HYDROLOGICAL AND HYDRAULIC ASSESSMENT OF RIVER NORMALIZATION FOR FLOOD MITIGATION IN THE BUNTUNG RIVER, SIDOARJO REGENCY, INDONESIA

Abstract

Buntung River is one of the rivers in Sidoarjo Regency that frequently experiences flooding due to insufficient channel capacity to convey flood discharge. This condition is mainly caused by sedimentation that reduces the effective cross-sectional capacity of the river, as well as inadequate channel dimensions. This study aims to assess the hydrological and hydraulic conditions of the Buntung River and evaluate river normalization as a flood mitigation measure. The hydrological analysis used rainfall data from six rainfall stations during the period 2016–2025. The analysis included rainfall consistency testing using the double mass curve method, areal rainfall estimation using the Thiessen Polygon method, rainfall frequency analysis using the Normal, Gumbel, Log-Normal, and Log-Pearson Type III distributions. The suitability of the distributions was evaluated using the Chi-Square and Smirnov–Kolmogorov goodness-of-fit tests. The Gumbel distribution, which produced the highest design rainfall values, was selected as a relatively conservative basis for subsequent analysis. Design flood discharges were estimated using the Mononobe method and the Nakayasu Synthetic Unit Hydrograph method. Hydraulic analysis was then performed using HEC-RAS 6.5 under steady-flow conditions to evaluate the existing and normalized channel conditions. The resulting design flood discharges were 74.04 m³/s (Q₂), 97.91 m³/s (Q₅), 113.71 m³/s (Q₁₀), 133.67 m³/s (Q₂₅), 148.48 m³/s (Q₅₀), and 163.18 m³/s (Q₁₀₀). The HEC-RAS simulation showed that the existing Buntung River channel experiences overtopping even under the 2-year return period flood discharge, indicating insufficient channel capacity. River normalization was proposed through sediment dredging and cross-sectional modification using precast concrete retaining walls. The normalized channel has a conveyance capacity of 284.48 m³/s, enabling it to safely accommodate the 25-year and 50-year design flood discharges without overtopping.