Unit
Hydrograph
A
unit hydrograph can be defined as a direct runoff (output) of a hydrograph from
a single unit rain fall, which is distributed evenly over the an area at the
same rate over a specified period known as the unit time or unit length.
The hydrograph unit is a direct response function of
a water runoff. It assumes that the time frame of the hydrograph remains
unchanged regardless of the rate at which the water flow is due to various
simultaneous storms.
Basically, the hydrograph shows us the ways in which
a river is affected by a storm. This helps us to understand the patterns of
drainage of certain water basins and helps to predict floods and to plan flood
prevention measures.
The Unit Hydrograph (UH) process is widely used for
runoff measurements, especially in determining maximum output.
1.
The
flood hydrograph can be calculated with the help of very short data records.
2.
In
addition to the flow unit hydrograph also provides the total flow and
distribution time.
3.
The
hydrograph unit process can be easily computerized to make the calculations
work.
Unit Hydrograph method of estimating maximum Probable Discharge.
Ø
The
Direct Runoff (DRO) is obtained by subtracting base flow from discharge values.
Ø
The
DRO is then multiplied by time interval to get total volume.
Ø
This
total volume is divided by catchment area to get depth of effective rainfall in
centimeters spread uniformly over whole catchment.
Ø
By
definition of unit hydrograph, its ordinates are obtained by dividing ordinates
of DRO by effective rain.
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