Almost any reservoir does have the ability to store water. Unless the reservoir is full and also the rainwater reaches the reservoir, the extent of its reservoir would then rise as well as the dam could ultimately overflow.
In order to overcome, this scenario the flood would have to be transferred downstream, and this has been completed by supplying a spillway that pumps water from its top of the reservoir. A spillway could be a component of or differentiate from of the dam.
Spillways may be governed or uncontrolled. Controlled spillways are supplied with gates that can still be brought up or reduced. Regulated spillways get some benefits. Whenever the reservoir is full, the level of water of the reservoir would be the same as the peak of the spillway.
When the flood reaches the river at this point, the water table would continue to increase although at the same time the water may begin to flow into the spillway. This increase of the water table in the river will persist for some time, as well as the spillway will be discharged.
Upon hitting the limit, the depth of the reservoir would descend and gradually return to the normal level of the reservoir.
The upper portion of the dam would have to be greater than that of the full depth of the reservoir referring to the configuration of the spillway flood, whereas the production capacity necessary is just part of the usual depth of the reservoir. Capacity below the highest reservoir level as well as the usual reservoir level is considered overload storage which is just temporary storage in unregulated spillways.
“Spillways are structural elements construct to provide safe release of floodwaters from the dam toward a downstream region”
Types of Spillway:
There are many various spillways which can be given based on the appropriateness of the location as well as other criteria. Through particular, the spillway consists of an integrated system, a conveyance stream as well as a terminals system, so the first two can be mixed in the very same manner with such forms.
The much more popular forms are listed below:
- Drop Spillway.
- Ogee Spillway.
- Siphon Spillway.
- Chute Spillway.
- Shaft Spillway.
- Side Channel Spillway.
#1. Drop Spillway-
Throughout a drop way, the flooded water flowed openly and then almost vertically mostly on a downstream side of a hydraulic structure. One such form of spillway is effective for small as well as weir dams. The nose is supplied to the crest of a spillway such that the water jet doesn’t always attack the downstream core of such a system.
In necessary to defend the system from its impact of scouring the horizontal impervious apron, it really should be supplied on the downstream side. Often a basin is built downstream to create a single synthetic pool recognized also as water cushion. The whole cushion provides a means of the energy dissipator.
#2. Ogee Spillway-
In particular, the Ogee spillway is given in rigid dams often constitute alongside the main dam on its own if adequate length is accessible. The top of the spillway is formed to correspond to the bottom nappe of a water sheet moving into an aeration tank, smooth, crested weir.
This is a revised type of drop spillway. In this, the downstream area of the spillway is designed to complement the form of the lowest nappe of the free-falling jet of water from either a sharp-crested weir.
Throughout this scenario, the form of the lowest nappe is identical to the projectile, as well as the downstream side of such ogee spillway follows the parabolic direction whereby “0” will be the root of the parabola.
That downstream side of the spillway creates a concave shape at the stage “T” which reaches the downstream area. Such a “T” pattern is called the tangential point. Therefore the spillway came in the form of a symbol “S” Such spillway is therefore referred to as the ogee spillway.
#3. Siphon Spillway-
Even as the title suggests, this spillway is based mostly on siphon theory. The hood given over the traditional spillway creates a duct. If the river level increases, the water continues to flood over the crest like in the “ogee” spillway. The running water, though, joins the air and, as soon as all the air in the crest is expelled, the siphon activity begins.
In this case, the discharge usually occurs at a far wider head. Therefore, the spillway does have a greater discharging capacity. That inlet side of the hood is usually held far below reservoir level to avoid floating water from accessing the duct.
#4. Chute Spillway-
Throughout this sort of spillway, the flood, since passing around a short slope or even other form of control mechanism, is transported to its downstream bank of the water through an open channel. In specific, the control system is common to a conveyance channel.
The channel is a built-in excavation of a steady steep slope but is typically lined. The channel flow is super-critical. The spillway may be given near the dam or in an acceptable saddle further again from the dam where requirements of the site authorize.
#6. Shaft Spillway-
Such a form of spillway uses a circular crest in design, a stream within which a vertical or sloping path leads to a horizontal tunnel almost at the stage of the stream bed but finally downstream. In certain instances, drainage tunnels built mostly during the building of the dam would be used as horizontal conduits.
#5. Side Channel Spillway-
Side-channel spillways are situated both upstream as well as to the side of the dam. Since running over a peak, the river reaches the side channel and is almost parallel to the crest. It will then be taken to the downstream side by a slide. Even a tube may be something other than a slide.
Essential Requirements of Spillway:
- Throughout regards to providing adequate power, the spillways should be pneumatically as well as structurally stable as well as positioned in a quiet manner that perhaps the discharge does not trigger unacceptable flooding downstream at just the toes of the dam resulting in the collapse of the dam as well as other appurtenant structures.
- Uncontrolled degradation of beds as well as deposit materials caused by poor construction of spillways and energy dissipation systems has triggered not just to significant safety issues, but also high maintenance expenses are associated yearly during the monsoon while the spillway has been in service.
- This bounding layer of the Spill Way shall be resistant to erosion in order to endure the high-speed flow produced by the decrease in the surface of the water from its reservoir stage upstream to that same drain water level downstream of the dam.
- These shall be built as transfer mechanisms for normal operation from upstream through downstream of even a water tank without risking disruption to the system or endangering the river.
Here we divide Spillway Design into three parts. Which are as follows.
- Function of Spillway.
- Factors Affecting Design of Spillway.
- Water and Site Conditions.
- Dam Type.
#1. Function of Spillway-
Spillways are supplied for processing and retention dams for the discharge of surplus or flooded water that cannot be stored in the allocated storage area and also in dams for this by streams that also surpass those that are converted into diversion dams.
The main purpose of a spillway would be to drain excess water through the reservoir to avoid overtaking and potential collapse of the dam.
The water discharged from over dam spillway does have a very high velocity owing to its static head, which would be typically higher than that of the safe, non-eroding speed in the downstream. This high-speed flow can cause severe scouring as well as erosion of the downstream river bed. Energy dissipators are being used as remedial operating systems to dissipate this extra energy and also to establish safe fluid flow in the downstream of a dam spillway.
#2. Factors Affecting Design of Spillway-
Safety considerations in accordance with the economic system. Most reservoir collapses also resulting from inadequately constructed spillways or with insufficient size.
Carefully constructed system of sufficient quality can be noticed to only be relatively higher priced than just a structure of insufficient infrastructure.
#3. Water and Site Conditions –
Its nature of the spillway as well as its capability rely upon this.
- Inflow release, its intensity and hydrographic form
- Dam’s Height
- Curve of capacity
- Environmental as well as other characteristics of the site
Significant topographic features that have an effect on the design of spillways are
- Steepness of the field
- The volume of drilling and the probability of its use as a material for the reservoir.
- The Possibility to Scour
- Slope stability, safe soil bearing capability.
- The permeability of the earth.
#4. Dam Type-
The form of dam determines the construction of the flood as well as a spillway. Earth as well as rock fill dams must have sufficient spillway power. The function of the dam including conditions of service. An ungated spillway may also be established, unless under exceptional situations where a gated spillway can be supplied.