Integrated empirical-hydrodynamic assessment of sediment regime transformation in a tropical cascade reservoir system

Abstract

Reservoir sedimentation threatens the long-term performance of multipurpose dams, especially in cascade systems where upstream flushing may intensify downstream deposition. This study aimed to diagnose sediment regime transformation in Sutami Reservoir within the Sengguruh-Sutami cascade system, East Java, Indonesia. An integrated framework combining the Borland–Miller empirical area reduction method (EARM), bathymetric differencing, and two-dimensional hydrodynamic modelling using the two-dimensional hydrodynamic-sediment transport solver in iRIC (Nays2DH) was applied using historical profile data (1972–2021) and bathymetric surveys from 2016 and 2021. The analyses showed that approximately 8.50 ∙ 10⁶ m³ of sediment was redistributed during 2016 to 2021, with about 62% accumulating below elevation 240 m, while the zero-capacity elevation rose by 0.24 m. Bathymetric differencing indicated a net storage loss of 7.06 ∙ 10⁶ m³ and confirmed downstream migration of deposition into the mid-reservoir and near-dam zones. The hydrodynamic simulation was used qualitatively to identify transport mechanisms rather than to hindcast the cumulative five-year sediment budget, and it showed that density-driven sediment plumes generated during upstream flushing can bypass shallow delta zones and promote deposition in deeper parts of the reservoir. The combined evidence indicates that the sediment distribution pattern in Sutami Reservoir has shifted from predominantly upstream/delta deposition toward increasing deposition in the deeper parts of the reservoir, broadly corresponding to a transition from Type I toward Type III–IV Borland–Miller distribution characteristics. So sedimentation in Sutami cannot be interpreted as an isolated single-reservoir process and support the need for coordinated cascade scale sediment management.