Planning policy guidance note (PPG) 25 defines the functional flood plain as the unobstructed or active areas within the land at risk of river and coastal flooding shown on indicative flood plain maps prepared by the Environment Agency where water flows regularly in times of flood. Areas that are defended are passive until such time as the defences are overtopped or breached and are not regarded as part of the functional flood plain.
The term functional flood plain was primarily coined in connection with river flooding, where the proper functioning of a river flood plain serves to reduce the impact of flooding on other areas. However, since most beaches are flooded by tides twice a day, and are undefended they could logically be considered to be functional flood plains, as could undefended areas above mean high water that are flooded by the sea on a regular basis. There may be instances where a storm beach created by extreme high waters has its crest above the flood level with an annual probability of occurrence of 0.5 per cent. In such a case the storm beach itself might not be classified as functional flood plain because of the very long periods between potential flooding events.