INRODUCTION:

HYDROLOGY:

Hydrology means science of water.

It is the science that deals with the occurance, circulation and distribution of water on the earth.

Hydrology is a broad subject of an inter-disciplinary nature drawing support from allied sciences.

HYDROLOGY DEALS WITH:

Estimation of water resources

The study of processes such as precipitation, runoff, evapotranspiration and their interaction.

The study of problems such as floods, droughts and strategies to combat them

ENGINEERING HYDROLOGY(APPLIED HYDROLOGY):

A study concerned with engineering applications

ENGINEERING HYDROLOGY APPLICATOINS:

In design

Operations of projects dealing with water supply

Irrigation and drainage

Water power

Flood control

Navigation

Costal works

Salinity control

Recreational uses of water etc.

WATER FROM

OCEANS

ATMOSPHEREGROUND

SUN

THE HYDROLOGIC CYCLE

THE HYDROLOGIC CYCLE:

The hydrologic cycle is the general continuous circulation

of water from the oceans to the atmosphere, to the

ground and back to the oceas again.

Sun is the main source of energy for hydrologic cycle.

Let us consider the cycle begins with oceans

The various stages of hydrologic cycle are

Evaporation

Precipitation

Infiltration

Traspitation

It is a continuous process

Each path of hydrologic cycle may have one or more of the following

Trasportation of water

Temporary storage

Change of state

TYPES OF PRECIPITATION

PRECIPITATION

CONVECTIVE OROGRAPHIC CYCLONIC

NON-FRONTALFRONTAL

WARM FRONT COLD FRONT

FORMS OF PRECIPITATION:

Rain

Snow

Drizzle

Glaze

Sleet

Hail

RAINFALL MEASURMENT:

Rainfall is expressed interms of the depth to which

rain water would stand on an area if allthe rain

were collected in it.

Rainfall is measured by rainguage

Rainguage is a cylindrical vessel assembly kept in

open to collect rain

TYPES OF RAINGUAGE

TYPES OF RAINGUAGE

NON-RECORDING RAINGUAGE SELF RECORDING(AUTOMATIC)

SYMON’S GUAGE

STANDARD NON RECORDING

TYPE RAINGUAGE

•TIPPING BUCKET

•WEIGHING TYPE

•FLOAT TYPE

•TELEMETERING

•RADAR MEASURMENT

NON RECORDING TYPE RAINGUAGES:

It does not record the rainfall directly but only

collect the rain water, which when measured gives

the total amount of rainfall at the given point of

time.

Precipitation gauge1 - pole2 - collector3 - support-galvanized

metal sheet 4 – funnel5 - steel ring

1. Non recording gauge

The graphic rain gauge1-receiver2-floater

3-siphon 4-recording needle5-drum with diagram6-clock mechanism

The rainguages that automatically record the intensity of

rainfall over a period of time in the form of pen trace or a

clock driven chart.

COMPUTATION OF AVERAGE RAINFALL OVER A BASIN:

Inorder to compute the average Rainfall over a

basin or catchment area, the rainfall is measured at

a number of stations located in that area.

If the basement area contains more than one

rainguage station then following methods are used

for computation of average rainfall.

Arithmetic mean method

Thiessen polygon method

Isohytel method

Arithmetic mean method:

Average rainfall is calculated by arithmetic average of recorded rainfall at various stations of selected area

This is the simplest method of computing the average rainfall over a basin. As the name suggests, the result is obtained by the division of the sum of rain depths recorded at different rain gauge stations of the basin by the number of the stations.

N

i

i

ni PNN

PPPPP

1

21 1..........

Thiessen Polygon Method:This is the weighted mean method. The rainfall is

never uniform over the entire area of the basin or catchment, but varies in intensity and duration from place to place. Thus the rainfall recorded by each rain gauge station should be weighted according to the area, it represents. This method is more suitable under the following conditions:

- For areas of moderate size.

- When rainfall stations are few compared to the size of the basin.

- In moderate rugged areas.

m

mm

AAA

APAPAPP

.....

.....

21

2211

M

i

ii

total

i

M

i

i

A

AP

A

AP

P1

1

The ratio Ai/A is called the weightage factor of station i

isohyetal Method:

An isohyetal is a line joining places where the rainfall amounts are equal on a rainfall map of a basin. An isohyetal map showing contours of equal rainfall is more accurate picture of the rainfall over the basin. This method is more suited under the following conditions:

- For hilly and rugged areas.

- For large areas over 5000 km2.

- For areas where the network of rainfall stations within the storm area is sufficiently dense, isohyetal method gives more accurate distribution of rainfall.

• P1, P2, P3, …. , Pn – the values of the isohytes

• a1, a2, a3, …., a4 – are the inter isohytes arearespectively

• A – the total catchment area

• - the mean precipitation over the catchment

Isohyetal Method

P

A

PPa

PPa

PPa

P

nnn

2

...22

1

1

32

221

1

F

B

EA

CD

129.2

4.0

7.0

7.2

9.110.0

12

8

6

a1a1

a2

a3

a4

a5

The isohyet method is superior to the other two methods especially when the stations are large in number.

• An isohyet is a line joining points of equalrainfall magnitude.

• Given annual precipitation values – P1, P2, P3,… Pm

at neighboring M stations of station X 1, 2, 3 & m

respectively

• The normal annual precipitation given by N1, N2,

N3,…, Nm, Ni… (including station X)

• To find the missing precipitation, Px , of station X

m

mxx

N

P

N

P

N

P

M

NP ...

2

2

1

1

Before using rainfall data, it is necessary to check the data for continuing and consistency

◦ Missing data

◦ Record errors

Estimation of Missing Data

Test for consistency record

• Let a group of 5 to 10 base stations in the neighbourhood of the problem station X is selected

• Arrange the data of X stn rainfall and the average of the neighbouring stations in reverse chronological order (from recent to old record)

• Accumulate the precipitation of station X and the average values of the group base stations starting from the latest record.

• Plot the against as shown on the next figure

• A decided break in the slope of the resulting plot is observed that indicates a change in precipitation regime of station X, i.e inconsistency.

• Therefore, is should be corrected by the factor shon on the next slide

xP

avgP xP

avgP

(Double mass curve techniques)

Double Mass Curve Analysis

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.5 1 1.5 2 2.5

Accumulated annual rainfall of neigbouring stns in 10^3 cm

accum

ula

ted a

nnual ra

infa

ll o

f X

stn

in 1

0^

3 c

m

a

c

M

M

a

c

a

c

xcxM

MPP

Pcx – corrected precipitation at any time period t1 at stationX

Px – Original recorded precp. at time period t1 at station X

Mc – corrected slope of the double mass curve

Ma – original slope of the mass curve

A few commonly used methods are

Mass curve of rainfall

Hytograph

Hyetograph of a storm

0

0.1

0.2

0.3

0.4

0.5

0 – 8 8 – 16 16 – 24 24 – 32 32 – 40 40 – 48

Time, hours

Intensit

y,

cm

/hr

Hyetograph

- is a plot of the accumulated precipitation against time, plotted in chronological order

Mass curve of rainfall

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Time, hour

accum

ula

ted p

recip

itation,

mm

Mass Curve of Rainfall:

The total accumulated percipitation is plotted against time

RUNOFF:

It is a part of precipitation which is transmitted through natural surface channels, streams etc.

Runoff is overlandflow and interflow which enters

stream immediately after precipitation

Runoff includes

surface flow

interflow

groundwater flow

RUNOFF

DIRECT RUNOFFBASE FLOW

RUNOFF

Rainfall characteristics

Metrological factors

Water shed factor

Storage characteristics

The Rational Method properly understood and applied can

produce satisfactory results for urban storm sewer and small on-

site detention design.

Rational Formula:

The Rational Method is based on the Rational Formula:

RATIONAL METHOD

Q =CIA

Q = the maximum rate of runoff (cfs)C = a runoff coefficient that is the ratio between the runoff volume from an area and the average rate of rainfall depth over a given duration for that areaI = average intensity of rainfall in inches per hour for a duration equal to the time of concentration, tcA = area (acres)

Sevaral empherical formulae, tables, curves relating to runoff and rainfall have been developed to estimate runoff over catchment area.

Binnie’s percentage

Barlow’s tables

Strange’s tables and curves

Inglis and Desouza formula

Lacy formula

Khosla’s formula etc