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<br />1 <br /> <br />I ., . <br /> <br />STATE OF COLORADO <br /> <br />COLORADO WATER CONSERVATION BOARD <br /> <br />. <br /> <br />Department of Natural Resources <br /> <br />721 Stale Centennial Building <br />1313 Sherman Street <br />Denver. Colorado 80203 <br />Phone {303) 666-3441 <br />FAX (303) 666-4474 <br /> <br />MEMORANDUM <br /> <br />Roy Romer <br />Governor <br /> <br />DanesC, Ue. P.E. <br />Diredor. cwes <br /> <br />TO: Donnette Oxley and Bob Jarrett <br />FROM: '. Brian Hyde 0f< I;-( <br /> <br />DATE: August 18, 1993 <br /> <br />SUBJECf: Hydrologic Analysis for Upper Dear Creek in Dear Creek County <br /> <br />I'm writing this memo in order to keep the hydrologic analysis for "Upper Dear <br />Creek" (Clear Creek from Idaho Springs upstream) from falling into a black hole. It's my <br />attempt at taking the next logical step after Bob's ~etter to me dated March 16, 1993 and <br />the meeting the three of us held in Breckenridge on June 30, 1993. fve enclosed a table <br />that can be considered a draft of our "homework assignments". Performing those homework <br />assignments will take us a long way toward completing the project. <br /> <br />FIRST THINGS FIRST <br /> <br />The first thing I'd like to do is to come to an agreement on a starting point for the <br />hydrology. Although I think the question of whether rainfall can be the primary cause of <br />flooding on stream reaches in the "Upper Foothills" portion of the Front Range (Le. do we <br />have rainfall floods above 7500 feet?) is extremely important, I do not want. to use this <br />particular study to try to answer it. So for now I'll move on. As you know, the Corps of <br />Engineers separated annual peak flows at the Lawson gage (snowmelt vs. rainfall) and <br />determined 2-year, 5-year, 10-year, 50-year, 100-year and 500-year peak flows for snowmelt <br />and rainfall flooding. They then combined those two probabilities into a composite <br />discharge frequency relationship. Interestingly enough, their gage analysis at Lawson showed <br />rainfall flood peaks to be smaller than snowmelt flood peaks. So to some extent the <br />question of significant rainfall in the "Upper Foothills" becomes moot for our specific <br />geographic area of interest. <br /> <br />There is not a great difference between the Corps' lOO-year snowmelt flows at <br />Lawson and their 100-year composite (rainfall & snowmelt) flows. The difference is <br />approximately 12%. For that reason and because the rainfall flows are the smallest of the <br />three that the Corps computed, I prefer that we all agree on ~ of those three numbers for <br />Lawson as an anchor for the remainder of our work. I do not want to reject the COE <br />report completely and perform some new estimate for Lawson. Once we come to an <br />agreement at Lawson, I propose that we also use the selected discharge frequency <br />