A hydrograph is a graph showing the rate of flow versus time at a specific point in a river, or other channel of flow.

1. Topic:
Hydrograph

2. Contents
 Definition
 Types of Hydro-graph
 Factors Affecting Hydrograph
 Uses of Hydrograph

3. Definition.
“A hydrograph is a graph showing the rate
of flow versus time at a specific point in a
river, or other channel of flow.
 Unit.
The rate of flow is typically expressed in
cubic meters or cubic feet per second (cms or
cfs).

4. Types of Hydrograph
 Natural hydrograph
 Unit hydrograph
 Dimensionless unit hydrograph
 Synthetic hydrograph
 Dam breach hydrograph (Special case - natural or
synthetic)

5. Natural Hydrograph
A natural hydrograph is one recorded at a stream
gauging site and is a fingerprint of the upstream
drainage area's response to rainfall

7. Unit Hydrograph
A unit hydrograph is a natural or synthetic hydrograph
representing one inch of runoff', uniformly from the
watershed during a specified time. In other words, the
area under the graph is actually a volume of one inch
of runoff.

8. Dimensionless Unit Hydrograph
A dimensionless unit hydrograph is a generic
combination of many natural unit hydrographs. The
ordinate and abscissa scales are ratios of the discharge
with respect to the peak discharge and the time
relative to the time ,to peak, respectively

10. Synthetic Hydrograph
A synthetic hydrograph is calculated based on
watershed and storm characteristics.
(In SCS, soils, land use, vegetative cover, size, slope,
and time of concentration are important watershed
characteristics.)
Synthetic hydrographs are used to simulate natural
hydrographs for ungauged watersheds .

12. Dam Breach Hydrograph
A dam breach hydrograph represents the sudden
release of water from the impoundment due to a
breach, followed by the draining of the reservoir.
The volume represented by the hydrograph is the
storage volume of the reservoir released during the
breach.

14. Factors
 Factors affecting the shape of the breach hydrograph
include:
 size and shape of breach,
 depth of water at the dam,
 volume of stored water,
 surface area of reservoir,
 and shape (especially length) of reservoir.
 A breach hydrograph can be natural (recorded) or
synthetic (a simulation)

15. Some Other Types of Hydro-graph
 Storm Hydrographs
 Flood Hydrographs
 Annual Hydrographs.

16. Storm Hydrograph
“Storm hydrographs are graphs that show how a
drainage basin responds to a period of rainfall.”
They are useful in planning for….
 Flood situations
 Times of drought
 It shows the variations in a river’s discharge over a
short period of time.

18. Flood Hydrograph
“Flood hydrographs is a visual representation of the
variation in the stream or river depth over time.
Flooding normally occurs when the depth of the
stream exceeds the depth from the stream bank to the
stream bottom.”

19. Annual Hydrograph
“An annual hydrograph shows the average daily flow of
a river over the course of one year rather than a period
of hours”
 It is affected by a number of controls depending on the
drainage basin concerned or the time of year.
 Hydrographs provided a means for computing a runoff
hydrograph over a single drainage area based on the
curve number and SCS Unit Hydrograph methods.

20. Factors Affecting Hydrograph
 The time distribution of runoff (the shape of the
hydrograph) is influenced by
 Climatic conditions
 Topographic and geological factors.
 The climatic and topographic factors mainly affect the
 “ Rising limb”
 The geological factors determine the
“Recession limb”

21. Climatic factors
 1) Rainfall intensity
 2) Rainfall duration;
 3) Distribution of rainfall on the basin;
 4) Direction of storm movement;
 5) Type of storm

22. Topographic and Geological Factors
 1) Catchment size;
 2) Catchment shape;
 3) Distribution of watercourses;
 4) Slope of the catchment;
 5) Storage in the catchment;
 6) Geology of the catchment;
 7) Land use.

23. Why we use Hydrograph..??
 Uses of Hydrographs include:
1. Watershed evaluations.
2. Design of structural works.
3. Flood plain management studies.
4. Emergency action plans.
5. Design of farm ponds.
6. Channel design.
7. Grade stabilization structures.