FLOOD MODELLING OF RIAM KIWA WATERSHED USING HEC-HMS

Abstract

At the end of 2021 to early 2022 several areas in Banjar Regency were flooded. This incident had a major impact on the surrounding area and became a national concern. One of the causes of the disaster was high-intensity rainfall which caused the water in the Riam Kiwa River to overflow. HEC-HMS software was used to simulate the designed flood discharge. This research aims to model the magnitude of the designed flood discharge with return periods of 5, 25, 50, and 100 years. Due to limitations of rainfall data from the field, satellite data from GSMaP were collected. In the distribution of rainfall frequency analysis, the Log Pearson type III method was chosen for calculating designed rainfall. Then, the Modified Mononobe Method was applied to calculate hourly rainfall intensity. Other data i.e., soil types, slope gradient and land use were analyzed using ArcGIS application. These three data produced hydrological parameters which were used as input values in the HEC-HMS using the Soil Conservation Service (SCS) method. Three parameters’ values show that Riam Kiwa watershed has more than 90% clay with a slope gradient of 83.58% (steep slope) and land use is predominantly consisted of mixed dryland agriculture (47.73%). From HEC-HMS simulation, it was found that Riam Kiwa watershed with an area of 1781.77 km² produces maximum flood discharge ranges from 8,822.3 m³/s to 17,058.6 m³/s; with a water volume ranges from 182,991.9 m³ to 340,889.6 m³ in 5 years to 100 years return periods, respectively. From the results, it can be concluded that impermeable clay soil and steep slope are the two major factors that contribute to the flooding occurrence in the area.