INTEGRATED HYDROLOGICAL-HYDRAULIC MODELING FOR FLASH-FLOOD INUNDATION EXPOSURE ASSESSMENT

Abstract

Flash floods in small tropical catchments require spatially explicit analysis, yet data scarcity often limits model verification. This study integrates HEC-HMS rainfall-runoff modeling and HEC-RAS two-dimensional hydraulic simulation to reconstruct the 19 February 2017 flood in the Nuay River watershed and to assess village-scale inundation exposure. Annual-maximum daily rainfall records for 2014-2024 were compiled from the Ranowangko Tara-Tara and Tondano-Paleloan stations. Missing Tondano-Paleloan observations in 2015, 2021, and 2024 were not treated as zero; Thiessen weights were renormalized to the available station for those years. The corrected annual-maximum series was analyzed with the Log Pearson Type III distribution, giving design rainfall from 111.39 mm for the 2-year return period to 283.57 mm for the 1,000-year return period. The reconstructed 2017 HEC-HMS event produced a peak discharge of 20.0 m3/s at 03:00. HEC-RAS 2D outputs show inundation concentrated near the river corridor and low-lying terrain, with displayed depth classes extending from less than 1 m to 5 m. Talikuran Satu and Tounelet Satu had the largest inundated-area proportions, 3.2% and 2.1%, respectively. These classes are interpreted as inundation exposure rather than comprehensive flood risk because population, assets, vulnerability, and velocity were not included. The integrated workflow supports screening-level mitigation prioritization, while uncertainty remains due to the short rainfall record, event-based non-gauged calibration evidence, and incomplete hydraulic-project metadata.