PERMFILE43453

• Permit No: M2002120
• IBM Index Class Name: Permit File
• Doc Date: 3/6/2003
• Doc Name: DMG staff review of potential for floodplain Impact
• From: ACS
• To: KAP
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman St, Room 215 - ~ 1
Denver, Colorado 80203 / 1 S I O N O F
Phone: [phone] Permit #: /I7-aoG~ /~ Confidental?: N E RA L S
FAX: [phone]
Class: >i~'n, ~ Type-Seq.: -&
S To: O L O G Y
F
DATE: March 6
2003 rom:
Doc. Name: - ~~~~ :F~'~~~1"--f ~--/ INGMSAFEDTY
, Doc. Date (if no date stamp): (r iii

T Kate Pickford yens
Governor
~j~ Greg E. Walther
FROM: Allen $OTensOn
(/Ji "~7 Executive Director
%r
~'-^'"~" Ronald W. Cattany
Division Director
RE: Potential for Floodplain Impacts Related to the Mining of the Bernhardt
Resource Gravel Pit, Hall-Irwin Corporation, File No. M-2002-720
The Division of Minerals and Geology (DMG) has completed a review of the floodplain information
contained in the Bernhardt Resource permit application and has reviewed the Flood Insurance RateMap
(FIRM) for the proposed mining operation location.- The DMG has not had the opportunity to review the
Flood Insurance Study that was used to prepare the FIRM. The flood hazazd legends included an.the maps
provided by the Applicant state that base flood elevations (BFE) for the subject property have noYbeen
determined. This statement is in error. The FIRM shows that BFE is at 4726 feet at the west end of the
permit area, 4721 feet in the easdwest center of mining pod 3, 4717 feet at the west edges of pods.4 and 5,.
and 4713 at the east edge of the permit area. The FIRM shows that proposed mining pods,l, 3; 4, and 5 are
in the 100-year floodplain.
The DMG is concerned with the potential for adverse off site impacts that may result if significant flood
flows from the Big Thompson River were to erode and down cut the proposed 100-foot wide strip of land
between the river and the mining pods and the strips of land that will be left between the pods..In some
cases, this type of erosion has caused significant river flows to be directed through the pits, even after the
floodwaters have receded. Comparison of the elevation contours on the maps provided by the Applicant
with the BFEs on the FIRM shows that base flood water depths of three feet will be prevalent at the
riverside margins of several of the proposed mining pods. Depending on the flow velocity of the
floodwaters, these flood depths may be sufficient to cause damaging levels of erosion between the pits and
the river and between the various pits. The Applicant must address this potential by researching and/or
developing sufficient site specific flood information to demonstrate that damaging levels of erosion will not
occur, or by developing plans for structures that would convey flood waters safely through the areas
proposed for mining. The following manuals may be helpful to the Applicant in addressing the floodplain
issues.
1. Colorado Division of Minerals and Geology, "In-Stream Aggregate Extraction and Reclamation
Guidance Document," August 1998.
2. Urban Drainage and Flood Control District, "Technical Review Guidelines for Gravel Mining
Activities within or Adjacent to 100-Year Floodplains," December 1987.
These manuals are available for purchase from the respective publishing agencies. The DMG document is
also available online at http://www.miningstate.co.us/Reports/dmQRe~orts.htm.
cc: Carl Mount, DMG (via electronic transmission)
c:~acsUly Documents~Bemhardt flooding.doc