MAPPING THE FLOOD INUNDATION AREAS USING HEC-GEORAS AT BATANG MAHAT RIVER

Abstract

The Batang Mahat region in Lima Puluh Kota Regency is highly prone to flooding, with significant events recorded in 1961, 1968, 1972, 1978, 1984, 1991, 1998, 2005, 2017, 2019, 2020, and 2021. The most catastrophic flood occurred in March 2017, inundating public areas and residential zones in the Pangkalan Koto Baru district. This study aims to simulate a flood inundation map for Batang Mahat using HEC-GeoRAS 10.8, ArcGIS, and HEC-RAS 6.2. HEC-GeoRAS is an ArcGIS extension designed for pre-processing geospatial data for HEC-RAS. For hydrological analysis, the study employed the Log Pearson Type III and Nakayasu Synthetic Unit Hydrograph models within HEC-RAS. The simulation covered a 9.26 km² area in Batang Mahat, from Batu Kisok upstream, analyzing discharge return periods of 2, 5, 10, 25, 50, and 100 years. The findings showed that flooding commenced at the Q5-year discharge return period and increased substantially at the Q25-year level, with an 84.44% rise in flood area compared to the Q10-year period. The 2017 flood was classified as a Q50-year event (2003.07 m³/s), affecting 2.47 km² of the 9.26 km² area studied. Validation through community surveys and interviews in Pangkalan Koto Baru yielded a data correlation value of 0.98, confirming the accuracy of the results.