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<br />Flood Stage. The stage or elevation at which overflow of the natural <br /> <br /> <br />banks of a stream or body of water begins in the reach or area in <br /> <br /> <br />which the elevation is measured. <br /> <br />Floodway. The minimum area of the flood plain that is required to <br /> <br /> <br />convey a flood peak of a selected magnitude. This usually consists <br /> <br /> <br />of the most hazardous area of the flood plain where water veloci- <br /> <br /> <br />ties are significant. Areas on the landward side of a floodway <br /> <br /> <br />normally convey little floodflow, although they are inundated during <br /> <br /> <br />floods. <br /> <br />Intermediate Regional Flood. A flood having an average frequency <br /> <br /> <br />of occurrence in the order of once in 100 years although the flood <br /> <br /> <br />may occur in any year. It is based on statistical analyses of <br /> <br /> <br />rainfall and funoff characteristics in the general region of the <br /> <br /> <br />watershed. <br /> <br />Low Steel or Underclearance. The lowest point of a bridge or other <br /> <br /> <br />structure over or across a river, stream, or watercourse that limits <br /> <br />the opening through which water flows. <br /> <br />Sheet Flow. A flow of water that has shallow depths, unpredictable <br /> <br /> <br />flow paths, and the areal distribution of the flooding is non-uniform. <br /> <br />Standard Project Flood. The flood that may be expected from the <br /> <br /> <br />most severe combination of meteorological and hydrological condi- <br /> <br /> <br />tions that is considered reasonably characteristic of the geograph- <br /> <br /> <br />ical area in which the drainage basin is located, excluding <br /> <br /> <br />extremely rare combinations. Peak discharges for these floods are <br /> <br /> <br />generally about 40% to 609 of the probable maximum floods for the <br /> <br /> <br />same basins. such floods, as used by the Corps of Engineers, are <br /> <br /> <br />intended as practicable expressions of the degree of protection <br /> <br /> <br />that should be sought in the design of flood control works, the <br /> <br /> <br />failure of which might be disastrous. <br /> <br />App. 4 <br />32 <br />