Performance Assessment of Taunsa Barrage Subsidiary Weir for Long Term Rehabilitation Planning

Abstract

Taunsa barrage has been rehabilitated by constructing a subsidiary weir at 800 ft distance from the barrage crest. The stilling basin floor was strengthened by replacing 2 ft top layer with 3 ft thick, high strength structural concrete, whereas the impact and friction blocks were replaced by chute blocks and end sill, respectively. While designing rehabilitation project at Jinnah barrage the Detail Design Consultants proposed a similar structure, the subsidiary weir to be constructed at 600 ft form the barrage crest. The replenishment of loose stone in gabion is recommended whereas the stilling basin floor was considered adequate. Arguments have emerged regarding hydraulic justification of subsidiary weir being proposed at the Jinnah barrage. It looks imperative that the energy dissipation systems of both the barrages are to be reviewed. Furthermore, suitability of the constructed subsidiary weir at Taunsa barrage is to be studied to establish its role in the barrage operation. This research indicated that both the barrages have distinct energy dissipation mechanism; therefore the provision of subsidiary weirs may independently be justified. The study revealed that the energy dissipation system at Taunsa Barrage is jump type, whereas it is impact/jump type at Jinnah barrage. Tail water levels at Taunsa barrage are adequate indicating that the subsidiary weir was not required. The proposed subsidiary weir at Jinnah barrage would change energy dissipation concept and will have serious hydraulic consequences. Furthermore, the subsidiary weir adversely affects hydraulic functioning of these barrages and reduces their discharging capacity.