TESTS OF BED ROUGHNESS MODELS USING FIELD DATA FROM THE MIDDLE ATLANTIC BIGHT

Four bottom roughness models are tested using field data from the inne r shelf of the Middle Atlantic Eight. Bottom roughness plays a signifi cant role in calculations of sediment concentration profiles and curre nt velocity profiles. The importance of each of the three parts in the roughness models (grain roughness, ripple roughness and sediment moti on roughness) vary depending on forcing conditions. Consistent with th e observations of others [e.g. Cacchione and Drake, 1990 (The sea, Vol . 9, pp. 729-773); Wiberg and Harris, 1994 (Journal of Geophysical Res earch, 99(C1), 775-7879)], our results show that the models of Smith a nd McLean (1977; Journal of Geophysical Research, 82, 1735-1746), Gran t and Madsen (1982; Journal of Geophysical Research, 87, 469-481) and Nielsen (1983; Coastal Engineering, 7, 233-257) overestimate the sedim ent transport roughness under sheet-flow conditions. However, the Niel sen (1983) model can predict the ripple roughness under moderate energ y conditions quite well. A refined bottom roughness model is proposed that combines Nielsen's ripple roughness model and a modified sediment motion roughness model k(b) = d + 8 eta (eta/lambda) + Omega d(psi(m) ' - psi(c)). This sediment motion roughness is defined in such a way t hat it is proportional to the maximum skin friction Shields' parameter . The proportionality constant, Omega, is determined by fitting the mo deled roughnesses and shear velocities with the field observations. Th e calculated velocity profiles and roughness using the refined roughne ss model, with Omega = 5, compare well to the field observations made under both moderate and high energy conditions at a sandy inner shelf site.