8/18/2019 unit hydrograph 19-03-14.ppt
1/13
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.
8/18/2019 unit hydrograph 19-03-14.ppt
2/13
Unit Hydrograph Contd.
8/18/2019 unit hydrograph 19-03-14.ppt
3/13
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) $rainage area iv) Base flow or basis for obtaining it.
8/18/2019 unit hydrograph 19-03-14.ppt
4/13
Derivation teps of UnitHydrographs i) Tabulate the total hydrograph with time
distribution
ii) Tabulate the base flow if given orseparate with method of our choice.
iii) Find the direct runoff hydrograph($%&) by
subtracting the base flow from the totalhydrograph.
iv) Find the volume of water under the $%&
8/18/2019 unit hydrograph 19-03-14.ppt
5/13
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.
8/18/2019 unit hydrograph 19-03-14.ppt
6/13
8/18/2019 unit hydrograph 19-03-14.ppt
7/13
Plot of the Total Hydrograph
(
)(((
*(((
+(((
,(((
-((((
-)(((
( . -( -. )( ). /( /. *(
Time (hrs)
F l o w R
a t e
( m ! s
8/18/2019 unit hydrograph 19-03-14.ppt
8/13
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 /
8/18/2019 unit hydrograph 19-03-14.ppt
9/13
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.
8/18/2019 unit hydrograph 19-03-14.ppt
10/13
8/18/2019 unit hydrograph 19-03-14.ppt
11/13
Non-Constant Storms
8/18/2019 unit hydrograph 19-03-14.ppt
12/13
Rainfall Intensity Duration Curve
Rainfall Duration (D)
! "# Return periods
$% $& $
ainfall
ntensity
8/18/2019 unit hydrograph 19-03-14.ppt
13/13
Runoff Coeffi&ient' C