HYDROGRAPHS

Sequence of lecture

• Introduction

• Types of Hydrograph

• Components of Hydrograph

• Effective Rainfall

• Basin Lag or Time Lag

• Parts of Hydrograph

• Hydrograph Analysis

• Factors Affecting Hydrograph Shape

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Introduction, HYDROGRAPHS

The graphical relation between any hydrological

quantity (stage, velocity, discharge, etc.) & the time is

known as a hydrograph.

The Hydrograph is a relationship of discharge and time at a

particular location on a stream.

A runoff hydrograph is a continuous record of stream flow

over time. A complete runoff hydrograph contains

information on runoff volume as the area under the

hydrograph and peak runoff rates as the maximum

flow or peak of the runoff hydrograph.

The hydrograph is “an integral expression of physio-

graphic and climatic characteristics that govern the

relation between rainfall and runoff of a particular

drainage basin” (Chow, 1964).

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Figure #1. Distribution of Precipitation input

(Viessman and Lewis, 1997)

TYPES OF HYDROGRAPH

• Discharge Hydrograph

It is the graphical representation of discharge against time.

Generally a hydrograph means discharge hydrograph.

• Stage Hydrograph

It is the graphical representation of stage against time. Stage hydrograph is useful only for the design of flood-protection works like embankment.

• Velocity Hydrograph

It is the graphical representation of velocity against time.

Duration

Lag Time

Time of Concentration

Rising Limb

Recession Limb (falling

limb)

Peak Flow

Time to Peak (rise time)

Recession Curve

Separation

Base flow

Parts of Hydrograph

Rising limb:

The rising limb of hydrograph, also known as concentration

curve, reflects a prolonged increase in discharge from a

catchment area, typically in response to a rainfall event.

The shape of the rising limb depends upon the storm

characteristics which are the duration of rainfall, intensity of

rainfall, areal distribution of the rainfall, etc.

Recession limb:

The recession limb extends from the point of inflection at the end

of the crest segment to the commencement of the natural

groundwater flow (base flow). It represents the withdrawal of

water from the storage built up in the basin during the earlier

phases of the hydrograph.

The falling limb depends upon the control on storage release,

which itself depends upon the geological structure.

Hydrograph Components

• Peak discharge:

The highest point on the hydrograph when the rate of discharge is

greatest. The crest segment is controlled by the storm characteristics

and the distribution of streams in the area, which further depends

upon geological structure of the area.

• Lag Time

Time interval from the center of mass of the rainfall-excess to the

peak of the resultant hydrograph

• Time to Peak

Time interval from the start of the rainfall-excess to the peak of the

resultant hydrograph

• Rainfall Duration

Time interval from the start to the end of rainfall-excess

• Time of Concentration

Time required for runoff to travel from the hydraulically most

distant point on the watershed to the point of interest

Hydrograph Components

EFFECTIVE RAINFALL

• All of the rain usually does not go into the stream but

a certain part of it reaches the stream & causes rise in

the stream flow while the remaining part of rain is

accounted for in various forms of precipitation losses.

• The portion of rainfall which contributes to stream

flow is called Effective Rainfall.

BASIN LAG OR TIME LAG

• A useful index to define the location of the peak of the hydrograph is the basin lag, which is the time difference between the center of mass of the rainfall and the center of the mass of the hydrograph.

• Rise of the stream flow depends upon the effective rainfall and type of the soil, and time lag depends upon type of area & also upon characteristics of rainfall.

Hydrograph

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1.0

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Time, t

Dis

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DRO Hydrograph

Lag

Continued..

COMPONENTS OF HYDROGRAPH

• A hydrograph normally consists of the four components:

Channel Precipitation

Direct Runoff (DRO)

Inter Flow or Sub-surface Flow

Ground or Base Flow

HYDROGRAPH ANALYSIS

• Separation of Hydrograph Components

Hydrograph Analysis means dividing total runoff indicated by the hydrograph into its (above-mentioned) components, of which direct runoff (DRO) and Base Flow are more important.

The separation of hydrograph components is done by one of the following four methods.

Straight Line Method

Fixed Base Length Method

Variable Slope Method

Recession Curve Equation

Continued..

Straight Line Method • A hydrograph as shown in

Figure. From point ‘A’ draw a straight horizontal line which meets the falling limb of hydrograph at point ‘B’.

• Above line ‘AB’ is the direct runoff and below this line is the base flow.

• The volumes of direct runoff and base flow (i.e. area under the curve) can be calculated by any suitable method.

Hydrograph

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Time, t

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DRO Hydrograph

A B

Horozontal Line from the

point of rise in rising limb

of hydrograph

•The Straight Line Method for separation of hydrograph has the

advantage of producing an extremely long time base for the direct

runoff hydrograph.

•The time base varies from storm to storm depending on the flow at

the point of rise.

•It is a computationally easy method.

Fixed Base Length Method • Let us have the hydrograph shown

in Figure 4. From point ‘A’ (existing prior to the storm) extend the curve to point ‘B’ which is vertically below the peak.

• Now draw a line from point ‘B’ to point ‘C’ on the hydrograph equal to a distance on X-axis given by equation below:

• N = 0.83*Ad0.2

Where,

N = Length of time in days, and

Ad = Drainage area in square miles

• This line will meet the falling limb at, say point ‘C’. Join the point ‘B’ to ‘C’ by a line ‘BC’.

• Area below ‘ABC’ is the base flow and that above it is the direct runoff.

Hydrograph

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Time, t D

isch

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DRO Hydrograph

A C

Extended

Recession

Curve

A0.2

B

Variable Slope Method

• This method is based on the fact that if water rises in the stream the water level in the stream becomes greater than the adjoining ground water table level.

• As a result the flow occurs from the stream to the groundwater and is stored there as bank storage.

• This method is not a well defined one. It depends upon experience, so it may vary from one expert to another.

Continued..

Contd.. • Take a point ‘A’ on the

previous recession curve, and draw the line ‘AE’ by extending the previous recession curve. Point ‘E’ is under the peak.

• Now take a point ‘B’ on the recession curve and extend it backward. Take a point ‘C’ on recession curve where the recession curve is changing slope.

• This point is called the point of inflexion. Draw a vertical line from the point ‘C’.

• It cuts the line drawn backward from point ‘B’, at point ‘D’. Join point ‘E’ and point ‘D’.

• The area under the line ‘AEDB’ shows the base flow component

Hydrograph

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Time, t

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DRO Hydrograph

A B

Extended

Recession

Curve Point of Inflexion

E

C

D

Contd..

• In general, Straight Line Method is the simplest and easiest but

also the least accurate.

• Fixed Base Method is widely used and gives practically good

results.

• Whereas Variable Slope Method is not commonly used and is

difficult but can give better results depending upon the expertise

of the person using it.

FACTORS AFFECTING HYDROGRAPH

SHAPE

• Hydrograph shape depends on Metrologic and catchment characteristics.

• Among the Metrologic characteristics the most important are the rainfall intensity, duration.

• Among the catchment characteristics, the topography and geological conditions are more important.

• These are discussed below: Storm Characteristics

Catchment Characteristics

Storm Characteristics

• Intensity of Rainfall

The intensity of rainfall has a predominant effect on the

shape of the hydrograph.

If other conditions remain the same, a higher intensity

storm will produce a rapid rise in the hydrograph and a

higher peak than that in case of a low intensity rainfall.

Such floods give little warning time and so are dangerous.

Continued..

Contd.. • Duration of Rainfall

The duration of rainfall is important if it is greater than the

time of concentration (defined as the time of travel from the

farthest point in the catchment area to the gauging station).

In such a situation, if the rainfall of certain intensity occurs

uniformly, the whole of the catchment area is contributing

runoff at the gauging station and the hydrograph attains its

high peak.

• The runoff will remain constant at the peak rate if the

rainfall occurs for duration longer than the time of

concentration.

• If rainfall of the same intensity occurs for duration less than

the time of concentration, the hydrograph will rise to a

smaller peak. Continued..

Contd.. • Weather

The shape of the hydrograph greatly depends on the

seasonal distribution of rainfall. During summer losses due

to evaporation may produce a small peak hydrograph.

On the other hand, in winter, losses are small with the

result that even a small intensity storm may produce a

relatively rapid rise and high peak of the hydrograph.

Antecedent soil moisture conditions (generally being

higher in winter) are important as for high soil moisture;

the storm will produce more runoff.

Catchment Characteristics

• Size of Catchment

The catchment area affects the stream flow in a variety of ways.

Even if the intensity and depth of precipitation is assumed to be

constant,

The hydrograph of a smaller catchment rises to its peak and then

recedes more rapidly than that for a larger catchment, because

for the latter, it takes longer for the runoff to reach the gauging

station.

The hydrograph of a larger catchment area, therefore, has

broader base than that of a smaller one. Moreover, the larger the

area, the greater will be the heterogeneity in soil and vegetation

cover, with a consequent effect on the stream flow of each part

of the catchment. The vegetation cover increases the time of

concentration.

Continued..

Contd..

The hydrograph of a larger catchment area,

therefore, has broader base than that of a smaller

one.

The vegetation cover increases the time of

concentration.

Continued..

Contd..

• Shape of the Catchment Area

The shape of the catchment area affects stream flow by altering

the time of concentration and the pattern of drainage tributaries.

If the length of the catchment along the main stream is less than

the width across the same stream, separate runoff peaks

generated by a heavy rainfall are likely to reach the gauging

station at the same time from the tributaries, with the result that

the flood peak in the main stream increases.

Thus a hydrograph with a high peak and narrow base is obtained.

Continued..

Contd.. However, if the length of the catchment along the main stream is

larger than width, the tributaries will be of shorter length and

contribution from each tributary area is likely to reach the

gauging stations at intervals.

After an intense storm over the whole of the catchment area

there will be some lag between the times at which the peak from

each tributary reaches the gauging station.

The hydrograph will therefore, have a low peak and broader

base, because the remote tributaries will continue contributing

for some time even after the rain fall has stopped.

Continued..

Contd.. • Elevation of the Catchment

The elevation of the catchment also plays an important role

in some hydrologic characteristics of hydrograph,

particularly due to variation in temperature and

precipitation.

The temperature reduces with the increase in elevation with

the result that above a certain elevation, the temperature

becomes so low that all precipitation falls as snow.

In the Indo-Pakistan Sub-continent, contribution from

areas, at altitude 15,000 ft. above the mean sea level, is

mainly due to snowmelt.

The floods from snowmelt are of usually low peak and

broader base.

Continued..

Contd..

• Catchment Slope

The slope of the catchment affects stream flow by

reducing the rate of infiltration due to increased

speed of water-movement towards drainage

channels.

The steeper the slope the faster will be the flow

and rise in the hydrograph. The hydrograph in such

a situation will have higher peak and smaller base.