Question

Is it true that rational process has to not be used for large catchments in estimating peak runoff?

Answer

Rational process is appropriate for small catchments only as time of absorption of small catchments is small. In Rational system peak runoff is calculated based on assumption that time of concentration is equal to rainfall duration. For small catchments, this assumption might hold true in mainly circumstances. One of assumptions of rational process is that rainfall intensity over entire catchment remains constant all through storm period. Though, in case of a large catchment it stands a high possibility that rainfall intensity varies in a variety of part of large catchment. In adding, for long period of rainfall, it is rare that rainfall intensity remains constant over total rainstorm and a shorter period but an additional intense rainfall might create a higher peak runoff. Furthermore, a decrease of peak runoff is also brought about by temporary storage of storm water similar to channels within catchment.

In genuine condition, runoff rate within catchment differs from place to place since of different soil properties and past conditions. Sometimes peak free occurs before all of drainage area is contributing. For example, when a significant portion of drainage area within catchment has very little time of concentration so that a higher rainfall strength be able to be used for this portion, runoff coming solely from this portion is higher than that of whole catchment in which a lower rainfall strength is adopted since remaining part of catchment has comparatively large occasion of concentration. Therefore, this results in wrong estimation of peak runoff of large catchments if rational process is adopted.