Abstract:
Development of storage facilities has been identified as one of the adaptation options for Climate
Change in Sri Lanka. The objective of the present study was to carry out a case study for Wemedilla
Reservoir in Dambulla, Sri Lanka to demonstrate the potential of using Piano Key Weir (PKW)
spillways to increase the reservoir capacity. The Ministry of Irrigation in Sri Lanka has identified
Wemedilla Reservoir as a key location in Diverting Mahaweli Water to North Western Province.
Incrreased storage capacity in Wemedilla Reservoir will provide benefits to many areas. So,
Wemedilla Reservoir was chosesn for the current Case Study. According to the Department of
Irrigation, Sri Lanka, Wemedilla Reservoir‟s Full Supply Level (FSL) is 221.34 m above mean sea
level (AMSL) and its High Flood Level (HFL) is 222.56m AMSL and therefore, the difference
between HFL and FSL is 1.22m. The proposed PKW crest elevation was taken as 222m AMSL, and
an amount of 0.56m was hence, kept for accommodating the afflux. Four (04) PKW configurations
were selected for the Case Study. Flood routing was performed for floods with 1,000 and 10,000
year return periods and the results were compared with that of the existing spillway. The PKW
configuration with an Upstream-downstream length of 7.2m and 11 PKW units could be identified as
the best out of the four proposed configurations. With the proposed PKW configuration, the capacity
of Wemedilla Reservoir can be increased by 11% while aflux reductions of 41% and 40% were
observed for 1,000 year and 10,000 year return periods respectively. The present work demonstrated
the capacity enhancement potential of Wemedilla Reservoir with the incorporation of PKW, without
raising the reservoir bund level or acquiring upstream land. Physical model testing, cost-benefit
analysis and a comprehensive study on environmental and social impacts are further required before
implementing this proposal.
Keywords: Piano Key Wier, Reservoir Capacity; Spillway Discharge, Climate Change Adaptations
