2014-04-22_REPORT - M1990057

• Permit No: M1990057
• IBM Index Class Name: REPORT
• Doc Date: 4/22/2014
• Doc Name: Drainage report
• From: Union Milling Company, LLC
• To: DRMS
• Email Name: TC1
• Media Type: D
• Archive: No

Flatirons, Inc. - Surveying & Engineering
basin was used to establish the sedimentation required for the contact area. The basin
covers 0.48 acres, and was assigned a RCN of 79. The basin was modeled as grass open
space in fair condition.
Basin Summary
Basin
Area (ac.)
RCN
Tc (min.)
Qio (cfs)
Qioo (cfs)
A
3.96
91
6.1
5.9
11.4
al
0.24
79
3.3
0.1
0.4
a2
0.29
92
9.2
0.4
0.8
B
4.08
79
8.9
1.7
5.3
b 1
0.48
79
3.3
0.3
0.8
OS2
13.17
77
18.8
2.6
10.4
OS3
1.47
77
17.1
0.3
1.2
Table 1
4.0 DRAINAGE FACILITY DESIGN
General Concept
The general concept for the drainage plan is to allow for the planned improvements without
altering the natural drainage patterns or water quality of the area. Runoff from non -
impacted areas will be kept separated and directed around impacted areas. All surface
flows from impacted areas will be directed to the Tailings Storage Facility. Water in the
TSF will be retained and not released into the surrounding environment. The TSF will also
function as the sedimentation area for Basin A. Small sedimentation areas will be added to
Basins al and bl to treat the runoff from the outside face of the TSF embankment. The
outer faces of the TSF embankment will be revegetated. Silt fence will be installed at the
outside toe of the embankment slope at the southwest corner of the TSF. The silt fence will
remain in place until a healthy vegetative cover is established on the outer slope of the
TSF.
Offsite runoff will be directed around the mill, TSF and other impacted areas using open
channels and natural surface drainage features. The drainage has been designed so the
surface flows leave the site at the same point after the improvements have been made.
Union Mill 9 March 5, 2014