2011-06-29_PERMIT FILE - C1981019A ‎(5)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 6/29/2011
• Doc Name: Attachment 1 to Attachment 12
• Section_Exhibit Name: Exhibit 07 Section 16 Pond 009A
• Media Type: D
• Archive: Yes

• EXHIBIT 7, ITEM SEC 16
SECTION 16 SEDIMENTATION POND
is
The location of the Section 16 Sedimentation Pond is presented on Map 41. The profiles
of the associated permanent and temporary ditches associated with this pond are
presented on Map 33B. These ditches include a series of permanent post- mining
conveyances draining to the southeast over reclaimed lands (Tributaries E, F, G, H and I
as described in and in Exh. 7, Item 20, Part E), and a temporary contour ditch composed
of Section 16 Pond Ditch and Section Pond Ditch Extension, described respectively in
Vol. 2C Exh. 7 -14B and Exh. 7 -140. This temporary conveyance routes flow from the
above described permanent ditches into Section 16 Sedimentation Pond. Channel designs
for all of these permanent and temporary drainage channels are provided in their
respective sections referenced above. Figures Exh.7, Sec. 16, Figure 1 and Exh.7, Sec. 16,
Figure 2 included it this section show the component hydraulic structures and networking
and the layout of the drainage areas and hydrologic conditions, primarily area,
reclamation status, and consequent runoff Curve Number for the inflow calculations for
this sedimentation pond.
The Section 16 Sedimentation Pond was originally described in Vol. 2E, Exh. Secl6 as
part of the January 2007 PR -02 for the South Taylor Permit Area. Pursuant to TR -81,
"South Taylor Fill Revision and PMT Change ", the limits of proposed mining
disturbance in Section 21 are being pulled back about a quarter mile to the northwest,
thereby reducing the portion of the Section 16 Sedimentation Pond watershed that is
disturbed. This warranted a review of the design of the sedimentation pond. Also, the
rationale for selection of runoff curve numbers (CN) was revised pursuant to TR -73. The
revised runoff calculations for this analysis were performed using the new TR -73 curve
numbers.
The following pages present the SEDCAD computer runs that were utilized to evaluate
the expected performance of the Section 16 Sedimentation Pond under the revised mining
and reclamation plan presented in TR -81. The "worst case" scenario that was evaluated
assumes the entire watershed is reporting to the sedimentation pond upon completion of
mining and reclamation effort but before full the revegetation is fully mature. However,
because the runoff is conveyed to the sediment pond via contour diversion ditches just
below the mining activity, almost all of the tributary watershed is disturbed and in some
stage of reclamation.
Two design storms were evaluated, a 10 year /24 hour storm to assess expected effluent
quality, and a 25 year /24 hour storm to assess the capacity of the emergency spillway to
safely discharge the design storm. The 25year /24hour storm was selected previously by
the dam's original designer and appears to have been based on the dam's dimensional
characteristics in accordance with Rule 2.05.3, which adopts by reference the design
criteria of the Colorado State Engineer's Office pursuant to C.R.S. 37 -87 -105.
Exh. 7- Sec16 -2 Revision Date: 12/31/10
Revision No.: TR -81