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INC
Engineering Consultants
2900 South College Avenue
Fort Collins, Colorado 80525
(308) 226-4955
March 4, 1991
Ms. Susan Duba. Hayes
Stormwater Utility Department
City of Fort Collins
235 Matthews Street
Fort Collins, CO 80522
RE: ARAPAHOE FARM MASTER DRAINAGE STUDY
Dear Susan:
RBD Inc. is pleased to submit to you the Master Drainage Study for the Arapahoe Farm
development. This site is located in the southwest part of Fort Collins, and is within the
McClella ,.is and Mail Creek Drainage Basin.
Please call me if you should have any questions concerning this report.
Sincerely,
nelle K, Ohms, P.E.
P oject Engineer
cc: Woodcraft Homes
434-001
h:\.\jko051 \arapfarm\masdrain. rep
Stan A. Myers, P.E.
Project Manager
11
Other Offices: Vail, Colorado (303) 476-6340 • Colorado Springs, Colorado (719) 598-4107 0 Longmont, Colorado (303) 678-9584
MASTER DRAINAGE STUDY
ARAPAHOE FARM
FORT COLLINS, COLORADO
MARCH 4, 1991
PREPARED FOR:
WOODCRAFT HOMES
PREPARED BY:
RBD, INC. ENGINEERING CONSULTANTS
2900 SOUTH COLLEGE AVENUE
FORT COLLINS, COLORADO 80525
INTRODUCTION
The results of a Master Drainage study for the Arapahoe Farm development are
presented in this report. A comprehensive plan for the management of stormwater is
proposed for use in the planning of future development.
The Arapahoe Farm, development is located in the southwest part of Fort Collins, and at
the northeast corner of the intersection of Taft Hill Road and Harmony Road. Arapahoe
Farm occupies most of the southwest quarter of Section 34, Township 7 North, Range
69 West, of the 6th Principal Meridian. The site is also bounded on the north by Imperial
Estates, and on the east by Regency Park P.U.D., Webber Junior High School, and
Johnson Elementary School. (See Master Drainage Plan in pocket.)
PREVIOUS REPORTS
Numerous drainage reports have incorporated flows from this area into their studies,
since this site is near the upper end of the McClellands and Mail Creek Basin, and
upstream of some existing developments. The McClellands and Mail Creek Major
Drainageway Plan, prepared by Cornell Consulting Company, December, 1980, includes
a hydrological analysis of the four historical drainage basins within the site. Each of these
basins originate within this quarter section. Basins 81 and 82 (refer to Exhibit 1) flow
through a culvert under Taft Hill Road, and then are routed to the north. Thus, no off -site
basins contribute flows to this area. The four historical basins in Arapahoe Farm are 75,
77, 79, and 80. The attached master drainage plan also shows the locations of these four
historical basins.
Since flows from these four basins will be directed in three different directions, the
following discussion deals with the separate basins accordingly.
Basin 75
The Final Drainage Report for Regency Park P.U.D., by Parsons and Associates
Consulting Engineers, dated March 30, 1987, states that 100-year developed runoff from
basin 75 will be routed through Regency Park in an open channel along the north side
of Harmony Road.
As described in the drainage report for Regency Park, 2nd Filing, prepared by Shear
Engineering; there had been plans to install a manhole at the east end of the 42" culvert
under Regency Drive, and extend an 18" pipe to the north, where the water would be
released into the Regional Detention Pond No. 1. These improvements, however, are
not currently in place. It is assumed that these improvements will be completed prior to
the construction of the first filing for Arapahoe Farm.
7-
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Basin 77
The following information was acquired from the "Final Drainage Report for Poudre School
District R-1 Elementary School", dated April 1987, and prepared by Engineering
Professionals, Inc.:
A. regional detention facility for Basins 77, 78, 83, and 84 was under consideration by the
City of Fort Collins Stormwater Utility. This facility would ultimately provide detention for
these undetained basins, when the necessary downstream improvements have been
made to handle the additional flows. On -site improvements at the elementary school were
designed to handle 100-year historic flows from Basin 77 upstream of the school site.
These off. -site flows are not be detained on the school site, but will be routed through the
on -site detention pond. Flows from upstream of the school exceeding the 100-year
historic rate will weir flow into Seneca Street, and enter the catch basin at the low point
in Seneca. They will then be released to the area on the east side of Seneca Street,
opposite from the elementary school.
Basins 79. and. 80
Based on the Preliminary Drainage. Report for the 1990 Junior High School, dated
February 5, 1988, and prepared%by RBD, Inc., provisions for the developed flows from
basins 79 and 80 were made in the design of the improvements at the junior high site.
Accordingto the "Final Drainage Report for Poudre School District. R-1 Elementary
9 p
School", dated April 1987, by Engineering Professionals, Inc., the channel along the north
side of Seneca Street and the east side of Regency Drive was sized for undetained on -site
as well as undetained off -site developed runoff from Basins 79, 80, and 85.
In summary, for basins 75, 79, and 80, the downstream improvements have provided for
the undetained conveyance of the developed flows into Regional Detention Pond No. 1.
For basin 77, however, the downstream improvements will only accommodate the historic
flow rate.
EXISTING CONDITIONS
The existing site is covered with native grasses, with little vegetation otherwise. Except
for a district waterline through the site, there are no improvements, save for several field
roads crossing through the area.. Most of the site drains naturally to the east, with most
slopes averaging from less than one percent to over three percent. A natural drainage
swale through the center of the quarter section intercepts much of the stormwater flows,
and transports the same toward the east boundary of the site.
At the east side of the Arapahoe Farm area, the stormwaters eventually flow toward the
west end of constructed Seneca Street, and then to the Webber Junior High School
property, where they are routed through the school site. Through three separate
K
segments of double 42" culverts and various types of open channels, the flows off -site
and on -site to the junior high are transported from the junior high school to the Regional
Detention Pond No. 1. This pond is located south of Wakerobin Lane, east of Regency
Drive, and west of Pleasant Valley and Lake Canal. Regional Detention Pond No. 1 is
designed for flows from basins 75, 79, 80, and 85.
The southeast portion of the Arapahoe Farm area drains toward an existing swale running
along the north side of Harmony Road, adjacent to the Regency Park development, A
42" culvert carries the flows under Regency Drive, but the outlet invert is a couple of feet
lower than the surrounding ground. With enough flows in the pipe, the water would
continue to flow to the east, toward the Regional Detention Pond No. 1.
A small portion of the quarter section drains to the very northeast corner, and then along
' the north side of Johnson Elementary School, located directly north of the junior high
school. The flows are routed through the site and then released into a detention area on
the east side of Seneca Street, opposite the school.
The southwest corner of the site has a distinctive ridge line. With grades over ten percent
sloping toward Harmony Road, this corner drains away from the area under study, and
is in an entirely separate drainage basin from the rest of the site.
PROPOSED DEVELOPMENT
The Arapahoe Farm area is planned as a single-family residential housing development,
with ultimately 432 lots. Harmony Road will be realigned to pass through the center of
the site, and connect with West Horsetooth Road at Taft Hill Road. Seneca Street, which
currently stops at the east boundary of Arapahoe Farm, will be extended across the
southeast corner of the site. The existing drainage swale through the center of the site
will be used as the primary means of stormwater conveyance through the site and for
delivery of flows to the existing drainage improvements at Webber Junior High School.
Stormwater flows from the northeast corner of the development will, as historically, flow
along the north side of,the elementary school. The southeast corner of the site will
continue to flow to the drainage channel in front of Regency Park.
HYDROLOGICAL ANALYSIS
Since the Rational Method is applicable for areas up to 200 acres, the Arapahoe Farm
site, covering about 130 acres, was analyzed by the Rational Method. The accompanying
-drainage map shows the boundaries of the developed basins, based on proposed street
layout and consistency with historical basins. The times of concentration were based on
approximate design slopes for the proposed streets.
3
DESIGN CRITERIA
With Arapahoe Farms designated as a residential development, the design storm
frequencies used were 2-year as the initial storm, and 100-year as the major storm.
The appropriate street encroachment criteria was used for the various types of streets
through the site. The arterial standards were used -for Harmony Road, collector standards
for Seneca Street, and local street standards for all other streets in the development.
Where the developed runoff exceeds the standards for street encroachment for the initial
and/or major storm, a storm sewer system has been defined. Inlets were located where
the street capacity was exceeded, and outlet pipes were sized by Manning's equation.
The basis used for the sizing of the temporary detention facility was the 100-year
developed storm for inflow, and the 100-year historic flow rate as the release rate. The
downstream facilities are designed to accommodate this release rate.
The calculations in the appendix dhow flow quantities generated for the various developed
basins.
PROPOSED DRAINAGE IMPROVEMENTS
The required portions of storm drain system, i.e., crosspans, inlets and pipe systems, are
shown on the Master Drainage Plan. The pipe systems will either outlet into the improved
drainage channel through the center of the development, or directly into existing culverts
at Seneca Street.
A box culvert will be installed where Harmony Road crosses over the drainage swale. A
pedestrian underpass will be incorporated into the design of the culvert crossing.
Three of the four on -site basins, numbered 75, 79, and 80, drain undetained toward the
Regional Detention Pond No.1. The on -site portion of the fourth basin, 77, will require its
own on -site temporary detention facility. When the anticipated downstream improvements
have been made to handle the 100-year developed flows from the entire basin, this
temporary detention facility will no longer be needed. In the meantime, a temporary
detention facility would be sized for a release rate of the 100-year historic rate. The outlet
pipe would discharge into the existing drainage channel on the north side of the
elementary school.
4
SUMMARY
The outlined methods of analysis are consistent with the storm drainage criteria used by
the City of Fort Collins. The proposed design will allow for the safe conveyance of
stormwater through the site and into existing drainage facilities. Most of the existing
downstream facilities have been designed to accommodate developed flow rates from the
developed basins in the Arapahoe Farm area. Where they are not designed for such,
temporary on -site improvements will be provided until the necessary downstream
improvements have been made.
5
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