_REVISION - M1973021 ‎(3)‎

• Permit No: M1973021
• IBM Index Class Name: Revision
• Doc Name Note: AM07 Final
• Doc Name: Adequacy Review Response #3
• From: Aggregate Industries
• To: DRMS
• Type & Sequence: AM7
• Media Type: D
• Archive: No

Aggregate Industries, Morrison Quarry
Amendment No. 7 to Permit M-1973-021
Morrison, CO

October 2021 | 31
Reclaimed wood debris, wildlife habitat structures, rockpiles, and other components of
reclamation (detailed in Exhibit F.4 through Exhibit F.6) will be placed on the benched or sloping
surfaces of the South and West Quarries at approximate 300-foot intervals. An access road will
be located on the steep upper reclamation benches to allow for hydroseeding and to assist in
monitoring revegetation programs in these difficult-to-access areas.
Stormwater and Erosion Control Features
With the proposed changes to the site grading plan, corresponding revisions to the approved
AM05 plan for channeling stormwater runoff and controlling erosion will be necessary. As with
the original plan, slopes equal to or less than 2H:1V will be stabilized as needed with
appropriate Best Management Practices (BMPs) in accordance with the Site Stormwater
Management Plan (Version 6, June 2021). Slopes steeper than 2H:1V may not be feasible to
install BMPs but will be graded to the nearest stormwater outfall or to the proposed
reservoir/dewatering intake point in the South Quarry. Drainage channels, which consist of
rundown channels with grouted boulders, will be used to collect and convey surface flow to
onsite sedimentation basins (see Appendix 3).
Channelizing and capturing surface runoff within the quarry area would further reduce impacts
to areas outside of the already impacted mining activity. A diversion channel, planned to be
installed during Phase III along the El. 7,000 contour of the South Quarry, will convey UD#1 to
UD#2 towards Turkey Creek (shown in Exhibit F.3 and Appendix 3). This diversions channel will
remain in place following reclamation. This diversion channel is designed to handle a 100-year
storm event, and more information regarding its design specifications can be found in Appendix
3. To the extent possible, stormwater flow below this new diversion will be directed towards a
proposed stormwater outfall to be located east of the South Quarry, draining into UD#3. This
proposed outfall is described in more detail in the following Dewatering section.
Dewatering
To handle stormwater accumulations within the quarry base after Phase III, where South and
West Quarry mining operations have lowered benches to maximum elevation before
groundwater is encountered (approximately El. 6,500), Aggregate Industries, Inc. proposes
installation of a horizontal drain borehole running from a sump location within the South Quarry
through the eastern wall and daylighting into UD#3 as proposed new Outfall 007. Dewatering
would include installing a borehole approximately 700 feet under the eastern wall of the South
Quarry. The outlet of the borehole could consist of an 8-inch stainless steel casing standpipe to
be able to handle up to 45 feet of head pressure. The remainder of the hole would be unlined
and drilled at a diameter capable of handling flows for a 100-year precipitation event. Note that
this proposed borehole is conceptual, and actual dimensions, flow capacities, alignments, and
other design criteria would be determined following extensive investigations. Hydrogeologic and
geotechnical investigations will be key to providing the necessary data for drainage analysis,
unlined borehole stability, borehole alignment, and other key components of the design.
A riprap rundown would be installed at the outlet of the discharge to dissipate energy and
reduce the likelihood of erosion in the immediate area. Because flow from UD#1 is already