Correct Answer - Option 3 : of unit rainfall excess precipitating instantaneously over the catchment
Explanation:
Instantaneous Unit Hydrograph(IUH):
- The instantaneous unit hydrograph is defined as a unit hydrograph produced by an effective rainfall of 1 cm and having an infinitesimal reference duration (in other words the duration tends towards zero).
- In terms of the systematic conception, this means that the instantaneous unit hydrograph represents the response of the catchment to an instantaneous impulse.
- It is useful as the IUH can be analytically computed and, on the basis of the "S" curve, a certain unit hydrograph of any reference duration may be derived from it.
- This ordinate of the instantaneous hydrograph is given by.
\(u\left( t \right) = \;\frac{1}{i}\frac{{ds}}{{dt}}\;\)Where,
I = Intensity of rainfall,
S = ordinate of S-Curve.
Uses:
- The development of flood hydrograph for extreme rainfall magnitude for use in the design of the hydraulic structure.
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Extension of flood flow records based on rainfall records.
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Development of flood record based on rainfall
- Development of flood forecasting and warning systems based on rainfall.
Limitations:
- Unit hydrograph cannot be used for catchment are greater than 5000 km2
- Unit hydrograph cannot be used for catchment less than 2 km2
- Precipitation must be in the form of rainfall only ie not be in the form of snow.
