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

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. Base discharge (for peak discharge). In the Geological Survey's annual reports on surface-water supply, the discharge above which peak discharge data are published. The base discharge at each station is selected so that an average of about three peaks a year will be presented. Flood-frequency curve. 1. A graph showing the number of tiroes 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 NGVD. 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. Stage-discharge curve (rating curve). A graph showing the relation between the gage height, usually plotted as ordinate, and the amount of water flowng in a channel, expressed as volume per unit of time, plotted or abscissa. Stage-discharge relation. The relation expressed by the stage-discharge curve. Stream gaging. The process measuring the depths, areas, velocities, and rates of flow in natural or artificial channels. vi 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.