Estimation of bed roughness from mean velocities measured at two levels near the seabed

A new method is developed to estimate the mean bed roughness from time series of mean velocities collected at two levels near the bed. Under the assumption of the law of the wall, the bed roughness is estimated by plotting time series of mean velocities measured at a lower level against those at an upper level. A simple criterion is also derived to censor the velocity data when only some of the data are of logarithmic distribution. This new method is then applied to estimate the bed roughness at three study sites (Site-1, 2, 5) in Moreton Bay, one of the semi-enclosed largest bays in Australia. The mean velocities were measured for 10 min every 30 min at 34 and 100 cm above the seabed at the northern entrance (h=16m, φ50=0.2mm, Site-1), the bay centre (h=15m, mud, Site-2) and Deception Bay (h=6m, mud, Site-5) in the Bay. Based on the mean velocities measured for 5 days, the bed roughness is estimated as ks=0.07, 1.92 and 0.52 cm at Site-1, 2 and 5, and correspondingly, the drag coefficient is CD=1.42×10-3, 2.96×10−3 and 2.13×10−3.