HomeMy WebLinkAboutDrainage Reports - 04/08/1980 (2)STORM WATER DRAINAGE REPORT
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THE RAINTREE P.U.D.
IN THE
CITY OF FORT COLLINS, COLORADO
APRIL 1980
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TARANTO
STANTON
'& TAGGE
CONSULTING ENGINEERS
Apri-1 8, 1980
Re: Stormwater Drainage Report
Raintree P.U.D.
' City of Fort Collins
Engineering. Services Department
300 LaPorte Avenue
Fort Collins, Colorado 80522
Attn: Mr. Mauri Rupel
Mr. Bob Smith
1
Gentlemen:
Submitted herein is the stormwater drainage
report on
the Raintree Planned Unit Development for your review.
If you should have any questions concerning
this report,
'
please do not hesitate to contact this office.
Sincerely,
'
John P. Cotten
Project Engineer
Larry E. Stanton, P.E.
TARANTO, STANTON & TAGGE
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CONSULTI14G ENGINEERS
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112 W. 11TH AVE. 6412 S. COLLEGE AVE. . P.O. BOX 703
HOLDREGE, NE 68949 FORT COLLINS, CO 80525 HAYDEN, CO 81639
(308) 995.6677 (303) 226-0557 (303) 276-3834
\l
DRAINAGE REPORT
for the
RAINTREE PLANNED UNIT DEVELOPMENT
FORT COLLINS, COLORADO
iI.
INTRODUCTION
'
The proposed development is a 37.39 acre tract located at the
northwest corner of.Shields Street and Drake Road; Southeast
Quarter, Section 22, Township 7 North, Range 69 West. The
purpose of this report is to provide a method of conveyance
and disposal of storm runoff from the proposed development.
Items to be -covered in this report are as follows:
1. Design.Criteria
2. Historic and Developed Runoff
'.
3. Storm Sewer Design.
4. Initial and Major Storm Runoff
5. Design Calculations
'
II. DESIGN CRITERIA
The design criteria.for this report was based upon.the guide-
'
dines and requirements put forth in the City of Fort Collins'
Storm Drainage Criteria manual and the Urban Storm Drainage
Criteria manual published by the Denver Regional Council of
Governments.
The "Rational Method" was used to determine all runoff flows
from and within the site; The following runoff coefficients
were used in ;the analysis':"
1
Undeveloped Land C 0.20
' Developed Residential C = 0.54
Runoff coefficients.are adjusted -for composite areas.
' III. HISTORIC AND DEVELOPED RUNOFF
' The proposed site was historically_.a portion of sub -basin
of'the Spring -Creek drainage area. This'area drained northerly
and northwesterly into Spring Creek. When the New Mercer ditch
' was constructed a portion of 'the sub -basin to the south of the
ditch was cut off from Srping',Creek and the drainage from this
area became tributary to the ditch This continued until
Shields Street and Drake Road were constructed, which further
' cut down the area by redirecting. the flow on the south side of
Shields Street.
There is a high point in' Drake Road approximately one thousand
' feet.(1000') west of the.intersection with Shields Street. The
drainage splits at this point, and the westerly portion flows
along Shields to Spring Creek... The eastern portion drains to
' the Drake -Shields intersection where it enters into the
Colorado State University farm -irrigation system.
From the time that these improvements were completed, until the
' present, only that parcel of"land which includes.Raintree P.U.D.
and the eight acre commercial site drained directly into the
New'Mercer ditch.
The proposed development will collect the runoff which is
' presently entering the New Mercer ditch in catch basins and
pipe the flow under the ditch to two detention.ponds. Release
1
t '
from•the ponds .wi11 be through .outlet structures into a common
' discharge line which terminates at Spring Creek.
In order to allow the main detention to be placed between the
two ditches and still keep the.water depth reasonable, the
upper portion of -the -site has its own detention system with a
'
controlled release into the main ponds. This includes Basins .
"F" and "G'.' which are detained :i.n.Ponds "C" and "D", as well
'
as a stub out in 'the storm sewer,.for therelease from the outlot
area (See Exhibit "A"). )\•four c.f.s. release rate from on- '
'
site detention in this outlot area -has been allowed for in
sizing the storm sewer.
The site contains a total of`37.39 acres,.of.which 35.00 acres
drains to detainage. The remaining 2.39 acres (6.8%) drains
into Drake Road Shields Street.
or
Detention pond sizing was accomplished by the.accumulated run-
off method. All runoff was calculated using the rational method
'
as described in the City of Fort Collins Storm Drainage Design
criteria. Detention pond volumes, -release rates, high water .
surfaces, etc. are shown on Exhibit "A" and in the design
drawings.
'
for the:100 is 97.40
The total developed runoff year storm c:f.s.
The two year undeveloped runoff is 14.31 c.f.s.
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IV. STORM SEWER DESIGN
In order to convey the drainage across the ditch to the deten-
tion ponds and also to,prevent.inundating the streets, a storm
`
sewer system was designed. Since the 100 year storm had to be
1
carried under the ditch in the pipes, a pressure system was
'
used to maintain reasonablc.pipe sizes. Pipe sizes were deter-.
mined by.a computer program using Bernoulli's.equation. A
discussion of the p.rogram.is'included in the calculation section
of.this report. In areas where the flow rates and elevations
'
allowed, gravity flow was used.
Also because of the high flow`requirements.a curb opening type
of inlet with a nine inch-(9") throat was designed. The calcu-
lations for this inlet are also included in the calculation
section. These inlets have been sized to accept the peak runoff
from the 100 year storm.',
V. INITIAL AND MAJOR STORM.RUNOFF
i
In order to protect the development against recurring minor
damage due to uncontrolled runoff and to reduce maintenance
t
costs, a two year'runoff period was used.to analyze the poten-
tial for damage.' A 100 year,return period was used to.analyze
for entire development. The.
the potential for major damage the
limiting criteria for the .initial storm on local streets was
'
that no curb -topping would be, allowed;, for the major storm the
maximum depth allowed.at'the.centerline would be 6":
The amount of runoff was determined at changes iri.grade and at
'
all intersections. The runoff was then routed to the detention
pond. Theoretical street capacities were calculated and then
reduced to an allowable capacity by the appropriate reduction
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factor. Where runoff exceeded the allowable runoff, inlets and
storm sewer was designed to be installed:
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