HomeMy WebLinkAboutDrainage Reports - 02/12/2014 (3)Cite of Ft. CollinyprovqjPlans
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January 16, 2014
Mr. Wes Lamarque
City of Fort Collins Stormwater Department
700 Wood Street
Fort Collins, CO 80521
RE: Drainage Memorandum — Harmony Technology Park Third Filing, Second Replat
Dear Wes:
The purpose of this drainage memorandum is to summarize the proposed drainage design and
provide calculations for the additional impervious area runoff being added by the 23 parking stalls,
which will be located along the south side of the existing Custom Blending Building. The stalls will
add approximately 3,870 square feet of additional impervious area to the site and will replace a
portion of the existing grass median island, located along the south side of building. Aspen is
providing the following analysis and supporting calculations to demonstrate the added parking stall
area (additional impervious area) has very little effect on the overall site runoff and masterplan
drainage area (HTP Masterplan Area). Please refer to the attached Drainage Exhibit and
accompanying rational method calculations for details.
The proposed 3,870 square foot parking stall area will tie directly to the southern drive aisle (at the
south end of the stalls), with the north side of the stalls fronting the existing green space, located
south of the existing building. The additional 3,870 square foot impervious area will increase the `c'
value for Basin 2 from 0.53 to 0.67, which also provides for a slight increase in the overall �site .`c'
value from 0.68 to 0.69 (overall site impervious ratio increases from 65% to 67%). It's important to
note that the existing site, including the added impervious area, continues to fall below the allowable
`c' value of 0.80, thereby, continuing to meet the intent of the HTP Drainage Masterplan.
The calculated 2-year time -of -concentration. for Basin 2 is 7.3 minutes, versus the previous 8.8
minutes. The 2-year rational method flow for Basin 2 will increase from 0.52 cfs to 0.70 cfs. The
calculated 100-year time -of -concentration for Basin 2 is 5.4 minutes, versus the previous 7.3
minutes, while the 100-year rational method flow for Basin will increase from 2.43 cfs to 3.40 cfs.
The decrease in time of concentrations (faster travel times) and slight increase in the 2-year and 100-
year runoff rates appear to be negligible and have very little effect on the drainage patterns or runoff
for the Custom Blending site, and in relation to the HTP Masterplan basin.
Stormwater runoff from Basin 2 will continue to follow the historic drainage path to the south and
will be routed through the LID basin(s), prior to flows spilling south and into the existing water
quality and detention pond. It's important to note that Aspen met with the City of Fort Collins
Stormwater and Stormwater Quality Departments, in which it was determined that additional LID
measures are not required for the increase in impervious area, as the LID measures proposed in the
HTP Third Filing, Second Replat Utility plans provide the necessary LID measures, all of which are
in addition to the existing water quality/detention pond, which provides all of the required water
quality and detention for the entire Custom Blending site.
In summary, the proposed 3,870 square feet of additional parking stall area on the south side of the
existing Custom Blending Building meets the intent of the "Final Drainage and Erosion Control
Report for Harmony Technology Park Third Filing, Second Replat", dated August 16, 2013, by
Aspen Engineering, and also the "Final Drainage and Erosion Control Report, Harmony Technology
Site Master Plan, by Stantec Consulting, dated May 19, 2008. The proposed improvements appear
to have little effect on the existing drainage system and existing facilities and meet the intent of
approved plans and drainage reports for the HTP Third Filing, Second Replat, and the HTP
Masterplan plans.
Should you have any questions or concerns, please contact me.
Sincerely,
ORA�O C�CF
Ov: '•FtYAN ' •. ti�
G'�P:
3251
NAL ECG\
John Gooch, P.E.
Aspen Engineering
Developed Weighted Runoff Coefficients
Custom Bfendmg
101-001
Basin RootAma(sqft)
•r
Asphalt Area (sqft)
r
Concrete Area (sqtf)
r•r
GmvelArea(sqft)
r,
LawnArea(sqh)
r
TatalArita
Composite
2
0
7,315
3.870
0
7.141
18,326
0.67
3
0
33.788
0
0
2,540
36,328
0.90
4
0
21.240
0
0
938
22,178
0.92
5
0
19.161
0
0
769
19,930
092
6
0
0
12.265
0
739
13,004
0.86
7
0
862
690
0
4,125
5,677
044
8
0
0
0
0
45.120
45,120
0.25
131
0
9,200
0
0
1,255
10,454
0.87
132
r29,255
0
0
10.820
40.075
076
Total SITE
122.840
16.825
0
78,218
217.883
Total Acreage
0.00
2.92
0.39
0.00
1.80
Si
a r = a, 53 ( P964IH.5 )
Impervious Katto
No.
1
0
2,019
0
0
4,771
(Sqft)
6.790
Ratio
33"6
2
0
7.315
3.870
0
7,141
18.326
6316,
3
0
33,788
0
0
2,540
38.328
93 A,
4
0
21.240
0
0
938
22.178
96 %
8
0
19,161
0
0
769
19,930
96%
6
0
0
12,265
0
739
13,004
95 2
7
0
962
690
0
4,125
5.677
314
8
0
0
0
0
45.120
45.120
5,
Total SITE
0
Re 3A5
16,825
0
68,143
167,353
677
Total Acreage
0.00
1.94
0.39
0.00
1.52
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