HomeMy WebLinkAboutRESOURCE RECOVERY FARM, SAND & GRAVEL MINE - PDP/FDP - 17-99 - CORRESPONDENCE - DRAINAGE RELATED DOCUMENTMr. Tom Shoemaker
October 18, 1999
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this mine pit were not next to two waterbodies subject to high water events, such setbacks as
mentioned by the engineers might be adequate. Given that both, the eastern and the western edges
of the mine pit adjoin Boxelder's easements that contain wastewater lines, the stability of the
setbacks under such a hydraulic head (of 15 to 25 ft), is a matter that MUST be given additional
consideration so as to ensure the integrity of the easement within which Boxelder's lines are laid.
I would urge you to please have the City's engineers revisit the issue of setbacks in light of what
transpired at the LaFarge mine pit, and preferably increase the degree of setbacks so that Boxelder's
lines do not end up being subjected to such a threat.
Thank you.
Sincerely,
Ravi Srivastava
District Engineer
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18 October, 1999
Mr. Tom Shoemaker
Natural Resources Director
City of Fort Collins
P. O. Box 580
Fort Collins, CO 80522
Dear Tom:
I thank you for the letter of 13 October 1999, in response to my comments dated 18 August 1999,
directed to Mr. Michael Hart, regarding the City's plans to mine at the Resource Recovery Farm site
that lies between Boxelder Creek to the east, and the mined out gravel pit ponds of ELC to the west.
It is good to know that i) the mine dewatering discharge would be routed to Boxelder Creek instead
of directly to Poudre River and ii) there would be no alteration of ground elevations along Boxelder's
wastewater lines.
There remains some concern relating to excavation setbacks from the edge of the District's
easements in that high water events in Boxelder Creek and Poudre River would result in a greater
than usual hydraulic head that drives water from these waterbodies into the pit, through the
intervening (porous) material, causing removal of fines and subsequent 'piping', thereby destabilizing
the slopes.
During the April 30, 1999 flood event, it is suspected that the presence of smaller setbacks than
desirable was partly responsible for the failure of the right bank of the Poudre (south of ELC bridge)
and the subsequent washout into the mine pit. If you remember the trip we took across the bridge
on 30 April 1999, you had yourself commented something to the effect that mines should not be
allowed to get this close to rivers. The situation at the proposed Resource Recovery mine site would
be similar to the LaFarge mine site, in terms of water surface elevations outside the pit, nature
(porosity) of soils, depths of cut in the mine.
In this light, the matter of setbacks and their role during high water events of a magnitude similar to
or greater than the flood event of 30 April 1999 remains a concern, given that:
(i) Boxelder Creek is subject to high water surface elevations during flood events, and
(ii) the ponds to the west of ELC are hydraulically connected to Poudre River and, during
floods, will experience water surface elevations in conjunction with Poudre River water
surface elevations.
Scott, Cox and Associates (City's consulting engineers) in their letter, state that the setbacks they
have computed (about 75 ft) would "afford adequate protection from the risk of slope instability." If
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