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Mark Mastin,
934 Broadway,
Suite 300
Tacoma, WA 98402

(253) 552-1609
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South Prairie Creek

Project Summaries

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WA391 - Flood Potential of South Prairie Creek, Pierce County, Washington - Completed FY1999

Problem - The lower portion of South Prairie Creek, which is located near Tacoma, Washington, has experienced floods in recent years that have overtopped the stream banks and inundated large portions of the valley. Residents of the area have claimed that increased logging and development in the upper portions of the basin have caused increased floods. The Pierce County Surface Water Management Utility is responsible for alleviating the impact of flooding caused by changes in land use. The Utility is interested in determining if there has been a change in the flooding potential in the basin and estimating the discharge and aerial extent of flooding in the lower portion of the basin that can be expected to occur under current conditions. Also, a particular area of streambank near the Spring Site Road has been identified by the Pierce County Storm Drainage Master Plan as a site in need of capital improvements in order to reduce erosion and overbank flooding. Several design options have been suggested, but the impacts of the various options on flooding are unknown.

Objectives - The objectives of this project are to compute the changes in acreage of development and clearcutting within the watershed and determine if any concurrent trend in the flooding potential of the basin exists. Based on the trend that is determined, estimates of flood discharges for various exceedence probabilities will be made and new flood profiles for lower South Prairie Creek will be developed for the present channel geometry and the altered geometry that would occur with the proposed set-back levee.

Approach - The relative change in land use over time will be compiled from photo mosaics that will be constructed for the South Prairie Basin from two to three available sets of aerial photos. The areas of development and clearcutting and lengths of roads will be delineated on an overlay and their acreage and lengths computed. The information will be tallied to provide a means of comparison of the change in land use from one to two or three points in time over the last two to three decades.

Several statistical tests will be performed to determine the trend, if any, that exists in the annual time series of peak discharges. A regression analysis using various climatological or hydrologic parameters will be made to develop a strong relationship between them and peak streamflows. Once these exogenous effects are removed, the trend in the data, if it exists, will be defined and the flood frequency for current conditions in the basin will be computed. A log-Pierson Type III analysis will be performed to compute the estimated discharges for various exceedence probabilities.

The estimated floods for various exceedence probabilities will be used for the flood profile analysis of the lower valley of South Prairie Creek. A standard step-backwater method of computing water-surface profiles will be performed for various floods. The water-surface profile computation model (WSPRO), a model developed by the U.S. Geological Survey, will be used to make the computations. The areas inundated by the estimated floods as computed by the step-backwater analysis will be delineated on a map. The average stream velocities will be shown at the cross-section locations. This procedure will be repeated for the altered channel geometry for the various design options supplied by the county.

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