Fill: A reference to an area or material that has been placed by mechanical equipment in the process of a grading operation.
Filter bed: The section of a constructed filtration device that houses the filtering media.
Filter strip: An area of vegetation, usually adjacent to a developed area, constructed to remove sediment, organic matter, and other pollutants from runoff in the form of sheet flow.
Fines: Very small particles, including silt and clay.
First flush: The first portion of runoff, usually defined as a depth in inches, considered to containing the highest pollutant concentration resulting from a rainfall event.
Flexible lining: Flexible channel linings include rip-rap, gravel, vegetation, and bare soil.
Flooding: When the volume or rate flow exceeds the capacity of the natural or man-made conveyance system and overflows onto adjacent lands, causing or threatening damage.
Floodplain: For a given flood event, that area of land adjoining a continuous water course which has been covered temporarily by water.
Flow: The quantity of water which moves through a stream, pipe, etc., in a given length of time. Flow is measured in gallons per minute (gpm) or cubic feet per second (cfs). gpm is used in a water plant, cfs in river flow.
Flow path: The distance between the most distant point in the watershed and the watershed outlet.
Flow regime: The type of flow. Examples: overland, shallow concentrated, channel.
Flow routing (or flood routing): The procedure used to determine the volume of water that will be stored in a detention basin during a rainfall event. Examples: Storage-Indication Method and the Graphical Storage Method.
Flow splitter: An engineered hydraulic structure designed to divert a portion of storm flow to a BMP located out of the primary channel, or to direct stormwater to a parallel pipe system, or to bypass a portion of base flow around a BMP.
Forebay: Storage space, commonly referred to as a sediment forebay, located near a stormwater BMP inlet that serves to trap incoming coarse sediments before they accumulate in the main treatment area.
Freeboard: Vertical distance between the surface elevation of the design high water and the top of a dam, levee, or diversion ridge. Usually, a 10% freeboard is adequate, with a minimum freeboard of 0.2 feet.
French drain: A type of drain consisting of an excavated trench filled with pervious material such as coarse sand, gravel or crushed stone, through whose voids water percolates and exfiltrates into the soil.
Frequency (design storm frequency): The recurrence interval of storm events having the same duration and volume. The frequency of a specified design storm can be expressed either in terms of exceedance probability or return period.
Exceedance probability: The probability that an event having a specified volume and duration will be exceeded in one time period, usually assumed to be one year. If a storm has a one percent chance of occurring in any given year, then it has an exceedance probability of 0.01.
Return period: The average length of time between events having the same volume and duration. If a storm has a one percent chance of occurring in any given year, then it has a return period of 100 years.