Estimation of bed material transport in gravel-bed streams using the virtual velocity approach: Insights from the North-Western Himalayas, India

This study focuses on evaluating the sediment mobility and transport patterns in two Himalayan rivers (Aglar and Paligad Rivers) during monsoon and non-monsoon flows. The virtual velocity approach involving the measurements of the bed proportional mobility (Y), active layer depth (d_s), displacement length and virtual velocity of mobilized grains was employed. Both local (0.5 m subsections) and wetted cross-sectional average parameters were used. While using local parameters the total annual bed material transport was estimated to be 67 100 (±20 400 t) and 18 400 t (±6000 t) in the Aglar and Paligad Rivers, respectively. Of this, nearly 60% of transport occurred during the monsoon and the overall contribution of partial transport (PT) remained low (<6%). However, based on cross-section average parameters, total transport was estimated to be 42 300 (±15 800 t) and 12 200 t (±4700 t), in Aglar and Paligad, respectively, with nearly 79% and 68% occurring during the monsoon. Moreover, the contribution of PT increased to nearly 18% and 29% for the Aglar and Paligad Rivers, respectively. Additionally, the dependence of PT on Y and full transport on d_s results in an abrupt shift in transport rates at the transition from partial to full transport, causing discontinuity in transport curves. Therefore, a unified function was proposed to represent the extent of transport for both partial and full transport, yielding continuous transport curves. These findings are particularly relevant for efficient river management as the region houses several hydropower plants and is highly vulnerable to climate change.