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RECONSTITUTION OF 1979 FLOOD PEAKS, NORTH CLEAR CREEK, GREGORY GULCH, AND CHASE GULCH INTRODUCTION The Colorado Water Conservation Board (CWCB) contracted with Love & Associates, Inc., to reconstitute the hydrologic analysis performed by the CWCB in 1979 for North Clear Creek, Gregory Gulch, and Chase Gulch for the City of Black Hawk, Colorado (City), using a computer model. The study tasks include; L Obtain information on soils, hydrologic parameters, and land use for the North Clear Creek, Gregory Gulch, and Chase Gulch watersheds; 2. Delineate sub-watersheds and probable flow paths using topographic maps provided by CWCB; 3. Calculate flood hydrology for the 10, 50, and 100-year storms for North Clear Creek, Gregory Gulch, and Chase Gulch using TR-20 with SCS Type II precipitation distribution and 1980 land use at approximately 10 design points within the study area; 4. Calibrate the TR-20 model to 1980 flood peak predictions for North Clear Creek, Gregory Gulch, and Chase Gulch using the curve number and, if necessary, rainfall distribution, 5. Define flood hydrology for the 500-year storm event using graphically-based probability analysis or other methods as approved by the CWCB; 6. Prepare a final report, and; 7. Coordinate with CWCB and perform a field review of the watershed, STUDY AREA The study area is located within a portion of the North Clear Creek watershed, part of the Clear Creek and South Platte River watersheds, in part of Townships 2 and 3 South and Ranges 71 and 72 West. Please refer to Location Map, Figure 1. The study area extends from the Continental Divide to the lower boundary of the City of Black Hawk, Colorado. The project area is located east of the Continental Divide, and is characterized by narrow valleys bordered by steep slopes ranging from approximately 20 to 60 per cent. Elevation within the study area ranges from approximately 7,500 to over 12,000 feet msl. The steep upslope areas are covered with a variety of rock outcroppings, thin residual soils on bedrock, and thicker debris, alluvium, and slope wash deposits and are vegetated with grasses, trees, and shrubs. Deeper soils and wetland vegetation are found on alluvial deposits adjacent to streams, The study area has been subject to underground and surface mining and residential and commercial developments. Roads, buildings, and mining- related features such as mine shaft openings and tailings have sometimes altered historic stormwater flow paths and runoff characteristics within the study area, The City of Black Hawk (City) is located within Gilpin County, Colorado. North Clear Creek, Gregory Gulch, and Chase Gulch flow through the City. North Clear Creek flows eastwardly through the City adjacent to State Highway 119, eventually joining Clear -1-