Turbulence Measurements in Open-Channel Flows over Artificial Bed Forms

Measurements of mean and turbulence characteristics in uniform open-channel flows of constant mean depth over artificial fixed one-dimensional periodic bed features were obtained using two-component laser-Doppler velocimetry. Two types of bed forms, based on triangular elements of the same height ( 20% of the flow depth) and wavelength ( 12.5 times the height) but of different shape, were studied: (1) 45° triangle ridges separated by a flat region; and (2) triangular elements approximating dunes with a mildly sloping upstream face instead of a flat region. The role of the total shear velocity, based on the mean energy slope and the mean flow depth, as a possible velocity scale is studied. Velocity defect and the Reynolds shear stress profiles were generally well approximated by the flat-bed profiles for regions of flow far from the bed. Sufficiently far from the recirculation and reattachment zones, turbulence characteristics collapsed fairly well when scaled with the total shear velocity over most of the depth, but do not follow the flat-bed behavior in the lower part of the flow. Possible implications of the clear-water measurements for the description of suspended sediment concentration profiles and total suspended load computations are explored.