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Filter |
filter zone |
A band of granular material which is incorporated in an embankment dam and is graded (either naturally or by selection) so as to allow seepage to flow across or down the filter zone without causing the migration of the material from zones adjacent to the filter. |
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Filter cake |
Mud cake |
A deposit of mud on the walls of a drill hole. |
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Financial analysis |
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Procedure that considers only tangible factors when evaluating various alternatives. |
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Fines |
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Portion of a soil finer that a No. 200 U.S. Standard sieve. Clay or silt particles in soil. |
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Finger drains |
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A series of parallel drains of narrow width (instead of a continuous drainage blanket) draining to the downstream toe of the embankment dam. |
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Finished grade |
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The elevation or surface of the earth after all earthwork has been completed (also finish grade). The final grade required by specifications. |
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Firm energy |
Power |
Non-interruptible energy and power guaranteed by the supplier to be available at all times, except for uncontrollable circumstances. |
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Fish ladder |
Fishway |
An inclined trough which carries water from above to below a dam so that fish can easily swim upstream. There are various types, some with baffles to reduce the velocity of the water and some consisting of a series of boxes with water spilling down from one to the next. |
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Fish weir |
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A type of fish ladder. |
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Fishing |
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The operation of recovering an object left or dropped in a drill hole. |
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Fixed-wheel gate |
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A gate consisting of a rectangular leaf mounted on wheels, particularly suited for high head situations. |
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Flail |
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A hammer hinged to an axle so that is can be used to break or crush material. |
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Flange |
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A ridge that prevents a sliding motion. A rib or rim for strength or for attachments. |
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Flashboards |
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Temporary barriers, consisting of either timber, concrete or steel, anchored to the crest of a spillway as a means of increasing the reservoir storage. Flashboards can be removed, lowered, or carried away at the time of flooding either by a tripping device or by deliberate failure of the flashboards or their supports. |
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Flat slab |
Slab and buttress dam |
A buttress dam with buttresses which support the flat slab of reinforced concrete which forms the upstream face. |
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Fleet angle |
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The angle between the position of a rope or cable at the extreme end wrap on a drum and a line drawn perpendicular to the axis of the drum. The fleet angle is used to indicate how effective or efficient the rope or cable is for raising a load. |
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Flexible pipe |
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Pipe designed to transmit the backfill load to the soil at the sides of the pipe. Flexible pipe must be supported on both the bottom and on the sides of the pipe. |
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Floatable days |
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The number of days during the recreation season on which it is safe to allow floating activities on recreation facilities |
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Floatable flows |
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River flows which make rafting and other floating recreation possible. |
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Floc |
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Loose, open-structured mass formed in a suspension by the aggregation of minute particles. |
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Flood |
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A temporary rise in water levels resulting in inundation of areas not normally covered by water. May be expressed in terms of probability of exceedance per year such as 1-percent chance flood or expressed as a fraction of the probable maximum flood or other reference flood. |
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Flood control capacity |
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Reservoir capacity assigned to the sole purpose of regulating flood inflows to reduce flood damage downstream. See exclusive flood control capacity. |
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Flood control pool |
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Reservoir volume above active conservation capacity and joint use capacity that is reserved for flood runoff and then evacuated as soon as possible to keep that volume in readiness for the next flood. |
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Flood plain |
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Nearly level land, susceptible to floods, that forms the bottom of a valley. |
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Flood pool index |
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Computed as the ratio of the flood control pool depth to the depth below the pool, multiplied by the percent of time the reservoir water surface will be within the flood control pool. |
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Flora |
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All plant life associated with a given habitat, country, or period. Bacteria are considered flora. |
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Flow |
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Volume of water that passes a given point within a given period of time. See base flow, discharge, enhancement flow, interstitial flow, minimum flow, peak flow, and return flow. |
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Flow augmentation |
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The release of water stored in a reservoir or other impoundment to increase the natural flow of a stream. |
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Flowage |
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Water that floods onto adjacent land. |
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Flowage easement |
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The right or easement to overflow, submerge, or flood certain lands; a right to prohibit building on certain floodways |
