1. Hydrology & Water Resources Engineering (2150602)
GOVERNMENT ENGINEERING COLLEGE RAJKOT
CIVIL ENGINEERING DEPARTMENT
Hydrology & Water Resources Engineering ( )
Hygrograph And Unit Hydrograph
Prepared By :
Gohel Maulik
Jadav Azad
Group No. :- 8
Faculty guide : Prof. V.G.Yadav

2. Hydrograph

3. Introduction
Discharge is the volume rate of water flow, including any suspended solids (e.g. sediment) or dissolved chemicals (e.g. CaCO3) which is transported through a given cross-sectional area.
A Hydrograph is a graph showing variation of discharge (Q) with time (t), at a particular point of stream or river.
Discharge is usually expressed in hectare meter per day and time is expressed in hours or days.
In Hydrograph discharge is plotted on y-axis and the corresponding time is plotted on x-axis.
Hydrograph shows the time distribution of total runoff at the point of measurement.
Hydrograph is also called Strom Hydrograph.

4. Figure 1. Component of a single peak hydrograph
Figure 2. Strom Hydrograph

5. Components of a Single peaked hydrograph
There are three essential component of the a single peaked hydrograph :
The rising limb (AD)
The peak of crest (D)
The reaction limb (DC)
A = Point of rise
D = Peak – Highest of the Hydrograph
E = Point of inflection

6. The Rising limb :
The rising limb is the ascending portion of the hydrograph corresponding to the increase of discharge due to gradual accumulation of storage in the storage in the channels existing in the area and also over the watershed surface.
The rising limb is also known as the concentration curve.
The Peak or Crest :
The peak or crest segment includes the part of the hydrograph from the inflection point on the rising limb to an inflection point on the recession limb.
The peak segment is the most important part of the hydrograph because it indicates the peak flow rate.
The peak represents the arrival of flow at the outlet from all parts of the basin.

7. The recession limb :
After the inflection point E, there is no inflow to the stream due to surface runoff.
The recession limb extends from the inflection point E, to the point C which represents the recommencement of the natural base flow or ground water flow.
The recession limb represents the withdrawal of water from the storage already built up in the catchment during the earlier phase of the hydrograph when surface runoff was occuring
Separation of base flow from direct runoff :
Straight line method
Two tangent method

8. Ordinate of hydrograph Total Q (cumecs)
Method of finding direct runoff from Hydrograph :
Date
and
Time
(1)
Ordinate of hydrograph Total Q (cumecs)
(2)
Base flow (cumecs)
(3)
Direct runoff
(o)
(4) = (2) – (3)
Date
05.00 14
08.00 25 12 13
11.00 51 11 40
14.00 65 10 55
17.00 54 43
20.00 28 15
23.00
∑ O = 166

10. UNIT HYDROGRAPH

11. Introduction
A unit hydrograph is a hydrograph representing 1 cm of runoff from rainfall of some unit duration.
For Example,
A 3-hour unit hydrograph indicates the hydrograph which gives 1 cm depth of direct runoff when a storm of 3-hour duration uniformly over the catchment.

12. Assumption of unit hydrograph theory
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 the drainage basin
The 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 common base period are directly proportional to the total amount of direct runoff resented by each hydrograph.
For a given drainage basin the hydrograph of direct runoff due to a given period of rainfall reflects all the combined physical characteristics of the basin

13. Derivation of unit Hydrograph
The unit hydrograph method is used for the estimation of the maximum flood discharge of a stream as well as for developing a flood hydrograph corresponding to any anticipated rainfall.
To derive unit hydrograph from storm hydrograph, following steps are followed.
From the past records, select a hydrograph resulting from an isolated, intense, short duration rainfall of uniform duration over the entire drainage basin.
Separate the base flow (ground water flow) from the direct runoff.
Subtracting the ordinates of base flow from the total ordinates, find the ordinates of direct runoff.

14. Figure 3. Construction of Unit Hydrograph

16. Limitation of Unit Hydrograph
The unit hydrograph theory cannot be applied to catchment areas greater than 5000 km2.
The unit hydrograph theory cannot be applied to very small catchments with area less than 2 km2.
This theory cannot be applied when the major portion of the storm is in the form of snow.
This theory is not very accurate. The accuracy obtained is ± 10 %
A large number of unit hydrographs are required for a catchment because as the unit duration changes, the unit hydrograph also changes.

17. Use of Unit Hydrograph
Once a unit hydrograph for a duration tr has been developed for a basin, the storm hydrograph for that basin for any other storm of different intensity but the same duration can be easily developed.
To draw maximum flood hydrograph for the design of spillways.
It can be used for flood forecasting and flood warning based on the observed rainfall in the basin.
To determine runoff from a storm of unit duration.

18. References http://hydro.nevada.edu/courses/gey711/week06.pdf
Hydrology And Water Resources Engineering by Dr. R. P. Rethaliya