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An instantaneous unit hydrograph is a direct run-off hydrograph:
1. of 12 cm magnitude due to a rainfall excess of 1-h duration
2. that occurs instantaneously due to a unit rainfall excess of duration 'D' h
3. of unit rainfall excess precipitating instantaneously over the catchment
4. occurring at any instant in a long storm

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Correct Answer - Option 3 : of unit rainfall excess precipitating instantaneously over the catchment

Explanation:

Instantaneous unit hydrograph (IUH):

  • A unit hydrograph of infinitesimal duration (Very small duration) is called an instantaneous unit hydrograph.
  • It mainly used when rainfall duration and intensity are variable because in that case, we need to divide the rainfall into various small durations and then find its response.
  • The advantage of IUH is that it is independent of the duration of effective rainfall hyetograph and has one parameter less than the D-hr unit hydrograph.
  •  IUH is independent of rainfall characteristics.
  • It is indicative of catchment storage characteristics.
  • An instantaneous unit hydrograph is also obtained from S- curve. But that is an approximate method.
  • Also called Clark's method


Uses and Limitations of unit hydrograph:

Uses:

  • The development of flood hydrograph for extreme rainfall magnitude for use in the design of the hydraulic structure.
  • Extension of flood flow records based on rainfall records.
  • 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.​

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