Reducing regional flood and erosion risk from wave action on the Channel Coast

SHINGLE-B, the online tool for prediction of shingle beach profiles developed as part of the following study, can be found at the Channel Coast Observatory website

1. Background

A region-wide assessment of design wave conditions and beach responses between Start Point, Devon and Pevensey, Sussex is being undertaken by the Channel Coastal Observatory. The three year study will investigate improvements to definition of standards of service for coastal schemes, improved flood forecasting, and improved definition of design wave conditions.

Regular occurrences of unexpected wave overtopping and breaching of barrier beaches have occurred at numerous sites along the English Channel Coast, leading to unpredicted damage and substantial maintenance bills. In every incident coastal flood forecasts have failed to identify that a significant event was impending. About 40% of storm events are affected by these conditions, yet they are not considered in any design or assessment methods. The implication is that many defence systems are seriously underdesigned. The two particular features of significance under bimodal wave periods are:

1. Increased overwashing of barrier beaches e.g. Chesil Beach
2. Increased wave run-up, overtopping and erosion of beaches e.g. Hayling Island

The sites at greatest risk are in the west of the region and generally face open sea to the west or south west, where swell waves can penetrate to the shoreline. The problem reduces towards the eastern end of the Channel Coast, where swell waves are rare. In broad terms, the problem is insignificant to the east of Pevensey. Most of the solent is unaffected by these conditions, as swell waves are unable to penetrate this area. The frequency of damaging events is much higher in western parts of the channel.

The project is highlighted as a high priority in the action plans of the North Solent Shoreline Management Plan and Poole and Christchurch Bays Shoreline Management Plan. It is relevant also to delivery of action plans for all Shoreline Management Plans on the English Channel Coast (total 7 SMPs). There is an urgent requirement to improve flood forecastring, definition of standards of service against wave overtopping and design of beach management schemes subject to bimodal wave conditions.

£250,000 of Flood Defence Grant in Aid funding has been attracted from the Environment Agency in support of the region-wide assessment to examine the impacts of combined swell and storm (bimodal) waves on the shingle beaches of the SCOPAC region. This funding follows the SCOPAC supported investigations into extreme wave conditions and which has acted as the catalyst for this funding [see Extreme Wave Conditions within the SCOPAC region]

The test programme has been revised following the recent winter storm events (2013/2014) and it is intended to attempt to replicate some of these events for sites at Chesil Beach, Hurst Spit and Hayling Island. The Environment Agency Wessex area have expressed some considerable interest in this following the recent problems at Chesil Beach and it is anticipated that some additional testing may be required to satisfy the requirements of this site.

The model test facility at HR Wallingford is now available and the model construction is expected to commence in late May. A visit to the test facilities will be arranged once the model has been established, which is now likely to be in early summer. A general SCOPAC visit will be arranged and additional site specific visits available for officers wishing to see tests for specific locations.

2. Aims & Objectives

The key aim of the study is to satisfy the Action Plan recommendation in the North Solent, Isle of Wight and Poole and Christchurch Bays SMPs which highlighted the need for a region-wide investigation of bimodal wave conditions at beach management sites. This will be achieved through an integrated study that bridges all of the SMPs and which benefits from an economy of scale in both cost terms and also in consistency of approach on a region wide basis.

Urgent investigations will be conducted to develop improved design and forecasting wave conditions and associated management techniques for beaches on the English Channel Coast.

The key objectives of the project are to:

• Produce definitions for design and forecasting, for bimodal wave period conditions; which are not currently part of standard design and assessment practice;
• Provide additional evidence of bimodal wave impacts through analysis of wave data from the regional monitoring programmes;
• Produce probability distributions and extreme conditions for bimodal conditions for at risk sites on the English Channel Coast;
• Quantify the impacts of a range of bimodal conditions on beach response;
• Produce generic guidance to be used in design and assessment of standards of service of defences;
• Incorporate findings from physical model tests in new beach management design guidance and flood forecasting guidance;
• Publish and publicise the outputs through the regional coastal groups and as through reviewed technical publications, with free and freely available access for all to use.

The project will:

• Be undertaken by a team which will be led by NFDC and involve various EA, Local Authority and Consultant teams to research and develop joint probability assessments and modify design approaches for sites that are subject to combined swell and windwave (bimodal) conditions. Work will involve analysis of field wave and beach data and physical modelling of beaches under bimodal conditions;
• Enable the analysis of existing wave data, measured at wave buoys along the Channel Coast since 2002;
• Test the response of beaches to bimodal wave conditions in physical model tests and using field data from the regional coastal monitoring programmes;
• Assess the results of physical model tests of beaches subject to bi-modal wave conditions;
• Inform the design, management and performance of existing Beach Management Plan sites;
• Identify direct resource and management implications for the maintenance of defence structures and beaches subject to bimodal conditions;
• Inform the development of coastal flood forecasting.

The project will deliver:

• Identification of improved triggers for coastal wave forecasting;
• Identification of sites at risk on the Channel Coast;
• Assess and quantify current standards of protection of at risk sites;
• Produce a probabilistic approach to the definition of frequency and intensity of bimodal conditions at sites throughout the English Channel;
• Identify the response of shingle beaches to these conditions;
• Identify additional measures required to improve standards of protection at key sites;
• Provide a definition of the conditions to be used in design where bimodal wave period conditions occur.