Evaluating flash flood vulnerability using combined deterministic and parametric model: A case in the upper Ciliwung watershed, Indonesia

On January 19, 2021, a flash flood hit the Upper Ciliwung watershed, a mountainous area in Indonesia, causing damage to seven buildings and displacing 1,800 residents. To minimize the impact of such disasters, there is a need for disaster risk awareness and management specifically focused on flash floods in mountainous regions. Therefore, this study aims to assess the flash flood potential index value in the Upper Ciliwung watershed. The research utilized deterministic (Soil and Water Assessment Tool-SWAT) and parametric (Flash Flood Potential Index-FFPI) modeling to analyze various factors, including slope, geology, ground movement, Antecedent Precipitation Index (API), and Runoff Coefficient (Curve Number-CN). The models showed satisfactory goodness-of-fit statistics with R and Nash-Sutcliffe efficiencies (NSE) values of 0.58 and 0.57, respectively. The findings indicated that 39.95% of the watershed had a moderate vulnerability index value of 3, while downstream areas (31.7%) had a low vulnerability index value of 2, and the middle and upstream regions had high vulnerability index values of 4 and 5, covering 23.65% and 2.49% of the total area. These results provide valuable insights to local authorities for implementing measures to reduce the Upper Ciliwung watershed's vulnerability to flash floods.