What is Ogee Spillway? Ogee Spillway Design | 4 Other Types of Spillways
What is Ogee Spillway? Ogee Spillway Design | 4 Other Types of Spillways
What is a Spillway?
A spillway is a structure constructed to control the level of water that flows over the top of an embankment by directing it away from the structure. They are used to regulate water levels in rivers, canals, reservoirs, and lakes.
A spillway may be controlled by gates or valves that allow flow in one direction, or by three-way gates at each end of a section with weirs in between.
A spillway structure may be as simple as a trench, or a complex structure that includes features like floodways and aprons.
Spillways are designed with future considerations in mind.
Types of Spillways
There are two main types of spillways; Controlled and uncontrolled spillways
A controlled spillway
A controlled spillway includes mechanical structures or gates to control the flow rate.
This design enables virtually the whole height of the dam to be used for water storage all year, with floodwaters released as needed by opening one or more gates.
An uncontrolled spillway
An uncontrolled spillway, on the other hand, lacks gates; as the water rises beyond the lip or crest of the spillway, it begins to flow out of the reservoir.
The depth of water over the reservoir’s spillway determines the rate of release.
The proportion of reservoir storage capacity above the spillway crest can only be utilized for temporary floodwater storage; it cannot be used for water supply storage since it is higher than the dam can hold.
The mechanical gates govern the reservoir’s normal level management in an intermediate kind.
In this scenario, the dam is not meant to function with water pouring over its top, either because of the materials used in its construction or because of the conditions right downstream.
If the reservoir’s intake exceeds the gate’s capacity, water will be sent down a constructed channel known as an auxiliary or emergency spillway.
Often, this is done on purpose using a fuse plug. The fuse plug, if present, is meant to wash out in the event of a major flood that exceeds the discharge capacity of the spillway gates.
Although it may take months for construction teams to rebuild the fuse plug and channel following such an operation, the overall damage and expense to repair is less than if the major water-retaining structures were overtopped. W
hen creating a spillway with the needed capacity would be too expensive, the fuse plug idea is employed.
Other Types of Spillways
- Drop Spillway
- Ogee Spillway
- Siphon Spillway
- Chute or Trough Spillway
- Shaft Spillway
- Side Channel Spillway
In a drop spillway, the flow descends freely from the top of a drop spillway. To divert minor flows away from the downstream face of the overflow section, the crest is sometimes extended in the shape of an overhanging lip.
The falling water jet’s underbelly is appropriately vented so that it does not pulse. A spillway of this type is better suited for a narrow arch dam with a virtually vertical downstream face.
Because the flow generally descends into the stream bed, unpleasant scour and a deep plunge pool may emerge in some circumstances.
If erosion is too extensive, a plunge pool is formed by building an auxiliary dam downstream of the main dam.
What is Ogee Spillway?
Ogee Spillway is a type of discharge structure, which is used to control the water level in a reservoir or other type of hydraulic structure.
An ogee spillway is a form of water release that will release with the same volume of water as the crest of the dam. It is used to minimize turbulence in the spillway and reduces the chances of cavitation.
There are many different types of ogee spillways. Some are water surface spillways and others are submerged or dry ogee spillways.
The Ogee spillway is typically used in rigid dams and, if sufficient length is available, becomes a component of the main dam itself.
The spillway’s crest is designed to fit the bottom nappe of a water sheet passing over an aerated sharp-crested weir.
It is a variant of a drop spillway. The downstream contour of the spillway is designed to match the form of the lower nappe of a free-falling water jet from a steep crested weir.
Because the form of the lower nappe is similar to that of a projectile in this scenario, the downstream surface of the ogee spillway will follow a parabolic route where “0” is the origin of the parabola.
From a point “T,” the downstream face of the spillway makes a concave curve and reaches the downstream floor.
This point “T” is referred to as the point of tangency. As a result, the spillway is shaped like the letter “S.” (i.e. elongated form). As a result, this spillway is known as an ogee spillway.
The curvature of the lower nappe differs throughout the entire head of water above the weir’s crest. It varies depending on the water’s head.
However, the maximum head is taken into account while designing the ogee spillway.
If the spillway is operating at full capacity, the spilling water simply follows the curved profile of the spillway, with no gap between the water and the spillway surface, and the discharge is maximum.
When the actual head exceeds the planned head, the lower nappe deviates from the ogee profile and separates from the spillway surface.
As a result, at the point of separation, a negative pressure occurs. Air bubbles develop within the moving water as a result of the negative pressure.
These air bubbles are the source of the frictional force (i.e., abrasion) that causes significant damage to the spillway surface.
Again, if the head of water is less than the intended head, the waterjet clings to the spillway body and increases positive pressure, reducing flow via the spillway.
A siphon uses the height difference between the intake and the exit to produce the pressure differential required to remove surplus water.
Siphons, on the other hand, require priming or the removal of air in the bend in order to operate, and most siphon spillways are built with a mechanism that uses water to remove the air and automatically prime the siphon.
The volute siphon is one such design, which uses water pushed into a spiral vortex by volutes or fins on a funnel that pulls air out of the system.
When the water level rises over the inlets used to drive the priming process, the priming procedure begins automatically.
A trough spillway is a form of spillway in which excess water from upstream is sent downstream via a sharply sloping open channel.
It is often built at one end of the dam or independently away from the dam on a natural saddle in the river’s bank.
The slope of the trough spillway is designed to keep the flow optimum at all times. Energy dissipators can be installed on the bed of the chute spillway to disperse the energy from the falling water.
Side Channel Spillway
Spillways for side channels are placed close upstream and to the side of the dam. After passing over a crest, the water enters a side channel that runs roughly parallel to the crest.
This is subsequently transported downstream by a chute. A tunnel may be used instead of a trough in some cases.
This form of spillway has a circular crest, over which the flow is transported via a vertical or sloping tunnel on to a horizontal tunnel at the stream bed level and subsequently to the downstream side.
In many situations, the diversion tunnels built during dam building can be utilized as the horizontal conduit.
Ogee Spillway FAQs
1. What is the function of the ogee spillway?
The ogee spillway’s primary role is to establish circumstances for the safe passage of floodwater from upstream to downstream.
Some dams have been destroyed due to improper spillway design and insufficient capacity. As a result, the spillway must be hydraulically and structurally sound.
2. How is ogee spillway designed?
For a design head, the ogee shape results in near-atmospheric pressure across the crest portion. Because of crest resistance, the discharge is smaller at lower heads than the design head.
Because the negative crest pressure suctions more flow at higher heads, the discharge is larger than that of an aerated sharp-crested weir.
3. Why is it called ogee?
Ogee, short for ogee spillway, is a concave, or curved-in-the-shape of the channel. The curve is caused by the weir’s shape.