HYDRAULIC STRUCTURES

A hydraulic structure is a structure submerged or partially submerged in any body of water, which disrupts the natural flow of water. They can be used to divert, disrupt or completely stop the flow. An example of a hydraulic structure would be a dam, which slows the normal flow rate of river in order to power turbines. A hydraulic structure can be built in rivers, a sea, or any body of water where there is a need for a change in the natural flow of water

1. SPILLWAYS

1. The height of the dam is always fixed according to the maximum reservoir capacity. The normal pool level indicates the maximum capacity of the reservoir. The water is never stored in the reservoir above this level. The dam may fail by over turning so, for the safety of the dam the spillways are essential. 

2. The top of the dam is generally utilized by making road. The surplus water is not be allowed to over top the dam, so to stop the over topping by the surplus water, the spillways become extremely essential. 

3. To protect the downstream base and floor of the dam from the effect of scouring and erosion, the spillways are provided so that the excess water flows smoothly.

TYPES OF SPILLWAY

Free Overfall Spill way:

A free overfall spillway (or a straight drop spillway) is a type of spill way in which flow drops straight or freely from crest into the stream bed, sometimes scours occur & thus there is a possibility of formation of a pool. This type of Spillway is suitable for Arch dam.

Ogee-shaped Spillway:

An Ogee-shaped (or Overflow) Spill way is the most commonly used spillway. It is widely used with gravity dams, arch dams & buttress dams. Several Earth & Rock fill dams are also provided with this type of spillway as a superstructure. An Ogee-shaped Spillway has a control weir of ogee-shaped, which is like the elongated English letter “S”. The upper part of the spillway surface matches closely to the profile of the lower nappe of a ventilated sheet of water falling freely from a sharp-crested weir.

Chute Spillway :

A Chute Spillway (or Open Channel Spill way) consist of a open channel , through which the water discharge. For earth dams and rock fill dams a separate spill way is generally constructed in a flank for saddle away from the dam if suitable site exists.

Side Channel Spill-way:
In a side channel spill-way, the control weir is placed approximately parallel to the upstream portion of the spill-way discharge. Thus the flow after passing over the crest is carried in a discharge channel running parallel to the crest. The spillway discharge flows over the weir crest and falls into a narrow trough.

Shaft Spillway:

A shaft spillway is used at the sites where the conditions are not favorable for an overflow or a chute spill-way.Shaf

Siphon Spillway:
A Siphon spillway operates on the principle of siphonic action. It is a closed conduit of the shape of an inverted U-tube with unequal legs. It is commonly used in practice. The siphon duct is formed by an air tight RCC cover, called hood over an oggy-shaped body wall made of concrete. The top of the body wall forms the crest of the spill-way and kept at the F.R.L. of the reservoir.

2. WIER : Structure constructed to raise the head of water 

 Different types of weir are: 

1. Sharp crested weir 

2. Broad crested weir 

3. Ogee weir 

4. Tyrolean weir 

5. weir with lateral intake etc

3. SURGE TANKS

- Surge tank (or surge chamber) is a device introduced within a hydropower water conveyance system having a rather long pressure conduit to absorb the excess pressure rise in case of a sudden valve closure. 

- It also acts as a small storage from which water may be supplied in case of a sudden valve opening of the turbine. In case of a sudden opening of turbine valve, there are chances of penstock collapse due to a negative pressure generation, if there is no surge tank.

4. CROSS DRAINAGE WORKS

In an Irrigation project, canals may have to cross the natural drainages like rivers, streams, nallahs, etc. at different points within the command area of the project. So, suitable structures must be constructed at the crossing point for the easy flow of water of the canal and drainage in the respective directions. These structures are known as cross-drainage works.

Types of Cross Drainage Works 

• Type I (Irrigation canal passes over the drainage) 

• (a) Aqueduct 

• (b) Siphon Aqueduct 

• Type II (Drainage passes over the irrigation canal) 

• (a) Super passage 

• (b) Siphon super passage 

• Type III (Drainage and canal intersection each other of the same level) 

• (a) Level crossing 

• (b) Inlet and outlet

Aqueduct 

• The hydraulic structure in which the irrigation canal is taken over the drainage (such as river, stream etc..) is known as aqueduct. 

• This structure is suitable when bed level of canal is above the highest flood level of drainage. 

• In this case, the drainage water passes clearly below the canal.

Siphon Aqueduct 

• In a hydraulic structure where the canal is taken over the drainage, but the drainage water cannot pass clearly below the canal. 

• It flows under siphonic action. So, it is known as siphon aqueduct. 

• This structure is suitable when the bed level of canal is below the highest flood level of the drain.

Super Passage 

• The hydraulic structure in which the drainage is taken over the irrigation canal is known as super passage. 

• The structure is suitable when the bed level of drainage is above the full supply level of the canal. 

• The water of the canal passes clearly below the drainage.

Siphon Super Passage 

• The hydraulic structure in which the drainage is taken over the irrigation canal, but the canal water passes below the drainage under siphonic action is known as siphon super passage. 

• This structure is suitable when the bed level of drainage is below the full supply level of the canal.

Level Crossings 

• When the bed level of canal and the stream are approximately the same and quality of water in canal and stream is not much different, the cross drainage work constructed is called level crossing where water of canal and stream is allowed to mix

Inlet and Outlet 

• When irrigation canal meets a small stream or drain at same level, drain is allowed to enter the canal as in inlet. 

• At some distance from this inlet point, a part of water is allowed to drain as outlet which eventually meets the original stream. 

5. DAMS

A dam is a barrier that stops or restricts the flow of water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, aquaculture, and navigability.

Dam Classification 

1. Embankment dams 

 Earth Fill Dams

 Rock Fill Dams 

2. Concrete dams 

 Arch Dams 

 Gravity Dams 

 Buttress Dams

Embankment failure modes 

 Overtopping 

 Internal erosion 

 Seepage 

 Piping Embankment and foundation settlement 

 Instability

Concrete dams defect mechanisms and failure modes 

 Over topping failure. 

 Seepage. 

 Landslide failure. 

 Collapse

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