HomeMy WebLinkAboutDrainage Reports - 06/28/2011City of Ft. Collins A roveq f lans
MPS BY
Date
June 2, 2011
Mr. Wes Lamarque
City of Fort Collins Stormwater Department
700 Wood Street
Fort Collins, CO 80521
RE: Drainage Memorandum — Poudre Valley Hospital Lemay Campus Therapeutic
Garden: Approximate 16,760 Square Foot improvement area
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 approximate
16,760 square foot therapeutic garden area, which will replace the existing grass median island,
located between existing parking lots on the east side of the existing hospital building. Please refer
to the attached Utility and Drainage Exhibit, Erosion Control Exhibit, and rational method and other
hydrologic calculations, all of which accompany this memorandum.
In general, the project will consist of an approximate 16,760 square foot therapeutic garden area on
the east side of the existing Poudre Valley Hospital Building. The garden will be approximately 55
feet wide by 305 feet long and will entail both pervious and impervious walking paths, as well as
landscaping, sitting areas, and other site amenities.
The proposed garden will replace the existing grass/sodded area island, with the existing sidewalk on
the north side of the island and majority of the existing trees being utilized and incorporated into the
garden project. The calculated 100-year time -of -concentration and rational method flow from the
garden area is 9.2 minutes and 2.03 cfs respectively, versus the calculated historic 100-year time -of -
concentration and rational method flow of 10.9 minutes and 1.23 cfs respectively (see attached
rational calculations). This is an increase of 0.8 cfs due to the new garden area.
Upon calculating the surface area of the existing ponds at their 100-year WSEL level (4961.00) and
utilizing the calculated FAA volume that is required for the new garden area, the estimated increase
in the ponds' elevations was arrived at. The change in volume due to the proposed garden is
approximately 0.022 ac-ft: Approximately 1/3 of the garden area will flow northeast and into the
existing northeast detention pond, while the remaining 2/3 will flow southeast and into the existing
southeast detention pond. The calculated/estimated rise in the northeast and southeast pond
elevations, due to the proposed garden, could be 0.01 feet and 0.02 feet, respectively, which is
considered negligible (see attached calculations). It's important to note the newly added impervious
areas within the garden will flow overland and across pervious areas within the garden which will
allow some infiltration and provide a mechanism for pollutants to settle out of the runoff. Therefore,
the potential rise in elevation within the ponds could be lower than the calculated/estimated rise in
elevation.
Due to the travel path and associated time of concentration for the developed flows to reach the
existing detention ponds, (NE Pond being approximately 160 linear feet to east/northeast of the
proposed garden area; SE Pond being approximately 200 linear feet to the east/southeast of the
proposed garden area), the additional 0.8 cfs of runoff from the therapeutic garden site will have
little effect on the existing detention ponds and storm drainage system.
Stormwater runoff from the proposed garden area will continue to follow the historic drainage paths
to the east/northeast and east/southeast and into the existing detention ponds, via overland flow and
routing in the existing curb and gutter on the east side of the street.
Because the site and proposed improvements are less than 1 acre, a Stormwater Management Plan is
not required. However, the contractor is urged to follow the proposed erosion control measures
shown in the provided Erosion Control Exhibit to mitigate erosion control problems until the
proposed landscaping/seeding becomes established.
In summary, the proposed 16,760 square foot therapeutic garden area on the east side of the existing
Poudre Valley Hospital building meets the intent of the "Final Drainage and Erosion Control Study
for the Poudre Valley Hospital P.U.D, Fort Collins, Colorado", dated September 24, 1992 by RBD,
Inc., and also the "Addendum to the Drainage and Erosion Control Report Poudre Valley Hospital
P.U.D., Fort Collins, Colorado", dated March 15, 2000, by The Sear Brown Group. The proposed
improvements appear to have little effect on the existing drainage system and facilities and meet the
intent of approved plans and drainage reports for the PVH Lemay Campus.
Should you have any questions or concerns, please contact me.
Sincerely,
VLYAN
John Gooch, P.E.
Aspen Engineering
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3
100 Year
Project
PVHS 4gM cGarden Condition
Design by.,
J.Goc
Date:
5f52m11
TI'M is 10 convert % Im . t0 a C value
1 ar
must insert % Imp. and C pervious).
I
R ufred detention
ft
acre-ft.
'C' value
0.34
�s�, 969:6_x3
I WO! 0223k{
V * 1.25
0.425
Area
0.38
acres
Modified
Modified
Release FatF
WLIFATER
D. JUDIS
C. LI
5195
Nov-97
Nov-98
DETENTION
POND SIZING
TIME
TIME
INTENSITY
0100
Runoff
Release
Required
Required
am
1o0 year
Volume
Cum total
Detention
Detention
mina
secs
Nh
cis
fl"3
ft"3
r3
ao-ft
0
0
0
0.00
0
0.0
0.0
0.0000
5
30C
9.950
1.61
482.0775
60.0
422.1
0.0097
10
600
7.720
1.25
748.088
120.0
628.1
0.0144
15
900
6.520
1.05
947.682
180.0
767.7
0.0176
20
1200
5.600
0.90
1085.28
240.0
845.3
0.0194
25
15001
4.980
0.80
1206.405
30D.0
906.4
0.0208
30
1800
4.520
0.73
1313.964
360.0
954.0
0.0219
351
21001
4.080
0.66
1383.732
420.0
963.7
0.0221
40
2400
3.740
0.60
1449.624
480.0
969.6
0.0223
45
2700
3.460
0.66
7508.733
540.0
968.7
0.0222
50
3000
3.230
0.52
1564.935
600.0
964.9
0.0222
55
3300
3.030
0.49
1614.839
660.0
954.8
0.0219
60
3600
2.860
0.46
1662.804
720.0
942.8
0.0216
65
3900
2.720
0.44
1713.192
780.0
933.2
0.0214
70
4200
2.590
0.42
1756.797
840.0
916.8
0.0210
75
4500
2.480
0.40
1802.34
900.0
902.3
0.0207
CO
4800
2.380
0.38
1844.976
960.0
885.0
0.0203
85
5100
2.290
0.37
1886.159
0020.0
866.2
0.0199
90
5400
2210
0.36
1927.341
1080.0
847.3
0.0195
95
5700
2.130
0.34
1960.772
1140.0
820.8
0.0188
100
6000
2.060
0.33 1996.14
1200.0
796.1
0.0183
105
6300
2.000
0.32 2034.9
1260.0
774.9
0.0178
110
6600
1.940
0.31 2067.846
1320.0
747.8
0.0172
115
6900
1.890
0.31 2106.122
3380.0
726.1
0.0167
120
7200
1.840
0.30 2139.552
1440.0
699.6
0.0161
125
7500
1.790
0.291 2168.138
1500.0
668.1
0.0163
130
7800
1.750
0.28 2204.475
1560.0
644.5
0.0148
135
8100
1.710
0.281 2236.937
1620.0
616.9
0.0142
140
8400
1.670
0.27 2265.522
1680.0
585.5
0.0134
145
8700
1.6301
026 2290232
1740.0
550.2fl0.0121
150
9000
1.600
0.26 2325.6
1800.0
525.E155
9300
1.570
0.25 2358.062
1660.0
498.1160
9600
1.540
0.25 2387.616
1920.0
467.6165
99M
1.510
024 2414.264
1980.0
434.3170
10200
1.480
0.24 2438004
2040.0
398.0175
10500
1.450
0.23 2458.838
2100.0
358.8180
10800
1.420
0.23
2476.764
2180.0
316.8185
11100
1.400
0.23
2509.71
2220.0
289.7
190
11400
1.380
0.22
2540.718
2280.0
260.7
0.0060
195
11700
1.360
0.22
2569.788
2340.0
229.8
0.0053
200
12000
1.340
0.22
2596.92
2400.0
196.9
0.0045
205
12300
1.320
0.21
2622.114
2460.0
162.1
0.0037
210
12600
1.300
0.21
2645.37
2520.0
125.4
0.0029
215
12900
1.280
0.21
2666.686
2580.0
80.71
0.0020
220
13200
1.260
U.201
213156.0581
2540.0
46A
0.0011
225
13500
1240
0.20
2703.51
2700.0
3.5
0.0001
230
13800
1.220
0.201
27moul
2760.0
41.0
-0.0009
235
141001
1.210
0.201
2756 3621
2820.0
$1.6
-0.0015
2401
144001
1.200
0.19
2790.72
2880.0
-89.3
-0.0020
�i1'rV� I�d•f�w5
Page 1
100 Year
Project
PVHS Lemay Therapeutic Garden Proposed Condition
Design by.,
J.Gooch
Date:
5/52011
This is to convert % imp. to a C value
10 ar
(must insert % imp. and C
Required detention
8s
acre -it.
'C' value
0.53
1924.4-..
',0f04421?
'C' • 125
0.6625
Area
0.38
acres
Modified
Modified
Release Rate
0.2
M. FATER
D. JUDISF
C. U
5/95
NOV-97
Nov-98
DETENTION
POND SONG
TIME
TIME
INTENSI
0100
Runoff
Release
Required
Required
crap
100 r
Volume
Crmr total
Detention
Detention
mina
secs
ds
ff"3
ff"3
8"3
aoff
0
0
0
0.00
0
0.0
0.0
0.0000
5
300
9.950
2.50
751.4738
60.0
691.5
0.0159
10
600
7.720
1.94
1166.106
120.0
1046.1
0.0240
15
900
6.520
1.64
1477.269
180.0
1297.3
0.0298
20
1200
5.600
1.41
1691.76
240.0
1451.8
0.0333
25
1500
4.980
1.25
1880.573
300.0
1580.8
0.0383
30
1800
4.520
1.14
2048.2381
360.0
1688.2
0.0388
35
2100
4.080
1.03
2156.994
420.0
1737.0
0.0399
40
2400
3.740
0.94
2259.708
480.0
1779.7
0.0409
45
2700
3.460
0.87
2351.849
540.0
1811.8
0.0416
so
3000
3230
0.811
2439.468
600.0
1839.5
0.0422
55
3300
3.030
0.76
2517248
660.0
1857.2
0.0426
60
3600
2.860
0.72
2592.018
720.0
1872.0
0.0430
65
3900
2.720
0.68
2670.564
780.0
1890.6
0.0434
70
42W1
ZS90
0.65
2738.537
840.0
1898.5
0.0436
75
4500
1 2.480
0.62
2809.53
900.0
1909.5
0.0438
80
48001
2.380
0.60
2876.9921
960.0
1916.0
0.0440
85
5100
2.290
0.58
2940.188
1020.0
1920.2
0.0441
90
5400
2210
0.56
3004.385
1080.0
1924.4
0.0442
95
5700
2.130
0.54
3056.497
1140.0
1916.5
0.0440
100
6000
2.060
0.52
3111.63
1200.0
1911.6
0.0439
105
6300
2.000
0.50
3172.05
1260.0
1912.1
0.0439
110
6600
1.940
0.49
3223.407
1320.0
1903.4
0.0437
115
6900
1.890
0.48
3283.072
1380.0
1903.1
0.0437
120
7200
1.840
0.46
3335.184
1440.0
18952
0.0435
125
7500
1.790
0.45
3379.744
1500.0
1879.7.
0.0432
130
7800
1.750
0.44
3436.388
1560.0
1876.4
0.0431
135
8100
1.710
0.43
3486.989
1620.0
1867.0
0.0429
140
8400
1.670
OA2
3531.549
1680.0
1851.5
0.0425
145
8700
1.630
0.41
3570.067
1740.0
1830.1
0.0420
150
9000
1.600
0.40
3625.2
1800.0
1825.2
0.0419
166
9300
1.570
0.40
3675.802
1860.0
1815.8
0.0417
160
96M
1.640
0.39
3721.872
1920.0
1801.9
0.0414
165
9900
1.510
0.38
3763.411
1980.0
1783.4
0.0409
170
10200
1.480
0.37
3800.418
2040.0
1760.4
0.0404
175
10500
1.450
0.37
3832.894
2100.0
1732.9
0.0398
160
10800f
1.420
0.36
3860.638
2160.0
1700.8
0.0390
185
111001
1.400
0.35
3912.195
2220.0
16922
0.0388
190
11400
1.380
0.35
3960.531
2280.0
1680.5
0.0386
195
11700
1.360
0.34
4005.846
2340.0
1685.8
0.0382
200
12000
1.340
0.34
4048.14
2400.0
1648.1
0.0378
205
12300
1.320
0.33
4087.413
2460.0
1627.4
0.0374
210
12600
1.300
0.33
4123.665
2520.0
1603.7
0.0368
215
12900
1.280
0.32
4156.896
2580.01
1576.91
0.0362
220
13200
1.260
0.32
4187.106
2640.0
1547.1
0.0355
225
135M
1240
0.31
4214.295
2700.0
1514.3
0.0348
230
13800
1220
0.31
4238.483
2760.0
1478.5
0.0339
235
14100
1.210
0.30
4295.107
2620.0
1475.1
0.0339
2401
144001
1.2001
0.30
4&-10.241
2880.0
1470.2
0.0338
0.099 AC-# cf
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