Laserfiche WebLink
<br />Flood Insurance Report is (7): <br /> <br />"The soils in the flood plains are moderately coarse-textured gravelly and cobbly <br />alluvium. Soils on the south-facing exposures of 40- to 70-percent slope are <br />characterized by many outcroppings of feldspars and granite. Soils on north - <br />facing exposures of 35- to 60-percent slope are deep and well drained, medium <br />textured, and are developing from the underlying feldspars and granite." <br /> <br />Figure 3 shows the Gregory Gulch catchment. Peak flow values for the drainage in <br />Black Hawk and the drainage in Central City are nearly identical since both cities share <br />the same catchment. The only difference in drainage areas is the exclusion by Central <br />City of the lower 1500 ft of the Gregory Gulch catchment. Black Hawk drainage <br />includes the entire catchment. The Black Hawk drainage (Gregory Gulch catchment) <br />is approximately 22,000 ft long, so the exclusion is small in comparison. <br /> <br />III. EXISTING INFORMATION <br /> <br />The results of a search for reports pertinent to the Gregory Gulch catchment are <br />presented chronologically in Table 1. Five of the reports shown in Table 1 contain <br />original hydrology studies with estimates of peak flow. The hydrology studies have <br />been highlighted in bold type. The remaining reports use existing peak floV'{ estimates <br />and make no new hydrology calculations. Table 2 contains the specifics on the <br />methods used to estimate peak flows in the five original hydrology studies. <br /> <br />A. Hydrology Studies <br /> <br />Five independent hydrology studies are significant to the Gregory Gulch catchment. <br />Three of the five studies apply directly to Gregory Gulch, while the remaining two <br />studies address the North Clear Creak catchment, downstream of the Gregory Gulch <br />confluence. The value of the studies on North Clear Creek lies in the method of <br />analysis used for this similar region. <br /> <br />One of two methods is employed in all five hydrology studies, to calculate the <br />estimated 100 year peak flow. The first method is a storm generated runoff system <br />using a curve number (CN). The second method is a regression equation developed <br />from stream gage data in a similar region. Specifics of the hydrology studies have <br />been summarized in Table 2. There are variations in the storm runoff methods, but <br />flow estimates are similar from the three studies using storm runoff methods. A large <br />discrepancy is apparent, however, between flow estimates from studies using storm <br />runoff methods and flow estimates from studies using statistical regression equations. <br /> <br />The first detailed hydrology study in this region was conducted by Owen Ayers and <br />Associates (OA&A) for the city of Black Hawk. The study estimated the 100-year <br /> <br />5 <br />