a reduced wave‐energy climate (as a result of bottom‐friction and

wave breaking on bank crests) the shorelines have greater rates of change than beaches not

protected by coastal banks (Figure 1.1). Additionally the demarcation between areas of eroding and

accreting coasts are sharp and long‐term records show that the coastal sections analysed to date

have highly variable erosion/accretion patterns through space and time (unpublished research by

the authors).

Figure 1.1: Shoreline trends along the Norfolk – Suffolk coastline for 1991 – 2007 (adapted from

Environment Agency Shoreline Management Group, 2007).


2 Data and methods

2.1 Data

Several data sources were investigated and requested (see Table 3.1).

Given the short lead time to gather data and produce this report, not

all data requested had arrived at the time of writing. The data types

sought were chosen primarily to quantify variability in shorelines. The

shoreline data acquired represent the medium‐term (years – decades)

beach behaviour. As local beach response is also controlled by the

wave climate and position, and by the elevation and shape of sand

banks, we also considered bathymetric and wave data. Bathymetric

data covering the sand banks had not arrived at the time of writing.

There are no long‐term wave records inshore of the bank, but wave

records are available from the West Gabbard wavebuoy in the

southern North Sea. An assessment of inshore wave climate and

longshore drift rates, which can control the erosion/accretion

behaviour of a beach, was not undertaken as this requires numerical

modelling which was not possible in the reporting time scale. The

required modelling could be undertaken at Cefas or one of a number

of other organisations that have wave/tide/sediment transport models

in the area (e.g., University of East Anglia, Tyndall Centre).

2.2 Methods

This sections briefly describes the methods used to examine variability

in the shoreline and nearshore environments.

2.2.1 Shoreline position from beach profiles – Excursion Distance

Analysis (EDA)

Excursion Distance Analysis (EDA) is a useful technique for extracting

shoreline position data, at a given elevation, from beach profiles. The

essence of EDA is to measure the distance from the beach profile

survey marker out to the point where the beach falls to a specified

elevation such as the Mean High Water Springs (MHWS) or Mean Sea

Figure 1.2: Location map


Level (MSL) datums. The excursion distances were calculated by the Environment Agency using the

SANDS (Shoreline And Nearshore Data System) software developed by Halcrow Group PLC. Excursion

distances were appended to the results of previous analyses (1991 ‐ 2007; Environment Agency

Shoreline Management Group, 2007) to give a record of c. 15 years prior to GYOH construction and

c. 3 years post construction (2008 ‐ 2010/11). The following elevation datums were used in the

analysis:

HAT (1.39 m ODN)

MHWS (1.02 m ODN)

MHWN (0.6 m ODN)

MSL (0.1 m ODN)

MLWN (‐0.37 m ODN)

MLWS (‐0.79 m ODN)

LAT (‐1.48 m ODN)

where ODN is Ordnance Datum Newlyn (approximately mean sea level).

Other outputs of the SANDS analysis, such as beach steepness, are not used in this report but could

be considered if a more detailed account is required.

Data type Data source Status

Beach profiles Environment Agency Received

Beach profiles Environment Agency on behalf

of East Port

Received

Aerial photography Environment Agency Received

Sea floor bathymetry HR Wallingford on behalf of

East Port

Received

LIDAR Environment Agency Requested. Dispatched and

awaiting delivery.

Recommended use in next

version of this report

Sea floor bathymetry (including

coastal sandbanks)

Maritime and Coastguard

Agency/ UK Hydrographic

Office

Requested. Awaiting response.

Recommended use in next

version of this report

Table 2.1: Data requested and status at time of writing this report.


Figure 2.1: Beach profile example indicating the shoreline positions (125 m at time A and 84 m at time B)

calculated using EDA. The shoreline change is a retreat in this example as indicated by the black arrows.

Adapted from Environment Agency Shoreline Management Group (2007).

2.2.2 Shoreline position from aerial photographs

The Environment Agency commission annual (late summer) low‐tide aerial photography for

monitoring the coastline and the state of coastal defences. From 2005 the annual aerial photography

was commissioned as an ortho‐rectified product (i.e., referenced to a geographic coordinate

system). The hardcopy plates for selected years prior to 2005 (1992, 1994, 1997, 2001) were

separately scanned and geo‐referenced by the Environment Agency and are also used here.

The aerial photographs from the study area were imported into Arc‐GIS and the low‐tide shoreline

from each image was digitised. An offset correction for variability in tidal level, such as that used by

Dolphin et al. (submitted), has not been applied here due to the short time‐scale for reporting. In

most cases the aerial photo data are in agreement with beach profiles, at least in a relative sense,

however an offset correction would be needed if the results were to be used in isolation.

The analysis of shoreline variability from low‐tide shorelines follows the method of Theiler et al.

(2009). The underlying dataset is a time‐series of shoreline positions (distances) measured along a

series of transects out to the intersection points of each transect and shoreline (Figure 2.2). In this

analysis the low‐tide shoreline locations were measured at transects spaced every 50 m along the

Shoreline retreat (at MHW)

Profile at time A

Profile at time B


18 km length of coastline between Caister Ness to Lowestoft Ness. The time‐series of shoreline

positions at each of the 367 transects was then used to statistically characterise the behaviour of the

coastline. The statistics were mapped so that the spatial variability in shorelines response could be

examined. As the transects are spaced every 50 m, the primary utility of the aerial photograph

analysis is its much higher spatial resolution (compared to the EDA on beach profiles).

Figure 2.2: Example map of shorelines (coloured lines) and their intersection (measurement point) with each

transect. The measurement distances to each intersection are then statistically analysed and mapped to

characterise the spatial variability in shoreline behaviour. Adapted from: Theiler et al. (2009).

2.2.3 Shoreline position statistics from EDA and aerial photographs

To investigate the spatial variability in beach behaviour four statistics were calculated to describe

the shoreline behaviour as determined from shoreline positions derived from EDA on beach profiles

and at the transects used in the aerial photograph analysis.

LRR is the Linear Regression Rate‐of‐change statistic (negative values indicate erosion)

LRR R2 is the R2‐value associated with the LRR

EPR is the End Point Rate‐of‐change (negative values indicate erosion)

SCE is the Shoreline Change Envelope (this is a distance not a rate)

The LRR was determined by fitting a least squares regression to all of the shoreline positions

(determined at beach profiles for EDA and at each transect for aerial photos). An example of the LRR

calculation is shown in Figure 2.3 (top panel). The LRR is the slope of the regression line determined

for shoreline positions on each transect. LRR uses all of the shoreline positions to estimate the rate

of change. It tends to under‐estimate the rate of change in comparison to other statistics such as the

EPR. As the R2‐value gives an indication of the closeness of fit to the curve, it can also be used to

determine if the erosion/accretion is a persistent trend (high R2).


See next page for figure caption


Figure 2.3: Graphical representations of the Linear Regression Rate, R2 (top panel), End Point Rate (middle

panel) and Shoreline Change Envelope (bottom panel) statistics. Source: Theiler et al., 2009.

The End Point Rate‐of change (EPR) was determined by dividing the distance of shoreline movement

by the time‐interval between the oldest and youngest (end points) shorelines. A disadvantage of the

EPR is that it ignores all intervening shoreline measurements and therefore can be unrepresentative

of the general shoreline change rate if either or both of the end points are anomalies.

The Shoreline Change Envelope (SCE) is a distance, not a rate. It is the distance between the closest

and farthest shorelines along each transect and indicates, as a range, the distance extents over

which the beach varies.

There is an unaccounted for bias in the aerial photograph analysis as conducted here because of the

time‐distribution of the data (photographs). That is, there are more data in the last 6 years of the

record (annual) than in the previous 14 years. The bias affects the LRR and SCE statistics described

below (Section 2.2.3), but not the EPR statistic. This bias can be easily removed by geo‐referencing

and digitising the other hardcopy plates so that the analysis can utilise the full aerial photograph

dataset. Due to the short reporting time scale the remaining aerials have not been prepared or

included in this analysis, however this could be done as required in any future edition of this report.

The bias described is not present in the EDA of beach profile data as the measurements are generally

regular, with the exception of EDA conducted at lower elevation datums (e.g., LAT). As a result of

missing data at lower elevations, EDA is presented for MHWS, MSL and MLWN, where there are few

data gaps.

2.2.4 GYOH Bathymetry

The GYOH bathymetric survey datasets were supplied by HR Wallingford as XYZ files. These data

have not been presented elsewhere in detail, as far as the authors are aware. Surveys were

conducted using a single‐beam echo‐sounder. The survey extents are highly variable and do not

consistently follow the same lines as recommended for bathymetric monitoring associated with

marine licences. These data do not appear to have undergone a thorough quality analysis; for

example, Figure 2.4 shows an artefact in the data which is probably due to squat. Squat can occur at

the beginning of a transect line (see Figure 2.4, black rectangle, for example) when the vessel (and

sounder) elevation change as a result of a change in vessel speed. The data are used here


nonetheless, but they should be subjected to an appropriate quality analysis before being used in

any further reporting.

Due to the limited reporting time only a selection of the bathymetric data provided were analysed.

These were the surveys from 2006, 2008 and 2009, representing pre and post construction surveys.

Bathymetric surfaces were produced as TINs (Triangulated Irregular Networks) and the 2008 and

2009 TINs were subtracted from the 2006 TIN to give a residual map, also known as an

erosion/accretion map. This analysis was conducted in Arc‐GIS using the 3D Analyst tool kit.

Figure 2.4: 2007 bathymetry showing an artefact (marked by a black rectangle) in the data probably due to

vessel squat. White lines mark the survey tracks.

2.2.5 MCA/UKHO bathymetry

Requests were made for the MCA and UKHO bathymetric datasets because the GYHO bathymetric

datasets are spatially limiting and do not include the margins of the adjacent sand banks. Without

some understanding of the variability in the local sandbanks the impacts of the GYOH may be

masked by changes in the banks, inshore wave climate and longshore sediment transport. Equally as

likely, changes in the bank system may alter medium‐term (years – decades) longshore transport

patterns that could be falsely attributed to the GYOH. The current monitoring program is weak in


this regard and provides insufficient evidence for identifying GYOH impacts on the surrounding

environment. If a precautionary stance were taken it is possible that changes in shoreline behaviour

could be attributed to the GYOH. Use of existing data from the MCA/UKHO is useful in this regard,

although the extents are very much linked to channels and may not always span the relevant areas.

To address these deficiencies an amended monitoring program that includes sections of the

adjacent sand banks is needed.

The full MCA and UKHO dataset has been requested under the MEMORANDUM OF

UNDERSTANDING ON THE EXCHANGE OF MARINE SURVEY DATA AND PLANNING OF FUTURE

SURVEYS that exists between the government organisations including Cefas, DEFRA, JNCC, MCA,

Natural England, British Geological Survey and the UK Hydrographic Office. These data are yet to be

delivered and therefore are not included in this version of the report.


3 Results and discussion

3.1 Overview

The general pattern of shoreline behaviour in the study area over the past 18 – 20 years can be

divided into two zones that exhibit similar shoreline trends (see Figure 3.1 – 3.3).

Northern beaches. North of the GYOH shorelines show a medium‐term accretion trend

(Wellington and Brittania Piers). To the south and at the location of the GYOH, shorelines

are stable or eroding. EA beach profiling ceased at the GYOH area in 2007 prior to

construction.

Southern beaches (Gorleston – Corton). Shorelines in the Gorleston area (Gorleston Golf

course to Great Yarmouth) are accreting in the medium term at a rate that increases with

distance to the north. To the south, shorelines are eroding in the medium term. At Corton

(south of Hopton), the beach is depleted with no intertidal‐elevation sediments seaward of

the rock revetments. As a result the shoreline there cannot presently retreat further.

Comparison of the results for EDA at different levels generally produces the same patterns (see

Figure 3.2 and Figure 3.3). The same spatial patterns are also evident when comparing the EDA

beach profile results with the low‐tide aerial photograph analysis (see Figure 3.1 and Figure 3.2). This

consistency adds confidence to the results and indicates that the same broad patterns of shoreline

change are evident regardless of the intertidal elevation used.

The following sections describe the results in the two zones (Section 3.2), followed by a comparison

of shoreline response and bathymetry before and after the GYOH construction (Sections 3.3 and 3.4

respectively).

Shorelines can also be generated from historical maps to identify the longer‐term beach behaviour

(including any long term cycles) and give context to the medium term patterns shown here.

However, as historical maps are not readily available, the longer‐term analysis has not been

conducted and any conclusions drawn in this report are valid only in the c. 20 year period (1991 –

2011). Long‐term cycles of erosion and accretion (several decades) may occur on some beaches in

connection with changes in the coastal sand banks (e.g., Park and Vincent, 2007 and Dolphin et al.,

2007), for example, but cannot be registered in this analysis.


3.2 Shoreline behaviour

3.2.1 Northern beaches: Great Yarmouth to Caister

The LRR, R2 and SCE are presented in Figure 3.1, and, along with Figure 3.2 and Figure 3.3 (red

arrows), highlight the similar spatial trends in the two zones. North of the GYOH the trend is beach

advance at a rate of 2.3 – 3.7 m/yr. The Shoreline Change Envelope is wide with the beach position

varying between 46 m and 68 m during the 1991 – 2010 period. Beaches around the Wellington and

Britannia Piers have a steady accretion rate as seen in the dark blue LRR and R2 areas on Figure 3.1

(marked by a ). The SCE is also high in this area (yellow – orange) as a result of persistent advance.

Toward the south the accretion rate decreases with the area just to the north of the GYOH exhibiting

persistent erosion (negative LRR and high R2; Figure 3.1 ). The closest beach profile to this area

(N4A5, South Beach, Figure 3.3) lies just to the north () and experiences low accretion rates (< 0.8

m/yr; Figure 3.2) and has a low R2 indicating that there is no erosion/accretion trend there. The

southward accretion to erosion trend is well‐illustrated in Figure 3.3 (LRR and EPR).

The GYOH is marked by a pink box (in all of the shoreline change figures) indicating its location and

the sections of the beach that were profiled up until construction began in 2007. These profiles are

no longer monitored but were eroding at ‐1.3 – ‐3.8 m/yr (Figure 3.2). Note that in Figure 3.1 the

breakwater walls were digitised and hence the aerial photo analysis indicates ‘shoreline’ advance.

This section of Figure 3.1 should be ignored when considering sandy shoreline change.


Figure 3.1: Shoreline change statistics based on the Environment Agency low‐tide aerial photographs. The

pink boxes mark transects N4A6 and N4A7 whose shoreline position is advanced (high accretion rates) due

to the construction of the Great Yarmouth Outer Harbour in 2007. Circled numbers are described in the text.

SCE LRR R^2


3.2.2 Southern beaches: Gorleston ‐ Corton

The shorelines from Gorleston to Corton follow a broad pattern of accretion in the north and erosion

in the south. The pivot point between the eroding and accreting coasts is just north of the Gorleston

Golf Course in the aerial photograph analysis (in Figure 3.1; ), however the EDA of beach profiles

suggests the pivot point is further south (in Figure 3.1; ), just to the north of Hopton where the

rate‐of‐change becomes negative (Figure 3.2, LRR = ‐0.4 m/yr at MSL). Although the spatial

resolution is higher for the photo‐derived shorelines, the absence of a correction for tidal level and

the bias due to missing data (Section 2.2.3) may explain the discrepancy with the beach profile EDA.

In the absence of a refined analysis of the aerial photographs (Section 4.3), the pivot point between

accretion in the north and erosion in the south is interpreted to be just north of Hopton at EA

transect SWG4 (Figure 3.3). The alongshore trend of accretion in the north and erosion in the south

is similar to the northern beaches, suggesting a medium‐term shoreline rotation (Figure 3.3).

The accretion rate at Gorleston is around 2.5 m/yr (aerial photographs) to just under 5 m/yr (EDA).

The shape of the shoreline and the build up of sediment against the harbour wall indicates trapping

of longshore drift sediments. The relatively high R2‐values show a reasonably consistent trend of

accretion which, in the light of net annual southerly longshore drift, suggests that sediments are

trapped during opposing northerly transport events and/or there is a circulation mechanism that

feeds sediments from the north into the Gorleston area where they are trapped. One possible

mechanism is a persistent eddy formed in the lee of the protruding harbour walls on each flood tide

delivering bypass sediments to the Gorleston area. Field data and/or numerical modelling are

required to test this mechanism.

Hopton and south of Hopton

The highest erosion rates observed (other than pre‐2007 at the GYOH location) are at south end of

Hopton (EA profile SWF1; Figure 3.3) where the shoreline is retreating at a rate of ‐1.7 m, ‐1.7 m and

‐1.9 m (MHWS, MSL and MLWN). The aerial photo evidence is in agreement, showing the highest

rate of retreat is just south of Hopton, between Hopton and the Broadland Sands Holiday Park (,

Figure 3.1 and Figure 3.2), where there is a band of relatively high retreat rates (‐1.7 – ‐2.7 m/yr).

The high R2 values in this area suggest that the erosion has been persistent over the past c. 20 years.


Figure 3.2: Shoreline change statistics based on the Environment Agency 1‐km series beach profiles. LRR, EPR and SCE are based on a time‐series of shoreline positions at the MHWS, MSL and MLWN elevations conducted using SANDS. The pink boxes

mark transects N4A6 and N4A7, which were not monitored beyond 2007 due to the construction of the Great Yarmouth Outer Harbour.


Figure 3.3: SCE, LRR and EPR bar charts with EA transect labels and location descriptions. The pink box marks the two profiles that were discontinued in 2007 prior to the GYOH construction.


Figure 3.4: Time‐series of shoreline position at the EA beach profile transect SWF2.

South of Hopton, in the Corton area, the beaches are severely depleted and in the surveys prior to

1994 and in 2004 – 2010 there was no beach above the MSL level. As a result the sea is acting

directly on the coastal defences (rock revetment) and the shoreline retreat rates are low, as

expected. It is important to note that the ‘shorelines’ there are hard defences and not beaches. For

example, the EPR at SWF2 is 0 m (Figure 3.2 and Figure 3.3) as highlighted in the beach profiles EDA

results (Figure 3.4).

3.3 Pre and post GYOH construction shoreline behaviour

Four key beach profiles were selected to examine the shoreline response for the periods before and

after the GYOH construction. A more detailed analysis could be conducted by considering all beach

profiles and by investigating the time‐series data from aerial photographs (see Recommendations,

Section 4.3). Pre‐construction is taken to be before 2008 and post‐construction after 2008. The

January 2008 data were included as post‐construction data, despite the GYOH being under

construction at that time, because the development is likely to have already been influencing

sediment transport patterns and because it increases the number of samples used to calculate post‐

construction statistics. Experimental exclusion of the January 2008 data had negligible effect on the


resultant statistics. It is important to note that the post‐construction data cover a significantly

shorter time‐period and it is likely that the wider system has not yet fully adjusted to the presence of

the GYOH. Of particular relevance is the variability in net longshore sediment transport and its

interaction with the GYOH. For example, if the period since GYOH construction has a net longshore

transport to the north we would expect to observe a different shoreline response to that from a

phase of transport to the south.

Table 2.1 presents the pre and post‐construction LRR rate of change statistics based on EDA at the

MSL elevation. Only MSL statistics are presented in this section. It is important to bear in mind that

the post‐construction data are based on a small number of samples. Figure 3.5 and Figure 3.6

present the EDA results at all seven levels and give detail to the shoreline position through time.

Note that the y‐axes do not have the same scale.

Profile N4A5 is the first EA 1‐km series profile to the north of the GYOH. Its importance is as an

indicator of build up of longshore drift sediments during phases of net southerly sediment transport.

HR Wallingford (2010) believes that the only detectable indicator that the GYOH is having an impact

on sediment transport is during phases of net southerly longshore transport in which there is a build

up of sediment north of the GYOH and erosion on the southern beaches.

Profile N4A5 does not have a strong trend of erosion or accretion. Shorelines accreted in 1992 –

1997 and 2001 – 2004, eroded in 1997 – 2001 and were stable in 2004 – 2010 (Figure 3.5; left panel).

There is no significant change in the shoreline position over the 2003 – 2010 period, which includes

the GYOH construction. At the time of writing there is no evidence of accretion to the north of the

port, although further confidence could be given to this statement by including the GY series beach

profiles and the aerial photograph shorelines.

Transect Pre‐GYOH period Post‐GYOH period Pre‐GYOH

LRR (m/yr)

Post‐GYOH

LRR (m/yr)

N4A5 08/91 – 01/07 01/08 – 03/11 0.78 (26) 1.13 (5)

SWG1 02/93 – 01/07 01/08 – 07/10 3.37 (24) 15.11 (6)

SWG3 08/92 – 01/07 01/08 – 07/10 1.50 (25) ‐1.0 (6)

SWF1 08/92 – 01/07 01/08 – 07/10 ‐1.62 (24) ‐1.39 (6)

Table 3.1: Pre and post‐construction Linear Regression Rate‐of‐change statistics. The number of samples for

each rate‐of‐change calculation is given in brackets.


Profile SWG1 is just south of the GYOH and the mouth of the Yare, and is also an important indicator

of GYOH impacts, according to HR Wallingford (2010), due to anticipated erosion if the GYOH is

interrupting the supply of sediments from the north. At some time during the period 1993 – 1997,

the upper parts (above MLWN) of the beach have eroded. Since that time (1997 – 2010), the

shoreline has been steadily advancing at a rate of 3.37 m/yr prior to construction until a step‐change

coincident with GYOH construction of 15.11 m/yr. As noted above, the time elapsed since

construction is short and the high post‐construction accretion rate at SWG1 is likely to be short‐

lived. An hypothesis as well as cause and effect evidence is required to determine whether this

result is coincidence or an impact. If it is an impact HR Wallingford’s (2010) single impact hypothesis,

which operates only under transport to the south, will require revision.

SWG3 is located adjacent to the Gorleston Golf course and experiences phases of erosion and

accretion without any persistent trend. The record is divided into four sections with no significant

change in shoreline position from 1992 – 2000, followed by accretion in 2000 – 2003, no significant

change/slow retreat in 2004 – 2008, and slow retreat during 2008 – 2010 (Figure 3.6, left panel). The

2008 – 2010 erosive period begins around 1 – 1.5 years after GYOH construction began and is

distinguished from the pre‐construction record by a switch from an accretion rate of 1.5 m/yr to

erosion of ‐1 m/yr (Table 3.1). Although this change appears quite marked, the pre‐construction

accretion rate is dominated by shoreline advanced of c. 15 m (MSL) in the summer of 2001 (Figure

3.6).

The SWF1 profiles near Hopton have the smallest LRR difference (Table 3.1), meaning that the

shoreline response has not changed significantly since the construction of the GYOH. The erosion

rate is slightly lower post‐construction (‐1.39 m/yr) than pre‐construction (‐1.62 m/yr). Although the

beach at SWF1 erodes fairly consistently, it does have a strong seasonal signal (alternating ups and

downs on Figure 3.6) and accreted over 10 m (MSL) and in 2004/5. Whilst there is currently no

evidence of a changing erosion rate or an GYOH impact on erosion rates at Hopton, potential

impacts may take several years to develop, particularly if the net longshore transport has been

primarily to the north in the post‐construction period to‐date.


Figure 3.5: Time‐series of shoreline position at the EA beach profile transects N4A5 and SWG1. The vertical black line is the nominal division of pre and post‐construction used in Table 3.1.


20

40

60

80

100

120

Figure 3.6: Time‐series of shoreline position at the EA beach profile transects SWG3 and SWF1. The vertical black line is the nominal division of pre and post‐construction used in Table 3.1.


3.4 Bathymetry

The sea floor bathymetry data supplied by HR Wallingford on behalf of East Port Ltd is a series of

single‐beam echo sounder surveys from June 2005 until September 2010 at half yearly intervals. The

data extents are highly variable, which is unusual for survey undertaken as part of a monitoring

agreement as consistency in survey is usually a condition of monitoring. In general the survey

extents enlarge with time; in June 2005 the survey area was 1.08 km2, in October 2005 it was 6.2

km2 and in September 2010 it was 17.3 km2. Although a detailed inspection has not been undertaken

here, a visual inspection of the data reveals that some of the records may not be fit for purpose (e.g.

Figure 2.4, Section 2.2.4). The 2010 surveys also use a different coordinate system (WGS84 UTM

Zone 31N) to all previous surveys (British National Grid). Survey tracks are not the same (repeat

lines) although the spacing is reasonably consistent.

Selected bathymetry datasets (2006, 2008, 2009) were chosen to permit a basic comparison

between the pre and post‐construction bathymetry. Previous analysis showed little change in the

position of bathymetric contours (HR Wallingford, 2010), however the contour analysis is not

revealing of the changes between contours and it is not possible to determine the changes in bed

elevation. The residual (erosion/accretion) maps produced here compare elevations across the

common area of two maps: 2006 – 2009 for pre minus post construction, and 2008 – 2009 for

changes in the first year post‐construction.

The erosion/accretion maps (Figure 3.7) in both cases show that most of the area surveyed is

unchanging as marked in grey (+/‐ 0.25 m change = within survey accuracy). In the nearshore zone of

the 2006‐2009 map (left panel) there is an accumulation of sediments on both sides of the GYOH

following construction and (up to 1.4 m) some bed lowering near the GYOH entrance probably

associated with dredging during construction. The nearshore accumulation to the north has a limited

alongshore extent (c. 500 m) and thus would appear to be due to trapping of longshore drift

sediments rather than a cross‐shore transport episode. There is also accumulation to the south,

which could be a cross‐shore or alongshore transport deposit. Closer inspection of the timing of

deposition is required to reveal the cause.

In the first year after construction there are no strong patterns of erosion or accretion. The survey

area is dominated by areas of no change (grey) and patches of slight minor accretion (c. 0.5 m). The

entrance to the GYOH has deepened by up to 2.8 m, presumably as a result of dredging.


Figure 3.7: Erosion/accretion maps for 2006 – 2009 (left) and 2008 – 2009 (right).


4 Conclusions and recommendations

4.1 Summary of conclusions

Based on the analysis undertaken it is our opinion that:

there is a build‐up of sediment to the north of the GYOH (seen in bathymetry data and

expected to be confirmed in as yet unanalysed beach profiles) that could be due to the

presence of the GYOH;

the change in accretion rate at Gorleston is coincident with port construction and requires

further investigation to determine cause and effect; and

the erosion in the Hopton area is persistent over the last 20 years, shows no change

following GYOH construction, and is therefore not an impact of the GYOH.

A caveat to the above summary points is that further analysis is needed to refine and give

confidence to our conclusions and the evidence base.

4.2 Conclusions

Shoreline change statistics in the Great Yarmouth area show that the beaches to the north and south

of the GYOH exhibit the same spatial pattern: accretion in the northern beaches and erosion in the

southern beaches (Figure 3.3 as highlighted by red arrows). This pattern suggests that the coastlines

on both sides of the Yare mouth have rotated toward the south over the course of the past c. 20

years, probably in response to changes in the inshore wave climate. The assessment of coastal

change as an impact due to the GYOH (or any other construction) must take into account the natural

variability of the shoreline (and coastal processes), which in this case the broader pattern of recent

beach rotation. An important limiting factor here is that no data have been gathered on changes in

adjacent banks and the inshore wave climate (see Section 4.3), which are important drivers of

sediment transport and shoreline change.

HR Wallingford (2010) suggest that the only impact the GYOH could have on the neighbouring

shorelines is an accumulation of southward travelling nearshore (longshore drift) sediments and a

subsequent disruption of sediment supply (erosion) on beaches to the south (e.g., Gorleston to

Hopton). There is some evidence in support of accumulation to the north (bathymetry; further

beach profile analysis required) but no evidence of erosion to the south. Instead, our observations


show that accretion occurs in the south (Gorleston) and aerial photos suggest there is some

shoreline retreat close to the GYOH (Figure 3.1, Figure 3.2, Figure 3.3), which contrasts with the

bathymetric accretion in the same area (Figure 3.7).

The concept of a blockage to the longshore transport system that results in accretion to the north

and erosion to the south of the GYOH can only occur when the net drift is to the south. Therefore,

this scenario (proposed by HR Wallingford, 2010) gives evidence of an impact but is only valid for

periods of net southerly transport. It does not consider other possibilities such as phases of net

longshore transport to the north and/or complex circulation patterns. Although the shoreline data

(profiles and aerial photographs) presented do not presently support accumulation, bathymetric

data do show accumulation 0 – 500 m updrift of the GYOH that may be due to a sediment transport

blockage. Inspection of the GY series of beach profiles and other bathymetric datasets is required to

determine whether this deposit, as well as the similar feature at Gorleston, is related to the

presence of the GYOH. We also note that only 3 years have elapsed since construction and, on a

coast with variable longshore transport direction, impacts may not be identifiable within this time

period.

Changes in the elevation of the sea floor bathymetry following the GYOH construction are localised

with no detectable impacts offshore. The build up of sediment north of the GYOH appears to be

slowing with time (smaller accretion area in right panel of Figure 3.7) suggesting bypassing

continues, but it is not possible to say whether the sediment does or does not re‐enter the beach

system. The subtidal accretion at Gorleston (Figure 3.7). Further analysis of the bathymetry time‐

series is required to distinguish if these deposits occur at different times (indicating longshore

transport blockages) or at the same time (indicating cross‐shore transport with no inference for

GYOH impacts).

Some of the shoreline change patterns close to the GYOH show step changes that occur around the

time of construction, namely the increase in shoreline advance at Gorleston and the switch from

accretion to erosion (1 – 1.5 years post‐construction) at the Gorleston Golf Course (Table 3.1, SWG1

and SWG3 respectively). Hypotheses need to be formulated and cause and effect determined before

these changes can be ascribed as impacts or coincidences. For cause and effect to be investigated

estimates of longshore drift magnitude and direction, along with general circulation patterns, needs

to be investigated. Both require field data and numerical modelling. Longshore transport rates

require the wave heights, periods and directions of the inshore wave climate, which in turn requires


hindcast modelling of waves over the Great Yarmouth sandbanks. Models of this area are already in

operation (e.g., Thurston et al., 2010; Stansby et al., 2006).

South of the Gorleston Golf Course the shoreline has been subject to persistent erosion for the last

c. 20 years. Of the profiles presented here, Hopton (SWF1) has the smallest difference in LRR before

and after the GYOH construction. There is no evidence that the erosion rate has changed

significantly nor is there evidence of any changes in erosion rate around the time of GYOH

construction. As Hopton is some distance from the GYOH, and given the potential variability in

longshore sediment transport rate and direction, any impact on sediment supply may not yet be

apparent there. Questions remain over the time‐ and space‐scales of potential impact. A continued

and appropriate monitoring plan is likely to quantify address these.

4.3 Recommendations for further investigation

This report was commissioned and conducted over a short time scale of around a month. Several

datasets were identified but were not delivered at the time of writing. Additionally there was not

sufficient time for a comprehensive analysis of the available/requested data. As a result the

following work (listed in order of priority) is recommended to be conducted and incorporated in a

revision of this report or as a separate document:

Beach volumes. The time‐series of beach volumes (or cross‐sectional areas) needs to be

carefully calculated and presented alongside shoreline change statistics. Volume

measurements are needed because beaches can change shape and position without

necessarily changing beach volume. That is shoreline movements may occur with an

unchanging beach volume, which signifies shoreline retreat but is not beach erosion per se.

The volume data presented in the HR Wallingford (2010) report were not supplied for use in

this report. The calculation method used there is also unclear; cross‐sectional areas were

calculated down to LAT, however very few profiles extend to LAT and an appropriate

method must be utilised to ensure that areas are not partly determined by the profile

length, which varies from survey to survey. It is unclear whether those volume estimates are

fit for purpose.

Shoreline positions and bathymetry. Only a selection of key profiles and bathymetric data

have been used in this report. Analysis of the GY series of beach profiles and remaining

bathymetric data will significantly increase the spatial resolution of the results and assist in

determining if there is/has been a longshore sediment transport blockage. Additionally, the


aerial photographs from missing period could easily be geo‐referenced to remove bias from

the low‐tide shorelines analysis. An adjustment for tidal levels is also recommended.

Inshore wave climate and longshore drift. An understanding of longshore sediment

transport magnitudes and directions is needed to quantify natural variability in sediment

transport and where/when impacts due to the GYOH may be expected to appear. For

example, if sediment transport has been primarily to the north since the construction then

no impact will be expected on beaches as far south as Hopton. Equally, a change in transport

direction could disrupt the sediment supply from the north and result in beach depletion

(although there is no evidence of this to date). The lack of inshore wave data is a significant

obstacle to identifying and impacts the GYOH may have (in the past and for the future). This

could be redressed using numerical wave models (e.g., Thurston et al., 2010) and/or making

directional wave measurements inshore of the banks.

Tidal circulation. Patterns of sediment transport may be more complicated than the simple

model of the GYOH as a barrier to longshore transport in which sediment builds up on one

side and erodes on the other. For example, sediments may bypass the barrier but not return

to the nearshore if they are directed off shore; or persistent tidal eddies may develop that

alter the downstream sediment transport pathways giving rise to areas of high and low

sediment supply. A tidal current model capable of resolving flows around structures could be

used to investigate the flow and sediment transport patterns, and to formulate hypotheses

regarding potential impacts that can then be examined in the shoreline response data.

Historical shoreline mapping. Historical maps and navigation charts can be used to plot

shorelines for decades to hundreds of years. An analysis of the longer‐term shoreline

behaviour is useful to identify natural variability in the position of the coast, including areas

that may be subject to cycles of erosion and accretion.


5 References

Dolphin, T.J., Vincent, C.E., Bacon, J.C., Dumont, E., and Terentjeva, A. Medium-term

impacts of a segmented, shore-parallel breakwater system. Submitted to Coastal

Engineering.

Dolphin, T.J., Vincent, C.E., Coughlan, C.E. and Rees, J.M., 2007. Variability in Sandbank

Behaviour at Decadal and Annual Time-Scales and Implications for Adjacent

Beaches. Journal of Coastal Research, SI50, 731 – 737.

Environment Agency Shoreline Management Group, 2007. Great Yarmouth Coastal

Monitoring: Coastal Trends Analysis.

HR Wallingford, 2010. Great Yarmouth Beach and Nearshore Monitoring Report. Report

EX6469 Release 2, February 2011.

Park, H.–B. and Vincent, C.E., 2007. Evolution of Scroby Sands in the East Anglian coast,

UK. . Journal of Coastal Research, SI50,868 – 873.

SANDS software by Halcrow Group PLC. (http://www.halcrow.com/sands)

Stansby, P., Kuang, C., Laurece, D. and Launder, B., 2006. Sandbanks for coastal

protection: implications of sea-level rise. Part 1: application to East Anglia.

Tyndall Centre Working Paper 86.

Thieler, E.R., Himmelstoss, E.A., Zichichi, J.L., and Ergul, A., 2009. Digital Shoreline

Analysis System (DSAS) version 4.0—An ArcGIS extension for calculating

shoreline change. U.S. Geological Survey Open-File Report 2008-1278.

Available online at http://pubs.usgs.gov/of/2008/1278/ .

http://woodshole.er.usgs.gov/project-pages/DSAS/index.htm

Thurston, K.J., Vincent, C.E., and Dolphin, T.J., 2010. The Influence of Storm Surges on

Sandbank Evolution: The Great Yarmouth Sandbanks, UK. In Proceedings of the

15th Physics of Estuaries and Coastal Seas (PECS) Conference, Colombo, Sri

Lanka, 14-17 September 2010.


6 Acknowledgements

The authors would like to acknowledge the other government agencies within the DEFRA

family that have provided data for this report: the Environment Agency, the Maritime and

Coastguard Agency and the UK Hydrographic Office. HR Wallingford also provided beach

profile and bathymetric data gathered under contract to East Port Ltd. In particular we would

like to thank Phillip Staley, Gary Watson and Will Riggs of the Environment Agency for

making data available at short notice and updating the EDA results.

© Crown copyright 2011

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Documents

Shoreline variability in the vicinity of the Great …...Figure 1.1: Shoreline trends along the Norfolk – Suffolk coastline for 1991 – 2007 (adapted from Environment Agency Shoreline