Sand Bed Roughness in the Nearshore, COAST 3D Experiment, Egmond aan Zee, the Netherlands

Sand bed roughness was measured in the near shore during a variety of hydrodynamic conditions using an array of seven 1 MHZ sonar altimeters mounted on the WESP (an amphibious vehicle used for measuring large-scale bathymetry) during the COAST 3D Experiment 1998, at Egrnond aan Zee, The Netherlands. Corollary waves were modeled using the Thronton and Guza (1983) wave transformation model. Wave height and current measurements were made in the surf zone using pressure sensors and electromagnetic current meters, and offshore wave heights were measured using an advectional wave-rider buoy. Measurements of sand bed roughness showed patterns similar to those observed by Clifton et al. (1971), but are highly variable both spatially and temporally with dependence on large scale morphology and wave and current conditions. Mobility number (Psi) calculated from the modeled wave field and also from measured currents revealed that roughness is a function of Psi. Roughness was observed to be highly variable at low Psi (< 100) calculated from the modeled wave heights. As Psi increased (100-150), roughness decreased gradually, but was still highly variable. As PSI reached values greater than 150, roughness was restricted to the lowest observed values (< 3 cm) implying planar beds under sheet flow conditions.Naval Postgraduate School Monterey CA is the author of 'Sand Bed Roughness in the Nearshore, COAST 3D Experiment, Egmond aan Zee, the Netherlands', published 2000 under ISBN 9781423536222 and ISBN 1423536223.