Evaluating the Effectiveness of Flood Control Scenarios in Reducing Flood Hazard Index Levels in the Ciletuh River, Indonesia

Abstract

Flood hazards in tropical watersheds are increasing due to extreme rainfall and rapid hydrological responses, particularly in low-lying agricultural regions. The downstream Ciletuh River in Ciemas District frequently experiences inundation that causes significant flood impacts. This study evaluates the effectiveness of structural flood control scenarios in reducing inundation extent, depth, flow velocity, and flood hazard levels. Hydrological analysis was conducted using rainfall frequency analysis and the SCS Unit Hydrograph, while hydraulic simulations were performed using coupled one-dimensional and two-dimensional HEC-RAS models for 2-, 5-, 10-, and 25-year return periods. Under the Q25 condition, the existing inundation area of 584 ha was reduced by 26% under the levee scenario and 32% under the normalization scenario. The combined levee–normalization scenario achieved the highest reduction of 74%, although this value is subject to uncertainty due to methodological limitations. The combined scenario also generated the highest peak velocity of 4.06 m/s, indicating improved hydraulic conveyance efficiency. This study integrates the BNPB Flood Hazard Index framework to support flood mitigation planning in the downstream Ciletuh Watershed.