HomeMy WebLinkAboutDrainage Reports - 02/19/2014City of Ft. Collins A love Plans
Approved By
Date
October 23, 2013
Mr. Wes Lamarque
City of Fort Collins
Storm Water Utility
700 Wood Street
Fort Collins, CO 80522
INTERYVEST CONSULTING GROUP
Re: Liberty Common Charter School — MINOR AMENDMENT
Storm Drainage Letter
Dear Wes,
Please accept the following letter on behalf of the Liberty Common Charter School to
demonstrate the site's compliance with the previously approved Drainage Letter for Liberty
Common Charter School dated December 11, 2011. The 2011 letter was based on previously
accepted Final Drainage Reports for Rigden Farm Filing One and Filing Six, as prepared by JR
Engineering, and the Drainage Letter for Pioneer Charter School dated August 20, 2003, as
prepared by Interwest Consulting Group.
The project was originally approved by the City in December 2011. Those plans were split into
two construction phases. Phase I was completed in the summer of 2012 and a drainage
certification for the Phase I improvements was approved by the City on August 27, 2012. The
minor amendment proposes to split Phase II as previously shown on the approved plans into a
Phase Ha and a Phase IIb. Phase Ha will include a 9,700 square foot building addition and will
align the pond in the ultimate configuration. Phase IIb will include an 11,120 square foot
building addition, a new parking lot located in the northeast corner of the site, and modifications
to the existing parking. lot. The purpose of this letter is to confirm that the improvements
associated with Phase IIa and IIb are in compliance with the original drainage reports and that
the volume of the reconfigured detention pond is in compliance as well.
In the previously approved plans, the existing pedestrian bridge that crosses the channel was
proposed to be relocated to provide access between the building addition and the new parking lot
in the northeast corner of the site. There were also a series of retaining walls tying into the
headwall of the proposed 30-inch pipe. The minor amendment eliminates the headwalls and the
bridge crossing and extends the 30-inch pipe to a flared end section. To make up for the lost
volume in the pond, the play field area north of the Phase I building expansion was lowered by
several feet.
Previously approved drainage reports established the peak storage requirement for this pond
which is 2.2 acre-feet at elevation 4924.00. With the grading revisions associated with this
Mr. Wes Lamarque
October 23, 2013
Page 2 of 2
minor amendment, the proposed pond volume at elevation 4924.00 is 2.43 ac-ft. Please refer to
the attached stage/storage calculations.
Previous drainage reports established an average imperviousness of 0.79. The actual
imperviousness of the site is 0.52 with a composite C Factor of 0.63. Therefore, it is requested
that the site be deemed in substantial conformance with the previously approved Final Drainage
Reports.
Although water quality for the site is downstream, low impact development (LID) is in use for
this development via the 3' by 3' filter channel along the bottom of the pond thereby managing
stormwater as close to its source as possible.
Thank you for your time in reviewing this minor amendment to the approved plans. If you have
any questions or comments please contact me at (970) 460-1091.
Sincerely,
Skylar Brower, P.E.
Colorado Professional Engineer 44248
Interwest Consulting Group
Attachments
I
RIGDEN FARM THIRTEENTH N
LOT i
EX PROPERTY
LINE (TYP) a
30" RCP
FL=4718.7
W FL=4719.1 SF
E..�
V,
ir
� 30" R.
P�
mac-
/
I ,•
Pond 217 REVISED
AS BUILT Detention Pond - Stage/Storage
LOCATION: LIBERTY COMMONS CHARTER SCHOOL
PROJECT NO: 1150-028-00
COMPUTATIONS BY: SMB
SUBMITTED BY: INTERWEST CONSULTING GROUP
DATE: 10/23/2013
PROPOSED POND PHASE I (keen existina bridael
Surface
Incremental
Total
Total
Stage
(ft)
Area
(ft2)
Storage
(ac-ft)
Storage
(ac-ft)
Storage
(cubic feet)
17.20
0
0.00
18.00
3480
0.02
0.02
928
19.00
7408
0.12
0.14
6250
20.00
10825
0.21
0.35
15312
21.00
15216
0.30
0.65
28271
22.00
20055
0.40
1.05
45851
23.00.
24897
0.51
1.57
68283
24.00.
30157
0.63
2.20
95768
AS BUILT POND PHASE I (keep existina bridae)
Surface
Incremental
Total
Total
Stage
(ft)
Area
(fe)
Storage
(ac-ft)
Storage
(ac-ft)
Storage
(cubic feet)
17.20
0
0.00
18.00
3055
0.02
0.02
815
19.00
8269
0.13
0.14
6265
20.00
12371
0.24
0.38
16516
21.00
16620
0.33
0.71
30959
22.00
21261
0.43
1.14
49852
23.00
26092
0.54
1.69
73488
24.00
31394
0.66
2.35
102190
NEW PHASE II W/ PIPE
Surface
Incremental
Total
Total
Stage
(ft)
Area
(ft2)
Storage
(ac-ft)
Storage
(ac-ft)
Storage
(cubic feet)
17.20
0
0.00
18.00
2028
0.01
0.01
541
19.00
5603
0.08
0.10
4208
20.00
8705
0.16
0.26
11305
21.00
15999
0.28
0.54
23474
22.00
26670
0.48
1.02
44582
23.00
30168
0.65
1.68
72983
24.00
35969
0.76
2.43
106009
detention -PHASE II.As
Interwest Consulting Group
RUNOFF COEFFICIENTS & % IMPERVIOUS
LOCATION: Liberty Common Charter School
PROJECT NO: 1150-028-00
COMPUTATIONS BY: SB
DATE: 1012N2013
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:
Landscape Areas:
Runoff
%
coefficient
Impervious
C
0.95
100
0.95
96
0.95
90
0.25
0
SUBBASIN
DESIGNATION
TOTAL
AREA
(so.)
TOTAL
AREA
(sq.ft)
ROOF
AREA
(sq.ft)
PAVED
AREA
(sq.ft)
SIDEWALK
AREA
(sq.ft)
LANDSCAPE
AREA
(sq.ft)
RUNOFF
COEFF.
(C)
%
Impervious
1
1.55
67,385
9,142
23,214
6,461
28,548
0.65
56
2
2.46
107,337
30,440
11,680
7,908
57,309
0.58
.43
3
0.71
30,783
0
16,520
6,180
8,083
0.77
73
Total 1
4.72 1
205,505
39,582 1
51,414 1
20,569 1
93,940
0.63
52
Equations
- Calculated C coefficients & % Impervious are area weighted
C=E(Ci Ai) /At
Ci = runoff coefficient for specific area, Ai
Ai = areas of surface with runoff coefficient of Ci
n = number of different surfaces to consider
At = total area over which C is applicable; the sum of all Ai's
PHSII-Cval and Imperviousness.xls