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ADDED IMPERVIOUS ARE
AREA 1 = 9,839 ft2
AREA 2 = 8,154 ft2
AREA 2 (ACTUAL)= 1,200 ft2
AREA 3 = 9,657 ft2
AREA 4 = 7,370 ft2
TOTAL ADDED IMPERVIOUS AREA
=35,020 ft2 (0.80 ACRES
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HOSPITAL
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0 60' 90' 120'
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EXISTING BASIN PROPOSED/MODIFIED BASIN
TOTAL AREA = 59.5 ACRES IMPERVIOUS = 32.34 ACRES
% IMPERVIOUS = 53% PERVIOUS = 27.17 ACRES
IMPERVIOUS = 31.54 ACRES % IMPERVIOUS = 54%
PERVIOUS = 27.97 ACRES
I (111'RIL:HT C 200)
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PROJECT
PROJLCi NO.
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POUDRE VALLEY HOSPITAL
187010241
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LEMAY CAMPUS
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209 South Me dNm
FORT COLLINS, CO
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FL COIIInS, CO. 80521-2603
Tel:(970) 482-5922
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HIS OR•.N o. Fax:(970) 482-6368
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ADDED IMPERVIOUS AREA MAP
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6/27/2007 10:41:13 MI
Stantec
November 21, 2007
' Page 6 of 6
Since the basin is already highly developed, and the potential for adversely impacting
' flooding conditions along Spring Creek may be limited in the case of some
developments, it may be acceptable to exempt certain infill developments from the
identified detention requirement if several conditions can be met: (a) the proposed
' development can be shown through rigorous engineering analysis and modeling that
100-year flows along Spring Creek would not increase; and (b) the 100-year release rate
from the development site and all upstream tributary areas can be conveyed to Spring
' Creek without adversely impacting existing structures, property and infrastructure.
(Anderson Consulting Engineers, Inc., Spring Creek Basin Master Drainage Plan, 2003;
pg X-18)
' Best Management Practices for Detention and Water Quality
' Detention and water quality will be provided for the proposed development in the form of a
porous landscape detention trench as recommended in Urban Drainage and Flood Control
District's Urban Storm Drainage Criteria Manual Volume 3. This is in the form of a pervious
' absorbent trench approximately 3 to 5 feet deep with a perforated pipe in the bottom to drain the
trench. The trench is located along the entire perimeter of the parking structure in order to
provide treatment for all of the additional roof area. Runoff will surface drain to the trench from
the building. The trench will convey flows into the existing storm sewer inlet located in the north
' flowline of Garfield Street. Detention will be provided in the void space of this trench per
discussions with the City of Fort Collins. The total added impervious area is determined to be
approximately 10,000 square feet and is considered to be one drainage basin for the
' calculations. The total required detention volume is 2100 cubic feet and the volume provided by
the proposed trench is approximately 2174 cubic feet. Calculations for this proposed trench can
be found in Appendix C.
' In conclusion, this project shows a minimal impact to the downstream drainage as a result of
this site. In addition, the improvements provide storm water detention and a water quality
' feature to improve the condition of the site runoff, resulting in a site that either meets or exceeds
the previous conditions.
1
Sincerely,
STANTEC CONSULTING INC.
Jim Allen -Morley, P.E.
Senior Project Manager
Tel: (970) 482-5922
Fax: (970) 482-6368
'
jallenmorley@stantec.com
I
Stantec
November 21, 2007
Page 5 of 6
10-Year Event
Conveyance
Channel
Scenario
Discharge
Depth
Flow
Wetted
ter
Perimeter
TopWidth
Element
Slope
(cfs)
(ft) Area (ft')
(ft)
422
0.004
Baseline
96.70
0.84
39.40
99.43
98.41
Modified
98.30
0.85
40.00
100.03
99.01
Difference
1.60
0.01
0.60
0.60
0.60
522
0.017
Baseline
293.10
0.99
55.10
114.99
113.97
Modified
294.40
0.99
55.30
115.19
114.17
Difference
1.30
0.00
0.20
0.20
0.20
524
0.006
Baseline
343.90
1.14
72.50
130.12
129.10
Modified
345.20
1.14
72.50
130.12
129.10
Difference
1.30
0.00
0.00
0.00
0.00
100-Year Event
Conveyance
Channel
Scenario
Discharge
Depth
Flow
Wetted
Perimeter
TopWidth
Element
Slope
(cfs)
(ft)
Area (ft')
(ft)
422
0.004
Baseline
265.70.
1.14
72.50
130.12
129.10
Modified
269.60
1.14
72.50
130.12
129.10
Difference
3.90
0.00
0.00
0.00
0.00
522
0.017
Baseline
894.90
1.34
101.00
151.64
150.61
Modified
P97.90
1.34
101.30
151.83
150.80
Difference
3.00
0.00
0.30
0.19
0.19
524
0.006
Baseline
1046.10
1.78
177.10
197.88
196.84
Modified
1049.40
1.78
177.60
198.13
197.09
Difference
3.30
0.00
0.50
0.25
0.25
These results indicate that the change in impervious area within Basin 222 result in very minor
hydraulic differences. Flow depth changes range from zero to 0.01 feet. Expected flow top
width increases are typically much less than a foot.
In general, the low level of hydrologic and hydraulic impacts associated with the proposed
modifications to Basin 222 suggests that no significant impacts to the existing conditions can be
expected. As discussed in the Spring Creek Master Drainage Plan, these results demonstrate
that it is reasonable to exempt the proposed development from any detention storage or other
requirements.
Stantec
November 21, 2007
Page 4 of 6
20
15
x
$ 10
a
G
5
0
0
Typical Lemay Street
Cross -Section
100 200 300 400 500 600
Station
To be consistent with the SWMM model the following additional hydraulic parameters were
used.
Conveyance
Element
Invert Slope
ft./ft.
Manning N
Street
Manning N
Overbank
422
0.0040
0.016
0.035
522
0.0170
0.016
0.035
524
0.0060
0.016
0.035
Using the open channel calculator in Haestad ".Method's FlowMaster the following resulting
channel depths and top widths were computed.
2-Year Event
Conveyance
Channel
Scenario
Discharge
Depth
Flow
Wetted
Perimeter
TopWidth
Element
Slope
(cfs)
(ft)
Area (ft')
(ft)
422
0.004
Baseline
48.00
0.64
21.60
77.99
76.97
Modified
48.90
0.65
22.00
78.45
77.44
Difference
0.90
0.01
0.40
0.46
0.47
522
0.017
Baseline
124.00
0.70
26.20
84.01
82.99
Modified
124.70
0.70
26.30
84.17
83.15
Difference
0.70
0.00
0.10
0.16
0.16
524
0.006
Baseline
145.20
0.92
47.10
107.30
106.28
Modified
146.00
0.92
47.30
107.52
106.50
Difference
0.80
0.00
0.20
0.22
0.22
Stantec
November 21, 2007
Page 3 of 6
Difference
422:4
1.6 1.7%
0
0
522:4
1.3 0.4%
0
0
524:4
1.3 0.4%
0
0
100-Year Event
Scenario
Conveyance
Element/Type
Peak (cfs)
Stage (ft)
Time
hr/min
Baseline
422:4
265.7
1.1
0 38
522:4
894.9
1.3
0 43
524:4
1046.1
1.7
0 43
Modified
422:4
269.6
1.1
0 38
522:4
897.9
1.3
0 43
524:4
1049.4
1.7
0 43
Difference
422:4
3.9 1.5%
0
0
522:4
3.0 0.3%
0
0
524:4
3.3 0.3%
0
0
These results show that the anticipated peak runoff increases due to the proposed small
increase in impervious area within Basin 222 are not expected to be large. The increases range
from a little under one cfs in the 2-year event to approximately 3 to 4 cfs during the 100-year
event._ In terms of a percent increase in peak runoff, the results range from under 2% in
Conveyance Element 422 to around than 0.5% in Conveyance Elements 522 and 524.
0
Hydraulic Analysis
To better define the hydraulic impacts on the street section the flow increases for each case
were examined using Haestad Open channel flow calculator. A typical street section was
developed based on a measured 62-foot, curb -to -curb width with a 2% crown slope. Extending
beyond the curb, the section was assumed to rise at a 2% slope as illustrated im'the following
table and figure.
Cross -Section
x
y
Comment
0
5.5
2% slope
250
0.5
2% slope
250
0
6-inch curb
281
0.62
CL Lemay
312
0
6-inch curb
312
0.5
2% slope
562
5.5
2% slope
N
Stantec
November 21, 2007
Page 2 of 6
4) Modify SWMM input data file and re -run the model. Compare computed discharge
estimates between the two cases (Baseline and Modified) for all downstream
conveyance elements.
5) To better define hydraulic impacts within each downstream conveyance element, an
open channel flow calculator was used to quantify impacts to flow depth and topwidth.
SWMM Model Results
Based on the increased impervious area illustrated in Figure 1, it was determined that the %
imperviousness for the basin (Basin 222) would increase from the Baseline value of 53% to
54%. The resulting SWMM estimated peak discharges for the basin and the three downstream
conveyance links ((422,522, and 524) are summarized in the following tables for the 2, 10, and
100-year events.
2-Year Event
M
Scenario
Conveyance
Element/Type
Peak (cfs)
Stage (ft)
Time
hr/min
Baseline
422:4
48.0
.6
0 40.
522:4
124.0
.7
0 48.
524:4
145.2
.8
0 49.
Modified
422:4
48.9
.6
0 40.
522:4
124.7
.7
0 48
524:4
146.0
.8
.0 49.
Difference
" 422:4
0.9 1.9%
0
0
522:4
0.7 0.6%
0
0
524:4
0.8 0.6%
0
0
10-Year Event
Scenario
Conveyance
Element/T pe
Peak (cfs)
Stage (ft)
Time
hr/min
Baseline
422:4
96.7
.8
0 40.
522:4
293.1
.9
0 47.
524:4
343.9
1.1
0 48.
Modified
422:4
98.3
.8
0 40.
522:4
294.4
.9
0 47
524:4
345.2
1.1
0 48.
stantec Consulting Inc.
209 South Meldrum Street
Fort Collins CO 80521-2603
Tel: (970) 482-5922 Fax: (970) 482-6368
stantec.c:orn
s
Stantec
November 21, 2007
City of Fort Collins
Storm Water Utility
700 Wood Street
Fort Collins CO 80521
Drainage Report Letter for the Parking Structure at the PVHS Lemay Campus
Attention: Stormwater Drainage Utility Staff
Dear Sir:
The purpose of this letter is three fold. One to provide the calculations that show minimal rise
due to the downstream drainage system when the improvements to the Lemay Campus are
undetained, Two to provide the drainage and water quality calculations for the proposed porous
landscape detention trench and Three to show that detention for the parking structure has been
adequately provided.
Downstream Drainage Analysis
The purpose of this section is to describe the method and results of an analysis of the impacts
to storm water discharge associated with the proposed plan to increase the impervious area at
several locations near the intersection of Garfield Street and South Lemay Avenue, as
illustrated in Figure 1 (this includes the parking structure as well as other proposed
improvements).
Approach
The project area is located entirely within Basin 222 of the Spring Creek Basin following the
conventions employed within the Spring Creek Basin Master Drainage Plan. To assess
increases in the impervious area within this basin the following actions were taken: '
1) Compute increased impervious area (Figure 1).
2) Obtain existing SWMM model from City of Fort Collins
3) Compute change in basin percent impervious parameter based on area obtained in Step
1.
Poudre Valley Hospital
Second Filing
Parking Structure
Drainage Report Letter
Prepared for:
Poudre Valley Hospital
Prepared by:
Stantec Consulting, Inc.
209 South Meldrum
Fort Collins, Colorado 80521
(970)482-5922
StanteC