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I I I I I I I I I I I I I I I I I I I -11- GLOSSARY Flood Frequency, (See Probabil ity.) The average recurrence i nterva 1 of specific discharges or water stages which cause flooding. Backwater Effect. The rise in surface elevation of flowing water upstream from and as a result of an obstruction to flow. Flood Peak. The maximum instantaneous discharge of a flood at a given loca- tion. It usually occurs at or near the time of the flood crest. Channel. definite flowing A natural or artificial watercourse of perceptible bed and banks to confine and conduct continuously or water. extent with periodically 100-Year Floodplain. An area in and adjacent to a stream which is subject to flooding as the result of the occurrence of a lOO-year flood and thus is so adverse to past, current, or foreseeable construction or land use as to constitute a significant hazard to public health and safety or to property. Flood. An overflow on lands not normally covered by water and that are used or usable by man, Floods have two essential characteristics; the inundation of land is temporary; and the land is adj acent to and inundated by overflow from a river, stream, ocean, lake, or other body of standing water. Normally a flood is considered as any temporary rise in streamflow or stage, but not the ponding of surface water, that results in significant adverse effects in the vicinity, Adverse effects may include damages from overflow of land areas, temporary backwater effects in sewers and local drainage channels creation of unsanitary conditions or other unfavorable situations , by deposition of materials in stream channels during flood recessions, use of groundwater coincident with increased streamflow, and other problems. Flood Profile, A graph showing the relationship of water surface elevation to location, the latter generally expressed as distance above mouth for a stream of water flowing in an open channel. It is generally drawn to show surface elevation for the crest of a specific flood, but may be prepared for conditions at a given time or stage. Flood, IOO-Year. A type of flood, including the water surface elevation and territorial occupation thereof, which can be expected to occur at any time in a given area based upon recorded historical precipitation and other valid data, but with an average statistical one percent chance of being equaled or exceeded during anyone year, The term is used interchangeably with a one percent flood or Intermediate Regional Flood. Floodway. The channel of a stream and those portions of the adjoining floodplain which are reasonably required to pass a lOO-year flood without raising the flood profile more than an acceptable amount. It is that por- tion of the floodplain most hazardous to personal safety and welfare. In this study, the floodway defines a flow area required to convey the 100-year flood without increasing the lOO-year water surface elevation by more than ten feet. Left Bank. The bank on the left side of a river, stream or water course as the observer looks downstream, Flood Crest. The maximum stage or elevation reached by the waters of a flood at a given location. Probability, The annual chance of occurrence of specific hydrologic events, such as rainfall over a specified area or peak discharge at a specified location expressed in percent, e,g., 5 percent representing one chance in 20 of the event occurring in any year or an average recurrence of once in twen- ty years.