Once the rainwater enters the pervious surface it may flow out of the construction through the base to groundwater or, if the construction is lined, the waters will be intercepted by a drainage network and discharged from the sub-base to a suitable receiving drainage system. Comparison of the outflow hydrograph at the drain outlet reveals marked differences in performance.
The porous surface and its sub-structure responds to rainfall much slower than the impermeable surface and continues to discharge for much longer, hours and even days after rainfall stops. For example, a typical hydrograph (as below) has shown that the first outflow from the porous surface was some three hours after start of rainfall and discharge continued into a second day, although there were additional, smaller rainfalls within the event. This effect is known as attenuation.
Pervious surfaces and their sub-structure may significantly modify hydrographs to drainage systems and downstream impacts are reduced unless there is runoff from extreme storm conditions. This is achieved by either infiltrating the rainwater through the surface and into the sub-soil or alternatively retaining the rainwater within the pervious construction.
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