HEC-RAS Based Modeling for the Flood Mitigation Analysis of the Karema River

Abstract

The Karema River, located in the Karema Watershed (DAS Karema) in the West Sulawesi Province, is classified as steep-sloping, with a topographical condition of 15%, causing rainfall from the upstream area to rapidly flow downstream, increasing the risk of flooding in low-lying areas with high population density. The objectives of this study are: analyzing the flood-affected areas along the Karema River and developing effective and efficient evacuation routes to mitigate the impact, using quantitative descriptive and spatial analyses with HEC-RAS and ArcGIS software. The results indicate that the Simboro rainfall station represents the upstream watershed area of the Karema River. The Log Pearson Type III method was used to determine the design rainfall, yielding a 25-year return period rainfall of 258.443 mm. The PSA 007 method (Genta model) was applied to obtain the hourly net rainfall. A flood design analysis was performed deploying the Synthetic Unit Hydrograph (SUH) Nakayasu method, resulting in a peak flood discharge (Q25) of 593.443 m³/s. A flood hydraulic simulation of the Karema River was performed using the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) version 6.4.1 with unsteady 2D flow analysis. This simulation produced a flood-affected area of 2.432989 km², with an average flood velocity ranging from 52.08 m/s to 61.67 m/s and an average flood depth ranging from 4.4 m to 4.9 m in two sub-districts: Karema and Simboro. This area falls into the high-hazard class for the flood category. Therefore, a flood disaster evacuation/mitigation route was developed based on spatial analysis, resulting in two types of evacuation routes: primary collector roads with two evacuation paths and primary local roads with eight evacuation paths. Additionally, eight checkpoint locations were marked to indicate the flood arrival times.

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