Assessing the effects of consecutive extreme rainfall events under future climate scenarios on reservoir storage and spillway requirements: a case study of Manankattiya reservoir, Sri Lanka

Abstract

Irrigation reservoirs in the dry zone of Sri Lanka are designed for a single design flood, so the damaging condition arises when a second heavy storm arrives while the reservoir is still full. This study examines how consecutive extreme rainfall events change under future climate scenarios and affect the storage and spillway of Manankattiya Reservoir. Daily observed rainfall from 1994 to 2024 generated 12 projection series from three global climate models, under two shared socioeconomic pathways and two future periods, giving 60 model-years per scenario; as record lengths differ, counts are compared as events per year. An extreme event was defined by a 200 mm event depth and a compound event by two extremes less than seven days apart. Of 50 compound events identified, 22 were routed through a HEC-HMS rainfall runoff model. By 2081-2100, compound events become over four times more frequent under the high-emission pathway, rising from 0.06 to 0.30 events per year, and the share of storms qualifying as extreme rises from 3.2% to 8.3%. Each event delivers 25 to 47 million cubic metres (MCM) to a reservoir holding 6 MCM at full supply, and a crest length near 24 m is indicated to keep the peak level below the bund top. As the routing uses daily rainfall for a catchment with a 185-minute lag and a derived elevation-storage relationship, this is a screening value requiring confirmation against the probable maximum flood. Reservoir safety and spillway design must therefore account for consecutive extremes rather than a single design flood.