What is a spillway?
The spillway is a structure constructed at a dam site for the effective disposal of surplus water from u/s to d/s. If the flood overtops the earthen dam, eventually the dam fails. So, a spillway is essential in every dam storage project.
Due to the high velocities of spilling water, some form of energy dissipation is usually provided at the base of the spillway.
Purpose or function of Spillway
Dispose of excess water without causing any damage to the dam. Works as a safety measure against overtopping & consequent damage & failures of the dam.
Location of spillway
The spillway is generally located in the dam body in certain sections. This section of the dam is called the overflow section of the dam.
Sometimes, the spillway may also be isolated from the dam body especially when the dam section is narrow & separate saddle is available. (saddle spillway)
Classification of spillway
- Based on the control mechanism
- Based on the Purpose
- Based on the prominent feature
1) Based on the control mechanism
a) Controlled spillway:-
In a controlled spillway, mechanical structures or gates are provided to regulate the rate of flow.
b) Uncontrolled spillway:-
Does not have a gate to control flow. When the water rises above the crest of the spillway, it begins to be released from the reservoir.
2) Based on purposes
a) Main spillway
Designed for safe passage of design maximum flood.
b) Auxiliary spillway
Supplementary spillway to the main spillway Favorable to provide an auxiliary spillway when there is a natural saddle existing.
c) Emergency spillway
When incoming discharge exceeds the maximum design flood or when the gate of the spillway malfunctions during a flood, to avoid serious accidents during such condition emergency spillway is provided.
3) Based on prominent features
a) Freefall spillway
Flow drops freely from the crest.
This type of structure is not an adaptable foundation because of the large impact of forces on d/s (downstream).
So, a pool of water can be created downstream in order to dissipate energy.
b) Ogee (overflow spillways)
Modified of vertical drop spillway which as control were that agrees (s) shaped in profile.
Generally used in high solid gravity dams.
Such spillways can be used on valleys where the width of the river is sufficiently more to provide sufficient crest length & river bed can be protected from scouring at moderated cost.
c) Side channel spillway
The control was is approximately parallel to the spillway discharge channel. When the dam is not rigid & if there is no place for the construction of a spillway in face of the dam, a side channel spillway is best.
d) Chute spillway
A chute or trough spillway passes the surplus water normally through a steep channel placed in dam body or saddle away from the dam.
e) Siphon Spillway
If the available space is limited & discharge is not large, siphon spillway is often superior to other forms.
It is based on the siphonic action in the shape of an invited pipe. An air vent pipe is provided at the crown to prevent the stopping of flow through the siphon.
f) Morning glory/drop inlet/shaft spillways
In this spillway, water drops through a vertical shaft & flows downstream the river channel through a horizontal conduit. Top-level of the vertical shaft is kept at normal water level. Excess water enters through the flared vertical shaft & discharges d
Cavitation
Cavitation is a phenomenon in which rapid changes in pressure in a liquid lead to the formation of small vapor-filled cavities, in places where the pressure is relatively low.
When subjected to high pressure, these cavities collapse & can generate a shock wave that is strong.
Spillways for medium & high head dams may be exposed to high-velocity flows & the associated destructive phenomenon of cavitation.
Preventive measures of cavitation in the spillway
The prevention of cavitation in spillways can be done by,
- Eliminating opportunities for flow separation
- Limiting flow velocities to non-cavitation levels
- Maintaining sufficiently high operating pressures
- Construction of smooth flow surface etc.
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