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<br />UNNAMED GULCH <br />This unnamed gulch is an ephemeral stream which drains a large portion of the mesa directly above <br />(south of) the Golden Gates Casino. This high gradient stream has its headwaters at an elevation of <br />about 8,700 feet, flows in a northerly direction down the hillside below the new Bobtail Street <br />Bridge, and discharges onto Backus Street. Flows from the stream are eventually discharged into <br />North Clear Creek within the City. The unnamed gulch basin consists of 0.11 square miles (69 <br />acres) of contributing watershed area at its mouth. <br /> <br /> <br />Floods along this unnamed gulch usually ongmate from stonnwater <br />runoff of the entire basin due to the small area of the drainage. An <br />inspection of the flood history and an analysis of the climatological data <br />show that the area's most severe flooding occurs in the late spring or <br />summer months, from May through August, due to intense localized <br />thunderstonns. These localized stonns and the physical features of the <br />basin can produce the rapid concentration of nmoff that characterizes <br />flash floods of high peak flows, small volumes, and short duration. The <br />steep slopes and short time of concentration of the unnamed gulch <br />watershed are conducive to flash flooding. A parking lot at the top of the <br />mesa also contributes to higher volumes of water draining to the gulch. <br /> <br />Problem Description <br />Intense localized thunderstonns on July 30, 1998, caused widespread small stream and street <br />flooding along Front Range communities, including Black Hawk. During the severe weather <br />events, a serious flood problem occurred in the unnamed gulch <br />above the Golden Gates Casino on Miner's Mesa Street in <br />Black Hawk. Approximately 2 inches of rain fell in less than <br />an hour over the subject basin. Floodwaters ran down the <br />gulch, and emptied onto Backus Street. The floodwaters <br />carried a large load of rock, sediment, and debris, which <br />damaged a patio and kitchen area of the Golden Gates Casino <br />and smashed an air conditioning unit against the building. <br />Rocks and sediment were washed down Backus Street onto <br />Main Street that closed it to traffic for several hours until the <br />Damage to Golden Gates Casino <br />debris could be cleaned up and hauled away. <br /> <br /> <br />Hydrologic and Hydraulic Determinations <br />A detailed engineering analysis of the hydrologic and hydraulic characteristics and parameters for <br />the unnamed gulch can be found in a Love & Associates report entitled "Draft Site Plan Level <br />Drainage Report - Bobtail Street, June 9, 1998." According the report, the I OO-year discharge for <br />the unnamed gulch. designated as Basin B I by Love & Associates, is 190 cfs. <br /> <br />CWCB staff visited the site on August 7, 1998 and measured cross-sectional areas, channel slopes, <br />and high water marks (HWM's) to estimate the July 30, 1998 flood discharge. The measurements <br />were perfonned using conventional field survey techniques. Good HWM's did exist at the subject <br />site. Due to the highly eroded nature of the channel, the staff was only able to survey one stable <br />channel and floodplain section, in addition to surveying the channel slope. From the field data, <br />CWCB staff computed a flood discharge from the July 30, 1998 flood event using the slope-area <br />method. The estimated discharge range for the event is 250 to 400 cfs. The low value represents a <br />critical depth calculation (Froude number of approximately 1.0), while the high value represents an <br /> <br />2 <br />