HYDROLOGY & WATER RESOURCES ENGINEERING TYPES OF SPILLWAYS Guide by :- Prof. Chetna M Vyas Prof. Drashti K Bhatt Created by :- AJAY BAGLE (140013106002)

Spillways A spillways is a structure constructed a or near the dam site to dispose of surplus water from the reservoir to the channel downstream. Spillways are provided for all dams as a safety measure against overtopping and the consequent damages and failure. A spillways acts as a safety valve for the dam, because as soon as the water level in the reservoir rises above a predetermined level, excess water I discharged safety to the downstream channel, and the dam is not damaged.

Types of spillway based on utility : Main spillways : A main spillway is the one which is the to come into operation and is designed to pass the entire spillway design flood. This spillway is necessary for all dams and in most of the dams , it is the only spillway. Auxillary spillway : In some cases, where site condition are favourable it may be economical to provide an auxillary spillway in conjunction with a smaller main spillway. The total capacity is then equal to the sum of the capacities of the main and the auxillary spillways. Emergency spillway : An emergency spillway is one which is provided in addition to the main spillway but it comes into operation only during emergency which may arise at any time and the same might not have been considered in the normal design of the main spillway.

Types of spillway based on most prominent features Free overflow spillway Overflow spillway Side channel spillway Open channel spillway Tunnel spillway Shaft spillway Siphon spillway

Free overflow spillway A free overflow spillway is a type of spillway in which the control structure consists of a low- height, narrow crested weir and less vertical. The overflowing water may discharge as a free jet clearly away from the d/s face crest of the spillway. If no artificial protection is provided on the down stream side of the overflow section.

Overflow spillway An overflow spillway is the most commonly used spillway. It is widely used with gravity dams, arch dams and buttress dams. An overflow spillway is an improvement upon the free overflow spillway. The profile of the spillway is ‘ogee’ or ‘s’. The overflowing water is guided smoothly over the crest and profile of the spillway so that overflow water does not break contact with the spillway surface.

Side channel spillway A side channel spillway is the one in which the flow, after passing over a weir, is carried away by the channel running essentially parallel to the crest. The side channel spillway is usually constructed in a narrow canyon where sufficient space is not available for an overflow spillway. Side channel spillway is also the best choice where a long overflow crest is desired in order to limit the surcharge head and the abutment are steep.

Open channel spillway Open channel spillway is provided in the following situation : (a) when it is not possible to provide overflow spillway. (b) when valley is narrow. (c) when separate site for spillway is not available. Generally this type of spillway is provided on earth or rock fill dam, and is isolated from the main dam. The open channel is sometimes of constant width , but usually narrowed for economy.

Tunnel spillway A tunnel spillway is the one in which a closed channel is used to convey the discharge around or near a dam. The control structure for this spillway may be in the form of an overflow crest, vertical or inclined orifice entrance and side channel crest. The tunnel is designed to flow partly full and it is not allowed to flow full because siphonic action may develop due to negative pressure being developed in the tunnel.

Shaft spillway Shaft spillway is the one which has horizontally positioned lip through which water enters and then drops through a vertical shaft and then to a horizontal tunnel which convey water past the dam. A shaft spillway consists of three main elements : (a) overflow control weir (b) vertical control (c) closed discharge channel The vertical shaft of large structures Is usually made of reinforced concrete and the horizontal tunnel is tunnelled in rock. There are two general types of shaft spillway : (a) standard crest type (b) flat crest type

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