[H-GM03] Geomorphology

[HGM03-P03] Characteristics of channel width and sediment particle size in the lower Stung Sen River

*Naoko Nagumo¹, Shinji Egashira¹, Sumiko KUBO² (1.International Centre for Water Hazard and Risk Management, PWRI, 2.School of Education, Waseda University)

Keywords:channel width, sediment transport, sediment sorting, Lake Tonle Sap, Cambodia

The Stung Sen River flows into the eastern end of Lake Tonle Sap in central Cambodia. Heavily affected by monsoonal rainfall, the river increases the water level and discharge in the rainy season. The lower areas within about 60 km from the lake coast are submerged every year when the lake area expands 3-to-5 folds as the lake water level also increases in the rainy season. In the lower Stung Sen River, the sediment particle size (D₅₀) at the river line during the dry season is around 400 µm at 180 km and around 350 µm at 80 km from the lake. The particle size becomes 250 µm at 40 km from the lake, showing longitudinal sediment sorting. This sediment sorting is probably influenced by changes in both the water level of Lake Tonle Sap and the discharge of the Stung Sen River. During the dry season, the river width is around 100 to 150 m up to 70 km from the lake coast (Section 1) and rapidly decreases between 70 km and 60 km from the lake (Section 2) to around 70 m at 60 km from the lake. It slightly decreases downstream after the 60 km point from the lake (Section 3) to roughly 50 m wide at the lake coast. Channel width and discharge seem to be related closely from the viewpoint of regime theory; therefore, the channel width in Section 1 is formed basically in relation to the maximum discharge of the Stung Sen River in the rainy season. On the other hand, the rapid decrease in the channel width in Section 2 is influenced by the decrease in the discharge of the Stung Sen River due to flooding in the rainy season and the decrease in discharge of the Stung Sen River and the water level of Lake Tonle Sap between the late rainy season and the early dry season. The river width in Section 3 results primarily because the river is totally submerged during the rainy season with little transportation of water with sediment taking place in the channel. The narrow channel width represents the river discharge between the late rainy season and the early dry season when the water and sediment transportation recovers in the channel as the lake water level decreases. In the lower Stung Sen River, the sediment transport capacity changes due to the cyclic changes in the water level of Lake Tonle Sap and the discharge of the Stung Sen River, which causes longitudinal sediment sorting and forms fluvial landforms, including channel width. Such changes also influence the land use pattern along the Stung Sen River.