Analisis Upaya Mitigasi Dampak Gerusan (Scouring) Aliran Sungai terhadap Struktur Pilar Jembatan

Abstract

A hydraulic analysis of the Basi 6 River revealed that the floodwater level during the Q100 design discharge nearly reaches the bottom elevation of the existing bridge. In 2025, a major flood event caused the water surface to rise to the underside of the bridge deck, posing a significant threat to the structural integrity of the bridge, which has been in service for several decades. The bridge is located at a sharp river bend, where the outer bank receives the highest flow velocity. This condition resulted in direct impact on the abutment, triggering local scouring that threatens the foundation's stability. Under existing conditions, the flow velocity was recorded at 4.86 m/s, which accelerates erosion around the bridge structure. To address this issue, a river normalization effort was carried out by realigning the river geometry to create a straighter flow path, alongside the construction of embankments extending 100 meters upstream and downstream of the bridge. These measures aim to reduce flow velocity and minimize scouring potential. Additionally, flow-reduction structures are planned, particularly at critical bends. Simulation results indicate that the flow velocity can be reduced to 2.03 m/s under the proposed conditions. This represents a significant reduction in flow energy and an improvement in structural stability near the abutment. Based on the study findings, it is recommended that a combination of river realignment, installation of gabion structures, and raising the bridge elevation by at least 1.5 meters above the Q100 flood level will effectively mitigate scouring risks and enhance the overall safety and resilience of the bridge structure.