Chapter 4 Hydrograph

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ANALYSIS HYDROGRAPH


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UNIT HYDROGRAPH

Sherman (1932) first proposed the unit hydrographconcept. The Unit Hydrograph (UH) of a watershed isdefined as the direct runoff hydrograph resulting froma unit volume of excess rainfall of constant intensity

and uniformly distributed over the drainage area. The duration of the unit volume of excess or effective

rainfall, sometimes referred to as the effectiveduration, defines and labels the particular unithydrograph.

The unit volume is usually considered to beassociated with 1 cm (1 inch) of effective rainfalldistributed uniformly over the basin area.


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HYDROGRAPH COMPONENTS

Total streamflow during a precipitation event

includes the baseflow existing in the basin prior to

the storm and the runoff due to the given storm

precipitation. Total streamflow hydrographs are

usually conceptualized as being composed of:

a) Direct Runoff, which is composed of contributions

from surface runoff and quick interflow. Unit hydrograph

analysis refers only to direct runoff.

b) Baseflow, which is composed of contributions fromdelayed interflow and groundwater runoff.


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Surface runoffincludes all overland flow as well as all

precipitation falling directly onto stream channels. Surface

runoff is the main contributor to the peak discharge.

Interflowis the portion of the streamflow contributed by

infiltrated water that moves laterally in the subsurface until it

reaches a channel. Interflow is a slower process than surface

runoff. Components of interflow are quick interflow, which contributes

to direct runoff, and delayed interflow, which contributes to

baseflow

Groundwater runoffis the flow component contributed to the

channel bygroundwater. This process is extremely slow ascompared to surface runoff.


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The streamflow hydrograph is subdivided into:

a) Rising Limb: rising portion of the hydrograph, composed mostly of

surface runoff.b) Crest: zone of the hydrograph around peak discharge.

c) Falling (or Recession) Limb: Portion of the hydrograph after the

peak discharge, composed mostly of water released from storage in

the basin. The lower part of this recession corresponds to

groundwater flow contributions.


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THEMAINFACTORSAFFECTINGHYDROGRAPH

SHAPEARE:

Drainage characterist ic s: basin area, basin shape, basin

slope, soil type and land use, drainage density, and drainage

network topology. Most changes in land use tend to increase

the amount of runoff for a given storm

Rainfal l characterist ics:rainfall intensity, duration, and their

spatial and temporal distribution; and storm motion, as storms

moving in the general downstream direction tend to produce

larger peak flows than storms moving upstream.


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UNIT HYDROGRAPHS: EMPIRICAL

DERIVATION

The following are essential steps in deriving a unithydrograph from a single storm: Separate the baseflow and obtain the direct runoff

hydrograph (DRH).

Compute the total volume of direct runoff and convert this

volume into equivalent depth of effective rainfall (incentimeters or in inches) over the entire basin.

Normalize the direct runoff hydrograph by dividing eachordinate by the equivalent volume (in or cm) of direct runoff(or effective rainfall).

Determine the effective duration of excess rainfall. To do this,obtain the effective rainfall hyetograph (e.g., use the -index,the Horton, Green and Ampt, or Philip equations, or someother method to determine infiltration losses) and itsassociated duration. This duration is the duration associatedwith the unit hydrograph.


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UNIT HYDROGRAPHSFOR DIFFERENT

EFFECTIVE DURATION

A unit hydrograph for a particular watershed is developed for a

specific duration of effective rainfall.

When dealing with a rainfall of different duration a new unit

hydrograph must be derived for the new duration.

The linearity property implicit in the UH analysis can be used

to generate UHs associated with larger or smallereffective

rainfall pulse duration.

This procedure is sometimes referred to as the S-curve

Hydrograph method.


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S-CURVE HYDROGRAPH METHOD

The S-curve, also known as S-hydrograph

produced by a continuous effective rainfall at a

constant rate for an infinite period.


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METHODOF SUPERPOSITION


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THATALL..

THANK YOU

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Chapter 4 Hydrograph