2004-07-08_REVISION - M2000158

• Permit No: M2000158
• IBM Index Class Name: Revision
• Doc Date: 7/8/2004
• Doc Name: Adequacy Review
• From: DMG
• To: Platte Sand & Gravel LLC
• Type & Sequence: AM1
• Media Type: D
• Archive: No

• • 1{{I{N{II{III!l111
STATE OF COLORADO

DIVISION OP MINERALS AND GEOLOGY _
Department of Natural Resources
1313 Sherman 51., Raom 215
Denver, Cobrado 80203 DIVISION O r,
Phone: [phone] M I IJ E R A L S ~
FA%: [phone] 6r
GEOLOGY]
May 1'1,'2001 RECLAMATION
HININ G~SAfETY
Rocky Hoffschneider Bi11o„~m
Platte Sand Rt Gravel LLC oo~rer^or
P.O, BOX 180 Grep E. walrher
Littleton, CO 80106 Erecurive Deecmr
Mrchael 6. LrMp
Dwisian piretlur
RE: Riverbank and Pit Slope Armoring Proposal, Technical Revision
to S&H Miae Section 112 Reclamation Permit Application, Platte
Valley Sand and Gravel, File No. M-2000.158
Dear Mr. Hoffschneider,
Platte Sand & Gravel LLC has provided a technical revision (TR) to further address concerns with erosion
potential through the gravel pit mining cells during a large flood event. The TR includes an illustration of
locations that the Applicant has determined to be sensitive areas with the potential for excessive erosion. 'ihe TR
includes a proposal and designs to stabilize certain ofthese potentially sensitive areas by armoring through the
placement of riprap. The TR also includes cost estimates for the placement of riprap and a description of the
timing of riprap installation relative to the sequence of excavation in the gravel pits. The Division's review of the
TR is broken out into four sections in this ntentorartdum as follows:
• The determination of locations that may be subject to erasion during a large flood.
• The specification and drawings that will govern the sizing and placement of riprap.
• The estimated cost to purchase and take delivery of riprap that meets specification, and to install the riprap in
accordance with designs and drawings approved by the Division.
• The timetable for riprap installation, in particular the installation of the riverbank riprap.
As a reminder, the purpose of the Flood control/mitigation plan is to minimize the potential for excessive scouring
and erosion of pit slopes and of berms protecting critical structures such as gas pipelines, and of offsite impacts
due to sedimentation or realignment of the existing river channel.
Ripmp Locations
In order to prevent excessive erosion, areas that may be subject to scour during a flood should be armored. To
identify areas of scour, the depth and velocity of floodwaters should be estimated and the gradation of the soil and
subsoil (sand and gravel) may be examined along with vegetation characteristics to determine if the erosive power
of the flood will cut into the vegetation and soil. Placement of larger and heavier material (riprap), that cannot be
lifted and moved by floodwater, will prevent erosion and down cutting in locations identified to be prone to scour.
There are a number of published computational methods available to determine the sizing and gradation of riprap
required to armor against erosive flows of different velocities.
Depth and velociq~ of floodwater increases exponentially in Flood events of increasingly lower probability of
occuvence.-The ten=percem annaa) chanceflood (dtesa=called ren-year flood) resuasirra substantiallyiower
flow, depth, and velocity than aorta-percent annual chance flood (the f 00-year return period event). Designs for
armoring against erosion during a 100-year flood would most likely fail during a much larger, but longer return