Hydrological Impacts and Design Solutions for Downstream Tank Affected by Upstream Tanks Breaches in Cascade Systems

Cascade tank systems play a crucial role in water management in Sri Lanka, particularly in districts of Anuradhapura, Vavuniya, and Mannar. However, the failure of the upstream tanks can trigger the failures in downstream tanks. This study assesses the hydrological impacts of such failures and proposes design considerations. The study area is Sinnakunchikulam, Mannar district. Four flood scenarios were analysed. Scenario 1 estimated inflows based on the gross catchment area of the downstream tank, while Scenario 2 considered the net catchment area of downstream tank and outflow from the upstream tanks. Scenarios 3a and 3b analysed breach flow using an empirical breach width formula and an assumed fixed width. Among the evaluated scenarios, Scenario 3b was identified as the most critical condition based on the analysis results. The impact on downstream tank is influenced by water depth, breach area and stored water volume of the upstream tanks, the spill length and attenuation capacities of the downstream tank. Under Scenario 3, the breach outflow calculation of upstream tanks situated immediate to the downstream tank, is sufficient to prevent overtopping of downstream tank. The minimum freeboard is recommended by the highest afflux from Scenarios 1 and 2 unless Scenario 3b exceeds them, in which case it is based on the maximum afflux from Scenarios 1, 2, or 3a. To mitigate sliding failure under overtopping conditions, the earth dam must be designed with stability measures that account for the water level reaching the bund top. The proposed bund top level is determined by selecting the highest value among three criteria added to the Full Supply Level (FSL). These criteria are: (1) the combined height of the minimum freeboard, wave height, wave ride, and an additional one-foot safety margin; (2) the afflux calculated under Scenario 3b, multiplied by a safety factor of 1.2; and (3) a nominal freeboard of four feet.