Assessing design hydrograph and spillway capacity sensitivity to antecedent soil moisture conditions, rainfall patterns and hydrological transformation uncertainty in the Knuckles basin

Abstract

Reservoir operation under extreme events requires reliable estimation of design floods while accounting for uncertainties in catchment runoff generation and rainfall temporal distribution. This study evaluates the flood response and capacity adequacy of the Maminiyawa Reservoir, Sri Lanka, using a scenario-based HEC-HMS modelling approach. A 50-year return period design storm was developed, and multiple hydrological scenarios were generated by systematically varying key runoff transformation parameters (Curve Number and the Peak Rate Factor) and rainfall temporal patterns representing front-loaded, centrally loaded, and backloaded storms. Peak discharge, reservoir pool elevation, and routing response were evaluated to identify the critical condition. Peak discharge ranged from 196.5 to 372.9 m³/s, representing an 89.8% variation among the scenarios. Variations in runoff transformation parameters produced increases of up to 24.8% in peak discharge, while rainfall temporal distribution had a comparatively smaller influence, with a maximum increase of 14.5% for the back-loaded pattern relative to the front-loaded pattern. The critical scenario produced a peak discharge of 372.9 m³/s and a pool elevation of 140.2 m. With a full supply level of 137 m and a bund level of 140 m, the simulated water level exceeded the bund by 0.2 m, indicating potential overtopping under the 50-year design event. The findings demonstrate that explicitly accounting for uncertainty in hydrological transformation and rainfall temporal distribution is important for robust reservoir flood-safety assessment and capacity enhancement planning.