Field and laboratory measurements of mean overtopping discharges and spatial distributions at vertical seawalls

Within the CLASH project, wave overtopping at the vertical seawall at Samphire Hoe was measured by HR Wallingford (HRW), and compared laboratory tests in 2 & 3 dimensions carried out at the University of Edinburgh and HRW. At Samphire Hoe, overtopping volumes were captured in three volumetric tanks capable of measuring wave-by-wave and total overtopping volumes. The three tanks were placed progressively farther back from the seawall edge so that the spatial distribution of the overtopping discharges could be determined. The field measurement equipment was successfully deployed on three occasions, and measured overtopping discharges ranged from that barely considered to be hazardous to the public to over q = 3.0 l/s/m. The 2d testing at Edinburgh was modelled at a scale of 1:40, and the 3d model at HRW was modelled at 1:20. For both sets of laboratory tests, a range of conditions, representative of the storm wave conditions and water levels, was reproduced in addition to a set of parametric conditions. The storm conditions allowed a direct comparison between the field and laboratory measurements, and the parametric conditions were used to test the generic overtopping behaviour of the structure. For both sets of laboratory tests, mean overtopping discharges and the spatial distribution were measured separately. Analysis of the distribution data relates the proportion of the discharge that has landed as a function of (x / Lo); where x is the distance behind the crest, and Lo is the offshore wavelength. Analysis of the field, 2d & 3d laboratory data, and empirical prediction methods have not identified any scale effects for overtopping discharges at vertical and near-vertical seawalls.