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<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 runoff 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,
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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,
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<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.
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<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 100-year discharge for
<br />the unnamed gulch, designated as Basin B I by Love & Associates, is 190 cfs.
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<br />CWCS 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
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