HomeMy WebLinkAboutDrainage Reports - 12/18/2015❑QUALITY
`x ENGINEERING
December 18, 2015
Mr. Shane Boyle & Mr. Dan Mogen
City of Fort Collins Engineering Department
281 N College Avenue
Fort Collins, Colorado 80524
W W W. QUALITY-ENGIN EERING. COM
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RE: Drainage Memo — 615 E. Plum Street Addition
615 E. Plum Street, Fort Collins (Larimer County), Colorado 80525
QE Project No. 6270-01
Mr. Boyle & Mr. Mogen:
The purpose of this letter is to address drainage impacts to the existing 0.21 acre property located at 615
E. Plum Street due to the proposed building addition and garage. The site is situated in Lot 5, Block
178, City of Fort Collins, County of Larimer, State of Colorado. Specifically, the property is located
midway between Smith Street and Stover Street and bound by East Plum Street to the north and a
common alley access -way to the south. The subject property is surrounded by residential properties to
the west, east, across the alley to south, as well as across Plum Street to the north.
The existing property currently consists of a house, sidewalks, a storage/shop building, and mature
landscaping. The existing site currently drains to a low -point in the lawn area of the southwest corner of
the lot. The existing shop/shed located south of the house will be removed and replace with the
proposed building addition footprint. The proposed work shall consist of an approximately 1,600 square
foot addition onto the existing home enlarging the building footprint, as well as the addition of a
proposed 24-ft x 24-ft detached 2-car garage in the southeast corner of the property.
The proposed grading plan for the site follows historical patterns as closely as possibly, while making an
effort to improve drainage and water quality issues that currently exist on the site. An existing high -
point, located within the northern third of the lot, splits the proposed runoff into north and south basins,
respectively.
The developed flows from the north basin shall continue to sheet flow north, as they do historically, to
the curb and gutter of East Plum Street. Runoff from the south basin will be routed into natural
flowlines conveying the flows into a proposed on -site runoff storage depression constructed with a `Dry -
Well' at the depression's low point. Due to this unique, historical, city drainage basin not having an
outlet, as it is the local low -point, the proposed depression and Dry -Well cannot have an underdrain
outlet. Therefore, the Dry -Well shall be considered as a point of infiltration to mitigate developed storm
water runoff.
QUALITY ENGINEERING, LLC (06) 4164891 2637 MIDPOINT DRIVE, SUITE E _ m FORT COLLINS, COLORAD0
Page 2
Drainage calculations are provided for this project due to the increase of impervious area on the exiting
subject lot and to verify that increased flows will not adversely impact adjacent properties. Since the
existing lot does not have any adverse impact to adjacent properties, a design is proposed to increase the
site's infiltration with the addition of the Dry -Well, in addition to detaining the difference in volume of
runoff due to the site's increased imperviousness. This design was also chosen based on the survey data
provided to the designer being limited to only the subject property, therefore limiting existing versus
proposed drainage conditions analysis.
For a comparison of the existing and proposed sites, the existing 2-year historical flow shall be used for
a baseline for the southern drainage basin. The 100-year, existing flow rate is 0.5 cfs/acre for the subject
property (See calculations in Appendix). Using this flow rate as the allowed release rate for the
proposed development, the site produces a runoff detention volume of 149 cubic feet of more storage
capacity than the existing site due to the proposed building/sidewalk additions.
The rear (south) yard area of the lot was selected as the location for the depression and Dry -Well to
follow existing drainage patterns as closely as possible as well as maintain as much distance as possible
to the surrounding adjacent homes. On December 10, 2015, Soilogic performed a borehole test in the
area of the proposed Dry -Well to determine the existing depth of the clay soil layer. The depth of clay
on -site is approximately 14-feet deep, and therefore the proposed Dry -Well shall also be a depth of 14-
feet, to allow maximum infiltration to the sand layer beneath the clay. Because the Dry -Well is being
extended to the depth of the sand soil layer, it is anticipated that the proposed release rate will be
increased from 0.5 cfs/acre to 0.8 cfs/acre. This difference of 0.3 cfs is accounted for by dividing said
flow by the area of the manhole base (pi*R^2 = 12.56 sf), leaving 0.024 vertical feet of drop in the
manhole per second. This is also equal to 1.4 vertical feet per minute of infiltration.
Per the proposed grading design, the volume capacity of the earthen depression is 128 cubic feet, leaving
21 cubic feet of storage un-accounted for. However, it is also proposed that the Dry -Well be constructed
of a typical 4-foot diameter manhole to a depth of 14-feet. The Dry -Well shall be built with 2.5-feet of
(+/-)9-inch Rip Rap at the bottom of the structure. The top 11.5-feet of the Dry -Well shall be left as
pure storage volume, providing 144 additional cubic feet for a total of 272 cubic feet, which is well over
the 149 cubic foot difference of developed storm water runoff. The development of the lot as proposed,
along with the depression and Dry -Well feature will enhance and improve the drainage on this site, as
well as the entire block, as the back yard of this lot appears to be the local low -point within the area.
The City of Fort Collins Stonnwater manual amending the Urban Stone Drainage and Flood Control
District (USDFCD) has been used for drainage design guidance.
Please see the attached hydrologic basin parameters, 2-year and 100-year existing & developed flow rates.
In summary, the grading and drainage design for the proposed home addition and garage at 615 East Plum
Street complies with the requirements of Urban Storm Drainage and Flood Control District as well as the
City of Fort Collins' Storm Drainage Criteria. The proposed changes have nearly negligible effects on
the historic drainage system of the lot. Please see the attached grading plan and drainage plan. If you
have any questions or require additional information, please call at (970) 416-7891.
Page 3
Sincerely,
QUALITY ENGINEERING, LLC
A& MOAU'"_
Mark Morrison, El
Project Manager
Attachments —
Reviewed by,
G�
Cody eisen rfer, E, CEM
Principal Engineer
Appendix A
-Existing and Proposed composite `C' values
-Existing and Proposed flows (cfs)
-Existing Detention Required (cubic feet)
-Proposed Detention Required (cubic feet)
-Grading/Drainage Plan
Appendix A
-Existing and Proposed composite `C' values
-Existing and Proposed flows (cfs)
-Fort Collins Existing Detention Spreadsheet
-Fort Collins Proposed Detention Spreadsheet
-Grading Plan (2406)
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615 E. Plum Street - Existing Drainage Conditions
cations: A trib. To pond = 0.19 acre
QD=CiA C11K)= 0.34
V; = T*CiA = T*QD Developed C*A = 0.06 acre
Vo=m*Q,.D*(60*T) Qro = 0.50 cfs
S = V, V„ tc = 7.07 min
Rainfall intensity from l.arimer County Area i IDF Curve
Storm
Rainfall
Qo
Vol. In
Outflow
Vol. Out
Storage
Storage
Duration, T
Intensity, i
(cfs)
V,
Adjustment
V.
S
S
(min)
(in/hr)
W)
Factor, in
(fe)
(re)
(ac-ft)
5
9.95
0.6
193
1.00
150
43
0.001
10
7.72
0.5
299
0.85
256
43
0.001
20
5.60
0.4
434
0.68
406
28
0.001
30
4.52
0.3
526
0.62
556
-30
-0.001
40
3.74
0.2
580
0.59
706
-126
-0.003
50
3.23
0.2
626
0.57
856
-230
-0.005
60
2.86
0.2
665
0.56
1006
-341
-0.008
70
2.62
0.2
711
0.55
1156
-445
-0.010
80
2.38
0.2
738
0.54
1306
-568
-0.013
90
2.22
0.1
774
0.54
1456
-682
-0.016
100
2.05
0.1
795
0.54
1606
-811
-0.019
110
1.93
0.1
823
0.53
1756
1 -933
-0.021
120
1.80
0.1
837
0.53
1906
-1069
-0.025
130
1.60
0.1
806
0.53
2056
-1250
-0.029
140
1.40
0.1
760
0.53
2206
-1446
-0.033
150
1.20
0.1
698
0.52
2356
-1658
-0.038
160
1.15
0.1
713
0.52
2506
-1793
-0.041
170
1.10
0.1
725
0.52
2656
-1931
-0.044
Igo
1.05
0.1
733
0.52
2806
-2073
-0.048
[Required Storage Volume: 43 ft'
0.001 acre-ft
615 E. Plum Street - Proposed Drainage Conditions
Equations: A trib. To pond = 0.19 acre
Q1) = CiA Cioo = 0.67
V; = T*CiA = T*Qp Developed C*A = 0.13 acre
V. =m*Qpo*(60*T) Quo = 0.80 cfs
S = Vi - Vp tc = 7.07 min
Rainfall intensity from Larimer County Area I IDF Curve
Storm
Duration, T
(min)
Rainfall
Intensity, i
(in/hr)
Qo
(cfs)
Vol. In
Vi
(ft)
Outflow
Adjustment
Factor, m
Vol. Out
V.
(ft)
Storage
S
(ft)
Storage
S
(ac-ft)
5
9.95
1.2
374
1.00
240
134
0.003
10
7.72
1.0
580
0.85
410
1 171
0.004
20
5.60
0.7
842
0.68
650
192
0.004
30
4.52
0.6
1019
0.62
890
130
0.003
40
3.74
0.5
1125
0.59
1130
-5
0.000
50
3.23
0.4
1214
0.57
1370
-156
-0.004
60
2.86
0.4
1290
0.56
1610
-320
-0.007
70
2.62
0.3
1379
0.55
1850
-471
-0.011
80
2.38
0.3
1431
0.54
2090
-658
-0.015
90
2.22
0.3
1502
0.54
2330
-828
-0.019
100
2.05
0.3
1541
0.54
2570
-1029
-0.024
110
1.93
0.2
1596
0.53
2810
-1214
-0.028
120
1.80
0.2
1624
0.53
3050
-1426
-0.033
130
1.60
0.2
1564
0.53
3290
-1726
-0.040
140
1.40
0.2
1473
0.53
3530
-2056
-0.047
150
1.20
0.2
1353
0.52
3770
-2417
-0.055
160
1.15
0.1
1383
0.52
4010
-2626
-0.060
170
1.10
0.1
1406
0.52
4250
-2844
-0.065
180
1.05
0.1
1421
0.52
4490
1 -3069
-0.070
Required Storage Volume: 192 ft3
0.004 acre-ft