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BACKGROUND On July 30, 1998 an intense thunderstonn occurred over the area surrounding the City of Black Hawk, resulting in a minor flood event. At the request of local officials, CWCB staff perfonned field investigations in the Blackhawk vicinity on August 5th and 7th as a result of the July 30 flood event. The flooding occurred along an unnamed gulch above Miner's Mesa Street and the Golden Gates Casino. In addition, flows in Gregory Gulch increased due to stonnwater runoff during the same event. Although the flows in Gregory Gulch were not that significant, debris blockage was a significant problem at the new CDOT inlet to the culvert beneath Gregory Street. GREGORY GULCH Gregory Gulch is a right bank tributary stream that drains directly into North Clear Creek within the corporate limits of Blackhawk, Colorado. The Gregory Gulch basin consists of 3.5 square miles of contributing watershed area at its mouth. Flooding has occurred a number of times along this stream including historical events in 1864, 1881, 1910, 1942, and 1975. Hydrologic and Hydraulic Determinations A detailed analysis of the hydrologic and hydraulic characteristics and parameters for Gregory Gulch can be found in a June 1980 report entitled "Floodplain Infonnation Report, North Clear Creek, Gregory Gulch, Chase Gulch, Blackhawk, Colorado" prepared by Owen Ayres and Associates, Inc. According the report, the 100-year peak discharge for Gregory Gulch at its mouth is 1040 cfs. The 100-year discharge upstream of the culvert inlet (approximately 1,600 feet upstream of the mouth) is 1240 cfs. The 10-year discharge for this location is reported to be 370 cfs. CWCB staff visited the site on August 7, 1998 and measured cross-sectional areas, channel slopes, and high water marks (HWM's) to estimate the July 30, 1998 flood discharge. The field measurements were perfonned using conventional field survey and slope-area techniques. Good HWM's did exist at the subject site. From the field data, CWCB staff computed an indirect flow measurement using the slope-area method. The estimated discharge for the event is 300 cfs (see Appendix). This value is representative of a flow having a recurrence interval of 5 to 10 years. An estimate of the "bulking factor" of the flow due to sediment loading was not scientifically detennined for this analysis; therefore the estimated discharge of 300 cfs represents the total water and sediment flow. The clearwater discharge was certainly less than the total flow, possibly on the order of 250 cfs. The July 30 peak flows in Gregory Gulch did not cause a major flood problem for the community, however the CDOT inlet to the culvert beneath Gregory Street was partially jammed with sediment, rocks, and debris and was in jeopardy of being overtopped. Maintenance crews were on-site during and after the event to clear debris away from the inlet that prevented overtopping and potential flood damage. CDOT Inlet Structure