Diversion and Impounding Structures

UNIT III

DIVERSION AND IMPOUNDING STRUCTURES

CONTENTS

3.WEIRS - TYPES & COMPONENTS …………………………………………..3.5

3.1. DEFINITION…………………………………………………………………..3.5

3.2. TYPES OF WEIRS………………………………………………………...….3.5

3.3. GRAVITY WEIRS…………………………………………………………….3.9

3.4.1. PURPOSES OF DIVERSION HEADWORK………………………….

3.4.2. C..OMPONENT PARTS OF DIVERSION HEADWORK……………

3.5. DAM……………………………………………………………………………3.8

3.5.1. TYPES OF DAM……………………………………………………………3.8

3.6. GRAVITY DAMS……………………………………………………………..3.9

3.6.1 FORCES ACTING ON A GRAVITY DAM

3.7. BUTTRESS DAM……………………………………………………..……...3.10

3.8. EARTH DAMS………………………………………………………………..3.11

3.8.1. VARIOUS TYPES OF EARTH DAM

3.9. ARCH DAM…………………………………………………………………..3.13

3.9.1. FORCES ACTING ON ARCH DAM

3.10. TANK…………………………………………………………………………3.14

3.10.1. TANK SLUICE…………………………………………………………3.14

3.11. SPILLWAYS………………………………………………………………….3.14

3.11.1. TYPES OF SPILLWAYS

3.12. HYDRAULIC STRUCTURES AND DAMS………………………………3.20

TECHNICAL TERMS:

Dam: A dam is a hydraulic structure constructed across a river to store water on its upstream side. It is an impervious or fairly impervious barrier put across a natural stream so that a reservoir is formed.

Spillways and energy dissipaters: Spillway is a channel that carries excess water over or around a dam or other obstruction. An energy dissipater is a device that is used to convert concentrated storm water runoff to sheet flow and is constructed at the end of all storm sewers or channels that outfall into a buffer.

Sluice and Outlet: A sluice is an artificial channel for conducting water, with a valve or gate to regulate the flow. An outlet is a small structure which admits water from the distributing channel to a water course of field channel.

Barrage: An artificial obstruction placed in a river or water course to increase the depth of water.

Canal Head Regulator: Any structure constructed to regulate the discharge, full supply level or velocity in a canal is known as a regulator work. This is necessary for the efficient working and safety of an irrigation channel. A canal head regulator regulates the supplies of the off taking channel and the present channel respectively. The head regulator is provided at the head of the distributaries and controls the supply entering the distributaries.

River Training Works: Various measures adopted on a river to direct and guide the river flow, to train and regulate the river bed or to increase the low water depth are called River Training works. The purpose of the river training is to stabilize the channel along a certain alignment.

Cross regulator: A regulator provided on the main channel at the downstream of the offtake to head up the water level and to enable the off-taking channel to draw the required supply is called a Cross Regulator.

Pump Unit: This takes water from the source and provides the right pressure for delivering into the pipe system.

The control head: This consists of valves to control the discharge and pressure in the entire system. It may also have filters to clear the water. Common types of filters include screen filters and graded some filters which remove fine material suspended in the water. Some control head units contain a fertilizer or nutrient tank. These slowly add a measured dose of fertilizer into the water during irrigation.

Mainlines, sub mains and laterals: These components supply water from the control head into the fields. They are normally made from PVC or Polyethylene Glucose and should be buried below ground because they easily degrade when exposed to direct solar radiation.

Emitters or drippers: These are devices used to control the discharge water from the lateral to the plants. They are usually spaced more than 1 meter apart with one or more emitters used for a single plant such as a tree. For row crops more closely spaced emitters may be used to wet a strip of soil. The aspect of an emitter that should be kept in mind to decide its efficiency is that it should provide a specified constant discharge which doesn’t vary much with pressure change and doesn’t block easily.

3.WEIRS - TYPES & COMPONENTS
3.1.Definition:
The weir is a solid obstruction put across the river to raise its water level and divert the water in to the canal. If a weir also stores water for tiding over small periods of short supplies, it is called a storage weir.

A solid obstruction put across river to raise its water level and divert water into canal (low head structure)
Vertical drop wall or crest wall
Upstream, downstream cut off wall at the ends of impervious floor
Launching apron for prevention of scour
Graduated inverted filter on downstream surface floor end to relieve the uplift pressure.

3.2.TYPES OF WEIRS :
The two main types of weirs are:

Gravity weir
Non Gravity weir

3.3.Gravity weirs:
Uplift pressure due to the seepage of water below the floor is resisted by the weight of floor.
Its further types are:

Vertical drop weir
Masonry or concrete slope weir
Dry stone slope weir
Parabolic weir

Explanation:

Vertical drop weir
Vertical drop weir or crest wall
Upstream and downstream cut off wall at the end of impervious floor.
Launching apron for scouring prevention
Graded inverted filter at downstream floor end to relieve the uplift pressure.
Masonry or concrete slope weir
Suitable for soft sandy foundation
Generally used where the difference in weir crest and downstream river is limited to 3m.
Hydraulic jump is formed on sloping crest.
Dry stone slope weir
Body wall or weir wall
Upstream and downstream rock fill laid in form of glacis, with few intervening care walls.
Parabolic weir
Similar to spillway section of a dam
Body wall designed as low dam.
Cistern to dissipate energy

Location of Weirs

A weir should be located in a stable part of the river where the river is unlikely to change its course.
The weir has to be built high enough to fulfill command requirements. During high floods, the river could overtop its embankments and change its course. Therefore, a location with firm, well defined banks should be selected for the construction of the weir.
Where possible, the site should have good bed conditions, such as rock outcrops.
Alternatively, the weir should be kept as low as possible.

FUNCTIONS

Weirs allow hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams or in industrial discharge locations. Since the geometry of the top of the weir is known and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow.
The calculation relies on the fact that fluid will pass through the critical depth of the flow regime in the vicinity of the crest of the weir.
If water is not carried away from the weir, it can make flow measurement complicated or even impossible.
A weir may be used to maintain the vertical profile of a stream or channel, and is then commonly referred to as a grade stabilizer such as the weir in Duffield, Derbyshire.
The crest of an overflow spillway on a large dam is often called a weir.

3.4. DIVERSION HEADWORK.

Any hydraulic structure, which supplies water to the off-taking canal, is called a headwork.

A diversion headwork serves to divert the required supply in to the canal from the river.

3.4.1. Purposes of diversion headwork.

It raises the water level in the river so that the commanded area can be increased.

It regulates the intake of water in to the canal.

It controls the silt entry in to the canal.

It reduces fluctuations in the level of supply in the river.

It stores water for tiding over small periods of short supplies.

3.4.2. Component parts of diversion headwork

Weir or barrage

Divide wall or divide groyne

Fish ladder

Head sluice or canal head regulator

Canal off-takes

Flood banks

River training works.

3.5. DAM

A dam is a hydraulic structure constructed across a river to store the supply for a longer duration and release it through designed outlets.

3.5.1. TYPES OF DAM

Solid gravity dam (masonry, concrete, steel and timber)

Arch dams

Buttress dams

Earth dams

Rock fill dams

Combination of rock fill and earth dams

Objectives:

Hydropower

Irrigation

Water for domestic consumption

Drought and flood control

For navigational facilities

FIGURE 1:Component parts of diversion headwork

•Heel: contact with the ground on the upstream side

•Toe: contact on the downstream side

•Abutment: Sides of the valley on which the structure of the dam rest

•Galleries: small rooms like structure left within the dam for checking operations.

•Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry.

•Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side

•Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.

3.6. GRAVITY DAMS:

A gravity dam is a structure so proportioned that its own weight resists the forces exerted upon it. It requires little maintenance and it is most commonly used.

These dams are heavy and massive wall-like structures of concrete in which the whole weight acts vertically downwards

figure 2: gravity dams

3.6.1 FORCES ACTING ON GRAVITY DAM

Water pressure

Weight of dam

Uplift pressure

Pressure due to earthquake

Ice pressure

Wave pressure

Silt pressure

figure 3: forces acting on gravity dam

•Chakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.

•Bhakra Dam is across river Sutlej in Himachal Pradesh

•The construction of this project was started in the year 1948 and was completed in 1963.

•It is 740 ft. high above the deepest foundation as straight concrete dam being more than three times the height of QutabMinar.

•Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the top is 9.14 m (30 feet)

•Chakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.

3.7. BUTTRESS DAM:

•Buttress Dam – Is a gravity dam reinforced by structural supports

•Buttress - a support that transmits a force from a roof or wall to another supporting structure

•This type of structure can be considered even if the foundation rocks are little weaker

FIGURE4:Buttress Dam

3.8. EARTH DAMS:

•They are trapezoidal in shape

•Earth dams are constructed where the foundation or the underlying material or rocks are weak to support the masonry dam or where the suitable competent rocks are at greater depth.

•Earthen dams are relatively smaller in height and broad at the base.They are mainly built with clay, sand and gravel, hence they are also known as Earth fill dam or Rock fill dam

3.8.1. VARIOUS TYPES OF EARTH DAM

Depending upon the method of construction, earth dam can be divided into,

Rolled fill dam

Hydraulic fill dam

FIGURE 5:Earth Dam

Types of failure that occur during construction of earth dam:

Hydraulic failures : 40%

Seepage failure : 30%

Structural failure : 30%

Advantages of Earth Dam

They can be designed and constructed to suit the soil available in the locality and the foundation conditions.

They can be constructed rapidly with relatively unskilled labour.

They are cheaper than other types.

They can be subsequently raised in height without much difficulty.

Disadvantages of Earth Dam

They are not suitable for greater heights.

They cannot be used as overflow dams.

They are not suitable for deep gorges.

They are not suitable in places of heavy rainfall.

They require heavy maintenance cost and constant supervision.

3.9. ARCH DAM

An arch dam is a dam curved in plan and carries a major part of its water load horizontally to the abutments by arch action. The part of the water load depends primarily upon the amount of curvature. The balance of the water load is transferred to the foundation by cantilever action.

•These type of dams are concrete or masonry dams which are curved or convex upstream in plan

•This shape helps to transmit the major part of the water load to the abutments

•Arch dams are built across narrow, deep river gorges, but now in recent years they have been considered even for little wider valleys.

FIGURE 6:Arch dams

3.9.1. Forces acting on arch dam

Water pressure

Weight of dam

Uplift pressure (negligibly small)

Pressure due to earthquake

Ice pressure

Silt pressure

3.10. TANK

They are small storage meant for irrigating the local area. They may receive their supply from their own catchments. They may also have supply from a nearby river.

3.10.1. TANK SLUICE

These are outlets that extend from the upstream face of a bund to the downstream face. They are provided to discharge the stored water either for irrigation or for any other purposes.

How will you select a site for a tank sluice?

The site to be selected should be such that,

The sluice commands the ayacut.

The sill level of the sluice is above the bed level of existing canal.

Good natural ground is available at the sill level.

It involves minimum cutting

It ensures the safety of the dam itself.

3.11. SPILLWAYS

A spillway is the overflow portion of dam, over which surplus discharge flows from the reservoir to the downstream. A spillway is therefore called as surplussing work, designed to carry this flood water not required to be stored in the reservoir, safely to the river lower down.

Spillways are provided for storage and detention dams to release surplus

Floodwater, which can not be contained in the allotted storage space.

In diversion works, like weirs and barrages, spillways bypass the flow exceeding that which is released in to the system like irrigation canals, power canals, feeder canals, link canals etc.
Ordinarily, the excess flow is drawn from the top of the pool created by the dam

And conveyed through an artificial waterway i.e. spillway, back into the same river or

To some other drainage Channel.

FIGURE 7:Spillways

3.11.1. TYPES OF SPILLWAYS:

1. Vertical Drop type Spillway

2. Ogee (overflow) Spillways

3. Chute (Open Channel or Trough) Spillways

4. Conduit and Tunnel Spillways

5. Drop Inlet (Shaft or Morning Glory) Spillways

6. Culvert Spillway

7. Siphon Spillways

1. Vertical Drop type Spillway

A vertical drop or free over fall type spillway is one in which the flow drops

Freely from the acres of the dam. This type is suited in a thin arch or a deck overflow

Type dam. Flows may be freely discharging, or they may be supported along a narrow

Section of the crest.

Occasionally, 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 ventilated sufficiently to prevent a

Pulsating, fluctuating jet.

Where no artificial protection is provided at the base of the

Over fall, scour will occur in most streambeds and will form a deep plunge pool. The

Volume and depth of the hole are related to the range of discharges, the height of the

Drop and the depth of tail water.

2. Ogee (overflow) Spillways

The ogee spillway as shown in Fig.3 (a) and Fig.3 (b) has a control weir, which

Is ogee or S-shaped in profile?

Ordinarily, the upper curve of the ogee spillway is made to conform closely to the profile of the lower of a ventilated sheet falling from a sharp-crested weir.
Flow over the crest is made to adhere to the face of the

Profile by preventing access of air to the under side of the 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.

FIGURE 8:

3. Chute (Open Channel or Trough) Spillways

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 trough type spillway.

These designations can apply regardless of the control device used to regulate the flow.
Thus, a spillway having a chute-type discharge channel, though controlled by an

Overflow crest, a gated orifice, a side channels crest, or some other control device

Might still be called a chute spillway.

However, the name is most often applied when the spillway control is placed normal or nearly normal to the axis of an open channel, and where the streamlines of flow both above and below the control crest follow the direction of the channel axis.

4. Conduit and Tunnel Spillways

Where a closed channel is used to convey the discharge around or under a dam,

The spillway is often called a tunnel or conduit spillway, as appropriate.

The closed channel may take the form of a vertical or inclined shaft, a horizontal tunnel through earth or rock, or a conduit constructed in open cut and backfilled with earth materials.
Most forms of control structures, including overflow crests, vertical or inclined orifice

Entrances, drop inlet entrances, and side channel crests, can be used with conduit and

Tunnel spillways.

Tunnel spillways may present advantages for dam sited in narrow canyons with steep abutments or at sites where there is danger to open channels from

Snow or rockslides.

5. Drop Inlet (Shaft or Morning Glory) Spillways

A drop inlet or shaft spillway, as the name implies, is one 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.