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