2011-10-11_PERMIT FILE - M2011040

• Permit No: M2011040
• IBM Index Class Name: PERMIT FILE
• Doc Date: 10/11/2011
• Doc Name: Preliminary adequacy review
• From: DRMS
• To: Rimrock Exploration and Development, Inc.
• Email Name: GRM
• Media Type: D
• Archive: No

Prince Albert Mine — General Stormwater Comments
Page 4
October 7, 2011
the selection of 0.028. Note flow velocities exceeding 5.0 fps will require a
discussion on armoring/erosion protection.
d. East Run On Diversion Swale, Sections E -E and F -F, Channel Data — Manning's
n: The value of 0.028 appears too high for bare soil and too low for riprap.
Please provide rationale for the selection of 0.028. Note flow velocities
exceeding 5.0 fps will require a discussion on armoring/erosion protection.
e. East Run On Diversion Swale, Section E -E, Channel Data — Slope: The
maximum slope is listed as 0.125 ft/ft. The Plan View shows a reach with an
18.08% slope. Please revise the calculations to reflect the steeper design slope.
f. East Run On Diversion Swale, Plan View: The note indicates topsoil will be
stripped and 4 -inch cobble will be placed to maintain "travel surface ". 4 -inch
cobble is unlikely to remain on bedrock either during a storm event or with
infrequent vehicle traffic on an 18% slope. Please provide some discussion on the
purpose and maintenance of the 4 -inch cobble.
East Run On Diversion Swale, Riprap Sizing: The table references Urban
Drainage's formula. The Urban Drainage riprap method is intended for
subcritical flow regimes. Please provide verification that the reaches for which
this riprap is intended are subcritical, or use an appropriate riprap sizing method
(e.g., USACE's EM 1110 -2 -1601, Steep slope riprap design, or other appropriate
method).
h. East Run On Diversion Swale, Riprap Sizing: Assuming the same source of
riprap as referenced on Exhibit U -2, riprap specification, the riprap size will need
to be adjusted accordingly to account for the low specific gravity (i.e., 2.24).
i. Please clarify whether the discharge from the Central Detention Overflow is
directed to the North Detention Pond or to a discharge point parallel to the East
Run On Diversion Swale discharge point.
10. Exhibit U -4 — Designs, calculations and methodology.
a. North Detention Plan View: The spillway is proposed to be located very close to
the West Diversion Swale inlet and well away from what appears to be a natural
drainage (near where the north end of Section I -I intersect natural ground. /the
former can lead to short circuiting in the sediment pond (lowering the sediment
trapping efficiency) and the latter may contribute to unnecessary erosion at where
the spillway discharges to native material. DRMS recommends the spillway be
moved to a location coincident with Section I -I to address both potential problems
unless there is a compelling reason to keep in the location shown on the drawing.
b. Spillway Riprap Design: The design flow is 5.86 cfs. The table on Exhibit U -1
indicates the design flow should be 3.01 cfs. Please provide some clarification on
the difference in design flows.
c. Related to Comment 7.b, if the riprap size is recalculated, note there is an error in
the Cu value after the adjustment for the low density rock (i.e., Cu = 1.95, instead
of 2.0).
d. West Diversion Swale Section H -H: There is a thick black line running diagonal
through this section. Please identify this line or remove it as appropriate.
c: \documents and settings \grmldesktop \current projects \prmCe albert m -11-
040\stormwatercomments mem07oct 1 1 .docx
g.