4.1.1 Type of spillways

Spillway structures

A spillway is a structure constructed at a dam site for effectively disposing of the dam surplus water from upstream to downstream. Spillway crest (top, peak) is generally kept at normal pool level when water gets filled up to this level water starts flowing over spillway. A spillway is essentially a safety value for a dam.

Location of a spillway :-

A spillway can be located either within the body of the dam or at one end of it or entirely away from it independently in a saddle (e.g. Panshet dam). For independent spillway a deep narrow gorge with steep banks separated from a flank by a hillock with its level above top of dam should be available. This is most suitable for earthen dams.

Many times a secondary safety arrangement is made for large dams called as subsidiary or emergency spillway.

The flow of water over a spillway may be controlled by installing gates over spillway crest. In that case spillway is known as controlled spillway.

VARIOUS TYPES OF SPILLWAY

1) Straight drop spillway

Simplest type may be constructed on small bunds or on thin arch dams. The d/s face of structure may be vertical or slightly inclined. Water, falls freely from the crest under the action of gravity. Since vacuum gets created in the underside portion of the falling jet sufficient ventilation of nappe is required in order to avoid pulsating and fluctuating effects of jet. This spillway is designed as weir.

2) Ogee spillway or overflow spillway

Widely used in concrete, masonry arch & buttress dams easily used on valleys where width of river is sufficient provide required crest length &river bed flow can be protected from scour at moderate costs. Profile of this spillway is made in accordance with the shape of the lower nappe of a free falling jet over duly ventilated sharp crested weir. Lower nappe shape can be determined from principle of projectile. It rises slightly (to point c) as it originates from crest (o)and falls making a parabolic form. Profile is similar to ogee (s curved shape in section) hence called ogee weir or ogee spillway. In an ogee spillway, the falling water glides over the curved surface of spillway & there is no normal design condition.

3) Chute spillway or trough spillway-

Ogee spillway is mostly suitable for concrete gravity dams when it is provided within dam body itself but for earthen dams where separate spillway is required chute trough spillway is simplest choice. It is sometimes known as waste weir. If it is constructed in continuation to dam at one end, called as flank weir. If it is constructed in a natural saddle in bank of a river separated from main dam by high ridge called as saddle weir. A chute spillway essentially consists of steeply sloping open channel placed along dam abutment or through a flank or a saddle.

The base of the channel is usually made up of RCC slabs 25to50cm thick. Light reinforcement 0.25% of concrete area is narrowed down for economy & then widened near end to reduce discharging velocity. Water tight expansion joints are provided. Under drains are also provided for water seeping through bottom & solid walls. Slope of the chute must be able to maintain super critical flow to avoid unstable flow conditions.

4) Side Channel Spillway

In this type flow of water after spilling over the crest is turn by 90° such that it flows parallel to weir crest.

This type of spillway is provided in a narrow valleys where no slide flanks of sufficient width to accommodate a chute spillway are available after the end of the crest the water is taken away as in an ordinary chute channel till it joins river downstream.

5) Shaft spillway

In shaft spillway the water from reservoir enters into a vertical shaft which conveys this water into horizontal tunnel which finally discharges the water into river downstream. Sometimes vertical shaft is excavated through natural rock or constructed entirely of metal concrete or clay tiles. Shaft spillway is adopted when possibility of overflow or through spillway has been ruled out because of non availability of space due to topography if suitable rock spur is available near reservoir upstream shaft spillway becomes economical first choice.

A gradual transition must be provided between vertical shaft & horizontal conduit in order to avoid danger of cavitations.

6) Siphon spillway

Siphon spillway essentially consists of a siphon pipe one end of which is kept on upstream side & is in contact with reservoir, while the other end discharges water on the downstream side.

Tilted outlet type- Installed within body of dam as shown in fig. when no space is available for constructing a separate spillway.

When water in reservoir in up to or below normal pool level air enters the siphon through the vent & siphon action can’t take place. When water level in reservoir goes above normal pool level & if one siphon is filled with water (Primed) the water will start flowing through siphon by siphon action. Out flow will continue toil water level in reservoir falls back to normal pool level. As soon as it happens air will enter siphon through new exposed air vent & flow will stop.

7) Hooded type

This type is commonly adopted. In this case a reinforced concrete hood is constructed over an ordinary overflow section of gravity dam. Inlet of this hood is kept submerged to prevent entry of debris etc. a small depriming hood is kept above the main hood & both this hood are connected by air vent. Inlet of deprimer is kept at normal pool level. At normal pool level water stand up to crest of spill way in case of floods water would start flowing over a spillway crest. The air entry at inlet is sealed. The air entry at outlet is also sealed by tail wailer. Water spilling over the crest all the remaining air from the hood siphon action gets established as air is completely exhausted & water will start flowing from upstream to downstream till level come back to normal pool level.