REPORT ( FINAL) Cornwall Beach & Dune Management Plans – Porthcothan Prepared for Cornwall Council October 2016 CH2M Ash House Falcon Road Sowton, Exeter EX2 7LB Contents Section Page Executive Summary ...................................................................................................................... v Introduction ................................................................................................................................ 1 1.1 Project Background ....................................................................................................... 1 1.2 Project Aims .................................................................................................................. 3 1.3 Dune Management Plan ............................................................................................... 3 1.4 Key Contacts ................................................................................................................. 3 Site Description ........................................................................................................................... 5 2.1 Location......................................................................................................................... 5 2.2 Setting ........................................................................................................................... 7 2.3 Key Characteristics ........................................................................................................ 7 2.3.1 Present Site ...................................................................................................... 7 2.4 Natural and Historic Environment ................................................................................ 8 2.4.1 Ecology ........................................................................................................... 10 2.4.2 Designated Geological Conservation Sites .................................................... 14 2.4.3 Landscape Setting .......................................................................................... 14 2.4.4 Archaeology and Cultural Heritage ................................................................ 15 2.5 Land Use ...................................................................................................................... 15 2.6 Value of the Dunes ...................................................................................................... 15 2.7 Key Problems Experienced.......................................................................................... 15 2.8 Other Studies .............................................................................................................. 16 2.8.1 Shoreline Management Plan (SMP) Policy .................................................... 16 Factors Affecting the Beach Dune System .................................................................................. 17 3.1 Wind, Wave Climate and Tides ................................................................................... 17 3.1.1 Wave Climate ................................................................................................. 17 3.1.2 Storm Waves .................................................................................................. 18 3.1.3 Tides ............................................................................................................... 19 3.1.4 Climate Change and Sea Level Rise ................................................................ 20 3.2 Sediment Budget and Linkages ................................................................................... 21 3.3 Historical Changes ....................................................................................................... 22 3.3.1 General Description ....................................................................................... 22 3.3.2 Long Term Evolution ...................................................................................... 22 3.4 Summary of Site Influences ........................................................................................ 28 3.5 Future Changes ........................................................................................................... 29 Beach & Dune Management Plan .............................................................................................. 30 4.1 Key Issues .................................................................................................................... 30 4.2 Management Techniques to Apply at Porthcothan.................................................... 30 4.3 Plan of Action .............................................................................................................. 31 4.4 Monitoring and Response ........................................................................................... 35 4.4.1 Survey Requirements ..................................................................................... 35 4.4.2 Trigger Conditions .......................................................................................... 36 References ................................................................................................................................ 39 Appendices Appendix A Sand Dune Management Techniques & Preliminary Decision Support Tool v2.0 III CONTENTS Section Page Appendix B An Overview of Coastal Sand Dunes Appendix C Baseline (Interim) Report Appendix D Site Actions Summary Map Appendix E Summary Leaflet Document history This document has been issued and amended as follows: Version Date Description Created by Verified by Approved by 0.1 06-09-16 Draft for CC comment AF, SB, EH JR JR 1.0 07-10-16 Final AF, SB, EH JR JR IV EXECUTIVE SUMMARY Executive Summary This Beach & Dune Management Plan (BDMP) for Porthcothan has been produced as part of the Cornwall Beach & Sand Dune Management Project commissioned by Cornwall Council in 2015. The BDMP has two purposes: 1. Identify the best management approach; in terms of monitoring and intervention (when trigger levels are reached) requirements for the beach and dune system at Porthcothan; based upon the best practice framework developed as part of the Cornwall Sand Dune and Beach Management Strategy produced by Halcrow in 2009, and updated as part of developing the new BDMPs by CH2M in 2015 (see Appendix A). 2. Provide a long-term (50 year) approach to beach and dune management at Porthcothan that is based upon an up-to-date understanding of the beach-dune system and coastal processes at the site, as well as predictions of future coastal evolution. For background information, Appendix B provides an overview of how beach-dune systems work. This BDMP report provides: • An introduction to the project and BDMP (Section 1); • A summary of the characteristics (Section 2) and coastal process drivers at Porthcothan (Section 3); and • Discussion and definition of suitable management techniques to apply at Porthcothan, including a plan of action and the recommended monitoring and response programme (Section 4). The key issue to be addressed by beach and dune management activities at Porthcothan Bay is the periodic relocation of the stream channel along the toe of the dunes and consequent perception that erosion of the dunes as a result of this is detrimental to the dune system, habitats and to public safety / access onto the beach. Porthcothan Bay, including the beach, dunes and stream, is a naturally functioning system, where periodically the dunes are eroded during storms, material is drawn down from the dunes and beach to the offshore. Periodically when conditions are conducive, the stream channel relocates along the dune toe. Evidence presented in Section 3 has shown that over time, material is returned to the beach and dune system and the stream channel relocates back to its position in the north of the bay. Even when the stream channel has been artificially relocated to a straight position, the channel tends to find its own equilibrium position, forming meanders again over time. This is a process that will repeat itself in the future. In the future, as sea levels rise, the risk of erosion of the bridge and road and resulting increases in flood risk upstream will be an issue. Artificial relocation of the channel to a straight position in the north will only exacerbate this risk, as storm waves will be funnelled up the channel and towards the bridge. However, the natural channel meanders down the beach and a berm of sand in the northern meander provides a line of defence by dissipating wave energy. It is therefore recommended that the beach, dune and stream system to the north of the bay should be left to behave naturally. There is little benefit to any intervention in this system from a flood and erosion risk management or environmental management point of view. The implication of this approach in the long-term for the road and bridge will need to be considered by the Highways Authority at a later date. If as a result of allowing natural processes to occur, safe access to the beach from either of the two current footpaths is an issue, then the following can likely be done by local groups, subject to obtaining any consents that may be required from the local planning authority (planning V SECTION 1 – INTRODUCTION permission), Natural England and the Marine Management Organisation (given the proximity of the MCZ to Porthcothan): • Re-profile the dune face at the access point when required (i.e. become over-steep for safe