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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. [ABSTRACT FROM AUTHOR]

An alluvial river system mostly comprises of series of turns, loops or bends in its course. This study presents the experimental investigation of the turbulence structure at the centre of a sinuous sand bed channel with rigid banks. To examine the turbulent flow, distribution of instantaneous velocities, Reynolds stresses and bursting events were analysed at the centre of the bend. In this regard, Reynolds shear stresses (RSS) distribution exhibits the existence of reversal of flow in a bend. Close to the channel bed, RSS presents a triangular positive distribution as has been observed in straight flow, and away from the bed, it attains negative value. Furthermore, an assessment of bursting events shows the ejection and sweep events prevail near the bed whereas away from the bed outward and inward events are prominent. In addition to this, analyses of these events reveal that contributions from sweep events (inrush of fluid moving towards the bed) are maximum nearer to the channel bed. Turbulence scale such as Taylor micro scale and Eulerian scale were also determined to measure the eddy size and its turn over time. The Eulerian length scale and Eulerian time scale are observed greater at the centre of the flow depth. [ABSTRACT FROM AUTHOR]

The presence of a small seepage velocity in streams and rivers induces transfer of mass and momentum near the sediment boundary, causing bank failure and instability of hydraulic structures. This experimental study is intended to report near-bed hydrodynamics and morphological characteristics under the impact of downward seepage. A laboratory investigation was conducted in a sinuous rectangular alluvial channel with rigid sides and movable uniform sand bed. From the distribution of Reynolds shear stress (RSS), the transverse or radial motion of the sediment across the bend was identified. High values of RSS in the outer region relate to higher momentum flux, eroding the outer bend. The contribution probability of ejection and sweep events are highest in the bend region. For seepage, the contribution probability of the events has increased. Moreover, the altered flow with seepage affected the transport of sediment and caused noticeable modifications along the channel bed. With seepage, the erosion rate has been enhanced along the outer bend of the channel. [ABSTRACT FROM AUTHOR]