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<br />upper threshold discharge assuming supercritical flow with an "n" value of 0,045, The staffs best <br />estimate of a peak flow value is approximately 300 cfs (see Appendix), <br /> <br />An estimate of the "bulking factor" of the flow due to sediment loading was not scientifically <br />determined for this analysis; therefore the estimated discharge represents the total water and <br />sediment flow, However, if one assumes that 20% of the total discharge was due to sediment and <br />debris, then the estimated clearwater discharge would be 240 cfs, <br /> <br />Study Findings <br />The CWCB staff finds that the July 30, 1998 flood event on the unnamed gulch was slightly greater <br />than the 100-year flood event computed by Love & Associates, Although storm runoff from the <br />gulch has historically flowed down Backus Street and onto Main Street, dumping of construction <br />ruble from nearby projects in the upper portion of the basin has contributed to a heavier sediment <br />load, and increased the damage potential from intense thunderstorms, The large paved parking lot <br />near the headwater area of the basin has also exacerbated runoff and flooding problems, In <br />addition, the CWCB staff believes that the 100-year desi!,'Tl discharge of 190 cfs for the unnamed <br />!,rulch may be too low, This conclusion is anecdotally based on the findings that an approximate <br />100-year storm event (2 to 2.5" of rain in an hour) caused a peak runoff (clearwater) of about 240 <br />cfs, The current design discharge would seem to be more realistic if a permanent detention facility <br />were constructed below the large parking lot. <br /> <br />Recommended Flood Mitigation Measures <br />. Outfall System <br />Plans are currently under review to build a parking structure in the area surrounding the mouth of <br />the gulch, which would effectively obliterate the lowest reach of the gulch itself. The design <br />engineers, Futura Engineering, have indicated that they plan to install a culvert under the parking <br />structure to redirect the flows into North Clear Creek, Upstream of the culvert inlet, a series of <br />debris and sediment basins have been designed to prevent the culvert from clogging, It is our <br />understanding that the proposed culvert would handle a portion of the 100-year discharge, while the <br />remaining overflow would be handled by the downstream street section as it did historically. <br /> <br />. <br /> <br />Review of the Basin Hydrology and Design Plans <br />Consideration should be given to revlsltmg the design <br />hydrology, Consequently, the current design capacity of the <br />proposed culvert near the Golden Gates Casino may be <br />inadequate unless detention storage is provided upstream, It is <br />also recommended that an engineered floodwall be constructed <br />on the back side of the Golden Gates Casino to protect that <br />structure from future flood damage in case of a project failure, <br /> <br />I oO\..lIlg IlI"'t,Il"OllH li\, it II: l..~t!h' . <br />"; , " 1" <br />. ; ~,~ '\ }., <br />.' .- . ." "- <br />'.r"-:?- . <br />",- ~ , --=~ <br />"~'~- <br />[f.t~.n :...~ <br />~"'~-:'J.-~ <br />"...'. ..' <br /> <br />-" J <br />..;.: , <br />, ,", <br /><<-;........ .-,~~ ",;';'" . Detention Pond and Slope Stabilization <br />.- ... In addition, slope stabilization techniques should be <br />implemented on the large fill embankment just below the large parking lot at the top of Miner's <br />Mesa. The easterly sector of the parking facility acts as a detention pond, However, the <br />embankment needs bank stabilization measures to resist any future overtopping and failure. This <br />action should take place in addition to the previously mentioned mitigation measures, Slope <br />stabilization would help prevent massive quantities of rock and sediment from being transported to <br />the lower part of the basin. <br /> <br />L\wctrans'-'eca\ I 998Flood,\Blackllawk <br /> <br />J <br /> <br />