1991-05-24_PERMIT FILE - C1981008A

• Permit No: C1981008A
• IBM Index Class Name: PERMIT FILE
• Doc Date: 5/24/1991
• Doc Name: Backfilling and Grading
• Section_Exhibit Name: Tab 20
• Media Type: D
• Archive: Yes

In the Nucla East area, the mined-out county road (see Exhibit 20-10) will be restored to
its original condition and approximate location as specified by agreement with Montrose
County (see Tab 13, Attachment Q.
Postmining Drainage Channels
All major drainages in the Nucla East area that are to be disturbed by mining activities
will be reclaimed to approximate the premining configuration and to blend with the
undisturbed drainage above and below the area to be reclaimed. The postmining drainage
channels will be designed, constructed, and maintained to the following specifications:
1. Gentle sloping banks which shall be stabilized by vegetation;
2. Prevent additional contributions of suspended solids to streamflow and to runoff
outside the permit area;
3. Blend with the undisturbed drainage above and below the area to be reclaimed;
4. Safely pass the runoff from a 10-year, 24-hour precipitation event.
The reclaimed drainages are designed to safely pass the 10-year, 24-hour precipitation
event and to be stable. The peak flow was determined by using the SEDIMOT 11 computer
program. Attachment A in Tab 13 contains a listing of the input parameters for obtaining
the peak flows for the reclaimed drainage channels. The peak flow determination assumed
the total watershed of the reclaimed channel and was determined at the downstream end of
the channel. Most of the reclaimed channel reach will not have to handle that large a
peak flow rate. This was done to ensure that the drainage channel capacity will be
adequate and conservatively designed.
A limiting permissible velocity of 5.0 feet per second (fps) was used to insure that the
postmining drainage channels would be nonerodible and stable. The permissible velocity
was obtained, Table 3.4, from "Red Book" by Barfield et at., 1981 for a grass mixture
vegetated channel. The shape of the reclaimed channel was assumed to be trapezoidal with
5h:ly side slopes as a worst case condition. The side slopes will be no steeper than
5h:ly.
The channel bottom was calculated using Dodson and Associates TRAP computer program (see
Attachment J, Tab 13). The channel width was increased to insure that the peak velocity
was below the limiting velocity. The typical trapezoidal channel section will be used for
the entire length of the reclaimed channel. Attachment B contains the specific design
20-6 Revised 05/24/91