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Use of the Unit Hydrograph
The UHs establish a relationship between the ERH and DRH for a catchment
They are of great use in
i. The development of flood hydrographs for extreme rainfall magnitudes (for use in the design of hydraulic structures)
ii. Extension of flood flow records based on rainfall records
iii. Development of flood forecasting and warning systems based on rainfall
Limitations of the Unit Hydrograph
• Unit hydrographs assume uniform distribution of rainfall over the catchment and uniform intensity during the duration of rainfall excess. In practice, these two conditions are never satisfied.
• Under conditions of non-uniform areal distribution and variation of in intensity, the unit hydrograph theory can still be used if the areal distribution is consistent between different storms.
• The size of the catchment imposes an upper limit on the applicability of the unit hydrograph theory (because the centre of the storm can vary from storm to storm and each of these storms can give a different DRH under otherwise identical conditions in very large basins).
• The upper limit for use of the unit hydrograph method is 5000km2
• In the case of very large basins, the flood hydrographs can be studied by dividing it into a number of smaller sub-basins, developing DRHs for these sub-basins by the UH method, and then routing these DRHs through their respective channels to obtain the composite DRH at the catchment outlet.