2003-10-17_REVISION - M1989120

• Permit No: M1989120
• IBM Index Class Name: Revision
• Doc Date: 10/17/2003
• Doc Name: Floodplain Issues
• From: DMG
• To: DMG
• Type & Sequence: AM1
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1373 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
DATE: October 17, 2003
TO: Kate Pickford
FROM: Allen Sorenson~~
RE: Floodplain Issues, Aggregate Industries, Platte Valley Pit,
Permit No. M-1989-120, Amendment Application AM-001
OLORADO
I V I S I O N O E
tIN sRALS
EOLOGY
<LAMATION•MINWG
SA FETY•SCIENCE
Bill Owens
Governor
Greg E. Watcher
Executive Director
Ronald W. Cottony
Division Director
Natural Resource Trustee
Much of the Platte Valley Pit is within the 100-yeaz floodplain. When the original permit application
was approved in 1990, it contained a number of requirements for installation of flood management
structures, including:
1. Riverside concrete rubble armor over a 400 foot reach along the southwest pit perimeter.
Rubble layer is to be 3 feet thick and buried a minimum of 24 inches by overburden and topsoil.
The fmished slope is to be no steeper than 3:1.
2. The existing dike is to be raised from a 4 foot high dike to a 6 foot high dike over a length of
300 feet out of the tota1400 foot long armored section, thus leaving a 100 foot wide and 2 foot
deep spillway in the top pf the dike. The side slopes of the spillway are to be armored, with the
rest of the dike vegetated. An armored rundown is to be installed on the pit side of the dike
below the spillway.
3. A lateral berm was to be installed 2000 feet north of the south end of the mining limits. This
berm is to have a 1.5 foot deep by 350 foot wide spillway with an integrated concrete rubble
cutoff wall. There is a berm in approximately the same location in the plan proposed under
amendment AM-001. However, due to the proposed change in the mining limits, the lateral
berm will now be 2600 feet north of the south mining limit. Also, in the original plan the berm
was shown on the maps to be 160 feet wide, where the new plan shows a width of 60 feet.
Presumably the widths depicted on the maps are crest widths.
4. The pit perimeter design is required to exclude the 10 year flood from entering the pit.
5. There was to be no diking azound the north end of the pit with the intention that the outlet for
floodwaters would benon-erosive flows that would spread out and return to the river.
6. The plan proposed in AM-001 includes a second lateral berm between phase 1 and 3 that was
not included in the original plan.
First, the Division of Minerals and Geology (DMG) should ascertain the extent to which flood
management structures described in the permit have been installed and whether any structures not
described in the permit have been installed. The Operator must provide a construction report describing
the as-built condition of structures that have been installed. Secondly, the flood management plan for
this pit must be updated to account for the changes to the pit configuration proposed under AM-001 and
to correct potential problems and vagueness in the original plan.
Office of Office of Colorado
Mined Land Reclamation Active and Inactive Mines Geological Survey