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<br />SIMULATION OF FOREST FIRES <br /> <br /> <br />A 50-square-mile area in the Idylwilde-Kinikinik area, about <br /> <br /> <br />midway between Spencer Heights and Rustic, was used to demonstrate the <br /> <br /> <br />adverse effects on runoff that could be caused by a forest fire. <br /> <br /> <br />Figure 9.6 in the SCS National Engineering Handbook, Section 4, <br /> <br /> <br />Hydrology, August 1972, indicates that a curve number of 60 for soil <br /> <br /> <br />group B is representative of a 45-percent ground cover density for a <br /> <br /> <br />juniper-grass complex. Reducing the ground cover to 25 percent raises <br /> <br /> <br />the curve number to 70. Ten-percent cover, the lowest that might be <br /> <br /> <br />expected in a burned forest, corresponds to a curve number of 77. <br /> <br />Curve numbers 70 and 77 were used in the 50-square-mile area while the <br /> <br /> <br />other 365 square miles of the basin remained at curve number 60. The <br /> <br /> <br />higher discharges caused by raising the curve numbers are shown as <br /> <br /> <br />profiles in figure 5. Since the plains area was not modeled, the <br /> <br /> <br />effects of a forest fire on discharge downstream from the North Fork <br /> <br /> <br />Cache la Poudre River are not shown. <br /> <br />. <br /> <br />CACHE LA POUDRE RIVER DOWNSTREAM FROM THE NORTH FORK CACHE LA POUDRE <br />RIVER <br /> <br />Synthetic unit hydrographs were developed for major subareas of <br /> <br /> <br />the plains portion of the basin. These unit hydrographs were based on <br /> <br /> <br />regional rplationships that were determined from unit graph studies of <br /> <br />other mountain and high plains tributaries in the South Platte River <br /> <br /> <br />basin. Snyder's unit hydrograph constants, derived from the regional <br /> <br /> <br />relationships, were used to develop un~t hydrographs for the subareas. <br /> <br /> <br />The I-percent flood or lOa-year flood hydrographs were developed by <br /> <br /> <br />using laO-year, I-hour rainfall values taken from U.S. Weather Bureau <br /> <br /> <br />Technical Publication No. 40. Th~ selected rainfall values were <br /> <br /> <br />reduced for size of area and for a loss rate of 0.60 inch per hour. <br /> <br /> <br />The resulting runoff value was applied to the respective unit hydro- <br /> <br /> <br />graph peak and adjusted for expected probability. The lOa-year flood <br /> <br /> <br />hydrographs were combined and routed along the Cache la Poudre River by <br /> <br />the modified PuIs S+Q/2 method. At selected locations, the routed <br /> <br />20 <br /> <br />. <br />