WSP04469 - Laserfiche WebLink

Home Browse Search

~ . ~'t .. ).' .,.. ~, ~~~> . ".~.~r::< .,1 1720 I. FEe:! (} APPLICATION OF THE FOREST SERVICE CHANNEL MAINTENANCE FLOW PROCEDURE 1)' !!1! [ - r :A~R. ~:9198S 1/ James D. Schall and Peter F. Lagasse- Co;L(~" -', \ . c: = __ - ,,' The U. S. Forest Service has recently adopted nationwide a channel maintenance flow procedure (see Chapter 30 of the Water Information Manage- ment System Handbook. FSH 2509.17). As described. the channel maintenance flow procedure is intended to quantify ". . . those streamflows necessary to provide for the self-maintenance of river. stream. or creek channels so as to retain their capability for passing flood-flow discharges" (Section 30.2. FSH 2509.17). A basic concept in the procedure is that natural channels are self-formed and self-maintained and have developed in s predictable manner as a result of 'upstream water and sediment loading. Two primary concerns addressed by the procedure are (1): .maintaining sediment transport capacity. to prevent aggradation. and (2) minimizing vegetation encroachment. . INTRODUCTION ", The Rock Creek Reservoir proj ect is one of two alternatives being considered as compensatory storage for the Windy Gap project. Rock Creek is tributary to the Colorado River near McCoy, Colorado. Preparation of the environmental impact statement for the Rock Creek project required applicaticn of the channel maintenance flo'" procedure since the proposed reservoir sites are located "'ithin the Routt National Forest boundary (see Fig. 1). APPLICATION OF CHAPTER 30 to ROCK CREEK The first step in the Chapter 30 procedure is determination of bank- full discharge at the point of quantification. The point of quantification is that location.(or reach) ",here prevention of aggradation or vegetation encroachment is judged to he most critical to maintaining channel capacity. For the Rock Creek proj ect. two alternative reservoir sites are under - consideration. The upper site (Site A) is located just belo'" Horse Creek "'hi1e the lo",er site (Site B) is near the entrance of a steep canyon about 4.000 feet belo", Site A. Based on the USFS stream classification proce- dure. the reach belo", Site A is a C3 reach ",hile the reach belo", Site B is a Bl reach. A C3 reach is a relatively mi~d sloped. gravel bed channel of moderately high sinuosity ",hile a Bl reach is a steep. relatively straight channel composed of large cobbles to small boulders. Fig, 2 is a plot of the Rock Creek bed profile illustrating the differences in channel slope downstream of the t"'o alternative sites. Evaluation of bankfull discharge was based on field observations and measurements. data from the USGS gage Rock Creek near Toponas (Station 09060500), and from previous analysis on Rock Creek completed by (8), Results indicated that a reasonable bankfull discharge below both sites (A and B) was approximately 190 cfs. lJ Water Resource Engineer and President. respectively. Resource Consul- tants. Inc.. P. O. Box Q. Fort Collins. CO.