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<br />--- <br /> <br />Creek near the intersection of State Highway 119 and V,S. Highway 6. Gregory Gulch <br />flows from the City of Central into the City of Black Hawk on State Highway 279, <br />passing through part of Black Hawk's business district. Chase Gulch flows through the <br />City along Chase Street, passing through a residential area. Gregory and Chase Gulches <br />join North Clear Creek above State Highway 119 within the Black Hawk city limits. <br /> <br />The North Clear Creek drainage catchment is approximately 34.9 square miles in area at <br />the lower City boundary. Gregory and Chase Gulches are approximately 3.5 and 4.2 <br />square miles, respectively, at their confluence with North Clear Creek. The three streams <br />are capable of transporting large volumes of water and debris at dangerously high <br />velocities and occasionally cause widespread damage. <br /> <br />BACKGROUND <br /> <br />North Clear Creek, Gregory Gulch, and Chase Gulch flood peaks were analyzed in 1979 <br />by the CWCB using graphical methods. The original data used for this analysis is no <br />longer available. However, graphical development of the triangular hydrographs used for <br />the 1979 analysis exist (Owens Ayers, 1980). Examining these data, it appears the 1979 <br />analysis of North Clear Creek used a 24-hour storm distribution with an SCS "Type IA" <br />distribution with a Curve Number of 79. The analysis of Gregory Gulch and Chase <br />Gulch used a 6-hour storm with an SCS "Type IIA" distribution. The Gregory Gulch <br />analysis used a Curve Number of between 81 and 85, and Chase Gulch analysis used a <br />Curve Number between 78 to 83. Curve numbers were estimated using the graphical <br />analysis, and a discussion of the methods used are presented in the Appendix. <br /> <br />The 100-year flood peak estimate for North Clear Creek from the 1979 analysis is 2,050 <br />cfs and 2,670 cfs at the upstream and downstream Black Hawk city limits, respectively. <br />The analysis indicated the 100-year flood peak estimate for Gregory and Chase Gulch at <br />their confluence with North Clear Creek equals 1,240 and 1,160 cfs, respectively. <br /> <br />Subsequent to the 1979 analysis, two reservoirs were constructed on Chase Gulch and <br />Gregory Gulch which could attenuate the magnitude of flood peaks in these streams in as <br />well as North Clear Creek. A water supply reservoir was constructed on Chase Gulch by <br />Central City in 1996 and provides inadvertent flood storage above the spillway. A small <br />detention facility, Dorothy Lee Reservoir, was constructed in 1991 above Central City for <br />flood protection. <br /> <br /> <br />The impact of the Dorothy Lee Reservoir on flood peaks was examined using an HEC-l <br />hydrologic model developed for the City of Central (RMC, 1991). This model was <br />subsequently extended through Black Hawk to North Clear Creek for a CDOT project <br />(J.F. Sato, 1996). The two studies used a 24-hour storm and an SCS "Type IIA" <br />distribution with an average Curve Number of 69. The 1996 study by Sato indicated the <br />Dorothy Lee Reservoir reduced flood peaks in Gregory Gulch by approximately 200 cfs <br />at its confluence with North Clear Creek. <br /> <br />-3- <br /> <br /> <br />