Numerical Modeling for Flood Hazard Mapping of Kedurus River Basin

Kedurus River serves to drain flood discharge from Gresik Regency and empties into Mas River Surabaya. In the rainy season, the West Surabaya area is often submerged by floods due to high rainfall and Kedurus River runoff. According to the results of the Surabaya City inundation monitoring survey by Public Works Agency for Highways at the end of 2017, the total area of flooding in Rayon Wiyung is 274.93 hectares. The biggest number contributors came from the Kedurus River area. So that a flood analysis of the downstream Kedurus watershed is needed to create a flood hazard map. Flood analysis in Kedurus River using HEC-RAS software, Flood simulation uses flow hydrograph as an upstream boundary condition with 5 years, 10 years and 50 years return periods. Meanwhile, rainfall is simulated as precipitation. In addition, secondary channels are simulated as lateral flows and reservoirs as storage areas. Flood simulation was carried out with three scenarios, the results of the three scenarios were compared with the results of the flood survey. From the simulation results obtained depth, velocity, area and duration of flooding, then mapping the flood map using ArcGIS software. The results of the HEC-RAS flood simulation with all three scenarios can be concluded that the flood depth is maximal 1.1 meters , the velocity in the inundation area is between 0.05 m / s to 0.21 m / s, the widest flood bank is 28.61 hectares. Subdistricts that have a high threat level are Wiyung District, especially in the Karangan Jaya area.