Large-scale roughness effect on free-surface and bulk flow characteristics in open-channel flows

Our ability to accurately predict the flow rate in open-channel flows using only the free-surface characteristics is currently limited by the capabilities of the measurement instruments and lack of adequate mathematical models to describe the free-surface velocity for the variety of situations encountered in field conditions. This dissertation is concerned with the description of open-channel flow over large-scale roughness and determination of bulk flow parameters in view of implementing non-contact, remote discharge estimation techniques. In general, flow discharge estimation requires information about flow depth, velocity distribution over the depth, bed topography, and roughness. Discharge estimation based on a pointwise velocity measurement at the free-surface requires to relate this velocity with the depth-averaged velocity using additional characteristics embedded in the signature of the bed geometry on the free-surface. These problems are addressed through experimental and numerical studies.; Experiments were performed in a hydraulic flume with varying flow depth and bed roughness. Rectangular ribs and two-dimensional dune-shaped obstacles were placed on the flume bottom to simulate different bed roughness conditions. Laser Doppler Velocimetry (LDV) measurements were made to obtain the vertical velocity profiles at various locations. Large-Scale Particle Image Velocimetry (LSPIV) measurements were made to determine the velocity distribution at the free-surface. Large-Eddy Simulation (LES) with "rigid-lid" approximation for the free-surface was used to numerically simulate the flow over rib and dune geometries.; The measured and calculated velocity fields were used to determine a representative spatially-averaged velocity profile over the roughness wavelength. A two layer, power-law model was then used to compare the flow over different roughness conditions and to establish an indexing parameter that links free-surface velocity to the bulk flow velocity. The experimental and numerical results presented here provide valuable insights into the flow structures at the free-surface and near-surface turbulence. The study therefore contributes to identification of velocity distribution laws over large scale roughness, and facilitates remote discharge measurements in open-channel flows.