Directorate of Economy & Environment Director Stuart Love Isle of Wight Shoreline Management Plan 2 Appendix C: Baseline Process Understanding C3: Baseline Scenarios (of future shoreline change) • No Active Intervention scenario • With Present Management scenario December 2010 Coastal Management; Directorate of Economy & Environment, Isle of Wight Council iwight.com Appendix C3: Page 1 www.coastalwight.gov.uk/smp iwight.com Appendix C3: Page 2 www.coastalwight.gov.uk/smp Appendix C: Baseline Process Understanding C3: Baseline Scenarios (of future shoreline change) • No Active Intervention (NAI) scenario • With Present Management (WPM) scenario Contents Page no. C3.1 Introduction and Methodology 3 1. Context 2. Aim and introduction 3. Geographical units (including a map of the units used in the tables) 4. Introduction to the Baseline Scenarios 5. Sea level rise 6. Methodology 6.1 Developing the ‘No Active Intervention’ and ‘With Present Management’ Scenarios 6.2 Future erosion risk -Allowing for sea level rise in the future predictions of coastal erosion rates 6.3 Future flood risk 6.4 Mapping of predicted shoreline change under the ‘No Active Intervention’ and ‘With Present Management’ Scenarios -Assumptions for mapping erosion zones in the Baseline Scenarios 7. Overall Conclusions 7.1 No Active Intervention 7.2 With Present Management 8. Key sources of reference used in the development of the Baseline Scenarios & Maps. C3.2 Table: North-east coast 21 C3.3 Table: Sandown & Undercliff coast 68 C3.4 Table: West Wight coast 114 C3.5 Maps: NAI & WPM erosion and flood mapping iwight.com Appendix C3: Page 3 www.coastalwight.gov.uk/smp iwight.com Appendix C3: Page 4 www.coastalwight.gov.uk/smp 1. Context This task follows the guidance published by Defra (2006) on the development of Shoreline Management Plans (including Volume 1, Volume 2 and Appendix D –Shoreline Interactions and Response), to provide an understanding of shoreline interactions and responses for two baseline scenario assessments: ‘no active intervention’ and ‘with present management’ (Task 2.2). 2. Aim and Introduction The aim of this task is to provide an understanding of how the shoreline is likely to evolve in the future and the influence that coastal management or intervention is likely to have on that behaviour. This provides the basis by which flood and coastal erosion risks are determined, to define the zone of assets within which features and issues are at risk over the next 100 years. This analysis is used by the SMP to develop and appraise the consequences of setting different shoreline management policies. This task delivers four outputs: • a description of the shoreline response to a scenario of ‘No Active Intervention’ (NAI). This assumes that defences are no longer maintained and will fail over time. • a description of the shoreline response to a scenario of continuing ‘With Present Management’ (WPM). This assumes that all defences are maintained to provide a similar level of protection to that provided at present. • maps illustrating predicted shoreline change if ‘No Active Intervention’ (NAI) occurs. • maps illustrating predicted shoreline change if continuing ‘With Present Management’ (WPM) techniques. The maps and scenarios describe coastal evolution over three future epochs or time periods: 1) 0-20 years (approx. 2025); 2) 20-50 years (approx 2055); 3) 50-100 years (approx. 2105). These three epochs reveal short, medium and long-term change and are examined by all SMPs in England & Wales. 3. Geographical units To describe the variation around the Isle of Wight coast, the coastline has been divided into 58 frontages, from 260m to 18km in length. These frontages characterise likely future patterns of change, based on geomorphological units, a change in the scale of active or potential cliff retreat, areas of flood risk and development patterns. This report also assesses the longshore interactions between these frontages, for example their reliance on one another for supply of beach sediments. iwight.com Appendix C3: Page 5 www.coastalwight.gov.uk/smp Erosion at Horestone Point, February 2009 Flooding surrounding Newport Harbour, March 2008 iwight.com Appendix C3: Page 6 www.coastalwight.gov.uk/smp Map showing the location of the ‘IW’ units (in purple) used in the tables throughout Appendix C. Nb. the map also shows the location of the new SMP2 Policy Units ‘PU1A.1’ (in blue) developed after the completion of Appendices C, D and E and used in the Main Report. iwight.com Appendix C3: Page 7 www.coastalwight.gov.uk/smp 4. Introduction to the Baseline Scenarios An understanding of shoreline interactions and response forms an integral part of the SMP. The Baseline Scenarios develop an understanding of how the shoreline is behaving and the influence that coastal management has upon this behaviour. This allows areas at risk from future flooding and erosion to be identified and the potential consequences of coastal management policies and structures to be understood, to inform the setting of future shoreline management policies. Assessments are required for two scenarios: a) No Active Intervention’ (NAI). This scenario assumes that defences are not maintained and will fail over time. The effectiveness of the defences will change across each time period as some fail sooner than others, depending on the residual life, for example a concrete sea wall will probably last longer than a timber revetment currently in a similar condition. This scenario takes account of the dates of defence failure defined in the Defence Appraisal (Appendix C2 of this SMP2). Residual life of current defences can also be affected by changes in beach morphology, e.g. accretion or erosion. Shoreline change following ‘defence failure is assessed over the next 100 years. b) ‘With Present Management’ (WPM). This scenario assumes that all defences are maintained to provide a similar level of protection to that provided at present, to identify when current practices will no longer be effective over the next 100 years (as sea level rises and coastal processes change, for example). For the purposes of this SMP Appendix C3, this has been defined as the standard of the defence structure being maintained but not improved –i.e. a seawall may become affected by overtopping in future epochs. c) Mapping of predicted shoreline change accompanies both scenarios described above (NAI maps & WPM maps). The short, medium and long-term evolution of the coastline is examined, using three epochs (0-20 years; 20-50 years; 50-100 years). A large-scale and long-term understanding of shoreline response is necessary to assess the sustainability of management options and to take into account any long-term trends or drivers of coastal change, which may vary from short-term and local observations. Appendix C1 –Assessment of Shoreline Dynamics- is a fundamental input to the Baseline Scenarios. As an understanding of coastal behaviour and dynamics both historically and present day, it identifies key linkages and interactions along the coast and past shoreline movement. The Baseline Scenarios take this work forward to predict the response of the coast to the failure or maintenance of coastal defences, to changes in forcing factors (waves, tides, exposure, etc.), to changes in sediment supply and storage, and it identifies erosion rates to determine the future shoreline position. The analysis of each stretch of coast in the ‘baseline scenarios’ must continually consider each of the following: • what is there? (i.e. features, geomorphology, sedimentology etc.); • how is it reacting to circumstances around it? (i.e. long-term trends; reactivation; typical response and response to extreme events); • why is it reacting in this manner? (e.g. is the reaction controlled by factors such as sediment supply, geological/geomorphological controls); • where will the shoreline be? (position); • what are the consequences elsewhere of this reaction? (e.g. features updrift and downdrift). iwight.com Appendix C3: Page 8 www.coastalwight.gov.uk/smp Under the ‘No Active Intervention’ (NAI) policy scenario, there is no expenditure on maintaining or improving existing coastal and flood defences, therefore defences will fail at a time dependent upon their residual life and the condition of the fronting beaches and inter-tidal areas. Under the ‘With Present Management’ (WPM) policy scenario, all existing defences and management practices are continued. Defences are maintained to provide a similar level of protection over the next 100 years to that provided at present, i.e. maintaining their current height. In some cases this will require considerable investment to existing defences to maintain their integrity and effectiveness. For this assessment it is the function of the defence ‘practice’ that is be considered rather than specifics of the structure itself. The assessment should also identify if a practice becomes technically impossible in the future, for example due to rising sea levels, or when the current practice (e.g. beach recharge at the current rate) becomes ineffective. It is important to highlight the reducing standards of service offered by these defences over time. When assessing the effects of continuing ‘with present management’, the standard of the defence structure is maintained but not improved –i.e. a seawall may become affected by overtopping in future epochs. Presently redundant structures are not maintained and do not form part of this analysis. The consequences of maintaining the defences and management practices are assessed in terms of how the coastline will change, for example, narrowing and steepening of beaches in front of a seawall through coastal squeeze, leaving high vertical structures with no useable beach, or exposing the toe of the seawall. There are standard assumptions for each defence type (e.g. seawall or timber revetment) under the WPM scenario, listed in figure 1 below. Defence Example Assumptions type Structure Linear Seawall • Continues to prevent cliff line retreat stoppers • Stops (reduces) sediment input • Structural integrity remains and the wall is rebuilt at a similar standard of effectiveness • Exposure may change, i.e.