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.

Understanding bedload transport fluctuations in rivers is crucial for complementing the existing knowledge on sediment transport theory. In this contribution, we use a natural-scale laboratory flume to analyse bedload transport fluctuations in non-uniform sand under normal flow conditions. Based on the significance of downward seepage, we incorporate the seepage effect on bedload transport over a non-uniform sand bed channel. The weight of the dry material was measured and the volumetric transport rate per unit width (bedload transport rate) was estimated. An important observation is that the bedload transport rate initially rapidly increases with time and reaches a maximum value. Based on experimental data, we propose an empirical expression to estimate temporal bedload transport. In addition, an empirical model for bedload transport is proposed by incorporating downward seepage among other variables. The performance of several existing bedload transport formulae was also taken into account by the experimental datasets.