Estimation of mean seabed roughness in a tidal channel with an extended log-fit method

An extended log-fit method is developed to estimate mean seabed roughness length z0 from a number of current profiles measured near the seabed in the absence of surface waves. With this new method, the mean seabed roughness length z0 is estimated by fitting the logarithmic distribution, k(z)=Aln(z/z0), to a time-independent dimensionless mean velocity profile k(z) that is derived from the number of current profiles, where A is a time-independent constant. The field data were collected at a mean water depth of 16 m in a tidal channel. An Acoustic Doppler Profiler (ADP) was used to continuously measure tidal current profiles at 16 levels above the seabed for 10 min, every 10 min over a period of 13 days. On the basis of collected field data, it is found that the bed roughness lengths estimated from individual velocity profiles are extremely fluctuating and profile-dependent. In contrast, the mean roughness lengths estimated with this extended log-fit method are shown much less fluctuating and proven more accurate. This study also investigates the sensitivity of the current shear velocity u¯* to different bed roughness lengths.