A spillway is a structure constructed at a dam site, for effectively disposing of the surplus water from upstream to downstream. Just after the reservoir gets filled up, up to the normal pool level, water starts flowing over the top of the spillway crest (which is generally kept at normal pool level). Hence, a spillway is essentially a safety valve 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. If a deep narrow gorge with steep banks, separated from a flank by a hillock with its level above the top of the dam (such as-shown in Fig.), is available, the spillway can be best built independently of the dam.

Parameters considered in Designing Spillways
Thus controlled spillways allow more storage for the same height of the dam. Many parameters need consideration in designing a spillway. These include:
1- The inflow design flood hydro-graph.
2- The type of spillway to be provided and its capacity.
3- The hydraulic and structural design of various components.
4- The energy dissipation downstream of the spillway.
The topography, hydrology, hydraulics, geology and economic considerations all have a bearing on these decisions.

Types of Spillways

1- Free Overfall Spill way

A free overfall spillway (or a straight drop spillway) is a type of spill way in which flow drops straight or freely from crest into the stream bed, sometimes scours occur & thus there is a possibility of formation of a pool. This type of Spillway is suitable for arch dam.

2- Ogee Spillway or Overflow Spillway
An Ogee-shaped (or Overflow), (S-shaped) Spillway is the most commonly used spillway. It is widely used with gravity dams, arch dams & buttress dams. Several Earth and Rock fill dams are also provided with this type of spillway as a superstructure.The crest of the spillway is shaped to conform to the lower nappe of a water sheet flowing over an aerated sharp crested weir.

3- Chute Spillway or the Trough Spillway (open channel)
In this type of spillway, the water, after flowing over a short crest or other kind of control structure, is carried by an open channel (called the “chute” or “trough”) to the downstream side of the river. The control structure is generally normal to the conveyance channel. The channel is constructed in excavation with stable side slopes and invariably lined. The flow through the channel is super-critical. The spillway can be provided close to the dam or at a suitable saddle away from the dam where site conditions permit. A chute spillway is sometimes known as a waste weir. If it is constructed in continuation to the dam at one end, it may be called a flank weir. If it is constructed in a natural saddle in a bank of the river separated from the main dam by a high ridge, it is called a saddle weir.

4- Side Channel Spillway
Side channel spillways are located just upstream and to the side of the dam. The water after flowing over a crest enters a side channel which is nearly parallel to the crest. This is then carried by a chute to the downstream side. Sometimes a tunnel may be used instead of a chute.

5- Shaft (Morning Glory or Glory hole or Bell Mouth) or (Drop Inlet)  Spillway
This type of spillway utilizes a crest circular in plan, the flow over which is carried by a vertical or sloping tunnel onto a horizontal tunnel nearly at the stream bed level and eventually to the downstream side. The diversion tunnels constructed during the dam construction can be used as the horizontal conduit in many cases.

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

Typical installations of siphon pipes
1- Tilted Outlet Type of a Siphon Spillway
The siphon pipe has been installed within the body of the dam. When the valley is very narrow and no space is available for constructing a separate spillway, the siphon pipes can be installed within the dam body An air vent may be connected with the siphon pipe

2- Hooded Type of a Siphon Spillway
The construction of a Hooded type of Siphon spillway is more commonly adopted. In this case a reinforced concrete hood is constructed over an ordinary overflow section of a gravity dam. The inlet of this hood is kept submerged so as to prevent the entry of debris, ice, etc.

7- Tunnel (Conduit)Spillway
A conduit Spill-way consist of a closed conduit to carry the flood discharge to the downstream channel . It is constructed in the abutment or under the dam . The closed conduit may take the form of a vertical or inclined shaft, a horizontal tunnel, or a conduit constructed in an open cut and then covered. Such a spill-way is suitable for dam sites in narrow canyons with steep abutments
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