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DRAINAGE STUDY
DARTMOUTH P.U.D.
PREPARED FOR
GEFROH ASSOCIATES
ONE DRAKE PARK, SUITE 23
333 WEST DRAKE
FORT COLLINS, COLORADO
PREPARED BY
JCR ENGINEERING
211 WEST MYRTLE
FORT COLLINS, COLORADO
JUNE, 1981
EXISTING CONDITIONS
The property to be considered is a 3.4 acre tract located in
the Northwest 1/4 of Section 24, Larimer County, Colorado, and
inside the City Limits of Fort Collins. The property is bounded
on the north by Spring Park Drive, on the south by Dartmouth
Avenue, on the east by Mathews Street, and on the west by Remington
Street. A more particular description of the property is given
by the legal description for the Dartmouth PUD.
The existing ground cover is an open field, medium vegetation
(grass and weeds) on heavy clay soil; the slope is from south to
north, and on the average is approximately 2.5%. The Sherwood
Canal borders the south boundary of the property.
Drainage is accomplished by overland flow to the existing
catch basin at the north end of the property (centered between
Remington Street and Mathews Street in Spring Park Drive). The
existing outlet from the catch basin is a 14" concrete pipe under
Spring Park Drive, and continuing to Spring Creek 75 feet to the
north. The capacity of the existing pipe under Spring Park Drive
is approximately 3.5 CFS.
ASSUMPTIONS AND PARAMETERS, DEVELOPED PROPERTY
Three mid -rise buildings are proposed for the 3.4 acre
parcel. A private parking area will be provided for access
to the buildings, and on site runoff will be collected through
this parking area.
For drainage purposes, the parking area will collect runoff
from the buildings and surrounding area, flow through swales and
under sidewalk drains to a new catch basin at the north end of
the property, through a new section of 10" concrete pipe to the
existing catch basin, and from there to Spring Creek.
The total calculated detention volume necessary using the
standard City of Fort Collins regulations is approximately 35,000
cubic feet, and the allowable release rate to the existing storm
drainage system is approximately 2.2 CFS.
The project will be developed in three phases, and the
detention area will be constructed as part of the first phase,
with drainage of the future phases having been planned for
during design of the first phase.
CALCULATIONS
Total Area = 3.4 acres (inside property boundaries)
.55 acres (outside property boundaries)
2 Year Storm Runoff = (undeveloped conditions)
Open field, medium vegetation on heavy clay soil: 3.95 acres
Tc (basin length 588 ft., average slope = 2.72%)
Cave = .35
t = 1.87(1.1-6)(D)1/2 = 1.87(1.1-.35)(588)1/2 24.38,
c S1 3 (2.72)1 3 use 25 min.
O = 1.6 in/hr
Q2 = CIA = .35(1.6)(3.95) = 2.2 cfs
100 Year Storm Runoff: (developed conditions)
driving surface, side walk, and roof top area 3.165 acres
established lawn, shrubbery .785 acres
factored "C" _ OS(3.165)95.2(.785)
3+ .82, use .8
t = 1.87(1.1-.8)(775)12 = 13.49 Use 13.5 min.
c (1.55)1 3
I = 5.95 in/hr
Q100 = CfCIA = 1.25(.8)(5.95)(3.95) = 23.50 cfs
Required Detention Volume Shown on Graph
2-YEAR STORM
(ft3 x 10-3)
TIME(MINUTES) (CA)(I)(SECONDS) VOLUME
5
(2.21)
(2.40) (300)
2.25
10
(2.21)
(2.50) (600)
3.31
20
(2.21)
(1.85) (1200)
4.91
30
(2.21)(1.47)(1800)
5.85
40
(2.21)(1.20)(2400)
6.36
50
(2.21)(1.05)(3000)
6.96
60
(2.21)(.90)(3600)
7.16
70
(2.21)(.82)(4200)
7.61
80
(2.21)(.75)(4800)
7.96
90
(2.21)(.68)(5400)
8.12
100
(2.21)(.60)(6000)
7.96
110
(2.21)(.56)(6600)
8.17
120
(2.21)(.52)(7200)
8.27
130
(2.21)(150)(7800)
8.62
140
(2.21)(.46)(8400)
8.54
150
(2.21)(,44)(9000)
8.75
160
(2.21)(.41)(9600)
8.70
170
(2.21)(.40)(10,200)
9.02
180
(2.21)(.39)(10,800)
9.31
d max. 29,810 ft3
or 30,000 ft detention required.
100-YEAR STORM
(ft3 x 10-3)
TIME(MINUTES)
(CfCA)(I)(SECONDS)
VOLUME
5
(3.95)(8.5)(300)
10.07
10
(3.95)(7.2)(600)
17.06
20
(3.95)
(5.20)
(1200)
24.64
30
(3.95)
(4.20)
(1800)
29.86
40
(3.95)
(3.50)
(2400)
33.13
50
(3.95)(3.00)(3000)
35.55
60
(3.95)(2.60)(3600)
36.97
70
(3.95)(2.30)(4200)
38.15
80
(3.95)(2.05)(4800)
38.86
90
(3.95)(1.85)(5400)
39.46
100
(3.95)(1.70)(6000)
40.29
110
(3.95)(1.55)(6600)
40.41
120
(3.95)(1.45)(7200)
41.24
130
(3.95)(1.32)(7800)
40.68
140
(3.95)(1.25)(8400)
41.48
150
(3.95)(1.20)(9000)
42.66
160
(3.95)(1.12)(9600)
42.46
170
(3.95)(1.10)(10,200)
44.32
180
(3.95)(1.05)(10,800)
44.79
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