Hydraulic Diagnosis of Urban Flood Mechanisms using HEC-RAS: A Case Study of the Bontang River, Indonesia

Abstract

Flooding along the Bontang River in Gunung Telihan Village, Bontang City, recurs almost annually, yet its causes had not been diagnosed mechanistically. This study applied one dimensional steady-flow HEC-RAS 4.1 modeling to a 1,936 m reach, using primary topographic data and design discharges (Q2, Q10, Q20) from the Nakayasu Synthetic Unit Hydrograph. Unlike prior single mechanism studies, the analysis resolved four co occurring flood mechanisms: bank-asymmetry overflow, right bank levee overflow, undersized sluice-gate backwater, and a downstream constriction. Although nominal channel capacity (13.83 m3/s) marginally exceeded Q2 (11.88 m3/s), the low right bank reduced the effective overflow threshold below Q2. A 95% confidence interval on design discharge, computed using Kite's asymptotic method, indicated the downstream constriction could approach overflow at the upper bound of Q10 uncertainty. An independent tidal sensitivity check showed the tidal range effect at the confluence (2.2 m) exceeded the sluice-gate backwater step alone (0.42-0.68 m). A planned condition simulation combining retention ponds, levee reinforcement, and gate augmentation achieved safe status at all critical points, indicating site differentiated intervention is a promising, land feasible direction pending detailed design verification.