HomeMy WebLinkAboutDrainage Reports - 12/04/2012NORTHERN
ENGINEERING
Drainage Addendum Letter
Date: November 27, 2012
City of FtCollins Approved Plans
Approved By
[date 1.1- 4-1J,..
ADDRESS: PHONE:970.221.4158 WEBSITE:
200 S. College Ave. Suite 10 www.northernengineering.com
Fort Collins, CO 80524 FAX: 970.221.4159
Project: Kechter Crossing Project No. 110-061
Regional Storm Sewer Revisions
Attn: Mr. Wes Lamarque
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
Dear Wes:
This letter serves to document the storm sewer revisions associated with the Kechter Crossing
development. Said revisions have primarily been made in order to provide a more regional stormwater
outfall capable of serving multiple properties in the vicinity of Kechter Crossing. The changes involve the
addition of an 18" RCP storm sewer in Tilden Street, as well as the upsizing of the off -site outfall line from
24" to 36" diameter.
The Final Stormwater Management Plan for Kechter Crossing, Amended September 3, 2008, by
BenchMark Engineers originally designed the outfall to McClellands Creek to accommodate 17.8 cfs.
However, based upon further analysis and consultation with the City of Fort Collins Stormwater Staff, the
methodology previously employed with Kechter Crossing was deemed invalid if additional properties were
to be connected to the system. In order for the City to consider the drainage outfall a regional facility, it
must sufficiently comport to the McClellands Basin drainage master plan.
While the off -site outfall pipe had the potential to serve multiple properties, there was insufficient evidence
to justify whether or not the system had adequate capacity. A marked -up copy of the Overall Basin Map
from the 2008 Kechter Crossing drainage report is attached to this letter. The area highlighted in yellow is
the approximate boundary of properties assumed to utilize the off -site drainage outfall pipe to be
constructed with Kechter Crossing. The overall tributary area was further divided into the following rough
sub -basins; Kechter ROW (5.76-ac), Northwest (28.08-ac), Kechter Crossing (30.54-ac), Northeast
(19.75-ac), and Lehman Property (37.63-ac).
The McClellands Basin drainage master plan prescribes a 100-year release rate of 0.5 cfs/ac. However, .
the Kechter Crossing development has already been designed and approved for a peak discharge of 8.6
cfs. Therefore, this rate has been maintained. The Kechter ROW was assumed to be developed; and thus,
a fixed 100-yr historic discharge of 11.2 cfs was held as a pass -through in both scenarios. The other
basins were evaluated for both a 0.2 cfs/ac and a 0.5 cfs/ac release rate. The resultant pipe sizes for each
scenario are depicted on the attached color -coded exhibit.
A 36" diameter outfall pipe was selected in order for this stretch to remain a regional system, and
potentially be eligible for ownership and maintenance by the City of Fort Collins Stormwater Utility. This
affords the ability for properties within the Northwest and Northeast sub -basins to release at 0.5 cfs/ac
when they develop in the future and connect to the Kechter Crossing off -site outfall.
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The Lehman Property, however, agreed to a self-imposed 0.2 cfs/ac restriction in the 100-year developed
condition. Since the Lehman property bears the sole burden, and is the sole benefactor, of constructing
the new storm sewer in Tilden Street, it was decided that a lower release rate was a reasonable offset for
the cost of installing a smaller diameter pipe. In this case, an 18" storm sewer was selected. This
concept was agreed to in a meeting with Fort Collins Stormwater staff on October 5, 2012. Stan Everitt
with the Everitt Companies was present representing the development interests of the Lehman Property,
which currently has a pending development proposal through Larimer County under the Mail Creek
Crossing application.
As originally approved with Kechter Crossing, Tilden Street was to be stopped approximately 30-feet short
of the southern property line. The interim condition provides an interceptor Swale to divert upstream flows
from the southwest around the Kechter Crossing development and towards the historic flow path on City -
owned property. The approved plans include an inlet and RCP storm sewer (SD-6) to allow stormwater to
pass under Tilden Street in the ultimate condition. However, the assumption is that the runoff previously
intended to utilize SD-6 will now be conveyed in the new 18" storm sewer (Line A6-1) in Tilden Street.
Therefore, Storm Line A6-1 will provide a pipe stub and flared -end section to the west to intercept flows
from the southern boundary of Kechter Crossing. It is anticipated that the Mail Creek Crossing
development (Lehman Property) will connect into the system at the southern terminus of Tilden Street
(STMH A6-5) at a future date.
The addition of an 18" RCP storm sewer in Tilden Street will provide a restricted outfall with 7.5 cfs
capacity to the Lehman Property for future development. STMH A6 will be shifted east approximately 17-
feet to accommodate the 18" RCP storm sewer incoming from the northwest corner of the City property.
The off -site outfall has been upsized from 24" to 36" diameter. This provides sufficient capacity for the
remaining undeveloped properties enclosed by the yellow line on the attached Overall Basin Map to release
at 0.5 cfs/ac. No additional analyses or modifications are expected with this proposed drainage revision.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely
7.
LEED
Nicholas W. Haws, PE AP
cc: Traci Shambo - Larimer County Engineering
Dave Oswandel - Brinkman Partners
Stan Everitt - Everitt Companies (Cloud Peak Ranch, LLC)
Michael Bird - Meritage Homes
enc.
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