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  UNIT HYDROGRAPH

 A standard hydrograph that relates to differentstorms can be produced.

The basis is a linear system. For rainfall of a given duration, t and intensity, i ,

a hydrograph (A) can be obtained. The principle of a linear system is that another

rainfall can be added to get a newhydrograph(C).

The single hydrograph (C) was obtained byadding the ordinates of A and B.

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Unit Hydrograph Contd.

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 Construction or Derivation of UnitHydrographsa) Desirable factors for derivation: niform

intensity and short duration storms are needed

for the construction.!deally, the storms should be of e"ual duration.

b) Specific information for derivation#  i) $uration of storm ii) Total hydrograph  iii) $rainage area  iv) Base flow or basis for obtaining it.

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  Derivation teps of UnitHydrographs i) Tabulate the total hydrograph with time

distribution

ii) Tabulate the base flow if given orseparate with method of our choice.

iii) Find the direct runoff hydrograph($%&) by

subtracting the base flow from the totalhydrograph.

iv) Find the volume of water under the $%&

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Derivation teps of Unit

Hydrographs Contd! v) $ivide the volume of water(step iv)

by the drainage area to get effective

rainfall (runoff) per unit area.

vi) $ivide the ordinates of the $%& by

the cm of effective rainfall(step v).

The result is a unit hydrograph(&')

for the duration of storm.

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Plot of the Total Hydrograph

(

)(((

*(((

+(((

,(((

-((((

-)(((

( . -( -. )( ). /( /. *(

Time (hrs)

   F   l   o   w    R

   a   t   e

   (   m      !   s

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Solution Contd.

Total volume of flow 0 1 m/  2s 3 hrs 01 m/  2s 3 3 /+ s

0 */.1 3 -+ m/

 Area of catchment 0 */ 4m 0 */ 3 -ha 0 */ 3 - 3 - m

 0 */ 3 -+  m  5ffective rainfall per unit area 0 .- m 0- cm

To obtain - cm i.e. - unit hydrograph, divide $%6column ordinates by -. !f + hr $%6 for / cm isre"uired, multiply &' ordinates by /

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Conversion of Unit Hydrograph fro"

horter to #onger Duration $%u&tip&eituation'

!t is possible to convert say a hr. unit

hydrograph to + hr unit hydrograph.

  This is by lagging hrs, two times,

adding up ordinate values to get +hr /

mm hydrograph.

$ivide ordinates by / to obtain a + hr

unit hydrograph.

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 Non-Constant Storms

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Rainfall Intensity Duration Curve

Rainfall Duration (D)

! "# Return periods

$% $& $

ainfall

ntensity

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Runoff Coeffi&ient' C