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GLOSSARY -- - This appendix defines those terms frequently encountered in floodplain management. t.pproximate.- Prepared by approximating mathematical analysis for one or more of the followin(l steps; topo(lraphic mapping, hydrologic calculations. and hydraulic calculations; arriving at floodplain delineations without precIse water surface profiles. tLreal u Pertaining to thE! land area covered, as rn88surHd in square miles or c,ther units of area, Backwater - Step F'roce,jure -- A mathematical methodology for calculating a water surface profile over a given stream reach by starting at the cownstream end of the reach and, through incmmental steps, balancing energy losses at upstream points to arrive at water sur1ace elevations at t10se pOints until the entire stream reach has been analyzed. Backwater Effect -.. The rise in water surface elevation caused by an obstruc- t on such as a building, fill material, or bridge that limits trle area through which water must flow. Basin -- The total land area from which surface run-off is transported away t,ya drainage system. Also known as a "waterslled." Benchmark -- A U.S. Coast and Geodetic Survey (National Geodetic Survey) E,levation mark established on some stationary object and used as a vertical reference pOint in other surveys. Breach u A gap or rift in a solid structure such as a dam or a levee. Channel.- The bee! of a stream or river. Channelization -- Improvement of flow characteristics or water carrying capacity of an artificial or natural channel by excavation, bank stabilization, clearing, or other means. Channel Capacity-.. The maximum flow which can pass through a channel without overflowin(l the banks. Contour Interval n The difference in elevation between acljacent contour lines on a topographic map, usually 1 foot, 2 fe"t, 5 feet. or some multiple of 10 feet. Constriction" A place or feature that makes th" channel and/or floodplain smaller or narrower, thereby reducing its ability to pass flow. Control Marker..- A U,S. Coast and Geodetic Survey (National Geodetic Sur- vey) position mark established on some stationary object and used as a ~orizontal reference point in other surveys. Corrective.. (see Structura0 Critical Depth -- That depth of flow for a given discharge of water, where the specific energy (energy per unit weight of water) is at a minimum. Cross Section -- A plot or graph of ground elevation across a stream valley or ~,ortion of it, along a line perpendicular to the stream or direction of flow. Cubic Feet Per Second (CFS) -- A unit of measurement that describes the amount of flow passing a given point in a stream channel at a given point in t'me. One cubic foot per second is equivalent to approximately 7,5 gallons j:.er second. Cyclical.. Recurring in a regularly repeated timl9 interval. Dam -. A barrier constructed across a waterway to control the flow or raise t'1e level of wate... Dam Failure Flood Zone.. That area downstream of a dam that would be sub- j'lct to flooding by the waters stored behind the dam if the dam were to fail. Datum.. An assumed. given, measured, or othelwise determined point, line, c,r surface used as a reference in surveying or mapping. Delineating.. (See Fioodplain Delineation.) Designation -- Approval and adoption by official action of a local governing body of the delineation of an area subject to flooding by a 1 OO'year flood; fo' which water surface elevations have been established by a detailed engineering study that has been reviewed and approved by an official action cf the Colorado Wahlr Conservation Board, as required by State Statutes, 53