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<br />II STUDY AREA DESCRIPTION <br /> <br />The Gregory Gulch flume is located within the city limits of Black Hawk, in Gilpin <br />County. Black Hawk is approximately 30 miles west and slightly north of Denver city <br />center (Figure 1). Central City borders Black Hawk to the west, and unincorporated <br />areas of Gilpin County border Black Hawk on the north, south, and east. <br /> <br />The city was established when gold was discovered in the area in 1859. Black Hawk <br />was a prosperous mining town, but prosperity faded along with production in the <br />mines in the 1900s. Low stakes gambling was legalized in 1992 and brought a <br />second boom to the city. The resident population of Black Hawk is 150, but the <br />casino industry generates a high level of visitor activity. The city is located in a steep, <br />narrow valley, and development has occurred primarily along State Highway 279 <br />which is named Gregory Street in Black Hawk. SH 279 is the connector between <br />Black Hawk and Central City, and the road parallels the main drainage channel of the <br />Gregory Gulch catchment. <br /> <br />A. Gregory Gulch Flume <br /> <br />The main channel and flood plain of Gregory Gulch have been severely affected since <br />the founding of the town in the 1800s. Mining operations, town expansion, and road <br />development have encroached on, and redefined, the natural cross sectiorl. The end <br />result is periodic flood events in the town. <br /> <br />To help reduce flood impacts, channel improvements were constructed at different <br />locations along Gregory Gulch throughout the 1900s. The lower section of the <br />drainage channel, from the intersection of High St and Gregory St., to the outlet at <br />North Clear Creek (Figure 2), was initially modified to a trapezoidal channel with <br />grouted rubble rock walls. The top of the channel was covered at a later date with <br />wood boards, and in 1979, CDOT replaced the boards with a concrete slab cap so the <br />flume could be incorporated into the roadway, The flume is approximately 1300 ft <br />long, and has a cross section approximately 9 ft wide by 6 ft high. The natural flood <br />plain had a larger cross section than provided by the flume, so flood flows tend to spill <br />over into the streets. <br /> <br />B. Gregory Gulch Catchment <br /> <br />The catchment of the Gregory Gulch flume lies within the south central region of <br />Gilpin County. The basin is steep and narrow, and is located in the mountains with <br />elevations ranging from 8,040 ft to 9,994 ft. Flood insurence studies by FEMA on <br />Black Hawk (7) and Central City (8) report that flood conditions are primarily a result <br />of intense summer cloudbursts, rather than the snow melt/rain storm combination <br />which is sometimes found in the spring. The basin is well timbered with douglas fir, <br />lodgepole pine, ponderosa pine and aspens. The soil description from the Black Hawk <br /> <br />2 <br />