BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 89

CHAPTER 9

WATER RESOURCES ENGINEERING

9.1 Hydraulic Structures of Storage

9.1.1 Introduction

In order to harness the water potential of a river optimally, it is necessary to

construct two types of hydraulic structures.

1. Storage structure : Usually a dam, which acts like a reservoir for storing

excess runoff of a river during periods of high flows (as during the

monsoons) and releasing it according to a regulated schedule.

2. Diversion structure : It may be a weir or a barrage that raises the water

level of the river slightly, not for creating storage, but for allowing the

water to get diverted through a canal situated at one or either of its

banks. The diverted water passed through the canal may be used for

irrigation, industry, domestic water needs or power generation.

3. Dams : A dam is a hydraulic structure of fairly impervious material

built across a river to create a reservoir on its upstream side for

impounding (storing) water for various purposes. A dam and a

reservoir are complements of each other.

Classification of Dams

(a) Based on Function Served

i. Storage dams.

ii. Detention dams.

iii. Diversion dams.

iv. Debris dams.

v. Coffer dams.

(b) Based on Hydraulic Design

i. Overflow dams ii. Non-overflow dams

i. Overflow dams : An overflow dam is designed to act as an overflow

structure. The surplus water which cannot be retained in the reservoir

is permitted to pass over the crest of the overflow dam which acts as a

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 90

spillway. The overf1ow dams made of a material which does not erode

by the action of over flowing water. Most of the gravity dams have

overflow section for some length and the rest of the length as a non-

overflow dam. The overflow dam is also called the spillway section.

ii. Non-overflow dams: A non-overflow dam is designed such that there

is no flow over it. Because there is non-over flow, a non-overflow dam

can be built of any material, such as concrete, masonry, earth, rockfill

and timber. As already mentioned, the non-overflow dam is usually

provided in a part of the total length of the dam. However, sometimes

the non-overflow dam is provided for the entire length and a separate

spillway is provided in the flanks or in a saddle away from the dam.

(c) Based on Materials construction: Based on the materials used in

construction, the dams are classified as follows :

(1) Masonry dam, (2) Concrete dam, (3) Earth dam, (4) Rockfill dam,

(5) Timber dam, (6) Steel dam, (7) Combined concrete-cum-earth- dam

and (8) Composite dam.

(d) Based on Rigidity

i. Rigid dams : A rigid dam is quite stiff. It is constructed of stiff materials

such as concrete, masonary, steel and timber.

ii. Non-rigid dams : A non-rigid dam is relatively less stiff compared to a

rigid dam. The dams constructed of earth and rockfill are non-rigid

dams.

Spillways: A spillway is the overflow portion of dams, over which surplus discharge

flows from the reservoir to the downstream side of dam. Common examples are

Straight fall or Drop spillway, Ogee spillway, Side channel spillway, Shaft spillway

and Siphon spillway.

Weir: The weir is a solid obstruction built across the river to raise its water level not

to store but to divert the water into the canal.

Canals: A canal is an artificial channel, generally trapezoidal in shape constructed on

the ground to carry water to the irrigation fields either from river or from a tank or

reservoir. Canal can be classified into Main canal, Branch canal, Distributary canal.

Canal Lining: It is thin covering provided to the bed and sides of canal. To prevent

the seepage loss and increase the discharge by increasing velocity, and also for to

increase the life of canal. Following material is used for lining stones, bricks, precast

concrete block, R.C.C.

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 90

Advantages of Canal lining :

1. To reduce the seepage loss.

2. To reduce excavation cost.

3. To reduce evaporation.

4. To increase life of canal.

5. To increase the discharge by

increasing velocity.

6. To reduce silt problems.

7. To reduce maintenance cost.

8. To decrease canal falls.

Disadvantage of Canal lining :

1. Heavy initial cost.

2. Difficult to repair.

3. Joint may create troubles in

flow.

4. More time required to

complete canal work.

Gravity dam

A gravity dam is a one in which the external forces such as water pressure,

wave pressure, silt pressure, uplift pressure etc. are resisted by the weight of

the dam itself.

Fig : Gravity Dam

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 90

Earth dam

Earth dam is a dam having the main section built of earth, sand or rock and a

core of impervious material such as clay. Earth dam resist the forces exerted

upon it mainly by its shear strength.

Fig : Earth Dam

9.1.1 COMPARISON BETWEEN GRAVITY DAM AND EARTH DAM

Sr. No. Gravity Dam Earth Dam

1

It is constructed with rigid

materials rigid materials such

as concrete

It is constructed with non-

rigid materials such as gravel,

sand; silt clay

2 It resists the forces exerted

upon it by its own weight

It resists the forces exerted

upon it mainly its shear

strength.

3

Gravity dams are relatively

more strong and stable than

earth dams

Earth dams are relatively less

strong and stable than gravity

dams

4 Overflow spillway crest is

used in gravity dam

Earth dam cannot be used as

overflow dam

5 It can be constructed of any

height

The height of the earth dam is

limited by the suitability of its

slope

6 It requires less maintenance It requires more maintenance

7 It gives enough warning time

before the failure

Earth dam generally fail

suddenly

8 Life is more because trap

efficiency is less

Life is less because trap

efficiency is higher than

gravity dam

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 91

9.2 WATER CONVEYANCE SYSTEMS:

Channel: A channel may be defined as a passage through which water flows

under atmospheric pressure.

The channels may be classified according to different considerations as given

below.

Closed channel: the channels having cover at the top are known as closed

channels. Closed channels will be running partly full of water. Common

examples are closed pipes, underground drain, tunnels etc.

Open Channel : The channels without cover at the top are known as open

channels

Natural channel: A. natural channel is the one which has an irregular sections

of varying shapes, which is developed in a natural way. The examples of

natural channels are rivers, streams etc.

Artificial channel: An Artificial channel is the one which is built artificially for

carrying water for various purposes. Artificial channels have their cross-

sections with regular geometrical shapes, which usually remain same

throughout the length of the channel. The artificial channels may be further

classified according to the shape of the cross-section as, rectangular channel,

trapezoidal channel, triangular channel, parabolic channel and circular

channel.

Among these cross sections, trapezoidal section is commonly used.

Note: Refer page no. for numerical based on Discharge carrying capacity of

channel

9.3 WATERSHED MANAGEMENT

Watershed :- It is defined as the land area from which water drains under the

influence of gravity into a stream or a lake or reservoir or any other body of

surface water.

Watershed management :- It is defined as the management of surface runoff

either by tapping by the surface water on the surface by allowing the surface

water to percolate to recharge the ground water table.

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 92

Necessity / Objectives:-

1. To reduce the risk of floods, droughts and land-slides.

2. To develop the economy of the rural areas through sustainable

development.

3. To convert the cultivate waste-land into cultivable lands through the

social forestry and horticulture.

4. To promote the drought prone areas economically, socially and

culturally.

5. To control the soil – losses through the surface runoff.

Methods of watershed management:-

To improve the ground level, several civil structures are constructed in the

watershed area they are as follows:-

1. Pits and Trenches:- The pits and trenches are dogged at equal interval

on the skipping surface to cut the surface flow and allow it to percolate

through these trenches to enrich the ground level.

2. Stone embankment or earthen dam:- Decreases the velocity of flow.

3. The farm Pound:- They are constructed near the agricultural field in

the catchment area to provide enough surface water to the field and

also to increase the ground water level.

4. Dyker or underground barriers:- These structure are constructed in a

small surface streams. Eg:- nallahs etc., to prevent the free ground

surface water flow and allow the water table to come up to help to

improve the irrigation through the dug-wells.

A Case Study of Relegan Siddhi

It is one of arid region located in Ahmednagar district of Maharashtra

state. One single person Mr. Anna Hazare a retired army man when

came back to his village, which is a typical settlement in the arid zone

with a traditional farming based on the rainfall only. During drought

years the villagers along with their domestic animals used to migrate

in search of food and water. Anna, could not bear it, he farmed a team

of volunteers from his own village and started his movement

concentrating on trapping every drop of rainfall (It is the base of

Draught Mitigation Practice).

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 93

Taking the help of the villages check dam and tanks were built for

conservation of soil, the trees were planted on the sloping grounds,

with his massive efforts within a period of 20 odd years, and he has

converted a dry tree-less, hopeless rural settlement into a green belt.

Now the settlement has a massive area of 1300 acres under irrigation.

(Subsurface irrigation). The out irrigation has stopped, per capital

income has increased from Rs. 225 to Rs. 2250.

This all was done without any outside help, Government, or the World

Bank, Now, Ralegan Siddhi has become a symbol of Self-development.

9.4 RAIN WATER HARVESTING

In rooftop harvesting, the roof becomes the catchments, and the rainwater is

collected from the roof of the house/building. It can either stored in the tank

or diverted to artificial recharge system.

The Main objective of rooftop rain water harvesting is to make water available

for future use. Capturing and storing rain water for use is particularly

important in dryland, hilly, urban and coastal areas.

Need for Rooftop Rain Water Harvesting:-

1. To meet the ever increasing demand for water

2. To reduce the runoff which chokes storm drains

3. To avoid flooding of roads

4. To augment the ground water storage and control decline of water levels

5. To reduce ground water pollution

6. To improve the quality of ground water

7. To reduce the soil erosion

8. To supplement domestic water requirement during summer, drought etc.

Advantages of Rooftop Rain Water Harvesting:-

1. Provides self-sufficiency to your water supply.

2. Reduces the cost for pumping of ground water.

3. Provides high quality water, soft and low in minerals.

4. Improves the quality of ground water through dilution when

recharged to ground water.

5. Reduces soil erosion in urban areas.

6. The rooftop rain water harvesting is less expansive

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 94

7. Rainwater harvesting systems are simple which can be adopted by

individuals.

8. Rooftop rain water harvesting systems are easy to construct, operate

and maintain.

9. In hilly terrains, rain water harvesting is preferred.

10. In saline or coastal areas, rain water provides good quality water and

when recharged to ground water, it reduces salinity and also helps in

maintaining balance between the fresh-saline water interfaces.

11. In Islands, due to limited extent of fresh water aquifers, rain water

harvesting is the most preferred source of water for domestic use.

12. In desert, where rain fall is low, rain water harvesting has been

providing relief to people.

Methods of Roof top Rain water harvesting:-

1. Storing water for direct use:-

In this method rain water collected from the roof of the building is

diverted to a storage tank. The storage tank has to be designed

according to the water requirements, rainfall and catchment

availability. Each drainpipe should have mesh filter at mouth and first

flush device followed by filtration system before connecting to the

storage tank. Water from storage tank can be used for secondary

purposes such as washing and gardening etc.

Fig : Storing rainwater for direct use

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 95

2. Recharging ground water aquifers from roof top run-off:-

Ground water aquifers can be recharged by various kinds of structures.

Commonly used recharging methods are:-

Recharging of bore wells.

Recharging of dug wells.

Recharge pits.

Recharge Trenches.

Soak ways of recharge shafts.

Percolation tanks.

Fig : Recharge ground water aquifers with roof top runoff

9.5 GROUND WATER RECHARGE

It is a hydrologic process where water moves downward from surface water

to groundwater. This is requisite to increase the ground water table elevation.

This can be done by many artificial techniques, say, by constructing a

detention dam called a water spreading dam to store the flood waters and

allow for subsequent seepage of water into the soil, so as to increase the

groundwater table.

Prof Asim

Ejaz

BASIC OF CIVIL ENGINEERING WATER RESOURCES ENGINEERING

w w w . a s i m s h e i k h 1 2 3 . w o r d p r e s s . c o m A . C . E . T . 96

Necessity:-

1. Groundwater forms in integral part of the water cycle.

2. Increasing water demands in urban and rural areas put increasing

pressures on the use of groundwater.

3. Increasing contamination of the resource as a result of urban, industrial

and agricultural expansion.

4. Soil salinization is often associated with irrigation practices but is also

driven by natural groundwater processes.

5. Integrated catchment management including groundwater is the key

to solving the continuously expanding environment problems of

salinity, water logging and land degradation as well as the

preservation of ecosystems.

Methods of ground water recharge:-

1. Water spreading method:- Water spreading involves diversion of

surface water to topographical lows such as abandoned pits or

detention ponds and reservoirs.

2. Recharging wells:- This method consists of injecting the water from

bore holes, called recharge wells. In this method, the water is fed into

recharging well by gravity or may be pumped under pressure to

increase recharge rate, if surface conditions permit. This method is

generally preferred in low permeable areas.

3. Induced infiltration method:- Injection wells are structures similar to

tube wells but constructed for augmenting the ground water storage in

deeper aquifers through supply of water either under gravity or under

pressure.

4. Recharge Pits and Shafts:- Recharge pits and shafts are artificial

recharge structures commonly used for recharging shallow phreatic

aquifers, which are not in hydraulic connection with surface water due

to the presence of impermeable layers.

Questions

1. Enlist different Hydraulic structures of storage and explain them.

2. Write down the different water conveyance systems.

3. Write short notes on

i. Rain water harvesting

ii. Watershed management methods

4. What are the different of Ground water recharge?

Prof Asim

Ejaz