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December 14, 1977
Wheeler Realty
1331 8th Avenue
Greeley, CO 80521
Attn: Mr. Dan Wheeler
Dear Dan:
Presented herein is a Storm Drainage Report for the Fifth Filing in
Brown Farm Subdivision, located in Fort Collins. This report conforms F
to both the subdivision regulations of Fort Collins and Federal Housing
Administration (FHA). All calculations pertaining to this report are
available upon request at our office. k
i
Please feel free to contact us with your questions or comments.
Very truly yours,
M & I, INC.
;a.
John R. Hutchinson
.Z
ames ichols, P.E., L.S. .
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4, 44-14
TABLE OF CONTENTS
I..
Introduction
II.
Study Area
III.
Existing Storm Drainage Facilities
IV.
Design Criteria
and Requirements
V.
Drainage Plan
VI.
Conclusions and
Recommendations
Exhibit No. 1
"Historical Flows"
Exhibit No. 2
"Design Flows"
Exhibit No. 3
"100 Year Storm Effects"
Exhibit No. 4
"Drainage Swale Cross -Sections"
Exhibit No. 5
"Offsite Detention Pond"
Exhibit No. 6
"Intensity Duration Curves"
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I. INTRODUCTION
The purpose of this report is to present the developer with a proposed
series of improvements which will provide for proper management of storm
water runoff from both the. subdivision itself and adjacent contributing
basins.
The scope of this report involves an analysis of the storm drainage
characteristics of this property in both its historical and improved
conditions, consisting of -,the following items:
1) Established rainfall intensity duration curves for the City of Fort
Collins.
2) Storm runoff quantities and their general direction of flow.
3) The size and slope of storm drains required.
4) Size and location,.of detention ponds and culverts.
II. STUDY AREA
The area on which this study is focused is the Brown Farm Fifth Filing
subdivision and its related contributary basins, located in the western
portion of Fort Collins. In the southwest corner and along the southern
boundary, a portion of future filings of Brown Farm is included for
purposes of this analysis. The Fifth Filing is part of a comprehensive
master plan for the development of approximately 500 acres into a self-
sufficient community.``
III. EXISTING STORM DRAINAGE FACILITIES
The general topography of the area is comprised of gently rolling.
features, basically sloping from west to east. Contiguous with the`
eastern boundary line of the Fifth Filing, the Pleasant Valley and Lake
Canal serves as the historical drainage relief. (See Exhibit No. 1).
-1-
A large drainage swale divides the area into two portions, while serving
as the major storm runoff outlet for an area encompassing as'far west
as Hughes Stadium. However, the runoff is limited by a 24" pipe running
under Overland Trail, which effectively utilizes a. small area west of
Overland Trail as a detention pond. This swale eventually intercepts
the Pleasant Valley and Lake Canal.
IV. DESIGN CRITERIA AND REQUIREMENTS
The initial and the major drainage system make up the storm runoff
drainage system for an urban area. The initial drainage system should be
designed to accomodate the runoff from a 2-year storm by utilizing ,
the maximum capacity of.the streets, overland swales and, if necessary,
underground conduits. The major drainage system must be designed to
handle a 100-year storm without extensive property damage or loss of life.
Areas which may be susceptible to flooding must be examined. For pur-
poses of the major storm analysis, it is assumed that the storm sewer has
been blocked and rendered ineffectual. Floor elevations should be set to
allow proper drainage between and around these houses without flooding.
(See Exhibit 3).
The City of Fort Collins requires an improved site to limit its runoff
to the amount resulting from a 2-year frequency storm, occurring while
the site is in its natural state. To accomplish this, it is usually
necessary to design a detention pond with a total storage capacity of
the maximum difference between the 50-year storm iunoff on the developed
area and 2-year historical flow. An outlet structure is designed to
limit the flow to the two-year historical runoff value.
The amount of storm water runoff for the area, individual basins and
streets was determined by the rational method. This procedure equates
the design peak flow to the total area, amount of rainfall on that area,
and a multiplier that is determined by the physical characteristics of
the area under study. The rational method is commonly employed in the
design of borrow ditches, storm sewer systems and combined sewers. The
bulk of the procedures, methods and numerical constants were obtained
-2-
from the Urban Storm Drainage Criteria Manual, published by the Denver
Regional Council of Governments. The rainfall intensity duration curves
used in this report were developed by the City of Fort. Collins Engineering
Department., (See Exhibit No. 6).
The ponding elevations, capacities of the channels and piping were
determined by Manning's Formula, which equates a constant, a roughness
factor, the area of the channel or pipe, the hydraulic radius and
the slope, to the capacity of the channel or pipe.
V. DRAINAGE PLAN
Historical, design, 100-year storm flows, and included improvements are
illustrated on Exhibits 1, 2 and 3, respectively.
Offsite areas to the west will contribute a great deal of runoff to
the Brown Farm Fifth Filing. To the west of the multi -family area,
Stadium Heights, a multi -family subdivision will be contributing runoff,
by means of Somerville Drive, Brown Farm is not obligated to detain
this water, but merely channel it to the nearest outlet. As previously
mentioned, a large drainage swale, which outlets into the Pleasant
Valley and Lake Canal, divides the Fifth Filing into two separate
areas. As in the case of the Stadium Heights runoff, Brown Farm's
only obligation is to make sure the flow of water in this swale will re-
main unimpeded.
i
The runoff from Stadium Heights will be carried by Somerville Drive to
a low point at its intersection with Langshire Drive. From there it
will flow down Langshire Drive and feed directly into the large drainage
swale through a series of curb openings.
The large drainage swale will run east along a proposed property line
under Langshire Drive through a set of three 30" culverts, continue east
along a proposed property line, and outlet into the Pleasant Valley and
-3
Lake Canal. The swale will be improved, for as it passes through the Fifth
Filing it is relatively undefined. The proposed cross-section for the
improved swale is shown on Exhibit No. 4.
In order to maximize utility of available open space, we propose creating
a detention pond in Tract "C", which is located across the Pleasant Valley
and Lake Canal from the Brown Farm, Fifth Filing Subdivision. In order
to accomplish this, runoff from Basins 1 and 4 will be collected and trans-
ported by means of proposed concrete lined drainage swales to underground
conduits installed beneath the Pleasant Valley and Lake Canal and outletting
into the detention area in Tract "C". These pipes are sized to handle a
50-year storm in order.to maintain city requirement for detention pond
storage. In the event of a storm in excess of a 50-year frequency, any
runoff not handled by the pipes will overflow directly into the Pleasant
Valley and Lake Canal..
The detention pond in Tract "C" will have an outlet of 10 C.F.S. and a
storage capacity of 2.75 acre feet, as determined by the "Mass Diagram"
procedure. This pond will outlet into an existing swale to be improved,
which will carry it to the main detention pond, serving the north half of
the Brown Farm Development, located in the southwest corner of the inter-
section of West Prospect and Taft Hill. The runoff is then transported
under Taft Hill Road to a small stream which eventually empties into Spring
Creek. (See Exhibit No. 5).
The channel connecting the Tract "C" detention pond and the main detention
pond exists but must be brought up to original design standards. This
cross section is illustrated on Exhibit No. 4.
The main detention pond was designed by Hogan & Olhausen of Loveland,
Colorado to store the difference between a 50-year storm and a 10-year
historical storm of three hours duration with a total storage capacity of
12.4 acre feet at a depth of 3 ft. The present detention pond size will
be adequate to handle the additional runoff water fostered by the Fifth.
Filing area. The present detention pond should be considered adequate
for two reasons. One, the Fifth Filing water will be released at a con-
-4-
trolled rate of 10 c.f.s. Two, the total time taken for this water to
reach the pond will be approximatelytwice the time of concentration -for
the natural basin. Therefore, its effect on the total storage require-
ments are negligible.
The runoff from Basin S, as indicated on Exhibit 2, will be detained in
a small detention pond in Tract "B". Its total storage capacity 0.29 acre
feet, with a restricted outlet of 2.0 c..f.s. This smaller, separate
pond was necessitated due to the lay of the land and the difficulty involved
in transporting the runoff,to the Tract "C" detention pond.
Basins 2 and 3, as indicated in Exhibit 2, will contribute a small amount
of runoff and will be intercepted. by the curb openings at the culverts
under Langshire Drive.
In the event of a'major storm (100-year frequency) there are areas
which would be susceptible to flooding. However, in all four areas no
damage would occur to housing in the immediate areas. The depth of water
at each area, in conjunction with its related "Q" value due to a storm of
100-year frequency, is shown on Exhibit No. 3.
VI. CONCLUSIONS AND RECOMMENDATIONS
This drainage design will satisfy all hydraulic considerations as
well as governmental regulations in providing this future community with
both a safe and equitable solution to the problem of handling storm water
runoff. The following items are recommended:
1) Storm runoff should be routed as shown on Exhibit No. 2.
2) All drainage facilities and detention ponds should be constructed as
shown on Exhibit No. 2.
3) The Federal Housing Administration (FHA) minimum requirements must be
maintained on overlot grading and street design.
4) The Pleasant Valley and Lake Canal Company be contacted concerning
the discharge of storm water runoff into, and the construction of
conduits under their canal.
BROWN FARM 5th. FILING
EXHIBIT No. I
HISTORICAL FLOW
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DESISY1T[S fXfXILf[ BASIN
INDICATES DIRECTION Of FLOW \ OUfXTIiY OF FLOW IN
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EXHIBIT No. 2
DESIGN FLOW
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BASIN 12
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5120
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CONCRETE LINED DRAINAGE SWALE DETAIL -AMWS
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INDICATES DIRECTION OF SLOW B QUANTITY OF FLOW IN
CUBIC FEET PER SECOND •C.F.S.' '
PROPOSED INPACYENENTS
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BF WN FARM 5th. FILING
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EXHIBIT No. 3
EFFECTS OF A 100 YEAR STORM
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\ TRACT'C' DETENTION POND
IDD YEAR POND a CAPACITY • 2.T5 ACRE FEET
MAX OEPTM•L L' g 9L^` •, OUTLET • 10 C.F.S.
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PROPOSED IMPROVEMENTS
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SLOPE 2.00 cyo tv SLOPE = 2.00 %
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MAX CAPACITY= 532 C.F.S.
DRAINAGE'. SWALE. BETWEEN LOTS I a 2
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MAX CAPACITY =648 C.F.S.
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MAIN DETENTION POND
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