Effect of downward seepage on turbulence and morphology in mobile boundary sinuous channel

Abstract

The porous boundary of alluvial channels allows water to interact with the surrounding groundwater. With reduction in groundwater level, transfer of water from the main channel to the groundwater is significant and referred to as downward seepage. The action of downward seepage causes morphological alteration along the sinuous alluvial channel. Laboratory experiments were conducted for no-seepage and seepage conditions to study the effect of downward seepage on turbulence and bed morphology in rectangular and trapezoidal sinuous channels. The deformation along the streambed and bank of the sinuous channels showed remarkable alterations with seepage. Downward seepage caused excess sediment transport in the sinuous channels showing high erosion along banks and streambed. With increased flow rate and downward seepage, prominent depositionalzones near inner region and erosion zones near outer region of the bend were observed in sinuous channel with rectangular cross section. Sinuous channel with trapezoidal cross section showed less morphological change as the side slope was almost equal to the angle of repose of the sand. However, with the application of downward seepage, the streambed of the channel has lowered in elevation, indicating sediment transport. Mean streamwise velocity at the center of the bend showed an average percentage increase in 26% in the rectangular channel and about 20% in the trapezoidal channel with downward seepage. The Reynolds shear stress estimated with downward seepage has increased near the channel bed, indicating high momentum flux. Higher bed shear stress was indicated near the channel bed with downward seepage, which results in downstream sediment transport.