Sedimentation Assessment and Mitigation Strategies for the Unda River, Bali, Indonesia

Abstract

The Unda River Basin has experienced substantial degradation due to deforestation, land-use change, landslides, soil erosion, and sediment accumulation within the river and its check dams. Excessive erosion and sedimentation reduce the hydraulic capacity of the river and the effectiveness of sediment-control infrastructure. This study aimed to estimate the potential sediment volume in the Unda River Basin and formulate structural and non-structural mitigation measures. The Universal Soil Loss Equation (USLE) was applied to estimate watershed erosion and the associated sediment volume. The results indicate that most check dams in the basin are heavily filled with sediment. The estimated annual sediment volume is approximately 118,115.70 m³, which may be used as the target volume for annual dredging. Under a five-year maintenance interval, approximately 249,554.37 m³ and 341,024.12 m³ of sediment should be removed from the two uppermost check dams, respectively. Under a ten-year interval, the required dredging volumes from the three uppermost check dams are approximately 249,554.37 m³, 378,787.88 m³, and 552,814.72 m³, respectively. Structural mitigation should therefore prioritize the initial removal of accumulated sediment followed by scheduled dredging to restore and maintain check-dam capacity. Non-structural measures should include regulatory oversight, sediment-removal monitoring, community participation, and village-level management. The findings provide a basis for establishing an integrated and adaptive sediment-management strategy for the Unda River Basin.