Evaluation of drainage capacity for various design discharges in the downstream Cibeber river

Abstract

Urban flooding in the Sukmajaya area of Cilegon frequently occurs during heavy rainfall, yet the hydraulic interaction between the local drainage network and the Cibeber River has not been comprehensively evaluated. This study aims to assess the hydraulic performance of the existing drainage system under various design flood scenarios and to identify the dominant mechanism governing flood occurrence. Hydrological inputs were derived using the Nakayasu Synthetic Unit Hydrograph and the Mononobe rainfall distribution method, while one-dimensional unsteady-flow simulations were performed in HEC-RAS based on field-surveyed channel geometries and high-resolution geospatial datasets. The results indicate that flooding is primarily influenced by downstream backwater effects, while local channel capacity limitations also contribute at specific locations. Under the 50-year design flood, corresponding to a peak inflow of 84.92 m³/s, the water level at the Kubang Laban–Cibeber junction reached a maximum of 5.46 m and remained above the existing channel-bank elevation for approximately 16 hours. During the critical period, reverse flow at the Kubang Laban junction reached ?2.02 m³/s, indicating a reversal of flow direction from the Cibeber River toward the local drainage network. These conditions restricted gravity-driven drainage and promoted prolonged inundation within the urban drainage network. Based on these findings, this study recommends an integrated polder system comprising continuous levees, temporary retention storage, and mechanical pumping to interrupt hydraulic connectivity with the receiving river and improve drainage performance during extreme flood events. The proposed framework provides a practical reference for flood mitigation in urban areas affected by river-induced backwater.