The effect of floodplains roughness on hydraulic flow in compound channel with non-prismatic floodplains

This paper investigates the effects of roughness on flood plain on hydraulic overbank flow in a compound channels with non-prismatic floodplains. Experiments are done using three divergence angles and three-roughness size on the floodplain. The velocity was measured using a three dimensional acoustic Doppler velocimeter and directional current meter in a lattice along the divergence channel in three portions of the flume (entrance, middle and end of the divergence) with different depth ratio. Using the experimental data, the values of the shear stress, depth–averaged velocity, local and zonal roughness coefficients, turbulence parameters and divided discharge between main channel and floodplain were evaluated. The results show that: With increasing the floodplains roughness, in non-prismatic section, the ratio of the velocity at floodplain on main channel decreased about 31% and turbulence flow increased about 25%. Also under these conditions, the shear stress at interface zone was increased from 26% to 48. As well as with increasing depth ratio, the above mentioned values are markedly reduced.