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Final Approved Report
Date
DRAINAGE REPORT
SIEGMUND MEDICAL OFFICE P.U.D.
PREPARED FOR
W. J. FRICK DESIGN GROUP
PREPARED
BY
STEWART & ASSOCIATES, INC.
103 SOUTH MELDRUM STREET
FORT COLLINS, COLORADO 80521
303/482-9331
MARCH 1, 1993
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DRAINAGE REPORT
SIEGMUND MEDICAL OFFICE P.U.D.
SITE DESCRIPTION:
The proposed Siegmund Medical Office P.U.D. is a reconfiguration of
Building Envelope "F" from the site plan for the "First Replat of Boardwalk
at the Landings P.U.D." The site is located adjacent to the south side of
Harmony Reservoir. The site is situate in the'McClellands and Mail Creek Major
Drainageway. It is part of the Drainage Study completedbyCornell Consulting
Company in 1980 and more specifically a part of the Landings Drainage Study
also completed by Cornell Consulting Company: The text from this study is
included with this report along with the approved grading plan for the "First -
Replat of Boardwalk at the Landings P.U.D." The original building pad "F"
was 10,539 square feet. This site is part of Basin "A" as shown on Sheet 5
of 6 of the Cornell study.
All construction in the "First Replat of Boardwalk at the Landings P.U.D."
has been completed except for this proposed building site. There is an existing
parking lot on the west. There are existing buildings on the south and east,
the developed Creeks ide.Estates on the northwest, and Harmony Reservoir directly
north.
DEVELOPED CONDITION:
The reason for the new P.U.D. is because the building is being rotated
to North from N 33' W causing part of the north side of the building to fall
out of the original building envelope.
The new proposed building is to be constucted in three phases. The
building for Phase I will be 4,617 square feet. The total building area of
all phases will be 7,953 square feet. The allowable building size on Building
"F" was 10,539 square feet, therefore reducing the overall building
imperviousness by 25% more or less and reducing the overall runoff from the
site. All drainage from this site is to Harmony Reservoir as designed by the
original overlot grading and drainage plan. A copy of this plan is included
with this report.
A grading plan for the proposed building layout is included with this
report. Runoff from this site is west to the existing parking lot, then to
Harmony Reservoir and also north, directly to Harmony Reservoir. This pattern
is the same as shown on.the original plan.
EROSION CONTROL:
The owner will need to protect Harmony Reservoir and the curb opening
at the .north end of the existing parking lot during construction. The owner
will be required to use a silt fence along the north side of the site between
Harmony Reservoir and the north side of the building. A gravel filter dike
will be used at the curb opening in the parking lot. Refer to the grading
and erosion control plan for exact location.
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Franklin D. Blake, P.E. & L.S. ,icy
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I. INTRODUCTION
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SCOPE: The purpose
of this report is to present the developer
or his planner with
a set of feasible alternatives to the storm
drainage problem in
the Landings/Whalers Cove proposed develop-
ments.
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This report will analyze the impact of a major storm on the
proposed development. It also will give guidelines as to allow-
able release rates for the separate phases of development, and
present several alternative solutions to the handling of storm
runoff in the natural drainage swale which lies south of Harmony
Rservoir and north of Colorado State Highway 68.
LOCATION: The study area is located in the South One -Half of
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Section 36, Township 7 North, Range 69 West. The present land
1. l!"', use is agricultural. Land currently being planned for rezoning
as the South College Properties is located upstream of the
Landings. Upstream property is included in the analysis; however,
no recommendations are included for off -site property.
j DEVELOPMENT SCHEDULE: The approximate development schedule is
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as follows: Construction is to begin on The boardwalk Commercial
z Center in the Spring or Sunnier of 1979. Development of the
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:I remainder of Landings properties will proceed during the next
1-2 years. At this writing the development schedule for the
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upstream properties is not known. However, total construction
i within five years is a conservative estimate.
II. METHODS
In general the analysis was completed using "A Preliminary
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:> Draft of Storm Drainage Design Specificationsfor the City of
Fort Collins." The Rational Method was used to determine peak
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I flows. Although the total area is over 200 acres, sufficient
drainage will be retained in Harmony Reservoir to reduce the
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area drained to the natural swale to less than 200 acres.
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Coefficients (C values) for use with the Rational Method
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were taken from Table 2 of the City specifications and were
applied according to zoning. No adjustments were made in the
coefficients for varying densities between different phases of
the overall zoned properties. For instance, any property zoned
;. RLP (Ligh Planned Residential) was represented by a coefficient
of 0.5 regardless of the layout of the, phases of development.
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y Time of concentration (Tc) used was derived from Kirpich's
formula. Kirpich's time of concentration is more accurate for
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larger basins than are the overland flow curves given in the
City specifications. Since some of the subbasins are 50-60
acres in size our staff feels using Kirpich is a better approach.
For documentation on the formula refer to Drainage Manual
published by the U.S. Bureau of Reclamation and "Civil Engineering"
June, 1940, for first Publication.
Midway through the analysis a meeting was held with the City
staff responsible for revi.ew of drainage plans. The staff requested
additional analysis to include storm runoff from all properties
east of the center line of College Avenue. To perform this anal-
Ysis, hydrograph routing was necessary. We developed straight line
hydrographs consistent with the Rational Method peak discharge
and the accumulated runoff method. Included in Appendix B is the
development of.this hydrograph. It does not satisfy the "feel"
of a natural hydrograph, but it allows consistent application of
the accumulated runoff method.
BASIN DESCRIPTION AND LAND USE
The basin thalweg is generally located between 500 and 1,000
feet north of Harmony Road falling at approximately one (1) per-
cent from west to east. In the eastern one -quarter mile it turns
southeasterly to the intersection of Lemay Avenue and Harmony Road,
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where a 40-inch by 29-inch elliptical concrete pipe culvert
system exists to carry storm runoff under the intersection.
Prior to any improvements, the basin extended upstream
=� approximately three miles and drained an area of one and a half
square miles. However, development has altered the historic
basin..
College Avenue effectively blocks all upstream flow. The
one 24 inch culvert under College will be unused after completion
of a storm sewer along the west side of the street.
The present upstream limit of the basin under consideration
is the Larimer County Irrigation Canal #2 which lies between
College Avenue and the study area. It is unknown whether developed
�. storm runoff between the canal and College Avenue will be allowed
to discharge into the canal or be required to bypass it. The
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canal discharges into Mail Creek immediately after leaving the
:r basin under consideration, and lends itself to use as a storm
drainageway both now and after development.
Harmony Road crosses the natural Swale at the intersection
with Lemay Avenue. The existing drainage structure (a 45X29 inch
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elliptical culvert) has a maximum capacity of 70 cfs before
innundating the intersection. This capacity of 70 cfs should
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equal the release amount of the entire basin to avoid flooding
I, the intersection or having to install additional expensive
culverts at the intersection.
�3 In planning for development in the area of the natural swale
r it should be recognized that storm waters have a natural easement
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along this swale. Therefore, it should be used as a local ,storm
! drainageway and as a location of major detention areas. A major
detention area is one that routes offsite water in addition to
5 detaining its own onsite water.
a The proposed collector street, Boardwalk Drive, will also
intersect the valley thalweg. In order to size the culvert under
r Boardwalk Dive to carry 2 y the two ( ) year undeveloped runoff
t+ quantity, a major detention area should be located upstream on
the 10 acre apartment site and the City Park site (basin ri1 on
rthe map). The size of this detention area is greatly affected by
the drainage pattern of upstream lands.
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;.; IV. HARMONY RESERVOIR
,; Harmony Reservoir is a man-made irrigation reservoir owned
,i and controlled by The Landings, Ltd. It is located on the northern
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edge of the drainage basin at the downstream edge of the proposed
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Landings Filing IV. It has a surface area of approximately 14
acres and a control gate outlet along the south side. Since the
lake is less than ten feet deep and holds less than 400 acre-feet
of water,.state agencies will not be involved in the review of
proposed use or improvements for
the reservoir.
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Presently only 14 acres of
land drain
naturally
into the
reservoir, and it has no special
spillway
to discharge
storm
water. After development, the Landings Filing
IV, Park Place, and
part of the City owned park will
flow into
the
reservoir (basins
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1 and 2 on the map). Undetained
discharge
from
this 42 acres
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of RLP zoned property during a 100 year, 3
hour
storm would cause
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the water level in the reservoir
to rise eight
.inches.
The only
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structural improvement necessary
to dandle
this
flow would be a
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spillway or other control device
to ensure
that
the lake returns
to normal operating level. The design of the improvements should
?� occur prior to development of Whalers Cove downstream of the
reservoir.
If storm runoff is accepted from a portion of the South
College Properties in the northwesterly portion of the basin
'.� (basins 3 and 4 on the map) a portion of the lake surface will
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stl be increased 20 inches during the design storm described above.
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Development in The Boardwalk and Landings, Filing IV, will
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establish the horizontal and vertical limits of the westerly
bank of the reservoir. Obviously building elevations will be
set above the maximum lake elevation. Final design of the develop-
ment adjacent to the reservoir and of the release structure will
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dictate the operating and 'maximum lake water surface elevations.
AHowever,
the maximum water elevation will never reach the visible
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limits of the historic high water level (5010t ft). Therefore,
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major dam improvements should not be necessary.
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V. HYDROLOGIC AND HYDRAULIC INVESTIGATION
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Refer to Appendix A for a map of the drainage subbasins and
a schematic master drainage plan showing probable flow patterns.
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Subbasins shown on the map are defined with respect to one or
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more of the following: zoning, land ownership, topography, and
proposed improvements.
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S.
An example of defining subbasins by property line is the
division between basins 2 and 6. Basin 2, owned by The Landings,
Ltd. is drained to Harmony Reservoir. Basin 6 drains to the
natural swale.
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Area 10 is a combination of Whalers Cove and a church site.
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Both are in the Swale and cannot be separated because of the
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existing topography.
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Storm discharges for this report were calculated using the
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Rational Method as explained in the "Methods" section of this
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report. Discharges calculated for each subbasin include the 2
a
year and 100 year frequencies for both the developed and unde-
veloped conditions. See Appendix B for a list of release rates.
The 2 year discharges given here are to be used as guidelines
in sizing the release from the individual areas. In addition,
the runoff from a.major storm of 60 minute duration was calcu-
lated for basins 3-10. Hydrograph routing in the drainage swale
was necessary to determine detention volumes. A 60 minute storm
waschosen for this routing as it is usually the critical duration
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in the accumulated runoff method. Finally, the discharge for
basin 6 was calculated for its present partially developed con-
dition.
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All capacity calculations on culverts were based on the
HEC-5 Manual, "Hydraulic•Charts for the Selection of Highway
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Culverts." At the point of final release, head was assumed to
build up to the point of overtopping lemay Avenue.
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To determine required detention volumes the Accumulated
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Runoff Method was used. For Harmony Reservoir the release rate is
so extremely small that the line representing inflow and outflow
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for the Accumulated Runoff Method were still diverging at three
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hours. Therefore, the entire volume of a 3-hour event was
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considered to be retained. Impact on the reservoir is expressed
as the rise of the water surface in inches to, store the entire
100 year storm runoff volume. The required size of detention
areas in the swale was determined by routing the controlled
release hydrographs of the upstream basin. Lag time was estimated
j as one-half the time of concentration, assuming channelization
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t in the basin during development. Detention calculations are
' included in Appendix B.
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Two (2) open grass -lined channels were designed to carry storm
water flow. Minimum slope was assumed at one-half percent to
minimize ponding of trickle flow. The channel cross sections were
selected using Manning's Formula applied to grassed channels,
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as outlined in Open Channel Hydraulics, Chow, 1959. The size and
location of easements required for the master drainage plan as it
affects this project have been coordinated with the planner.
VI. SUMMARY A1ID RECOMMENDATIONS
The complexity and number of possible drainage patterns is
such that it is unrealistic to investigate only one pattern.
Therefore, we chose to present five alternatives, demonstrate their
feasibility, and suggest what we feel is the most economical final
solution.
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Three alternatives are based on the existing capacity of
the culvert at Harmony and Lemay, and vary only in the upstream
rpdrainage
pattern. The first assumes that all developed storm
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runoff uphill of the irrigation canal be allowed to flow into
the canal and immediately carried to Mail Creek. The second
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would consider all runoff by-passing the canal with areas 3 and
4 being directed toward Harmony Reservoir. The third would have
runoff from all property above the canal bypass it and flow into
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the existing swale.
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The last two alternatives assume the culvert at Lemay is
improved to carry the historic 2 year flood or overtopping of
Lemay is alowed to create the historic 2 year discharge out of
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the basin at that point. The alternatives accept the same upstream
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contributions as the second and third schemes described below.
Basically, the alternatives are either land intensive to
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provide extra space for
detention or capital intensive in the
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amount of improvements
required. These five alternatives are
listed in tabular form
with the required improvements in Appendix A.
The use of Harmony
Reservoir as detention plays a major role
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in the drainage system.
As discussed, the maximum water elevation
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will be set by proposed
construction near the lake's edge. The
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i water line will be set somewhere between the present water
surface and the historic high water line. With the maximum
high water set, the operating elevation will be set at a lower
elevation equal to at least the rise expected from a major storm.
By setting the maximum high water mark below the historic high
water mark, required improvements to the dam at Harmony Reservoir
are minimal. Since cosmetic improvements will be applied to the
dam face, hydraulic improvements should be done at the same time
by controlling the composition and compaction of imported material.
We recommend the following:
1. Larimer County Irrigation Canal k2, due to its immediate
release into Mail Creek be used as a point of final
discharge for upstream storm waters. Development according'
to City criteria will actually lessen the impact on the
canal, since a 100-year frequency storm would only release
an undeveloped 2 year runoff rate into the canal after
upstream detention is provided.
2. The detention pond in Basin N7 be a phased construction
to be reduced from 7.8 ac-ft. to 1.0 ac-ft. at the time,
of development with detention in Basin #6.
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