Spillway Modernization for Improving Robustness of Village Tank Cascades for Floods, Sri Lanka

Village tanks contribute significantly to the irrigation sector of Sri Lanka as a large number of rural population depends on them for their livelihoods. However, these systems are vulnerable to water scarcities during dry periods and excess inflows during floods as they are small in size. Most of the tanks are located in cascades, and a large number of their bunds breach during flood events due to their limited spillway discharge capacities. Hence, structural intervention to improve spillway discharge capacity is vital to make them robust for floods. This study assesses the ability to enhance the spillway discharge capacity of the lowermost tank in 216 village tank cascades in the Southeastern region of the country by adopting Piano Key Weir spillways. When the clear overflow spillways at the lowermost tanks in the cascades are replaced by Piano Key Weirs of higher discharge capacities, the number of cascades that can stand peak discharges of 50 year return period increased from 12% to 47% and from 60% to 84% for 0.3 m and 0.6 m affluxes respectively. Thus, the modernization of spillways by adopting Piano Key Weirs is a viable solution to improve the robustness of the village tank cascades for floods.