Hydraulic Design of Spillway Approach Channel of Polavaram Irrigation Project, Andhra Pradesh

The approach channel is meant for diversion of flood towards the spillway. Proper alignment of approach channel minimizes oblique flows in the approach channel and in front of spillway and also improves the discharging capacity of spillway because of favourable approach flow conditions and also, avoid endanger of spillway abutments. An upstream guide bund contributes to this by smoothly guiding the flow towards spillway thereby improving the flow conditions infront of spillway. CWPRS, Pune carried out hydraulic model studies on a geometrically similar 1:140 scale Froudian model of spillway of Polavaram dam to optimize the layout of approach channel and guide bund. Various alternatives of approach channel and guide bund have been incorporated and model studied on the model of spillway to assess the flow pattern in approach channel and performance of upstream guide bund for different flood conditions. The layout comprising of about 2000 m long approach channel with width varied from 1000 m to 450 wide (at narrow section) with 500 m long straight guide bund was recommended. The model studies showed that, with this layout, there was reduction in the velocities near left abutment of spillway when compared with other layouts. Frequent and periodic formation of swirling flows in front of spillway; vortexes shedding along the guide bund, sharp rise of water surface on the outer boundary of vortices were eliminated. Water surface profiles were contained in front of spillway piers and overtopping was eliminated. On the upstream of spillway, maximum velocities of the order of 1.6–7.7 m/s were observed along Ch. 100 m. The layout has been adopted by the project authorities and construction of project is in progress. In this paper, the authors explained the results of hydraulic model studies conducted on various layouts approach channel and guide bund of Polavaram irrigation project, Andhra Pradesh.