Optimizing gravity flow for flood management in urban watersheds on kelani river bank

Abstract

The drainage designs and operation procedures in coastal cities located on river banks are planned according to the worst-case flood scenario; the peak flood level at the urban watershed coincides with the flood peak of the river and the peak surge height at the sea. The annual exceedance probability of each event is considered separately to match the project design return period, without considering the joint probability of events. Though this approach is conservative, it always overestimates the flood risk and therefore leads to overdesign. This study aims to develop a design and operation procedure to optimize the pumping by incorporating the possibilities of phase differences of high rainfall events, high flood events, and high water levels at sea. The Kalu Oya-Mudun Ela sub-watershed at the downstream reach of the Kelani River is selected for the analysis. The probability of occurrence of a rainfall event above the 25-year return period and 10-year return period in Colombo gauging station, while Kelani River is at a minor flood level of 1.5 m MSL at Nagalagam Street is found to be 1.4% and 2.2%. The probability of occurrence of storm surge events and rainfall events at the Colombo gauging station is 100% and the occurrence of storm surge and rainfall while downstream reach is above minor flood level is 4%. It is shown that the gravity drainage of the local catchment is possible until the flood level of Nagalagam Street reaches 1.5 m MSL. Therefore, it is economical to design the pumping stations to cater to the discharges of a 10-year return period and enhance the gravity drainage capacities to drain out the stormwater under gravity before the Kelani flood level increases beyond 1.5 m MSL.