Post-normalization sedimentation in the Citarum Majalaya River: Patterns, impacts, and mitigation strategies

• The Majalaya area suffers from frequent annual flooding • Normalization effort was carried out in 2020 by deepening the river • Flooding was recurrent in 2023 due to river shallowing by sedimentation • Results show that the sedimentation volume in two years post-normalization reached 50% of the volume observed 11 years pre-normalization, reducing river capacity by 23% • There is a need for improved monitoring and maintenance strategies in the river The Citarum River in Majalaya frequently experiences annual flooding due to extreme weather, land cover changes, and sediment accumulation that reduces river capacity. Before 2020, the area recorded at least six floods annually, with the February 2018 flood being among the most severe. A river normalization project was completed in April 2020, reducing major floods from 2020 to 2022. However, flooding returned in 2023, driven by rapid sediment reaccumulation. This study evaluates sediment transport in a 1.3 km river segment from May 2020 to January 2022 using HEC-RAS 2D modeling. The model incorporates river geometry, satellite-derived DEM, field-measured bathymetry, SWAT+ discharge, and primary sedimentation observation data. River bathymetry comparison found that normalization removed 38,009 m 3 of sediment (averaging 2.36 m in thickness) over 11 years, but within just two years post-normalization, 17,081 m 3 of sediment reaccumulated, nearly 50% of the pre-normalization. The modeling result closely aligned with the observed result, with a 3.20% difference, attributed to variability in field sediment distribution and manual estimation methods. Post-normalization reduced the river’s capacity by 23%, from 76,963.75 m 3 to 59,336.33 m 3 , with an average sedimentation rate is 1.9 cm/month. Flood simulations with different bathymetric data indicate that a 5-year recurrence period could increase the flooded area by 279.72%. The effect expands over time with increasing recurrence periods, affecting residential areas on both the left and right banks of the river. These findings highlight the timescale of sediment reaccumulation following normalization and emphasize the need for improved river monitoring and maintenance strategies.