Sediment accumulation owing to backwater effect in the lower reach of the Stung Sen River, Cambodia

Abstract

We examined channel bars at two sites in the lower reach of the Stung Sen River, which flows into Lake Tonle Sap, Cambodia, to identify sediment transport and accumulation processes during monsoon-related flood events and to investigate how sediment transport capacity changes as a result of enhanced backwater effect of the lake. Channel bars in the lower reach of the Stung Sen River that emerge in the dry season were classified into type A (lateral bars), type B (point bars), type C (concave-bank benches) and type D (diagonal and island bars, or fluvial dunes) based on Nagumo et al.'s previous study. Type B, type C, and type D channel bars were at our study sites of SEN-01 and SEN-02, about 83 and 77km from the river mouth respectively. Computation of water surface profiles showed that sediment transport capacity of the Stung Sen River decreased abruptly after the peak flow stage because of the backwater effect from Lake Tonle Sap during the decreasing flow stage. Our observations suggest that alternating layers of sand and silt to clay layers accumulate to form type C channel bars, corresponding to changes in sediment transport capacity controlled by backwater effects from Lake Tonle Sap and by changes in flow depths and associated slackwater systems. The accumulation of alternating silt and sand layers of type B channel bars results from lateral sediment transportation that is accelerated with an increase of secondary flow, whereas development of type D channel bars is related to bedload mobility.