Flood is a common natural disaster, especially in Indonesia, and causes a huge amount of material and social loss. West Sumatera is one of the provinces that is often affected by flooding, and one of the most affected area is Solok Selatan District. In this district, there is a junction of two watersheds that often affected by floods, which are Batang Suliti watershed and Batang Bangko watershed. The junction of the Batang Suliti river and Batang Bangko river is in Sungai Pagu Sub-District. One example of a huge flood occurred on February 8, 2016 where the rain occurred for 17 hours long which caused a 0.5–3 m deep of flood inundation and covering 11.9 km2 area. Hence, it is necessary to know how much is the capacity of the river’s cross-sections to accommodate the water flow before runoff discharge occurs. In this study, we calculate the runoff discharge using HEC-RAS 4.0 software and Q25 discharge which are 332.42 m3 /s for the Batang Suliti river and 336.43 m3 /s for the Batang Bangko river.

Keywords : Batang Bangko; Batang Suliti; Flood; HECRAS.