F

FEH
Flood estimation handbook, produced by Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology)

filter drain
A linear drain consisting of a trench filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage, to store and conduct water, but may also be designed to permit infiltration.

filter strip
A vegetated area of gently sloping ground designed to drain water evenly off impermeable areas and filter out silt and other particulates.

filtration
The act of removing sediment or other particles from a fluid by passing it through a filter.

first flush
The initial runoff from a site or catchment following the start of a rainfall event. As runoff travels over a catchment it will pick up or dissolve pollutants and the "first flush" portion of the flow may be the most contaminated as a result. This is especially the case in small or more uniform catchments, however, in larger or more complex catchments pollution wash-off may contaminate runoff throughout a rainfall event.

flood frequency
The probability of a flowrate being equalled or exceeded in any year.

floodplain
Land adjacent to a watercourse that would be subject to repeated flooding under natural conditions (see Environment Agency's Policy and practice for the protection of flood plains for a fuller definition).

flood routeing
Design and consideration of above-ground areas that act as pathways permitting water to run safely over land to minimise the adverse effect of flooding. This is required when the design capacity of the drainage system has been exceeded.

flow control device
A device used to manage the movement of surface water into and out of an attenuation facility, eg a weir.