HomeMy WebLinkAboutDrainage Reports - 03/01/1979DRAINAGE STUDY
FOR
INDIAN HILLS WEST P.U.D.
PREPARED FOR:
DAN BAILEY, DEVELOPER
FIRST NATIONAL TOWER BUILDING
OAK STREET AT MASON STREET
FORT COLLINS, COLORADO
PREPARED BY:
JOSEPH C. ROESSER, JR., P.E.
BRADFORD L. SMITH AND ASSOCIATES
155 NORTH COLLEGE AVENUE
FORT COLLINS, COLORADO
MARCH 1979
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TABLE OF CONTENTS
EXISTING CONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
DEVELOPED PROPERTY, PARAMETERS AND ASSUMPTIONS . . . . . . . . . . . . . 2
AREA IA, IB, IE - CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . 3,4
AREA IC - CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 5,6,7,8
AREA ID - CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 9,10,11,12
AREA II - CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . ... . . 13
AREA III - CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . 14
AREA IV - CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
DEVELOPED PROPERTY CHANNEL CAPACITIES . . . . . . . . . . . . . . . . . . . 16
EXISTING CONDITIONS
This drainage study includes a 12.488 acre parcel of property located in
Section NW 24 of Larimer County inside the City limits of Fort Collins, bounded on
the north by Stuart Street, on the south by the Arthur Ditch, near Stover Street
to the east, and near Busch Court to the west.
Conditions existing before development are, on the northern 7 acres, a
packed gravel parking lot, and surrounding shrubbery known as the Sunset Drive -In
Movie Theater. The south 5.5 acres before development is an open field, sparsley
vegitated on heavy soil, bordered on the east by a mature stand of juniper trees.
The property slopes, on the average, 2% from the southeast to the north-
west, and drainage is accomplished by way of the existing curb and gutter on Stuart
Street, east to a catch basin on the south side of Prospect Street at Stover Street,
then through the City storm drainage system to Spring Creek.
-1-
DEVELOPED PROPERTY
PARAMETERS AND ASSUMPTIONS
Development of the 12.488 acre parcel includes 107 unit townhouse
project complete with 20 foot private street, parking areas, sidewalks, tennis
courts, and landscaped open space.
For drainage purposes, Areas IC and ID have been designed to collect
developed property drainage; required retention volumes and discharge piping are
included in these areas. Swales "A" and "B" on the east property boundary are
designed to drain directly onto Stuart Street, allowing adjoining property drainage
from the east and south (Area II and IV) to flow through Indian Hills West property.
Swale "D" on the west property boundary is designed to carry controlled runoff from
Area ID to Stuart Street. Existing portions of the property located in the Stuart
Street right-of-way will remain unchanged after development, and therefore, are not
included in the calculations.
The November 1978 City of Fort Collins Drainage Manual has been used to
design this project together with locally accepted engineering principals, and infor-
mation from the Data Book for Civil Engineers by Elwyn Seelye Vol I.
All drainage piping and drainage channels have been sized for the 100-
year storm runoff, using standard rainfall data for the City of Fort Collins. It
is assumed that during the 100-year storm, the Arthur Dtich at the south property
boundary is flowing full, and Area II to the south of the Arthur Ditch will flow
onto Indian Hills West property. Area IV to the east will also flow onto Indian
Hills West property and the drainage channel along the east property boundary will
have this added capacity. Area III to the south of the project is collected by
street, curb and gutter, and eventually to the bridge on Stover Street. It will
be shown that the curb and gutter on the west side of Stover Street at the bridge
is sufficient to carry the 100-year storm runoff from Area III and allowances will
not have to be made to carry this runoff through Indian Hills West property.
-2-
AREA IA, IB, IE
Area IA, and IB,.and IC, and ID, and IE = 12.488 acres = 543,977
ft2
Area IA _ Stuart Street, curb, gutter and sidewalk (existing) =
15,598
ft2
Area IB Area Total - 104,102 square feet = 2.39 acres
2-Year Storm Runoff:
Existing Conditions:
Packed
"C"
Gravel
Light Vegitation, Soil
41,050
ft2
.90
Heavy
Heavy Vegitation,
10,000
ft2
.40
Heavy Soil
533052
ft
.30
Factored "C" Value 41,050(.9)+10,000(.40)+53,052(.30)
=
104,102
.60
T,= (basin length = 720 ft) = 1.87(1.1-.6)(720I2-
25.088
= 19.93
minutes
(2)r3
1.259
I = 1.82 in/hr
Q = CIA = (.6)(1.82)(2.39 acres) = 2.61 CFS
100-Year Storm Runoff:
Developed Conditions:
-
- - 2-,
"C"
Roof Area
'-;,2,-526_ftz
.95
Street, curb and gutter, and sidewalk
6,900
ft
.95
Established lawn and shru-bery
94�676
ft 2
.20
Factored "C" Value .95(6900+2526)+.2(94,676) = 27
104,102
Tc,=basin length = 720 ft) = 1.87(1.1-.27)(720)Z
=
33.1 minutes
(2 )'3
I = 3.9 in/hr
Q = Cf Cia = 1.25(.24)(3.9)(2.39) = 2.80 CFS
Q%00 - QZ = 2.80-2.61 = .19 CFS
Area IE Area Total = 19,009 square feet = .436 acres
2-Year Storm Runoff:
Light Vegitation, Soil
"C"
Heavy
Heavy Vegitation, Soil
4,000
ft2
2
.40
Heavy
15,009
ft
.30
Factored "C" Value 4000(.4)+15009(.3) = 32
19009
Tr _ (basin length = 460 ft) = 31 minutes
I = 1.45 in/hr
Q = CIA = (.32)(1.45)(.436 acres) = .20 CFS
-3-
100-Year Storm Runoff: ACTT
Established lawn and shrubbery 19,009 ft2 .20
T,_ _ (basin length = 460 ft) = 31 minutes
I 4.1 in/hr
Q = CfCIA = 1;26(.2)(4.1)(1.436) = .44 CFS
Q,00- QZ = (.44-.20) = .24 CFS
Conclusion: Runoff from Area IB and IE will have a combined rate of .4 CFS
which will not be retained during the 100-Year Storm. Therefore, .4 CFS will be sub-
tracted from the allowable runoff in retention area in IC or ID.
-4-
AREA IC
Total Area = 189,179 square feet = 4.34 acres
2-Year Storm Runoff: licit
Packed Gravel 180,079 ft2 .90
Heavy Vegitation, Heavy Soil 95100 ft .40
Factored "C" Value 180079(.9)+9100(.4) = 88
189179
T, _ (basin length = 730 ft) = 1.87(1.1-.88)(730)2= 8.3 minutes
(2.5)Y3
I = 2.7 in/hr
Q = CIA = (.88)(2.7)(4.34) = 10.31 CFS
100-Year Storm Runoff:
2
licit
Street, Curb and Gutter, and Sidewalk
54,958
ft2
.95
Roof Area
94,841
ft
2
.95
Lawn and Landscape Area
39,380
ft
.20
Factored "C" Value (54958)+94841).95+39380(.20)
= 79
189179
T, _ (basin length = 770 ft) = 1.87(1.1-.79)(770)�=
13.52
minutes
(1.7)
I = 6.4 in/hr
Q = C+CIA = (1.25)(.79)(6.4)(4.34) = 27.43 CFS
Required retention volume shown on graph.
-5-
INDIAN HILLS WEST
AREA IC
Minutes
(CA)(I)(TIME)
2-Year Storm
Volume(Ft3 x 10-3)
5
(3.819)(3.2)(300 sec)
3.666
10
(3.819)(2.50)(600)
5.729
20
(3.819)(1.85)(1200)
8.478
30
(3.819)(.1.45)(1800)
9.968
40
(3.819)(1.20)(2400)
10.998
50
(3.819)(1.05)(3000)
12.030
60
(3.819)(.90)(3600)
12.37
70
(3.819)(.80)(4200)
12.83
80
(3.819)(.75)(.4800)
13.75
90
(3.819)(.65)(5400)
13.41
100
(3.819)(.60)(6000)
13.75
110
(3.819)(.55)(6600)
13.86
120
(3.819)(.52)(7200)
14.30
130
(3.819)(.50)(7800)
14.89
140
(3.819)(.45)(8400)
14.44
150
(3.819)(.42)(9000)
14.44
160
(3.819)(.40)(9600)
14.67
170
(3.819)(.39)(10200)
15.19
180
(3.819)(.38)(10800)
15.67
I Me
INDIAN HILLS WEST
AREA IC
(CfCA)(I)(TIME)
Minutes
100 Year Storm
Volume (Ft 3 x 10-3)
5
(4.286)(8.5)(.300)
10.93
10
(4.286)(7.15)(600)
18.37
20
(4.286)(5.1)(1200)
26.23
30
(.4.286)(4.2)(1800)
32.40
40
(4.286)(3.5)(2400)
36.00
50
(4.286)(3.0)(3000)
38.57
60
(4.286)(2.60)(.3600)
40.12
70
(4.286)(2.30)(4200)
41.4
80
(4.286)(2.05)(4800)
42.17
90
(4.286)(1.85)(5400)
42.82
100
(4.286)(1.70)(6000)
43.72
110
(4.286)(1.55)(6600)
43.85
120
(4.286)(1.45)(7200)
44.75
130
(4.286)(1.30)(7800)
43.46
140
(4.286)(1.25)(8400)
45.00
150
(4.286)(1.20)(9000)
46.29
160
(4.286)(1.12)(9600)
46.08
170
(4.286)(1.10)(10,200)
48.09
.180
(4.286)(1.05)(10,800)
48.60
-7-
9
�0L-.UME
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-a-
AREA ID
Total Area = 216,089 square feet =
4.96 acres
2-Year Storm Runoff:
2
licit
Packed Gravel
8,850
ft2
.90
Heavy Vegitation, Heavy Soil
3,800
ft2
.30
Light Vegitation, Heavy Soil
203,439
ft
.40
Factored "C" Value .9(8850)=.3(3800)+.4(203,439)
= 42
216089
T, _ (basin length =.840
ft) = 1.87(1.1-.42)(840)?- = 29.27
minutes
(2)"3
I = 1.45 in/hr
Q = (.42)(1.45)(4.96) =
3.02 CFS
100-Year Storm Runoff:
2
licit
Street, Curb and Gutter,
and Sidewalk
43,964
ft
.95
Roof Area
68,878
ft2
2
.95
Established Lawn and Shrubbery
103,247
ft
.20
Factored licit Value .95(43964+68878)+.2(103,247)
= 59
216,089
T, = (basin length = 940
ft) = 1.87(1.1-.59)(940)"A=
23.22 minutes
(2) �3
I = 4.6 in/hr
Q = 1.25(.59)(4.8)(4.96)
= 17.56 CFS
Required retention volume shown on graph.
M
INDIAN HILLS WEST
AREA ID
(CIC)(I)(TIME)
Minutes
100-Year Storm
Volume(x10-3Ft3
5
(3.50)(8.5)(300)
8.93
10
(3.50)(7.15)(600)
15.02
20
(3.50)(5.10)(1200)
21.42
30
(3.50)(4.20)(1800)
26.46
40
(3.50)(3.50)(2400)
29.40
50
(3.50)(3.0)(3000)
31.50
60
(3.50)(2.60)(3600)
32.76
70
(3.50)(2.30)(4200)
33.81
80
(3.50)(2.05)(4800)
34.44
90
(3.50)(1.85)(5400)
34.97
100
(3.50)(1.70)(6000)
35.70
110
(3.50)(1.55)(6600)
35.81
120
(3.50)(1.45)(7200)
36.54
130
(3.50)(1.30)(7800)
35.49
140
(3.50)(1.25)(8400)
36.75
150
(3.50)(1.20)(9000)
37.80
160
(3.50)(1.12)(9600)
37.63
170
(3.50)(1.10)(10200)
39.27
180
(3.50)(1.05)(10800)
39.69
-11-
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-12-
AREA II
Total Area = 133,206 square feet = 3.058 acres
Assume: C = .5 (Residential Area)
Tc = (basin length = 400 ft) = 1.87(1.1-.5)(400)_ 52.5 minutes
(.0175)1/3
I = 2.85 in/hr
Q = (1.25)(.5)(2.85)(3.085) = 5.45 CFS
This runoff rate will be.added to the channel capacity at the East property
boundary.
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AREA III
Total Area = 226,294 square feet = 5.195 acres
Assume: C = .5 (Residential Area)
T, _ (basin length = 1000 ft) = 1.87(1.1-.5)(1000)= 42 minutes
(•6)"3
I = 3.55 in/hr
During the 100-Year Storm, developed:
Q = 1.25(.5)(.3.55)(5.2)•.= 11.54 ft3/seconds
�--Lim its of C.BG.
n
E— 14"
IYAI EZ
1
Flow � - - o..�. :• •'•'.•.-.:•'.-; -.
I ine
D.
' v
' (I.50)
DRIVE -OVER CURB, GUTTER AND SIDEWALK
Existing drive over curb and gutter at the Southwest section at the bridge
(over the Arthur Ditch) on Stover Street is flowing at a slope of .008.
Total runoff capacity at this point:
Q = 1n49 (A)(R)jS�2
Q = 1n49 3.(3.31)(16379)3(.008)2 = 11.50 CFS
Therefore, at the bridge on Stover Street during the 100-Year Storm
(assuming the Arthur Ditch is flowing full) the existing street section (including
curb and gutter) will carry the runoff. No provision will be necessary to allow
Area III runoff to flow through Indian Hills West Property.
-14-
AREA IV
Total Area IV, = 43,600 square feet = 1 acre
Assume: C = .5 (Residential Area)
T, = (basin length = 500 ft) = 1.87(1.1-.5)(500)�2
= 32 minutes
(1.6)"3
I = 3.86 in/hr
Q,= (1.25)(.5)(3.86)(1) = 2.4 CFS
00
Total Area IV? = 70,500 square feet = 1.618 acres
Assume: C = .5 (Residential Area)
T, = (.basin length = 700 ft) = 1.87(1.1-.5)(700)�2=.21.66 minutes
(2.57)"
I = 4.95 in/hr
Q,= (1.25)(.5)(4.95)(1.618) = 5 CFS
00
These runoff rates will be added to the channel capacity at the East
property boundary.
-15-
DEVELOPED PROPERTY
CHANNEL CAPACITIES
CHANNEL A (Southeast property boundary) must carry Area II and Area IV and
IE
5.45 CFS + 2.4 CFS + .44 CFS
= 8.29 CFS
CHANNEL B (Northeast property boundary) must carry Channel A + Area IV and
2(IB)
8.29 CFS + 5 CFS + (2.8) CFS
= 14.69 CFS
CHANNEL C (Off Cheyenne Drive, between lots 47 and 48, flowing toward the tennis
courts)
Area ID (Developed)
= 17.56 CFS
CHANNEL D (Northwest property boundary) must carry the controlled runoff from
retention area of ID
R2 Year (Area ID) = 3.02 CFS
-16-
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