PERMFILE53867

• Permit No: M1977004
• IBM Index Class Name: Permit File
• Doc Date: 8/31/1984
• Doc Name: Response to Adequacy
• From: Homestake
• To: DMG
• Media Type: D
• Archive: No

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The existing spillway structure has been examined and, during the time
period leading up to final reclamation, Homestake will conduct futher exploration
and will ensure the design for the spillway will be adequate for long-term
operations. As will be discussed later in response to observations of
the Forest Service, it presently appears possible to revise the spillway
alignment and obtain a more gentle slope which may also position the
spillway in bedrock.
Homestake agrees that it should include this investigation and design
evaluation in the calculations for surety and has done so in Exhibit L
by adding $17,000 in the Section titled "Engineering Investigations and
Design."
Item: The possibility of breaching the Crown Ditch diversion only partly
and the potential that other proposed drainage channels may not be maintenance
free was raised.
R~espo_ns~e: It is difficult to envision how the diversion could be partially
breached without utilizing some form of structure requiring some maintenance
to compensate for development of obstructions. The objective of breaching
the existing diversion is to remove reliance on drop boxes, pipes, and
culverts in areas where steep gradients exist. The design flow calculations
have not indicated advantages in developing flow-initiated diversions. It
was also anticipated that reclamation of the disturbed areas would be
performed before the existing diversions were breached, thus allowing the
channels to become stabilized prior to subjecting them to the flows from
undisturbed areas.
The diversions that are to be retained in final reclamation do not
rely on conduits and thus the issue of maintenance does not arise. Further,
these channels are designed for ]arge flows and should readily adapt to
normal flows if some obstructions should develop.
Item: The SCS run-off curve number 60 for the mine appears somewhat low.
Response: The curve number of 60 was derived from premining conditions
and was submitted with the design for mine wastewater treatment reservoir
(Bingham Engineering Report dated April 1978). The CN60 represents a soil
of slow infiltration rate in an aspen grove with about 45 percent cover
as well as Ponderosa Pine areas of about 40 percent cover, with soils
possessing moderate infiltration capacities. Homestake is advised that
strict application of the SCS CN's to high altitude basins generally
gives higher run-off values than given by actual measurements. In summary,
Homestake believes the CN used is representative of reclaimed area run-off.
Based on a curve number of 90 for areas disturbed by mining and the
CN 60, a composite curve number of 72 was determined by Bingham Engineering
for the drainage area during mining. Since the disturbed area has now
changed only slightly under this amendment, the composite curve number
remains the same.