Abstract:
In irrigation engineering, weir is the most extensively used hydraulic structure for diversion of river flow. A diversion of water may be required for the purposes of irrigation, hydro electric power generation, drinking water supply or industrial, navigation, inter basin transfer of water or combination of any of these purposes.
The problems involved in hydraulic design of structures are linked to surface and subsurface flow conditions. In general design of weir on permeable foundation, for a given surface flow criteria, the cost of the apron can be minimized with respect to subsurface flow consideration to safeguard against undermining, the exit gradient must not be allowed to exceed a certain safe limit, the uplift force must not exceed the weight of the structures but the floor thickness should be at minimum from economic consideration. The provision of sheet piles or cutoffs play an important role in the distribution of the uplift pressure. The stability of a weir demands provision of an upstream sheet pile and downstream sheet pile to prevent slipping of the soil under the weir to anticipated scour holes at the upstream and downstream reaches. Further more, the downstream sheet pile prevents undermining but uplift pressures at pucca floor is increased with length of downstream sheet pile. Therefore, some intermediate sheet piles or cut off walls should be built for decreasing uplift pressure and exit gradient. However, the mapping the flow region onto the half-plane becomes complicated when the weir has several cut-offs. The number of parameters to be determined increases, since each cut off adds three new vertices to flow region.
