Water and Environment Management Framework Lot 3 – Engineering and Related Services West Wight Coastal Flood and Erosion Risk Management Strategy Appendix C - Coastal Processes and Geotechnics Summary August 2015 Document overview Capita | AECOM was commissioned by the Isle of Wight Council in October 2014 to undertake a Coastal Flood and Erosion Risk Management Strategy. As part of this commission, a brief review of coastal processes and geotechnics has been undertaken to inform the option development phase of the Strategy. Document history Version Status Issue date Prepared by Reviewed by Approved by George Batt – Assistant Coastal Jonathan Short Engineer Tara-Leigh Draft for – 1 30th March 2015 Jason McVey – comment Senior Coastal Drummond – Associate Specialist Principal Flood and Coastal Specialist George Batt – Assistant Coastal Updated Jonathan Short Engineer Tara-Leigh following – 2 4th August 2015 Jason McVey – client Senior Coastal Drummond – Associate comments Specialist Principal Flood and Coastal Specialist Scott House, Alencon Link, Basingstoke, Hampshire, RG21 7PP. i Limitations Capita Property and Infrastructure Ltd (“Capita”) | URS Infrastructure & Environment UK Limited (“AECOM”) has prepared this Report for the sole use of the Isle of Wight Council in accordance with the Agreement under which our services were performed. No other warranty, expressed or implied, is made as to the professional advice included in this Report or any other services provided by Capita | AECOM. This Report is confidential and may not be disclosed by the Client nor relied upon by any other party without the prior and express written agreement of Capita | AECOM. The conclusions and recommendations contained in this Report are based upon information provided by others and upon the assumption that all relevant information has been provided by those parties from whom it has been requested and that such information is accurate. Information obtained by AECOM / Capita has not been independently verified by Capita | AECOM, unless otherwise stated in the Report. The methodology adopted and the sources of information used by Capita | AECOM in providing its services are outlined in this Report. The work described in this Report was undertaken between December 2014 and March 2015 and is based on the conditions encountered and the information available during the said period of time. The scope of this Report and the services are accordingly factually limited by these circumstances. Where assessments of works or costs identified in this Report are made, such assessments are based upon the information available at the time and where appropriate are subject to further investigations or information which may become available. Capita | AECOM disclaim any undertaking or obligation to advise any person of any change in any matter affecting the Report, which may come or be brought to Capita | AECOM’s attention after the date of the Report. Certain statements made in the Report that are not historical facts may constitute estimates, projections or other forward-looking statements and even though they are based on reasonable assumptions as of the date of the Report, such forward-looking statements by their nature involve risks and Capita | AECOM specifically does not guarantee or warrant any estimate or projections contained in this Report. ii Contents Glossary 1. Introduction 1 1.1 Context 1 1.2 Purpose of this Report 1 1.3 Data/Reports Reviewed 1 2. Overview of Coastal Processes and Geotechnics by SMZ 4 2.1 SMZ1: Needles headland (Fort Redoubt to southern limit of Totland Bay) 4 2.2 SMZ2: Totland and Colwell bays (Southern limit of Totland Bay to Fort Victoria) 5 2.3 SMZ3: Yarmouth and the Western Yar (Fort Victoria to Port la Salle, including Freshwater Bay) 6 2.4 SMZ4: Newtown coast (Bouldnor cliff to Thorness Bay, including Newtown Estuary) 7 2.5 SMZ5: Gurnard and Cowes headland (Gurnard Luck to Cowes Parade) 8 2.6 SMZ6: Cowes, East Cowes and Medina (Cowes Parade to Old Castle Point, East Cowes) 9 Figures Figure 1-1: Map of the study area showing delineation of Strategic Management Zones 3 Appendices Appendix A – Coastal Processes Report – Isle of Wight Council, November 2014 Appendix B – Existing and Predicted Water Levels Appendix C – Future Erosion Predictions iii Glossary Accretion – The accumulation of sand or other beach material due to the natural action of waves, currents and wind Aspect – The direction that the section of frontage faces or points towards Astronomical tide – The tidal levels and character which would result from gravitational effects, e.g. of the Earth, Sun and Moon, without any atmospheric influences Back beach – The section of beach extending landwards from the high water mark to the point where there is an abrupt change in slope or material; also referred to as the backshore Beach profile – A cross-section taken perpendicular to a given beach contour; the profile may include the face of a dune or seawall, extend over the backshore, across the foreshore, and seaward underwater into the nearshore zone Bed load – Sediment transport mode in which individual particles either roll or slide along the seabed as a shallow, mobile layer a few particle diameters deep Climate change – Refers to any long-term trend in mean sea level, wave height, wind, speed, drift rate etc. Coastal defence – A term used to encompass both coastal protection against erosion and sea defence against flooding Dredging – Actions such as excavation, digging, scraping, draglining, suction dredging etc. to remove sand, silt, rock or other underwater sea-bottom material Epochs – The three periods of time in which the Strategy is reviewed in. The first epoch is 0-10 years, the second epoch is 10-40 years and the third epoch is 40-100 years Erosion – Coastal erosion can be defined as the removal of material from the coast by wave action, tidal currents and/or the activities of man, typically causing a landward retreat of the coastline Estuary – Mouth of a river, where fresh river water mixes with the seawater Event – An occurrence meeting specified conditions, e.g. damage, a threshold wave height or a threshold water level Extreme – The value expected to be exceeded once, on average, in a given (long) time period Fetch – The distance over which a wind acts to produce waves – also termed fetch length Foreshore – The intertidal area below highest tide level and above lowest tide level Groyne – Shore protection structure built perpendicular to the shore; designed to trap sediment and/or to reduce longshore currents Landslide – A coastal landslide can be regarded as the movement of sediment from an area of elevated topography to the foreshore Littoral zone – Zone from the beach head (the cliff, dune or seawall forming the landward limit of the active beach) seawards to the limit of wave-induced sediment movement Response factors – Used in the Walkden and Dickson equation to estimate future shore recession. Represents the response time of the coast as a result of changing sea level rise rates Return period – Average period of time between occurrences of a given event Sea level rise – The long term trend in mean sea level Sediment – Particles of rock covering a size range from clay to boulders iv Seepage erosion – Can be defined as the condition when finer particles are carries out of the soil mass under certain hydraulic gradients. The consequence of seepage erosion is to cause progressive failure for a slope and finally slope failure occurs Significant wave height – The average height of the highest one third of the waves in a given sea state SMP (Shoreline Management Plan) – a high-level non-statutory planning document which provides a large scale assessment of the risk associated with coastal processes and presents the a long-term policy framework to reduce these risks to people and the developed, historic and natural environment in a sustainable manner SMZ (Strategic Management Zone) – A group of Policy Units (divisions of the Strategy frontage arising developed in the SMP) with similar characteristics in which overarching, wider scale options to manage the flood and erosion risk are developed Spit – A long, narrow accumulation of sand or shingle, lying generally in line with the coast, with one end attached to the land the other projecting into the sea or across the mouth of an estuary Still water level – Average water surface elevation at any instant, excluding local variation due to waves and wave set-up, but including the effects of tides and surges Surge – Changes in water level as a result of meteorological forcing (wind, high or barometric pressure) causing a difference between the recorded water level and that predicted using harmonic analysis: may be positive or negative Swell waves – Remotely wind-generated waves. Swell characteristically exhibits a more regular and longer period and has longer crests than locally generated waves Tidal current – The movement of water associated with the rise and fall of the tides Wave refraction – Process by which the direction of approach of a wave changes as it moves into shallow water v 1. Introduction 1.1 Context The Coastal Processes Report, 2014 (Appendix A) and additional key documents received from the Isle of Wight Council (‘the Council’) have been reviewed to identify, understand and apply the most pertinent coastal and geotechnical processes that should be taken into consideration during strategic option development. To compliment this, additional work on predicting still water levels and estimating erosion was also undertaken. 1.2 Purpose of this Report This summary document is primarily intended to inform the baseline understanding underpinning the appraisal of Strategy options. Additionally, the review has provided outputs which have been used in the hydraulic modelling and the economic damage and benefit calculations. The technical details for the development of the predicated extreme water levels are included in Appendix B. The technical approach to the prediction and mapping of future recession through changes in erosion rate are included in Appendix C.