Department of Civil Engineering

Faculty of Science & Engineering

Dhaka, Bangladesh
Full Module Specification
Module Title/Course Name Hydrology
Module Code: CE 353
Module Level: 3
Academic Year: 2023
Module Preparation by: Umme Tahmina Toma, Mohammad Ferdaush and
Jannatul Ferdaus Dola
Module Preparation Date: 25/10/2022
Module Modification by:
Module Modification Date:
Contact: 528, 328
Counselling hour: Before or after class or by appointment
Module Credit 3.0
Pre-Requisites: Fluid Mechanics, Open Channel Flow
Co Requisites: -
Duration 6 Months
Grading: As outlined in the University policy
Teaching Methodology Class room lecturer, multimedia presentation,
discussion, group study, assignment, presentation
etc.
Method of Evaluation Attendance =10
Continuous Assessment =10
Term-1 =25
Term-2 =25
Final =30
TOTAL =100

1
 Introduction
Hydrology is a diverse field that plays a vital role in society and, in particular, the
work of Civil Engineers in developing water resources infrastructure. At its most
basic definition, hydrology is the scientific study of movement, distribution and
water quality on Earth and other planets, including the water cycle, water resources,
and environmental watershed sustainability. It is an important field where the people
who study hydrology (which are called hydrologists) use their knowledge and
expertise to combat water pollution, protect the earth’s water resources, and provide
engineering hydrology – which is an engineering specialty focusing on water
resources

Message from the Teacher

This course has been updated by appropriate, interesting and important contents.
Though lecture would be given to enlighten about the related topics and its
application in modern engineering, this module would be a useful summary for quick
solutions. Lectures will be given to highlight the important topics or giving
information in an effective way. This course will work as an introduction for
irrigation and other modelling related engineering. So, we will be looking forward
for you to attend the course and other materials.

Best of luck…

2
 Course Contents:
Module Topics/Module/Chapter Course Teacher
Term-1
Module 1 Introduction to Hydrology ● Umme Tahmina Toma
Module 2 Precipitation ● Mohammad Ferdaush
● Jannatul Ferdaus Dola
Term-2
Module 3 Abstractions from Precipitations Readings Text:
Module 4 Hydrograph 1. Engineering Hydrology
By K. Subramanya
Final
Module 5 Stream flow Measurement
Module 6 Runoff
Module 7 Floods

3
 Chapter-1
Introduction to Hydrology
Introduction

Hydrology means the science of water. It is the science that deals with the
occurrence, circulation and distribution of water of the earth and earth's atmosphere.
As a branch of earth science, it is concerned with the water in streams and lakes,
rainfall and snowfall. snow and ice on the land and water occurring below the earth's
surface in the pores of soil and rocks.

Objectives of Hydrology

In general sense engineering hydrology deals with

❖ estimation or water resources.
❖ the study of processes such as precipitation, runoff. evapotranspiration and their
interaction and
❖ the study of problems such as floods and droughts and strategies to combat them.

Hydrologic Cycle:

The various aspects of water related to the earth can be explained in terms of a cycle
known as the hydrologic cycle.

The hydrologic cycle begins with the evaporation of water from the surface of the
ocean. As moist air is lifted, it cools and water vapor condenses to form clouds.
Moisture is transported around the globe until it returns to the surface as
precipitation.

Once the water reaches the ground, one of two processes may occur; 1) some of the
water may evaporate back into the atmosphere or 2) the water may penetrate the
surface and become groundwater. Groundwater either seeps its way to into the
oceans, rivers, and streams, or is released back into the atmosphere through
transpiration. The balance of water that remains on the earth's surface is runoff,
which empties into lakes, rivers and streams and is carried back to the oceans, where
the cycle begins again.

4
Figure 1.1 is a schematic representation of the hydrologic cycle. The sequence of
events as above is a simplistic picture of a very complex cycle that has been taking
place since the formation of the earlier. It is seen that the hydrologic cycle is a very
vast and complicated cycle in which there are a large number of paths of varying
time scales. Further, it is a continuous recirculating cycle in the sense that there is
neither a beginning nor an end or a pause.

Catchment Area:

The area of land draining into a stream or a water course at a given location is known
as catchment area. It is also called as drainage area or drainage basin. In USA, it
is known as watershed. Catchment area is separated from its neighboring areas by
ridge called a divide in USA and watershed in UK

5
 Figure 1.2 Catchment Area
Water Budget Equation

6
7
Assignment-1:

8
 Chapter-2
Precipitation
Precipitation:

Precipitation is any product of the condensation of atmospheric water vapor that

falls under gravity. The main forms of precipitation include
drizzle, rain, sleet, snow and hail. Rainfall being the predominant form of
precipitation causing stream flow. Precipitation occurs when a portion of the
atmosphere becomes saturated with water vapor, so that the water condenses and
"precipitates". Thus, fog and mist are not precipitation but suspensions, because the
water vapor does not condense sufficiently to precipitate. Two processes, possibly
acting together, can lead to air becoming saturated: cooling the air or adding water
vapor to the air. Precipitation forms as smaller droplets via collision with other rain
drops or ice crystals within a cloud.

To form precipitation
There are four conditions that must be present for the production of precipitation
(i)Atmosphere must have moisture.
(ii)Must be sufficient nuclei present to aid condensation.
(iii)Weather conditions must be good for condensation of water vapor to take place.
(iv) Product of condensation must reach the earth.

Under proper weather conditions. The water vapor condenses over nuclei to form
tiny water droplets of sizes less than 0.1 mm in diameter. The nuclei arc usually salt
particles or product of combustion and are normally available in plenty.

Forms of Precipitation

9
Figure 2.1 Forms of Precipitation

10
 Drizzle Glaze

Hail Sleet

Figure 2.2 Different forms of Precipitation

Weather Systems for Precipitation

For the formation or clouds and subsequent precipitalion it is necessary that moist
air mass cool to form condensation. Some of the terms and processes connected with
the weather systems associated with precipitation are given bclow.
There are 3 types of precipitation that occur on earth are Cyclonic
Precipitation, Convective Precipitation, and Orographic Precipitation.

Cyclonic Rainfall
Cyclonic rainfall is occurs when air mass rise up due to pressure difference. When
there is the formation of a low-pressure area, air from the other surrounding spaces
flows to less pressure zone.
It is phenomenon forces warm and colder air to meet. As warm air is lighter in
comparison to colder ones it rises above the colder air. Then the warmer air starts
cooling beyond saturation point which results in heavy rain. Such rainfall is
called Cyclonic Rainfall.

11
 Figure 2.3 Warm and Cool front

Figure 2.4 Cyclonic Storm

The cyclonic rainfall itself is categorized into two types,

• Frontal Rainfall
• Non-Frontal Rainfall

Convective Rainfall
Convective Precipitation or Convectional Rainfall generally occurs in equatorial
areas. The surface areas in these zones get heated frequently and constantly because
of the sun’s heat. In turn the air near the surface gets hated and spreads. Heating also

12
makes the air lighter hence it tends to rise up. The air starts cooling as it moves up
and reaches its saturation limit, resulting in precipitation.

Figure 2.5 Convective rainfall

Orographic Precipitation
Orographic Precipitation occurs when the moist mass of air, strikes natural barriers
of the topography area. These barriers are like mountains, hills, etc which causes the
air to rise up, condense and then precipitate. Hence mountains are the sites of higher
precipitation than plain lands.

Figure 2.6 Orographic storm

13
Measurement of Precipitation:
Precipitation is measured as the amount of water that reaches horizontal ground or
the horizontal ground projection plane of the earth’s surface, and is expressed as a
vertical depth of water.
Rain gauges are classified into recording and non-recording types. The latter include
cylindrical and ordinary rain gauges, and measurement of precipitation with these
types is performed manually by the observer. Some recording types such as siphon
rain gauges have a built-in recorder, and the observer must physically visit the
observation site to obtain data. Other types such as tipping bucket rain gauges have
a recorder attached to them, and remote readings can be taken by setting a recorder
at a site distant from the gauge itself to enable automatic observation.
Important considerations for setting a raingauge
In a flat surface
❖ It must be placed near the ground
❖ Placed in an open space (area of 5.5 m x 5.5 m).
❖ No obstruction within 30 m.
Symons rain gauge:
The non-recording type gauge mostly used in the world is Symon’s gauge. Non-
recording gauges don’t record the rain but only collect the rain. Once the rain is
collected, it is measured using a graduated cylinder.
1. It consists of the circular receiving area which is 127mm in diameter and
connected to a funnel.
2. The rim of the collector is set at a height of 30.5 cm above ground level.
3. The funnel discharges the rainfall into receiving funnel.
4. The funnels and receiving vessels are housed in a metallic container.
5. The receiving bottle normally holds a maximum of 10 cm of rain and in case of
heavy rainfall, the measurement must be done frequently.
6. The last reading must be taken at 8:30 AM and the sum of the previous readings
in the last 24 hours entered as the total rainfall of the day.
7. In Symon’s rain gauge, the concrete blocks which act as a foundation are 600
mm x 600 mm x 600 mm.

14
 Figure 2.7: Symon’s Raingauge
Advantage of Symon rain gauge
1. It helps to measure all forms of precipitation or rainfall which comprised snow
and rain.
2. Removing its funnel collector permits the accumulation of solid rainfall and also
plays a significant role in estimating cold climates.
3. Rainwater collected is measured on daily basis.
Disadvantage of Symon rain gauge
1. This type of gauge only collects rain.
2. This gauge does not record and measure the rainfall simultaneously.
3. Symons rain gauge only shows how much rain has fallen.
4. One of the main disadvantages of Symons rain gauge is that we cannot determine
when the rain began, the intensity with which the rain was falling, when the rain
stopped and variation in the intensity of rain throughout the rainy period.
Recording Gauges Advantage
Recording raingauges give a permanent automatic record of rainfall. It has a
mechanical arrangement by which the total amount of rainfall since the start of
record gets automatically recorded on a graph paper. It produces a plot of cumulative
rainfall vs time
❖ We can identify storm event.
❖ We find intensity of rainfall.
❖ We find storm duration.

15
1. Tipping Bucket Type
This is ideally suited for use as a telemetering rain gauge. The catch from the funnel
falls onto one of a pair of small buckets. These buckets are so balanced that when
0.25mm of rain falls into one bucket, it tips bringing the other bucket in position.
The water from the tipped bucket is collected in a can.
Tipping actuates an electrically driven pen to trace a record on the graph paper
mounted on a clockwork driven drum.
2. Natural Syphon Type
The rainfall collected in the funnel shaped collector is led into a float chamber,
causing the float to rise. As the float rises, a pen attached to the float through a lever
system records the rainfall on a rotating drum driven by a clockwork mechanism.

Raingauge Network
World Meteorological Organization (WMO) recommends the following densities.
1. Flat regions: ideal –1 station for 600 –900 Km2 acceptable –1 station for 900 -
3000 Km 2
2. Mountains regions: ideal –1 station for 100 –250 Km 2 acceptable - 1 station for
250 –1000 Km 2
3. Arid and Polar Zones: 1- station for 1500 –10,000 Km2

16
17
18
Mean Precipitation over an area
To convert an average value over a catchment, following three methods are used.
• Arithmetical- mean method
• Thiessen- mean method
• Isohyetal method
1. Arithmetic Mean:
When the area of the basin is less than 500 km2 this method implies summing up of
all the rainfall values from all the raingauging stations and then dividing it by the
number of stations in that basin.

Thiessen- mean method

• Any point in the watershed receives the same amount of rainfall as that at the
nearest gage
• Rainfall recorded at a gage can be applied to any point at a distance halfway to the
next station in any direction.
Steps in Thiessen Method
1.Draw lines joining adjacent gages.
2.Draw perpendicular bisectors to the lines created in step 1.
3. Extend the lines created in step 2 in both directions to form representative areas
for gages.
4. Compute representative area for each gage.
5. Compute the areal average using the following formula.

19
 Figure 2.9 Thiessen Polygon
Station Bounded Area Area Weightage
1 abcd A1 A1/A
2 kade A2 A2/A
3 edcgf A3 A3/A
4 fgh A4 A4/A
5 hgcbj A5 A5/A
6 jbak A6 A6/A

Example 2.3 For the catchment area shown in figure 2.9, details of each Thiessen
polygons surrounding each rain gauge and the recording of the rain gauge on the
month of August 2011 are given below,

Rain gauge
1 2 3 4 5 6
station
Thiessen Polygon
720 380 440 1040 800 220
Area (km2)
Rainfall (mm) 121 134 145 126 99 115

Determine the average depth of rainfall on the watershed by i) Arithmetical-Mean

method and ii) Thiessen-Mean method.

Solution:
Station 1, 2 and 4 within the basin. So arithmetic mean is obtained by these stations
only.

20
 121+134+126
Monthly mean = = 127mm
3
Calculations for Thiessen method are given below

Rain gauge Thiessen Weightage Rainfall (mm) Weighted

station Polygon Factor Station Rainfall
Area (km2) (mm)
1 720 720/3600= 0.2 121 24.2
2 380 0.106 134 14.1
3 440 0.122 145 17.7
4 1040 0.289 126 36.4
5 800 0.222 99 22
6 220 0.061 115 7
Total= 3600km2 Total= 121.5mm

Thiessen Mean= 121.5mm

Isohyetal method

The isohyet method is superior to other methods in terms of large rainfall stations
Steps
❖ Construct isohyets (rainfall contours)
❖ Compute area between each pair of adjacent isohyets (Ai)
❖ Compute average precipitation for each pair of adjacent isohyets (pi)

Figure 2.10 Isohyetal Method

21
Example 2.4 For the watershed given in figure 2.10, compute the precipitation mean
by isohyet method
Isohyets (cm) Area (km2)
Station 12 30
12-10 140
10-8 80
8-6 180
6-4 20

Solution:
For the first area consisting of the station surrounded by a closed isohyet,
precipitation is taken as 12 cm. For other stations mean of two bounding isohyets
are taken.

Isohyets (cm) Average Value Area (km2) Weightage Weighted

of P (cm) Factor Station Rainfall
(cm)
Station 12 12 30 30/450=0.067 0.8
12-10 11 140 0.322 3.422
10-8 9 80 0.177 1.6
8-6 7 180 0.4 2.8
6-4 5 20 0.0444 0.222
Total= 450km2 Total= 8.84cm

Isohyet Mean= 8.84 cm

22
 Chapter 3
Abstractions from Precipitation
Evaporation drives the water cycle
Evaporation from the oceans is the primary mechanism supporting the surface-to-
atmosphere portion of the water cycle.
After all, the large surface area of the oceans (over 70 percent of the Earth's surface
is covered by the oceans) provides the opportunity for large-scale evaporation to
occur.
On a global scale, the amount of water evaporating is about the same as the amount
of water delivered to the Earth as precipitation.
This does vary geographically, though. Evaporation is more prevalent over the
oceans than precipitation, while over the land, precipitation routinely exceeds
evaporation.
Most of the water that evaporates from the oceans falls back into the oceans as
precipitation.
Only about 10 percent of the water evaporated from the oceans is transported over
land and falls as precipitation.
Once evaporated, a water molecule spends about 10 days in the air.
The process of evaporation is so great that without precipitation runoff, and
groundwater discharge from aquifers, oceans would become nearly empty.

Potential evapotranspiration: Maximum quantity of water capable of being

evaporated in a given climate from a continuous expanse of vegetation covering the
whole ground and well supplied with water.
It includes evaporation from the soil and transpiration from the vegetation from a
specific region in a specific time interval, expressed as depth of water.

Evaporation:
Evaporation is the process by which water changes from a liquid to a gas or vapor.
Water boils at 212 ⁰F (100 ⁰C), but it actually begins to evaporate at 32 ⁰F (0 ⁰C); it
just occurs extremely slowly. As the temperature increases, the rate of evaporation
also increases.

Transpiration:
Water vapor is also emitted from plant leaves by a process called transpiration.
Every day an actively growing plant transpires 5 to 10 times as much water as it
can hold at once.

23
Factors influencing the rate of evaporation:
1.Concentration of the substance evaporating in the air
2.Concentration of other substances in the air
3.Flow rate of air
4.Inter-molecular forces
5.Pressure
6.Surface area
7.Temperature of the substance

Atmospheric factors affecting transpiration: The amount of water that plants

transpire varies greatly geographically and over time. There are a number of factors
that determine transpiration rates:
Temperature:
➢ Transpiration rates go up as the temperature goes up, especially during the
growing season, when the air is warmer due to stronger sunlight and warmer air
masses.
➢ Higher temperatures cause the plant cells which control the openings (stoma)
where water is released to the atmosphere to open, whereas colder temperatures
cause the openings to close.
Relative humidity:
➢ As the relative humidity of the air surrounding the plant rises the transpiration
rate falls. It is easier for water to evaporate into dryer air than into more saturated
air.
Wind and air movement:
➢ Increased movement of the air around a plant will result in a higher transpiration
rate. Wind will move the air around, with the result that the more saturated air
close to the leaf is replaced by drier air.
Soil-moisture availability:
➢ When moisture is lacking, plants can begin to senesce (premature ageing, which
can result in leaf loss) and transpire less water.
Type of plant:
➢ Plants transpire water at different rates. Some plants which grow in arid regions,
such as cacti and succulents, conserve precious water by transpiring less water
than other plants.

MEASUREMENT OF EVAPORATION
Units and scales: The rate of evaporation is defined as the amount of water
evaporated from a unit surface area per unit of time.
It can be expressed as the mass or volume of liquid water evaporated per area in unit
of time, usually as the equivalent depth of liquid water evaporated per unit of time
24
from the whole area
The unit of time is normally a day. The amount of evaporation should be read in
millimeters (WMO, 2003).
Depending on the type of instrument, the usual measuring accuracy is 0.1 to 0.01
mm.

This can be done by the following methods

➢ Using evaporimeters
➢ Using empirical equations
➢ By analytical methods

Evaporimeters – These are pans containing water which are exposed to the
atmosphere.
Loss of water by evaporation from these pans are measured at regular intervals
(daily).
Meteorological data such as humidity, wind velocity, air and water temperatures,
and precipitation are also measured and noted along with evaporation.

a. USWB Class A Evaporation Pan

A pan of diameter 1210mm and depth 255mm. Depth of water is maintained between
18 and 20cm.The pan is made of unpainted GI sheet. The pan is placed on a wooden
platform of height 15cm above ground level to allow free air circulation below the
pan. Evaporation is measured by measuring the depth of water in a stilling well with
a hook gauge

25
b. ISI Standard Pan
Specified by IS: 5973 and known as the modified Class A Pan. A pan of diameter
1220mm and depth 255mm. The pan is made of copper sheet 0.9mm thick, tinned
inside and painted white outside. The pan is placed on a square wooden platform of
width 1225mm and height 100mm above ground level to allow free air circulation
below the pan. A fixed-point gauge indicates the level of water. Water is added to or
removed from the pan to maintain the water level at a fixed mark using a calibrated
cylindrical measure. The top of the pan is covered with a hexagonal wire net of GI
to protect water in the pan from birds. Presence of the wire mesh makes the
temperature of water more uniform during the day and night. Evaporation from this
pan is about 14% lower as compared to that from an unscreened pan

c. Colorado Sunken Pan

920mm square pan made of unpainted GI sheet, 460mm deep, and buried into the
ground within 100mm of the top.
Main advantage of this pan – its aerodynamic and radiation characteristics are
similar to that of a lake
Disadvantages – difficult to detect leaks, expensive to install, extra care is needed to
keep the surrounding area free from tall grass, dust etc.

26
d. USGS Floating Pan
A square pan of 900mm sides and 450mm deep. Supported by drum floats in the
middle of a raft of size 4.25m x 4.87m, it is set afloat in a lake with a view to simulate
the characteristics of a large body of water. Water level in the pan is maintained at
the same level as that in the lake, leaving a rim of 75mm. Diagonal baffles are
provided in the pan to reduce surging in the pan due to wave action Disadvantages
– High cost of installation and maintenance, difficulty in making measurements.
Evaporation pan are not exact models of large reservoirs and have the following
drawbacks:
1. They differ in the heat storing capacity and heat transfer from the sides and
bottom. The sunken pan and floating pan aim to reduce this deficiency. As a result
of this factor the evaporation from a pan depends to a certain extent on its size.
2. The height of the rim in an evaporation pan affects the wind action over the
surface.
3. The heat transfer characteristics of the pan material is different from that of the
reservoir.

Lake Evaporation = Pan Coefficient (Cp) X Pan Evaporation

Evaporation Stations
WMO recommends the following values of minimum density of evaporimeters
➢ Arid Zones – 1 station for every 30,000 sq.km
➢ Humid Temperate Zones – 1 station for every 50,000 sq.km
➢ Cold regions – 1 station for every 1,00,000 sq.km

A typical hydro-meteorological station has the following:

1. Recording rain gauge and non-recording rain gauge
2. Stevenson box with maximum, minimum, wet, and dry bulb
thermometers
3. Wind anemometer and wind vane
27
 4. Pan evaporimeter
5. Sunshine Recorder etc.

EMPIRICAL EQUATIONS: Most of the available empirical equations for

estimating lake evaporation

1. Dalton type equation

2. Meyer’s Formula

3. Rohwer’s Formula: Accounts for the effect of pressure in addition to the wind

28
 speed effect.

In the lower part of the atmosphere, up to a height of about 500m above the ground
level, wind velocity follows the one-seventh power law as

ANALYTICAL METHODS OF EVAPORATION ESTIMATION

a) Water Budget Method
b) Energy Budget Method
c) Mass Transfer Method
a) Water Budget Method

29
b) Energy Budget Method
It involves application of the law of conservation of energy. Energy available for
evaporation is determined by considering the incoming energy, outgoing energy, and
the energy stored in the water body over a known time interval. Estimation of
evaporation from a lake by this method has been found to give satisfactory results,
with errors of the order of 5%, when applied to periods less than a week.

30
Ha is estimated using Bowen’s ratio

31
 INFILTRATION
INFILTRATION is the movement of water through the soil surface and into the
soil as distinguished from percolation, which is the movement of water
within/through the soil. It determines the part of the precipitation that would become
the surface runoff. It is also an important property of soil which affects surface
irrigation. It not only controls the amount of water entering the soil but also the
overland flow.
In the first few minutes of a rainstorm, water will accumulate on vegetation, and then
it will reach the ground and start infiltrating into the soil.
Infiltration is a complex process which depends on:
❖ Soil properties
❖ Initial soil moisture content
❖ Previous wetting history
❖ Permeability and its changes due to surface water movement
❖ Cultivation practices

32
❖ Type of crop being sown
❖ Climatic effects.

Factors affecting infiltration:

❖ Precipitation: The greatest factor controlling infiltration is the amount and
characteristics (intensity, duration, etc.) of precipitation that falls as rain or snow.
Precipitation that infiltrates into the ground often seeps into streambeds over an
extended period of time, thus a stream will often continue to flow when it hasn't
rained for a long time and where there is no direct runoff from recent
precipitation.
❖ Base flow: To varying degrees, the water in streams have a sustained flow,even
during periods of lack of rain. Much of this "base flow" in streams comes from
groundwater seeping into the bed and banks of the stream.
❖ Soil characteristics: Some soils, such as clays, absorb less water at a slower rate
than sandy soils. Soils absorbing less water result in more runoff overland into
streams.
❖ Soil saturation: Like a wet sponge, soil already saturated from previous rainfall
can't absorb much more ... thus more rainfall will become surface runoff.
❖ Land cover: Some land covers have a great impact on infiltration and rainfall
runoff. Vegetation can slow the movement of runoff, allowing moretime for it to
seep into the ground. Impervious surfaces, such as parking lots, roads, and
developments, act as a "fast lane" for rainfall - right into storm drains that drain
directly into streams. Agriculture and the tillage of land also changes the
infiltration patterns of a landscape. Water that, in natural conditions, infiltrated
directly into soil now runs off into streams.
❖ Slope of the land: Water falling on steeply-sloped land runs off more quickly
and infiltrates less than water falling on flat land.
❖ Evapotranspiration: Some infiltration stays near the land surface, which is
where plants put down their roots. Plants need this shallow groundwater to grow,
and, by the process of evapotranspiration, water is moved back into the
atmosphere.

Rate of Infiltration: In an initially dry soil, the infiltration rate is high at the
beginning of rain (or irrigation), but rapidly decreases with time until a fairly steady
state infiltration is reached. This constant rate of infiltration is also termedthe basic
infiltration rate and is approximately equal to the permeability of the saturated soil.

33
Infiltration Rate and Infiltration Capacity: The infiltration rate f expressed in cm/hr
is the rate at which water enters the soil at the surface. If water is ponded at the
surface (an ample supply), then infiltration occurs at the potential infiltration rate,
which is also called as infiltration capacity fc that is the maximum rate at which a
given soil at a given time can absorb water. If the rate of supply is less than the
potential rate, then the actual infiltration rate will be less than the potential rate and
equal to the rate of supply of water i.e. rainfall intensity.

ESTIMATION OF PARAMETERS OF INFILTRATION MODEL

1. Horton Equation:

It implies that infiltration begins at some rate f o and exponentially decreases until it
reaches a constant rate fc (saturated soil hydraulic conductivity). β is a decay constant
(1/T). The actual rate of infiltration is also governed by the rate of supply.

2. Phillip’s Equation:

Where S = sorptivity which is a function of the soil suction potential, K = hydraulic

conductivity

34
35
 Chapter 4
Hydrograph
Introduction:
A hydrograph is a graph showing the rate of flow (discharge) versus time past a
specific point in a river, or other channel or conduit carrying flow. The rate of flow
is typically expressed in cubic meters or cubic feet per second (cms or cfs).

Figure: Hydrograph

It can also refer to a graph showing the volume of water reaching a particular outfall,
or location in a sewerage network. Graphs are commonly used in the design of
sewerage, more specifically, the design of surface water sewerage systems and
combined sewers

36
Terminology:
The discharge is measured at a specific point in a river and is typically time variant.

● Rising limb: The rising limb of hydro graph, also known as concentration curve,
reflects a prolonged increase in discharge from a catchment area, typically in
response to a rainfall event
● Recession (or falling) limb: The recession limb extends from the peak flow rate
onward. The end of storm flow (aka quick flow or direct runoff) and the return
to groundwater-derived flow (base flow) is often taken as the point of inflection
of the recession limb. The recession limb represents the withdrawal of water
from the storage built up in the basin during the earlier phases of the hydrograph.
● Peak discharge: the highest point on the hydro graph when the rate of discharge
is greatest
● Lag time: the time interval from the center of mass of rainfall excess to the peak
of the resulting hydrograph
● Time to peak: time interval from the start of the resulting hydro graph
● Discharge: the rate of flow (volume per unit time) passing a specific location in
a river or other channel

Types of hydrograph can include:

● Storm hydrographs
● Flood hydrographs
● Annual hydrographs aka regimes
● Direct Runoff Hydrograph
● Effective Runoff Hydrograph
● Raster Hydrograph
● Storage opportunities in the drainage network (e.g., lakes, reservoirs, wetlands,
channel and bank storage capacity)

Unit hydrograph:
The unit hydrograph is defined as the hydrograph of storm runoff resulting from an
isolated rainfall of some unit duration occurring uniformly over the entire area of the
catchment, produces a unit volume (i.e. 1cm) of runoff.

37
38
39
40
41
42
43
Base flow separation method
Method I –Straight-Line Method
In this method the separation of the base flow is achieved by joining with a straight
line the beginning of the surface runoff to a point on the recession limb representing
the end of the direct runoff.
N = 0.83 A0.2
Where A= drainage area in Km2 and N is in days
Method II
In this method the base flow curve existing prior to the commencement of the surface
runoff is extended till it intersects the ordinate drawn at the peak (point C in Fig.).
This point is joined to point B by a straight line.
Method III
In this method the base flow recession curve after the depletion of the flood water is
extended backwards till it intersects the ordinate at the point of inflection (line EF in
Fig.). Points A and F are joined by an arbitrary smooth curve.

44
Direct Runoff Hydrograph (DRH)
The surface runoff hydrograph obtained after the base-flow separation is also known
as direct runoff hydrograph (DRH).

45
46
47
48
49
50
51
Unit hydrograph of Different Durations
Two methods are available for this purpose
(i) Method of superposition, and
(ii) the S –curve
The S –Curve is desired to develop a unit hydrograph of duration mD, where m is a
fraction, the method of superposition cannot be used.

52
 Chapter 5
Stream flow
Measurement
Stream flow - one of the most important topics in engineering hydrology, because it
directly related to water supply, flood control, reservoir design, navigation,
irrigation, drainage, water quality, and others. The volume of water moving down a
stream or river per unit of time, commonly expressed in cusec or cumec.

Stream gauge provides continuous flow over time at one location for water resources
and environmental management or other purposes.
Stream stage (also called stage or gauge height) - the height of water

surface above an established altitude where the stage is zero. Types of

gauges measuring river stage:

o Staff gauge – vertical or inclined

o Suspended – weight gauge
o Recording gauge -
automatic stagerecorder
o Crest-stage gauge

53
 Discharge is the volume of water moving down a stream or river per unit of time,
commonly expressed in cusec or cumec.

54
 Discharge Measurement

1. Meters: Weirs, Venturi flumes or Standing wave flumes, Parshall flume,

etc. are used for small channels (a few meters wide or smaller).
Discharge is calculated as
Q =𝐶𝐿𝐻3/2,Where Q = stream discharge,
C = coefficient of weir L = length Flow of weir,
H = head over the weir crest.

2. Slope-Area method: Discharge, Q = AV,

Where mean velocity is estimated using some expressions, like, mean velocity,
V = C√ (RS) (Chezy formula) and according to
1
Manning’s, V= 𝑅2/3 𝑠1/2
𝑛

1
Chezy’s, C= 𝑅1/6
𝑛
Hydraulic radius, R = A/P
S = slope of water surface (measuring difference in water levels)

55
56
57
 3) Area-Velocity method: Discharge, Q = AV
Here the mean velocity of flow (V) may be determined by making
velocitymeasurements.

Velocity Measurements Pitot Tubes -

Suitable only for clean water, V = sqrt(2gh)
Floats - Suitable for straight channel, V =
L/T
Velocity rods – consist of a wooden rod (3-5 cm size), weighted by
means oflead or cast iron rings
Current Meters
Cups, Propellers (The rate at which the cups revolve is directly
related to thevelocity of the water) V = a + bN
Where V = flow velocity;
a = starting velocity to overcome
mechanical friction;b= equipment
calibration constant;
N = revolutions/sec.

Acoustic Doppler Current Profilers ADCP uses Doppler Effect determine water
velocity Sending sound pulse water measuring change frequency that sound pulse
reflected back ADCP sediment other particulates being transported water

58
Discharge Computation: Discharge = Area X Mean velocity
Channel cross section is divided into several subsections to measure depth and
velocity; Discharge is determined by summing discharges in these subsections.
Mean velocity in a vertical can be approximated by making velocity observations
and using a known relation between those velocities and the mean in the vertical.

59
60
61
62
 Stage-Discharge Relation

• Typical relationship: Q = m(H -h0)n , here h0 is supposed to be the stage

corresponding to zero discharge.
• The functional relationship between H & Q have to be calibrated locally for
different stations.
Determining the values of K, n, h0

• The values of K, n, h0 can be obtained from regression analysis taking a trial

value for h0 . Their best values are those for which the sum of the squared
deviations is minimum.
• The values of K, n, h0 may also be obtained from graphical procedure. Then a
trial value of h0 is assumed and the plot is made between Q and (h- h0) on log-
log plot.
A vertical through B and a horizontal through C are drawn to meet at D, and a
vertical through A and a horizontal through B are drawn to meet at E.

63
Then lines BA and DE are extended to meet at F. The ordinate of the point F
gives the desired value of h0. The values of K and n can then be determined from
rating equation.

64
65
 Chapter 6
Runoff
Runoff means the draining or flowing off of precipitation from a catchment area
though a surface channel. It thus represents the output from the catchment in a given
unit of time.
❖ Overland flow is the movement of water over the land, downslope toward a
surface water body.
❖ The flow where the water travels all the time over the surface as overland flow
and through the channels as open-channel flow and reaches the catchment outlet
is called Surface runoff.

Figure: Hydrologic cycle

Based on the time delay between the precipitation and the runoff, the runoff is
classified into two categories; as
Direct runoff (DRO): It is that part of the runoff which enters thestream
immediately after the rainfall.
Base flow: The delayed flow that reaches the stream essentially as groundwaterflow is
base flow.
RO = DRO + BF
DRO = OF + IF = SRO + IF
BF = GRO RO = OF (SRO) + IF + BF

66
Classification according to sources; as
Surface runoff (SRO): includes all overland flow (OF) as well as all precipitation
falling directly onto stream channels.
Interflow (IF): Interflow is the portion of the streamflow contributed by infiltrated
water that moves laterally in the subsurface until it reaches a channel.
Base flow (groundwater runoff or GRO).

Meteorological factors affecting runoff:

• Type of precipitation (rain, snow, sleet, etc.)

• Rainfall intensity
• Rainfall amount
• Rainfall duration
• Distribution of rainfall over the watersheds
• Direction of storm movement
• Antecedent precipitation and resulting soil moisture
• Other meteorological and climatic conditions that affect
evapotranspiration,such as temperature, wind, relative humidity,
and season.

Physical characteristics affecting runoff:

• Land use
• Vegetation
• Soil type
• Drainage area
• Basin shape
• Elevation
• Slope
• Topography
• Direction of orientation
• Drainage network patterns

The area contributing to runoff becomes greater with increased down slope distance,
thus larger discharge may result at greater slope lengths.
Drainage density is defined as the ratio of the total channel length [L] in the
watershed to total watershed area [A].

Drainage density = L / A

67
The effect of drainage density on runoff volume is associated with the time during
which the runoff remains in the watershed. Low densities allow for long residence
times; therefore, abstraction mechanisms have more time to remove water.
High densities usually allow fast runoff removal. Therefore, greater peaks and
hydrographs with shorter durations are expected for watersheds with higher drainage
densities.

Tributary: A stream that flows to a larger stream or other body of water. Distributary:
An outflowing branch of a stream or river, typically found in a delta (as opposed to
tributary).

Rainfall-Runoff Correlation

68
Solution:

69
70
Assignment:

71
 Chapter 7
Floods
A flood is an unusual high stage in a river, normally the level at which the river
overflows its banks and inundates the adjoining area.

The damages caused by floods

❖ loss of life,
❖ loss of property
❖ economic loss

The methods to estimate the magnitude of peak floods are

❖ Rational method
❖ Empirical method
❖ Unit-hydrograph technique
❖ Flood-frequency studies.

Rational method

The rational method is a simple technique for estimating a design discharge from a
small watershed. It was developed by Kuichling (1889) for small drainage basinsin
urban areas. The rational method is the basis for design of many small structures.

Assumptions and Limitations:

❖ Watershed area < 200 acres
❖ The method is applicable if time of concentration (tc) for the drainage areais
less than the duration (t) of peak rainfall intensity. (tc<t)
❖ The time of concentration (tc) is the time required for water to travel fromthe
hydraulically most remote point of the basin to the point of interest.
❖ The calculated runoff is directly proportional to the rainfall intensity.
❖ Rainfall intensity is uniform throughout the duration of the storm.
❖ The frequency of occurrence for the peak discharge is the same as the
frequency of the rainfall producing that event.
❖ Rainfall is distributed uniformly over the drainage area.
❖ The minimum duration to be used for computation of rainfall intensity is 10
minutes.

72
The runoff coefficient, C, is a dimensionless ratio intended to
indicate the amount of runoff generated by a watershed given an
average intensity of precipitation fora storm

Equivalent Runoff Coefficient,

73
74
Assignment:

75