WireWall: a new approach to coastal wave hazard monitoring

Many countries with a sea border need manmade defences to protect them from coastal hazards such as flooding. In the UK 3200 kilometres of coastline are defended, particularly in seaside towns and cities. This is to prevent flooding and to protect people, property and infrastructure from the harm caused by large waves that can occur when a severe storm happens at the same time as a high tide. Building strong coastal defences can be costly, often about £10,000 per meter, and needs careful planning.

When planning coastal defences a lot of data are needed to understand the potential hazards that might occur in decades to come. To obtain this data for a particular site usually means monitoring the local tides, wave heights, and beach levels for a period of 5 to 10 years. These data are used in numerical tools (e.g. EurOtop) to test which seawall design is most suitable and how high it needs to be to provide protection for the next 100 years. The tools do this by estimating the "overtopping hazard" for each design, i.e. what volume of water might come over the wall during storm conditions. Accuracy of the tools is assessed by checking outputs against measurements of overtopping volumes during storms. Field experiments have previously used large tanks placed behind the seawall to catch the water that comes over. Such experiments are very costly and can be difficult to do, so only a few have been made - usually at sites with very different structures (e.g. dikes) and for only a few days. They also only provide a limited amount of data and none at all on the speed of the water that overtops: an important factor for public safety. This lack of measurements means there is large uncertainty in the numerical estimates of the hazards, so sea defences are overdesigned to have large safety margins and may therefore cost much more than they need to.

This project aims to take a low-cost instrument that has previously been used to measure waves in the open ocean, and convert it into a system ("WireWall") that will measure coastal overtopping hazard. Recent improvements in technology now make it possible to measure at the very high frequencies required to record the fast moving overtopping water (a few hundred times a second for a jet of water travelling up to 100 mph). The system will employ a 3-dimensional grid of capacitance wires that sense contact with saltwater. This signal will be used to measure the volume and speed of overtopping at vulnerable locations on the 900-meter-long seawall at Crosby in the North West of England. This seawall is reaching the end of its design life and intense monitoring of the local conditions has begun to aid the design of a new wall.

This project includes engineers, environmental hydraulics experts and oceanographers who have complementary field, laboratory and modelling expertise. Our project partners (Sefton Council, Environment Agency, Balfour Beatty, Marlan Maritime Technologies and Channel Coastal Observatory) are involved in commissioning, designing and constructing coastal defences, and include government authorities and private consultancies. They will provide existing monitoring data at Crosby, and will advise on the methods and tools routinely used in the design of a new seawall. We will use this information to optimise the configuration of the WireWall system and its deployment at Crosby. Data obtained by WireWall will improve the tools used when designing the new seawall by calibrating the numerical estimates of overtopping hazards to those observed.

In the future WireWall could be incorporated into new seawall structures to enable long-term monitoring. The ability to observe trends and abrupt changes in hazardous conditions (due to defence degradation, climate change and sea level rise) would support shoreline management plans and provide data to validate operational flood forecasting systems.

Keywords: Shoreline monitoring; Coastal defence; Wave overtopping hazard

The observations of key overtopping parameters collected by the WireWall system at the Crosby seawall will be used to reduce the uncertainty in overtopping estimates derived from the numerical tools and methods used in industry when designing new coastal defences. In the next 5 years funding for a new 900 m long coastal defence at Crosby will need to be secured through grant aid administered by the Environment Agency and from the Sefton Council's Flood and Coastal Erosion Risk Management resources. The dataset collected at this site will allow validation of the numerical tools and methods used by project partners when designing the new Crosby defence. Use of the tools to simulate a wide range of past storm events that caused overtopping will allow calibration of the site-specific safety thresholds that are needed to inform a) the new seawall design and b) the timing of promenade and car park closures for public safety. By having these valid tools and calibrated safety tolerances our partners will have reduced uncertainties in their wave overtopping estimates and hence a reduced need to overdesign the new defence at Crosby. If construction costs are estimated at £10,000 per linear meter and the WireWall project reduces uncertainty in the overtopping estimates by 10%, this would result in a direct saving of about £500,000 for our partners for this one seawall alone. Scaled up nationally, a similar calculation would give a saving of £125 million on the £2.5 billion that the UK Government has committed to flood defences over the next 6 years. Additional on-going savings will be made through reduced maintenance costs over the 100-year design life of the new scheme. Where the height of a new scheme can be limited the aesthetics of the shoreline may also be enhanced, thus improving the scheme's dual purpose as a public amenity.

The deployment of WireWall at Crosby will be the first step towards the development of an overtopping monitoring system that could ultimately be integrated into new coastal schemes as part of the UK's regional shoreline monitoring programmes. These programmes support shoreline management plans (SMPs). SMPs were adopted in 2001 by Coastal Groups across England and Wales, to deliver management activities that prevent future generations being tied to costly and unsustainable management practises 100 years from now. A long-term national dataset of key wave overtopping parameters (as obtained by WireWall) could provide an evidence base to assess the effectiveness of shoreline management policies and schemes in support of flood hazard management activities. Promotion of the WireWall capabilities through our project's Wider Interest Group (which represents many parties who are involved with the management of, or research into, coastal hazards across the UK) will provide potential opportunities to up-scale the deployments of WireWall to enable cost savings in the implementation of other new defence schemes nationally. Dissemination of the WireWall approach through the Channel Coastal Observatory will ensure consistency in future data collection, processing and management across the National Network of Regional Coastal Monitoring Programmes. This will ensure the value of the dataset grows with each supplementary data collection. Once large enough the observed dataset will enable development of improved empirical methods for use by industry to estimate overtopping hazard specifically for seawall structures. This will be of particular value to our industry partner Balfour Beatty and other consultancies, since current tools are developed primarily from dike and flume experiments.

Continued WireWall deployments at Crosby post-project, and deployments of the system at other sites across the UK, could be achieved through intellectual property (IP) licensing that would allow commercialisation opportunities with our partners Marlan Maritime Technologies and expand their business opportunities.