2021-09-03_PERMIT FILE - C1981019 ‎(6)‎

• Permit No: C1981019
• IBM Index Class Name: Permit File
• Doc Date: 9/3/2021
• Doc Name: Collom Haul Road Culverts
• Type & Sequence: TR149
• Section_Exhibit Name: Exhibit 7, Item 25F
• Media Type: D
• Archive: No

Page Exh. 25F-1 Revision Date: 6/16/21
Revision No.: TR-149
Exhibit 7, Item 25F
Collom Haul Road Culverts
Introduction

The location of the Collom Haul Road is presented on Map 25E. Sheet 1 of that Map presents
the general location of the road, and the individual culverts presented in this exhibit.

As discussed with DRMS staff during initial permitting for the Collom Mine, it was agreed and
subsequently approved that an alternate flood prediction tool could be used for these two larger
watersheds because of SEDCAD’s inability to accurately predict peak flows from such a large
drainage areas. A program known as Trihydro was chosen. This program has developed and
maintained by WWC Engineering of Sheridan, Wyoming. It is based on the NRCS Triangular
Unit Hydrograph method and has been in widespread use in the mining field for over 25 years.

Wilson Creek Crossings

For the Wilson Creek culvert, the upstream drainage area consists of Wilson Creek for about 10
miles south to the ridgeline known as the Danforth Hills. Map 12B shows the boundaries of this
drainage basin. At the proposed crossing point, the upstream drainage area is about 9,241 acres
(14.43 square miles).

Rule 4.03.1(4)(e) requires that crossings be designed for a 20 year-24 hour event if the average
end area of the culvert is expected to exceed 35 square feet (an 80-inch diameter culvert). In
spite of this, for a more conservative approach, the culvert at Wilson Creek has been designed for
the 100 year event. Based on the drainage areas, the stream lengths, elevation changes and other
parameters as shown in Figures 1, Trihydro predicts 100 year peak flows for Wilson Creek to be
404 cfs and the total runoff volume to be 242 acre-feet.

Figure 2 presents a generally accepted FHWA nomograph for flow through culverts operating
under the entrance control condition. The nomograph is marked up to illustrate acceptable pipe
diameters for the Wilson Creek crossing. Acceptable solutions have been limited to diameters
that can deliver the design flow with a Headwater/Diameter (HW/D) ratio of 1.5 or less. Pipe
ends mitered to conform to the slope have been assumed, and the pipe will have riprap placed at
the sides and inlet to a height 4 feet about the top of the crown.

At Wilson Creek, it can be seen on Figure 2 that a 7-foot diameter pipe can pass the 404 cfs
design flow at a HW/D of about 1.3. An alternate design of twin 66 inch diameter pipes would
also be adequate. In the final road design however, it was determined that a 12 foot diameter
pipe will be installed at this crossing which surpass the minimum requirements. This pipe size
was selected as approximately 3 foot of fill will be placed in the pipe bottom for a natural
channel surface to comply with other regulatory requirements.

Jubb Creek Crossings

For the Jubb Creek Culvert, the upstream drainage area consists of the combined streamflows
from both the West Fork and East Fork of Jubb Creek. Map 12B shows the boundaries of this