<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 />
|