WireWall: a new approach to coastal wave hazard monitoring

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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