Spillways are needed to pass surplus flood flows safely from the reservoirs developed on a river for various purposes, such as for power generation, irrigation, and flood control, navigation waterways, water supply to town and cities etc.
A spillway is an important part of any large reservoir project. From engineering point of view its suitability as per the site condition, geology of the area and the type of dam and other parameters should be studied, keeping in view the relevant design and planning procedures. Hence a careful consideration of these points, before selection of the suitable type of structure is required. In general overflow spillways are classified as high or low head as per IS-6943-1998 depending on whether the ratio of height of spillway crest measured from river bed to the design head is greater than or equal to or less than 1.33 respectively. High head spillways need to negotiate a large drop between reservoir level and down stream river bed.
To design a high head spillway, the main components to be designed are control structure gated or un-gated, conveyance structure open channels or shaft or tunnels and terminal structure containing energy dissipators.
The proposed Pancheshwar Multipurpose Project, is a high dam (280.0 m height) scheme planned on Mahakali River, a border river between India and Nepal under the Mahakali River Treaty signed in 1997. The dam site is in District Champawat, Uttranchal, India and District Baitadi, Nepal. The total catchment is 12,100.0 km square. The average flow in the river is 582.0 cumecs, the project will produce 970.0 MW of power. A review of the existing high head spillway structures of various types, evaluation of the techno-economic feasibility of proposed side channel spillway, and comparison with various other alternatives for the Pancheshwar Multipurpose Project (PMP) high head spillway is presented in this paper
The following alternative types of high head spillways for Pancheshwar dam are considered.
1. Gated Ogee chute spillway
2. Shaft tunnel and gated- chute spillway combination
3. Side channel ogee type overflow spillway.
A ski-jump bucket type of energy dissipator for all the three alternatives is assumed. In shaft- tunnel alternative swirling flow arrangement is proposed to kill the high velocities. This device minimises the cost of energy dissipator and also suits the environment.
The shaft- tunnel and chute spillway combination is found techno-economically feasible alternative.
