Slapton Coastal Zone Management Main Study Volume 1: Phase 1 Report (I) Prepared by Scott Wilson For Slapton Line Partnership August 2006 DOCUMENT APPROVAL FORM Document Title: Slapton Coastal Zone Management Main Study Volume 1: Phase 1 Report (I) Author: Scott Wilson, with University of Plymouth, Roger Tym and Partners and Dr Mark Lee. Amendments Record and Author Approvals: Version Date Description Prepared Reviewed Approved 1 July 2004 Draft Scott Wilson Dr Damon D C Dales University of Plymouth O’Brien Atlantic Consultants E Williams Dr Mark Lee R Hein 2 October 2004 Scott Wilson Dr Damon D C Dales University of Plymouth O’Brien Atlantic Consultants E Williams Dr Mark Lee R Hein 3 October 2004 Revised Scott Wilson Dr Damon D C Dales with minor University of Plymouth O’Brien comments Atlantic Consultants E Williams Dr Mark Lee R Hein 4 August 2006 Re-titled as Scott Wilson Dr Damon D C Dales Volume 1 University of Plymouth O’Brien Roger Tym and Partners E Williams Dr Mark Lee R Hein Project D101963 This document has been prepared in accordance with the scope of Scott Wilson’s appointment with its client and is subject to the terms of that appointment. It is addressed to and for the sole use and reliance of Scott Wilson’s client. Scott Wilson accepts no liability for any use of this document other than by its client and only for the purposes, stated in the document, for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions, or recommendations within this document should be read and relied upon only in the context of the document as a whole. The contents of this document are not to be construed as providing legal, business or tax advice or opinion. © Scott Wilson Group plc 2006 Scott Wilson Ltd. Scott House Basing View Basingstoke RG21 4JG [email protected] Tel: 01256 461161 Preface The Slapton Coastal Zone Management Study has been commissioned by the Slapton Line Partnership. The Slapton Line Partnership is made up of the following organisations: English Nature Devon County Council South Hams District Council Whitley Wildlife Conservation Trust Slapton Ley Field Centre Environment Agency Defra The Partnership has been formed to decide how best to deal with potential future erosion of Slapton Line and its effect on the road, the environment, and the community. The objective of this study is to compile a single reference document that will form the basis for all future decision-making that affects the shingle ridge. Scott Wilson has undertaken the study in collaboration with the University of Plymouth, Roger Tym and Partners and Dr Mark Lee. The University of Plymouth provided material for the chapters on coastal processes and modelling, Roger Tym and Partners focused on the socio-economic aspects of the study and Dr Mark Lee provided specialist geomorphology information. This report should be read in conjunction with Volumes 2,3 and 4. Volume 2 provides figures to accompany Volume 1. Volume 3 is the Phase 2 Report and Volume 4 the overall Executive Summary. Contents Preface 1 Phase 1 Executive Summary 1-1 2 Introduction 2-1 3 Consultation 3-1 4 Ecology Baseline 4-1 5 Geomorphology Baseline 5-1 6 Archaeology and Cultural Heritage Baseline 6-1 7 Landscape Character and Visual Receptor Baseline 7-1 8 Socio-Economic Baseline 8-1 9 Transportation Baseline 9-1 10 Literature Review and Historical Change 10-1 11 Conceptual Beach Process Model 11-1 12 Option Development 12-1 13 Environmental Appraisal 13-1 14 Option Economic Appraisal 14-1 15 Option Appraisal 15-1 16 Funding Review 16-1 17 Conclusions 17-1 18 The Way Forward 18-1 Appendices A Futurecoast B Consultation Responses C SSSI Designation D References Slapton Coastal Zone Management Main Study Volume 1: Phase 1 Report (I) 1 Phase 1 Executive Summary 1.1 This report presents the findings of the Phase 1 Report for the Slapton Coastal Zone Management Main Study. This report has been prepared by Scott Wilson on behalf of the Slapton Line Partnership. The terms of reference for the study were set out in detail in the brief for the study. 1.2 This report forms part of the Slapton Coastal Zone Management Main Study. Phase 2 will address refining the range of shoreline management options, and will result in selection of the final preferred shoreline management option. Aim 1.3 The aim of the Main Study is to provide a comprehensive evaluation of the issues relating to coastal processes at Slapton Sands for the purposes of determining an appropriate shoreline management response to the recent erosion and from this, to establish a robust long-term coastal zone management strategy for the area. The objective of the Study is therefore to compile a single reference document that will form the basis for all future decision making that affects the shingle ridge. Background 1.4 The A379 offers an important transport link between local communities, as well as providing a secondary emergency route into Dartmouth. The principal emergency route is via the A381 and A3122, narrow lanes offer the only other alternative, suitable for small vehicles only. 1.5 In previous years storms have caused significant damage to the shingle ridge, forcing road closure. The winter of 2000/2001 was no exception; a series of storms caused the loss of 5m of shingle beachhead over a length of 1000m. The erosion undermined a 200m section of the A379 that runs along the shingle ridge. This resulted in closure of the road. 1.6 Following the damage to the Coast Road, the worst affected section was realigned. The realignment extended over a length of about 200metres and moved the road approximately 20metres landwards. In addition some 12,000 tonnes of shingle were transported from the Strete Gate area and deposited in front of the affected area at Slapton, with shingle ‘bastions’ formed at each end of the protected length. Since 2001 there has been an accretion of shingle on the beach at the affected locations. August 2006, Scott Wilson 1- 1 Slapton Coastal Zone Management Main Study Volume 1: Phase 1 Report (I) Data Collection and Environmental Baseline 1.7 For the Phase 1 report a review was made of the data available. From the review it was decided to undertake a programme of surveys to provide base data for the Study. The surveys included: • Topographic survey of the road and beach • Hydrographic survey of the sea bed in the southern part of Start Bay and over Skerries Bank • Sidescan sonar survey of the seabed • Origin and destination traffic survey on the A379 1.8 In addition a programme of profile monitoring was started to supplement the monitoring already undertaken by the Field Studies Centre, South Hams District Council and the Environment Agency. 1.9 An environmental baseline was prepared, using a combination of previously published reports, data held by key stakeholders, and observations from filed inspections. The Baseline was addressed under the following headings: Ecology Geomorphology Archaeology and Cultural Heritage Landscape Socio-Economics Transportation Coastal Processes and Geomorphology 1.10 A detailed study was also made of the coastal processes affecting the shingle barrier. The study of coastal processes is in far greater detail than has previously been undertaken. This study included: An assessment of the wave climate, based on transformation inshore using a numerical wave model of offshore data from the Meteorological Office Analysis of water levels using data from Devonport Analysis of wind data Modelling of sediment transport along the beach and in the cross-shore direction August 2006, Scott Wilson 1- 2 Slapton Coastal Zone Management Main Study Volume 1: Phase 1 Report (I) Modelling of breach probability Assessment of historical changes in beach position and rates of erosion 1.11 The key beach processes affecting Slapton Sands are: Short-term storm impacts Medium term longshore transport gradients Long-term sea level rise causing the shingle barrier to rollback landwards 1.12 Each of these processes was studied. Short- and medium-term impacts are coastal process issues, whereas long-term changes are best considered within a geomorphological framework. 1.13 The cross-shore profile predicted by Powell’s model, at the time and location of the road collapse in January 2001, shows that the storm event was of a sufficient severity (wave height) and occurred in conjunction with a sufficiently high still water level to impact on the crest of the shingle ridge. The breach prediction model of Bradbury suggests that at the time of the road collapse in January 2001, the loss of road was due to beach cut back not due to the mechanism of over washing and roll back. The sensitivity tests showed that neither a sea level rise of 250mm, nor a reduction in barrier cross-sectional area of 30% would significantly increase the chances of this mechanism becoming dominant. 1.14 The January 2001 erosion event can clearly be categorised as cut-back of the edge of the shingle barrier as indicated on Figure 10.25 in Volume 2, and not as a breach of the barrier. 1.15 Examination of aerial photographs from 1999 shows that, compared to adjacent areas, the face of the shingle ridge was further seawards at the central car park and at the location where the road was undermined. Therefore these areas had steeper beaches and were at higher risk of greater erosion than other areas.