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<br />I <br /> <br />Flood frequency-A statistical expression of the average time <br />period between floods equaling or exceeding a given <br />magnitude. For example, a 100-year flood has a magnitude <br />expected to be equaled or exceeded on the average of once <br />every 100 years; such a flood has a I-percent chance of <br />being equaled or exceeded in any given year. Often used <br />interchangeably with "recurrence interval." <br /> <br />Flood plain-The relatively flat area or lowlands adjoining the <br />channel of a river, stream, or watercourse or an ocean, lake, <br />or other body of standing water that has been or may be <br />covered by floodwater. <br /> <br />Flood proofing-Any combination of structural and <br />nonstructural additions, changes, or adjustments to <br />properties and structures which reduce or eliminate flood <br />damage to lands, water and sanitary facilities, structures, <br />and contents of buildings. <br /> <br />Flood stage-The stage or elevation at which overflow of the <br />natural banks of a stream or body of water begins in the <br />reach or area in which the elevation is measured. <br /> <br />Flood warning-The issuance and dissemination of information <br />about an imminent or current flood. <br /> <br />Freeboard-A factor of safety expressed in feet above a design <br />flood level for flood protective or control works. Freeboard <br />is intended to compensate for the unknown factor which <br />could increase design heights, such as wave action, <br />flood way obstruction, or future changes in the watershed. <br /> <br />Hydrodynamic loads-Forces imposed on structures by <br />floodwaters due to the impact of moving water on the <br />upstream side of the structure, drag along its sides, and <br />eddies or negative pressures on its downstream side. <br /> <br />Hydrostatic loads-Those loads or pressures resulting from the <br />static mass of water at any point of floodwater contact with <br />a structure. They are equal in all directions and always act <br />perpendicular to the surface on which they are applied. <br />Hydrostatic loads can act vertically on structural members <br />such as floors, decks, and roofs and can act laterallv on <br />upright structural members such as walls, piers, and <br />foundations. <br /> <br />12 <br />