2001-05-17_REVISION - M2000158

• Permit No: M2000158
• IBM Index Class Name: Revision
• Doc Date: 5/17/2001
• Doc Name: RIVERBANK AND PIT SLOPE ARMORING PROPOSAL TR TO S&H MINE SECTION 112 RECLAMATION PERMIT APPLICATION
• From: DMG
• To: PLATTE SAND & GRAVEL LLC
• Type & Sequence: TR1
• Media Type: D
• Archive: No

•
STATE • iii iiiiiiiiiiiu iii
OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Nanl ral Resources
131 3 Sherman 51., Room 215
Denver, Coora[10 80203 I
DIVISION O F i
Phone: [phone] MINERAL 5' ~
FA%: [phone] &
I
G EOLOGY
Ma 17,2001
y RECLAMATION
MINING•SAFETY
Rocky Hoffschneider gill Owens
Platte Sand & Gravel LLC ~°vefnOr
P.O. BOX 180 ereF E. w,irher

Littleton, CO 80106 Eeecwive Dv°aor
Michael 8. Long
Division DireUUr
RE: Riverbank and Pit Slope Armoring Proposal, Technical Revision
to S&H Mine 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 coltcerns 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. 'The TR
includes a proposal and designs to stabilize certain of these potentially sensitive areas by armoring through [he
placement of riprap. The TR also includes cos[ estimates for [he 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 [he
TR is broken out into four sections in this memorandum as follows:
• The deterntination of locations that may be subject to erosion 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 [he riprap ill
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 protectiltg critical structures such as gas pipelines, and of offsite impacts
due [o sedimentation or realignment of the existing river channel.
Riprnp Locutions
In order [o 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 velocity of floodwater increases exponentially in flood events of increasingly lower probability of
occurrence. The ten-percent annual chance Flood (tlte so-called ten-year flood) results in a substantially lower
flow, depth, and velocity than aone-percent annual chance flood (the 100-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