SPILLWAY.

The various discharging facilities provided in a storage dam are spillway, intake and outlet works. Spillway is the escape provided in conjunction with a dam to dispose of surplus flood water from a reservoir. The surplus water is drawn from the reservoir and conveyed back to the river downstream or some natural drainage. Spillway is provided at storage dam and diversion dam to serve the designed purpose. For a storage dam, spillway is provided to release surplus food water which cannot be contained in the allotted storage space in the reservoir. For a diversion dam, spillway is provided to bypass the flood water in excess of that turned into the diversion system. The spillway thus acts as a safety valve in the reservoir against failure of dam due to over-topping. It implies that the spillway is of adequate capacity to cater for spillway design flood. It is particularly applicable to earth and rock fall dams where over topping is disastrous, whereas concrete dams may be able to withstand moderate over-topping. Many failures of dams have been caused by improperly designed spillways or spillways of insufficient capacity. The cost of providing spillway of adequate capacity is only moderately higher than that of one which is obviously too small but the consequence of an inadequate spillway may be catastrophic. Besides safety of the dam, the spillway governs,

(1) The length and height of dam, (2) Maximum flood levels attained both upstream and downstream of the dam, (3) Submergence in the upstream, and (4) Areas susceptible of flooding downstream.

The inlet and outlet works are devise to release water from the reservoir for irrigation and for power generation, etc.

SPILLWAY COMPONENTS.

Spillway may be constructed as an integral part of the dam or as independent structure but essentially compromises components discussed as under:

Approach channel: It is an entrance structure to draw water from the reservoir and convey it to the control structure. The approach channel may be a straight or curved in plan. Its banks may be parallel, convergent, divergent or combination of these and may be vertical or sloping. In order to ensure minimum head loss through the channel (which has the effect of reducing spillway discharge) and to obtain uniformity of flow over the control structure (which too has the effect of reduction in spillway discharge), the entrance velocities are limited and channel curvatures and transitions, if any, are gradual.

Control structure: It consists of an overflow crest provided with a bridge and gates to regulate and control the outflows from the reservoir. The control limits the outflows below fixed reservoir levels and also regulated release when the reservoir rises above that level. The length of the crest is finalized from economic considerations and hydraulic model studies as for a given level of the crest, the height of dam increases if the crest is made of short length (Fig. 20.1)

Discharge carrier: It is the waterway provided to convey the flows released from the control structure to the river below the dam, except in the case of arch dam wherein the discharge falls free from the crest or where the flow is released directly along the abutment hill side to cascade down the abutment face. The structure of the discharge carrier may be the downstream face of a concrete dam, an open excavated through an abutment. The dimensions of the discharge carrier are primarily governed by hydraulic requirements besides geologic and topographic characteristics of the site which influence the profile, shape of cross section, width, length, etc. The profile may be variably flat or steep; cross section may be variably rectangular, trapezoidal, circular or of other shape and the waterway may be wide or narrow, long or short.

Terminal Structure: At the downstream end of the discharge carrier where flow from reservoir to downstream river level manifests in the form of high velocities enough to cause scour, erosion and subsequent damage to the adjacent structures, a terminal structure usually in the form of stilling basis is provided for dissipation of residual energy.