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Flood Frequencies

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WA386 - Flood Frequencies in Washington - Completed FY1999

Problem - Water managers need to know how frequently floods of various sizes can be expected in order to advise on measures to avoid potential destruction. Typically, managers are most interested in large floods that may occur rarely but with enough frequency to be of real concern in the scale of human lifetimes.

Objectives - The objective of the study is to update the estimates of specified flood flows associated with various frequencies of recurrence, at selected gage sites, and to update regression equations for flood flows for ungaged sites. The new estimates will incorporate streamflow data that have been collected since the publication of the earlier studies.

Approach - The data base of flood peaks at gaged sites, upon which the estimates are made, will be updated to ensure that the most recent flood peaks are included. Standard procedures for calculating the flows at gaged sites to associate with given flood frequencies will be followed. These procedures are given in Bulletin 17B of the Hydrology Committee of the U.S. Water Resources Council, "Guidelines for Determining Flood Flow Frequency." The procedures have been incorporated into U.S. Geological Survey computer program J407, which will be used for the computations of this study. To extend flood-frequency information to ungaged sites, regression equations will be developed that estimate flood flows from basin area, incident precipitation, and percent forest cover.

WA166 - Magnitude and Frequency of Floods In Washington - Completed FY1974

Problem - The need for data to aid in the proper design of structures and installations in or along stream channels within the State has been critical for many years. The flood potential at any site, whether gaged or ungaged must be defined. Water Supply Papers 1687, 1688, and 1689, magnitude and frequency of floods in parts 12, 13, and 14, and the Open-File Report, floods in Washington, magnitude and frequency, 1960, by Bodhaine and Thomas, partially answered this need. However, better definition of flood potential for small drainage areas is needed, and is a recognized deficiency in the above reports. In addition, new techniques have been developed for regionalizing flood data, allowing better interpretation of available data.

Objectives - Provide data on flood magnitude and frequency, particularly in the small streams in Washington, and provide formulation to allow the determination of these data at any point on a given stream where selected data inputs are available.

Approach - Streamflow data from all gaging station and crest-stage gages will be used in the evaluation, unless the stream has significant regulation. Station flood-frequency will be determined by log-pearson type III techniques, as recommended by the Water Resources Council. The peak discharge at various recurrence intervals will be regionalized using standard multiple regression method as described in techniques of Water Resources Investigations, regional analysis of streamflow characteristics, utilizing basin and climatic parameters such as precipitation, stream slope, forest cover, basin elevation, etc.

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