Quantifying reservoir capacity enhancement and spillway design under future climate scenarios in the Kattiyawa Wewa, Knuckles basin, Sri Lanka

Abstract

This study assessed future climate impacts on Sri Lanka’s Kattiyawa Wewa Reservoir, located downstream of the Knuckles Basin. It quantified the reservoir capacity augmentation and spillway design upgrades required under those scenarios. Future daily precipitation projections from three Coupled Model Intercomparison Project Phase 6 (CMIP6) global climate models (GCMs) (CNRM-CM6-1, HadGEM-GC31-LL, and MRI-ESM2-0) under SSP2-4.5 and SSP5-8.5 scenarios were statistically downscaled to the watershed level for the 2041–2060 and 2081– 2100 periods using LARS-WG software. The downscaled daily data were disaggregated to subdaily durations using to develop Intensity-Duration-Frequency curves. These rainfall inputs were incorporated into the HEC-HMS hydrological model, configured with SCS Curve Number loss method and unit hydrograph transforms, to simulate design flood hydrographs and estimate critical peak discharges for various return periods. Results show peak discharges rising drastically, up to 1,378 m³/s under the HadGEM-GC31-LL SSP5-8.5 scenario by 2081– 2100, representing a 215% increase from baseline conditions. Using area–capacity curves derived from the non-dimensional relationship developed for small tanks in Sri Lanka, required storage enhancement ranges from 55% to over 200%, depending on the climatic scenario. Spillway capacity must be upgraded to 1,100–1,200 m³/s with concurrent storage expansion, or up to 1,380 m³/s without it. Existing infrastructure is shown to be inadequate under most scenarios. Adaptive strategies include reservoir capacity enhancement, combined with operational flexibility to manage variable inflows, preventing overtopping, reducing downstream flooding, and ensuring long-term dam safety.