HomeMy WebLinkAboutDrainage Reports - 03/14/1975i
_.� DRAINAGE REPORT
BROWN FARM 3RD FILING
�.� FORT COLLINS, COLORADO
1
Prepared For:
Wheeler Realty Company
I1331
8th Avenue
1
Greeley, Colorado 80631
.
March 14, 1975
File No. 39-14-11
HOGAN & OLHAUSENO P.C.
2300 West Eisenhower Boulevard
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Loveland, Colorado 80537
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March 14, 1975
File No. 39-14-11
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Wheeler Realty Company
1331 8th Avenue
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Greeley, Colorado 80631
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Gentlemen:
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The enclosed report contains the results of a
drainage study
made for the Brown Farm 3rd Filing, a proposed
subdivision in
Fort Collins, Colorado.
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If there are anyquestions concerning the data
or recommendations
presented, or if we may be of further service,
please call.
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Very truly yours,
HOGAN & OLHAUSEN, P.C.
Fred L. Ja.'Ckks```on
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TABLE OF CONTENTS
Letter of Transmittal
Table of Contents
General
Design Criteria
Storm Drainage Design
Conclusions
Recommendations
Location Map
Rainfall Intensity Frequency Curves
Head Slope Capacity Chart
Reduction Factor Chart
Street Cross Section.
Calculations
Drainage Area Map - Exhibit A
ii
Page
i
ii
2
3
4
6
7
8
9
10 - 11
GENERAL
The Brown Farm 3rd Filing, Fort Collins, Colorado, is a proposed residential
subdivision located in Section 21, Township 7 North, Range 69 West of the
6th P.M. The site is presently a hayfield with topography gently rolling in
a west to east direction. The Pleasant Valley and Lake Canal Company's
irrigation canal is located along the westerly boundary of the site and
prevents offsite drainage from the west from flowing onto the site. Waters
which are carried in this canal flow southerly to Spring Creek. The Pleasant
Valley and Lake Canal Company irrigation canal is designated as a future
drainage -way for the City of Fort Collins. The offsite contribution from the
north is limited by a ridge which parallels the northerly boundary of the
site. The majority of the offsite contribution from the north is generated
within Brown Farm 2nd Filing and is contained within the street section. The
offsite contribution from the south is limited by a ridge which approximately
parallels the southerly boundary. The contribution generated offsite from
the south is to be carried within the street section and drainage swales.
All improvements proposed to carry the offsite contributions have been designed
to consider these areas in the developed state.
The site proper has a gentle west to east swale running through it from the
Pleasant Valley and Lake Canal irrigation ditch to Taft Hill Road. The
majority of the proposed subdivision has been designed to drain to this swale
by way of street flow to a proposed detention pond located in the southeast
corner of the site. The overall drainage pattern for the proposed development
will be maintained as in the present state with additional retention capacity
provided in the detention pond to provide for additional flows generated by
development.
DESIGN CRITERIA
The rational method of design as outlined in "Design and Construction of
Sanitary and Storm Sewers" prepared by a Joint Committee of Water Pollution
Control Federation and the American Society of Civil Engineers is the basis
of computation used in this study. This method is based on the formula Q = CIA
explained as follows:
Q = The discharge of storm water in cubic feet per second.
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C = A runoff factor - the runoff factor is a coefficient that
approximates the amount of rainfall running off of a particular
area and takes into consideration various factors that effect
this runoff such as soil features and type of improvement. For
this report, a runoff factor of 0.5 was used for all land that is
to be developed as single family residences and a runoff factor
of 0.3 was used for irrigated farm land.
I = Intensity rate of rainfall - for this study we have used an
intensity
intensity frequency curve developed by Black and Veatch developed
for the Fort Collins area for storms with frequencies of 50 years
or less. Additional rainfall data was needed in order to determine
an intensity curve for a storm of 100 year frequency. This data
was obtained from the Colorado State University recording station
in Fort Collins. The 100 year rainfall intensities as plotted on
the Rainfall Intensity Frequency Curves (Page 6 of this report) are
within 10 percent of data taken from the U.S. Department of
Commerce, Weather Bureau, Technical Paper No. 40, repaginated and
reprinted January 1963. Therefore, the 100 year intensity curve
was used as the design basis throughout this report.
A = The area contributing storm water runoff to any particular point
in the design considerations.
For the 100 year storm, runoff in streets shall not exceed a depth of 18
inches above the flowline of the curb and gutter. Since flow in the gutter
is not always unobstructed, a reduction factor shall be applied to theoretical
street capacities taken from the Head Slope Capacity Chart. See Reduction
Factor Chart on Page 8 of this report.
STORM DRAINAGE DESIGN
The drainage basin has been divided into nine design study areas. (See
Drainage Area Map - Exhibit A.
The collection point for Areas 1 & 2 is the intersection of Hampshire Road and
Charolais Drive. At this point the runoff has reached 67 cfs for the 10 year
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storm (Q10) and 95 cfs for the 100 year storm 0100)• The depth above
flowline for a street at minimum grade (0.4%) with a reduction factor of
0.5 is 1.17 feet for Q100 = 190 cfs. (See Head Slope Capacity Chart, Page 7,
and Reduction Factor Chart, Page 8).
Areas 1, 2 and 3 have a common collection point at the East end of Ayrshire
Drive at which point the runoff exits from the street into the retention
pond. The flow at this point has reached 81 cfs for the 10 year storm and
116 cfs for the 100 year storm. The depth above flowline at this point
for a street at minimum grade is 1.26 feet for Q100 = 232 cfs.
Area 4 has its collection point on Manchester Drive between Taft Hill Road
and Rambovillet Drive. The runoff exits south into the retention pond from
Manchester Drive. The rate of runoff has reached 26 cfs for Q10 and 37 cfs
for Q100. The depth above flowline at this point for a street at minimum
grade is 0.81 feet for Q100 = 74 cfs.
onsidering areas 1 through 7 contributing runoff to the retention and
the total runoff for the 100 year storm is 7.3 acre-feet and the rate of
runoff fort -he 10-_year storm would be 80 cfs. Upon complete development
of the areas South of the 3rd Filing, the total runoff increases to 8 5
acre-feet for Qinn and the rate of runoff is 127 cfs for Q10
CONCLUSIONS
The storm runoff for the Brown Farm 3rd Filing can be handled by flow
along proposed streets without exceeding -a depth of 18 inches above the
gutter flowline. Runoff exits from the streets into the proposed detention
pond located in the Southeast corner of the development. The runoff will be
conveyed underground via a storm sewer from the retention pond to Spring
Creek along Taft Hill Road. The proposed storm sewer and detention facilities
are to be designed for the entire contributing area in the fully developed
state.
3
01
RECOMMENDATIONS
It is recommended that the detention pond and storm sewer be built as
shown on the attached plan. The detention pond combined with the 24-inch
diameter storm sewer will accommodate the runoff from the 100 year storm.
When the proposed areas to the south are developed, the runoff from them
should be collected at the corner of Taft Hill Road and Drake Road and
the storm sewer should be increased to a 36 inch diameter at this point.
It is recommended that:
I. The retention pond and storm sewer be constructed in accordance
with details shown on the drawings.'
2. The minimum street grade .be 0.4%.
3. The minimum overlot. grading standards of the Federal Housing
Administration be observed in the construction of residences.
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NOTE: Flowline Slope (%)
GRAPH CAN BE USED FOR ROLLOVER CURB SECTIONS. WITH SUFFICIENT
ACCURACY FOR DESIGN PURPOSES.
GRAPH BASED ON FOLLOWING CRITERIA:
40
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Cross -sections ; S=200 STANDARD CURB
40, SECTION
Mannings n's Side .slope above curbs =0.037 street =0.016
No crossfall
Actual Capacity=Theoretical Capacity x Reduction Factor
When flow is below crown of street , each side of street carries one-half
of the theoretical capacity_
M-40 Figure III•E-1
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