Conwy Tidal Flood Risk Assessment

Stage 1 – Final Report (Includes new sea level and future climate information )

Report EX 4667 (release 3.0) May 2008

Conwy Tidal Flood Risk Assessment

Stage 1 – Final Report (Includes new sea level and future climate information)

Report EX 4667 (release 3.0) May 2008

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Address and Registered Office: HR Wallingford Ltd . Howbery Park, Wallingford, OXON OX10 8BA Tel: +44 (0) 1491 835381 Fax: +44 (0) 1491 832233

Registered in England No. 2562099. HR Wallingford is a wholly owned subsidiary of HR Wallingford Group Ltd.

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Contract - Consultancy

This report describes the first phase of work commissioned by Conwy County Borough Council, whose representative was Mr Dyfed Rowlands. The HR Wallingford job number was MCR 3844. This version of the report includes updated new sea level and future climate information and supersedes and replaces all earlier versions, and should be used for all new work in the geographical area and scope of use to which it applies. This work was carried out by members of the Flood Management Group. The report was written by Mohamed Ahmed Ali Mohamed Hassan. The HR Wallingford Project Manager was Paul Sayers.

Prepared by ......

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Approved by ......

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Authorised by ......

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© HR Wallingford Limited 2008

HR Wallingford accepts no liability for the use by third parties of results or methods presented in this report.

The Company also stresses that various sections of this report rely on data supplied by or drawn from third party sourcesHR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data.

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Summary

Conwy Tidal Flood Risk Assessment

Stage 1 – Final report

Report EX 4667 May 2008

This report details the work undertaken under Stage 1 of the Conwy Tidal Flood Risk Assessment study and provides a scientific appraisal of the extreme loading on the defences and their likely response in terms of overtopping and breaching. This version of the report includes updated new sea level and future climate information in addition to new survey data of the River Clwyd undertaken by the EA. It supersedes and replaces all earlier versions, and should be used for all new work in the geographical area and scope of use to which it applies. The following Seven areas were identified for examination within this study:

• Pensarn to Kinmel Bay (including the western embankment of the River Clwyd) • Penrhyn Bay/Rhos-on-Sea • Llandudno North Shore • Llandudno West Shore • Deganwy • Conwy Quay • Glan Conwy

These areas are largely, but not entirely, defended by extensive coastal defences.

The analysis undertaken as part of Stage 1 has enabled a series of priority defences to be identified based on their likelihood of breaching and/or the severity of overtopping. The selection was based on the computed overtopping rates and the probability of defence failure and breach for the 100-year event. These include:

Group 1 Priority: 1. North Wales Golf Club (1) 2. Llandudno West Promenade (1) 3. Colwyn Road, Craigside 4. Penrhyn Bay (2) 5. Llandrillo-yn-Rhos 6. River Clywd Banks coded 5C, 5D1, 5D2, 5F and 5E (For details see Appendix 2)

Group 2 Priority: 1. Gogarth Wall 2. Llandudno Promenade (1) 3. Penrhyn Bay (1) 4. Kinmel Bay 1 (C) 5. Kinmel Bay 1 (E)

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Summary continued

In this version of the report, the following items were updated or added to include recent updates to extreme sea levels and future climate change assumptions in addition to the new survey data of River Clwyd:

• Table 1 in Chapter 3 • Sections 5.4, 5.5 and 5.6 in Chapter 5 • Tables 5,6,7 and 8 in Chapter 5 • Tables 9a, 9b, 10, 11a,11b,11c,11d,12a,12b,15 and 16 in Chapter 6 • Table 17 in chapter 8 • The references section

More details about the above updates are given in the Section 1 of the report.

Following on from this report, Stage 2 applied detailed numerical modelling to simulate different flood scenarios at Pensarn to Kinmel Bay, Penrhyn Bay/Rhos- on-Sea, and Llandudno North and West Shores with more limited modelling elsewhere.

Contents

Title page i Contract iii Summary v

1. Introduction...... 1 1.1 Changes from previous versions...... 1 1.2 Aims and objectives...... 2 1.3 Report structure...... 3

2. Task 1: Inspection of existing data ...... 4 2.1 Introduction...... 4 2.2 Description of the work undertaken...... 4

3. Task 2: Updating of defence data ...... 5 3.1 Introduction...... 5 3.2 Description of the work undertaken...... 5

4. Task 3: Examination of historical flooding...... 11 4.1 Introduction...... 11 4.2 Description of the work undertaken...... 11 4.3 Review of sea conditions during the historic events...... 11 4.4 Conclusions from the historic event analysis...... 13

5. Task 4: Wave and water level conditions ...... 14 5.1 Introduction...... 14 5.2 Description of the work undertaken...... 14 5.3 Offshore sea conditions ...... 14 5.4 Nearshore sea conditions ...... 17 5.5 Water levels and waves in Conwy estuary ...... 21 5.6 Future sea level rise and wave climate change ...... 22

6. Task 5: Overtopping and breach predictions for each idenitfied defence.26 6.1 Introduction...... 26 6.2 Analysis of the defence overtopping...... 26 6.3 Assessment of defence breach probability...... 33 6.3.1 Defence breach probability...... 33 6.3.2 Breach size and invert level...... 37

7. Task 6: Future maintenance...... 40

8. Conclusions...... 41

9. References...... 42

Tables Table 1 Summary of structural failure modes 6 Table 2 Distribution and extremes of surge residual (1970-83) 14 Table 3 Distribution and extremes of high water level (1970-83) 15 Table 4 Joint occurrence of large offshore waves and high water levels results 16

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Contents continued Table 5 Location of the nearshore wave prediction points 18 Table 6 Wave conditions for nearshore prediction points (without wave breaking) 18 Table 7 Extreme sea levels for prediction points (mOD, present-day) 18 Table 8 Joint probability wave and water level conditions for nearshore prediction points 19 Table 9a Present day “mean” overtopping results – Defence by defence (Lower Bound) 27 Table 9b Present day “mean” overtopping results – Defence by defence (Upper Bound) 28 Table 10 Present day “peak” overtopping results – Defence by defence 29 Table 11a “Mean” overtopping results by 2056 - Defence by defence (Lower Bound) 30 Table 11b “Mean” overtopping results by 2056 – Defence by defence (Upper Bound) 31 Table 11c “Mean” overtopping results by 2106 – Defence by defence (Lower Bound) 32 Table 11d “Mean” overtopping results by 2106 – Defence by defence (Upper Bound) 33 Table 12a “Peak” overtopping results by year 2056 – Defence by defence 34 Table 12b “Peak” overtopping results by year 2106 – Defence by defence 35 Table 13 Sd values vs. the probability of failure 34 Table 14 Critical values of C w (Terzaghi et al, 1996) 35 Table 15 Summary of structural failure modes and associated modelling approach 36 Table 15 Summary of structural failure modes and associated modelling approach (continued) 37 Table 16 Summary of expected defence annual breach probabilities and breach dimensions 38 Table 16 Summary of Expected defence annual breach probabilities and breach dimensions (continued) 39 Table 17 Defence priority groups 41

Figures Figure 1 Location map 1 Figure 2 Towyn Flooding in 1990 Event 13 Figure 3 Average wave conditions for 5 years for the first set of data 24 Figure 4 Average wave conditions for 5 years for the second set of data 25

Appendices Appendix 1 Identified Data and Sources Appendix 2 Database of Defence Details Appendix 3 Historical Data Appendix 4 Defence Management and Maintenance Actions Appendix 5 Brief Description of the Overtopping Spreadsheet Appendix 6 Defence details

Drawings (Foldout at the end of the report) Drawing 1: Location map 1 Drawing 2: Location map 2

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1. INTRODUCTION

This report was formerly released in 2004 to detail Stage 1 of the work undertaken by HR Wallingford Ltd. and Coastal Engineering UK Ltd and commissioned by Conwy County Borough Council (CCBC) Engineering and Environmental Services to examine tidal flood risk in County of Conwy (See Figure 1). The 2004 report provided a scientific appraisal of the extreme loading on the defences and their likely response in terms of overtopping and breaching. This version of the report is released to include recent updates to extreme sea levels and future climate change assumptions.

1.1 Changes from previous versions The following items were updated or added in this version of the report:

• Table 1 to update the failure modes for the River Clwyd embankments based upon the new survey data undertaken by the EA. • Sections 5.4, 5.5 and 5.6 in Chapter 5 to include recent updates to extreme sea levels and future climate change assumptions. • Tables 5, 6, 7 and 8 in Chapter 5 to include recent updates to extremes sea levels and future climate change assumptions and incorporate new wave point for the River Clwyd. • Tables 9a, 9b, 10, 11a, 11b and 12a in Chapter 6 to update the overtopping rates and volumes based upon the above changes. • Tables 11c, 11d and 12b in Chapter 6 to include the overtopping rates and volumes in 2106. • Table 15 and 16 in Chapter 6 to update the failure mode(s) and failure probability of defences based upon the above changes. • Table 17 in Chapter 6 to update Group 1 and 2 defences based upon the above changes. • The references section to incorporate new references used for this release of the report.

Figure 1 Location map

ABCD 1 EX 4667 19/05/2008 Initially four areas were identified for examination in the study:

• Pensarn to Kinmel Bay (including the western embankment of the River Clwyd) • Penrhyn Bay/Rhos-on-Sea • Llandudno North Shore • Llandudno West Shore.

This was later extended to include:

• Deganwy • Conwy Quay • Glan Conwy.

The landward limit of the study areas is described by the Indicative Floodplain Maps as identified by the Environment Agency in 2001. These areas are highlighted in more details in Drawings 1 and 2 (see A0 foldout at the end of the report).

The following tasks form Stage 1 of this study:

Task 1: Inspection of existing data Task 2: Updating of defence data Task 3: Examination of historical flooding Task 4 Wave and water level conditions Task 5: Overtopping and breach predictions for each identified defence length Task 6: Future maintenance

1.2 Aims and objectives The general aim and objective of this study is to provide a robust scientific appraisal of risk of flooding from tidal inundation. The areas of Indicative Floodplain identified for examination within this study are largely, but not entirely, defended by extensive coastal defences. It is particularly relevant to note that many of the defence lengths associated with the areas under examination have been reconstructed in the past 20 years, with associated improvements being provided in the Standard of Protection, including:

• New shore connected breakwaters and beach recharge were provided at Penrhyn Bay and Llandudno West Shore in the early 1990’s whilst beach recharge and promenade/sea wall elevation improvements were made across the main section of Llandudno North Shore between 1996-2000. Also in the early 1990’s a new armour stone revetment was constructed between Penrhyn Bay and Rhos Point.

• Across the Pensarn frontage rear flood walls have been constructed along the crest of the shingle bank, whilst the Towyn section received extensive armour stone revetments, following breaching of the defences and hinterland flooding as a result of the February 1990 storms. Also in the late 1990’s improvement works to the sea wall were carried out across the Kinmel Bay frontage.

Along significant lengths of the CCBC frontage the beach plays an important part in providing primary defence. Robust flood risk assessment will need to recognise the level of service that this provides and identify those sections that are most vulnerable to future breaching.

The Indicative Floodplain Maps provided by the Environment Agency are developed from a projection of the 200-year return period water level and ignores the presence of defences. Therefore it is reasonable to assume that the estimate of the flood extent in such an event is overestimated. Using more realistic risk assessment methods (based on those developed as part of a project funded by the EA titled Risk Assessment for Strategic Planning (RASP)) would provide a better estimate.

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This project aims to provide a more detailed assessment of risk and the likelihood of inundation that recognises the role of the coastal defences in reducing flood risk.

1.3 Report structure This report comprises a number of sections. Each section covers a specified task(s) and contains an introduction to this task followed by a description of the work undertaken. Where significant data has been collated or reviewed this is referred to in the main text and reported in full in the Appendices.

ABCD 3 EX 4667 19/05/2008 2. TASK 1: INSPECTION OF EXISTING DATA

2.1 Introduction In this task, the basic information relating to the database of coastal defence structures was reviewed through comparison with existing as-built drawings and by visual inspection (Task 2). In particular, an assessment was made of the geometry and condition of the defences.

2.2 Description of the work undertaken Data was collected from different sources to inspect and confirm the adequacy of the data held by CCBC Engineering and Environmental Services including:

• Eighty-nine past reports • Foreshore and bathymetric surveys • Hinterland topography data • Floodplain and OS mapping data • Photographic records of coastal development and past failures • Defence details, as-built drawings, past surveys and existing databases • Environmental data on waves and water levels.

Sources of the data collated and reviewed under these headings are listed in Appendix 1.

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3. TASK 2: UPDATING OF DEFENCE DATA

3.1 Introduction In this task, the information collated in Task 1 has been reviewed and supplemented by visual inspection to provide a description of the construction, present condition and geometry of the existing defences.

3.2 Description of the work undertaken The data collated in Task 1 (See Appendix 1) has been supplemented by data gathered through a ‘walk- over’ survey of all defences undertaken in the areas identified by Coastal Engineering UK Ltd. Based on this visual inspection the CCBC database has been updated. The details of the revised database are presented in Appendix 2.

In particular the survey focused on the following aspects:

• Establishing a local defence coding linked to NFCDD and past CCBC studies • Defence location (OS grid references) • Defence length • Description of the primary, secondary and tertiary defence details (i.e. shingle beach, backed by a secondary sea wall etc.) • Beach gradient • Levels (mOD) of primary, secondary and tertiary defence crests • Year of construction(s) if known • Condition of primary, secondary and tertiary defences • Comments on likely defence performance and other general issues • Profile and cross-section • Details of on-going monitoring • Maintenance responsibilities.

In addition to the above, cause consequence diagrams have been produced (See Appendix 6) for each identified defence length. Each diagram outlines:

• The sources of risk • The risk pathways • The risk receptors • Potential harm that can be experienced in the event of a flood.

Based upon this analysis a dominant failure mode has been identified for each identified defence length. This dominant failure mode is later used in Task 5 as an indicator of breach probabilities. The dominant failure mode of each defence is provided below in Table 1.

ABCD 5 EX 4667 19/05/2008 Table 1 Summary of structural failure modes Defence Critical Failure Mode Rationale Length Llandudno West Shore 1A Overtopping damage behind crest of shingle bank Failure due to changes in 2D profile of shingle bank causing increased overtopping. Probabilities linked to tolerable discharges for embankment sea walls, as EA Manual (Besley, 1999). 1B Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 1C No failure No failure - Shoreline protected by root of shore connected breakwater 1D Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 1E No failure No failure - Shoreline protected by root of shore connected breakwater 1F Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 1G Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999).

Llandudno North Shore 2A Overtopping damage to promenade Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). 2B Overtopping damage to promenade Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). 2C No failure No failure - Concrete slip way 2D Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 2E Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 2F Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 2G Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 2H Overtopping damage to rear of defences Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999).

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Table 1 Summary of Structural Failure Modes (continued)

Defence Critical Failure Mode Rationale Length Llandudno North Shore (continued) 2I Overtopping damage to earth bank behind shingle Failure due to changes in 2D profile of crest shingle bank causing increased overt opping. Probabilities linked to tolerable discharges for embankment sea walls, as EA Manual (Besley, 1999). 2J Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward

Penrhyn Bay/Rhos-on-Sea

3A Overturning/Breaching as a result of lowering Assessment of potential for scour of beach beach levels (scour) in front of hard defence in front of wall leading to lowering of beach and increased probability of geotechnical failure. 3B No failure No failure - Shoreline protected by root of shore connected breakwater 3C Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). 3D Damage to rock armour facing leading to failure Probabilities linked to changes in Van der of defences Meer damage criteria (S d) due to different wave conditions 3E Damage to rock armour facing leading to failure Probabilities linked to changes in Van der of breakwater Meer damage criteria (S d) due to different wave conditions 3F Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999).

Pensarn to Kinmel Bay 4A Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 4B Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 4C Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). 4D Damage to rock armour facing leading to failure Probabilities linked to changes in Van der of defences Meer damage criteria (S d) due to different wave conditions 4E Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward

ABCD 7 EX 4667 19/05/2008 Table 1 Summary of Structural Failure Modes (continued)

Defence Critical Failure Mode Rationale Length Pensarn to Kinmel Bay (continued) 4F Damage to rock armour facing leading to failure Probabilities linked to changes in Van der of defences Meer damage criteria (S d) due to different wave conditions 4G Damage to rock armour facing leading to failure Probabilities linked to changes in Van der of defences Meer damage criteria (S d) due to different wave conditions 4H Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 4I Crest retreat of shingle beach in front of defences Increased exposure and probability of failure as crest of beach moves landward 4J Damage to rock armour facing leading to failure Probabilities linked to changes in Van der of defences Meer damage criteria (S d) due to different wave conditions 4K Run up over dunes causing lowering of crest Probabilities linked to limit of wave run up on dune face in relation to crest of structure 4L Run up over dunes causing lowering of crest Probabilities linked to limit of wave run up on dune face in relation to crest of structure 4M No failure No failure - frontage sheltered from predominant wave conditions. Exposure water level driven 4N No failure No failure - frontage sheltered from predominant wave conditions. Exposure water level driven

River Clwyd 5A Head difference across flood bank causing Increased probability with increase in water destabilization by overflow causing erosion of levels linked to potential pathways for embankment material and leading to failure overflow. Exposure water level driven 5B Head difference across flood bank causing Increased probability with increase in water destabilization by overflow causing erosion of levels linked to potential pathways for embankment material and leading to failure overflow. Exposure water level driven 5C Head difference across flood bank causing Increased probability with increase in water destabilisation by piping and possible slip failures levels linked to potential pathways for water at extreme events or overflow causing erosion of beneath or above the embankment. embankment material and leading to failure Exposure water level driven 5D1 Head difference across flood bank causing Increased probability with increase in water destabilisation by piping and possible slip failures levels linked to potential pathways for water at extreme events or overflow causing erosion of beneath or above the embankment. embankment material and leading to failure Exposure water level driven 5D2 Head difference across flood bank causing Increased probability with increase in water destabilisation by piping and possible slip failures levels linked to potential pathways for water at extreme events or overflow causing erosion of beneath or above the embankment. embankment material and leading to failure Exposure water level driven

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Table 1 Summary of Structural Failure Modes (continued)

Defence Critical Failure Mode Rationale Length River Clwyd (continued) 5E Head difference across flood bank causing Increased probability with increase in water destabilisation by piping and possible slip levels linked to potential pathways for water failures at extreme events or overflow causing beneath or above the embankment. Exposure erosion of embankment material and leading to water level driven failure 5F Head difference across flood bank causing Increased probability with increase in water destabilisation by piping and possible slip levels linked to potential pathways for water failures at extreme events or overflow causing beneath or above the embankment. Exposure erosion of embankment material and leading to water level driven failure

Glan Conwy 6A Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). Railway immediately behind sea wall Conwy Quay 6B Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 6C 1999). Overturning/Breaching as a result of lowering Assessment of potential for scour of beach in beach levels (scour) in front of hard defence front of wall leading to lowering of beach and increased probability of geotechnical failure. 6D Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). Deganwy 6E Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). 6F Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 1999). Railway immediately behind sea wall Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 6G 1999). Crest retreat of shingle beach in front of Increased exposure and probability of failure defences (SHINGLE model) as crest of beach moves landward

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Table 1 Summary of Structural Failure Modes (continued)

Defence Critical Failure Mode Rationale Length Deganwy (continued) Overtopping damage to rear of structure Failure due to increased overtopping. Probabilities linked to tolerable discharges for revetment sea walls, as EA Manual (Besley, 6H 1999). Crest retreat of shingle beach in front of Increased exposure and probability of failure defences (SHINGLE model) as crest of beach moves landward

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4. TASK 3: EXAMINATION OF HISTORICAL FLOODING

4.1 Introduction A number of significant flood events have occurred in the past relating to the areas under examination, including:

• Tidal flooding of the centre of Llandudno simultaneously from both the West and North shores during the early part of the 20 th century • Flooding of the low lying parts of Rhos on Sea in the mid to late 20th century • Breaching of the British Rail defences at Towyn in 1990, which led to extensive hinterland flooding and damage estimated to be in excess of £50 million.

In this task, the CCBC database of reports relating to coastal defence and flooding issues was examined together with other sources to identify dates of known past flood events. In addition, archives of local newspapers were also accessed to identify any articles and press coverage relating to these events.

4.2 Description of the work undertaken Based upon the review of historical data on flooding, forty-nine records of past flooding have been identified. These range from minor tidal incursions through to more significant and extensive breaching of the coastal defences and subsequent widespread inundation (for example the event of 26-28 February 1990 when high tides and winds caused overtopping and a 450m long breach in the seawall.)

The details of the information collated on historical flooding are recorded in Appendix 3 under the following headings:

• Event date • Area affected • Event details • Hinterland consequences – where known • Source of information.

4.3 Review of sea conditions during the historic events The historical event data were compared against the analysis of present day extreme wave and water levels as discussed later in Task 4. This comparison provides an interesting insight into the return period of past events and the degree to which the recent climate is similar to that experienced in the past. To aid this comparison, key tidal flood events identified above that have an associated quantifiable water level or wave height have been reviewed in light of the analysis of extremes provided below in Task 4 and assigned an appropriate return period.

Events prior to 1970 Three ‘events’ between 1899 and 1924 are said to have involved a predicted tide of 9-9.5 m at Holyhead. Presumably this is to an older unknown datum, as HAT at Holyhead is only 6.3 m above Chart Datum. On two of these three occasions a surge of 1 m was noted, which has a return period of around 5 years. One entry for 1937 notes a surge of 0.45 m at Holyhead that would have a return period of about 0.02 year. Measured sea level and modelled wave conditions are available from 1970 onwards, in a form suitable for direct comparison with design sea conditions used in this study.

November 1974 This event seems to have involved only wind-blown sand, and did not register as a marine flooding event. The sea level was around MHWS and a predicted H s of 3.5m was associated with the strong winds.

ABCD 11 EX 4667 19/05/2008 February 1975 There were four successive high waters over 10mCD at Liverpool at the end of January 1975, peaking at th 10.67mCD (about 2 years return period) at 14.00 on the 30 of January with an associated H s of 1.2m. There were four successive high waters over 10mCD at Liverpool at the end of February 1975, peaking at th 10.18mCD at 13.00 on the 27 of February with an associated H s of 0.6m.

November 1976 There was no obvious wind ‘event’ in the Blackpool Airport record for October to December 1976.

11 November 1977 There were six high waters over 10mCD at Liverpool 11-14 November with around three metre waves. The two highest records were at:

th • Midnight on 11 of November, 10.88mCD at Liverpool (about 10 years return period) with an H s of 2.88m (about 20 years joint exceedance return period), and

th • At 14.00 on the 14 of November, 10.58mCD at Liverpool (about 5 years return period) with an H s of 3.5m (about 10 years joint exceedance return period).

31/1-1/2/1983 There were five successive high waters over 10mCD at Liverpool, peaking at 10.74mCD, whilst H s peaked at 4.52m (nearly 1 year return period). The highest record was at 2.00 on the 1st of February, 10.74mCD at Liverpool (about 3 years return period) with an H s of 3.52m (about 15 years joint return period).

11-12 February 1990 Precise times and measurements are not available for this event, but the sea level had previously been th shown to have a return period of about 1.5 years, and the predicted H s was about 3.5m throughout 12 of February (about 10 years joint return period).

26-28 February 1990 There were five successive high waters, each of which might have been considered an ‘event’ in its own right. The two highest records were at:

• At 12.00 on the 26 th of February, 11.06mCD at Liverpool (about 80 years return period based on data prior to that occurrence, and 30 years when Feb 1990 included) with an H s of 4.32m (about 1.5 year return period for wave height; 500 years joint exceedance return period based on data prior to that occurrence, and 100 years when Feb 1990 included), and

th • At 11.00 ob the 27 of February, 10.82mCD at Liverpool (about 7 years return period) with an H s of 4.22m (about 1.2 year return period for wave height; 100 years joint exceedance return period based on data prior to that occurrence, and 25 years when February 1990 included).

(The recorded water level at Deganwy Dock of 5.15mOD suggests about a 7 year return period sea level when February 1990 data is included.)

In this event, Towyn was inundated when 450 m of seawall was breached by a 1 in 500 year event when a 1.3 m storm surge coincided with high tide and 4.5 m high waves (Dawson et al, 2003). A lack of natural protection meant that the seawall, which had been targeted for maintenance in the near future, felt the full force of the waves. The nature of the topography resulted in the flood reaching as far as 2km inland with a maximum depth of 1.8m. Although there were no direct fatalities, 5000 people were evacuated from nearly 3000 properties. Immersion of agricultural areas resulted in damage to crops. The total of flood damage was estimated as being in excess of £50 million. Figure 2 shows the flood extents in Towyn.

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Figure 2 Towyn Flooding in 1990 Event

10 February 1997 The highest record was at 14.00 on the 10 th of February, 10.79mCD at Liverpool (about 5 year return period) with an Hs of 2.5m (about 10 years joint return period). (The Conwy Quay water level of 5.10mOD also indicates about a 5 year return period sea level.)

4.4 Conclusions from the historic event analysis The sea condition predictions outlined in Task 4 below are based mainly on data for the period 1970-83 plus the known severe stormy spell in February 1990. The historic event analysis suggests that marine flooding was at least as common during this period as in the longer term. Indeed, inclusion of the severe flooding in February 1990 probably means that the source data used will slightly over-estimate the longer-term flood risk.

It is also interesting to note that most of the marine flooding events were caused by extreme sea levels coupled with high wave conditions. This indicates that the worst marine flooding is driven primarily by very high sea levels, sometimes persisting over a number of successive high waters, but that high wave conditions at the same time are also a contributory factor.

ABCD 13 EX 4667 19/05/2008 5. TASK 4: WAVE AND WATER LEVEL CONDITIONS

5.1 Introduction Extreme waves and extreme water levels have been estimated for several locations between Llandudno and Rhyl (See Drawings 1 and 2) for a range of return periods up to one thousand years. In view of the large number of previous studies in the area, the approach adopted was not to do any new modelling of sea conditions, but to develop all necessary results by re-interpretation of existing results and reports.

5.2 Description of the work undertaken The widely quoted HR Wallingford Report EX2133 on extreme waves and water levels for the North Wales coast (HR Wallingford,1990) was taken as the starting point. This was supplemented by more recent extreme water levels predictions based on research at the Proudman Oceanography Laboratory (POL, 1997) to provide extreme water levels for the study area. Offshore wave conditions predicted within EX2133 have not been revised, however they have been transferred nearshore using site-specific wave transformation functions to provide nearshore wave climates at a series of points. The analysis of joint exceedance within EX2133 has also been extended to include return periods of combinations of wave and water levels up to 1000 years.

The above steps are discussed in more detail below.

5.3 Offshore sea conditions Extreme water levels The distribution and extremes of surge residual and high water level at Princes Pier, Liverpool are reproduced from EX2133 in Tables 2 and 3 below. These values were used only indirectly in the present study to assist in estimating water levels with the same return period areas across the study area.

Deep water wave conditions Deep water wave climate and extremes are quoted in several previous HR Wallingford reports (references). Results in EX2133 compared well with those in HR other reports, and the addition of later years of data appeared to have little impact on the results. Wave conditions from EX2133 are therefore accepted as best estimates of present-day deep water waves.

Table 2 Distribution and extremes of surge residual (1970-83)

Distribution Extremes Surge (m) % > Return period (yrs) Surge (m) All data High water Max At high values only water 0.00 46.4 44.8 1 1.66 0.97 0.125 24.7 21.5 0.25 12.0 9.5 5 1.99 1.25 0.375 5.60 3.92 0.50 2.84 1.74 20 2.27 1.49 0.75 0.75 0.35 1.00 0.21 0.07 50 2.45 1.64 1.25 0.070 0.01 1.50 0.020 0.01 100 2.59 1.76 1.75 0.011 0.01 2.00 0.004 0.00 2.25 0.001 0.00 Note: Based on 112610 hourly values derived from Princes Pier water level records, of which 9013 were at high water.

ABCD 14 EX 4667 19/05/08

Table 3 Distribution and extremes of high water level (1970-83)

Distribution Extremes High water Return Period High water level (mCD) % > (yrs) level (mCD) 8.0 77.4 1 10.56 8.3 66.2 5 10.77 8.6 53.9 20 10.93 8.9 40.2 50 11.02 9.2 25.5 100 11.08 9.5 13.9 150 11.12 9.8 6.24 200 11.17 10.1 1.77 500 11.27 10.4 0.291 1000 11.34 10.7 0.043 11.0 0.000 Note: Based on 9267 measured high water levels at Princes Pier

Joint occurrence of large offshore waves and high water levels The main results of EX 2133 are expressed in terms of the probability of joint occurrence of large waves (offshore Rhyl) and high water levels (at Liverpool) for joint return periods between 1 and 100 years. The dependence between large waves and high water levels is higher than the UK average. Results compared well with a later analysis of the same data set using a different analysis technique (HR, 2000). Hence, they have not been updated for the present study.

Results for additional joint return periods were inferred assuming the same dependence observed in EX 2133.

Table 4 represents the joint probability for return periods from 1 year to 1000 years. The wave conditions are offshore and their most likely direction is west-north-west. The water levels are at Liverpool.

ABCD 15 EX 4667 19/05/2008 Table 4 Joint occurrence of large offshore waves and high water levels results

Joint Exceedance High Water Level, Hs(Wave height), off Tp(Peak Wave Return Period (in years) Princes Pier (m CD) Rhyl (m) period), off Rhyl (s) 1 9.50 3.62 8.14 9.75 3.24 7.70 10.00 2.80 7.16 10.25 2.20 6.35 10.50 0.75 3.70 5 9.50 5.01 9.58 9.75 5.00 9.57 10.00 4.83 9.40 10.25 4.50 9.07 10.50 3.90 8.45 10.75 1.75 5.66 20 9.50 5.64 10.16 9.75 5.64 10.16 10.00 5.58 10.11 10.25 5.30 9.85 10.50 4.98 9.55 10.75 4.22 8.79 11.00 0.37 2.60 50 9.50 6.05 10.52 9.75 6.05 10.52 10.00 6.00 10.48 10.25 5.80 10.30 10.50 5.50 10.03 10.75 4.85 9.42 11.00 2.80 7.16 100 9.50 6.36 10.79 9.75 6.36 10.79 10.00 6.25 10.69 10.25 6.12 10.58 10.50 5.79 10.29 10.75 5.32 9.87 11.00 4.45 9.02 200 9.75 6.76 11.13 10.00 6.74 11.11 10.25 6.64 11.03 10.50 6.35 10.78 10.75 5.80 10.31 11.00 5.15 9.71 500 10.00 7.20 11.48 10.25 7.14 11.44 10.50 6.90 11.24 10.75 6.50 10.91 11.00 6.00 10.48 11.25 4.70 9.28 1000 10.25 7.60 11.80 10.50 7.42 11.66 10.75 7.00 11.32 11.00 6.40 10.83 11.25 5.45 9.99

ABCD 16 EX 4667 19/05/08

5.4 Nearshore sea conditions The extreme deep water sea conditions discussed above were transformed inshore as follows:

• Individual sea levels (at Liverpool) were replaced by site-specific sea levels with the same return periods. • Individual significant wave heights were replaced by the site-specific wave heights with the same return periods. • Wave periods were reproduced unchanged from offshore. The results from this analysis are discussed below.

Nearshore wave conditions Numerous studies were reviewed to extract information on the relationship between nearshore and offshore extreme wave conditions for the locations of interest. Wave directions generally change from west-north-west offshore to north-north-west nearshore. Wave periods are slightly changed between offshore and nearshore and were taken to be the same. Wave heights are reduced by between about 10% and 50% for different nearshore locations. The location of each point is listed in Table 5 and shown in Drawings 1 and 2, with predictions for each point given in Table 6.

Sea levels Nearshore extreme sea levels were taken from predictions in Section 3.2.9 of Environment Agency (2007), the relevant figures being reproduced below for convenience.

Location Event probability (% per year) and sea level (mAOD) 10% 4% 2% 1.33% 1% 0.5% 0.2% 0.1% Rhyl 5.30 5.50 5.61 5.71 5.81 6.00 6.12 6.29 Kinmel Bay 5.23 5.42 5.53 5.63 5.72 5.91 6.03 6.18 Colwyn Bay 5.04 5.22 5.32 5.41 5.50 5.67 5.78 5.93 Llandudno 5.05 5.21 5.32 5.40 5.48 5.65 5.75 5.89 Conwy 5.18 5.34 5.44 5.53 5.61 5.77 5.87 6.01

Kinmel Bay, Colwyn Bay and Llandudno are taken as being the same as Points A, D and F used in this study (see Table 5), and the extreme sea levels above are used directly. Conwy is taken as equivalent to the Conwy Estuary locations used in this study. Rhyl is introduced into this study following advice from Conwy CBC that the Environment Agency uses this point as representative of levels at Clwyd Bank. In compiling Table 7, Points B and C were taken at the one-third and two-thirds positions between Kinmel Bay and Colwyn Bay, Point E was taken at the one-third position between Colwyn Bay and Llandudno, and the 1 year return period levels were inferred from tidal levels and from the 10 year levels.

Joint occurrence of large nearshore waves and high water levels The content of Table 4 (off shore Joint probability) was re-worked for each near shore point in turn. Joint probability results for each nearshore location are listed in Table 8. In using Table 8 the following should be noted:

• Influence of local wave breaking All significant wave heights quoted in Table 8 are unbroken (since allowance for breaking is specific to the exact location and sea level being used). Wave breaking has therefore been taken account of in the overtopping and other defence response models used later in this report. However, for guidance, and for use outside of a coastal response model that includes the influence of breaking, the significant wave heights quoted above should be reduced to about 60% of the local water depth (e.g. at the toe of a structure) to allow for local depth-limitation of wave height.

• Use of multiple sea conditions - For each prediction point and each joint return period, a number of combinations of large wave height and high water level are quoted. Each is expected to be equalled or exceeded once, on average, in the specified return period. All such combinations should be considered

ABCD 17 EX 4667 19/05/2008 as potential ‘worst cases’. Exactly which condition proves to be worst may vary from one coastal response to another.

Table 5 Location of the nearshore wave prediction points

Nearshore Point Location Name (EA 2007) Name (EX 4667) Rhyl Use for Clwyd Bank A 3°31’54’’W, 53 °19’19’’N Kinmel Bay Kinmel Bay B 3°34’21’’W, 53 °18’08’’N ---- Pensarn C 3°39’19’’W, 53 °17’41’’N ---- Llanddulas Jetty D 3°43’24’’W, 53 °18’08’’N Colwyn Bay Victoria Pier E 3°44’50’’W, 53 °19’16’’N ---- Rhos on Sea F 3°49’20’’W, 53 °19’47’’N Llandudno Llandudno North shore G 3°50’24’’W, 53 °17’54’’N ---- Llandudno West shore Conwy Conwy Estuary

Table 6 Wave conditions for nearshore prediction points (without wave breaking)

Nearshore Point Conwy Estuary Return Offshore Point A Point B Point C Point D Point E Point F Point G Down Up Period stream Stream (years) Hs(m) Hs(m) Hs(m) Hs(m) Hs(m) Hs(m) Hs(m) Hs(m) Hs(m) Hs(m) Tm(s) Tm(s) Tm(s) Tm(s) Tm(s) Tm(s) Tm(s) Tm(s) Tm(s) Tm(s) 1 4.78 4.00 3.77 3.51 2.04 3.25 2.4 3.16 0.69 0.39 10.1 9.2 9.0 8.7 6.6 8.3 7.2 8.2 2 .0 2.0 10 5.56 4.74 4.46 4.13 2.27 3.84 2.86 3.68 0.78 0.44 10.9 10.1 9.8 9.4 7.0 9.1 7.8 8.9 2.1 2.1 50 6.11 5.26 4.95 4.57 2.43 4.24 3.18 4.04 0.86 0.48 11.4 10.6 10.3 9.9 7.2 9.5 8.2 9.3 2.2 2.2 100 6.33 5.48 5.15 4.75 2.5 4.4 3.32 4.2 0.89 0.50 11.6 10.8 10.5 10.1 7.3 9.7 8.4 9.5 2.2 2.2 200 6.57 5.70 5.36 4.94 2.57 4.57 3.46 4.36 0.93 0.52 11.8 11.0 10.7 10.3 7.4 9.9 8.6 9.6 2.2 2.2 500 6.88 6.00 5.64 5.19 2.66 4.80 3.64 4.56 0.97 0.54 12.1 11.3 11.0 10.5 7.5 10.1 8.8 9.9 2.3 2.3 1000 7.11 6.22 5.85 5.37 2.73 4.97 3.78 4.72 0.99 0.56 12.3 11.5 11.2 10.7 7.6 10.3 9.0 10.0 2.3 2.3

Table 7 Extreme sea levels for prediction points (mOD, present-day)

Return Rhyl Nearshore coastal points Conwy Estuary Period Point Point Point Point Point Point Down Up Point F (years) A B C D E G Stream Stream 1 4.84 4.77 4.72 4.66 4.60 4.61 4.63 4.56 4.76 4.76 10 5.30 5.23 5.17 5.11 5.04 5.04 5.05 4.98 5.18 5.18 50 5.61 5.53 5.46 5.39 5.32 5.32 5.32 5.24 5.44 5.44 100 5.81 5.72 5.65 5.58 5.50 5.49 5.48 5.41 5.61 5.61 200 6.00 5.91 5.83 5.75 5.67 5.66 5.65 5.57 5.77 5.77 500 6.12 6.03 5.95 5.87 5.78 5.77 5.75 5.67 5.87 5.87 1000 6.29 6.18 6.10 6.02 5.93 5.92 5.89 5.81 6.01 6.01

ABCD 18 EX 4667 19/05/08

Table 8 Joint probability wave and water level conditions for nearshore prediction points

Nearshore Point Return Point A Point B Point C Period (years) WL Hs Tm WL Hs Tm WL Hs Tm (mOD) (m) (s) (mOD) (m) (s) (mOD) (m) (s) 2.88 2.89 6.35 2.88 2.74 6.35 2.85 2.58 6.35 3.37 2.53 6.01 3.36 2,40 6.01 3.31 2.28 6.01 1 3.85 2.11 5.58 3.84 2.01 5.58 3.77 1.93 5.58 4.34 1.54 4.95 4.32 1.47 4.95 4.24 1.45 4.95 4.67 0.16 2.89 4.65 0.18 2.89 4.55 0.29 2.89 3.37 4.51 7.70 3.36 4.25 7.70 3.31 3.94 7.70 3.85 4.40 7.61 3.84 4.14 7.61 3.77 3.84 7.61 10 4.34 4.11 7.38 4.32 3.88 7.38 4.24 3.60 7.38 4.67 3.67 7.02 4.65 3.47 7.02 4.55 3.24 7.02 5.09 2.28 5.63 5.02 2.17 5.63 4.96 2.07 5.63 3.37 5.21 8.21 3.36 4.90 8.21 3.31 4.52 8.21 3.85 5.16 8.17 3.84 4.85 8.17 3.77 4.48 8.17 50 4.34 4.97 8.03 4.32 4.68 8.03 4.24 4.32 8.03 4.67 4.68 7.82 4.65 4.41 7.82 4.55 4.08 7.82 5.09 4.07 7.35 5.02 3.83 7.35 4.96 3.56 7.35 5.56 2.11 5.58 5.50 2.01 5.58 5.43 1.93 5.58 3.37 5.50 8.42 3.36 5.17 8.42 3.31 4.77 8.42 3.85 5.40 8.34 3.84 5.08 8.34 3.77 4.68 8.34 100 4.34 5.28 8.25 4.32 4.96 8.25 4.24 4.58 8.25 4.67 4.96 8.03 4.65 4.67 8.03 4.55 4.32 8.03 5.09 4.51 7.70 5.02 4.25 7.70 4.96 3.94 7.70 5.56 3.68 7.04 5.50 3.47 7.04 5.43 3.24 7.04 3.85 5.87 8.67 3.84 5.51 8.67 3.77 5.08 8.67 4.34 5.77 8.60 4.32 5.42 8.60 4.24 5.00 8.60 200 4.67 5.49 8.41 4.65 5.17 8.41 4.55 4.76 8.41 5.09 4.97 8.04 5.02 4.68 8.04 4.96 4.32 8.04 5.56 4.35 7.57 5.50 4.10 7.57 5.43 3.80 7.57 3.85 6.30 8.95 3.84 5.92 8.95 3.77 5.44 8.95 4.34 6.25 8.92 4.32 5.87 8.92 4.24 5.40 8.92 500 4.67 6.02 8.77 4.65 5.65 8.77 4.55 5.20 8.77 5.09 5.64 8.51 5.02 5.30 8.51 4.96 4.88 8.51 5.56 5.16 8.17 5.50 4.85 8.17 5.43 4.48 8.17 6.05 3.92 7.24 5.98 3.70 7.24 5.89 3.44 7.24 4.34 6.68 9.20 4.32 6.28 9.20 4.24 5.76 9.20 4.67 6.51 9.09 4.65 6.12 9.09 4.55 5.62 9.09 1000 5.09 6.11 8.83 5.02 5.74 8.83 4.96 5.28 8.83 5.56 5.54 8.45 5.50 5.21 8.45 5.43 4.80 8.45 6.05 4.64 7.79 5.98 4.36 7.79 5.89 4.04 7.79

ABCD 19 EX 4667 19/05/2008 Table 8 Joint probability wave and water level conditions for nearshore prediction points (continued)

Nearshore Point Return Point D Point E Point F Period (years) WL Hs Tm WL Hs Tm WL Hs Tm (mOD) (m) (s) (mOD) (m) (s) (mOD) (m) (s) 2.82 1.70 6.35 2.82 2.44 6.35 2.79 1.71 6.35 3.28 1.58 6.01 3.26 2.17 6.01 3.22 1.49 6.01 1 3.73 1.45 5.58 3.71 1.85 5.58 3.65 1.23 5.58 4.18 1.28 4.95 4.15 1.41 4.95 4.08 0.87 4.95 4.49 0.85 2.89 4.48 0.37 2.89 4.49 0.02 2.89 3.28 2.20 7.70 3.26 3.67 7.70 3.22 2.72 7.70 3.73 2.16 7.61 3.71 3.58 7.61 3.65 2.65 7.61 10 4.18 2.08 7.38 4.15 3.37 7.38 4.08 2.47 7.38 4.49 1.94 7.02 4.48 3.03 7.02 4.49 2.20 7.02 4.90 1.51 5.63 4.93 1.98 5.63 4.91 1.33 5.63 3.28 2.41 8.21 3.26 4.20 8.21 3.22 3.15 8.21 3.73 2.40 8.17 3.71 4.16 8.17 3.65 3.12 8.17 50 4.18 2.34 8.03 4.15 4.02 8.03 4.08 3.00 8.03 4.49 2.25 7.82 4.48 3.80 7.82 4.49 2.83 7.82 4.90 2.06 7.35 4.93 3.33 7.35 4.91 2.44 7.35 5.38 1.45 5.58 5.37 1.85 5.58 5.34 1.23 5.58 3.28 2.51 8.42 3.26 4.42 8.42 3.22 3.33 8.42 3.73 2.47 8.34 3.71 4.34 8.34 3.65 3.27 8.34 100 4.18 2.44 8.25 4.15 4.25 8.25 4.08 3.19 8.25 4.49 2.34 8.03 4.48 4.01 8.03 4.49 3.00 8.03 4.90 2.20 7.70 4.93 3.67 7.70 4.91 2.72 7.70 5.38 1.94 7.04 5.37 3.04 7.04 5.34 2.20 7.04 3.73 2.62 8.67 3.71 4.70 8.67 3.65 3.56 8.67 4.18 2.59 8.60 4.15 4.62 8.60 4.08 3.50 8.60 200 4.49 2.50 8.41 4.48 4.41 8.41 4.49 3.33 8.41 4.90 2.34 8.04 4.93 4.02 8.04 4.91 3.00 8.04 5.38 2.15 7.57 5.37 3.55 7.57 5.34 2.62 7.57 3.73 2.75 8.95 3.71 5.03 8.95 3.65 3.83 8.95 4.18 2.74 8.92 4.15 4.98 8.92 4.08 3.80 8.92 500 4.49 2.67 8.77 4.48 4.81 8.77 4.49 3.65 8.77 4.90 2.55 8.51 4.93 4.52 8.51 4.91 3.42 8.51 5.38 2.40 8.17 5.37 4.16 8.17 5.34 3.12 8.17 5.80 2.02 7.24 5.82 3.22 7.24 5.77 2.35 7.24 4.18 2.87 9.20 4.15 5.32 9.20 4.08 4.07 9.20 4.49 2.82 9.09 4.48 5.19 9.09 4.49 3.96 9.09 1000 4.90 2.70 8.83 4.93 4.88 8.83 4.91 3.71 8.83 5.38 2.52 8.45 5.37 4.45 8.45 5.34 3.36 8.45 5.80 2.24 7.79 5.82 3.76 7.79 5.77 2.80 7.79

ABCD 20 EX 4667 19/05/08

Table 8 Joint probability wave and water level conditions for nearshore prediction points (continued)

Nearshore Point Conwy Estuary Return Point G Downstream bridge Upstream bridge Period (years) WL Hs Tm WL Hs Tm WL Hs Tm (mOD) (m) (s) (mOD) (m) (s) (mOD) (m) (s) 2.53 2.38 6.35 2.73 0.52 1.55 2.73 0.29 1.55 2.96 2.13 6.01 3.16 0.46 1.47 3.16 0.26 1.47 1 3.39 1.84 5.58 3.59 0.40 1.36 3.59 0.23 1.36 3.82 1.43 4.95 4.02 0.31 1.21 4.02 0.18 1.21 4.34 0.46 2.89 4.54 0.10 0.71 4.54 0.06 0.71 2.96 3.52 7.70 3.16 0.75 1.80 3.16 0.42 1.80 3.39 3.44 7.61 3.59 0.73 1.78 3.59 0.41 1.78 10 3.82 3.24 7.38 4.02 0.69 1.73 4.02 0.39 1.73 4.34 2.93 7.02 4.54 0.62 1.64 4.54 0.35 1.64 4.84 1.96 5.63 5.04 0.42 1.32 5.04 0.24 1.32 2.96 4.01 8.21 3.16 0.85 1.89 3.16 0.48 1.89 3.39 3.98 8.17 3.59 0.84 1.88 3.59 0.47 1.88 50 3.82 3.84 8.03 4.02 0.81 1.85 4.02 0.46 1.85 4.34 3.64 7.82 4.54 0.77 1.80 4.54 0.43 1.80 4.84 3.21 7.35 5.04 0.68 1.69 5.04 0.38 1.69 5.27 1.84 5.58 5.47 0.39 1.28 5.47 0.22 1.28 2.96 4.22 8.42 3.16 0.89 1.91 3.16 0.50 1.91 3.39 4.14 8.34 3.59 0.88 1.89 3.59 0.49 1.89 100 3.82 4.06 8.25 4.02 0.86 1.87 4.02 0.48 1.87 4.34 3.83 8.03 4.54 0.81 1.82 4.54 0.46 1.82 4.84 3.52 7.70 5.04 0.75 1.75 5.04 0.42 1.75 5.27 2.94 7.04 5.47 0.62 1.60 5.47 0.35 1.60 3.39 4.47 8.67 3.59 0.95 1.94 3.59 0.53 1.94 3.82 4.40 8.60 4.02 0.93 1.93 4.02 0.52 1.93 200 4.34 4.21 8.41 4.54 0.89 1.88 4.54 0.50 1.88 4.84 3.84 8.04 5.04 0.81 1.80 5.04 0.46 1.80 5.27 3.41 7.57 5.47 0.72 1.70 5.47 0.41 1.70 3.39 4.78 8.95 3.59 1.01 1.98 3.59 0.57 1.98 3.82 4.74 8.92 4.02 1.00 1.97 4.02 0.56 1.97 500 4.34 4.58 8.77 4.54 0.97 1.94 4.54 0.55 1.94 4.84 4.31 8.51 5.04 0.91 1.88 5.04 0.51 1.88 5.27 3.98 8.17 5.47 0.84 1.81 5.47 0.47 1.81 5.70 3.11 7.24 5.90 0.66 1.60 5.90 0.37 1.60 3.82 5.04 9.20 4.02 1.07 1.09 4.02 0.60 1.09 4.34 4.92 9.09 4.54 1.04 1.07 4.54 0.59 1.07 1000 4.84 4.64 8.83 5.04 0.98 1.04 5.04 0.55 1.04 5.27 4.24 8.45 5.47 0.90 1.00 5.47 0.50 1.00 5.70 3.61 7.79 5.90 0.77 0.92 5.90 0.43 0.92

5.5 Water levels and waves in Conwy estuary Water levels in the study area of the Conwy estuary are dominated by tidal levels. Field data from a previous HR report (EX 1251) showed that high water level in the downstream area and the upstream area

ABCD 21 EX 4667 19/05/2008 of the Conwy bridge were the same, both during spring tide and neap tide; which are relevant to the extreme water levels with return periods of 0.04 year and 35 years.

Data collected from an HR previous study (EX 651) showed that mean annual flow and mean annual flood are 18.5 m 3/s and 450 m 3/s, respectively; the 100 year and 350 year floods are 920 m 3/s and 1150 m 3/s, respectively. Observed data on 11 December 1973 showed that the maximum instantaneous tidal flow through the bridge section on the flood tide was 2500 m 3/s. The study considered that the astronomical tide alone controls high water level in the estuary, that is, river inflow is not stored during the flood tide. Hence, it is considered that at high water levels, water levels in the study areas both up stream and down stream of the Conwy bridge are the same.

With mean high water spring data in and out of the estuary, published on Admiralty Chart Tables (Conwy and Llandudno), it is estimated that the extreme water levels in the estuary should be about 20 cm higher than the extreme water levels at Point G. With the limited data to hand, it is difficult to tell the differences of water levels between the four prediction points. So, they are considered to be same values.

Environment Agency (2007) gives two different locations for its “Conwy” sea level prediction point. In Version1 of this note, the open coast location was assumed. Following discussion between Conwy CBC and HR Wallingford and confirmation (Andrew Wilkinson, personal communication, 9 October 2007) that the anecdotal evidence of 5.5mAOD sea levels at Conwy in Environment Agency (2007) could be disregarded, this was changed. The Environment Agency (2007) “Conwy” sea level point is now assumed to be representative of Conwy Estuary, and the open coast sea levels to be 20 cm lower than this.

Waves downstream of the Conwy bridge are composed of both waves entering the estuary from offshore and local wind generated waves. Wave conditions upstream of the bridge are decided mainly by local generated waves. The wave conditions are decided based on results from previous HR reports (EX1532, EX1526). With the data we have, it is difficult to tell the difference between the four predicted locations; just dividing them into two areas; i.e. the upstream and downstream areas of the bridge. Results within each area are considered to be the same throughout that area.

In the absence of specific local data in the estuary, the dependence of high waves and water levels in the estuary is assumed as the dependence at coastal points.

5.6 Future sea level rise and wave climate change Sea level rise Defra (2006) recommends an allowance for future mean sea level rise for the North Wales coast of 3.5, 8.0, 11.5 and 14.5mm/year for the periods 1990-2025, 2025-2055, 2055-2085 and 2085-2115, respectively. There is no specific recommendation for future extreme sea level, but this is usually taken to be the same as for mean level.

All sea levels quoted in this section are best estimates of present-day conditions. Before use in calculations representing future conditions, water levels should be increased appropriately for the relevant period into the future. For example, to represent sea level rise over 50 years (relative to the 2006 values given in Environment Agency, 2007) add 0.318m to all present-day sea levels, or for 100 years add 0.956m.

Wave climate - Inter-annual variability in the deep water wave conditions The uncertainties involved in the prediction of future wave climate change are too great to attempt a “prediction” of future wave conditions. However, it is useful to consider inter-annual variability and use this to assist in the consideration of future wave climate scenarios. Two offshore data sets were therefore analysed to assess the trend (if any) that can be observed from the past wave climate. The first set covers a period from January 1970 to January 1993. The second set of data covers the period from January 1987 to December 2001. Although the two sets are each internally consistent they cannot be considered as one as they are not exactly at the same location. The analysis consists of calculating the average of the wave

ABCD 22 EX 4667 19/05/08

heights for the 10% and 1% exceedance levels for a period of 5 years (a five year rolling average is considered to smooth the annual variability and highlight any underlying trend).

Using the first set of data from 1970 to 1993 (See Figure 3) the following can be deduced:

Trend • +5% increase in the height of the 10% exceedance level in 18 years • +7% increase in the height of the 1% exceedance level in 18 years Largest • +6.8% above the average 10% exceedance level in 18 years • +3.8% above the average 1% exceedance level in 18 years Smallest • -5.7% below the average 10% exceedance level in 18 years • -7.0% below the average 1% exceedance level in 18 years 90% bounds • from-4.6% to +4% variation from average 10% exceedance level in 18 years • from –6.5% to +2.4% variation from average 1% exceedance level in 18 years

Using the second set of data from 1987 to 2001 (see Figure 4) the following can be deduced:

Trend • -13.5% increase in the height of the 10% exceedance level in 10 years • -22.5% increase in the height of the 1% exceedance level in 10 years Largest • +7.7% above the average 10% exceedance level in 10 years • +12% above the average 1% exceedance level in 10 years Smallest • -5.1% below the average 10% exceedance level in 10 years • -11.7% below the average 1% exceedance level in 10 years 90% confidence • from-4.4% to +7.6% variation from average 10% exceedance level in 18 years • from –9.5% to +8.6% variation from average 1% exceedance level in 18 years

Note - It is important to realise that the storms of early 1990 have an excessive weight in this comparison. Without those storms, the trend is close to 0 for both sets of data with the first set showing a slightly upward trend in wave height and the second set showing a slightly downward trend.

It can be concluded from this analysis, that:

• The period of the data used in this study is a reasonably representative sample • Any overall trend from 1970-2000 is small • The natural variability amongst the 5 years blocks of data is of the order of ± 10%. Therefore + 10% would be a realistic sensitivity allowance for future wave height change. (Hence, + 5% on wave period to retain the same wave steepness). This is now recommended in Defra (2006) as a sensitivity allowance to possible future wave climate change.

ABCD 23 EX 4667 19/05/2008 ABCD wave Average conditions 5yearsfor for the first set data of Figure 3

Figure 3 Average wave conditions for 5 years for the first set of data

8 10% exceedance 6 1% exceedance 4

2

0

24 -2

-4

of the whole record (Hs) record whole the of -6

Percentage deviation from the mean mean the from deviation Percentage -8 70 72 74 76 78 80 82 84 86 88 90 92 Time (central year of the 5 year moving average)

EX 4667 19/05/084667 EX

ABCD wave Average conditions 5yearsfor for the second of dataset Figure 4

Figure 4 Average wave conditions for 5 years for the second set of data

15 10%exceedance 10 1% exceedance 5

0

25 -5

-10

-15 Percentage deviation from the the from Percentagedeviation mean of the whole record (Hs) whole the of mean -20

89 91 93 95 97 99 Time (central year of the 5 year moving average).

EX 4667 19/05/2008 EX

6. TASK 5: OVERTOPPING AND BREACH PREDICTIONS FOR EACH IDENTIFIED DEFENCE

6.1 Introduction Under this task, two primary failure modes have been considered:

• Overtopping (non-structural failure) • Breaching (structural failure).

The analysis of both of these failure modes is discussed below together with results.

6.2 Analysis of the defence overtopping For each identified coastal defence length, an upper and lower bound estimate of overtopping discharge has been predicted for a range of joint return period events over a tidal cycle. The upper estimates were calculated using the lowest wall crest level and beach level. The lower estimates were calculated using the highest wall crest level and beach level.

The results of this analysis are provided as present day ‘mean’ discharge rates in Tables 9a and 9b and ‘peak’ overtopping volumes in Table 10. Tables 11a and 11b show lower and upper bound ‘mean’ discharges for the year 2056 whilst Tables 11c and 11d show that in 2106. Tables 12a and 12b provide ‘peak’ overtopping volumes in 2056 and 2106. The estimated overtopping was computed based on the equations and approaches outlined in Besley (1999) modified where necessary with additional site-specific information to enhance the reliability of the overtopping predictions.

A spreadsheet based overtopping model, based upon the Besley (1999), has been developed using the VBA programming language in excel. It is a spreadsheet based and incorporates all loads and geometric defence details to enable overtopping discharges to be predicted. A brief description of the overtopping spreadsheet is given in Appendix 5.

To Calculate the overtopping (i.e. overflow) for the Clwyd river, the broad crested weir equation was used (assuming that the effect of waves in the river is negligible) as follows:

where: Q : Discharge (m3/s). Cw : Coefficient equals 1.7. B : Embankment length (m). H : Total water head over the crest of the embankment (m).

Volume was calculated assuming a triangular hydrograph with a peak value calculated using the above equation.

ABCD 26 EX 4667 19/05/08

Table 9a Present day “mean” overtopping results – Defence by defence (Lower Bound)

ABCD 27 EX 4667 19/05/08 Table 9b Present day “mean” overtopping results – Defence by defence (Upper Bound)

ABCD 28 EX 4667 19/05/08

Table 10 Present day “peak” overtopping results – Defence by defence

ABCD 29 EX 4667 19/05/08 Table 11a “Mean” overtopping results by 2056 - Defence by defence (Lower Bound)

ABCD 30 EX 4667 19/05/08

Table 11b “Mean” overtopping results by 2056 – Defence by defence (Upper Bound)

ABCD 31 EX 4667 19/05/08

Table 11c “Mean” overtopping results by 2106 – Defence by defence (Lower Bound)

ABCD 32 EX 4667 19/05/08

Table 11d “Mean” overtopping results by 2106 – Defence by defence (Upper Bound)

ABCD 33 EX 4667 19/05/08 Table 12a “Peak” overtopping results by year 2056 – Defence by defence

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Table 12b “Peak” overtopping results by year 2106 – Defence by defence

ABCD 35 EX 4667 19/05/08

6.3 Assessment of defence breach probability With regard to more catastrophic failure modes, breach probabilities are difficult to predict. Predicting breach growth and maximum size is equally problematic and at present beyond the capabilities of existing numerical tools. However, breach events represent the most significant of flood scenarios and are of considerable importance in determining flood risk. Therefore, for this study the following approach was used:

• Event breach probability - Defence fragility tables, relating load to breach probability, were developed for each identified defence length based upon the condition survey undertaken in Task 2. This enabled event breach probability to be determined.

• Breach size and invert level – Equally important as determining the likelihood of breach is to determine the likely extent and invert level should a breach occur. This was done through evidence based reasoning and consensus within the project team utilising the evidence from historical records.

6.3.1 Defence breach probability In considering the likelihood of breach, the critical failure modes outlined in the cause-consequence diagrams and summarised in Table 1 have been used as an indicator of the probability of breaching. The following criteria have been used to determine the likelihood of failure associated with each dominant failure mode:

1. Overtopping Damage caused by overtopping water to the promenade or the rear of the structure. The overtopping spreadsheet (See Section 6.2) was used to estimate the overtopping rates for each defence then the following criteria (Besley, 1999) were used to estimate the probability of breaching:

• For an embankment:

p=0 Q ≤ 0.002 (m 3/s/m) p=0.1 0.002 (m 3/s/m)< Q ≤ 0.05 (m 3/s/m) p=0.2 0.05 (m 3/s/m) < Q ≤ 0.5 (m 3/s/m) p=0.3 0.5 (m 3/s/m) < Q ≤ 5 (m 3/s/m) p=0.3 Q > 5 (m 3/s/m)

• For a revetment:

p=0 Q ≤ 0.05 (m 3/s/m) p=0.1 0.05 (m 3/s/m) < Q ≤ 0.2 (m 3/s/m) p=0.2 0.2 (m 3/s/m) < Q ≤ 2 (m 3/s/m) p=0.3 2 (m 3/s/m) < Q ≤ 20 (m 3/s/m) p=0.3 Q >20 (m 3/s/m) where: Q = Overtopping flow (m 3/s/m) p = Probability of defence failure and breach

ABCD 33 EX 4667 19/05/2008 2. Crest retreat Breaching due to the retreat of the beach crest in front of the defence. The HR Wallingford SHINGLE model was used to determine the crest retreat of the beach. The following criteria were then used to estimate the probability of breaching:

p=0 CR ≤ 30 % p=0.1-0.2 30 % < CR ≤ 60 % p=0.2-0.6 60 % < CR ≤ 100 % where: CR = Crest retreat as a percentage of the initial crest width. p = Probability of defence failure and breach

3. Overturning \ collapse of the structure Scour at the toe of the structure can undermine its foundation and reduce its factor of safety against overturning and finally lead to its collapse. The HR Wallingford SCOUR model was used to determine the scour depth at the toe of the structure and criteria similar to that used in the crest retreat failure mode was used for this failure mode as follows:

p=0 Sc ≤ 0.5

p=0.05 0.5 < S c ≤ 1 p=0.2 1 < S c ≤ 2 p=0.4 Sc > 2 where: Sc = Scour depth (m). p = Probability of defence failure and breach

4. Damage to rock armour Damage of the rock armour was evaluated using Van der Meer’s equation (Simm, 1991). Van der Meer’s equation enables the likely degree of damage to be calculated through the stability parameter(S d). S d has been related to the probability of failure (p) as shown in the following table:

Table 13 Sd values vs. the probability of failure Revetment p=0 p=0.15 p=0.3 Slope 1:2 < 3 3-7 > 7 1:3 < 4 4-10 > 10 1:3.5 < 5 5-12 > 12 1:4 to 1:10 < 6 6-14 > 14

5. Wave run up Wave run up over the dunes can lower the dune crest and finally lead to breaching. In the absence of a criterion to quantify the probability of failure for dunes, expert judgement has been therefore used to relate the run-up limit (R u)to the likelihood of crest lowering and hence the probability of failure (p).

p=0 Ru ≤ 7 mOD p=0.05 7 mOD < R u ≤ 8 mOD p=0.2 Ru > 8 mOD

6. Piping The pressure head difference across a flood embankment can lead to piping (i.e. water moves freely through the body of the defence) and finally breaching of the whole embankment (Mohamed, 2002). This process is quite complex and is still an active area of research. It can also occur concurrently with overflow

ABCD 34 EX 4667 19/05/08

and \ or slip failures. Combined with the lack of detailed data, the following criteria have therefore been used to quantify the effect of piping (Terzaghi et al, 1996): B 3 + ∑t C = w H where: C w = Weighted creep ratio (For critical values See Table 14). B = Width of the structure. t = Depth of impervious layers or sheet piles below the embankment H = Pressure head difference across the embankment.

To quantify the overflow effect, the broad crested weir equation (See Section 6.2) has been used.

Table 14 Critical values of C w (Terzaghi et al, 1996)

Embankment Material Critical C w (C wr ) Very fine Sand or Silt 8.5 Fine Sand 7 Medium Sand 6 Coarse Sand 5 Fine Gravel 4 Medium Gravel 3.5 Coarse Gravel and cobbles 3 Boulders 2.5

Using the values listed in the above table, Cw can be related to the probability of failure (p) with and without overflow as follows:

p=0 Cw > 1.25 C wr

p=0.05 Cwr < C w ≤ 1.25 C wr p=0.1 Cw ≤ C wr or Overflow occurs

p=0.2 Cw ≤ C wr with associated overflow and / or slip failures

To evaluate the above range of critical failure modes a number of different models have been applied. The models used for each defence length are outlined in the Table 15. The results of this analysis for each identified defence length are provided in Appendix 6. The resulting annual breach probabilities are summarised in Table 16.

ABCD 35 EX 4667 19/05/2008 Table 15 Summary of structural failure modes and associated modelling approach

Defence Length Model Llandudno West Shore 1A Overtopping failure (Overtopping spreadsheet) 1B Crest retreat of shingle beach in front of defences (SHINGLE model) 1C NA 1D Crest retreat of shingle beach in front of defences (SHINGLE model) 1E NA 1F Crest retreat of shingle beach in front of defences (SHINGLE model) 1G Overtopping damage to rear of structure (Overtopping spreadsheet)

Llandudno North Shore 2A Overtopping damage to promenade (Overtopping spreadsheet) 2B Overtopping damage to promenade (Overtopping spreadsheet) 2C NA 2D Crest retreat of shingle beach in front of defences (SHINGLE model) 2E Crest retreat of shingle beach in front of defences (SHINGLE model) 2F Overturning/ Collapse as a result of lowering beach levels in front of hard defence (SCOUR model) 2G Overturning/Collapse as a result of lowering beach levels in front of hard defence (SCOUR model) 2H Overtopping damage to rear of defences (Overtopping spreadsheet) 2I Overtopping damage to earth bank behind shingle crest or Crest retreat of shingle beach in front of defences (SHINGLE model) 2J Crest retreat of shingle beach in front of defences (SHINGLE model) or undermining leading to failure (SCOUR model)

Penrhyn Bay/Rhos-on-Sea 3A Overturning/Breaching as a result of lowering beach levels in front of hard defence (SCOUR model) 3B NA 3C Overtopping damage to rear of structure (Overtopping spreadsheet) 3D Damage to rock armour facing leading to increased overtopping damage to rear structure (Van der Meer equation) 3E Breakwater failure leading to increased overtopping damage to rear of secondary structure (Van der Meer equation) 3F Overtopping damage to crest of structure (Overtopping spreadsheet) Pensarn to Kinmel Bay 4A Crest retreat of shingle beach in front of defences (SHINGLE model) 4B Crest retreat of shingle beach in front of defences (SHINGLE model) 4C Overtopping damage to crest of structure (Overtopping spreadsheet) 4D Damage to rock armour facing leading to failure of defences (van der Meer equation) 4E Crest retreat of shingle beach in front of defences (SHINGLE model) 4F Damage to rock armour facing leading to failure of defences (Van der Meer equation) 4G Damage to rock armour facing leading to failure of defences (Van der Meer equation) 4H Crest retreat of shingle beach in front of defences (SHINGLE model) 4I Crest retreat of shingle beach in front of defences (SHINGLE model)

ABCD 36 EX 4667 19/05/08

Table 15 Summary of structural failure modes and associated modelling approach (continued)

Defence Length Model Pensarn to Kinmel Bay (continued) 4J Damage to rock armour facing leading to failure of defences (Van der Meer equation) 4K Run up over dunes causing lowering of crest (expert judgement) 4L Run up over dunes causing lowering of crest (expert judgement) 4M No structural failure condition 4N NA

River Clwyd 5A Head difference across flood bank causing destabilization by overflow (Broad crested weir equation) 5B Head difference across flood bank causing destabilization by overflow (Broad crested weir equation) 5C Head difference across flood bank causing destabilisation by piping (Creep ratio equation) and overflow (Broad crested weir equation). 5D1 Head difference across flood bank causing destabilisation by piping (Creep ratio equation) and overflow (Broad crested weir equation). 5D2 Head difference across flood bank causing destabilisation by piping (Creep ratio equation) and overflow (Broad crested weir equation). 5E Head difference across flood bank causing destabilisation by piping (Creep ratio equation) and overflow (Broad crested weir equation). 5F Head difference across flood bank causing destabilisation by piping (Creep ratio equation) and overflow (Broad crested weir equation).

Glan Conwy 6A Overtopping damage to rear of structure (Overtopping spreadsheet) Conwy Quay 6B Overtopping damage to rear of structure (Overtopping spreadsheet) 6C Overtopping damage to rear of structure (Overtopping spreadsheet) 6D Overtopping damage to rear of structure (Overtopping spreadsheet) Deganwy 6E Overtopping damage to rear of structure (Overtopping spreadsheet) 6F Overtopping damage to rear of structure (Overtopping spreadsheet) Overtopping damage to rear of structure (Overtopping spreadsheet) 6G Crest retreat of shingle beach in front of defences (SHINGLE model) Overtopping damage to rear of structure (Overtopping spreadsheet) 6H Crest retreat of shingle beach in front of defences (SHINGLE model)

6.3.2 Breach size and invert level Determining the likely extent and invert level of the breach for each defence was done through a combination of the following methods:

• The HR Breach Model, when applicable, was used to model the breach failure and predict the breach width and depth • Evidence based reasoning and consensus utilising the evidence from historical records for coastal defences which showed that for soft defences breach length would in the range of about 80% of the whole defence and for hard defences the ratio would be 20% • The visual inspection of the defences undertaken in Task 2 allowed the identification of hard defence foundation levels which was considered as the ultimate invert of the breach.

ABCD 37 EX 4667 19/05/2008 Table 16 Summary of expected defence annual breach probabilities and breach dimensions Location Expected Annual Probability Breach Invert (mOD) Breach Length (m) Llandudno West Shore 1A 0.01 2 148 1B 0.31 5.9 30 1C < 0.01 NA NA 1D 0.12 5.9 106 1E < 0.01 NA NA 1F 0.22 5.9 9 1G 0.01 5.5 20 Llandudno North Shore 2A 0.01 7 19 2B < 0.01 6.3 12 2C < 0.01 6.3 5 2D 0.06 5.5 170 2E 0.01 7 108 2F 0.01 7 28 2G 0.01 7 36 2H < 0.01 6.4 29 2I < 0.01 6.5 158 2J 0.12 6 19 Penrhyn Bay/Rhos-on-Sea 3A < 0.01 5.5 84 3B < 0.01 NA NA 3C 0.02 5.5 8 3D < 0.01 7 110 3E < 0.01 6 50 3F 0.10 5.9 48 Pensarn to Kinmel Bay 4A 0.01 6.1 248 4B 0.11 5.9 194 4C < 0.01 5.7 152 4D < 0.01 7 145 4E < 0.01 5.9 60 4F < 0.01 7 240 4G 0.08 6.9 66 4H 0.06 6.1 76 4I 0.12 6.3 144 4J 0.17 6.9 66 4K < 0.01 5 60 4L < 0.01 5 16 4M < 0.01 5.4 56 4N < 0.01 NA NA River Clwyd 5A < 0.01 5.65 5-25 5B < 0.01 5.65 5-25 5C 0.06 4.2 5-25 5D1 0.06 3.5 5-25 5D2 0.06 4.2 5-25 5E 0.06 4.7 5-25 5F 0.06 4.4 5-25

ABCD 38 EX 4667 19/05/08

Table 16 Summary of Expected defence annual breach probabilities and breach dimensions (continued)

Location Expected Annual Probability Breach Invert (mOD) Breach Length (m) Glan Conwy 6A 0.01 3.8 195 Conwy Quay 6B 0.17 5 20 6C 0.17 5 35 6D < 0.01 6 13 Deganwy 6E 0.12 5.5 140 6F 0.02 5 106 6G 0.17 5 94 6H 0.01 5.3 55

ABCD 39 EX 4667 19/05/2008 7. TASK 6: FUTURE MAINTENANCE

The principal elements of the future monitoring and maintenance plan are outlined, defence by defence, in Appendix 4 and include the following generic measures:

• Regular collection of beach monitoring data • Management of the existing beach resource, subject to analysis of monitoring data • Inspection of the condition of hard defences and identification of defects • Repair of defects and appropriate structure maintenance.

ABCD 40 EX 4667 19/05/08

8. CONCLUSIONS

A detailed review of the coastal defences boarding flood plains has been undertaken. This has been supplemented by a review of past flood events and analysis of waves and water levels. The results of these studies provide a sound basis for undertaking detailed assessment of flood inundation proposed for stage 2, and highlight the following defences as likely to be the most important in terms of their flood defence function. The selection is based on the computed overtopping rates and the probability of defence failure and breach for the 100-year event.

As shown in Table 17, two groups of defences were identified. Group 1 defences have a higher priority than Group 2 defences. This is due to the short length of Group 2 defences compared to Group 1 or low overtopping.

Table 17 Defence priority groups

Code Defence Location Area Overtopping Probability Length (litre/s/m) of Failure (m) Group 1 Priority Defences 1A North Wales Golf Club (1) Llandudno West Shore 230 0.2 369 1B Llandudno West Promenade (1) Llandudno West Shore 1 0.4 148 2J Colwyn Road, Craigside Llandudno North Shore 93 0.4 94 3A Penrhyn Bay (2) Penrhyn Bay/Rhos-on-Sea 57 0.05 421 3F Llandrillo-yn-Rhos Penrhyn Bay/Rhos-on-Sea 170 0.10 244 5C River Clwyd 5C River Clwyd 17 0.2 218 5D1 River Clwyd 5D1 River Clwyd 25 0.2 1140 5D2 River Clwyd 5D2 River Clwyd 25 0.2 1725 5F River Clwyd 5F River Clwyd 25 0.2 631 5E River Clwyd 5E River Clwyd 25 0.2 469 Group 2 Priority Defences 1G Gogarth Wall Llandudno West Shore 40 0.1 97 2A Llandudno Promenade (1) Llandudno North Shore 0 0.1 95 3C Penrhyn Bay (1) Penrhyn Bay/Rhos-on-Sea Near 0.3 42 Overflow 4I Kinmel Bay 1 (C) Pensarn to Kinmel Bay 0 0.4 718 4J Kinmel Bay 1 (E) Pensarn to Kinmel Bay 0 0.3 593

ABCD 41 EX 4667 19/05/2008 9. REFERENCES

1 Besley, P. (1999). Overtopping Of Seawalls: Design and Assessment Manual. Environment Agency (TR W178).

2 Dawson, R.J., Hall, J.W., Sayers, P.B. and Bates, P.D. (2003). Flood risk assessment for Shoreline Management Planning, in Proc. Int. Conf. Coastal Management , pp83-97, Brighton.

3 Defra (2006). Flood and coastal defence appraisal guidance: FCDPAG3: Economic appraisal: Supplementary note to operating authorities: Climate change impacts October 2006. Defra website, http://www.defra.gov.uk/environ/fcd/pubs/pagn/Climatechangeupdate.pdf.

4 Environment Agency (2007). North Wales tidal flood mapping: Phase II report. Version 1 of a report by Atkins to the Environment Agency, dated 16 March 2007, and provide for use in this update of EX 4667.

5 HR Wallingford and Lancaster University (2000). The joint probability of waves and water levels: JOIN-SEA: A rigorous but practical new approach. HR Wallingford Report SR 537.

6 HR Wallingford Report EX 2367.

7 HR Wallingford (1974). A55 Trunk Road Improvement, Hydrographic survey and hydraulic investigation of a bridge crossing near Deganwy. Report EX 651.

8 HR Wallingford (1985). Conwy Estuary Crossing Field data collected by Hydraulics Research. Report EX 1251.

9 HR Wallingford (1986). Conwy Estuary Crossing, Waves at the reclamation areas. HR Wallingford Report EX 1526.

10 HR Wallingford (1986). Conwy Estuary Crossing Waves at the Reclamation Areas. Report EX 1526.

11 HR Wallingford (1987). Conwy Estuary Crossing Waves at the Deganwy Jetty. Report EX 1532.

12 HR Wallingford (1990). Joint probability of waves and water levels on the North Wales coast. HR Wallingford Report EX 2133.

13 HR Wallingford (1991). Colwyn Borough Sea Defence Review.

14 HR Wallingford (1993). Mobile bed physical model study of proposed developments. HR Wallingford Report EX 2754.

15 MAFF (1999). Flood and Coastal defence project appraisal guidance: Volume 3: Economic appraisal. MAFF publication FCDPAG3.

16 Mohamed, M. (2002). Embankment Breach Formation and Modelling Methods. PhD. Thesis, the Open University.

17 Proudman Oceanographic Laboratory (1997). Estimates of extreme sea conditions: Spatial analyses for the UK coast. POL Internal Document No 112.

18 Simm, J. (1991). Manual on the use of rock in coastal and shoreline engineering. CIRIA special publication 83. CUR report 154.

ABCD 42 EX 4667 19/05/08

19 Terzaghi, K., Peck, R.B. and Mesri, G. (1996). Soil Mechanics in Engineering Practice. 3 rd Edition, John Wiley & Sons Inc., New York.

ABCD 43 EX 4667 19/05/2008 Appendices

ABCD EX 4667 19/05/08

ABCD EX 4667 19/05/2008 Appendix 1

Identified Data and Sources

ABCD EX 4667 19/05/08

ABCD EX 4667 19/05/2008 ABCD Appendix 1 Identified Data and Sources

Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By

REPORTS 1 13 Report on Review of Coast 1983 Scott Wilson, Kirkpatrick and Hard Copy CCBC CECS Protection (Green Cover Report) Partners 2 20 Report on Coast Protection Flood 1982 Scott Wilson, Kirkpatrick and Hard Copy CCBC CECS Study (White Cover Report) Partners 3 27 Aberconwy Coastal Study 1987 BMT Hard Copy CCBC CECS Summary Report (White Cover Report) 4 28 North Shore Coastal Works - 1994 Shoreline Management Hard Copy CCBC CECS Craig y Don Scheme Justification Partnership (Cream Cover Report) 5 35 Aberconwy Coastal Study Final 1987 BMT Hard Copy CCBC CECS Report Part 3 (Blue Cover Report) 6 36 North Shore Llandudno Mobile 1993 HR Wallingford Hard Copy CCBC CECS Bed Physical Model Study of Proposed Developments (Photocopied Report) 7 42 North Shore Coastal Works 1994 Shoreline Management Hard Copy CCBC CECS (Cream Cover Report) Partnership 8 44 Structural Survey - West Shore 1990 Shoreline Management Hard Copy CCBC CECS Pre 1991breakwaters and recharge Revetment (Yellow Cover Report) Partnership scheme visual evaluation of condition of existing RC sea wall. Subsequent report and further inspection provided by Concrete Society (letter).

9 47 Impulse Radar Survey of Penrhyn 1988 Ampthill Geophysical Consulting Hard Copy CCBC CECS Bay and Llandudno promenade (White Cover Report) 10 67 Llandudno Coastal Works - 1987 BMT Hard Copy CCBC CECS Contains typical cross sections Engineer’s Statement Appendix G through sea wall defences. Drawings (Blue Cover Report) 11 68 Breakwater protection within 1987 BMT Hard Copy CCBC CECS Penrhyn Bay produced by British Maritime Technology (Blue

EX 4667 19/05/2008 EX Cover Report) 12 78 Llandudno North Shore Coastal 1987 Shoreline Management Hard Copy CCBC CECS Same as 27 Protection Capital Works (Yellow Partnership Cover Report)

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By 13 79 Aberconwy Borough Council - 1987 BMT Hard Copy CCBC CECS Coastal Study - Summary Report (Blue Cover Report) 14 84 North Shore Coastal Works 1994 Shoreline Management Hard Copy CCBC CECS Same as 42 Partnership 15 101 Development Plan for Tidal 1986 Welsh Water Authority Hard Copy CCBC CECS Discharges produced by Welsh Water Northern Division (Blue Cover Report) 16 105 Report on the Problem of Floods 1983 Welsh Water Authority Hard Copy CCBC CECS in the Conwy Valley produced by the Welsh Water Authority - Gwynedd Division (Red Cover Report) 17 108 North Shore Coastal Works 1994 Shoreline Management Hard Copy CCBC CECS 2 No. Copies received (Yellow Cover Report) Partnership

18 110 North Shore Coastal Works 1993 Shoreline Management Hard Copy CCBC CECS (Yellow Cover Report) Partnership 19 116 Report areas of Present Flooding 1993 NRA, Welsh Water, Gwynedd Hard Copy CCBC CECS in Aberconwy (Blue Cover County Council & Aberconwy Reports) Borough Council

20 117 Condition survey of Waun Culvert 1993 Mott MacDonald Hard Copy CCBC CECS (blue cover report) 21 128 Report on Coast Protection at 1984 Scott Wilson Kirkpatrick & Hard Copy CCBC CECS Llandudno West Shore Penrhyn Partners Bay (Brown Cover Report) 22 133 North Shore Llandudno Second 1992 HR Wallingford Hard Copy CCBC CECS Opinion on Proposed Coast Protection Scheme (White Cover Report) 23 140 Aberconwy Coastal study - Final 1987 British Maritime Technology Hard Copy CCBC CECS Report Parts 1,2,3,4 & 5 (Blue Cover Reports) 24 149 Pensarn Coast Protection Scheme - 1993 Colwyn Borough Council Hard Copy CCBC CECS Cost Estimate (white cover reports) 25 160 Llandudno North Shore Sea 1995 Scott Wilson Kirkpatrick Hard Copy CCBC CECS

EX 4667 19/05/2008 EX Defences - Retaining Wall from Consulting Engineers for Stepped Revetment to Pier - Llandudno Town Council Structural Inspection Report (white cover report)

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By 26 162 Llandudno North Shore Coast Protection - Shoreline Management Hard Copy CCBC CECS Capital Works (Yellow Cover Report) Partnership for Aberconwy Borough Council

27 172 West Shore Coastal Works - 1992 Shoreline Management Hard Copy CCBC CECS Gogarth Breakwater Inspection Partnership (yellow cover report) 28 176 Introductory Report on the 1979 Colwyn Borough Council Hard Copy CCBC CECS Proposed Extension to the Promenade and sea defences at Pensarn Beach, Abergele (blue cover report) 29 177 Pensarn Sea Defence - Report on 1980 Colwyn Borough Council Hard Copy CCBC CECS Beach Conditions (blue cover report) 30 210 Kinmel Bay Coast Protection 1994 Williamson Technical Services Hard Copy CCBC CECS Topographic Survey, XS & LS of Scheme - Survey of Sea Wall and Ltd original KB defences (dxf files) Beach (blue cover in a brown A4 envelope)

31 214 Development Feasibility Study 1994 Welsh Development Agency Hard Copy CCBC CECS Foryd Harbour, Rhyl - Summary Statement of Proposals (white cover report and drawings) 32 215 West Rhyl - Regeneration and 1990 Tibalds/Colbourne/Karski/Willia Hard Copy CCBC CECS Development (white cover A3 ms Consultants report) 33 218 Proposed Coast Protection Scheme 1993 Soils and Material Testing Ltd, Hard Copy CCBC CECS Borehole Logs and test details - No Pensarn Beach - Report of Soils Llanddulas plan. This CCBC Ref. is "Pensarn and Foundation Conditions Coast Protection Scheme file gineer's Report etc (pink folder) containing Engineer's Report etc (pink folder).

34 221 Pensarn Coast Protection Scheme - Jan-94 Colwyn Borough Council Hard Copy CCBC CECS Full scheme proposals - Beach Engineer's Report (white cover recharge, groynes and wave wall. report) Only wall section constructed.

35 222 Llandudno - Slip in Happy Valley 1990 Scott Wilson Kirkpatrick and Hard Copy CCBC CECS Road, Proposals for Remedial Partners Measures (drawings included)

EX 4667 19/05/2008 EX (white cover reports) 36 258 Report of Soils and foundation May 1992 Soils and Material Testing Ltd, Hard Copy CCBC CECS GI records and testing details conditions for proposed sea Llanddulas including plan defences at Rhos on Sea

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By 37 264 Borough of Colwyn Sea Wall East Feb 1990 Traverse Morgan Hard Copy CCBC CECS Immediate post Feb 1990 inspection of Pensarn records relating to defences between Abergele and Towyn

38 265 An Assessment of the Clwyd Jun 1990 Environmental Research Centre, Hard Copy CCBC CECS Contains useful background data to Coastal Lowlands after the Floods University of Durham and analysis of Feb 1990 event of February 1990

39 266 Towyn and Belgrano Permanent Jun 1991 Posford Duvivier Hard Copy CCBC CECS All calculations for post 1990 Towyn Works - Design Calculations defences

40 291 Craig y Don Coastal Defences, March 1998 Shoreline Management Hard Copy CCBC CECS Maintenance Inspection Partnership 41 304 Aberconwy Coastal Study Final Report Parts 1 British Maritime Technology Hard Copy CCBC CECS and 2 42 305 Llandudno Coastal Works Nov-87 British Maritime Technology Hard Copy CCBC CECS Engineers Statement 43 329 Llandudno - North Shore Synopsis Mar-92 Shoreline Management Hard Copy CCBC CECS Partnership 44 330 Report on Inspection of Happy 14 May, Mott MacDonald Hard Copy CCBC CECS Valley Rock Slope (Opposite 1998 Grand Hotel), Llandudno (DRAFT) 45 337 Report of Soils and Foundation December, M. J. Chettleburgh & Associates Hard Copy CCBC CECS Provides criteria for Pensarn wave Conditions - Further Tests of Soils 1994 wall design for Proposed New Sea Defences at Pensarn Beach

46 340 Summary of Structural Inspection June, 1997 Engineering Management, Hard Copy CCBC CECS and Report on the Underground Housing & Technical Services, Reservoir at Happy Valley, Great Conwy County Borough Council Orme, Llandudno EX 4667 19/05/2008 EX

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By 47 351 Conwy County Borough Council September, Environmental Advice Centre Hard Copy CCBC CECS Small scale and limited land Site Investigation of Land Off 1999 Ltd., Unit 1, Long Barn, Pistyll use/condition information First Avenue, Rhos-on-Sea Ref.: Farm, Nercwys, Mold, CH7 4EW J209/V2/10.99

48 364 Concrete Testing of Sea Defence 19 June, Celtest Ltd., Cyttir Lane, Bangor Hard Copy CCBC CECS Thorough report on the condition of Wall, Kinmel Bay - Factual and 1998 the concrete wall at Kinmel Bay. Conclusive Proposals for minor repairs. Have these been carried out?

49 369 Report of Soils and Foundation October, M. J. Chettleburgh Associates Hard Copy CCBC CECS Trial pits & lab testing details and Conditions - Proposed Beach 1993 foundation recommendations for Defence Works - Pensarn - proposed works. Scheme 2, Volume 1

50 370 Mochdre Flood Alleviation Study Feb-94 Travers Morgan Hard Copy CCBC CECS Forest Glade Watercourse 51 372 Mochdre Flood Alleviation Study, Feb-94 Travers Morgan Hard Copy CCBC CECS Upper Afon Ganol 52 393 Aberconwy - West Shore - Aug-90 Shoreline Management Hard Copy CCBC CECS supplementary engineers report Partnership 53 395 Pensarn Coastal Protection May, 1993 Dr. A. B. Poole Hard Copy CCBC CECS Provides useful results of degradation Scheme - Degradation of Beach of beach nourishment material used at Nourishment Materials Penrhyn Bay

54 396 A Statistical Survey of Storm Winter The Institution of Civil Engineers Hard Copy CCBC CECS Damage to Coastal Defences 1989-1990

55 399 Report on Damage to Coast January, Scott Wilson Kirkpatrick & Hard Copy CCBC CECS Protection Works - Llandudno 1988 Partners West Shore 56 415 Status Report West Shore, Dec-99 Mott MacDonald Hard Copy CCBC CECS Report highlighting condition and Llandudno and Penrhyn Bay RC need for further investigation into

EX 4667 19/05/2008 EX Sea Defences defects in existing RC sea walls at West Shore and Penrhyn Bay. In both cases these structures form the back line of defences behind a recharged shingle beach.

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By 56 74 A Review of Sea Defences on The 1974 Hydraulics Research Station, Hard Copy HRW Their vulnerability will be an North Wales Coast (green cover Wallingford important aspect in determining risk report) of failure in the future.

57 260 Colwyn Bay Beach and Groyne Dec 1988 HR Wallingford Hard Copy HRW Study 58 262 Colwyn Bay Beach Profiles 1980- 1988 HR Wallingford Hard Copy HRW 1988 59 326 Colwyn Borough Sea Defence Oct-91 HR Wallingford Hard Copy HRW Review 60 155 Colwyn Borough Sea Defence HR Wallingford Hard Copy HRW Review (Blue Cover Reports) 61 272 Design and Analysis of Urban Oct 1981 HR Wallingford Hard Copy HRW Storm Drainage - The Wallingford Procedure Volume 3 Maps 62 39 Joint Probability of Waves and 1990 Hydraulics Research Station, Hard Copy HRW Water levels on the North Wales Wallingford Coast (White Cover Report) 63 224 Kinmel Bay Coast Protection 1996 HR Wallingford Hard Copy HRW Scheme - project appraisal (blue/white cover report) 64 225 Kinmel Bay Coast Protection 1995 HR Wallingford Hard Copy HRW Scheme - project appraisal (blue/white cover report) 65 213 Kinmel Bay Coast Protection 1994 HR Wallingford Hard Copy HRW Works - Hydraulic Model Studies (blue cover report) 66 80 North Shore Llandudno - Mobile 1991 HR Wallingford Hard Copy HRW "Physical Model Study" Report EX Bed Physical Model Study of 3715 dated Dec 1997 Proposed Developments (blue cover report) 67 36 North Shore Llandudno Mobile 1993 HR Wallingford Hard Copy HRW Bed Physical Model Study of Proposed Developments (Photocopied Report) 68 133 North Shore Llandudno Second 1992 HR Wallingford Hard Copy HRW Opinion on Proposed Coast Protection Scheme (White Cover Report) 69 269 Pensarn Beach - Numerical Model Aug 1993 HR Wallingford Hard Copy HRW Studies 70 146 Pensarn Beach - Numerical Model 1993 HR Wallingford Hard Copy HRW EX 4667 19/05/2008 EX Studies (Blue Cover Reports) 71 148 Pensarn Coast Protection Scheme 1994 HR Wallingford Hard Copy HRW - Project appraisal (Blue Cover Reports)

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By 72 147 Pensarn Coast Protection Scheme - 1993 HR Wallingford Hard Copy HRW Benefit Cost Assessment (blue cover reports) 73 252 Rhos on Sea Coast Protection Apr 1992 HR Wallingford Hard Copy HRW Scheme 74 232 Rhos on Sea Coast Protection 1992 HR Wallingford Hard Copy HRW Scheme - Industrial Model Study (blue cover report) 75 150 The North Wales Coast - A 1974 HR Wallingford Hard Copy HRW Review of the Sea Defences (Green Cover Reports) 76 267 Towyn Sea Defences - Hydraulic Jun-91 HR Wallingford Hard Copy HRW Model Tests 77 233 Towyn Sea Wall Overtopping June 1992 HR Wallingford Hard Copy HRW 1992 (blue cover report) 78 53 Western Breakwater North Shore 1992 HR Wallingford Hard Copy HRW Llandudno - Hydraulic Model Study (Blue Cover Report) 80 229 Proposed Sea Defences Rhos on John Chettleburgh of Soils & Hard Copy CCBC CECS Sea - Report on soils and Materials Testing Ltd foundation conditions (white cover report) 81 230 Pensarn Beach - Report on Soils John Chettleburgh of Soils & Hard Copy CCBC CECS and Foundation Conditions (white Materials Testing Ltd cover report) 82 273 West Shore Coastal Works for 1990 Shoreline Management Hard Copy CCBC CECS Aberconwy Borough Council - Partnership Draft Tender Document 83 373 Rhos on Sea Coast Protection Jan-93 D. N. Gough, Director of Housing Hard Copy CCBC CECS Scheme & Technical Services

84 151 Tides, Waves and Defence Works 1945 C.R. Irving Hard Copy CCBC CECS on the North Wales Coast (Green Cover Reports)

85 178 Report on a Survey of the 1969 Sandford Fawcett, Wilson & Bell Hard Copy CCBC CECS Foreshore from Llanddulas to Consulting Engineers Towyn (white cover report)

86 194 Notes on the Storms and Coastal 1990 R A Flather and R Proctor Hard Copy CCBC CECS Floods in February 1990 (white

EX 4667 19/05/2008 EX cover report)

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By 87 234 Report on the Breach in the Sea 1990 G M Roe Hard Copy CCBC CECS Wall in Towyn on 26 Februayr 1990 (white cover report)

88 235 The Towyn Experience and the 1990 G M Roe Hard Copy CCBC CECS Lessons Learned (white cover report)

89 Monitoring of Beach Levels Aug-95 Williamson Technical Services Hard Copy CCBC CECS Only contains station and RO details Pensarn to Llandulas Topographic Ltd Survey

FORESHORE/BATHYMETRIC SURVEYS Present and Historical Admiralty Hard Copy HRW/CECS/Ad HRW Historical Charts to be copied to Charts miralty HRW CD ROM containing Shoreline Survey Operations Digital CCBC CECS To only be used in connection with Monitoring Data November 1997 - study April 1999 CD ROM containing Shoreline Survey Operations Digital CCBC CECS To only be used in connection with Monitoring Data: April 2000 & study October 2001

HINTERLAND TOPOGRAPHIC DATA

MAPPING Indicative Flood Plain Mapping - Environment Agency Digital/Hard CCBC/EA/HR HRW Section 105 etc Copy W Present OS 1:50,000 Raster Ordnance Survey Digital CCBC HRW To only be used in connection with study

EX 4667 19/05/2008 EX Present OS 1:10,000 Raster Ordnance Survey Digital CCBC HRW To only be used in connection with study Present OS 1:1250, 1:2500 Vector Ordnance Survey Digital CCBC HRW To only be used in connection with 'Landline' study

ABCD Project Seq. External Data Date Data Author Data Received Data Comment Number Ref. No. Originated Format(s) From Held By

PHOTOGRAPHS Vertical Aerial Photographs - 1992 Digital CCBC HRW Colour. CCBC coastline Study shoreline Vertical Aerial Photographs - 1950/1967 Digital CCBC HRW B/W Kinmel Bay Kinmel; Bay Vertical Aerial Photographs - 1969 Digital CCBC HRW B/W Colwyn Bay Colwyn Bay Miscellaneous Oblique Aerial 1990's Digital CCBC HRW Colour Photographs - Study Shoreline Stills: Pensarn CP Progress Photos 1995/96 Digital CCBC HRW Colour Stills: Towyn Coast Protection Mar-90 Digital CCBC HRW Colour Emergency Works Stills: West Shore Sea Wall Aug-90 Digital CCBC HRW Colour Stills: Colwyn Bay Wall & Groyne 1990? Digital CCBC HRW Colour CP Progress Photos Stills: Miscellaneous CP Photos Unknown Digital CCBC HRW Colour Stills: Old Photos - Colwyn Bay Digital CCBC HRW Colour Prom., Llandudno Prom., Miscellaneous (1946) DEFENCE DETAILS Extracts from Welsh Office Digital CCBC CECS Coastal Defence Survey Extracts from NFCDD Digital HRW HRW

ENVIRONMENTAL DATA Wind data Liverpool Bay Digital HRW HRW Wave data Liverpool Bay Digital HRW HRW Recorded Tidal level Data Digital HRW HRW (Liverpool) to 1987 Recorded Tide Level Data from Digital POL Liverpool and Llandudno (post 87)

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Appendix 2

Database of Defence Details

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Appendix 2 Database of Defence Details

This Appendix summarises the results of the visual condition survey undertaken by between August and September 2002 and contains the following four tables:

- Table A2.1 Condition survey definitions - Table A2.2 Conwy Tidal Flood Risk Assessment Defence Details - Table A2.3 Conwy BCC – Defence database - Table A2.4 Defence details from National Flood and Coastal Defence Database (2002)

ABCD EX 4667 19/05/08 Table A2.1 Condition survey definitions

Condition Survey

Primary Beach/Foreshore/Saltmarsh

S Stable M Mobile but restrained e.g. by groynes, breakwaters etc E Eroding A Accreting

Secondary Structures / Tertiary

1 Very Good Fully Serviceable 2 Good Minor Defects 3 Fair Some cause for concern, requires careful monitoring 4 Poor Structurally unsound now or in the near future. 5 Very Poor Completely failed or derelict

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Table A2.2 Conwy Tidal Flood Risk Assessment Defence Details

ABCD EX 4667 19/05/08 Table A2.2 Conwy Tidal Flood Risk Assessment Defence Details (continued)

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Table A2.2 Conwy Tidal Flood Risk Assessment Defence Details (continued)

ABCD EX 4667 19/05/08 Table A2.2 Conwy Tidal Flood Risk Assessment Defence Details (continued)

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Table A2.3 Conwy BCC – Defence database

ABCD EX 4667 19/05/08 Table A2.3 Conwy BCC – Defence database (continued)

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Table A2.3 Conwy BCC – Defence database (continued)

ABCD EX 4667 19/05/08 Table A2.4 Defence details from National Flood and Coastal Defence Database (2002)

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Appendix 3

Historical Data

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ABCD Appendix 3 Historical Data

Event Date Area Event Details Hinterland Consequences Further Comments Source 1585-1606 Pensarn to First coastal defences erected. A 1

Kinmel Bay clay bank approximately 1.0 mile

long between Pensarn and Towyn 1606 Pensarn to Sea broke through clay bank Damage to land 1 Kinmel Bay 1722 Pensarn to Sea flooded marsh 1 Kinmel Bay 1845-48 Pensarn to Construction of sea wall to protect 1 Kinmel Bay railway between Pensarn and Sandy

Cove (east end of Towyn Defences) 9/1/1849 Pensarn to Sea broke through defences Track flooded to a depth of 1 Kinmel Bay approximately 18 inches 1869 Pensarn to Flooding at Kinmel Bay end of 1 Kinmel Bay frontage where there weren’t any

defences

1877 Pensarn to Sea broke through (exact location Flooding of marsh Railway company took control of 1 Kinmel Bay not identified, but assumed to be sea defences

where no defences existed) 17/8/1879 Pensarn to Torrential Rainstorm lasting two Flooding of marshland between Event known as ‘Y DWR MAWR Kinmel Bay days caused the River Gele to burst Abergele and Rhyl. Train – THE GREAT FLOOD’ its banks. services halted. Extensive 1 damage to property and land. Access across marsh could only be gained by horse. 1894-1906 Llandudno First coastal defences constructed North Shore

13/2/1899 Pensarn to Predicted tide level at Holyhead Kinmel Bay was 30.08 feet. Level at 4 EX 4667 19/05/084667 EX Rhuddlan Marsh was 3.34 feet above its predicted level. 1890’s Rhos on Sea Existing sea wall built 8

ABCD Event Date Area Event Details Hinterland Consequences Further Comments Source 1905 Llandudno First coastal defences constructed West Shore 7

1906 Pensarn to Further improvements made to 5 Kinmel Bay railway defences. 1921 Pensarn to Sea broke through undefended 1 Kinmel Bay section 1922-24 Pensarn to First coastal defence embankment 1 Kinmel Bay built between Sandy Cove to Kinmel 28/11/1924 Recorded tide level was 1.71 feet above predicted level (30.84 ft) at 4 Holyhead

2712/1924 Recorded tide level was 3.37 feet above predicted level (30.75 ft) at 4 Holyhead

1925 Pensarn to Embankment between Sandy Cove Kinmel Bay and Kinmel destroyed by storms 1

15/12/1926 Conwy High tide and winds 1. Conwy Quay inundated with Estuary water reaching the doors of the cottages. Benarth Hall cut off. 2. Deganwy promenade flooded with water reaching houses on 2 Marine Crescent.

3. Road from Llandudno Junction to Deganwy under water 4. River Conwy burst its banks.

EX 4667 19/05/084667 EX 27/10/1927 Llandudno High tide and winds Flooding of hinterland. Driftwood West Shore and seaweed found as far inland 2 as Maelgwyn Road

ABCD Event Date Area Event Details Hinterland Consequences Further Comments Source Llandudno High tide and winds Nant y Gamar Road and Queens North Shore Road flooded together with houses in Victoria Street and 2

Pleasant Street at Craig-y-Don end Pensarn to High tide and winds 50 yds of embankment wall Kinmel Bay between Pensarn and Foryd 2 washed away Nov 1927 Llandudno High tides and NW wind Flooding of Llandudno to Mostyn Street estimated at 4 feet (1.22 metres). Underground toilets at 2 North Western Gardens submerged, trapping people inside. Dec 1935 Pensarn to Portion of embankment on Kinmel Sea water able to flood up River Kinmel Bay side of River Clwyd collapsed Gele flooding the surrounding 1 area on each high tide. Dec 1936 Pensarn to Sea defences hit by three successive Sea wall held but holes appeared Kinmel Bay high tides under railway tracks and land was 1 flooded for about 3½ miles along the track. 1936 Llandudno Major remedial works carried out to 7 West Shore defences 27/2/1937 Llandudno High tide and winds Promenade damaged between Recorded tide level was 1.5 feet North Shore Bandstand and Neville Hydro. above predicted level at Holyhead 2,4 Planks stripped from Pier. Jan 1938 Pensarn to River Gele broke way from its Flooding of land in river Kinmel Bay course. River Clwyd obstructed in catchments 1 two places 1938 Llandudno Existing stepped concrete revetment North Shore constructed from Trevor St. to Craig 7

EX 4667 19/05/084667 EX Y Don 1950-52 Llandudno Existing stepped sea wall 7 West Shore constructed

ABCD Event Date Area Event Details Hinterland Consequences Further Comments Source 1954 Pensarn to Flooding of land in Morfa 1 Kinmel Bay Rhuddlan 1954-56 Penrhyn Existing stepped sea wall Includes section of Rhos on Sea

Bay constructed. defences – Old County of 7,8 Caernarfonshire 1956 Pensarn to Flooding of land in Morfa 1 Kinmel Bay Rhuddlan Winter 1963 Pensarn to Sea defences breached along Kinmel Unknown 1 Kinmel Bay Bay frontage 1966-67 Pensarn to Stage 1 of Kinmel Bay Coastal 1 Kinmel Bay defences constructed 1967-68 Pensarn to Stage 2 of Kinmel Bay Coastal 1 Kinmel Bay defences constructed Nov 1974 Pensarn to Gale force winds Extensive wind blown sand Kinmel Bay transported inland, trapping some residents in their homes. Sand 1 reached the roofs of bungalows in places Feb 1975 Conwy High water levels Llandudno Junction and Deganwy Estuary suffered flooding. House in Conway Road flooded. Flooding 2 to a depth of 1 foot (300mm) at Pen Twyn Hill. Slight flooding on Conwy Quay 1975 (date Pensarn to Sea wall breached approximately 1- Unknown) Kinmel Bay 1.5 km east of Pensarn station. 6

1976 (Nov?) Pensarn to Hurricane Force Winds buffeted Towyn and Kinmel Bay flooded 1 Kinmel Bay coast breaching the railway sea wall to a depth of 7 feet (2.1 metres) 11/11/1977 Rhos on Sea High tide and winds Basement of Rhos Abbey Hotel 2 flooded

EX 4667 19/05/084667 EX Pensarn to Overtopping of Towyn Defences 3 Kinmel Bay causing limited flooding 1982-83 Rhos on Sea Offshore breakwater constructed

ABCD Event Date Area Event Details Hinterland Consequences Further Comments Source 31/1-1/2 Feb Pensarn to High tide and winds Serious flooding took place. 1983 Kinmel Bay Caravan parks under several feet 1, 2 (1 metre+) of water. Dozens of

houses cut off. Rhos on Sea 30 foot (10 metre) long hole in promenade. Promenade surfacing 2 damaged. Deganwy Promenades littered with debris 2 Penrhyn 2 Bay Llandudno 2 North Shore 1989-90 Penrhyn Shore Connected Breakwaters and Bay Beach Recharge constructed

11/12 Feb Pensarn to Overtopping of Towyn Defences 3 1990 Kinmel Bay causing limited flooding

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ABCD Event Date Area Event Details Hinterland Consequences Further Comments Source 26-28 Feb Pensarn to High tide and winds caused Consequences of sea wall breach: 1990 Kinmel Bay overtopping and then 450 metre long • 2,800 homes flooded under at breach in sea wall at Towyn least 6ft (1.8 Metres) of water • > 5,000 people evacuated • 1,000 people still homeless in July 1990 • Approximately 6.5 km2 area flooded, including all of Towyn and parts of Kinmel Bay 1,2,5 • Flood water levels reached +5.39m AOD in the centre of Towyn. • Flood water levels reached +5.24m AOD at Gors Farm, 1 km south of Towyn. • Flood waters flowed up to two

kilometres inland

Rhos on Sea High tide and winds Waves overtopped breakwater and sea wall causing flooding of roads 2 and property on Marine Drive and Penrhyn Avenue Penrhyn Cobbles and shingle thrown up 2 Bay onto promenade Llandudno Section of minor defence at Craig- North Shore Y-Don collapsed eroding promenade behind. Llandudno

West Shore Conwy Water levels over top of quay Water level of 5.15m AOD 2 EX 4667 19/05/084667 EX Estuary recorded at Deganwy Dock Glan Conwy Waves overtopped BR wall 2 removing ballast from the track

ABCD Event Date Area Event Details Hinterland Consequences Further Comments Source 1991/2 Llandudno Shore Connected Breakwaters and West Shore Beach Recharge constructed

1992 Pensarn to New armour stone revetment built in Kinmel Bay front of BR Towyn sea wall, including section breached in 1990. 1993/94? Pensarn to New armour stone revetment built in Kinmel Bay front of section of BR Belgrano sea wall. 1994/95 Rhos on Sea New armour revetment constructed

in front of existing wall 1996 Pensarn to Rear Flood wall built along crest of Kinmel Bay shingle bank from Ty Cryn to Pensarn station 1996/97 Llandudno Phase 1 coastal defence works North Shore constructed between Vaughan St. and Craig Y Don Paddling pool, comprising: • Improved flood wall • New promenade surfacing • Beach Recharge 10/02/97 Conwy High Tides and Strong Winds Water overtopped Conwy quay Estimated water level from Estuary causing flooding of roads and photographs of 5.10m AOD 2 property

Shingle and debris thrown up onto Deganwy promenade ???? River Clwyd River broke its banks (exact location unknown), causing 2 flooding of properties in Kinmel Bay EX 4667 19/05/084667 EX

ABCD Event Date Area Event Details Hinterland Consequences Further Comments Source 2000 Llandudno Phase 2 coastal defence works North Shore constructed between Vaughan St. and Trevor St Slipway comprising: •

Improved flood wall • New promenade surfacing • Beach Recharge 1999/2000 Pensarn to Rock Revetment and beach recharge Kinmel Bay carried out between Sandy Cove and Horton’s Nose. New rock groynes constructed across Horton’s Nose frontage

Sources identified in the above are as follows:

1. “When the Sea Came By” 2. North Wales Weekly News, Various Dates. 3. Flather R.A. & Proctor R., 1990. “Notes on the storms and coastal floods in February 1990”. 4. Irving C.R., 1945. “The Influence of Tides and Wind on the North Wales Coast – Tides, Waves and Defence Works on the North Wales Coast”. 5. Englefield G.J.H., Tooley M.J., & Zong Y, June 1990. “An Assessment of the Clwyd Coastal Lowlands after the Floods of February 1990”. 6. James Williamson and Partners, January 1980. “Pensarn Sea Defence – Report on Beach Conditions”. 7. British Maritime Technology, November 1987. “Llandudno Coastal Works – Engineer’s Statement”. 8. HR Wallingford, October 1991. Colwyn Borough Sea Defence Review

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Appendix 4

Defence Management and Maintenance Actions

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ABCD Appendix 4 Defence Management and Maintenance Actions Table A4.1: A Summary of the Proposed Management and Maintenance Actions for each Defence CTFRA Specific Structure Management

Location Length Monitoring Beach Management Elements to be Inspection Responsibility Ref. examined Existing Proposed Llandudno West Shore North Wales Golf Club (1) 1A Beach Survey Beach Survey Yes, subject to analysis Not Required Beach profile and CCBC volume Llandudno West Promenade 1B Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC (1) volumes in front of sea wall; structural defects in sea wall Lloyd St.Breakwater 1C Beach Survey Beach Survey Yes, subject to analysis Yes Movement and CCBC breakage of armour blocks Llandudno West Promenade 1D Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC (2) volumes in front of sea wall; structural defects in sea wall Gogarth Breakwater 1E Beach Survey Beach Survey Yes, subject to analysis Yes Movement and CCBC breakage of armour blocks Llandudno West Promenade 1F Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC (3) volumes in front of sea wall; structural defects in sea wall Gogarth Wall 1G Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC volumes in front of sea wall; structural defects in sea wall Llandudno North Shore Llandudno Promenade (1) 2A Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels CCBC against wall; EX 4667 19/05/084667 EX structural defects in sea wall and promenade

ABCD CTFRA Specific Structure Management Location Length Monitoring Beach Management Elements to be Inspection Responsibility Ref. examined Existing Proposed Llandudno Promenade (2) 2B Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels CCBC

against wall; structural defects in sea wall and promenade Llandudno Promenade (3) 2C Beach Survey Beach Survey Yes, subject to analysis Yes Structural defects in CCBC slipway and promenade Llandudno Promenade (4) 2D Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC volumes in front of sea wall; structural defects in sea wall and promenade Llandudno Promenade (5) 2E Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC volumes in front of

sea wall; structural defects in sea wall and promenade Craig-y-Don (1) 2F Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC volumes in front of sea wall; structural defects in sea wall and promenade Craig-y-Don (2) 2G Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC volumes in front of sea wall; structural defects in sea wall and promenade Craig-y-Don, Boating Pool 2H Beach Survey Beach Survey Yes, subject to analysis Yes Beach levels and CCBC volumes in front of sea wall; structural

EX 4667 19/05/084667 EX defects in sea wall and promenade Craigside Road 2I Beach Survey Beach Survey Yes, subject to analysis Yes Beach profile and CCBC volume

ABCD CTFRA Specific Structure Management Location Length Monitoring Beach Management Elements to be Inspection Responsibility Ref. examined Existing Proposed Colwyn Road, Craigside 2J Beach Survey Beach Survey Yes, subject to analysis Not Required Beach levels and CCBC – Beach

volumes in front of Private - sea wall; structural Structures defects in sea walls Penrhyn Bay/Rhos-on-Sea Penrhyn Bay (2) 3A Beach Survey Beach Survey Yes Yes Beach levels and CCBC volumes in front of sea wall; structural defects in sea wall Afon Ganol Breakwater 3B Beach Survey Beach Survey Yes Yes Movement and CCBC breakage of armour blocks Penrhyn Bay (1) 3C Beach Survey Beach Survey Yes Yes Movement and CCBC breakage of armour blocks; structural

defects to sea wall Rhos-on-Sea (2) 3D Beach Survey Beach Survey No Yes Lowering of beach CCBC levels at toe and movement/breakage of armour blocks Rhos-on-Sea Breakwater 3E Beach Survey Beach Survey No Yes Movement and CCBC breakage of armour blocks on breakwater; natural or artificial lowering of beach levels behind breakwater; structural defects in sea wall Llandrillo-yn-Rhos 3F Beach Survey Beach Survey No Yes Movement and CCBC

EX 4667 19/05/084667 EX breakage of armour blocks; structural defects to sea wall

ABCD CTFRA Specific Structure Management Location Length Monitoring Beach Management Elements to be Inspection Responsibility Ref. examined Existing Proposed Pensarn to Kinmel Bay

Pensarn Shingle Bank (W) 4A Beach Profiles Beach Survey Yes, subject to analysis Yes Beach recession CCBC and volumes; structural defects in sea wall Pensarn Shingle Bank (E) 4B Beach Profiles Beach Survey Yes, subject to analysis Yes Beach recession CCBC and volumes; structural defects in sea wall BR Abergele Embankment 4C Beach Profiles Beach Survey Yes, subject to analysis Yes Beach levels Railtrack (W) against wall; structural defects in sea wall BR Abergele Embankment 4D Beach Profiles Beach Survey Yes, subject to analysis Yes Lowering of beach Railtrack (C) levels at toe and

movement/breakage of armour blocks BR Abergele Embankment 4E Beach Survey Beach Survey Yes, subject to analysis Yes Beach profiles and Railtrack (E) volumes; structural defects in rear defences Towyn 4F Beach Survey Beach Survey No Yes Lowering of beach Railtrack levels at toe and movement/breakage of armour blocks Kinmel Bay 2 4G Beach Survey Beach Survey No Yes Lowering of beach CCBC levels at toe and movement/breakage of armour blocks; structural defects in rear sea wall

EX 4667 19/05/084667 EX Kinmel Bay 1 (W) 4H Beach Survey Beach Survey Yes, subject to analysis Yes Beach recession CCBC and volumes; structural defects in sea wall

ABCD CTFRA Specific Structure Management Location Length Monitoring Beach Management Elements to be Inspection Responsibility Ref. examined Existing Proposed Kinmel Bay 1 (C) 4I Beach Survey Beach Survey Yes, subject to analysis Yes Beach recession CCBC

and volumes; structural defects in sea wall Kinmel Bay 1 (E) 4J Beach Survey Beach Survey Yes, subject to analysis Yes Beach recession CCBC and volumes; movement and breakage of armour blocks; structural defects to sea wall Foryd Dunes 3 4K Beach Survey Beach Survey Yes, subject to analysis Yes Beach recession CCBC and volumes; dune erosion; exposure of toe defences Foryd Dunes 2 4L Beach Survey Beach Survey Yes, subject to analysis Yes Beach recession CCBC

and volumes; dune erosion; exposure of toe defences Foryd Dunes 1 4M Beach Survey Beach Survey Yes, subject to analysis Not Required Beach recession CCBC and volumes; dune erosion; exposure of toe defences Foryd Harbour 4N None Beach Survey No Yes Beach levels CCBC against walls; structural defects in defences River Clwyd River Clwyd 5A None Beach Survey No Yes Beach levels CCBC/Private? against walls; structural defects in defences

EX 4667 19/05/084667 EX River Clwyd 5B None Beach Survey No Yes Beach levels Private against walls; structural defects in defences

ABCD CTFRA Specific Structure Management Location Length Monitoring Beach Management Elements to be Inspection Responsibility Ref. examined Existing Proposed River Clwyd 5C None Saltmarsh No Not Required Saltmarsh extent CCBC/EA?

monitoring River Clwyd 5D None Saltmarsh No Yes Saltmarsh extent; EA monitoring damage to floodbank integrity River Clwyd 5E None Saltmarsh No Yes Saltmarsh extent; EA/Private? monitoring damage to floodbank integrity River Clwyd 5F None Saltmarsh No Yes Saltmarsh extent; EA monitoring damage to floodbank integrity Conwy Estuary Glan Conwy 6A None Saltmarsh No Yes Saltmarsh Extent; Railtrack monitoring structural defects in rear defences

Conwy Quay (NW) 6B None Topograhic/ No Yes Beach volumes CCBC Hydrographic Survey Conwy Quay (Centre) 6C None Topograhic/ No Yes Estuary bed levels; CCBC Hydrographic structural defects in Survey rear defences Conwy Quay (SE) 6D None Topograhic/ No Yes Estuary bed levels; CCBC Hydrographic structural defects in Survey rear defences A55 Tunnel (Portal) 6E None Topograhic/ No Yes Estuary bed levels; CCBC Hydrographic structural defects in Survey rear defences Deganwy Glan y Mor 6F None Topograhic/ No Yes Estuary bed levels; Railtrack Road Hydrographic structural defects in Survey rear defences Deganwy Marine Crescent 6G Beach Profiles Beach Survey Yes Yes Beach profiles and CCBC EX 4667 19/05/084667 EX and Promenade volumes; structural defects in rear defences

ABCD CTFRA Specific Structure Management Location Length Monitoring Beach Management Elements to be Inspection Responsibility Ref. examined Existing Proposed Beach Profile Beach Survey Yes Yes Beach profiles and CCBC Deganwy Sea Wall 6H volumes; structural defects in rear defences

EX 4667 19/05/084667 EX

ABCD

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Appendix 5

Brief Description of the Overtopping Spreadsheet

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ABCD EX 4667 19/05/08

Appendix 5 Brief Description of the Overtopping Spreadsheet

The overtopping spreadsheet encapsulates the equations presented in the Environment Agency technical report W178 ‘Overtopping of Seawalls, Design and Assessment Manual’. It computes the potential mean and peak overtopping rates for sloped, vertical and return seawalls. It also incorporates the Goda’s formula to transfer offshore wave heights to their corresponding inshore heights.

The spreadsheet is developed using the Visual Basic for Application (VBA) under Microsoft Excel. The user interface of the spreadsheet is designed to be friendly and allows the user to enter the input data through a series of dialog boxes to facilitate the input data procedure. It also produces a sketch of the wall details and allows the user to debug the calculations. It includes global updating capabilities should the user modified the data entered through the dialog boxes manually.

The spreadsheet starts with splash screen showing the title and the version of the spreadsheet.

Figure 5. 1: Splash Screen

This is followed by a control screen that allows the user to add, delete, and update seawall locations or to exit.

Figure 5. 2: Control Screen

ABCD EX 4667 19/05/08 To add a location, the following dialog box is used to input the details on this location. This creates a sheet with the name of this location.

Figure 5. 3: Add location dialog box After selecting the wave conditions the user can select the seawall type. Three types are available as shown in the following dialog box. Each type will allow the user to enter the data for each the selected type as shown below.

Figure 5. 4: Seawall Type Selection

ABCD EX 4667 19/05/08

1) Sloped seawall.

Figure 5. 5: Sloped Wall Data

2) Vertical Seawall.

Figure 5. 6: Vertical wall with a mound data

ABCD EX 4667 19/05/08 3) Return Seawall.

Figure 5. 7: Return wall data

As shown above a typical cross section is drawn next to the data to explain what each parameter is. In the case of a sloped seawall, it covers the cases of a simple slope, double slopes, double slope with berm, or simple slope with berm. In the case of vertical seawall, two options are available, the simple vertical wall with or without mound. For return walls, only a simple slope can be used to model the sea defence.

Once all of the parameters are entered, The user can select the roughness that depends on the wall type. A selection of materials is given to the user as guidance to choose this parameter as shown below.

Figure 5. 8: Roughness selection

ABCD EX 4667 19/05/08

Once chosen, the user can either have a brief or detailed output. The latter allows debugging of the calculation whilst the first does not. The Spreadsheet automatically summarises the input data and the calculations of the different location to allow direct comparison between them.

ABCD EX 4667 19/05/08 ABCD EX 4667 19/05/08

Appendix 6

Defence details

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Appendix 6 Defence details

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Drawings

Drawing 1: Location map 1 Drawing 2: Location map 2

ABCD EX 4667 19/05/08

ABCD EX 4667 19/05/08