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floodplain - A nearly flat, alluvial lowland bordering a stream that is subject to inundation by floods. Flow-Control Structure - A structure either within or outsid" a channel that acts -as a countermeasure by controlling the direction, depth, or velocity uf flowing water. flow-Duration Chart - A graph indicating the percentage of time during which a qiven discharge is exceeded. Flow HClzard - ~ low. chClracteristics (dischClrge, 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 uf a countermeasure. Flow Slide - Saturation of a bank to the point where the soil materiClI behaves more like Cl liquid than a solid; the soil/water mixture may then move downslope, resulting in a bank failure. Freeboard - 1 he vertical distance above a design starJe that is allowed for waves, surges, drift, and other contingencies. Froude Number - A dimensionless number (expressed at V/,r gy) that re- presents the ratio of inertial to gravitational forces. High F roude numbers can be indicative of high flow velocity and scour potential. Gabion - A basket or compartmented rectangular container made of steel wire mesh. When filled with cobbles or other rock of suitable size, the qabion becomes a flexible and permeable block with which flow-control structures can be built. General Scour - Scour in a channel or on a floodplain that is not localized at a pier, abutment, or other obstruction to flow. In a channel, general scour usually affects all or most of the channel width. Geomorpholoqy - 1 hat branch of both physiography and geology that deals with the form of the earth, the general configuration of its surface, and the changes that take place due to erosion of the primary elements and in the buildup of erosional debris. Grade-Control Structure (sill, check dam) - Structure placed bank to bank across a stream channel (usually with its central axis perpendicular to flow) for the purpose of controlling bed slope and preventing scour or head -cutting. Gravel - Particles, usually of rock, whose diameter is between 2 and 64 mm. The term gravel is also applied to a mixture of sizes (gravel with sand or gravel with cobbles) in which the dominant or modal fraction is the gravel size range. Groin - Embankment or wall constructed more or less square to a stream bank or shoreline (also 'spur'). Grout - A fluid mixture of cement and water or of cement, sand, and water used to fill joints and voids. xlvi I J I I I . I I I .- I I I I I I I -. I