HomeMy WebLinkAboutDrainage Reports - 08/09/1993 (2)S4
Mr. Glen Schlueter
City of Fort Collins
Stormwater Utility
235 Mathews
Fort. Collins, CO 80524
RE: Park Central Parking Lot Modifications
Project No. 10-648-007
Dear Mr. Schlueter:
July 2, 1993
STORMWATER REVIEW COPY
Tina# Approved Report
Date
We are sending you this letter to summarize our investigation into the impacts of the
proposed parking lot modifications at the Park Central P.U.D. on the existing drainage
facilities. Attached to this letter, you will find an exhibit which shows the existing parking
lot configurations, the proposed modifications, and the basin delineation which was shown
on the original drainage report. We are also sending you copies of our supporting
calculations.
The "Final Drainage Report for Park Central P.U.D., Phase 11" was submitted to the City
of Fort Collins in March of 1983, by TST, Inc. A letter addressing parking lot modifications
to the "Park Central Food Court" dated July 9, 1992, was also sent to the City by TST, Inc.
Both the report and the letter were referenced while preparing this investigation.
The purpose of our investigation was to identify the impacts of the proposed addition of 33
parking spaces which will increase the sites overall impervious area. Basins were delineated
based generally on the delineation shown in the original drainage report, but also reflecting
the impacts of the proposed modifications. The basins shown on the attached exhibit were
labeled the same as they were in the original report for continuity. The proposed
modifications will occur in Areas 2, 3, 4, 5 and 6 with the majority being done in Area 3.
Runoff was calculated using the rational method standard equation:
Q=CCfIA
where: Q = Runoff (cfs)
C = Runoff Coefficient for Land Use (unidess)
C f = Storm Frequency Coefficient (unitless)
I = Rainfall Intensity (in/hr)
A = Drainage Area (Ac.)
TST, INC. 748 Whalers Way • Building D 102 Inverness Terrace East
Consulting Engineers Fort Collins, CO 80525 . Englewood. CO 80112
(303) 226.0557 (303) 792.0557
Metro Denver (303) 595-9103 FAX (303) 792-9489
FAX (303)226.0204
TST, INC.
Mr. Glen Schlueter
July 2, 1993
Page 2
In calculating the runoff, the following assumptions were made:
1. C = 0.95 (for parking lots, 0.90 was used in the original report)
2. The 10-year and 100-year storm frequencies would be examined
3. The total time of concentration would be less than five minutes, so the maximum
values of rainfall intensity (1) for the 10-year storm (5.6 in/hr) and the 100-year storm
(7.0 in/hr) would be used from Figure 3-1 of the SDDC manual.
The calculations made to determine the increase in runoff for each basin are at the end of
this letter. We made a comparison of the total existing runoff and the additional runoff from
the proposed modifications (Sheet 2 of 2) for each basin and found the percent increase in
runoff to be less than 2% in Areas 2, 4, 5 and 6. Area 3 runoff was increased about 15%
(0.43 cfs for 10-year, 0.56 cfs for 100-year). Area 3 has a standard Fort Collins Area Inlet
(originally a curb inlet) which will have a capacity of 7.0 cfs at a 6" ponding depth. The
total runoff to the inlet will only be 3.86 cfs during the 100-year event.
We believe that our investigations have shown negligible impacts for Areas 2, 4, 5 and 6 and
that the area inlet in Area 3 can easily accommodate the increased runoff from these
modifications. _ It is our opinion that the current storm drainage facilities in the Park Central
P.U.D., Phase II site will accommodate the proposed parking lot modifications without
making any modifications to the drainage system.
If you have any questions or comments about the methodology or depth of our investigation,
please feel free to contact us.
Respectfully,
TST, INC., CONSULTING ENGINEERS
Donald N. Taranto, P.E. Dave Lindsay
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Existing is"
Curb Cut And
Swale To Remain
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lNsrAu taenn. mvcr
nticx alRac ccwsr
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w.a, s/PPc a. III
'•.x� �-� 2.OaPhalf
c'loae Ceu'a T ' o Compc<fed .51 bu.e�
Note ' Atakr Ta 7Ae 6eo/ecn neo/ n„y own. A-<w saral/be
Repe f` Any Y9rI CAN "I ,C Pad P/rh t7a'
Fveinah eel
TYPICAL PARKING LOT SECTION
A api. /r 4 /medic
cov'Ja ✓, S-Al"'d
Po✓[ncn
1. All Grades And dimensions Shown Are From Flow Line.
1. Contractor Shall ✓ed/x All Existing Grades (Spot Elevations
Designated With An b') And Shall /nlorm Engineer As
Soon As Possible Of Any lnconsistencres.
J. All Pr000sed Pocking Stalls Are To Be 9- Wide And
Will Be Marked With A 4' Wide White Stripe.
4. All Concrete Work Shall Conform With City O/ Fort Collins
Construction Standards.
T—
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Am
Ei PON MA
GONMow, walN Flow,
r11I MIM'mI
_ MN Of IN M , Mptn all NIP'^
al Shall M I,-
MINIIIm�n!III Ia lk.
CITY OF FOR'
M 0 M M
Will 1._>0' (I
TST
TST, INC.
'a�Or aM�•
xo 10-6A6-o03
1"-30'
APRIL 1993
1 1