Kelani Ganga Conservation Barrage and Results of Model Studies

Abstract

Kelani Ganga is the main source of water supply for domestic and industrial areas around Colombo, Sri Lanka. An extraction rate of 6.0m3/s (114 mgd) is presently necessary at the Ambatale intake on the Kelani Ganga and it is planned to increase this to 12m3/s in the future to cater for the natural increase of demand and for extending the supply to a wider area. However, pumping at the Ambatale intake has been occasionally suspended due to salinity intrusion during dry periods and this has been an increasingly frequent occurrence at this intake due to various reasons. The construction of a conservation barrage downstream of the Ambatale intake has been identified as the most feasible solution to this problem. However, one of the major impacts that could result from the construction of a conservation barrage is that it might cause an unacceptable rise in flood levels upstream. A mathematical model study was conducted to assess the impact of the proposed conservation barrage on flood levels in the Kelani Ganga. This model was based on an earlier flood model study carried out in 1992, and updated using the latest river cross-sections. The model also benefited from the results of a physical model study, which was carried out to optimise the scour and the siltation patterns, and improve the hydraulic performance of the conservation barrage. There has been a rapid lowering of the bed level of the Kelani Ganga since the 1960s because of increased sand mining. The Kelani Ganga Flood Study of 1992 concluded that this decline, though it had a beneficial effect of lowering the low and medium flood levels, kept the high flood levels (eg. similar to levels in 1947) unchanged. However, the present study shows that further deepening of the river in the last 12 years has an effect even on the high flood levels. The combined physical and mathematical model study was used to predict the water level rise due to the construction of the conservation barrage at Ambatale and to investigate the impact of changes in channel geometry on flood levels. Some historical floods and constant discharges were used for this analysis. This paper describes the results of the mathematical model making reference to the impact of sand mining on flood levels.