28. Hydraulic Investigations to Optimize the Design of Spillway and Energy Dissipator for Dri Dam, Arunachal Pradesh—A Case Study

Etalin Hydroelectric Project (3097 MW) is a run-of-the-river scheme upstream of 3000 MW Dibang Multipurpose Project, utilizing the waters of Dri and Tangon rivers in Dibang Valley of Arunachal Pradesh. Dri limb diversion structure consists of a 101.5 m high and 213.7 m long concrete gravity diversion dam. An orifice spillway has been provided to pass a design flood (PMF) of 11,811 m³/s along with Glacial Lake Outburst Flood (GLOF) of 1,170 m³/s. A ski-jump bucket is provided for energy dissipation. Hydraulic model studies were conducted at CWPRS, Pune, to determine the efficacy of orifice spillway and energy dissipator for various ranges of discharges and reservoir water levels. The original design of the orifice spillway was found to be suitable in terms of discharging capacity of the spillway and performance of the energy dissipator. However, pressures on the spillway were not found acceptable while passing the low discharges for the gated operation of the spillway. The fluctuating upper nappe was intermittently hitting the trunnion of the radial gate for the design flood. Flow conditions in the bucket were violent due to strong swaying rooster tails generated at the end of piers for higher discharges. Based on the findings from the model studies, the design was modified by raising the crest by 5 m and extending the pier length up to the lip of the bucket to improve the overall performance of the orifice spillway. Modified design helped in finalizing the trunnion elevation of the gate, improving the pressures on the spillway surface and flow conditions in a ski-jump bucket. A need for providing a pre-formed plunge pool was also identified by analyzing the scour profiles. The location and size of the plunge pool were suggested based on the extent of scour observed downstream of the ski-jump bucket. Hydraulic model studies played a very important role in assessing and optimizing the hydraulic performance of spillway and energy dissipator of Dri dam spillway.