Reservoir Capacity Analysis for Flood Mitigation in the Upper Kuranji Watershed

Abstract The objective of this study is to assess the hydrological behavior of the upper Kuranji River watershed to inform reservoir capacity planning for effective flood mitigation. Four probability distributions, Normal, Log-Normal, Gumbel, and Log Pearson Type III, were used to assess maximum daily rainfall data from 2002 to 2022 at the Gunung Nago and Batu Busuk stations. The results were confirmed using the Chi-Square and Kolmogorov-Smirnov tests. Flood discharge was estimated using the Nakayasu SUH and Rational Method. The most appropriate techniques were determined to be the Gumbel and Log Pearson Type III methods, which had a peak design discharge of 1,178.67 m³/s over a 100-year return period. The nine reservoirs studied have a total storage capacity of 24,258 m³, with Siratal Mustakim 2 being the largest. According to simulation results, the 2-year return term had the greatest reservoir filling time (39.31 seconds), whilst the 100-year return period had the shortest filling time (20.58 seconds). The novelty lies in combining empirical hydrological methods, statistical rainfall modeling, and theodolite-based field surveys to estimate reservoir volume for flood mitigation in complex tropical catchments. This approach offers a more accurate and practical framework for flood control planning in tropical urban watersheds with complex terrain and rapid land-use change.