Spilways are structures constructed toprovide safe release of flood waters from adam to a downstream are, normally theriver on which the dam has beenconstructed.Every reservoir has a certain capacity tostore water. If the reservoir is full and floodwaters enter the same, the reservoir levelwill go up and may eventually result inover-topping of the dam.
To avoid thissituation, the flood has to be passed to the downstream and this is done by providing a spillway which draws water from the top ofthe reservoir.

A spillway can be a part of the dam or separate from it. optimum combination.

Types of Spillways –

Classification of Spillways

There are different types of spillways thatcan be provided depending on the suitabilityof site and other parameters. Generally aspillway consists of a control structure, aconveyance channel and a terminalstructure, but the former two may becombined in the same for certain types. The more common types are briefly described below.

Ogee Spillway:-

The Ogee spillway is generally provided inrigid dams and forms a part of the maindam itself if sufficient length is available.The crest of the spillway is shaped toconform to the lower nappe of a water sheetflowing over an aerated sharp crested weir.

Chute (Trough) Spillway:-

In this type of spillway, the water, afterflowing over a short crest or other kind ofcontrol structure, is carried by an openchannel (called the “chute” or “trough”) tothe downstream side of the river. The control structure is generally normal to theconveyance channel. The channel isconstructed in excavation with stable sideslopes and invariably lined. The flowthrough the channel is super-critical.Thespillway can be provided close to the damor at a suitable saddle away from the damwhere site conditions permit.

Side Channel Spillway:-

Side channel spillways are located justupstream and to the side of the dam. Thewater after flowing over a crest enters aside channel which is nearly parallel to thecrest. This is then carried by a chute to thedownstream side. Sometimes a tunnel maybe used instead of a chute.Shaft (Morning Glory or Glory hole)SpillwayThis type of spillway utilizes a crestcircular in plan, the flow over which iscarried by a vertical or sloping tunnel on toa horizontal tunnel nearly at the stream bedlevel and eventually to the downstreamside. The diversion tunnels constructed during the dam construction can be used asthe horizontal conduit in many cases.

Siphon Spillway:- 
-As the name indicates, this spillwayworkson the principle of a siphon. A hoodprovided over a conventional spillwayforms a conduit. With the rise in reservoirlevel water starts flowing over the crest asin an “ogee” spillway. The flowing waterhowever, entrains air and once all the airin the crest area is removed, siphon actionstarts. Under this condition, the dischargetakes place at a much larger head. The spillway thus has a larger dischargingcapacity. The inlet end of the hood isgenerally kept below the reservoir level toprevent floating debris from entering theconduit. This may cause the reservoir to bedrawn down below the normal level beforethe siphon action breaks and thereforearrangement for de-priming the siphon atthe normal reservoir level is provided.