module 2 ch-1 heytograph and hydrology analysis

MODULE-2

HYETOGRAPH AND HYDROGRAPH ANALYSIS

PREPARED BY:-Prof. ANKIT N PATEL

HYETOGRAPH A hyetograph is a graphical representation of the

relationship between the rainfall intensity and time. It is the plot of the rainfall intensity drawn on the

ordinate axis against time on the abscissa axis. The hyetograph is a bar diagram. The area under the hyetograph gives the total rainfall

occurred in that period. This chart is very useful in representing the

characteristics of storm, and is particularly important in developing the design storm to predict extreme floods.

Bits Edu Campus Prof. Ankit Patel 2

Mass curve of rainfall

0

10

20

30

40

50

60

0 20 40 60 80 100 120Time, hour

accu

mul

ated

pre

cipita

tion,

mm

Mass Curve of Rainfall:

The total accumulated precipitation is plotted against time

Bits Edu CampusProf. Ankit Patel 3

Hyetograph of a storm

00.10.20.30.40.5

0 – 8 8 – 16 16 – 24 24 – 32 32 – 40 40 – 48Time, hours

Inte

nsity

, cm

/hr

Hyetograph-A hyetograph is a graphical representation of the relationship between the rainfall intensity and time


Rainfall Hyetograph


RUNOFF Runoff is that portion of rainfall that is not

evaporated. The runoff is defined as the portion of the rainfall

that makes its way towards river or ocean as surface or subsurface flow.

The discharge flowing in a river is the runoff from the basin drained by that river.

Runoff is usually expressed as volume per unit time, the common unit being m3/s or cumec.


Classification of Runoff Surface runoff: Water flows over the land and is first to reach the streams

and rivers which ultimately discharge the water to the sea.

Inter flow or subsurface flow: A portion of rainfall infiltrates into surface soil and

depending upon the geology of basins, runs as subsurface runoff and reaches the streams and rivers.

Ground water flow or base flow: It is that portion of rainfall which after infiltration,

percolates down and joins the ground water reservoir which is ultimately connected to the ocean.


PROCESS OF RUNOFF When rainfall a part of it is intercepted by vegetation some of it is

stored as depressions on the ground surface and is known as depression storage which later infiltrate or evaporates some of the rain is absorbed by soil.

If the rain continues further, the water starts infiltrating into the surface soil and if rate of rainfall exceeds the infiltration rate then this excess water start collecting on the surface as surface detention and this water flows overland and joins the stream, river oceans.

The water which percolates without joining the water table and flows below the ground surface and then joins the stream or river as a sub surface flow and is considered as a part of surface runoff.

The water that percolates to the ground water table and later after long time joins the river or stream is known as ground water flow or base flowBits Edu Campus Prof. Ankit Patel 8

PROCESS OF RUNOFF


FACTORS AFFECTING RUNOFF

1. Climatic Factors Type of precipitation Rain intensity Duration of rainfall Distribution of rainfall Direction of storm movement Soil moisture deficiency Other climatic conditions2. The physiographic Factors Size of basin Shape of the basin Topography of the basin Soil characteristics of basin Cultivation and vegetative cover in basin area


Characteristics of drainage basin1. Divide Lines: The boundary line, along a topographic ridge separating two adjacent drainage basins is called drainage divide.2. Concentration Point: The single point or location at which all surface drainage form a basins comes together or concentrates as outflow from the basin in the stream channel is called concentration point3. Time of concentration: the time of a concentration of a drainage basin is the time required by the water to reach the outlet from the most remote points of the drainage area.


HYDROGRAPH A hydrograph is a graphical plot of discharge of a

natural stream or river versus time. It shows variations of discharge with time, at a

particular point of a stream. It also shows the time distribution of total runoff at

the point of measurement. Discharge is plotted on Y-axis and the corresponding

time is plotted on X-axis. Flood analysis and derivation of unit hydrograph a

single peaked hydrograph is required. Hydrograph analysis is the most widely used method

of analyzing surface runoff.Bits Edu Campus Prof. Ankit Patel 16

HydrographRecord of River Discharge over a period of timeRiver Discharge= cross sectional area

rivers mean (average) velocity

X(at a particular point in its course)

Storm HydrographsShow the change in discharge caused by a period of rainfall

WhyConstruct & Analyse

Hydrographs ? To find out discharge patterns

ofa particular drainage basinHelp predict flooding events,therefore influence implementation of flood prevention measures

Construction

Of Storm (flood) Hydrographs

0 12 24 36 48 30 72Hours from start of rain storm

3

2

1

Disc

harg

e (m

3 /s)

Base flow

Through flow

Overland flowRi

sing

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Flood

Hyd

rogr

aph


3

2

1

Disc

harg

e (m

3 /s)


3

2

1

Disc

harg

e (m

3 /s)

mm4

3

2

Rainfall shown in

mm, as a bar graph


3

2

1

Disc

harg

e (m

3 /s)

mm4

3

2

Discharge in m3/s, as a line graph


3

2

1

Disc

harg

e (m

3 /s)

Risi

ng

limb

mm4

3

2

Rising limbThe rising

flood water in the river


3

2

1

Disc

harg

e (m

3 /s)

Risin

g lim

bmm4

3

2

Peak flow

Peak flowMaximum

discharge in the river


3

2

1

Disc

harg

e (m

3 /s)

Risin

g lim

b

Recession

limb

mm4

3

2

Peak flow

Recession limbFalling flood water in the

river


3

2

1

Disc

harg

e (m

3 /s)

Risin

g lim

b

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Basin lag timeTime

difference between

the peak of the rain

storm and the peak

flow of the river


3

2

1

Disc

harg

e (m

3 /s)

Base flow

Risin

g lim

b

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Base flowNormal discharge

of the river


3

2

1

Disc

harg

e (m

3 /s)

Base flow

Through flow

Overland flowRi

sing

limb

Recession

limb

Basin lag time

mm4

3

2

Peak flow

Overland flow

Through flow

+

=Storm Flow

Volume of water

reaching the river from

surface run off

Overland flow

Through flow

Volume of water reaching the

river through the soil and

underlying rock layers

COMPONENTS OF A HYDROGRAPH• 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 (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


SEPERATION OF BASE FLOW

The total runoff comprises direct runoff and base flow1. Direct runoff: it is that water which reaches the river shortly

after it falls as rain. It is a overland flow2. Base Flow: It is the initial flow of the river before the rainfall

comes. It is sustained or dry weather flow of the river resulting from the outflow of perennial or almost permanent ground water contribution that reaches the river.


Unit Hydrograph

Unit Hydrograph is defined as the hydrograph of surface runoff of a catchment area resulting from unit depth of rainfall excess or net rainfall occurring uniformly over the basin and at uniform rate for a specified duration.


UNIT HYDROGRAPH The effective rainfall also called rainfall excess is that part of total

rainfall that enters the stream directly. The specified or unit duration is that period within which the

effective rainfall is considered to be uniformly distributed. A unit hydrograph is designed according to the unit duration Since the area under a hydrograph represents the total runoff, it is

evident from the definition of unit hydrograph that the area under the unit hydrograph represents a direct runoff of 1 cm


Text in here

Text in here

ASSUMPTIONS

The effective rainfall is uniformly distributed within its duration or specified period of time

The effective rainfall is uniformly distributed over the entire area of drainage basin.

The base or time duration of the hydrograph of direct runoff due to an effective rainfall of unit duration is constant.

The ordinates of all the direct runoff hydrographs of a common base period are directly proportional to the total amount of direct runoff represented by each hydrograph.


Text in here

Text in here

For a given drainage basin the hydrograph of direct runoff corresponding to a given period of rainfall reflects all the physical characteristics of the basin.


LIMITATIONS OF UH This theory is not applicable to large areas because uniformly

distributed effective rainfall cannot be expected in large area. The UH method cannot be applied when an appreciable portion of

the storm precipitation falls as snow Practically no two storms have the same nature in space and time

period. So it is not possible to construct UH for each pattern.


39

Derived Unit Hydrograph

0.0000

100.0000

200.0000

300.0000

400.0000

500.0000

600.0000

700.0000

0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000

Total Hydrograph

Surface Response

Baseflow

Bits Edu Campus Prof. Ankit Patel

APPLICATION OF UH Use of UH for deriving a flood hydrograph resulting from rainfall

of unit duration Use of UH for deriving a flood hydrograph resulting from a series

of rainfalls each of same unit duration Use of UH for deriving a UH of other duration.


module 2 ch-1 heytograph and hydrology analysis

Engineering