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<br />I <br />. <br />. <br />I <br />I <br />I <br />. <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />i' <br />I <br /> <br />EV ALUA nON OF RUNOFF CURVE NUMBERS (CN) FOR NORTH CLEAR <br />CREEK, GREGORY GULCH, AND CHASE GULCH <br /> <br />The reconstruction ofthe TR-20 evaluation of the three watersheds requires that the CN's <br />be reestablished if possible. The information available does not explicitly show the CN <br />values for the watersheds, but sufficient information is shown on the hydrographs to <br />estimate the CN values at the watershed outlets, giving the weighted CN's for the three <br />tributaries at the reference location of the hydrographs. <br /> <br />North Clear Creek <br /> <br />The 1980 Black Hawk Floodplain Information Technical Addendum (BFIT A) by Owens <br />Ayers, page 2-9, shows the hyetograph and triangular hydrographs for the North Clear <br />Creek Watershed. The information also shows the Creek was evaluated for a 100-year <br />24-hour storm. The drainage area is shown as 33.88 square miles with a I DO-year peak Q <br />of 2,640 cfs occurring at 11.5 hours from the beginning of the storm event. The <br />hydrograph shows runoff occurring at 4 hours after the beginning of the rainfall. The <br />hyetograph shows that 0.5" of rainfall has occurred at 4 hours. This is the Initial <br />Abstraction (Ia). <br /> <br />The USDA National Engineering Handbook Section 4 (NEH 4) equation 10.9 gives the <br />empirical relationship between Ia and the variable S, a function of the total retention (the <br />rainfall that does not become runoff due to interception and infiltration). In turn, S is <br />directly related to the runoff curve number (CN) by equation 10.11 ofNEH 4. <br /> <br />Ia = 0.2 8 <br />CN = 1000 / (8+ 10) <br /> <br />(10.9) <br />(10.11) <br /> <br />For Ia = 0.5, 8 = 2.5, and the related curve number is 80. <br /> <br />For comparison, we computed the CN by taking the BFITA qp = 484*A*Q/Tp, and <br />computing Q, then 8, and CN for each watershed. For North Clear Creek the CN from <br />BFITA qp values is 79. These values are close, and support the anecdotal evidence that a <br />CN value of79 was used. <br /> <br />Chase Gulch <br /> <br />The BFIT A, page 2-8 shows the hyetograph and triangular hydro graphs for the Chase <br />Tributary. The information also shows that the creek was evaluated for a 100-year 6- <br />hour storm. BFITA page 2-3 shows the I DO-year peak runoff value of 1,160 cfs, which is <br />consistent with the hydrograph shown on page 2-8. The drainage area is shown as 4.17 <br />square miles with the peak occurring about 3.95 hours from the beginning of the storm <br />event. The hydrograph shows runoff occurring 3 hours after the beginning of the rainfall. <br />The hyetograph shows that 0.4" of rainfall has occurred at 3 hours. This is the Initial <br />Abstraction (Ia). <br />