Flood Characteristics in the Lower Citarum River, Indonesia and Their Possible Management Practices

Citarum River is the third longest river in Java.It is classified as a nationally strategic river area since it is essential for the national economy.However, annual flooding occurs upstream and downstream of the river.Lower Citarum River (LCR) often experiences flooding due to the high flow release of the reservoir and the contribution from the other basins.The most significant recent extreme flow was in 2021, with 1,141 m³/s flow, which inundated parts of many regencies, posing a significant risk to property, economic disruption, and human health.Even though floods in the LCR occur frequently, studies regarding the characteristics of floods there are still limited.Floodway scenarios simulated using HEC-RAS 1D-2D with SWE-ELM solver regarding flood characteristics in LCR and involving selected potential management to predict its performance, impacts, or behavior under specific conditions.The scenarios considered were floodway 30 m, floodway 60 m, flood estate, and combination scenario, along with evaluating benefits and costs.The benefit calculation evaluates the economic loss of a single flood event and, thus, the benefit if the flood is avoided by implementing the evaluated construction.The floodway 30 m scenario had the best benefit-cost ratio (0.87).This scenario reduced the flood inundation from 13,602 ha to 6,527 ha, and economic loss decreased from 16.8 million USD to 5.9 million USD.However, other scenarios with 1.5 times higher construction costs show lower economic losses (thus higher benefits) but a lower benefit-cost ratio.The benefit of the measure grows as more flood events occur.