HomeMy WebLinkAboutDrainage Reports - 09/01/2005i1N
Drainage Letter — Amendment
for
Value Plastics PUD
Fort Collins, Colorado
September 1, 2005
t1wNORTHERN
ENGINEERING
SERVICES, INC.
Drainage Letter — Amendment
for
Value Plastics PUD
Fort Collins, Colorado
September 1, 2005
Prepared For:
Value Plastics
3325 Timberline Rd
Fort Collins, CO
Prepared By:
NENORTHERN
ENGINEERING
SERVICES, INC.
420 S. Howes, Suite 202 Fort Collins, Colorado 80521
Phone: (970) 221-4158 Fax: (970) 221-4159
Project Number: 126-002
First Submittal
I ICE
NORTHERN
ENGINEERING
SERVICES, INC.
' September 1, 2005
126-002
'
Mr. Basil Hamden, P.E.
City of Fort Collins
Utilities - Stormwater Dept.
700 Wood St.
Fort Collins, CO 80522
RE: Value Plastics PUD — Stormwater Report Addendum — Additional Parking Stalls
' Fort Collins, Colorado
' Dear Basil:
This letter is regarding the request to add twelve (12) additional parking stalls to the existing
' parking area along the west side of the existing Value Plastics building located at 3325
Timberline Drive (see attached photos). This letter compares the existing site, in particular the
original drainage Basin 2, to the proposed conditions. For simplicities sake, this letter report will
' compare the increase/decrease in impervious area from existing to proposed conditions and
compare the runoff rate from Basin 2. I have included a copy of spreadsheet calculations for
review and a drawing showing the proposed improvements.
The drawing included with this letter indicates where the proposed parking stalls are to be
constructed. The drawing also indicates the area previously planned to be impervious, being the
future connection between the existing building and the future two story building in Phase II.
The drawing indicates, conservatively, the impervious area that would be eliminated within
Basin 2 but on the lot of the current Value Plastics facility. The attached spreadsheet indicates
that there is a decrease in impervious area of 1,134sf and in increase in impervious area of
2,341sf for a net increase of 1,207sf. Using the previously calculated pervious and impervious
areas and same methodology from the original drainage report from Water, Waste & Land, Inc
dated February 13, 1995 the calculated weighted "C" is 0.71. This is in comparison to the
reports original calculated weighted "C" of 0.72. It appears as the though the original report had
a different "C" value due to calculation based on a variation of units between acres and square
feet and the effects of rounding. Based on the revised "C", the calculated runoff, Q100 equals
14.7cfs which is slightly less than the original calculation. These calculations are shown on the
attached spreadsheet.
As part of this site review a comparison of the site, Basin 2, was made with the revised rainfall
intensity. Since the original report was completed in 1995 prior to the revised rainfall intensities
420 SOUTH HOWES, SUITE 202 / FORT COLLINS, COLORADO 80521 / 970.221.4158 / FAX 970.221.4159
' Basil Hamden
Value Plastics
' September I, 2005
for Fort Collins the intensity for the Basin 2 time of concentration of 6.8 minutes has increased to
8.9 in/hr in comparison to the original 7.9 in/hr. Due to the change in intensity the runoff
increased to 16.5 cfs in comparison to the original 14.9 cfs and the revised 14.7 cfs. This is a
' maximum net difference increase of 1.8cfs in runoff which flows to the concrete pan at the east
end of the parking area (see attached photos) located between the garage area and the north end
of the Value Plastics facility. At the east end of the pan the site discharges from Design Point B.
' Design Point B was designed to discharge a maximum 40.45cfs leaving an excess of 7.35cfs
between the maximum,and existing runoff rates. The proposed site revision will increase the
runoff by 1.6cfs and reducing the excess difference to 5.75.
In summary, the proposed improvements do not cause a significant impact on the site and
existing improvements. As a mater of fact the additional stalls, analyzed with the 1995 rainfall
' intensities, do not increase site runoff.
If you should have any questions or comments as you review this report, please feel free to
' contact us at your convenience.
Sincerely,
SERVICES, INC.
D:\Projects\126-002\Drainage\Reports\126-002-DRNG-RPRT#001.d6c 2
Basil Hamden
Value Plastics
September I. 2005
3
cis
DAProjects\126-002\Drainage\Reports\126-002-DRNG-RPRT#001.doc 3
Basil Hamden
Value Plastics
September 1, 2005
Co -
I
F&000.1 1 14 :59
DAProjects\ 126-002\Drainaae\Reports\ 126-002-DRNG-RPRT#001.doc
Weighted "C" calculations / Comparison to Original
pervious
impervious
TOTAL
Original
30,012.84
60,548.40
90,561.24
Additions/subtraction
-2341.73
2341.73
Additions/subtraction
1134.13
-1134.13
28,805.24
61,756.00
90,561.24
Runoff Coefficient - C =
0.25
0.95
CALCULATED Wei hted runoff coefficient -SITE
0.7147
ORIGINAL Wei hted runoff coefficient -SITE
0.72
text = this information was taken directly from Basin 2 of the original drainage report dated 2-13-95 by
Water, Waste, & Land, Inc.
Time
Intensity i h
5
9.95
5.5
9.63
6
9.31
6.5
9.055
7
8.8
6.8
8.8966
From Interpolation
7.5
8%9
8
8.38
Tc = 6.8min
0 cfs
i in/hr
Cc
Cf
A
0100 revised to new rainfall =
16.52
8.8966
0.71
1.251
2.079
0100 1995 rainfall =
14.67
7.91
0.71
1 1.251
9
Difference in Runoff due to rainfall
1.85
Difference in Runoff (original calc and new talc
Qorginal 1 14.9 From Table 1 of the original report
Qnew(1995 rain) 1 14.67 1 From calculations above
D:\Projects\126-002\Drainage\Reports\Weighted C calculations.xls
No Text