Brief description of Hydrology, various spheres of Hydrology, applications and utilities of Engineering Hydrology and understanding possibilities of flood.

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Fluid Mechanics
SY Mech (Sem – IV)
Jan - June 2021
IA-II (Report Writing)
Topic: HYDROLOGY
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Name of Student Batch Roll No
1.Yashika Tank B4 1915126
2.Siddharth Upadhyay B4 1915127
3.Manjiri Vichare B4 1915128
Date Of Presentation: 05/04/2021
Grade: AA/ AB/ BB/ BC/ CC Sign:

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Content
SN Topic Page No
1 Definition of Hydrology 3
2 Engineering Hydrology 3
3 Features of Hydrology 3
4 Scopes of Engineering Hydrology 3
5 Applications of Engineering Hydrology 4
6 Divisions of Hydrology 4
7 Hydrological Cycle 5
8 Hydrological Processes 5
9 Precipitation and forms of Precipitation 6
10 Mechanism Producing Precipitation 6
11 Types of Precipitation 7
12 Precipitation Measurement 8
13 Snowfall Measurement 10
14 Calculating Stream flow 10
15 Hydrograph 12

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HYDROLOGY
❖ Definition of hydrology:
The study of water in all its forms (rain, snow and water on the earth’s surface), and from its
origins to all its destinations on the earth is called hydrology.
❖ Engineering Hydrology
It uses hydrologic principles in the solution of engineering problems arising from human
exploitation of water resources of the earth. The engineering hydrologist, or water resources
engineer, is involved in the planning, analysis, design, construction and operation of projects for
the control, utilization and management of water resources.
Hydrologic calculations are estimates because mostly the empirical and approximate methods are
used to describe various hydrological processes.
❖ Features of Hydrology
The engineering hydrology deals with the following features:
1. Estimation of water resources
2. Study the components of the hydrological cycle like precipitation, runoff, transpiration,
and their interactions.
3. Study the problems of floods and droughts and preventive actions.
❖ Scope of Engineering Hydrology
The main scope of engineering hydrology is:
➢ Determination of Maximum Probable Flood
➢ Determination of Water yield from a basin
➢ Study the groundwater development
➢ Determination of maximum intensity of the storm
1. Determination of Maximum Probable Flood
The study of hydrology can help in determining the maximum probable flood that can occur at a
particular location. Its frequency is also determined that is essential for the design of hydraulic
structures like dams and reservoirs, channels and other flood control structures.
2. Determination of Water yield from a basin

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For the design of dams and municipal water supply units, river navigation etc. it is necessary to
determine the occurrence, the frequency and the quantity of water that can be yielded from a
basin. This is performed in hydrology.
3. Study the ground water development
The knowledge on hydro-geology of the area helps to understand the groundwater development
that influences the recharge facilities like reservoirs and streams, climate, cropping pattern etc.
4. Determination of maximum intensity of storm
The maximum intensity of storms influences the design of drainage projects, which are studied
in hydrology in depth.
❖ Applications of Engineering Hydrology
The main applications of engineering hydrology are:
• Calculates rainfall, surface runoff, and precipitation.
• It determines the water balance for a particular region.
• It mitigates and predicts flood, landslide and drought risk in the region.
• Enables real-time flood forecasting and flood warning.
• Hydrology analyses the variations observed in the catchments by bringing a
relationship between the surface water and groundwater resources of the
catchment.
• Hydrology studies the required reservoir capacity that is necessary for irrigation
and municipal water supply purpose during drought conditions.
• It is used in the design and operation of hydraulic structures
• It is used for hydropower generation.
• Brings measures to control erosion and sediments.
❖ Divisions of Hydrology
Hydrology can generally be divided into two main branches
1. Engineering Hydrology
Engineering hydrology deals with the planning, design and Operation of Engineering projects for
the control and use of water
2. Applied Hydrology

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Applied hydrology is the study of hydrological cycle, precipitation runoff, relationship between
precipitation and runoff, hydrographs, Flood Routing
❖ Hydrological cycle
• The hydrologic cycle describes the continuous re-circulating transport of the waters of the
earth, linking atmosphere, land and oceans.
• Water evaporates from the ocean surface, driven by energy from the Sun, and joins the
atmosphere, moving inland as clouds. Once inland, atmospheric conditions act to
condense and precipitate water onto the land surface, where, driven by gravitational
forces, it returns to the ocean through river and streams.
• Engineering Hydrology takes a quantitative view of the hydrologic cycle.
• The quantification of the hydrologic cycle which is an open system can be represented by
a mass balance equation, where inputs minus outputs are equal to the change in storage.
• It is a basic Hydrologic Principle or equation that may be applied either on global or
regional scale.
I - O = ΔS
The water holding elements of the hydrological cycle are: Atmosphere, Vegetation, Snow packs,
Land surface, Soil, Streams, lakes and rivers, Aquifers, Oceans.
❖ Hydrological Processes
1. Precipitation 5. Infiltration
2. Overland flow 6. Evaporation
3. Transpiration 7. Surface Runoff
4. Groundwater outflow

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❖ Precipitation and forms of precipitation
Precipitation is any type of water that forms in the Earth's atmosphere and then drops onto the
surface of the Earth.
➢ Rain
Rain is precipitation that falls to the surface of the Earth as water droplets. Raindrops form
around microscopic cloud condensation nuclei, such as a particle of dust or a molecule of
pollution.
Rain that falls from clouds but freezes before it reaches the ground is called sleet or ice pellets.
➢ Hail
Hail forms in cold storm clouds. It forms when very cold-water droplets freeze, or turn solid, as
soon as they touch things like dust or dirt. The storm blows the hailstones into the upper part of
the cloud. More frozen water droplets are added to the hailstone before it falls.
Unlike sleet, which is liquid when it forms and freezes as it falls to Earth, hail falls as a stone of
solid ice.
➢ Snow
Snow is precipitation that falls in the form of ice crystals. While hail is just a collection of frozen
water droplets, snow has a complex structure; the ice crystals are formed individually in clouds,
but when they fall, they stick together in clusters of snowflakes.
Snowfall happens when many individual snowflakes fall from the clouds. Unlike a hail storm,
snowfall is usually calm. Hailstones are hard, while snowflakes are soft.
➢ Other Types of Precipitation
Sometimes, different types of precipitation fall at the same time. During harsh winter storms, for
instance, it is not unusual for sleet and rain to fall at the same time.
Other times, precipitation doesn't fall at all. Virga is a type of precipitation that begins to fall
from a cloud, but evaporates before it reaches the surface of the Earth.
❖ Mechanism Producing Precipitation
Three mechanisms are needed for formation of precipitation.
1. Lifting and Cooling - Lifting of air mass to higher altitudes causes cooling of air.
2. Condensation - conversion of water vapor into liquid droplets.
3. Droplet Formation - Growth of droplets is required if the liquid water present in a cloud is
to reach ground against the lifting mechanism of air.

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Types of Precipitation
Depending upon the way in which the air is lifted and cooled so as to cause precipitation, we
have three types of precipitation, as given below:
➢ Cyclonic Precipitation:
Cyclonic precipitation is caused by lifting of an air mass due to the pressure difference. Cyclonic
precipitation may be either frontal or non-frontal cyclonic precipitation.
➢ Frontal precipitation:
It results from the lifting of warm and moist air on one side of a frontal surface over colder,
denser air on the other side. A front may be warm front or cold front depending upon whether
there is active or passive accent of warm air mass over cold air mass.
➢ Non-frontal precipitation:
If low pressure occurs in an area (called cyclone), air will flow horizontally from the surrounding
area (high pressure), causing the air in the low-pressure area to lift. When the lifted warm-air
cools down at higher attitude, non-frontal cyclonic precipitation will occur.
➢ Convective Precipitation
Convective precipitation is caused by natural rising of warmer, lighter air in colder, denser
surroundings. Generally, this kind of precipitation occurs in tropics, where on a hot day, the
ground surface gets heated unequally, causing the warmer air to lift up as the colder air comes to
take its place. Convective precipitation occurs in the form of showers of high intensity and short
duration.
➢ Orographic Precipitation
Orographic precipitation is caused by air masses which strike some natural barriers like
mountains, & rise up due to constraint, causing condensation and precipitation. This is the

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phenomenon behind precipitation in Himalayas.. It is rich in moisture because of their long travel
over oceans.
❖ Precipitation measurement
All the forms of precipitation are measure on the basis of vertical depth of water that would
accumulate on a level surface of precipitation remained where it fell measured in millimeter.
Any open receptacle with vertical side can be used as a gauge for measuring rainfall.
Rain gauges for measurement of precipitation are of two types
➢ Recording rain gauges
➢ Non-Recording rain gauges
The main difference between these rain gauges is that with the help of recording rain gauges we
get the rain recorded automatically with respect to time, so intensity of rain fall is also known.
1. Non-recording Rain gauges:
Non-recording rain gauges are commonly used. They do not record the data and collect only rain
and this collected rain is then measured in a graduated cylinder.
The standard gauge of U.S. Weather Bureau has a collector of 200 mm diameter. Rain passes
from a collector into a cylindrical measuring tube inside the overflow can. The measuring tube
has a cross sectional area 1/10th of the collector, so that 2.5 mm rain fall will fill the tube to 25
mm depth. A measuring stick is marked in such a way that 1/10th of a cm depth can be
measured. The collector and tube are removed when snow is expected. The snow collected in the
outer container or overflow can is melted, poured into the measuring tube and then measured.
This type of rain gauge is one of the most commonly used rain gauges.
Depth of rain = volume of rain collected in cm3
/area of aperture of gauges in cm3

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It consists of:
➢ Collector (receiving 8" 20.3 cm diameter)
➢ Overflow can
➢ Cylindrical measuring tube of area of 1/10th of collector
➢ Measuring scale
2. Recording gauges:
Are those which automatically record rainfall without any bottle reading. The worker is not
required to record the reading but instead mechanical arrangements are there by which total
rainfall is recorded automatically on graph paper. A graph of total rainfall VS time which is
known as mass curve of rainfall is plotted by the gauges.
Weighing bucket type rain gauge is most common self-recording rain gauge. It consists of a
receiver bucket supported by a spring or lever balance or some other weighing mechanism. The
movement of bucket due to its increasing weight is transmitted to a pen which traces record or
some marking on a clock driven chart. Weighing bucket type rain gauge instrument gives a plot
of the accumulated (increased) rainfall values against the elapsed time and the curve so formed is
called the mass curve.
Its three types commonly used are:
➢ Tipping bucket gauges
➢ Weighing type gauges

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➢ Float recording gauges
Snowfall Measurement:
Snowfall is often measured with regular rain gauges. Snowfall is measured by the depth of snow
using snow survey. Such survey is particularly useful in mountains.
❖ Calculating Streamflow
Streamflow is a measurement of the amount of water flowing through a stream or river over a
fixed period of time. Streamflow cannot be measured directly, it must be calculated in a
process known as stream gaging.
The USGS splits stream gaging into a three-step process: measuring stream stage, measuring
discharge and determining the stage-discharge relation.
1. Measuring Stream Stage
The first step in calculating streamflow involves measuring stage, which is the height of the
water surface at a particular point in a stream or river. Stage is sometimes known as gage
height, and can be measured several ways. Among the most common of these approaches
uses a stilling well installed in the river bank or attached to a stationary structure such as a
pier or bridge support. A float or a sensor — whether pressure, optical or acoustic — then
measures the stage inside the well. An electronic recording device or data logger records
stage measurements at regular intervals.
Stage must always be measured relative to a constant reference elevation, or datum.
Depending on the duration of your project, it may be necessary to routinely survey the
elevation of your stream gage structure and its datum, to ensure that elevations have not
shifted due to settling or natural erosion.
2. Measuring Discharge

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Discharge is the volume of water moving down a waterway per unit of time. It is most
commonly expressed in cubic feet per seconds or gallons per day. To calculate discharge,
multiply the area of water in a channel cross section by the average velocity of water in that
cross section.
In short: discharge = area X velocity
The simplest way to measure discharge is to divide the channel cross section into vertical
rectangular subsections. Once the area (width X depth) of each of these subsections is
established and multiplied by velocity to determine subsection discharge, the results can be
added together to calculate total discharge.
Subsection width is best measured with a cable or steel measuring tape, while depth can be
measured by a wading rod in shallower channels and suspended sounding weights in deeper
waters. Velocity, on the other hand, should be measured with a current meter. Many current
meters rely on a wheel formed of several cups revolving around an axis. Each revolution
generates an electronic signal that is counted and timed by the meter, which translates to
water velocity.
A faster, but more expensive method to measure velocity involves the use of an Acoustic
Doppler Current Profiler (ADCP) which can be mounted in a small watercraft. The ADCP
sends a pulse of sound into the water and measures changes in the pulse’s frequency as it
returns to the instrument. The ADCP speeds discharge calculations by measuring velocity
and depth at the same time.
3. Determining the Stage-Discharge Relation
Stage-discharge relation, or “rating,” is a dynamic variable that is determined by comparing
stage at a stream gage to discharge at the same point. Accurate stage-discharge relations can
only be developed by measuring discharge across many ranges of stage. Furthermore,
channels should be continually surveyed for changes caused by erosion, sediment deposition,
vegetation growth and ice formation.
When discharge has been established across enough stages, stage-discharge relation can be
visualized in the form of a graph. When this relation is properly maintained through periodic
updates, it can provide useful streamflow information for a given stream or river.

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❖ Hydrograph
A storm hydrograph is a way of displaying how the discharge of a river can change over time in
response to a rainfall event. The discharge of a river is just the amount of water passing a certain
point every second, and is calculated by multiplying the cross sectional area of the river by its
velocity. Because the cross section is measured in metres2 and the velocity is measured in
metres per second the discharge is measured in metres 3 per second.
The graph shows base flow which are the contributions made to the river via soil and ground
water flows. The runoff or storm flow is the water that arrives in the river via surface runoff or
rapid throughflow through the rock. The rising limb gives an indication of how fast water is
reaching the channel and represents the level of water rising in the channel. The steeper the
rising limb the more likely a flood is to occur, this is vital knowledge for flood forecasters. The
falling limb shows the river as its level falls. Peak discharge is the maximum amount of water in
a river after a rainfall event, if this level surpasses the bankfull discharge then a flood will occur
where the river overtops its banks. The last item indicated on the hydrograph is the lag time, this
is the amount of time between the peak amount of rainfall and the peak discharge in the river.
Generally, the less the lag time the quicker the river rises, the more FLASHY the graph and the
more likely a flood.