HomeMy WebLinkAboutDrainage Reports - 04/21/2011City of Ft. Collins v Plans
Approved BY
Date L+--a- I -11
January 11, 2011
Mr. Wes Lamarque
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, CO 80521
J•R ENGINEERING
A WesMnn Company
RE: Lot 2 Tower Shoppes PUD - Drainage Addendum for Firehouse Express
Carwash
Dear Wes,
JR Engineering is pleased to submit, for your review and approval, this addendum that
describes the proposed drainage conditions for the revised layout for Lot 2 of the Tower
Shoppes PUD. This addendum references the originally approved drainage report
entitled "Final Drainage and Erosion Control Report for Tower Shoppes PUD" prepared
by RBD Engineering Consultants dated June 7, 1995 and an addendum to the study from
2005. The addendum, entitled "Final Drainage and Erosion Control Report for Sonic
Drive -In — Lot 4, Tower Shoppes P.U.D.,." prepared by Interwest Consulting Group and
dated June 29, 2005, updated the original design with current City drainage requirements.
All computations within this report have been completed in compliance with the City of
Fort Collins Storm Drainage Design Criteria dated May 1984, revised April 1997.
The revised plans for Lot 2 includes an automatic car wash building and self serve
vacuum bays in the paved parking lot. Calculations included in the attachment show that
the runoff coefficient of the proposed site will remain within 1% of the coefficient for the
same site under the previously approved plans. Although a slight increase is indicated, it
is our opinion that the increase is negligible. In addition, the intent of the proposed design
is to maintain the drainage patterns associated with the previously approved plans. This
includes stormwater entering the swale in the southeast corner of the site.
In the approved addendum for the Sonic Drive -In, the report discusses the regional
detention pond capacity and states the following: "Thus detention for Lots 2 and 4 will be
provided in the regional pond and on -site detention for Lots 2 and 4 will not be required."
The report goes on to discuss water quality for which it offers a design that, "is located in
its final location and is sized to provide the required amount of water quality
enhancement for all of the Tower Shoppes P.U.D. to be developed."
The above mentioned revisions have been designed with provisions for safe and efficient
control of stormwater runoff in a manner that will not adversely affect the stormwater
system. Due to the fact that the attached calculations indicate a negligible difference in
discharge when compared to the original design, and the approved design states that both
water quality and detention is provided by existing facilities, it is evident that the
proposed project does not need to provide additional storm water improvements.
Therefore, the storm water management design approved with the Sonic Drive -In, and
updated with this addendum is assumed adequate for the construction of the project.
We greatly appreciate your time and consideration in reviewing this submittal. If you
have any questions or required additional information please call (970) 222-1317.
Sincerely,
JR Engineering
Prepared by,
John E. Meyers Jr.,1
Project Engineer an
Reviewed by,
Michael S. Brake, P.E., P.L.S.
Principal and Vice President
RUNOFF COEFFICIENTS & % IMPERVIOUS
LOCATION: Lot 2 - Tower Shopper PUD
PROJECT NO: 39671.00
COMPUTATIONS BY: jmj
DATE: 111112011
Recommended Runoff Coefficients from Table 3-3 of City of Fort Collins Design Criteria
Recommended % Impervious from Urban Storm Drainage Criteria Manual
Streets, parking lots (asphalt):
Sidewalks (concrete):
Roofs:
Lawns (flat <2%, sandy soil):
Runoff
%
coefficient
Impervious
C
0.95
100
0.95
96
0.95
90
0.10
0
JR Engineering
2620 E. Prospect Rd., Ste. 190
Fort Collins, CO 8525
SUBBASIN
DESIGNATION
TOTAL
AREA
(ac.)
TOTAL
AREA
(sq.ft)
ROOF
AREA
(sq.ft)
PAVED
AREA
(sq.ft)
SIDEWALK
AREA
(sq.ft)
LANDSCAPE
AREA
(sq.ft)
RUNOFF
COEFF.
(C)
PROPOSED
0.90
39,153
3626.67
22,372
4,133
9,022
0.75
Equations
Calculated C coefficients & % Impervious are area weighted
C=y(C;A1)
0 At
C; = runoff coefficient for specific area, A
A; = areas of surface with runoff coefficient of C;
n = number of different surfaces to consider
A, = total area over which C is applicable; the sum of all A;'s
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