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PROPERTY OF/
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Final Drainage Report
for
CENTER GREENS AT SOUTHRIDGE GREENS P.U.D.
Fort Collins, Colorado
Prepared for
ALBRECHT HONES
4836 South College Avenue
Fort Collins, Colorado 80525
PREPARED BY:
SHEAR ENGINEERING CORPORATION
Project No: 1137-7-90
Date: February 28, 1991
4836 S. College, Suite 12 Fort Collins, CO 80525 (303)226-5334
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PAGE 1
Project No: 1137-07-90
Center Greens at Southridge Greens P.U.D.
•
I GENERAL LOCATION AND DESCRIPTION
A. Location
1. Center Greens is a part of the Southridge Greens P.U.D.
development located in south Fort Collins in Section 7, T6N,
R68W of the Gth P.M.
2. The site is bounded on the north, west and south by
Southridge Greens Golf Course; more specifically Southridge
Greens P.U.D. Tract B-2, Fairways 8, 9 and 10, respectively.
The property is bounded on the east by Southridge Greens
Boulevard which was platted with Greenridge at Southridge
Greens P.U.D.
B. Description of property
1. Center Greens contains 12.01 gross acres, more or less. A
land trade between the Center Greens owners and the Golf
Course has been completed. The land trade has been an even
one and the acreage of Center Greens remains 12.01 acres.
2. The site slopes moderately toward the south and west at
slopes varying from 0.5% to 8%. Street design slopes reach
up to 6.6% for the interior streets. Specifically, at the
lower portion of Glen Eagle Court.
3. Center Greens will consist of 37 single family lots. The
original Southridge Greens Master Plan indicated that this
• tract was originally intended for 130 cluster townhomes.
Many of the lots will abut the Southridge Greens Golf
Course. 28' wide interior streets will be provided. These
will be compatible with the rest of the Southridge Greens
street system.
4. A green area concept will be utilized between backlots.
These areas will serve as open space and will be utilized
for storm drainage conveyance as well.
II DRAINAGE BASINS AND SUB -BASINS
A. Major Basin description
1. Center Greens is situated south of the Mail Creek Ditch. The
Mail Creek Ditch traverses a ridge dividing the Southridge
Greens development and two (2) major drainage ,basins. The
McClelland Drainage Basin is north of the ditch and the
Fossil Creek Basin to the south. Center Greens ip part of
the Fossil Creek Basin.
B. Sub -basin description
1. Center Greens was a part of the original Southridge Greens
P.U.D. Master Plan, designated as Tract I. It was also
included in the original drainage report for Southridge
Greens P.U.D. which was prepared by Engineering
Professionals, Inc. of Fort Collins in 1982. Runoff from
Center Greens, as well as a majority of the Southridge
• Greens development south of the Mail Creek Ditch, is
directed to an existing golf course pond, then to Fossil
Creek. Although the primary purpose of the pond is for golf
course irrigation storage, the pond plays a role in the
overall drainage system of the Southridge Greens P.U.D.
development.
PAGE 2
Project No: 1137-07-90
• Center Greens at Southridge Greens P.U.D.
B. Sub -basin description (continued)
2. There will be little change to historic flow patterns for
the Center Greens property. Runoff will continue to flow to
the south and west to the existing golf course pond.
3. There is a minor amount of offsite flow that enters the site
from the north (8th Fairway, approximately 1.18 acres).
Generally, however, most offsite flows are routed around the
site within the limits of the golf course fairways according
to the original grading and drainage intent of the golf
course. Refer to the exhibit (EXHIBIT C) in Appendix III.
This is a portion of the original Southridge Greens
PUD-Phase 1 Grading Plan, Sheet 5 of 7, which was prepared
by Engineering Professionals, Inc.
4. Storm runoff from the existing development north of Center
Greens, which is conveyed southerly via Southridge Greens
Boulevard, will continue to be conveyed via Southridge
Greens Boulevard to the four existing Type R inlets located
near the southeasterly corner of Center Greens..These inlets
redirect flows down the north side of the 10th Fairway to
the south golf course pond.
III DRAINAGE DESIGN CRITERIA
A. Regulations:
•
1. City of Fort Collins Storm Drainage Criteria was used.
B. Development criteria reference and constraints:
I. There is major drainage conveyance proximate to the
property. This is Southridge Greens Boulevard. This street
receives a very minor contribution of runoff from the Center
Greens project. This would be only from the landscaped areas
of Tracts A and S. No discharges from Southridge Greens
Boulevard contribute to the Center Greens onsite drainage
system. The site generated runoff is conveyed through the
internal street system and grass swales within green areas,
to a point of concentration on the golf course which is
approximately 75 feet from the existing golf course pond.
The runoff is then collected in a channel and ultimately
conveyed to the pond. A golf cart path will cross the
channel. An underdrain system to convey nuisance flows to
the pond, under the golf cart path will be provided.
C. Hydrological criteria
1. The Rational Method for determining peak flows was used for
the evaluation of the 2 and 100 year event. The calculation
of such flows for several basins and combinations thereof
appear in Appendix I.
D. Hydraulic criteria
1. Figure 5-2 (Nomograph for capacity of curb opening inlets in
sumps) was utilized to verify the Type R inlet opening
•
requirements or verifications. See Appendix II.
PAGE 3
Project No: 1137-07-90
• Center Greens at Southridge Greens P.U.D.
D. Hydraulic criteria (continued)
2. Figure 804-1D (Headwater Depth for Concrete Pipe Culverts
with Inlet Control) taken from the State Highway Department
Roadway Design Manual, was utilized to verify pipe culvert
capacities.
3. The Mannings formula was utilized for determining capacities
for streets, storm sewers and open channels according to the
requirements of Sections 4, 5 and 7. Mannings °n" values as
indicated in the criteria manual were utilized.
IV DRAINAGE FACILITY DESIGN
A. General concept
1. Generally, all storm runoff will be conveyed to the south
golf course pond via a series of street and greenway
conveyance systems. Refer to the Grading and Drainage Plan
included in the back envelope of this report.
2. Actual drainage facilities will include conveyance systems
and erosion protection. A separate erosion Protection design
and plan is included in the final plan set for Center
Greens.
V CONCLUSIONS
• A. Compliance with standards
1. All drainage calculations and facility design comply with
the requirements of the City of Fort Collins Storm Drainage
Criteria Manual and the Southridge Greens P.U.D. Master
Drainage Plan.
B. Drainage concept
1. The drainage concept for Center Greens, Tract I of
Southridge Greens P.U.D. is consistent with the original
Southridge Greens Master Drainage Plan and the existing
surrounding developments.
2. The grading plan allows for the effective conveyance of
storm water runoff throughout the site and within the limits
of the golf course to the existing south golf course pond.
The golf course pond was designed and constructed with a
pipe invert overflow to maintain a certain high water
working elevation. Flows which exceed the capacity of the
overflow structure spill over the pond and is conveyed via
an overflow channel provided with the original construction
of the golf course. Refer to the exhibit (EXHIBIT C) in
Appendix III. This is a portion of the original Southridge
Greens PUD-Phase 1 Grading Plan, Sheet 5 of 7, which was
prepared by Engineering Professionals, Inc.
3. The overall downstream impact potential will be reduced
because of the reduction in density from the original
Southridge Greens P.U.D. Master Plan. The original
• Southridge Greens P.U.D. Master Plan indicated that 130
cluster townhouses was planned for this tract. This has been
modified to accommodate the current plan which provides for
37 single family detached units.
PAGE 4
Project No: 1137-07-90
Center Greens at Southridge Greens P.U.D.
•
V CONCLUSIONS (continued)
B. Drainage concept
4. Detention will not be provided.
5. There is a total 40.4 cfs generated by the proposed
development of the site for the 100 year event and 10.8 cfs
for the 2 year event. Additional details for all sub -basins
and concentration points may be found in the appendix.
G. The peak runoff discussed in item 5 concentrates at the
intersection of two (2) drainage swales (Concentration Point
F). This runoff is collected in a channel (Channel A) and is
conveyed to the pond. The existing golf cart path will be
w reconstructed in order to cross the channel. An underdrain
system to convey nuisance flows to the pond, under the golf
cart path is provided. This consists of a 4" underdrain
placed in gravel. All storm events will convey storm water
over the golf cart path. Rip rap protection on both sides of
the golf cart path will be provided. See Channel "A"
profiles in the construction drawings. The Channel "A"
Profile includes the 4" diameter underdrain profile.
VI REFERENCES
1. City of Fort Collins Storm Drainage Criteria Manual
2. "Supplemental Drainage Report for the Southridge Greens PUD",
Engineering Professionals, Inc., March, 1982
3. "Southridge Greens PUD-Phase 1 Grading Plan", Sheet 5/7,
Engineering Professionals, Inc., Job 217-7
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APPENDIX I
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Summary of Sub -basin Areas
Residential Composite Runoff Coefficient Determination
Summary of street travel distances
Peak runoff determinations / Facility requirements
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APPENDIX II
Figure 3-1; Rainfall Intensity Duration Curve
Figure 3-2; Estimate of average flow velocity
Figure 4-2; Reduction factor for allowable gutter capacity
Figure 5-2; Nomograph for capacity of curb opening inlets in sumps
Figure 604-1D; Headwater depth for concrete pipe culverts (inlet control)
EXHIBIT A - Allowable street capacity determination
EXHIBIT B - Channel "C" Sections
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DRAINAGE CRITERIA MANUAL
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*MOST FREQUENTLY OCCURRING "UNDEVELOPED"
LAND SURFACES IN ,THE DENVER REGION.
REFERENCE: "Urban Hydrology For Small Watershads" Technical '
Release No.'55, USDA, SCS Jan. 1975.-
5 -1-84
URBAN DRAINAGE 6 FLOOD CONTROL DISTRICT
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(From: U.S. Dept. of Commerce, Bureau of Public Roads,1965)
MAY 1984
4-4
DESIGN CRITERIA
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MAY 1984 5-10 DESIGN CRITERIA
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APPENDIX III
Center Greens Master Grading and Drainage Plan (Stuffer Envelope)
Supplemental Drainage report for Southridge Greens P.U.D. by Engineering
Professionals, Inc., dated March, 1982
EXHIBIT C; a portion of the original Southridge Greens PUD-Phase 1 Grading
Plan, Sheet 5 of 7, which was prepared by Engineering Professionals, Inc.
•
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SOUTHRIDGE GREENS P.U.D. PHASE 1
SOUTHRIDGE GREENS P.U.D. PHASE 1
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EXHIBIT
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SUPPLEMENTAL DRAINAGE REPORT
for
SOUTHRIDGE GREENS P.U.D.
5 March 1982
Project No.: 217-2
City of Fort Collins
P.O. Box 580
Fort Collins, Colorado
80522
Attn: Mr. Bob Smith, Engineering Services
Dear Bob:
Transmitted herewith is the drainage study for the
SouthRidge Greens, P.U.D., located in the SS•i; of Section
6 and the NA of Section 7, Township 6 North, Range 68
West. The drainage study was prepared using the 1981
edition of the Fort Collins "Storm Drainage Criteria"
manual.
If you have any questions or comments, or if I can be
of any assistance, please advise.'•
5
J',
ENC,
i
�
d
3031221-3760 . 2020 Airway Ave. ! Fort Collins;
CO 80524
SCOPE:
This report presents the methods, assumptions and, results
of the calculations relating to storm drainage at South Ridge
Greens, P.U.D. Situated in south Fort Collins between Portner
Reservoir and the Union Pacific railroad right-of-way, SouthRidae
Greens is a 205 acre proposed development. Although submitted with
Phase One, this report constitutes a drainage master plan for
the entire proposed development.
Following sections of this report will deal with the site's
existing conditions and proposed development plan; the hydrology
of the future developed site; and detention requirements and
designs.
EXISTING SITE CHARACTERISTICS:
Two natrual and one man-made features dominate the hydrology
of the site. First, Fossil Creek traverses the southern quarter
of the site. Second, a high ridge divides the site into north
and south basins creating natural slopes in excess of eight
precent. Finally, Mail Creek Irrigation Ditch and two laterals
cross the northern basin slightly below the ridge line. This
ditch supplies irrigation water to the pasture and cropland pre-
sently covering the site.
The ridge, being up to one hundred feet above Fossil Creek flowline,
is the boundary between two major basins being studied by the
City of Fort Collins. The northern area of SouthRidae Greens lies
in McClelland basin, the southern in Fossil Creek basin. Interim
flood plains for Fossil Creek as it crosses the site show that
a one hundred year flood does not overtop the creek's relatively
deep banks. For complete information on major basins, the reader
should refer to the studies mentioned above.
South Ridge Greens has an uncommon development plan that affects
its impact on urban drainage. A core of low, medium and high
density residential phases will be surrounded with an eighteen
hole golf course. The 126 acre first phase consists of the golf
course, club house and maintenance buildings. How the golf course
J
and Fossil Creek impact the proposed detention plan will be
discussed in a later section.
DEVELOPED SITE HYDROLOGY:
Developed site hydrology for this master plan is based on
probable horizontal and vertical street alignments for the
future residential areas. Because of the steep slopes and. the
unusual shapes created by the golf course, the developer has
conducted extensive studies to establish an adequate and efficient
street system. (This street plan is based on present market
analysis and is subject to change as market forces change. To
maintain this flexibility to change, the street plan is not
presented on any documents that will go on public record.)
The street plan was used to define the basins and flow patterns
shown on the drainage plan sheet 6 of 10, of the utility plans.
Note that all residential phases south of Mail Creek Ditch will
drain to the south. The Rational Method was used for all hydrologic
calculations prepared for this report. Time of concentration 'calcu-
lations and rainfall intensities are tabulated in the appendix.
All other assumptions and results are listed on sheet 6 of 10.
Open channels required for phase one are of two types. Most of
the drainage channels will run parallel with the around slope.
In two areas, the channels will cut across the natural slope.
Calculations for both channel types are included in the appendix.
Probable culvert locations shown on the drainage plan are based
on an -analysis of gutter capacity for a two year storm.
Based on the probable street plan the major and minor storm systems
will be the same with one exception. During a minor storm there
will be cross basin flow from basin 3b to 4b via the culvert at
a proposed street intersection.
Some change in the major storm runoff pattern of the southern
basin pattern may occur during final design. However, additional._
runoff caused by such changes will not exceed the capacity of phase
one channels.
DETENT I Ott :
Immediately south of the southern property line Fossil Creek forms
a major backwater during a one hundred year'flood. Even with
possible improvements, this area will be ponded during majro
storm events. Due to the site's proximity to Fossil Creek and
the short times of concentration arising from steep slopes, no
detention is proposed for the southern basin.'
Much of the area of the northern basin.will be developed as a
golf course. One hundred percent perviousness will be maintained
in these areas. Also, golf course construction will create no
points of concentration. Existing slopes of four percent in the
north basin eliminate depression storage often associated with
undeveloped ground. Since construction of the golf course will
not change existing hydrology, no detention is proposed for
these areas.
Detention is proposed for basin 1, as shown on the drainage plan.
Calculations for release rate, required detention volume, provided
detention volume and controlled outlet are appended to this report.
Construction details are shown on the "Drainage Plan" and. "Cradina
Plan" sheets in the utility plan.
S UM14ARY :
Existing conditions at South Ridge Greens P.U.D. present no major
obstacles to safe and effective storm water management. Probable
regrading of future developed areas will direct some flows that
presently discharge to Mail Creek ditch southward to Fossil Creek.
Because of the site's location in the Fossil Creek basin and
because of the perviousness of phase one construction, no detention
is proposed for most of the site. Detention will be provided for
that area of residential development that contributes to McClellands
basin.
APPENDIX
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