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. . . GLOSSARY acre-foot -- A volume of water covering 1 acre to a depth of 1 foot; 43,560 cubic feet. cross section -- A representation of the area between a streambed and an imaginary level string stretched across a stream at right angles to the streamflow. The size and shape of the cross section is determined by using surveying Instruments to measure distances down to the streambed at various locations along the imaginary string. Floods usually will change the cross-section configuration unless the streambed is composed of stable material. cubic foot per second.-- A unit of discharge passing a cross section of a stream. The unit of discharge represents 1 cubic foot of water passing a given point in 1 second and is equivalent to approximately 7 Y, gallons per second or 450 gallons per minute. discharge.-- The water flowing past a particular point, such as a pOint at a cross section. The rate of flow generally Is measured in cubic feet per second: the volume of flow, In acre-feet. drainage area. -- A land area. usually measured In square miles in a horizontal plane, which collects precipitation and allows the resultant runoff to flow towards the most downstream point in the area. envelope curve .-- A line drawn above plotted points on a graph to enclose the largest floods ever measured In a region; the points show stream discharges for given drainage areas. Larger floods are possible; if they occur, the envelope curve is moved upward to enclose the new, higher values. flood plain.-- An area that is adjacent to the streambed and is covered with water when the stream overflows its banks during floods. flood-prone area map.-- A map showing areas along the streambanks which are subject to flooding. Areas expected to be covered by water at least once in any 1 DO-year period are shown on the Elbert County flood-prone area maps. flood velocity.-- Speed with which water moves downstream. A velocity of 10 feet per second (nearly 7 miles per hour) would move an object floating In the center of the stream one township (6 miles) farther downstream in less than 1 hour. floodwater-retarding structure.-- A dam placed across a small upstream water course to provide floodwater storage, Earthen dams constructed In Elbert County by the U.S. Sol] Conservation Service in the 1950's were designed to hold about 1 inch of runoff from the basin above and to release it gradually through pipes over a period of 2 to 3 days, Excess runoff passes over the emergency spillways at one side of the dams. The structures delay stormwater traveling towards the larger creek channels and help reduce consequent flooding. hundred-year (1 DO-year) flood.-- Peak discharges so large that they may be expected to be equaled or exceeded on the long-term average of about once every 100 years and have a 1-percent chance of occurring in any given year. The recurrence interval is an average; no schedule of regularity is implied. National Geodetic Vertical Datum of 1929 (NGVD of 1929): A geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called "Mean Sea Level." NGVD of 1929 is referred to as sea level in this report. runoff.-- That part of the precipitation that appears in surface streams. One inch of runoff is equal to about 27 cubic feet per second of water flowing for one day from each square mile of drainage area above the location considered. stream slope.-- The vertical distance a stream drops in traveling a given horizontal distance. When the stream falls more than about 50 feet per mile, floodflows are shallow and very fast and may cause considerable channel erosion. As the slope flattens to less than about 50 feet per mile (a 1-foot drop about every 100 feet), floodflows become relatively deep and spread out as the water velocity slows. topographic map.-- A map that shows surface features of a given area. 10