Analyses of flood-flow frequency for selected gaging stations in South Dakota through September 1985

Abstract

Backwater. Water backed up or retarded in its course as compared with its normal or natural condition of flow. In stream gaging, a rise in stage produced by a temporary obstruction such as ice or weeds, or by the flooding of the stream downstream. The difference between the observed stage and that indicated by the stage-discharge relation, is reported as backwater. Flood-frequency curve. 1. A graph showing the number of times per year on the average, plotted as abscissa, that floods of magnitude, indicated by the ordinate, are equaled or exceeded. 2. A similar graph but with recurrence intervals of floods plotted as abscissa. Flood peak. The highest value of the stage or discharge attained by a flood; thus, peak stage or peak discharge. Gage height.-The water-surface elevation referred to some arbitrary gage datum. Gage height is often used interchangeably with the more general term stage although gage height is more appropriate when used with a reading on a gage. When added to the gage datum, gage height gives the water surface elevation in feet above National Geodetic Vertical Datum of 1929. Gaging station.-A particular site on a stream, canal, lake, or reservoir where observations of gage height or discharge are obtained. Partial-duration flood series (partial peaks).-A list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year. Recurrence interval (return period).-The average interval of time within which the given flood will be equaled or exceeded once. Regulation. The artificial manipulation of the flow of a stream. Stage.-The height of a water surface above an established datum plane; also gage height. Stream gaging.-The process measuring the depths, area, velocities, and rates of flow in natural or artificial channels. Stream-gaging station.-A gaging station where a record of discharge of a stream is obtained. Within the Geological Survey, this term is used only for those gaging stations where a continuous record of discharge is obtained. XI Water year.-In Geological Survey reports dealing with surfacewater supply, the 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months.