<br />VIII Contents
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<br />Flood plain: A flood'prone area, as identified on Federal Emergency Management
<br />Agency (FEMA) flood insurance rate maps, generally contains a floodway district
<br />and flood way fringe district, or contains areas of land adjoining (or near) the channel
<br />of a water course which has been, or may be, covered by floodwaters. A flood plain
<br />functions as a temporary channel or reservoir for overbank flows, The lowland that
<br />borders a river, usually dry but subject to flooding (Hoyt and Langbein, 1955, p, ] 2),
<br />Flow regime: A range of flows producing similar bed forms, resistance to flow, and mode
<br />of sediment transport. The lower flow regime occurs with tranquil flow and produces
<br />bed forms of ripples, ripples on dunes, or dunes, The upper flow regime produces bed
<br />forms of plane bed with sediment moving, standing waves, or antidunes. Water-
<br />surface undulations are generally in phase with bed undulations. Between these two
<br />stable regimes is the transition regime, which produces instability in the stage-to-
<br />discharge relation and in the typical bed forms,
<br />Froude number: A dimensionless number used as an index to characterize the type of
<br />flow (subcritical, critical, and supercritical) in an open channeL The Froude number
<br />is the ratio of the inertial forces to the gravitational forces, and is computed as the
<br />mean flow velocity divided by the square root of the product of the mean depth times
<br />the acceleration of gravity.
<br />Grain size, coarse and fine: Coarse-grained bed material generally refers to those
<br />particles (gravel, cobble, boulder) whose size can be individually measured with a
<br />graduated rule or caliper; fine, grained material (sand, silt, clay) is measured by
<br />passage through a sieve or by rate of sedimentation. See also particle size.
<br />High-water marks: Evidence of the highest stage reached by flow, High'water marks
<br />generally consist of debris and scour marks found along the channel boundaries,
<br />Hydraulic radius: The ratio of the stream channel's cross-sectional area to its wetted
<br />perimeter in a plane normal to the direction of flow.
<br />Hydrograph: The functional relation between time and flow discharge, as observed at a
<br />particular point within a drainage basin. In the case of a detention and (or) retention
<br />facility, an inflow hydrograph depicts the relation of time and runoff inflow to the
<br />facility, and an outflow hydrograph is a graph of flow discharge from the facility
<br />compared to time,
<br />Intermittent stream: A stream that flows only at certain times of the year when it
<br />receives water from some surface source (rainfall or snowmelt) or from an
<br />intermittent spring and ceases to flow during other periods of the year, The channel is
<br />usually above the water table,
<br />Manning's roughness coefficient (n value): A measure of the frictional resistance
<br />exerted by a channel on the flow. The n value also can reflect other energy losses such
<br />as those resulting from unsteady flow, extreme turbulence, and transport of
<br />suspended material and debris, that are difficult or impossible to isolate and quantify,
<br />Particle-size: The size of material on the bed of a stream, referenced to a specific diameter
<br />(either maximum, intermediate, or minimum) of the measured particles.
<br />Perennial stream: A stream that discharges continuously all year during dryas well as
<br />wet years.
<br />Relative roughness: Relative roughness is the ratio of mean depth (usually represented by
<br />hydraulic radius, R) to the size of roughness elements (usually represented by the
<br />median value of the intermediate diameter of the streambed material, dso),
<br />Runoff: The portion of precipitation on land that ultimately reaches streams-especially
<br />water from rain or melted snow that flows over the land surface.
<br />Scour: Erosion due to flowing water, usually considered as being localized as opposed to
<br />general bed degradation,
<br />Slope, water-surface: The slope of the water surface, computed as the change in
<br />elevation per unit change in the channel's length.
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