JAGADISH S H, ASSISTANT PROFESSOR ASSIGNMENT TOPICS WITH MATERIALS UNIT: I 1. Introduction to Engineering Hydrology and Its Applications Hydrology: It is the science which deals with the occurrence, distribution and movement of including that in the atmosphere and below the earth surface. Water occurs in the atmosphere is in the form of vapour. On the ground surface as water or ice. Below the earth surface as ground water occupying all the voids with in the geological stratum. Hydrological Cycle and their Components Hydrological Cycle: Except for very deep ground waters, the total water supply of earth is in contact circulation circulatory system is known as hydrological cycle. (Or) hydrological cycle is the process of and evaporation of water back to the atmosphere. Diagram Showing Hydrological Cycle 2. Hydrological cycle consists of the following Processes. 1. Evaporation and Transpiration (E)
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JAGADISH S H, ASSISTANT PROFESSOR
ASSIGNMENT TOPICS WITH MATERIALS
UNIT: I
1. Introduction to Engineering Hydrology and Its Applications Hydrology: It is the science which deals with the occurrence, distribution and movement of
including that in the atmosphere and below the earth surface.
Water occurs in the atmosphere is in the form of vapour.
On the ground surface as water or ice.
Below the earth surface as ground water occupying all the voids with in the
geological stratum.
Hydrological Cycle and their Components Hydrological Cycle: Except for very deep ground waters, the total water supply of earth is in contact circulation
circulatory system is known as hydrological cycle. (Or) hydrological cycle is the process of
and evaporation of water back to the atmosphere.
Diagram Showing Hydrological Cycle 2. Hydrological cycle consists of the following Processes. 1. Evaporation and Transpiration (E)
JAGADISH S H, ASSISTANT PROFESSOR
The water from the surface of the ocean, rivers and also from the moist soil evaporates. The
vapours are carried over the land by air in the form of clouds. Transpiration is the process of
water being lost from the leaves of the plants from their pores. Total evaporation (E)
including transpiration consists of the following:
d. Surface evaporation: Evaporation from moist soil e. Water surface evaporation: Evaporation from river surface, from oceans f. Evaporation from plants and leaves ( Transpiration ) g. Atmospheric evaporation. 2. Precipitation (P)
s surface in any form. Precipitation may be two forms
Liquid Precipitation - rain
Frozen form consist of - Snow, Hail, sleet, freezing rain.,
3. Run off (R) Runoff is that portion of precipitation that is not evaporated. When moisture falls to the ground in the form of precipitation, a part of precipitation will be evaporated by means of soil surface, water surface and plants. The remaining part of precipitation is available as a runoff which ultimately reaches ocean through surface and sub surface streams. The run off may be classified as following:
Surface Run-off
Water flow over the land surface and First it reaches the streams and rivers ultimately
this quantity of water is discharged in oceans.
Inter flow (or) Sub-surface Run-off
depends on the geology of the basin, runs as a sub-surface flow ultimately reaches to oceans
through rivers and streams.
JAGADISH S H, ASSISTANT PROFESSOR
Ground Flow (or) Base flow Run-off:
It is also a portion of precipitation, which after infiltration, percolates down and Joins the ground water reservoir. This ultimately reaches the oceans. Precipitation = Evaporation + Run off 3.Measurement of Rain fall Rain fall is the main source of water used for various purposes. Knowledge of rain fall
quantity, intensity of rain fall & distribution of rain fall is extremely useful for irrigation
engineering. The amount of rain fall is measured in centimeters (inches).which is falls on a level
surface and is measured by rain gauges.
The intensity of rain fall is the rain fall per unit time. It is expressed as cm/hr. The following
are the main types of rain Gauges used for measurement of rain fall.
Non-automatic rain gauges
Non automatic rain gauges are non-recording type.
Eg: Symons rain gauge.
Automatic rain gauges.
Weighing bucket type rain gauge.
Tipping bucket type rain gauge.
Float type rain gauge. Symons rain gauge: Symons rain gauge is commonly non recording type rain gauge in India
and is used in meteorological department of government of India. It consists of cylindrical
vessel 127 mm (or 5") in diameter with a base enlarged to 210 mm (or 8") diameter. The top
section is a funnel provided with circular brass rim exactly 127 m (5") internal diameter. The
funnel shank is inserted. in the neck of a receiving bottle which is 75 to 100 mm ( 3 to 4")
diameter. A receiving bottle of rain-gauge has a capacity of about 75 to 100 mm of rainfall
and as during a heavy rainfall this quantity is frequently exceeded, the rain should be
measured 3 or 4 times in a day on day of heavy rainfall lest the receiver fill should overflow.
A cylindrical graduated measuring glass is furnished with each instrument, which reads to
0.2 mm. The rainfall should be estimated to the nearest 0.1 mm.
JAGADISH S H, ASSISTANT PROFESSOR
Weighing bucket type rain gauge: it is self recording type rain gauge this is used to
determine the rate of rainfall (or) intensity of rain fall over a short period of time. The
weighing bucket rain-gauge essentially consists of a receiver bucket supported by a spring or
lever balance or any other weighing mechanism. The movement of the bucket due to its
increasing weight is transmitted to a pen which traces the record on a clock-driven chart.
Tipping bucket type rain gauge: The tipping bucket type rain-gauge consists of 30 em
diameter sharp edge receiver. At the end of the receiver is provided a funnel. A pair of
buckets are pivoted under the funnel in such a way that when one bucket receives 0.25
JAGADISH S H, ASSISTANT PROFESSOR
(0.01 inch) of precipitation it tips, discharging its contents into a reservoir bringing
the other bucket under the funnel. Tipping of the bucket completes an electric circuit
causing the movement of pen to mark on clock driven revolving drum which carries a
record sheet.
Float Type Automatic Rain-gauge: The working of a float type rain-gauge is similar to the
weighing bucket type gauge. A funnel receives the rain water which is collected in a
rectangular container. A float is provided at the bottom of the container. The float is raised as
the water level rises in the container, its movement being recorded by a pen moving on a
recording drum actuated by a clock work. When the water level in the container rises so that the
float touches the top, the siphon comes into operation, and releases the water; thus all the water in the
box is drained out.
JAGADISH S H, ASSISTANT PROFESSOR
Site Selection For Rain Gauge:
The rain gauge should be installed on a plain surface.
The distance between rain gauge and nearest objective should be at least twice the height of the objective
The rain gauges should not be provided side and top of the hill. 4.Advantages of Recording type rain fall over non Recording type rain ruage:
Rain fall is recorded automatically and hence no attender is necessary.
By using automatic rain gauge records the intensity of rain fall, where as
in the case of non-automatic rain fall rain gauges, only rain fall depth is
inaccessible points also.
Human errors are avoided. Disadvantages
It is costly.
Fault may be developed in electric &mechanical mechanism for
recording the readings.
JAGADISH S H, ASSISTANT PROFESSOR
UNIT-II
1. Hydrograph Analysis
Hydrograph is a graph showing variations of discharge with time, at a particular point of a
stream. It shows the time distribution of total run-off at the point of measurement.as shown in
fig 1.1 Fig 1.1 Storm Hydrograph Component Parts of Storm Hydrograph A single peaked hydrograph resulting from a isolated storm is shown in Fig.1.2. A is the
point of rise, D is the peak and E is the point of inflection on the recession curve. As started
earlier, rain water, exclusive of the water withheld as basin recharge. May follow three paths
to stream:
Overland flow (surface run-off,
Interflow (influent stream), and
Ground water flow (effluent stream). Overland flow and interflow are frequently grouped together as direct run-off. Much of the
low water flow of streams is derived from ground water. In a storm hydrograph, the direct
run-off and the ground water run-off are separated by the following procedure. Extend the recession of the flow existing prior to the storm to a point under the peak (or crest) of the hydrograph (line AB). Select a point C on the recession limb of the hydrograph N days after the peak. Join point
Band C.
JAGADISH S H, ASSISTANT PROFESSOR
. Fig 1.2 Components of Single Peak hydrograph A rough selection of N
(in days) is given by
N= O.84A0.2 Where A = drainage area is sq. km.If the drainage area of the basin is expressed in sq. miles.
N= A10.2
Where A1= drainage area in sq. miles. Thus, ABC is the ground water divide line. Area ABCD gives volume of direct run-off. In
Fig.1.2, at any time instant t, the ordinate ab represents the flow due to ground water
storage or base flow and bd represents the flow due to both surface run-off as well as
influent stream.
Fig.1.2 also shows heytograph along with infiltration curve, at the top. It also shows the
following important time factors useful in hydrograph analysis.
Effective Duration: It is the time between which the rainfall rate is more than the
infiltration rate.
Basin Lag (tp): It is the time between the centroid of the net rainfall (Pnet) and the
peak of the hydrograph.
Recession Time (Tr): It is the duration of direct runoff after the end of effective
duration.
JAGADISH S H, ASSISTANT PROFESSOR
Time of Concentration (tc): Time of concentration is the time in hours taken by rain
water that falls at the farthest point to reach the outlet of a catchment. This is equal to the
time between point of inflexion and end of effective duration. Valley Storage: The discharge in a stream goes on increasing as water enters the stream, and
the levels go on increasing. The difference between the water level prior to the flood and the
flood water level indicates the volume of water that is temporarily stored.The water stored is
not standstill, but moves as sheet of water and is known as valley storage. Channel Storage:
If the flood water level is within the banks of the stream, the valley storage is known as
channel storage. Experience in America indicates that flow to the right side of the point of
inflexion of the falling side of the hydrograph above ground water line is due almost entirely
to channel storage.
Base flow or Depletion Curve: In Fig 1.2, ABC is the estimated or assumed base flow or
depletion curve. Actual contribution of ground water is shown by dotted lines in,,Fig. 1.3(b).
As shown in Fig. 1.3(a) when the water level before the storm is low, there is a flow from
ground water to stream. But as the water level in the stream is raised due to the storm, there
is flow from the stream to the ground water. As the flood level in the stream recedes, the
ground water again starts contributing to the stream. Thus ABC denotes the ground water
line only if there were no flood. However since the ground water contribution is small
percentage of the total flow, it is usually considered-justified that the flow above the line
ABC is due to direct run-off.
Hyetograph: Hyetograph is the graphical representation of average rainfall and rainfall
excess (i.e., rainfall minus infiltration) rates over specified area during successive unit time
intervals during a storm. Such hyetographs are plotted above the run-off hydrographs. Fig 1.3(a) Stages in storm before and after
JAGADISH S H, ASSISTANT PROFESSOR
(b) Actual base flow 2. Calculation of Direct Run-off for Rain-fall Excess From the Storm Hydrograph. Procedure (Refer Table 1.1). Table 1.1
Date and Time Ordinate of hydrograph Base flow Direct run-off (0)
(1) Total Q (cumecs) (4) . (cumecs) (2) (3)
14-8-65
0500 14 14 0
0800 25 12 13
1100 51 11 40
1400 65 10 55
1700 54 11 43
2000 28 13 15
2300 14 14 0
Q=166
Find the ordinates of storm hydrograph, representing total discharge Q at a
given time interval, say t hours.
Separate the ground water flow. Find the ordinates of the base flow at the same
time interval
Find the ordinates of direct run-off by subtracting the ordinates of base flow
from total discharge ordinates. .4. The direct run:-off in depth of water (in cm) is found from the expression Direct run-
off = Total volume of direct run-off /Area of basin
C. 0.36 (
Where t = Time interval between successive ordinates in hours A = Area of basin in sq. km.
JAGADISH S H, ASSISTANT PROFESSOR
Q = Discharge in cumecs.
O = Sum of discharge ordinates (direct run-off) in cumecs. For the above example, if area of the basin is 30 sq. km. . Directly run off = 0.36 (
0.36 166
3. Unit Hydrograph
A unit hydrograph is a hydro graph representing 1 cm (or 1 inch) of run off
from a rainfall of some unit duration and specific areal distribution.
Unit duration refers to the duration of a run-off producing rainfall-excess,
that results in a unit hydrograph.
For example, if a unit hydrograph results from a 3 hours unit rainfall duration,
it is known as a 3-hours unit hydrograph, meaning thereby a hydrograph produced
by surface run-off from a storm lasting for 3 hours and yielding rainfall excess of 1
cm.
The unit duration may vary from one day for large catchments to a few
hours for a small catchment. Experience has shown that best unit duration is
about one fourth of the basin lag, the time from the centre of mass of rainfall to
the peak of the hydro graph.
The basis of the unit hydrograph concept, first given by L.K.Sherman
(1932) is that if two identical rain storms could occur over a drainage basin with
identical conditions prior to the rain, the hydrographs of run-off from the two
storms would be expected to be the same.
The basic premise implies that the rainfall excess of n centimeters with in
the selected unit duration unit will give a run-off hydrograph having ordinates n
times as great as those of unit hydro graph of the same unit duration.
JAGADISH S H, ASSISTANT PROFESSOR
4. Assumptions of Unit Hydrograph Theory. The assumptions of unit hydrograph theory must be properly understood before applying it to
actual problems, as these assumptions impose certain limitations which must be carefully
noted. The various assumptions are given below.
The effective rainfall is uniformly distributed within its duration of specified
period of time.
The effective rainfall is uniformly distributed throughout the whole area of the
drainage basin.
The base or time duration of the hydrograph of direct run-off due to an effectiverainfall of unit duration is constant.
The ordinates of direct run-off of common base-time are directly proportional
to the total amount of direct run-off represented by each hydrograph.
JAGADISH S H, ASSISTANT PROFESSOR
UNIT-III
1. Types of geologic formations and Aquifers
There are basically 4 types of geological formations:
Aquifers
Aquiclude
Aquifuge
Aquitard
h. Aquifer: An aquifer is a water-bearing geologicfonnation which stores water and is
also capable of transmitting water through its pores at a relatively large rate which is
sufficient for economical extraction of ground water by wells. Thus an aquifer is a
saturated formation which not only stores water but also yields it in sufficient quantity
when extracted because of its high permeability.
There are two types of aquifers:
Unconfined aquifers
Confined aquifers
Unconfined aquifers: An unconfined aquifer is one in which a free surface (i.e. water
table) exists (as shown in figure 1.1(a)).The unconfined aquifer extends from the ground
surface upto the impervious stratum underneath. However, only the saturated zone of this
aquifer below the water table is of importance in ground water hydrology. A well drilled
into an unconfined aquifer indicates the water level corresponding to the water table at
that location. The unconfined aquifer is also called the water table aquifer, and a well
drilled in this type of aquifer is known as a water table well.
Confined aquifers: A confined aquifer is an aquifer which is sandwiched between
two impervious strata (or aquiciludes) (as shown in figure below 1.1 (b)). The water in a
confined aquifer is under pressure. The piezometric surface is much higher than the top
level of the aquifer at that point. The confined aquifer js also called the artesian aquifer.
If the water level in the well drilled in a confined aquifer rises above the ground
surface, water flows out freely without any pump. Such a well is called as flowing
artesian well (as shown in figure 1.2). On the other hand, if the water level is below the
ground surface, the well water does not flow freely and the well is called a non-flowing
artesian well.
Fig 1.2 Artesian Well
Aquiclude: An aquiclude is a geologic formation which contains water but cannot
transmit it in sufficient quantity because of its low permeability. It is essentially a
geologic formation which is impermeable to the flow of water. Clay and shales are the
examples of aquicludes.
JAGADISH S H, ASSISTANT PROFESSOR
Aquifuge: An aquifuge is a geologic formation which does not have interconnected pores. It
is neither porous nor permeable. Thus it can neither store water nor transmit it. Examples of
aquifuge are rocks like basalt, granite, etc. without fissures.
D. Aquitard: An aquitard is a partly permeable geologic formation. It transmits water at
such a slow rate that the yield is insignificant. Pumping by wells is not possible. For
example, sand lenses in a clay formation will form an aquitard.
2. Darcy's Law
In 1856, Henry Darcy proposed a law for the velocity of flow through a porous
medium. According to Darcy's law, the velocity of flow in soil is proportional to. the
hydraulic gradient, and is given by
v= ki .
Where k is the coefficient of permeability and i is the hydraulic gradient.
As the hydraulic gradient is dimensionless, the unit of the coefficient of
permeability is the same as that of the velocity. The coefficient of permeability is also
called the hydraulic conductivity. The velocity v in the above equation is the discharge
velocity (or apparent velocity or superficial velocity) and is obtained as
v= Q/A .
Where Q is the discharge and A is the gross cross-sectional area of the soil mass,
including the area occupied by the solid particles.
Because the water can flow only through the pores of the soil mass and not through
the solid particles, the actual velocity (vs) through the pores is different from the apparent
velocity (v). The actual velocity of flow (based on macroscopic scale) through the pores is called the seepage velocity, and is given by
vs = v/n
Where Vs = actual velocity, v discharge velocity and n is the porosity.
As it is difficult to measure the seepage velocity, it is rarely used in ground water
hydrology. Discharge velocity (v) is commonly used in practice, and the velocity in
general means the discharge velocity, unless mentioned otherwise. The discharge can be
expressed as (Fig. 1.4).
JAGADISH S H, ASSISTANT PROFESSOR
Fig. 1.4 Darcy's Law
Q= (ki) A
Where A is the gross cross-sectional area of the porous medium (aquifer). The-hydraulic
gradient (i) is given by ( h/L), where h is the drop in head in a length L.
Range of validity of Darcy's law: Darcy's law is a particular case of the general
laminar flow. The upper limit for the validity of the law is usually taken as Reynolds number equal to unity, where Reynolds number is given by
Re =pvd/µ
Where Re = Reynolds number, v = discharge velocity (cmls), d is the effective diameter
(cm), which is taken as d10 size (i.e. 10% of the particles are finer than this size), p is the
mass density (g/ml), µ is the coefficient a) The flow in aquifers generally follows Darcy's law.
b) It has been found that Darcy's law can be applied even upto Re of 10 or so.
c) In aquifers with large fissures. coarse gravels. rockfills, etc.. the Reynolds
number is usually greater than 10, and therefore, Darcy's law is not strictly applicable.
3. Measurement of coefficient of permeability: In a laboratory, the coefficient of
permeability is determined with an instrument called permeameter. There are two
types of permeameters: (1) Constant-head permeameter. and (2) Falling-head
bpermeameter.
JAGADISH S H, ASSISTANT PROFESSOR
UNIT- IV
1. Necessity and Importance of Irrigation Definition
Irrigation may be defined as the process of artificially supplying water to soil for
raising crops. It is a science of planning and designing an efficient, low-cost, economic
irrigation system tailored to fit natural conditions. It is the engineering of controlling and
harnessing the various natural sources of water, by the construction of dams and
reservoirs, canals and headworks and finally distributing the water to the agricultural
fields. Irrigation engineering includes the study and design of works in connection with
river control, drainage of water-logged areas, and generation of hydroelectric power.
Necessity
India is basically an agricultural country, and all its resources depend on the
agricultural output.
Water is evidently the most vital element in the plant life. Water is normally
supplied to the plants by nature through rains.
However, the total rainfall in a particular area may be either insufficient, or
ill-timed. In order to get the maximum yield,
it is essential to supply the optimum quantity of water, and to maintain correct
timing of water.
This is possible only through a systematic irrigation system-by collecting water during
the periods of excess rainfall and releasing it to the crop as and when it is needed. Thus,
the necessity of irrigation can be summarised in the following four points:
i. Less Rainfall
When the total rainfall is less than needed for the crop, artificial supply is
necessary.
In such a case, irrigation work may be constructed at a place where more water is
available, and then to convey the water to the area where there is deficiency of
water.
JAGADISH S H, ASSISTANT PROFESSOR
3. Commercial Crops with Additional Water
The rainfall in a particular area may be sufficient to raise the usual crops, but more water
may be necessary for raising commercial and cash crops.
4. Controlled Water Supply
By the construction of proper distribution system, the yield of the crop may be increased.
Application of water to the soil by modern methods of irrigation serves the
following purposes:
It adds water to the soil to supply the moisture essential for the plant growth.
It saves the crops from drying during short duration droughts.
It cools the soil and the atmosphere, and thus makes more favourable
environment for healthy plant growth.
It washes out or dilutes salts in the soil.
It reduces the hazard of soil piping.
It softens the tillage pans.
2. Importance of Irrigation
The scope of irrigation is not limited to the application of water to the soil. It
deals with all aspects and problems extending from the watershed to the
agricultural farms.
It deals with the design and construction of all works, such as dams, weirs,
head regulators etc. in connection with the storage or diversion of water, as well
as the problems of subsoil drainage, soil reclamation and water-soil crop
relationships
An irrigation engineer is also required to have the knowledge of cultivation of
various crops, their maturing and protection from pests.
Briefly speaking, the scope of irrigation can be divided into two heads: Engineering aspect. Agricultural aspect. Engineering Aspect Storage, Diversion, or Lifting of. Water
This is the first phase of irrigation engineering. By the construction of a dam across
the river, a suitable reservoir can be created and water can be stored.
JAGADISH S H, ASSISTANT PROFESSOR
Alternatively,if river is perennial and carries sufficient discharge, suitable diversion
works, such as a weir, barrage and bandhara can be constructed across the river and water
can be diverted to the canal.
Places where ground water table is high, suitable wells can be dug and water can be
lifted and fed to small channels or pipes.
Conveyance of Water to the Agricultural Fields The stored or diverted water is conveyed to the agricultural fields through some
suitable distribution system.
If the project is big, the distribution system will consist of two or more main canals
and a number of distributaries, and minors.From the canal, the water is led to the field
through 'water course' or 'field channel'.These field channels take off from the distributaries
or minors through modular or non-modular outlets, so that water supply can be regulated.
Thus, this second phase includes the design and construction of suitable canal
system, along with various regulatory works such as head regulators, cross regulators, falls
etc. for the efficient working of the canal.
In addition to these, suitable cross drainage works, such as aqueducts, super
passages, level crossings, bridges etc. will have to be designed and constructed at the places
where the canal crosses a natural drain or a road.
3. Types of Irrigation
Irrigation has the following main types or classes
JAGADISH S H, ASSISTANT PROFESSOR
Flow Irrigation
Flow irrigation is that type of irrigation in which the supply of irrigation water
available is at such a level that it is conveyed on to the land by the gravity flow. Flow
irrigation may further be divided into two classes:
(i) Perennial irrigation system (ii) Inundation or flood irrigation system.
Perennial Irrigation
In perennial irrigation system, the water required for irrigation is supplied in
accordance with the crop requirements throughout the crop period.
For such a system, therefore, some storage head works, such as dams and storage
weirs or barrages are required to store the excess water during floods and release
it to the crops as and when it is required.
Inundation Irrigation
Inundation irrigation is carried out by deep flooding and thorough saturation of
the land to be cultivated which is then drained off prior to the planting of the crop.
Depending up on the source from which the water is drawn, flow irrigation can be further
subdivided into three types:
Direct irrigation (River canal irrigation): know as Diversion scheme
Storage irrigation (Reservoir or tank irrigation): know as Storage scheme Combined storage and Diversion scheme.
Direct Irrigation or River Canal Irrigation
In this direct irrigation system, water is directly diverted to the canal
without attempting to store the water.
JAGADISH S H, ASSISTANT PROFESSOR
Fig1.1 Direct Irrigation Scheme
For such a system, a low diversion weir or diversion barrage is constructed
across the river. This raises the water level in the river and thus diverts the water
to the canal taking off upstream of the weir, as shown schematically in Fig. 1.1.
Generally, a direct irrigation scheme is of a smaller magnitude, since there are
no rigid controls over the supplies. One or two main canals may take off directly
from the river.
Cross-drainage works are constructed wherever natrual drains or distributary
streams cross the canals. In a bigger scheme, there may be branch canal taking off
from the main canal.
JAGADISH S H, ASSISTANT PROFESSOR
4. Choice between the Systems
Direct irrigation scheme is adopted in the circumstances where the river is
perennial and has a normal flow throughout the irrigation season, never less .at
any time than the requirements of the field.
On the contrary, storage irrigation system is adopted when the river flow is
either not perennial, or where flow is insufficient during certain parts of the crop
season for irrigation requirements.
In a multistage river -valley development, a combined storage-cum-diversion
scheme is more useful.
Lift Irrigation
Lift irrigation is practiced when the water-supply is at too low a level to run
by gravitation on to the land.
In such a circumstances water is lifted up by mechanical means. Irrigation
from wells is an example of lift irrigation, in which sub-soil water is lifted up to
the surface and is then conveyed to the agricultural fields.
5. Quality of Irrigation Water
Irrigation water usually contains some silt as well as certain dissolved salts. These
ingredients in the irrigation water may be useful for the crops if they supplement or
replenish the nutrients taken by the plants from the soil. On the other hand, some of
these ingredients may be injurious to the plant growth and may render the soil infertile.
The effects of silt and salts are separately discussed below:
2. Effect of silt in water The effect of silt on the quality of irrigation water depends
upon the type and amount of silt material as well as on the characteristics of the silt. If
the silt contains a large amount of nutrients required for the plants, it is quite useful,
especially for virgin soils of the agricultural land. Such soils usually have low content of
plant nutrients and a very low water-holding capacity. When the silty water is applied to
such soils, gradual accumulation of silt occurs and the fertility is improved. On the other
hand, the silt carried by the irrigation water may also be injurious to the land. This is
usually the case when the silt is not rich in plant nutrients and the land is already fertile.
Another disadvantage of the silt is that it may reduce the permeability of the soil and
make irrigation more difficult and time consuming.
JAGADISH S H, ASSISTANT PROFESSOR
2. Effect of salts in water The quality of irrigation water is mainly determined by
the concentration of the soluble salts such as chlorides, sulphates and borates of sodium
(Na), potassium (K),calcium (Ca) and magnesium (Mg). Salts of some heavy elements
such as lead, zinc, selenium, arsenic etc. are injurious to plants even when in small
quantities. Fortunately, under normal conditions, the solubility of heavy element salts is
quite low. Hence the concentration of such salts is usually much less than that which
may be injurious to plants. Thus the quality of irrigation is mainly controlled by salts of
sodium, potassi;-'m, calcium and magnesium.
In general, the irrigation water must have low concentration of salts. A high
concentration of dissolved salts affects the water intake of the plant. The plant may result
in dehydration or wilting due to high osmotic pressure. As such, water with a high
concentration of salt should not be used. However, the tolerable concentration of salts in
water will also depend upon the type of soil. The water with salts may however be
tolerable or even useful for irrigation on some types of soils. For example, heavy clayey
soil with a low permeability and plenty of soluble salts will not tolerate the water with a
concentration of salts, but light sandy soil may tolerate such water.
high concentration of a particular salt may destroy the soil structure due to
continuous exchange of ions between the irrigation water and the irrigated soil
For example, if the irrigation water contains a high concentration of sodium ions,
they may replace calcium and magnesium ions from the clayey soil.
Calcium and magnesium are beneficial for the soil because they keep it in a
coalgulated clod form.
This occurs because the silicates and aluminates of calcium and magnesium are
insoluble in water and they possess also cementing properties.
After the exchange of calcium and sodium ions by sodium ions of irrigation
water, soluble sodium silicates are formed which do not have cementing
properties.
The clods of the soil therefore crumble and fine silica grains are released which
clog the pores of the soil, resulting in the reduction of permeability and
destruction of the soil structure.
Exchangeable Sodium Ratio (ESR) The exchangeable sodium ratio (ESR) is equal to the
concentration of exchangeable sodium ions divided by the sum of the concentrations of
JAGADISH S H, ASSISTANT PROFESSOR
exchangeable ions of sodium, calcium, magnesium and potassium. Thus Classification of
Irrigation Water On the basis of suitability for irrigation, water may be classified into 3
classes as follows:
4. Class I water: This type of water is suitable for most crops under general growing
conditions. The water is rated to be excellent to good.
5. Class II water: This type of water is suitable only for sandy permeable soils and
for moderate leaching conditions. The water is rated to be good to injurious, depending
upon the type of soil.
6. Class III water: This type of water is unsuitable for most crops except a few more
tolerant crops. The water is rated to be injurious to unsatisfactory.
UNIT-V
1. Maximum probable flood and Design flood
Any flow which is relatively high and which overtops the natural or artifical
banks in any reach of a river may be called a flood. Floods are produced when the
capacity of the river channel is inadequate to carry off the abnormal quantity of water
arising from heavy rainfall causing the river to overflow its banks and inundate the
surrounding low-lying country.
Maximum probable flood
The maximum probable flood, abbreviated as MPF, has been defined as the flood that
may be expected from the most severe combination of critical meteorologic and
hydrologic conditions that are reasonably possible in the region. This is very large
flood indeed and is very rarely used in design except for reservoir spillways where
failure would lead to great damage and loss of life. The determination of the MPF
involves a detailed study of storm patterns, transposition of storms to a position that
will give maximum runoff and computation of maximum flood by the unit
hydrograph method.
Design flood
A design flood is the flood discharge adopted for the design of a structure after
careful consideration of economic and hydrologic factors. As the magnitude of the
design flood increases, the capital cost of the structure also increases but the
probability of annual damages will decrease.
The most economical design flood can be found after studying the various
alternatives. The design flood in many a case is less than the maximum probable flood
because the MPF is not economically practicable. Several design floods may be
adopted in a single structure, as for example the design flood for a spillway of a
reservoir may be much greater than the design flood adopted for temporary coffer
darns at the initial stages of the structure. Central water commission has given certain
recommendations regarding the design flood to be adopted with due consideration to
the size and importance of the project.
In general the methods used in the estimation of the design flood can be grouped as
under. Increasing the observed maximum flood by a certain percentage
Envelope curves
Empirical flood formulae
Rational Method
Unit hydrograph application
Frequency analysis (or Statistical methods).
For an economic design consistent with safety it is necessay to have the
complete design flood hydrograph rather than merely the value of flood peak.
Methods (i) to (iv) of the above list can give only the flood peak. Method (v) gives the
complete flood hydrograph but it cannot be used for very large areas. In method (vi)
the flood hydrograph may be constructed by making its peak discharge equal to the
estimated design flood, and other features to resemble the observed hydrographs in
the available record. The method to be adopted in any specific case is mostly decided
by the nature of data that is available.
2. Increasing the observed flood, Envelope Curves and Empirical
Formulae Increasing the observed Flood
When the flood data is not available, or when the available data is short, the
maximum flood that occurred during the past, and the number of years during which
this flood was the highest are found out by local enquiries or from the observation of
past records. This is then increased by a certain percentage and adopted as the design
flood. The percentage increase naturally depends on the number of years over which
the observed flood was maximum. If it is over a longer period the percentage increase
may be less and for shorter periods the increase may have to be more. This method is
adhoc in nature and has not much theoretical justification and should be resorted to
under helpless situations
Envelope Curves
When the observed peak floods in different catchments of a hydro
meteorologically homogeneous region are plotted against their respective areas on a
log-log sheet it will usually show that all the points lie below an enveloping curve
which can be drawn by eye. After the envelope curve is drawn the maximum flood for
the catchment under consideration can be read by entering the plot with the known
area of the catchment. Fig. 1.1shows the envelope curves for Indian Catchments,
presented by Kanwarsain and Karpov (1967). Baird and Mcilluraith (1951) gave the
following equation for the enveloping curve of maximum floods throughout the
world.
Q = 3010A
(277 + A) 0.78
where Q is in m3/s and A is in km2.
3. Probability Plotting Methods
In the probability plotting methods, the recurrence intervals are first computed
for various flood peaks.
A plot is then made between the flood discharge as ordinate and the recurrence
interval as abscissa.
For the computation of the recurrence intervals, the data are arranged in the
descending order of the magnitude.
The flood data are assigned the order or rank (m), depending upon the position
of a particular flood in the series.
Thus the highest observed flood is given the rank of 1, and the lowest observed
flood, the rank of n, where n is the number of years of record.
The recurrence interval (Tr) is usually computed by one of the following methods:
California formula
Hazen's formula
Gumbel's Distribution Method
Gumbel's distribution is perhaps the most widely used distribution for the estimation'
of floods of various recurrence intervals. Gumbel considered the annual flood series.
According to Gumbel, the probability of occurrence of an event X equal to or larger
than a value of Xo.
Flood routing
Flood routing may be defined as the procedure whereby the shape of a flood
hydrograph at a particular location on the stream is determined from the known or
assumed flood hydrograph at some other location upstream.
As the discharge in a stream increases stage also increases and with it the volume of
water in temporary storage in the channel. When the flood wave recedes water is
depleted from the channel storage as the stage in the stream falls. As a result, a flood
wave moving down a channel lengthens its base time and consequently its peak gets
reduced. The flood wave is then said to be attenuated. The effect of reservoir storage
or lake storage is also similar. Flood routing is the technique in hydrology to compute
the effect of storage on the shape and movement of the flood wave, Flood routing is
used in establishing the height of a flood peak ata downstream location in short term
flood forecasting, estimating the protection that would result from construction of a
reservoir, determining required levee heights for flood protection, determining the
adequacy of spillways, predicting the behavior of a river after a change in channel
conditions and in the derivation of synthetic unit graphs.
Flood routing may be divided into two basic types namely
The reservoir routing and
The channel routing or the stream flow routing.
The reservoir routing analyses the effect of reservoir storage on the flood
hydrograph, while the channel routing analyses the effect of storage of a specified
channel reach on the flood hydrograph.
Reservoir routing
Letting I and Q to denote the inflow into and outflow from a reservoir, and S the
storage in the reservoir, the continuity equation in the differential form for the
reservoir is given by
I-Q = ds
Alternatively, the same can be written as
O is the average outflow rate in the same time interval and
time interval.
If suffixes 1 and 2 are used to denote a given quantity at the beginning and the end of the time interval and if the inflow and outflow have straight line variation within the time interval, the above equation can be written as
(I1 + I2/ (O1 + O2 2 S1
Proper units must be chosen for storage to maintain compatibility in the above
equation For example, if / and Q are express
must be expressed in cumec-days.
the first term on left hand side represents the volume of water entering the storage,
(I1 + I2/
the second term represents the volume of water leaving the storage and
(O1 + O2
the term on the right hand side represents the change in storage,
S2 S1
which can be either Positive or negative depending on whether 1 is more than or less
than O. if I> O Positive storage
. if I< O Negative storage
In the above equation I1 and I2 are known from the given inflow hydrograph to be
routed through the reservoir, O1 and S1 are the initial outflow from the reservoir and
the initial storage in the reservoir which are either known or assumed and O2 and S2
are the two unknown quantities which must be determined. Thus to solve for O2 and
S2 one mor.e relation is needed.
In reservoirs the water surface is level (or horizontal) and it can be assumed that the storage in the reservoir is independent of inflow and the outflow is dependent onJy on storage. And it is this relation between storage and outflow which provides the
additional relationship required for the solution of S2 and O2.. For this reason, the
reservoir routing is also caIJed level pool routing. All the reservoir routing methods will make use of this storage discharge relation in some convenient form or other.
4. Storage Discharge Relation
Before we discuss the routing methods, we describe how the storage discharge
relation can be obtained from the topographical data of the reservoir site and the
discharge characteristics of the reservoir outlets such as spillways and sluices.
Establishing Storage-Discharge Relationship
The area at the reservoir site is surveyed in detail and a contour map is
prepared with a contour interval of 2 to 3 m depending on the size of the
reservoir.
From this map the areas enclosed by various contours are .planimetered and a
curve of elevation versus area is prepared.
Such a curve is very useful in estimating the evaporation losses from the
reservoir since the area enclosed by any contour will also represent the water
spread area
if the water is stored in the reservoir up to that contour' elevation, though it is
not directly useful in the routing procedure.
Once the areas enclosed by contours are known the incremental volumes of
water stored between any two successive contours can be determined using
one of the following equations.
16). UNIT WISE-QUESTION BANK
UNIT-I
TWO MARKS QUESTION WITH ANSWERS
1. Draw the hydrological cycle.
2. Name the types of rain gauges?
Types of Rain Gauges
There are two types of rain gauges:
1) Non-recording type
2) Recording type
a) Weighing bucket type
b) Tipping bucket type
c) Floating or natural syphon type rain gauge
3. What do mean by term HYDROLOGY?
The sequence of conditions through which water passes from vapor in the atmosphere
through precipitation upon land or water surfaces and ultimately back into the atmosphere as
a result of evaporation and transpiration called also hydrological cycle
4. Define readily available soil moisture
Readily available water (RAW) is the water that a plant can easily extract from the soil.
RAW is the soil moisture held between field capacity and a nominated refill point for
unrestricted growth
5. Define permanent wilting point.
Permanent wilting point (PWP) or wilting point (WP) is defined as the minimal point of soil
moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant
wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12
hours.
THREE MARKS QUESTIONS WITH ANSWERS
1. Define rainfall double mass curve.
Double-Mass Curves. ...
The double-mass curve can be used to adjust inconsistent precipitation data. The graph of the
cumulative data of one variable versus the cumulative data of a related variable is a straight
line so long as the relation between the variables is a fixed ratio.
2. How can you measure the infiltration?
The infiltration rate is the velocity or speed at which water enters into the soil. It is
usually measured by the depth (in mm) of the water layer that can enter the soil in one hour.
An infiltration rate of 15 mm/hour means that a water layer of 15 mm on the soil surface will
take one hour to infiltrate.
3. What is transpiration?
Transpiration is the process where plants absorb water through the roots and then give off
water vapour through pores in their leaves. An example of transpiration is when a plant
absorbs water in its roots.
4. How will you measure flow in stream and rainfall?
Stream flow, or channel runoff, is the flow of water in streams, rivers, and other channels,
and is a major element of the water cycle. ... The discharge of water flowing in a channel
is measured using stream gauges or can be estimated by the Manning equation. The record
of flow over time is called a hydrograph.
5. What are infiltration indices?
Infiltration Index is a method used for Measurement of Infiltration. It represents average rate
of infiltration. There are two types of infiltration index: - - Index It represents
average infiltration rate during the period of rainfall excess.
FIVE MARKS QUESTIONS WITH ANSWERS
1. Explain the balanced equation for precipitation.
The precipitation reaction is defined as the Chemical reaction occurring in aqueous
solutions where two ionic bonds combine forming up insoluble salts. The salts formed are
precipitates and are the products of precipitation reaction.
The precipitation reaction is the double displacement reactions involving the production of a
solid form residue called the precipitate. The reaction also occurs when two or more solutions
with different salts are combined, resulting in the formation of insoluble salts that precipitates
out of the solution.
One of the best examples of precipitation reactions is the chemical reaction between
potassium chloride and silver nitrate, and solid silver chloride is the precipitate or the
insoluble salt formed as a product of the reaction. The chemical equation for this
The turning of any liquid into vapour is called Evaporation. The factors affecting the
evaporation are:
Temperature. As the temperature increases, the rate of evaporation also increases.
Surface Area. As the Surface area increases, the rate of evaporation increases.
Density. As the density increases, the rate of evaporation decreases.
Wind Velocity. The velocity of the wind also affects the rate of evaporation. As the
velocity of wind increases, the rate of evaporation increases.
3. Explain the following in brief.
(a) Probable maximum precipitation
(b) Rain gauge density
(a) Probable maximum precipitation:-
Probable Maximum Precipitation (PMP) is defined by the Manual for Estimation of Probable
Maximum Precipitation (WMO,2009) as: "...the theoretical maximum precipitation for a
given duration under modern meteorological conditions."
(b) Rain gauge density:-
Rainfall is an extremely variable parameter in both space and time. Rain gauge density is
very crucial in order to quantify the rainfall amount over a region. The level
of rainfall accuracy is highly dependent on density and distribution of rain gauge stations
over a region
4. Describe the terms
i) Interception and
ii) Depression storage.
i) Interception:-
Interception refers to precipitation that does not reach the soil, but is instead intercepted by
the leaves, branches of plants and the forest floor. It occurs in the canopy (i.e. canopy
interception), and in the forest floor or litter layer (i.e. forest floor interception . Because
of evaporation, interception of liquid water generally leads to loss of that precipitation for
the drainage basin, except for cases such as fog interception, but increase flood protection
dramatically, Alila et al., (2009).
5. Definition of canopy and forest floor interception
Intercepted snowfall does not result in any notable amount of evaporation, and most of the
snow falls off the tree by wind or melts. However, intercepted snow can more easily drift
with the wind, out of the watershed. Conifers have a greater interception capacity
than hardwoods. Their needles gives them more surface area for droplets to adhere to, and
they have foliage in spring and fall, therefore interception also depends on the type of
vegetation in a wooded area.
Mitscherlich in 1971 calculated the water storage potential as interception values for different
species and stand densities. A storm event might produce 50 - 100 mm of rainfall and 4 mm
might be the maximum intercepted in this way. Grah and Wilson in 1944 did sprinkling
experiments where they watered plants to see how much of the intercepted is kept after
watering stops.
ii) Depression storage:-
Depression storage capacity, in soil science, is the ability of a particular area of land to
retain water in its pits and depressions, thus preventing it from flowing. Depression
storage capacity, along with infiltration capacity, is one of the main factors involved
in Horton overland flow, whereby water volume surpasses both infiltration and depression
storage capacity and begins to flow horizontally across land, possibly leading
to flooding and soil erosion. The study of land's depression storage capacity is important in
the fields of geology, ecology, and especially hydrology.
5. Write short notes on:
(i) Double-mass curve
(ii) Cold and warm fronts
(iii) Cyclones and anticyclones.
(i) Double-mass curve:-
Double mass analysis is a commonly used data analysis approach for investigating the
behaviour of records made of hydrological or meteorological data at a number of locations. It
is used to determine whether there is a need for corrections to the data - to account for
changes in data collection procedures or other local conditions. Such changes may result from
a variety of things including changes in instrumentation, changes in observation procedures,
or changes in gauge location or surrounding conditions. Double mass analysis for checking
consistency of a hydrological or meteorological record is considered to be an essential tool
before taking it for analysis purpose. This method is based on the hypothesis that each item of
the recorded data of a population is consistent.
An example of a double mass analysis is a "double mass plot", or "double mass curve".[2] For
this, points and/or a joining line are plotted where the x- and y- coordinates are determined by
the running totals of the values observed at two stations. If both stations are affected to the
same extent by the same trends then a double mass curve should follow a straight line. A
break in the slope of the curve would indicate that conditions have changed at one location
but not at another. This technique is based on the principle that when each recorded data
comes from the same parent population, they are consistent.
(ii) Cold and warm fronts:-
A cold weather front is defined as the changeover region where a cold air mass is replacing a
warmer air mass. Cold weather fronts usually move from northwest to southeast. The air
behind a cold front is colder and drier than the air in front. When a cold front passes through,
temperatures can drop more than 15 degrees within an hour.
On a weather forecast map, a cold front is represented by a solid line with blue triangles along
the front pointing towards the warmer air and in the direction of movement.
There is usually an obvious temperature change from one side of a cold front to the other. It
has been known that temperatures east of a cold front could be approximately 55 degrees
Fahrenheit while a short distance behind the cold front, the temperature can go down to 38
degrees. An abrupt temperature change over a short distance is a good indicator that a front is
located somewhere in between.
A warm weather front is defined as the changeover region where a warm air mass is replacing
a cold air mass. Warm fronts usually move from southwest to northeast and the air behind a
warm front is warmer and moister than the air ahead of it. When a warm front passes, the air
becomes noticeably warmer and more humid than it was before.
On a weather forecast map, a warm front is represented by a solid line with red semicircles
pointing towards the colder air and in the direction of movement.
Again, there is typically a noticeable temperature change from one side of the warm front to
the other, much the same as a cold front.
If colder air is replacing warmer air, it is a cold front, if warmer air is replacing cold air, then
it is a warm front.
(iii) Cyclones and anticyclones:-
A cyclone is a storm or system of winds that rotates around a center of low atmospheric
pressure. An anticyclone is a system of winds that rotates around a center of high atmospheric
pressure. Distinctive weather patterns tend to be associated with both cyclones and
anticyclones. Cyclones (commonly known as lows) generally are indicators of rain, clouds,
and other forms of bad weather. Anticyclones (commonly known as highs) are predictors of
fair weather.
Winds in a cyclone blow counterclockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere. Winds in an anticyclone blow just the opposite. Vertical air
movements are associated with both cyclones and anticyclones. In cyclones, air close to the
ground is forced inward toward the center of the cyclone, where pressure is lowest. It then
begins to rise upward, expanding and cooling in the process. This cooling increases the
humidity of the rising air, which results in cloudiness and high humidity in the cyclone.
In anticyclones, the situation is reversed. Air at the center of an anticyclone is forced away
from the high pressure that occurs there. That air is replaced in the center by a downward
draft of air from higher altitudes. As this air moves downward, it is compressed and warmed.
OBJECTIVE QUESTIONS WITH ANSWERS
1) which of the followings methods of applying water may be used rolling land?
a) boarder flooding
b) check flooding
c) furrow flooding
d) free flooding
2) The value of sodium absorption ratio for high sodium water lies between
a) 0 to 10
b) 10 to 18
c) 18 to 26
d) 26 to 34
3) Irrigation water having concentration of Na++, Ca++, and Mg++ as 20, 3 and 1 milli
equivalent per litere respectively
a) sodium water
b) medium sodium water
c) high sodim water
d) very high sodium water
4) The duty is largest
a) At the head if water course
b) On the field
c) At the head of main canal
d) Same at all places
5) The outlet discharge factor is the duty at the head of
a) Main canal
b) Branch canal
c) Water house
d) distributory
6) The water utilized by plant is available in soil mainly in the form of
a) Gravity water
b) Capillary water
c) Hydroscopic water
d) Chemical water
7) Hydrograph is the graphical representation of
a) Runoff & time
b) Surface runoff & time
c) Ground water flow & time
d) Rainfall & time
8) Infiltration rate is always
a) more than the infiltration capacity
b) less than the infiltration capacity
c) Equal to or less than the infiltration capacity
d) Equal to or more than the infiltration capacity
9) Infiltration is the
a) Movement of water through
b) Absorption of water by soil surface
c) Both (a) and (b)
d) None of the above
10) Which of the following is non-recording rain gauge?
a) Tipping bucket type rain gauge
b) rain gauge
c) weighing type rain gauge
d) Floating type rain gauge
KEY ANSWERS
Question no Answers 1 Boarder flooding 2 18 to 26 3 Medium sodium water 4 On the field 5 distributary 6 Capillary water 7 Runoff & time 8 Equal to or less than the
infiltration capacity 9 Movement of water through 10
FILL IN THE BLANKS WITH ANSWERS
1
2 the head of.
3.
4.
5 is the movement of water through the soil.
KEY ANSWERS
Question no Answers 1 Boarder Flooding
2 Watercourse
3 Capillary water
4 Runoff and time
5 Infiltration
UNIT - II
TWO MARKS QUESTIONS WITH ANSWERS
1) Explain Hydrograph analysis?
A hydrograph is a continuous plot of instantaneous discharge v/s time. It results from a
combination of physiographic and meteorological conditions in a watershed and represents
the integrated effects of climate, hydrologic losses, surface runoff, interflow, and ground
water flow.
2. What do you mean by Base flow?
Base flow (also called drought flow, groundwater recession flow, low flow, low-water flow,
low-water discharge and sustained or fair-weather runoff) is the portion of stream flow that
comes from "the sum of deep subsurface flow and delayed shallow subsurface flow"
3. Define unit hydrograph
It can be defined as the direct runoff hydrograph(DRH) resulting from one unit (e.g., one cm or
one inch) of effective rainfall occurring uniformly over that watershed at a uniform rate over
a unit period of time
4. Define design flood?
Definition of Design Flood: Design flood is the value of the instantaneous peak discharge
adopted for the design of a particular project or any of its structures.
5. Define annual series?
The equivalent annual cost (EAC) is the annual cost of owning, operating and maintaining an
asset over its entire life. EAC is often used by firms for
THREE MARKS QUESTIONS WITH ANSWERS
1) Define partial series?
Let us define things a little better now: A Sequence is a set of things (usually numbers) that
are in order. A Partial Sum is the sum of part of the sequence. The sum of infinite terms is an
Infinite Series. And Partial Sums are sometimes called "Finite Series.
2) Write the formulae used to calculate unit hydrograph.
The instantaneous unit hydrograph is defined as a unit hydrograph produced by an effective
rainfall of 1 mm and having an infinitesimal reference duration (in other words the duration
tends towards zero).
3) What is recession time?
A recession is a decline of economic activity, more specifically, a decline in gross domestic
product (GDP) for two or more consecutive quarters. GDP is the market value of all goods
and services produced within a country in a given period of time.
4) What is flood frequency?
Flood frequency analysis is a technique used by hydrologists to predict flow values
corresponding to specific return periods or probabilities along a river. The application of
statisticalfrequency curves to floods was first introduced by Gumbel
5) Write dickens formula for flood discharge
DICKENS FORMULA (1865) Qp = CD A 3/4
whereQp = maximum flood discharge (m3 /s) A = catchment area (km2)
CD = Dickens constant with value between 6 to 30 .The following are some guidelines in
selecting the value of CD:
RYVESFORMULA (J884)
Qp = CR 2/3
where Q = maximum flood discharge (m3 /s) A = catchment area (km2) and CR = Ryves
coefficient This formula originally developed for the Tamil Nadu region, is in use in Tamil
Nadu and parts of Karnataka and Andhra Pradesh. The values of CRrecommended by Ryves
for use are:
CR = 6.8 for areas within 80 km from the east coast
=8.5 for areas which are 80 -160 km from the east coast.
FIVE MARKS QUESTIONS WITH ANSWERS
1) Explain the terms:
(i). Recurrence interval
(ii). Probable maximum precipitation
(i). Recurrence interval:-
A return period, also known as a recurrence interval (sometimes repeat interval) is an
estimate of the likelihood of an event, such as an earthquake, flood, landslide, or a river
discharge flow to occur.
It is a statistical measurement typically based on historic data denoting the average recurrence
interval over an extended period of time, and is usually used for risk analysis (e.g. to decide
whether a project should be allowed to go forward in a zone of a certain risk, or to design
structures to withstand an event with a certain return period). The following analysis assumes
that the probability of the event occurring does not vary over time and is independent of past
events.
1. What are the limitations of unit hydrograph?
Limitations of the Unit Hydrograph
intensity during the duration of rainfall excess. In practice, these two conditions are never
satisfied.
-uniform areal distribution and variation of in intensity, the unit
hydrograph theory can still be used if the areal distribution is consistent between different
storms.
theory (because the centre of the storm can vary from storm to storm and each of these storms
can give a different DRH under otherwise identical conditions in very large basins).
2
basins, the flood hydrographs can be studied by dividing it into a
number of smaller sub-basins, developing DRHs for these sub-basins by the UH method, and
then routing these DRHs through their respective channels to obtain the composite DRH at
the catchment outlet.
2.
Barlow, the first Chief Engineer of the Hydro-Electric Survey of India (1915) on the basis of
his study in small catchments (area ~ 130 km2) in Uttar Pradesh expressed runoff R a shown
in (19.1)
Where Kb is the runoff coefficient which depends upon the type of catchment and nature of
monsoon and P is the rainfall.
Table 19.1. b in percentage
(Developed for use in UP) (Source: Subramanya, 2008)
Class Description of catchment
Kb (percentage)
Season I Season II Season III
A Flat, cultivated, and absorbent soil 7 10 15
B Flat, partly cultivated, and stiff soil 12 15 18
C Average catchment 16 20 32
D Hills and plains with little cultivation 28 35 60
E Very hilly, steep and no cultivation 36 45 81
Season I: Light rain, no heavy downpour
Season II: Average or varying rainfall, no continuous downpour
Season III: Continuous downpours
Strange (1892) studied the available rainfall and runoff in the border areas of present-day
Maharashtra and Karnataka and has obtained yield ratios as functions of indicators
representing catchment characteristics. Catchments lie classified as good, average and bad
according to the relative magnitudes of yield they give. Two methods using tables for
estimating the runoff volume in a season are given.
(I) Runoff Volume from Total Monsoon Season Rainfall
A table giving the runoff volumes for the monsoon period (i.e. yield during monsoon season)
for different total monsoon rainfall values and for the three classes of catchments (viz. good,
average and bad) is given in Table 19.2a. The correlation equations of best fitt.ing lines
relating percentage yield ratio (Y)to precipitation (P)could be expressed as
Table 19.2a.Strange's Table of Total Monsoon Rainfall and estimated Runoff(Source:
Subramanya, 2008)
Since there is no appreciable runoff due to the rains in the dry (non-monsoon) period, the
monsoon season runoff volume is recommended to be taken as annual yield of the catchment.
This table could be used to estimate the monthly yields also in the monsoon season.
However, it is to be used with the understanding that the table indicates relationship between
cumulative monthly rainfall starting at the beginning of the season and cumulative runoff, i.e.
a double mass curve relationship.
3. Explain in detail about synthetic unit hydrograph?
Snyder's synthetic unit hydrograph
A synthetic unit hydrograph retains all the features of the unit hydrograph, but does not
require rainfall-runoff data. A synthetic unit hydrograph is derived from theory and
experience, and its purpose is to simulate basin diffusion by estimating the basin lag based on
a certain formula or procedure.
The first synthetic unit hydrograph was developed by Snyder in 1938.1 In order to provide
sufficient flexibility for simulating a wide range of diffusion amounts, Snyder devised two
parameters: (1) a time parameter Ct, and (2) a peak parameter Cp. A larger Ct meant a greater
basin lag and, consequently, greater diffusion. A larger Cp meant a greater peak flow and,
consequently, less diffusion.
4. What are the applications of unit hydrograph?
The Unit Hydrograph (UH) technique is widely used for runoff estimation, especially for
determining peak discharges. In this paper, a geomorphological based UH has been applied.
Its most remarkable characteristic is that it includes the watershed structure in its formulation.
This is defined from the drainage network, each sub watershed being considered as a linear
reservoir. In this method, the fact of considering this reservoir sequence according to the
drainage network leads to the formulation of the model only depending on a single parameter.
The Geomorphological Unit Hydrograph of Reservoirs (GUHR), proposed in this paper, is
compared with Nash's Instantaneous Unit Hydrograph (Nash's IUH), by applying the two
methods to the Aixola watershed. This 4.7-km2 watershed is located in Gipuzkoa (Northern
Spain). It is characteristic of the headwaters watersheds of the area. Most of them are forested
and have steep slopes. Annual rainfall is over 1500 mm and many intense rainfall events are
observed, among which 18 were selected for this study. Both UH techniques were applied to
the rainstorms. The resulting hydrographs were plotted against registered data and the best
fits were determined. According to these results, the GUHR model behaved similarly to
Nash's IUH. However, the GUHR method seemed able to reproduce a wider range of
rainstorms than Nash's IUH. While analyzing the UHs calculated, seasonal behavior was
observed in runoff generation, and different average UHs were proposed for two different
periods. This variability was also observed in values adopted by the GUHR model parameter,
providing some information about the watershed time response. The dynamic character of the
only uncertain parameter, and the model formulation, in which the watershed morphology is
reflected, together with the model's simplicity, leads us to consider GUHR as being a
promising UH model.
OBJECTIVE QUESTIONS WITH ANSWERS
1. Water contains
a) One hydrogen atom and one oxygen atom
b) Two hydrogen atoms and one oxygen atom
c) One hydrogen atom and two oxygen atoms
d) Three hydrogen atoms and two oxygen atoms
2. Unit Hydrograph theory was enunciated by
a) Merril Bernard,
b) W.W. Horner,
c) Le-Roy K. Shermen,
d) Robert E. Horte
3. Hydrology helps in
a) Predicting maximum flows,
b) Deciding the minimum reservoir capacity,
c) Forecasting the availability of quantity of water at reservoir site,
d) All the above.
4. The surface Run-off is the quantity of water,
a) Absorbed by soil,
b) Intercepted by buildings and vegetative cover,
c) Required to fill surface depressions,
d) That reaches the stream channels.
5. Pick up the correct equation from the following :
a) Run off = Surface run off + Ground water flow,
b) Run off = Surface run off - Ground water flow,
c) Run off = Surface run off / Ground water flow,
d) Run off = Surface run off x Ground water flow.
KEY ANSWERS Question no Answers
1 Two hydrogen atoms and one oxygen atom
2 Le-Roy K. Shermen,
3 All the above.
4 That reaches the stream channels.
5 Run off = Surface run off + Ground water flow
FILL IN THE BLANKS WITH ANSWERS
1.
2.
3.
4.
5. is used for measuring rain in remote hilly inaccessible.
KEY ANSWERS
Question no Answers
1 In an open space
2 Rain deficiency of 30%
3 Cubic meter/sec
4 Increase in intensity of rain
5 Tipping bucket type
UNIT-III
TWO MARKS QUESTIONS WITH ANSWERS
1. Define Aquifer?
A body of permeable rock which can contain or transmit groundwater
.
2. What are different types of aquifer?
Aquifers come in two types which are shown below: unconfined and confined. Unconfined
aquifers are those into which water seeps from the ground surface directly above the aquifer
3. Define Specific yield?
Specific yield is defined as the volume of water released from storage by an unconfined
aquifer per unit surface area of aquifer per unit decline of the water table.
4. Define specific retention?
Definition of specific retention.The ratio of the volume of water that a given body of rock or
soil will hold against the pull of gravity to the volume of the body itself. It is usually expressed
as a percentage.
5. Define permeability?
The ability of a substance to allow another substance to pass through it, especially the ability
of a porous rock, sediment, or soil to transmit fluid through pores and crack.
THREE MARKS QUESTIONS WITH ANSWERS
1. Define porosity?
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the
volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and
100%
2. Define transmissibility?
Transmissibility is the ratio of output to input. Transmissibility (T) = output/input. T>1
means amplification and maximum amplification occurs when forcing frequency (ff) and
natural frequency (fn) of the system coincide
3. Define storage coefficient?
Storability or the storage coefficient is the volume of water released from storage per unit
decline in hydraulic head in the aquifer, per unit area of the aquifer. Storability is a
dimensionless quantity, and ranges between 0 and the effective porosity of the aquifer.
4. What are the types of wells?
There are three types of private drinking water wells.
Dug/Bored wells are holes in the ground dug by shovel or backhoe. ...
Driven wells are constructed by driving pipe into the ground. ...
Drilled wells are constructed by percussion or rotary-drilling machines.
5. Define well development?
Well development of drilled wells is a part of normal well drilling procedure after the
completion of the well and before the final disinfection. ... It refers to the cleaning of oil wells
to improve performance and requires utterly differing methods than the rehabilitation of
water wells.
FIVE MARKS QUESTIONS WITH ANSWERS
1. Write short notes on:
(a) Specific capacity of a well
(b) Specific yield of an aquifer
(c) Aquifer and aquiclude .
(d) Specific capacity of a well
200 gpm.The maximum pumping rate of a well can be estimated using the initial Specific
Capacity. The maximum pumping rate is calculated as the Specific Capacity times the
maximum available drawdown. For example, with a well that has 40 feet of drawdown and is
pumping 200 gpm has a Specific Capacity of 4.9.
(e) Specific yield of an aquifer-
Specific yield is defined as the volume of water released from storage by an
unconfined aquifer per unit surface area of aquifer per unit decline of the water table. ...
Thus,specific yield, which is sometimes called effective porosity, is less than the total
porosity of an unconfined aquifer (Bear 1979).
(f) Aquifer and aquiclude
Porous water-bearing formation capable of yielding exploitable quantities of water. (WMO,
2006)A formation, group of formations, or part of a formation that contains sufficient
saturated permeable material to yield significant quantities of water to wells and springs
(LOHMANN, 1972)
Aquitard
A confining bed that retards but does not prevent the flow of water to or from an adjacent
aquifer; a leaky confining bed.It does not readily yield water to wells or springs, but may
serve as a storage unit for ground water.
Aquiclude
A hydrogeologic unit which, although porous and capable of storing water, does not transmit
it at rates sufficient to furnish an appreciable supply for a well or spring (WMO, 1974)
2) Distinguish between Groundwater and Perched groundwater.
Water table:
Water table is a free water surface in an unconfined aquifer.
A well driven into an unconfined aquifer will indicate a static water level. This water level is
corresponding to the water table level at that location. It is constantly in motion adjusting its
surface to achieve a balance between the recharge and outflow.
Fluctuations in water table level
Fluctuations in water level occur due to:
Lowering of ground water due to heavy pumping of the wells.
Rise in the water of an irrigated area with poor drainage.
During various seasons of the year.
3. Distinguish between Open wells and tube wells.
Water wells can be classified either open wells or tube wells. Open wells have relatively large
diameter (generally 1 to 5 m) and low yields (generally less than 30 m3/hr), and are not very
deep (generally 3 to 15 m). Since these wells are constructed by digging, they are also known
as dug wells. The walls of an open well may be of either brick (or stone) masonry or precast
concrete rings.
Open wells can be either shallow open wells or deep open wells. A shallow open well drains
its supplies only from the top upper aquifer. But deep open wells are those which rest on an
impervious stratum and draw their supplies from the previous water-bearing stratum lying
below the impervious stratum through a bore hole made in the impervious stratum. As shown
in Fig. 4.4, a deep open well can have its depth smaller than a shallow open well in the
vicinity. A tube well is in the form of a long pipe with holes (or slots) at suitable locations
and which is sunk into the ground intercepting one or more aquifers. The diameter of a tube
well is usually in the range of about 8 to 60 cm. The depth of a shallow tube well is usually
limited to about 30 m. A deep tube well can have depths up to about 600 m.
4. Distinguish between Water table and artesian aquifers.
The Water Table
The water table is the upper most section of the saturation zone in the ground. The saturation
zone is the area of ground in which water has penetrated and fills all the gaps in the ground,
completely saturating it. As time progresses, the saturation zone may raise or lower
depending on levels of precipitation. As the saturation zone changes, so does the water table
level. For example, if the weather is dry, the water table may become deeper as less water is
available. An aquifer is the water beneath the water table.
Aquifer
An aquifer is a body of saturated rock through which water can easily move, according to the
Idaho Museum of Natural History. Water moves through the pores of the rock. The pores act
as a natural filtration system, removing even viruses and bacteria from the water. Aquifers
can be considered unconfined or confined. An unconfined aquifer's bottom is a layer of
nonporous rock, which restricts water flow, creating a barrier to the aquifer. The water table
is the top layer of the unconfined aquifer. A confined aquifer sits below a unconfined aquifer
and layer of nonporous roc
5) Distinguish between Confined aquifer and water table aquifer
A confined aquifer is an aquifer below the land surface that is saturated with water. Layers of
impermeable material are both above and below the aquifer, causing it to be under pressure
so that when the aquifer is penetrated by a well, the water will rise above the top of the
aquifer.
A water-table, or unconfined, aquifer is an aquifer whose upper water surface (water table) is
at atmospheric pressure, and thus is able to rise and fall. Water-table aquifers are usually
closer to the Earth's surface than confined aquifers are, and as such are impacted by drought
conditions sooner than confined aquifers.
OBJECTIVE QUESTIONS WITH ANSWERS
1. Infiltration capacity of soil depends upon,
a. Number of voids present in the soil,
b. Shape and size of soil particles,
c. Arrangement of soil particles,
d. All the above.
2. The main factor which affects the infiltration capacity is
a. Thickness of saturated layer,
b. Depth of surface detention,
c. Soil moisture,
d. All the above.
3. According to Robert E. Horton, the equation of infiltration capacity curve is
(where letters carry their usual meanings),
a. F = fc (fo - fc) ekt,
b. F = ft - (fo - fc) e-kt,
c. F = ft + (fo - fc) e-kt,
d. F = f + (fo - fc) ekt.
4. Absolute humidity in air,
a. Decreases at higher altitudes,
b. Increases at higher altitudes,
c. Remains constant at all altitudes,
d. None of these.
5. Precipitation caused by lifting of an air mass due to the pressure difference, is
called,
a. Cyclonic precipitation,
b. Convective precipitation,
c. Orographic precipitation,
d. None of these.
e.
KEY ANSWERS Question no Answers
1 All the above.
2 All the above
3 F = ft + (fo - fc) e-kt,
4 Decreases at higher altitudes,
5 Cyclonic precipitation,
FILL IN THE BLANKS WITH ANSWERS
1.
2. The
3. A
4.
5. -recoding rain gauge.
KEY ANSWERS Question no Answers
1 Velocity of flow of water
2 One fourth of basin lag
3 Deep well
4 Infiltration
5
UNIT-IV TWO MARKS QUESTIONS WITH ANSWERS
1. Define irrigation.
Irrigation is the application of controlled amounts of water to plants at needed intervals.