1997-06-13_REPORT - M1981302 ‎(16)‎

• Permit No: M1981302
• IBM Index Class Name: Report
• Doc Date: 6/13/1997
• Doc Name: EXHIBIT A 12
• Media Type: D
• Archive: No

Between 1973 and 1979, the US Army Corps of Engineers re-visited the hydrologic
conditions of the South Boulder Creek drainage basin and increased the 100-year flood
discharge from its 1969 findings of 5,000 cubic feet per second to 6,200 cfs. This
revised 100-year discharge was used in studies that followed this reported change.
In 1979, prior to commencement of mining operations, Flatiron Companies contracted
with Leonard Rice Consulting Water Engineers (LRCWE) to prepare an "Application for
a Special Permit for Floodplain Construction - South Boulder Creek" in order to obtain a
' Boulder County floodplain construction permit for the site perimeter berm which was to
be constructed on the southern and eastern edges of the Property. The LRCWE study,
using a 100-year flood discharge of 6,200 cfs, initially determined the 1-foot rise
' floodway and then set the site perimeter berm location so that the berm, in conjunction
with an excavated channel comprised of a 50-foot base width located adjacent to and
' outside of the berm within the floodplain, would together not cause a rise in the
floodplain of more than 1-foot. The purpose of this site perimeter berm was to protect
the gravel mine from flood damage should a flood occur during the time mining
operations were underway at the site. See Figure 3 for the LRCWE 100-year delineation
of the South Boulder Creek floodplain based upon the constructed site perimeter berm.
No request was made by the Flatiron Property owners or their consultants to modify the
' regulatory floodplain based upon the LRCWE floodplain analysis nor to cause the berm
to meet FEMA's flood levee criteria.
' The most recent flood study and the one currently being used by FEMA to regulate the
South Boulder Creek floodplain was a study completed in July, 1986 by Greenhome &
O'Mara, Inc. (G&O) and published in a document entitled South Boulder Creek
Flood Hazard Area Delineation (FHAD). This study was undertaken through a
contract with Boulder County and the Urban Drainage & Flood Control District
' (UD&FCD) and is based upon 1980 topographic mapping. The G&O 100-year
floodplain delineation is shown on Figure 4. The current FEMA Flood Insurance Study
(FIS) regulatory floodplain for South Boulder Creek is based upon the G&O study,
which utilized a 100-year discharge of 6,160 cfs at U.S. Highway 36. The G&O
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