Geographia Technica, Vol 20, Issue 1, 2025, pp. 127-142
HEC-RAS-BASED URBAN FLOOD SIMULATION FOR ENHANCING MITIGATION SCENARIOS IN THE GAJAHWONG RIVER, INDONESIA
Suci Purnama SARI 
, Slamet SUPRAYOGI , Satrio BUDIMAN
ABSTRACT: Rapid urban development in Yogyakarta, including increased impermeable surfaces and inappropriate land use, has intensified surface runoff and flood risks, particularly along highways and riverbanks. The study of flooding on the Gajahwong River is significant due to the urban residential areas situated along its banks. This research aims to analyze the peak discharge and flood inundation model and estimate the ideal embankment height for flood mitigation in the Gajahwong River section. The peak discharge is calculated using the rational method, employing daily maximum rainfall data from 2001 to 2021, and flood inundation modeling is conducted using the Hydrological Engineering Center River Analysis System (HEC-RAS) software with a one-dimensional (1D) model for steady flow analysis. The Gumbel distribution was selected in the maximum rainfall frequency analysis with the Gumbel probability plot results showing a very high level of fit (R2 = 0.9792). The high peak discharge in the Gajahwong Watershed is primarily driven by extensive built-up land, which dominated by 48% of the watershed’s area, generating the C-value of 0.57. The research results indicate that the inundation area increases for flood return periods of 2-, 5-, 10-, 25-, and 50-years respectively by 52,891.3 m2; 62,142.1 m2; 67,702.6 m2; 74,269.8 m2; and 79,396 m2. To deal with flood disasters in the Gajahwong River section, a 4-meters embankment is required. The implementation of both scenarios should be accompanied by nonstructural mitigation, such as cliff erosion control using natural vegetation or additional protective structures.
Keywords: Return period; Peak discharge; Flood inundation; Channel discharge capacity; River embankment.
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