HomeMy WebLinkAboutDrainage Reports - 04/19/2005TST
748 Whalers Way, Bldg, D
Fort Collins. CO 80525
970.226.0557
metro 303.595.9103
fax 970.226.0204
info@tstinc.com
w .tstinc.com
TST. INC. CONSULTING ENGINEERS
p r(l" Pd Repell
�FMIA
J - gate April 11, 2005
Qs
Mr. Basil Hamden
City of Fort Collins Stormwater Department
Utilities
P.O. Box 580
Fort Collins, CO 80522
RE: TST, Inc. Building Expansion
Project No. 0000.0000.00
Dear Mr. Hamden:
TST, Inc. Consulting Engineers is processing a proposed building expansion of
our office on Whalers Way, in the Boardwalk Business Park. In support of that
application we have prepared an exhibit and some calculations to demonstrate
the revised runoff regime that the proposed expansion will generate. This letter
is provided to summarize the proposed improvements and explain the revised
runoff.
TST currently occupies Building D of the Boardwalk Business Park and has
recently acquired Building E, adjacent to the northwest. Both buildings are two
story and are currently connected by an uncovered walkway. The proposed plan
is to connect the two buildings with a new enclosure creating one large office
space in place of the two existing ones. The improvements will also extend the
first floor of the buildings further north towards the existing recreational pond and
create an uncovered concrete patio area. Very little will change regarding the
sidewalk and existing landscape areas in front of the buildings, although we
anticipate replacing much of the existing landscaping and sidewalk because it will
not likely survive the building reconstruction. However, those replacements are
proposed to be generally in -kind and will preserve the existing drainage pattern.
Accompanying this letter is an exhibit that shows the proposed site plan in
comparison to the existing layout. A table summarizes the impervious area for
the existing and proposed site plans. The proposed improvements will result in
approximately 2,914 square feet of additional impervious surface. That increase
equates to an increase of 0.8 cfs runoff during the 100-YR event storm and 0.3
cfs in the 10-YR. Those runoff calculations also accompany this letter. The
basic drainage patterns have not changed. The new roof lines will continue to
convey runoff in the same pattern as they do now in combination with the ground
level sidewalks and landscape areas.
The majority of the runoff from the site discharges to an existing private pond
called the Harmony Reservoir to the north and east of the buildings. Runoff is
currently detained by this facility and the increased runoff resulting from the
proposed improvements will be accepted by the Harmony Reservoir Recreation
Association. Current City of Fort Collins criteria requires stormwater quality be
addressed. In this case, all of the runoff from the proposed improvements that
drain to the pond must sheet flow across at least 55 feet of grass area. Most of
the runoff will be to the back of the building (north side) and must sheet flow
across over 100 feet grass area before entering the pond. The filtering effect of
traveling across this much grassed area should be at least as effective as any
type of extended detention and would not require alteration of the existing
drainage patterns. No additional consideration to treatment of storm runoff is
recommended.
A silt fence barrier at the edge of the pond will prevent eroded sediment from
leaving the site during construction. After construction has been completed there
will be no bare soils to erode. The area will either be building, concrete, or sod.
Thank you for your attention to this application. We have addressed the
comments from your previous review which were dated March 4. Please feel
free to contact me if you need anything else.
Sincerely,
TST, Inc. Consulting
32135V'c o `-
David B. Lindsay, PE
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City of Fort Collins
Rainfall Intensity -Duration -Frequency Table
for using the Rational Method
(5 minutes - 30 minutes)
Figure 3-1a
Duration
(minutes)
2-year
Intensity
in/hr
10-year
Intensity
in/hr
100-year
Intensity
in/hr
5.00
2.85
4.87
9.95. '
6.00
2.67
4.56
9.31
7.00
2.52
4.31
. 8.80
8.00
2.40
4.10
8.38
9.00
2.30
3.93
8.03
10.00
2.21
3.78
7.72
11.00
2.13
3.63
7.42
12.00
2.05
3.50
7.16
13.00
1.98
3.39
6.92
14.00
1.92
3.29
6.71
15.00
1.87
3.19
6.52
16.00
1.81
3.08
6.30
17.00
1.75
2.99 •
6.10
18.00
1.70
2.90
5.92
19.00
1.65
2.82
5.75
20.00
1.61
2.74
5.60
21.00
1.56
2.67
5.46
22.00
1:53
2.61
5.32
23.00
1.49
2.55
5.20
24.00
1.46
2.49
5.09
25.00
1.43
2.44
4.98
26.00
1.40
2.39
4.87
27.00
1.37
2.34
4.78
28.00
1.34
2.29
4.69
29.00
1.32
2.25
4.60
30.00
1.30
2.21
4.52
0
PROPOSED
SITE PLAN
EDIT OF
EXISTING LAKE
t '
s°
PROPOSED °Pp`
BOUNDARY WALL.
�S / �92_ a \
o° �s
�S Q\
ANOSCAPE AREA
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4 EK15
v1
\Ary\ TREES ( YIP)
~ 0I
y ~ a 4 • PROPOSED
1 a .59 I BOUNDARY
rc t t� WALL
I
a
— sills, sills
5p0 -- I E=5002.05 I I I
y t t t I
D
S9»_-- I °9
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PERNWS LANDS APING ---- no no try
o run
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• „ _! LDSCAPING AN
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L ^ __.__________ � a .__ ___ -_ ' .Z_• RETAINING WALL
PARKING LOT ten~
BUILDING FOUTFRINT = 10.63T.6 S.F.
CONCRETE FLATYIORK 3551 SF
TOTAL = 14,194.0 SE.
EXISITNG
SITE PLAN
I
_-499P \ I WU� EDGE OF
EXISTING LAKE
En
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�TREIS YP)
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BU/LL�IYO E Col. 1 t T a II i
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PERVIWS L DWA%NG • ` ( S " �g g �'s /
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Will
•__v __ ___._t___ �___�___------ _ --__1----- _._a___
a. .
PARKING
LOT
SUMMARY
BUILDING D
OF IMPFRWMI$
AREA
3,226.6 S.F.
FOOTPRINT =
BUILDING E
FOOTPRINT =
3,821.3 SF.
CONCRETE
FLATWJRK =
3,732.2 BE
TOTAL =
11,280.1 S.F.
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