filter:
Layer of fabric (geotextile) or granular material (sand, gravel,
or graded rock) placed between bank revetment (or bed
protection) and soil for the following purposes: (1) to prevent
the soil from moving through the revetment by piping,
extrusion, or erosion; (2) to prevent the revetment from sinking
into the soil; and (3) to permit natural seepage from the
streambank, thus preventing the buildup of excessive
hydrostatic pressure.
filter blanket:
A layer of graded sand and gravel laid between fine-grained
material and riprap to serve as a filter.
filter fabric (cloth):
Geosynthetic fabric that serves the same purpose as a
granular filter blanket.
fine sediment load:
That part of the total sediment load that is composed of particle
sizes finer than those represented in the bed (wash load).
Normally, the fine-sediment load is finer than 0.062 mm for
sand-bed channels. Silts, clays and sand could be considered
wash load in coarse gravel and cobble-bed channels.
flanking:
Erosion around the landward end of a stream stabilization
countermeasure.
flashy stream:
Stream characterized by rapidly rising and falling stages, as
indicated by a sharply peaked hydrograph. Typically
associated with mountain streams or highly disturbed
urbanized catchments. Most flashy streams are ephemeral,
but some are perennial.
flood-frequency curve:
A graph indicating the probability that the annual flood
discharge will exceed a given magnitude, or the recurrence
interval corresponding to a given magnitude.
floodplain:
A nearly flat, alluvial lowland bordering a stream, that is subject
to frequent inundation by floods.
flow-control structure:
A structure either within or outside a channel that acts as a
countermeasure by controlling the direction, depth, or velocity
of flowing water.
flow hazard:
Flow characteristics (discharge, stage, velocity, or duration)
that are associated with a hydraulic problem or that can
reasonably be considered of sufficient magnitude to cause a
hydraulic problem or to test the effectiveness of a
countermeasure.
flow slide:
Saturated soil materials which behave more like a liquid than a
solid. A flow slide on a channel bank can result in a bank
failure.
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