WireWall – laboratory and field measurements of wave overtopping

Abstract

In the UK £150bn of assets and 4 million people are at risk from coastal flooding, whilst the construction of sea wall defence schemes typically cost £10,000 per linear meter. With reductions in public funding and 3200 km of coastal defences, cost savings are required that do not cause a reduction in flood resistance. Increasingly there is a requirement to design new coastal flood defences with site specific tolerable hazard thresholds, with regard to wave overtopping during storms of varying severity. The traditional and preferred method for establishing these thresholds has always been physical modelling, but it is recognized that these can cost many 10s thousands of Euros. This is not always feasible, and coastal asset managers have long been looking for affordable methods that can be used to assess overtopping in the field. Recent advances in technology mean existing wave height sensors can now measure at the high frequencies (a few 100 Hz) required to obtain overtopping data, making this the ideal time to initiate a step-change in coastal hazard monitoring capabilities. By converting the existing wave measurement technology into an overtopping monitoring system WireWall, we can measure the excursions of overtopping volumes and velocities in the lee of a structure. These then can be readily integrated to obtain wave-by-wave volumes and overtopping discharges (l/s/m). At Crosby in the north west of England, the 900 m sea wall will reach the end of its design life in the next 5 years. Deployments of WireWall at this site will provide site-specific data and calibrated overtopping that will feed into the design of a new sea wall. Before deployment in the field, an extensive set of tests were carried out in a 2D wave flume. Starting with known wave conditions from a buoy near the Crosby sea wall, and values from a joint probability wave and water level study, a representation of the sea wall has been tested. Extensive testing was performed to calibrate the WireWall rig. Using traditional methods of assessing wave overtopping in the flume, the WireWall measurements could be directly calibrated against the known volumes collected in the overtopping tanks. At the time of writing, analysis of the laboratory and the flume wave overtopping data is ongoing. The paper describes how WireWall works, describes the laboratory measurements, the field deployments and presents and compares the analysis from the two systems. A successful deployment of the calibrated WireWall rig at Crosby was during the winter of 2018/2019, where waves can be seen overtopping the sea wall is shown in Fig. 1.