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Probability Flows for Streams in Eastern WA

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WA426 - Development of Equations for Determining 10 Percent Exceedence Probability Flows for Eastern Washington Streams - Completed FY2000

Problem - Washington Administrative Code (WAC) 220-110-070 requires that all water-crossing structures (bridges and culverts) be designed to facilitate fish passage. Culverts must be designed such that maximum flow velocities in them are not exceeded. Maximum flow velocities are to be determined using the 10 percent exceedence probability flow (the flow that is equalled or exceeded only 10 percent of the time), hereafter referred to as the Q10 flow, during the month(s) of adult fish migration. The Washington State Department of Natural Resources (DNR), which maintains culverts at over 20,000 road crossings in eastern Washington, needs to know the Q10 flow at many of these sites during January, the primary month of adult fish migration according to DNR, to determine whether maximum flow velocities are being exceeded.

WAC 220-110-070 allows the use of the 2-year peak discharge, a flow statistic for which estimating equations have already been derived, in place of the Q10 flow if the latter values are not available. DNR, however, would prefer to use Q10 flows, because 2-year peak discharges are frequently 3 or 4 times greater than Q10 flows.

Objectives - Develop equations from which Q10 flows for January, the primary month of adult fish migration, can be determined for ungaged sites on eastern Washington streams.

Relevance and Benefits - Fish passage design flow equations for eastern Washington streams are needed by all levels of government that build and maintain stream culverts to ensure that their current and future culvert designs are in compliance with Washington Administrative Code (WAC) 220-110-070, which sets the maximum flow velocities that may occur in culverts during the passage of design flows. The primary agencies involved with the maintenance and design of culverts are transportation agencies, such as The Washington State Department of Transportation, and federal agencies, such as the U.S. Forest Service and BLM. Compliance with the WAC helps ensure the free and unimpeded passage of fish, especially salmon, through culverts. This study also furthers the understanding of the characteristics of streamflow in eastern Washington, especially in small streams.

Approach - Derive equations using regression analyses for one or more eastern Washington regions using discharge data and basin characteristics for eastern Washington gaging stations on unregulated streams. The Generalized Least Squares regression method will be used unless initial evaluations of the data indicate that some other method of regression analysis would be more appropriate. There are approximately 180 station records for gages on unregulated eastern Washington streams. About half of the records are from continuous record gages for which Q10 flows will be calculated from January daily mean discharge data by the use of flow duration curves as described by Searcy (1959). The other half of the records, which are from crest stage gage (CSG) sites, contain only annual peak discharge data. One or more regression equations will be developed for the continuous record sites that relate the 2-year peak discharges to the Q10 flows, and those equations will be used to estimate Q10 flows corresponding to 2-year peak discharges at the CSG sites. Nearly all of the CSG sites have small drainage basins while most of the continuous record gage sites have medium to large drainage basins. (See plot of data used for one of the six eastern Washington regions in the 1998 flood frequency study.) Because the equation(s) to be developed in this study will be used primarily to estimate Q10 flows for small basins, it is of critical importance that the records from the CSG sites be included in the analyses.

The only basin characteristics found to be statistically significant in developing peak discharge equations in the recently completed flood frequency report were drainage area and mean annual precipitation. Therefore, because this study also involves the development of equations for a high flow streamflow statistic, only these two basin characteristics will be used as independent variables in the regression analyses.

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