CHUTE (TROUGH SPILLWAY)





A spillway whose discharge is conveyed from the reservoir to the downstream river level through an open channel, placed either along a dam abutment or through a saddle, might be called a chute, open channel, or through type spillway. The chute spillway has been used with earth-fill dams. Sometimes, even for gravity dams, a separate spillway is required because of the narrowness of the main valley. In all such circumstances, a chute spillway is provided. It is lighter and adaptable to any type of foundations and hence provided easily on earth and rock fill dams.

Factors affecting the selection of chute spillway are the simplicity of their design and construction. a chute spillway is sometime known as Waste Weir. If it is constructed in continuation of the dam at one end, it may be called a Flank Weir. If it is constructed in a natural saddle in a bank of a river separated from the main dam by a high ridge, it is called a Saddle Weir.

Chute spillways ordinarily consist of an entrance channel, control structure, a discharge channel, a terminal structure, and an outlet channel. The simplest form of chute spillway has a straight centerline and is of uniform width. Often, either the axis of the entrance channel or that of a discharge channel must be curved to fit alignment to the topography. in such cases the curvature is confined to the entrance channel is possible, because of low approach velocity. Where the discharge channel must be curved, its floor is sometimes super elevated to guide the high velocity flow around the bend, thus avoiding a piling up of flow toward the outside of the chute. The base for the channel is usually made of reinforced concrete slabs, 25 to 50 cm thick. Expansion joints are usually provided in the chute at intervals of about 10 meters in either direction. The expansion joints should be made watertight so as to avoid any under-sewage. Under-drains are also provided, so as to drain the water which may seep through trough bottom and side walls. These under-drains may be in the form of perforated steels pipes, clay tiles or rock-filled trenches. If the slope of the chute can confirmed to available topography, the excavation shall be minimum. But the slope of the chute must be high enough, and should at least be able to maintain super critical flow to avoid unstable flow conditions.

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