Partial area effect
In rational method calculations, the storm duration used to define a rainfall intensity of a given average recurrence interval is commonly set equal to the time of concentration of the catchment.
This is based on the assumption that, in the rational method equation, Q = C.I.A, as the time after the start of a storm increases, the contributing area A increases faster than the statistical intensity I decreases,
In many cases this is not the case, due to catchment shape, slope and land-use factors.
The land-use changes, and the alterations to flow paths and streams brought about by urbanisation, create many "partial area effects" where the greatest calculated Q = C.I.A occurs at a shorter time than the time of concentration. In the DRAINS rational method calculations, a search is made automatically to define the greatest value of the flowrate Q.
Usually, the pervious area will have a longer time of travel than the impervious area. For a selected time shorter than this, DRAINS assumes that the contributing area is:
The full impervious area + full pervious area x (selected time / pervious area time of travel).
The shape of the pervious area is considered to be such that its time-area diagram is linear.
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