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41 -mountain shrub communities <br />Oak-aspen cover type, 70% ground cover/good hydrologic <br />condition, hydrologic soil group C <br />5. Two separate models were ran, one using a weighted average of curve numbers <br />for each watershed, the other designating each cover type as a subwatershed. <br />Null structures were placed at the location of ditch cross section measurements <br />taken on May 27, 1993. <br />The results of the modeling and comparisons are listed below. Detailed documentation of the <br />modeling inputs are attached. <br />X-Section 1 <br />Predicted flow <br />l0yr - 24hr 1.68 cfs <br />weighted CN <br />Predicted flow <br />l0yr -24hr 29.76 cfs <br />subwatersheds <br />Calculated flow <br />May 16, 1993 91 cfs <br />X-Section 2 X-Section 3 X-Section 4 <br />2.04 cfs 2.04 cfs 7.15 cfs <br />14.61 cfs 12.33 cfs 89.58 cfs <br />103 cfs 39 cfs 167 cfs <br />In addition to assisting in the determination of whether the May 16th storm was greater than <br />that which the ditch was designed for, this comparison also revealed a couple of other points <br />which I believe should be considered when Colowyo finalizes the South Collection Ditch and <br />Prospect Pond designs. First, we should find out the maximum area that will be stripped at <br />any given time, and the approximate stage of pit development which can be expected at that <br />time. This would enable us to more accurately establish the maximum runoff potential for <br />which the structures should be designed. Second, we should not utilize the practice of <br />combining watersheds using weighted curve number averages. As can be seen in the data <br />above, weighted averages alter the timing of peak discharge, resulting in a significant <br />difference in total discharge. The June 15, 1993 correspondence from Colowyo notes that the <br />ditch design is based on a maximum peak flow of 6.99 cfs. This discharge value is a result of <br />modeling with weighted curve number averages. I believe this method resulted in undersizing <br />of the ditch. Had the ditch been designed using an aggregate maximum peak flow from <br />separate watersheds, the ditch capacity would have been roughly 175 cfs (based on previous <br />CDMG model). <br />c: Janet Binns <br />SLM\070893.WP <br />rry Outten 2 u y , 1 3 <br />