HomeMy WebLinkAboutSTOCKBRIDGE P.U.D. - PRELIMINARY - 27-94 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT..... ....
.
4 Scale I In feet I
VW 500 0 Uw 11* 7=
Legend:
major drainageway
pipe only
. . .. ...... ..
channel only
existing detention area
detention area
TablePaee 61
drainage n concentration points
eIleble 51
FOOTHILLS BASIN I BASIN G I
Storm Drainage Improvements
West End
Figure 8
RESOURCE CONSULTANTS, INC. - Ft. Collins, Colorado
28
Legend: FOOTHILLS BASIN l BASIN G 1
drainage divides
open channel with Existing Drainage Patterns
direction of flow I West End
with di
1
Scale ( in feet ) ewers and drains
with direction of flow
1000 Soo 0 1200 general direction of flow Figure 3
ofsurfacedrainage
tr pending areas,7= detention ponds, etc. RESOURCE CONSULTANTS, INC. • Ft. Collins, Colorado
drainage concentration points
I see Table 3 1
R
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8-99
Table 8
Cost Estimates for Drainage Improvements to Eliminate
Existing Drainage Problems for Foothills Basin
Storm sewer south of
Channel improvements
New Mercer Canal and
Drake Road west of
between Lochwood Drive
Larimer County No. 2 Canal
Item
Union Pacific Railroad
and Lemay Avenue
drainageway crossings
Construction cost
$26,000
$207,000
$175,000
Contingencies 20%
5,200
41,400
35,000
Engineering 10%1/
3,100
24,800
21,000
1/
Legal and administrative 8%
2,500
19,900
17,000
2/
Maintenance 3%
1,100
8,800
7,500
Total cost
$37,900
$301,900
$255,500
Annual net benefits
9,800
11,900
1,900
Annual total costa/
2,700
21,500
18,200
i
r Benefit -cost ratio
0
3.63
0.55
0.10
Total contributing acreage
Developed
222 ac.
890 ac.
232 ac.
Total
244 ac.
1,168 ac.
499 ac.
Cost/acre
Currently developed area
$166/ac.
$339/ac.
$1,101/ac.
Total drainage area
$151/ac.
$258/ac.
$512/ac.
1/Percentage base on construction and contingencies costs.
/Percentage for life of project based on total construction cost.
3/Assumes a 50-year project life and a 6-7/8 percent discount rate.
Table 7
Unit Costs Used for
Cost Estimates of Drainage Improvementsl/
Item
Unit
Excavation and backfill
CY
Concrete (reinforced)
CY
Concrete (flatwork)
CY
Reinforced concrete pipe
T & G Class II
18"
LF
24"
LF
30"
LF
36"
LF
48"
LF
Manholes 6' deep
4' diameter
Ea
5' diameter
Ea
6' diameter
Ea
Riprap and filter material
CY
Pavement patching
SY
Culvert boring
36"
LF
48"
LF
Seeding and mulching
SF
Right-of-way costs
Undeveloped land
SF
Developed land
SF
1/Based on March 1980 prices.
S
Cost
3.00
250.00
150.00
22.00
29.00
45.00
62.00
98.00
840.00
1,020.00
1,240.00
30.00
98.00
140.00
0.10
0.50
2.00
-39- RESOURCE CONSULTANTS INC
Railroad track indicated that a detention pond in this area would not
be feasible. Therefore a major storm sewer is proposed under the Colorado
& Southern Railroad and under the Larimer County No. 2 Canal. Several
alternatives were analyzed before recommending this solution including
development of a major detention facility in this area. The size of the
detention facility would have required the purchase of approximately 15
acres of land along with reconstruction of approximately 1,500 feet of the
New Mercer Canal. With that expenditure the peak flows would have been
reduced to only about 25 percent. Therefore it is recommended that the
drainageway improvements proposed be constructed and that detention
facilities be constructed upstream of this area in order to minimize the
peak flows through these drainage facilities.
Three major detention areas are proposed west of Shields Streetas
shown in Fig. 10; two of these would capture drainage waters from existing
developments west of Shields Street and south of Horsetooth Road (Ponds 17,
18, and 19, Fig. 8). The largest facility would be immediately above the
Pleasant Valley & Lake Canal (Pond 19, Fig. 8). This would capture
drainage waters from the Imperial Estates area and release them under the
Pleasant Valley & Lake Canal in a 24-inch storm sewer. A second detention
facility would be constructed at the corner of Shields Street and Horse -
tooth Road within the current development. This detention facility would
release waters into a 30-inch storm sewer along the west side of Shields
Street. An additional detention facility is proposed for the currently
undeveloped parcel immediately north of Horsetooth Road west of Shields
Street and this would then release into the 30-inch storm sewer on Shields
Street. This storm sewer would then cross Shields Street into the major
drainage channel that is proposed north of Horsetooth Road.
Since many drainage improvements are proposed for the Foothills Basin,
the costs were analyzed in order to develop drainage basin fees for future
developments within the basin.
-37- RESOURCE CON ULTAI`17 INC
Table 6
On -Site Detention Requirements
for Foothills Basin
Pondl�
Location
Maximum
storage
(ac.ft.)
Maximum
outflow
(cfs)
1
Intersection of County Rd. 9 and
10.8
85
Drake Rd.
2
Pond on south drainage outfall
8.9
33
3
It 1t 11
5.5
24
4
n It to n n
6.1
24
5
Pond east of Timberline Rd. in Sec. 29
2.1
24
6
11 1t 11 11 11 II II It
1.4
11
7
It to It IT it it it it
4.6
44
8
to if It it to it it it
1.6
24
9
Pond north of Drake Rd.
2.9
11
10
if It to is it
3.7
24
11
,t if It it IT
5.0
24
12
Pond south of Larimer Co. No. 2 Canal
1.3
24
in Sec. 36
13
Pond at corner of Horsetooth Rd. and
2.7
2
College Ave.
14
Pond west of College Ave. south of
1.15
4
Larimer Co. No. 2 Canal
15
Pond on undeveloped tract in Sec. 26
6.1
11
16
if n it to to
0.8
4
17
Pond in Sec. 27
3.5
11
18
Pond at corner of Shields St. and
7.2
11
Horsetooth Rd.
19
Pond west of Pleasant Valley 5 Lake Canal
14.0
23
1/Exact
locations are shown on Figures 9 and 10.
-36- RESOURCE CON ULTMTS INC
carry this peak flow of storm water during the short time in'which it
would occur assuming no drainage enters the canal upstream of the basin.
Another area of existing drainage problems is immediately west of
College Avenue between Harvard Street and Swallow Road. This study pro-
poses the development of an 18-inch storm sewer from the low area along the
frontage road of College Avenue to the 54-inch storm sewer near College
Avenue north of Foothills Parkway. This would still allow some ponding
during major storms but would eliminate ponding during storms of less
than 10-year frequency.
Proposed development west of College Avenue immediately south of the
Larimer County No. 2 Canal plans to construct a detention pond with a
12-inch storm -sewer outfall into the 54-inch storm sewer north of Foot-
hills Parkway. The detention requirements for this pond are shown in
Table 6 (pond #14). Another area of potential ponding occurs north of
Foothills Parkway from Stanford Road to McClelland Drive (between Points
L and P, Fig. 8). No drainage channel exists in this area and the analysis
indicates the capacity of the existing storm sewer would be exceeded during
major storms. Since the area of flooding is entirely in parking lot areas,
only minor damage would occur during these major storms. One area of par-
ticular concern is the area immediately west of College Avenue. This low
area long the frontage road could pond water 3 feet deep before overtopping
College Avenue. Therefore it is recommended that buildings constructed
in this area be built above the area of potential flooding as shown in the
preliminary plans (see appendix of this report).
Reach 4
Proposed improvements to Reach 4 of this drainage basin include con-
struction of major drainage outfall facilities from Meadow Lark Avenue to
McClelland Drive. Also, detention facilities and storm -sewer facilities
west of Shields Street are recommended.
Since there is potential for overtopping the New Mercer Canal, facilities
are proposed that would transport storm water under it. The analysis of
storm -water detention facilities immediately west of the Colorado & Southern
-35- 1 RESOURCE CONSULTANTS INC
Pointy
Table 5 (Cont'd)
Computed Peak Flows at Various
Points Within the Foothills Basin
with Improvements
(Fully Developed Conditions)
Peak
Location 2-yr 10-yr
Flows (cfs)
25-yr 50-yr 100-yr
R Shields St. storm sewer 45 46 46 46 46
S Storm sewer under Pleasant 23 23 23 23 23
Valley & Lake Canal
1/Exact location is shown on Figures 7 and g,
-30- RESOURCE CONSULTANTS IIIC
Table 5
Computed Peak Flows at
Various
Points Within the
Foothills Basin
with Improvements
(Fully Developed Conditions)
Peak
Flows
(cfs)
Pointl/
Location
2-yr
10-yr
25-yr
50-yr
100-vr
A
Major drainage outfall at
200
410
510
610
710
Poudre River
B
Major drainage channel
92
120
140
150
170
north of Horsetooth Rd.
C
Major drainage channel
180
340
420
530
630
west of County Rd. 9
D
North tributary 1/4 mile
56
65
66
69
73
south of Drake Rd.
E
South tributary storm sewer
49
49
49
49
49
1/4 mi. north of Horsetooth
Rd.
F
Meadows East pond at
4
4
4
4
11
Timberline Rd.
G
Storm sewer at Drake Rd.
28
30
32
36
39
and Timberline Rd.
H
Major drainageway at
120
260
310
380
430
Timberline Rd.
I
Major drainageway at Union
120
260
320
390
440
Pacific Railroad
J
Warren Lake drainageway
26
72
100
120
160
at Horsetooth Rd.
K
Major drainageway at
129
190
230
270
307
Lemay Ave.
L
Major drainageway at
190
350
430
510
580
Stanford Rd.
M
Warren Lake drainageway
73
160
190
230
270
at Horsetooth Rd.
N
Horsetooth Rd. storm sewer
7
20
31
38
47
0
Larimer Co. No. 2 Canal
19
46
61
76
90
storm -welter flow at
Warren Lake
P
Culvert under Colorado &
100
110
110
120
130
Southern Railroad and
Larimer Co. No. 2 Canal
Q
Culvert under New Mercer
76
84
85
86
87
Canal
Cont'd
-29- RESOURCE CON ULTNNTS INC
Table 3
Computed Peak Flows at Various
Points Within the Foothills Basin
(Existing Conditions)
Peak
Flows
(cfs)
Pointy
Location
2-yr
10-yr
25-yr
50-yr
100 yr
A
Fossil Creek Reservoir
24
230
330
420
520
Inlet Ditch at County
Rd. 9 crossing 2/
B
Intersection of Drake
4
56
100
140
180
Rd. and County Rd. 9
C
Major drainageway at
20
190
270
340
410
Fossil Creek Reservoir
Inlet Ditch
D
North tributary drainage-
4
18
34
45
57
way at Timberline Rd.
E
Major drainageway at
22
180
240
280
330
Timberline Rd.
F
Drake Rd. at Union
13
19
48
50
62
Pacific Railroad
G
Major drainageway at
23
180
240
280
330
Union Pacific Railroad
H
Intersection of Horsetooth
25
75
110
140
160
Rd. and Lemay Ave.
I.
Major drainagway at Lemay
82
160
200
230
280
Avenue
J.
Major drainageway at
120
260
330
390
450
Stanford Rd.
K.
Warren Lake drainageway
28
120
170
220
270
at Horsetooth Rd.
L.
Major drainageway at
5
110
140
180
220
Larimer County No. 2 Canal
M.
Major drainageway at
28
100
140
170
210
New Mercer Canal
N.
Major drainageway at
5
70
100
140
180
Shields St.
0.
Major drainageway at
19
78
110
140
170
Pleasant Valley & Lake Canal
l/The
The
exact locations of these points
are indicated on
Figures 2 and
3.
flows for Fossil Creek Reservoir
Inlet
Ditch are
storm -water
flows only and do.not include normal
ditch
diversions.
-14-
Table 2
Impervious Area for Various Land Uses
Land Use Zoning Z Impervious
Parks, Green Belts
and Farmland
_
02 1/
Low Density
Residential
RL, RP 5 RLP
40Z
Medium Density
Residential
RM 6 RMP
50%
High Density
Residential,
RE
60%
Mobile Homes
MM
50%
Industrial
IP h IL
70%
Business 6 Commercial
BP, BG & HB
90X
1/ Parking Lots, Roads and Farmsteads considered separately.
-13-
Table 1
Design Storms for Foothills Basin
Time
Period
Rainfall
Intensities
(inches/hr.)
Ending
(min.)
2yr
10yr
25yr
50yr
100 y r
5
0.12
0.48
0.48
0.60
0.60
10
0.36
0.60
0.72
0.84
0.96
15
0.48
0.72
0.96
1.08
1.44
20
0.60
0.96
1.32
1.68
1.68
25
0.84
2.16
2.28
2.40
3.00
30
1.80
3.12
3.72
4.44
5.04
35
3.24
5.64•
6.84
7.92
9.00
40
1.08
2.28
2.88
3.24
3.72
45
0.84
1.12
1.56
2.04
2.16
50
0.48
0.84
1.08
1.44
1.56
55
0.36
0.72
0.84
0.96
1.20
60
0.36
0.60
0.72
0.72
0.84
65
0.36
0.60
0.72
0.72
0.60
70
0.24
0.48
0.48
0.48
0.48
75
0.24
0.36
0.36
0.36
0.36
80
0.24
0.24
0.36
0.36
0.36
85
0.24
0.24
0.36
0.24
0.24
90
0.12
0.12
0.36
0.24
0.24
95
0.12
0.12
0.24
0.24
•0.24
100
0.12
0.12
0.12
0.24
0.24
105
0.12
0.12
0.12
0.24
0.24
110
0.12
0.12
0.12
0.12
0.24
115
0.12
0.12
0.12
0.12
0.12
120
0.12
0.12
0.12
0.12
0.12
-12-
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Scale I In feet I
1000 500 O 1200
7=
Legend:
drainage divides
open channel with
direction of flow
stormsewers and drains
with directon of flow
general direction of flow
of surface drainage
®pending areas,
detention ponds, etc.
Odrainage concentration points
Isee Table 3 1
FOOTHILLS BASIN I BASIN G I
Existing Drainage Patterns
West End 1
Figure 3
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In summary,
reflected on the
Plan (subject to
drainage system
the site.
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
the proposed storm drainage for Stockbridge as
enclosed exhibits is consistent with the Master
some modification), and the resulting storm ,
should adequately protect life and property on
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
Due to the relatively small area of the subbasins that will
contribute storm runoff from within Stockbridge, the
concentration of discharges in the street gutters will be
somewhat minor - other than when the 100 year storm enters
Stockbridge from Subbasin 0-2.
At that location, flows over what the 24 inch storm sewer
system will pass to Pond No. A will run in a swale over the storm
sewer and - possibly - down the gutter of Highbridge Ct.
Other than the effect of the Subbasin 0-2 flows, the largest
flow in a gutter will be from Subbasin No. 1. There, a 2 year
"minor" design discharge of 2.3 cfs is estimated, and a 100 year
"major" design discharge of 8.5 cfs is estimated. The slope of
the street is 0.6 percent.
According to Figure 4-1 of the design manual and using the
following criteria: Cross Slope of 2 %, so a Reciprocal (Z) of
50; a Roughness Coefficient (n) of 0.016 from Table 4-3; an
allowable depth of flow in the gutter of 6 inches during a two
year storm: and an eighty percent reduction factor, the gutter
will carry about 20 cfs before exceeding City criteria for the
two year, or Minor, storm.
That allowable capacity is much larger (nine times the
estimated 2 year storm discharge in the curb and gutter).
Obviously, street flows in Stockbridge are well within City of
Fort Collins criteria.
Storm drainage facilities, or improvements, for the project
include cross pans at the intersections where flows are
concentrated, storm drain inlets and storm sewers, a limited
amount of 2 foot wide concrete trickle pans, and the previously
discussed regional drainage and detention facilities.
When completed, landscaped, and subject to a warranty period
for the improvements, the City of Fort Collins has agreed to
provide for future maintenance of the integrity of the drainage
capacity. It is uncertain at this time if the ponds will be
owned and maintained as to landscaping by the City or a
homeowners association.
Figure 5-2 of the City Drainage Criteria manual shows a
theoretical capacity of 1.2 cfs per foot of length within the
allowable 6 inch depth and 6 inch heigth of opening for a 2 year
storm. A Reduction Factor of 80 percent shows 0.96 cfs per foot
actual capacity. The inlets have all been preliminarily designed
to accomodate the estimated 2 year flows divided by 0.96 cfs per
linear foot.
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
The Time of Concentrations were determined using the
equation for overland flow in the City design manual, and then by
adding the time of concentrated gutter flow. The time of
concentrated flow was estimated using Figure 3-2 "Estimate of
Average Flow Velocity for Use with the Rational Formula" provided
in the "Urban Hydrology for Small Watersheds" Technical Release
No. 55, USDA, SCS January, 1975.
The formula for overland flow Time of Concentration is:
Tc = [1.87 (1.1 - CCf) D1/23 / S1/3
The subbasins within Stockbridge are relatively small. The
maximum time of concentration for any of the sub basins in this
filing is 19.6 minutes for sub basin # 1. A 10 minute minimum
time of concentration was used for several of the subbasins,
since the overland and concentrated times of concentration for
those basins did not exceed 10 minutes. Several subbasins have
times of concentration around 15 minutes.
The subbasins contributing storm water runoff from off -site
in Imperial Estates have significantly longer times of
concentration since they have long overland flow lengths at
relatively flat slopes. There is no curb and gutter in Imperial
Estates, so the concentrated flows occur in roadside ditches.
The concentrated flow times were calculated as though curb
and gutter does exist.- someday it might. The longest time of
concentration for the off -site basins is 50.6 minutes (2YR) and
47.4 (100YR) in Subbasin 0-2, which extends over 3,000 feet to
Taft Hill Drive on the west side of Imperial Estates.
The Time of Concentration for combined subbasins at the
Design Points is determined using the longest time of
concentration for any of the subbasins contributing at that
point. The longest time of concentration for combined flows is
15.9 minutes during a two year storm and 14.3 minutes during a
100 year storm for on -site Subbasins 11 and 13 at Design Point
II, which is the 4 foot inlet on the west side of Seneca just
south of Horsetooth Road..
Rainfall intensities vary in the on -site subbasins from 1.9
to 2.5 inches per hour for a two year storm and from 5.6 to 7.2
inches per hour for the 100 year storm. The off -site Subbasin
rainfall intensities vary from 1.1 to 1.9 and 3.2 to 5.6 for the
same events. "C" values (Runoff Coefficients) were chosen to be
reflective of the overall subdivision density - 0.5 on -site and
0.35 off -site.
III. DESCRIPTION OF STREET FLOWS AND DRAINAGE IMPROVEMENTS
TABLE 1
HYDROLOGIC CHARACTERISTICS
STOCKBRIDGE P.U.O.
2 YEAR 100 YEAR
BASIN AREA C VALUE TIME OF TIME OF
DESIGNATION CONCEN- CONCEN-
TRATION TRATION
(ACRES) (MIN) (MIN)
i 2-YR i t00-YR 9 2-YR R 100-Y DESIGN
POINT
(inlhr) (inlhr) (CFS) (CFS)
No.
1
2.42
0.5
t9.6
17.6
1.9
5.6
2.3
8.5
No.
2
1.11
0.5
15.4
13.9
2.1
6.3
t.2
4.4
No.3
1.65
0.5
14.7
13.2
2.1
6.4
t.7
6.6
No.4
1.99
0.5
15.7
14.t
2.t
6.2
2.t
7.7
No.
5
1.4t
0.5
17.t
15.4
2.0
6.t
1.3
5.4
No.6
1.33
0.5
16.t
t4.5
2.0
6.0
t.3
5.0
No.7
1.43
0.5
t7.1
15.4
2.0
6.1
1.4
5.5
No.8
1.t2
0.5
14.4
13.0
2.1
6.0
1.2
4.2
No.9
1.03
0.5
10.8
10.0
2.4
7.2
t.2
4.6
No.
10
t.03
0.5
12.4
tt.2
2.3
7.0
t.2
4.5
No.
It
0.93
0.5
15.9
14.3
2.1
6.t
t.0
3.5
No.12
2.13
0.5
16.1
14.5
2.t
6.1
2.2
8.1
No.13
0.25
0.5
t0.0
10.0
2.5
7.2
0.3
1.1
No.
t4
t.19
0.5
10.2
10.0
2.5
7.2
1.5
5.4
No.
t5
4.51
0.5
16.2
14.6
2.1
6.t
4.7
17.2
No.16
1.2t
0.5
t0.0
t0.0
2.5
7.2
1.5
5.4
No.17
t.27
0.5
10.0
10.0
2.5
7.2
t.6
5.7
No.18
0.56
0.5
10.0
10.0
2.5
7.2
0.7
2.5
No.
t9
0.57
0.5
t0.5
t0.0
2.5
7.2
0.7
2.5
No.20
0.64
0.5
t0.0
t0.0
2.5
7.2
0.8
2.9
No.2t
1.61
0.5
t0.0
t0.0
2.5
7.2
2.0
7.2
No.
0-1
15.5
0.35
42.7
39.0
t.2
3.6
6.5
24.6
No.
0-2-a
2.0
0.35
19.3
t7.5
1.9
5.6
t.3
4.9
No.O-2-b
3.3
0.35
19.3
17.5
1.9
5.6
2.2
8.t
No.
0-2
46.2
0.35
50.6
47.4
t.1
3.2
t9.8
72.5
COMBINED FLOWS
0-1612
17.63
0.37
19.2
17.5
1.2
3.6
12.3
27.4 I
1tE13
1.18
0.5
t5.9
14.3
2.1
6.1
1.2
4.5 II
9 d t0
2.06
0.5
12.4
it.2
2.3
7.0
2.4
9.0 1II
81 9 6 t0
3.18
0.5
14.4
13.0
2.1
6.0
3.3
11.9 IV
0-2, 9-1-abb,
60.42
0.37
50.6
47.4
1.1
3.2
24.6
89.4 V
20,415,610
NOTE; "C" VALUES ARE BASED ON 0.35 FOR THE LOW DENSITY
DEVELOPMENT 1N IMPERIAL ESTATES AND 0.5 FOR THE
MEDIUM DENSITY DEVELOPMENT PROPOSED IN STOCKBRIDGE
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
No. A, a 15 inch RCP culvert will be installed in the sag curve
of Seneca to drain into Pond No. B, a 15 inch ADS N-12 culvert
from Highbridge Court to Pond No. C. and a 15 inch ADS N-12 storm
sewer from the end of Bridge Court to Detention Pond No. C.
A 24 inch ADS N-12 will extend downstream from Pond No. C
under the Pleasant Valley ditch. This office will investigate
how to either get the remainder of the 24 inch storm sewer
extended downstream to Shields and Horsetooth or to make
temporary release provisions that will daylight the detention
ponds.
Additional drainage improvements include 2 foot concrete
trickle channel with additional swale capacity for larger flows
in backyard areas where two percent slopes are not achievable,
culvert end sections, area inlets, street gutter inlets/catch
basins. and storm drain manholes.
Discharge quantities were estimated for each of the
subbasins using the Rational Method. The 2 year Minor storm and
100 year Major storm discharges are identified on the storm
drainage chart included as Table 1 on the following page - and on
the Preliminary Storm Drainage Plan, which is at Exhibit B to
this report.
Where storm drainage runoff from individual subbasins
converge, the combined, or concentrated, flows are identified as
Design Points. At those points the peak flows from the combined
subbasins were estimated. Those discharges are also shown on
Table One and the drainage plan. The Design Points are indicated
on the chart and are located on the plan.
The Hydrologic Characteristics for the subbasins in Mountain
Ridge, as well as for the combined subbasins at the respective
Design Point are included on the following page as Table I.
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
be provided with the development. There are also two ponds to be
constructed on the east side of Seneca above the irrigation
ditch.
The proposed Detention Ponds at the southwest corner (No. A)
of Seneca and Horsetooth and at the southeast corner (No. D)
detain storm runoff from Subbasins 0-1, 11, 12, 13 and 14. They
contain one to one and one-half acre feet of volume each.
The detention ponds (No. B and No. C) on the west and east
side of Seneca Street approximately 600 feet south of Horsetooth
detain storm runoff from Subbasin No.'s 0-2-a, 0-2-b, 0-2, 2, 3,
4, 5, 6, 7, 8, 9, 10, 15, 17, 18, 19 and 20. Detention storage
volumes totaling 10.5 acre feet are provided in ponds B and C.
The total of the provided volumes for storm water detention
are close to the 14 required in the Foothills Master Storm
Drainage Basin. Subbasin No.'s 1 and 16, which are mostly rear
lot areas for the lots backing up to Mountain Ridge, will drain
southerly. Mountain Ridge Detention facilities to the south will
be sized to accomodate these very minor flows.
This office is also providing the engineering services for
the Mountain Ridge project. We will work with RBD Engineering to
finalize the regional detention facilities to be built in and
adjacent to Mountain Ridge. We anticipate also working with
Resource Consultants, which did the Foothills Basin study and
report for the city. If necessary, another firm such as RBD will
be retained to assist in finalization of the regional detention
facilities for Stockbridge.
There are numerous other storm drainage improvements to be
provided with the Stockbridge development. Sidewalk culverts
(i.e. curb chases) will be provided where drainage Subbasin No.'s
3 and 5 contribute runoff to the west side of Seneca. A 24 inch
Reinforced Concrete Pipe (RCP) culvert will extend upstream from
Pond No. B under Bridgeplace Court and then as ADS N-12 plastic
pipe culvert to the upper end of Subbasin No. 4 and Highbridge
Court.
At this location, storm runoff from Imperial Drive in
Imperial Estates is contributing storm runoff to the Stockbridge
site from Subbasin 0-2. The 24 inch pipe will carry the majority
of the flow from Subbasin 0-2. Major 100 year storms will cause
some overflow of the inlet area and cause some of the storm
runoff to be carried in the street gutters and in a 2 foot
concrete trickle channel and associated Swale that is to be
provided in the rear lot area of Subbasin No. 5.
Other storm sewer to be provided include a 15 inch RCP
culvert crossing of Seneca Street between Detention Pond No.'s A
and D, and a 24 inch RCP culvert crossing of Seneca between Pond
No.'s B and C. Also, a 15 inch ADS N-12 culvert will be
installed in the sag curve of Horsetooth Road to drain into Pond
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
inch water distribution system in the Seneca Drive alignment, and
future sanitary sewer service to Imperial. Estates via a sewer
main to be stubbed to the Imperial Drive alignment.
The detention facilities to be provided in Stockbridge will
further benefit the neighborhood as a means to assist in
protection of the properties downstream/easterly of Stockbridge.
The storm drainage exhibits supporting this report are as
follows:
A. Preliminary Plat, which shows the street and lot layout - as
well as information on tracts, easements, etc.
B. Preliminary Storm Drainage Plan, which shows the drainage
Subbasin boundaries, flow arrows, slopes, culverts, inlets, and
the basic storm system in general.
C. Preliminary Grading and Erosion Control Plan, which shows the
Proposed contours at 2 foot intervals.
Generally speaking, erosion control measures for the entire
site will be designed in final form with each filing - as the
final plans are processed. Improvements will include gravel
filters and hay bales at storm sewer pipe inlets, re -seeding with
temporary grass within developed lots, and permanent grass
seeding or coding in greenbelt areas.
II. Proposed Storm Drainage Plan
The proposed drainage plan for the Stockbridge P.U.D. is in
general compliance with the City's Master Drainage Plans. The
on -site sub basin boundaries are close to those boundaries
depicted on the Master Plans, except the extreme southwest
portion of the site as designed flows southerly in Seneca St. to
the regional detention ponds in Mountain Ridge.
All of the subbasins within Stockbridge P.U.D. are
identified at Exhibit B, which is the Preliminary Storm Drainage
Plan. Additional grading and drainage information is provided at
Exhibit C, which is the Preliminary Grading and Erosion Control
Plan. Both are included with this report and were also submitted
separately as a part of the preliminary plan submittal.
Storm runoff from drainage basins within the site are
designated as Subbasin Ws 1 through 20, and basins contributing
storm runoff from off -site (Imperial Estates to the west) are
designated as Subbasin Ws 0-1, 0-2-a, 0-2-b and 0-2.
Storm drainage runoff from all of the subbasins on the west
side of the project (i.e. west of Seneca), except Subbasin No. 1
that drains southerly, will be routed through detention ponds to
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
low density (1 lot per 5 acres) residential neighborhood -
Richmond Estates - to the intersection of Horsetooth Road and
Shields Street.
The existing irrigation ditch crosses the property from
north to south approximately 1,400 feet west of Shields Street.
The irrigation ditch then extends from north to south through
Mountain Ridge P.U.D. and beyond to its confluence with Fossil
Creek. It is discussed in detail in the above mentioned reports,
but currently the ditch is a non -factor when discussing storm
drainage flows on the Stockbridge site. It is presumed full of
water during the 100 year, or "Major" Storm.
Generally, the Stockbridge site slopes from west to east at
one to two percent. Off -site storm drainage will enter the site
from the west from Imperial Estates. The off -site contributing
drainage basins and flows are identified on the Foothills Master
Drainage Basin Report. This office used a city orthophoto to
further delineate the off -site subbasins as they contribute storm
drainage to Stockbridge.
The on -site storm drainage system is designed to accommodate
the storm water flows from off -site, as well as provide the
facilities for the storm water flows originating within the
Stockbridge P.U.D.
The site has previously been processed for residential
development through the City of Fort Collins. The previous
development plan is modified in this proposal. Some improvements
other than storm drainage are described as follows:
A 15 inch "trunk/outfall" sanitary sewer system will be
constructed in the north/south aligned Seneca Street to provide
sewer service to the project, and further to extend northerly
under Horsetooth Road to serve properties north and west of the
site. An existing 12 inch water main system also exists in
Seneca, which will provide the needed domestic water tie-ins for
the project.
Basin wide improvements are required for all of the City
Designated Storm Drainage Basins. Certain basin storm drainage
improvements are identified in each of the Drainage Basin Utility
Construction Plans. Drainage Basin Fees are collected in each
City designated basin to offset the anticipated cost of basin
wide improvements. As stated, additional information regarding
the City's Foothills Basin can also be obtained in current form
at the City Storm Water Utility Department.
Some of the benefits of the project include allowing a
portion of Horsetooth Road to be widened, Seneca Street to be
extended for 1,400 feet, the important 15 inch trunk sewer system
to be installed, provide better utilization of the existing 12
STOCKBRIDGE P.U.D.
PRELIMINARY STORM DRAINAGE REPORT
I. GENERAL INFORMATION
Stockbridge P.U.D. is a proposed residential subdivision in
the City of Fort Collins, Colorado. The site is generally
located on the south side of Horsetooth Road about one —quarter
mile west of Shields Street in southwest Fort Collins. A general
Vicinity Map is provided at the end of this report.
This preliminary storm drainage report is for the entire
Stockbridge property, which consists of the majority of the West
1/2 of the of the North 1/2 of the Northeast 1/4 of Section 34,
T7N, R68W, Sixth Principal Meridian, Larimer County, Colorado.
The Pleasant Valley and Lake Canal Company irrigation ditch
exists along the east boundary, Horsetooth Road on the north,
Imperial Estates (a low density county residential neighborhood)
on the west, and the proposed Mountain Ridge P.U.D. residential
development on the south.
The proposed development for the Stockbridge site is for
approximately 130 single family homes on relatively small lots
(50 feet by 100 feet -typical). The residential uses planned for
the property are consistent with the approved Master Plan for the
property. However, when the property was originally platted in
the early 1980's, condominium units were proposed. Now the plan
is for detached single family homes.
Stockbridge P.U.D. is a part of the properties identified in
the city's Foothills Storm Drainage Basin. A small portion of
the site on the south side that is located in this "Master
Drainage Basin" will drain southerly into the city designated
McClleland — Mail Creek Storm Drainage Basin.
City funded reports on both basins are quite lengthy, and
are not attached in full as a part of this report. Both reports
are available at the Stormwater Utility Department of the City of
Fort Collins. Certain pertinent portions of the Foothills Basin
are included in Addendum A to facilitate review of the storm
drainage system for Stockbridge.
The Foothills report identifies the need to construct
Regional Detention facilities within the Stockbridge P.U.D. The
description of those improvements are generally defined as
follows:
1. Build storm water detention facilities on the west side
of the.Pleasant Valley Ditch that wiil provide 14 acre feet of
detention volume and reduce the peak discharge from the 100 year
"Major" storm from 170 to 23 cubic feet per second (cfs).
2. Construct a 24 inch diameter storm sewer from the
detention facilities downstream under the irrigation ditch and,
ultimately, through undeveloped land to the east and an existing
THIS PRELIMINARY STORM DRAINAGE STUDY AND REPORT FOR STOCKBRIDGE
P.U.D. WAS PREPARED BY LAND DEVELOPMENT SERVICES USING THE CITY
OF FORT COLLINS STORM DRAINAGE DESIGN CRITERIA, dated May, 1984.
The results of this drainage study are reported in the enclosed
text and supporting documents, 'and are shown in further detail on
the Preliminary Plans submitted concurrently with this report.
Signed this 41h-�QY-Qi-April, 1994.
Dennis L. Donovan, P.E.
Land Development Services
309 W. Harmony Road
Fort Collins, Colorado 80526
Phone (303) 225-9244