Near-bottom flow and flow resistance for currents obliquely incident to two-dimensional roughness elements

Measurements of the three components of the velocity when a steady current is flowing over a rippled bottom are presented. The bottom consisted of equally spaced triangular bars placed at angles of 0 degrees, 30 degrees, 45 degrees, and 60 degrees to the incident flow. The experiments show that close to the bottom, there is a transverse component of the velocity, which, for the larger angles of incidence, can be of the same order of magnitude as the main flow. The spatial average velocity profiles obtained from the measurements performed between two ripple crests show that close to the bottom, there is a well-defined logarithmic region. The bottom roughness obtained by fitting a logarithmic law in the near-bottom region to the longitudinal component of the flow depends strongly on the angle of incidence. In contrast, the analysis of the near-bottom velocity component in the direction perpendicular to the ripple axis shows that the bottom roughness is independent of the angle of incidence. These experimental observations support the concept of a direction-dependent equivalent bottom roughness when currents are obliquely incident to two-dimensional bottom roughness elements, such as wave-generated ripples.