Model Studies for Conceptual Design of Low Flow Weir across Mahaweli Ganga

Water extraction at Meewathura Intake has become difficult or impossible without the assistance of submersible pumps during the dry seasons since the river water level drops below the bottom level of the lowest opening of the Intake. A new intake is also planned to be located on the left bank around 300 m downstream of Meewathura Intake to extract an additional 32,000 m 3 / d a y to expand Kandy South Water Supply Scheme. In order to ensure a total of 41,000 m 3 / d a y from a new intake and from the existing intake, it is envisaged that a weir across the Mahaweli is required to raise the river water level during the dry weather and to extract silt free water. The analysis also shows that weir construction is a better proposal compared to other methods available.

Construction of such weir across Mahaweli Ganga would cause flood level rise in the upstream and change the river bed and river bank geometry in the area s u r r o u n d i n g the weir, particularly downstream of weir. To investigate these effects both Mathematical (MIKE 11) and Physical model studies were conducted. Mathematical Model was used to optimize weir dimension and to predict water level rise due to weir construction. Physical Model was used to optimize weir and intake layout, to investigate river bed scouring and bank erosion pattern, and to determine erosion protection measures. The study concludes that modeling techniques could be applied very effectively for such and similar studies.