HomeMy WebLinkAboutBROOKFIELD SECOND REPLAT - Filed GC-GENERAL CORRESPONDENCE - 2016-08-02NORTHERN
ENGINEERING
September 16, 2015
Sherri Langenberger, P.E.
City of Fort Collins Engineering
281 North College Ave
Fort Collins, Colorado 80522
Re: Brookfield 2"d Replat
Subdrain Size Justification
Dear Sherri:
This letter is to fulfill the requirements stated in the comments originated on 8/4/15 that a
size justification letter be provided for the underdrain system proposed with sanitary line A
of the Brookfield 2"d replat. The subdrain is currently shown to be an 8" perforated
underdrain. The line will tie into the existing subdrain located on Northern Lights Drive.
This subdrain was previously designed with the plans for Brookfield subdivision by TST,
Inc. revised on 9/11/01. The developer is not aware of any issues with the existing
subdrain and it appears to be installed and functioning as intended within those plans. The
proposed extension is shown in these plans the same as proposed in the 1st filing version of
these plans, including length of pipe and size to be installed, except for an increase in slope
to increase cleanout velocity.
We believe it is reasonable to install this subdrain per the original plans and size provided
that there appears to be no known defect of this subdrain. We also do not feel it
appropriate to upsize this line as it we are tying into the same size line downstream of the
proposed extension. We also feel the capacity is adequate for the proposed extension of this
subdrain according to the attached calculations.
It is assumed in these calculations that the existing subdrain within the proposed Brookfield
Drive works in conjunction with the existing subdrain and the proposed extension within
Northern Lights Drive. Between the two subdrains, we feel it appropriate to assume that
only infiltrative recharge would be collected within the subdrains. Outside of the perimeter
of these subdrains, a generous assumption of one independent well per foot of pipe,
pumping to maintain drawdown at the depth of the subdrain pipe was used.
Provided one well per foot of pipe were modeled for a 30' deep unconfined aquifer, using a
drawdown extinction radius of 2,5OO',a depth of 11.5' (to the subdrain invert), and a
hydraulic conductivity of 1.6x10-6ft/sec (average for Loess soil type) we have calculated
that the groundwater collection from the 600-LF of subdrain adjacent this project, would be
301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com
NORTHERN
ENGINEERING
around 0.1-cfs divided between 2- 8" subdrain pipes. Each of these subdrain pipes has a
capacity of around 0.6-cfs for a total excess capacity of 1.1-cfs between both pipes. We
feel the assumption of one well per foot of pipe is generous because it ignores the principles
of superposition in drawdown calculations.
Assuming a high value of 10% of the total seasonal precipitation combined with a generous
irrigation requirement of 36" per season would infiltrate, the total recharge within the
proposed development would be around 0.0028-cfs. This assumes 17" of yearly
precipitation on average. The resulting subdrain discharge is negligible in our judgement to
the overall pipe flow and excess capacity.
Evaluation of the Brookfield Subdivision in its entirety is beyond the scope of this project s
and is hovered under the overall subdivision design. We trust this letter will meet your
needs for the additional 265-LF of subdrain proposed with this development.
Please contact Northern Engineering with any questions or concerns regarding this matter.
Sincerely,
Ryan Banning, P.E.
Project Manager
301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com
NORTHERN
ENGINEERING
PROJECT: JOB #: - J I- , ,
21/
DATE:
rtlorl it per
Y(e 0-4 Poll- S-pa Mal seoson
40 Mec 3 soor,50vj 1 In I'mpe(VA'o
5a v uc or 4 e q, 3
LzPled l 5M bd (a 1,/I -Flo 4
2 2
ylb 63
p r( V;,o S ok r H
---- ------ W�(
sn cc r)
ms,
8,y
'� (NORTHERN
ENGINEERING
A,5
5 7
jo,
P I (,-" r
PROJECT: JOB #:
DATE:
i
r
r
rip
nc ofel t, 2
we po 1") s.
le
�o q