Flood Risk Assessment and Sustainable Mitigation Strategies for the Wulan River Using HEC-RAS 1D-2D Modeling

The frequency and intensity of flood disasters in Indonesia have shown an increasing trend due to climate change and rapid urbanization. This situation has led to significant moral and material losses and damaged infrastructure. This study focuses on the flooding problem in Demak Regency, Central Java, which regularly experiences floods due to heavy rainfall and insufficient river capacity, resulting in embankment breaches at several points along the Wulan River. The study aims to analyze the hydrological conditions of the Wulan River in Demak Regency and assess flood potential in the area through hydraulic analysis. Various datasets, including daily rainfall, digital elevation models (DEM), land use, maximum tidal levels were used as inputs in this study. The methodology involved hydrological analysis, including rainfall frequency analysis, synthetic unit hydrographs, and hydraulic modeling using HEC-RAS 6.6 1D and 2D model. The hydrological analysis of the Wulan River revealed its characteristics, with a watershed area of 4.047 km2 and a main river length of 189.47 km. The 50-year return period rainfall, used for planning, was determined through frequency analysis and tested for distribution fit using the Chi-Square and Smirnov-Kolmogorov tests, resulting in an R50 value of 112.92 mm. The 50-year flood discharge, derived from the Nakayasu Synthetic Unit Hydrograph, was calculated at 1,422.67 m3/s. Hydraulic analysis using HEC-RAS provided insights into the river's capacity, flood inundation depth, and affected areas. Based on the findings, this study recommends river channel modification by deepening the riverbed to enhance the Wulan River's capacity to accommodate the design flood discharge.