Page - 644 - in Book of Full Papers - Symposium Hydro Engineering

km north-east of the capital city of Colombo. The installed capacity of the HPP will be 30.2 MW for the rated head of 69 m and design discharge of 50 m³/s. The expected mean annual energy generation is 100 GWh/a. The project comprises a reservoir inundated by a gravity dam equipped with spillway, bottom outlet and fish ladder, a lateral intake, a low-pressure headrace tunnel, a surge shaft, a steel lined penstock tunnel and a surface powerhouse accommodating two Francis units. The general layout is shown in Fig. 2. Fig. 2 Project location and general layout The reservoir will be slightly longer than 3 km with gross storage capacity of 4.66 million m³ at the normal operation water level of 548 masl and active storage of 2.68 million m³. The minimum operating water level will be at 542 masl. The dam will have maximum height of 37 m and length of 236 m. The overflow spillway will be equipped with five radial gates with the ogee crest level at 534 masl. A sand sluice followed by a steel lined conduit embodied in the gravity dam will be installed next to the lateral intake with purpose the removal of sediment deposits. A bottom outlet is also incorporated in the dam body for dam safety reasons. Finally, a micro- hydro power plant will be installed at the dam toe for the exploitation of the hydropower potential of the ecological flow. The catchment area draining at the Moragolla dam site is 809 km². Upstream of the envisaged Moragolla reservoir is located the Kotmale dam, which inundated the year 1984 a reservoir with gross storage capacity of 177 million m³. The latter regulates the runoff from 562 km², i.e. approximately 70% of the catchment area. It has been calculated that the trap efficiency of Kotmale reservoir is approximately 99.5%. Hence, the regulated part of the catchment will not contribute to the sediment inflow at the envisaged Moragolla reservoir. The unregulated part, i.e. the catchment area excluding Kotmale dam catchment area is 247 km². The hydrologic analysis concluded that the mean annual flow at the Moragolla dam site will be 22.4 m³/s. The mean annual sediment inflow has been estimated to be 145,000 m³/a. This assessment was based on the specific sediment yield at Kotmale reservoir as calculated from the measured deposits, the trap efficiency and the size of the regulated catchment area. The bulk of sediment Headrace tunnel Reservoir Concrete gravity dam Surge shaft Surface Powerhouse Penstock tunnel 644