A spillway is structure constructed at dam site, effectively disposing off the surplus water from upstream to downstream.

Just after the reservoir filled up to the normal pool level water starts flowing over the spillway crest (which is generally kept at normal pool level).

Depending upon the inflow rate, water will start rising above the pool level, and at the same time it will be let off over the spillway.

SPILLWAYS

Spillways are provided for storage and detention dams to release surplus or flood water which cannot be contained in the allotted storage space, and diversion dams to bypass flows exceeding those which are turned into the diversion system.

In addition to provide sufficient capacity, the spillway must be hydraulically and structurally adequate and must be located so that spillway discharges will not erode or undermine the downstream toe of the dam.

The spillway's bounding surfaces must be erosion resistant to withstand the high scouring velocities created by the drop from the reservoir surface to tail water, and usually some device will be required for dissipation of energy at the bottom of the drop.

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FUNCTION

The frequency of spillway use will be determined by the runoff characteristics of the drainage flows will result during flood and periods of sustained high runoff when the capacities of other facilities are exceeded.

At diversion dams where storage space is limited and diversions are relatively small compared to the normal river flows, the spillway will be used almost constantly

It must have adequate discharge capacity.

It must he hydraulically and structurally safe.

The surface of the spillway must be erosion resistant.

The spillway must be so located that the spillway discharge does not erode or undermine the downstream toe of the dam.

It should be provided with some device for the dissipation of excess energy

The spillway discharge should not exceed the safe discharge capacity of the downstream channel to avoid its flooding.

ESSENTIAL REQUIREMENTS OF A SPILLWAY

A spillway can be located either within the body of a 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, is available the spillway can be best built independently of the dam.

Under such circumstances, a concrete or an earthen dam can be constructed across the main valley and spillway can be constructed independently into the saddle. Sometimes a concrete or masonry dam along with its spillway can be constructed in the main valley, while the flanks are closed by earthen dikes or embankments. A separate independent spillway is generally preferred for earthen dams, although due to non availability of sites, a concrete spillway sometimes constructed within or at one of the ends of an earth dams

LOCATION OF SPILLWAY

TYPES OF SPILLWAY1. Free Overfall (Straight Drop) Spillway 2. Overflow (Ogee) Spillway 3. Chute (Open Channel/Trough) Spillway 4. Side Channel Spillway 5. Shaft (Drop Inlet/Morning Glory) spillway 6. Tunnel (Conduit) spillway 7. Siphon spillway

Free Overfall Spillway

Free over fall type spillway is one in which the flow drops freely

This type is suited in a thin arch ,deck overflow type dam, low concrete or masonry dam The crest is extended in the form of an overhanging

lip to direct small discharges away from the face of the overflow section

In free over fall spillways, the underside of the nappe is ventilated

sufficiently to prevent a pulsating, fluctuating jet.

Overflow Spillway

The ogee spillway has a control weir which is ogee or S-shaped in profile.

The upper curve of the ogee ordinarily is made to conform closely to the profile of the lower nappe of the ventilated sheet falling from a sharp-crested weir.

Flow over the crest is made to adhere to the face of profile by preventing access of air to the underside of sheet.

For discharges at designed head, the flow glides over the crest with no interference from the boundary surface and attains near-maximum discharge efficiency.

Since the lower nappe of the free falling jet will be different for different heads over the crest of the sharp crested weir, the profile of the ogee weir is generally confined to the lower nappe that would be obtained for maximum head over the spillway (i-e up to the maximum reservoir level).

This type of spillway can be easily used on valleys where the width of the river is sufficient to provide the required crest length and the river bed below can be protected from scour at moderate costs.

Used in arch and buttress dam

Chute (Open Channel/Trough) Spillway

A spillway whose discharge is conveyed from the reservoir to the downstream river level through an open channel, placed either along a dam abutment or through a saddle, might be called a chute, open channel, or through type spillway.

The chute spillway has been used with earth-fill dams. Sometimes, even for gravity dams, a separate spillway is required because of the narrowness of the main valley. In all such circumstances, a chute spillway is provided.

It is lighter and adaptable to any type of foundations and hence provided easily on earth and rock fill dams.

Chute spillways ordinarily consist of an entrance channel, control structure, a discharge channel, a terminal structure, and an outlet channel.

The simplest form of chute spillway has a straight centerline and is of uniform width.

Often, either the axis of the entrance channel or that of a discharge channel must be curved to fit alignment to the topography

Side Channel Spillway

The side channel spillway is one in which the control weir is placed along the side of and approximately parallel to the upper portion of the spillway discharge channel flow over the crest falls into the narrow trough opposite the weir, turns an approximate right angle, and then continues into the main discharge channel.

side channel type of spillway commonly used for earthen and rock fill type dams

Shaft (Drop Inlet/Morning Glory) spillway

A drop inlet or a shaft spillway also called morning glory is a spillway in which the water enters over a horizontally positioned lip, drops through a vertical or sloping shaft, and then flows to the downstream river channel through a horizontal or near horizontal conduit or tunnel.

The structure may be considered as being made up of three elements; namely, an overflow control weir, a vertical transition, and a closed discharge channel.

A drop inlet spillway can be used at dam sites in narrow canyons where the abutments rise steeply or where a diversion tunnel or conduit is available for use as the downstream leg.

This type of spillway may be adapted when the possibility of an overflow spillway and a trough spillway is ruled out because of non-availability of space due to topography.

Used for concrete and other types of dams

Tunnel (Conduit) spillway

It is a spillway in the form of a closed channel used to convey the discharge around or under the dam.

The closed channel may take the form of vertical or inclined shaft, a horizontal tunnel through earth or rock, or a conduit constructed in open cut and back filled with earth materials.

Most form of control structures can be used with conduit and tunnel spillways.

Ample aeration must be provided in this spillway in order to prevent a make-and-break symphonic action which would result if some part of the tunnel or conduit tends to seal temporarily because of an exhaustion of air caused by surging of the water jet, or by wave action or backwater to guarantee free flow in the tunnel, the ratio of the flow area to the total tunnel area is often limited to about 75%.

Air vents may be provided at critical points along the tunnel or conduits to insure an adequate air supply which will avoid unsteady flow through the spillway.

Siphon spillway

A siphon spillway is a closed conduit system formed in the shape of an inverted U, positioned so that the inside of the bend of the upper passageway is at normal reservoir storage level.

The initial discharges of the spillway as the reservoir level

rises above normal are similar to flow over a weir. Siphon action takes place after the air in the bend over the crest has been exhausted.

Continuous flow is maintained bythe suction effect due to the gravity pull of the water in the lower leg of the siphon.