HomeMy WebLinkAboutDrainage Reports - 03/15/1988FINAL DRAINAGE INVESTIGATION
FOR
SUNSTONE VILLAGE - 3RD SUBDIVISION
(A PLANNED UNIT DEVELOPMENT)
FORT COLLINS, COLORADO
Prepared For:
Sunstone Village Associates
401 W. Mulberry
Ft. Collins, Colorado 80521
Date: March 15, 1988
Project No. 3721-CD1J-01-568
CONSULTING ENGINEERS
LANDMARK ENGINEERING LTD.
2300 W. Eisenhower Boulevard
Loveland, Colorado 80537
Ph: (303) 667-6286
Laadmarh
' ENGINEERING LTD.
March 15, 1988
' Project No. 3721-CD1J-01-568
Sunstone Village Associates
' 401 W. Mulberry
Ft. Collins, CO 80521
' RE: Final Drainage Investigation for Proposed "Sunstone
Village - P.U.D.", 3rd Subdivision
Ft. Collins, Colorado
Gentlemen:
As requested, this office has performed a final drainage
' investigation for the proposed "Sunstone Village - P.U.D.",
3rd Subdivision, Ft. Collins, Colorado. The existing and
proposed conditions have been analyzed based upon site
' observations, topographic mapping, previous reports and the
proposed site improvement plans.
Enclosed you will find the results of this investigation.
If you should have any questions or desire additional
information, please feel free to contact this office.
Sincerely,
Landm r Engineering Ltd.
Den its R. Messner
DRM/ej
Enclosure
I hereby affirm that this report and plan for the final
drainage design of "Sunstone Village - P.U.D.", 3rd
Subdivision, was prepared by me or under my direct
supervision, for the s thereof.
Kn%yt+s
rt
00.
David Shupe/t Q/ L
Colo. P.E. 5914; 5914 �; • s
'
0
es-�0?JA�
Main Office y��.-.__...•
2300 West Eisenhower Blvd. h OF %o �t Dale D. Olhousen, P.E. & L.S.
' Loveland, Colorado 80531 �� w�"`` President ( '�
ENGINEERS / ARCHITECTS / PLANNERS / SURVEYORS d� -
Loveland (303) 667-6286 Greeley (303) 356-6286 • Fort Collins (303) 221-5550 • Denver (303) 629-7124
TABLE OF CONTENTS
Letter of Transmittal
Table of Contents
General
Design Criteria
Storm Run-off Analysis
Existing Conditions
Proposed Developed Condition
Conclusions and Recommendations
Exhibits
Design Calculations
i
ii
1
2
3
3
3
5
0
'
GENERAL
'
The "Sunstone Village - P.U.D.", 3rd Subdivision, is a
portion of a proposed mixed usage development, the major
portion of which is to be developed into single family
residential lots. The site is located in the Northwest
Quarter of Section 32, Township 7 North, Range 68 West of
the 6th Principal Meridian, Larimer County, Colorado. More
generally, the site is situated along the Eastern side of
Timberline Road approximately 1/4 to 1/2 mile south of
'
Horsetooth Road. The 3rd Subdivision consists of an area of
5.6 acres. The site is proposed to be developed into 25
single family residentia.1 lots. The timetable for develop-
ment of the site is anticipated to be over the next two to
three years.
'
The site is accessed on the West from Timberline Road. A
developed area, the Sunstone Village 2nd Subdivision, abuts
the site to the South. The 1st Subdivision of Sunstone
Village adjoins the site on the East. Future phases of the
'
Sunstone Village project abut the site on the West. The Fox
Meadows Subdivision and the Fox Meadows Basin drainageway
abut the site on the North. The topography generally slopes
'
from West to East at relatively flat slopes, from 0.5% to
1%. The site has been farmed recently. The site has a
relatively high ground water table which is anticipated to
have seasonal fluctuation due in part to continuing agricul-
tural irrigation in the vicinity. As previously stated, the
Fox Meadows Basin Drainageway has been rough graded and is
situated adjacent to the Northern boundary of the 3rd
'
Subdivision.
Runoff from a part of the 2nd Subdivision to the South is
routed around the site to the East along Caribou Dr., and
through an existing detention facility which was developed
as a part of the 1st Subdivision. Approximately 7 acres of
the existing 2nd Subdivision and future development which
lies to the South and West of the proposed 3rd Subdivision
drains to the Northeast and will be routed along Sunburst
Drive. The area to the West of the site is to be developed
with future phases of the subdivision. Run-off from these
areas presently drains from West to East. The area to the
West of the 3rd Subdivision will continue to drain easterly
to Sunburst Drive where the flow will be intercepted and
directed to the North to the Fox Meadows Basin drainageway
located along the Northern Boundary of the site. The area
to the North of the site drains internally to the Fox
Meadows Subdivision and is directed to the detention facili-
ty which was constructed with the development of that
'
Subdivision.
DESIGN CRITERIA
In order to determine the rate and volume of runoff, both
'
under the existing conditions and the proposed developed
conditions, an acknowledged method of calculation has been
used. The design method used was the "Rational Method" as
'
outlined in the "City of Fort Collins Storm Drainage Design
Criteria and Construction Standards" manual. This method
enables the designer to determine the rate of runoff to be
anticipated. The "Rational Method" is based on the formula
Q = CIACf, explained as follows:
Q = the rate of runoff, in cubic feet per second
'
(cfs) .
C = runoff coefficient, which is the ratio between the
maximum rate of runoff from the area and the
average rate of rainfall intensity, in inches per
hour, for the period of maximum rainfall of a
given frequency of occurrence having a duration
'
equal to the time of concentration.
I = average intensity of rainfall, in inches per hour,
for a duration equal to time of concentration.
A = area, in acres.
C f = frequency factor to account for antecedent precip-
itation conditions, dimensionless.
The Time of Concentration is that time required for runoff
to flow from the most remote point of the drainage basin to
the point of concentration being considered. The Time of
'
Concentration is the total of the overland flow time and
flow time within channel, if any.
The equation used to determine the overland flow is:
Tc = 1.87 (1.1 - C C f ) D1/2/S1/3 where:
Tc = Time of concentration, in minutes.
S Slope of basin, in percent.
D Length of basin, in feet.
C Cf = Run-off coefficient and frequency factor, as
'
used in the "Rational Method" formula.
2
The in -channel flow time is based on the flow velocity, and
is dependent on the configuration of the channel and magni-
tude of flow.
The volume of each storm evaluated has been based on the
"Rational Method Hydrograph". The volume is based on the
following formula:
Vol. = Q x Tc x 60
Vol. = Volume in feet (cu. ft.)
of runoff, cubic
'
Q = Rate of runoff, in cubic feet per second (cfs)
Tc = Time of concentration, in minutes
Storm Runoff Analysis
The analysis for the site has considered the existing and
proposed conditions. The design storm return periods con-
sidered are the "Initial Design Storm", 2 Year, and "Major
Design Storm", 100 Year.
Existing Conditions
The runoff from this phase is included with the total 74.4
acre site which has been estimated to be 5.4 cfs for the 2
Year Storm Frequency and 19.2 cfs for the 100 Year Storm
'
Frequency.
'
Proposed Developed Condition
The runoff from the total 74.4 acre site has been estimated
to be 61.4 cfs for the 2 Year Storm Frequency and 214.8 cfs
'
for the 100 Year Storm Frequency.
The minimum detention storage volume to be provided for the
site paly is estimated to be 10.82 Ac-ft. with a release
rate of 10.4 cfs. The volume designed to be provided is 29
acre-feet, more or less, with a maximum release rate of 24
cfs. 7"IJe. MHP p�en./(dWU-5 IKCPenc¢c/ , Th'eW 6�1/470-Ae =C,1.5J`on�,_- jro^d a
2�r. Pe w coPar- 9 Cne<d ac(' �S �m-✓-� 6'r"
The propoggd plan is fAt' i19ea to retain the existing drain-
age pattern with runoff on the site being routed internally
from West to East. All of the runoff from the site will be
directed into the existing detention facility which is
3
1
located at the Northeast corner of the 1st Subdivision. The
construction of the detention facility was performed in
conjunction with the initial development phase. The drain-
'
ageway channel along the Northern boundary was rough graded
with the initial phase of construction. The determination
'
of the location and size of storm drainage pipes, inlets and
associated facilities has been determined as a part of the
subdivision improvement design process. All facilities are
designed to meet the City of Fort Collins criteria and
specifications. The development of the project has been
designed to construct facilities of adequate size to handle
all future development on the upstream area of the site.
4
I
CONCLUSIONS AND RECOMMENDATIONS
'
1.
The existing drainage pattern for the site is to be
generally retained.
2.
The proposed run-off volume due to the proposed devel-
opment of the site has been considered. On -site deten-
tion is proposed for the development in conjunction
with an existing regional facility.
3.
Drainage structures, that 'is, catch basin, pipe, and
overland channels are designed as a part of the final
subdivision improvement plans to be filed with the City
of Fort Collins Engineering Department.
4.
Overlot grading is to be performed in accordance with
the site grading and drainage plan.
5.
All proposed structures and streets' flow capacities
have been investigated and have been found to meet or
exceed the criteria set forth in the "City of Fort
Collins Storm Dranage Design Criteria and Construction
Manual."
6.
The site is not located within a designated "Flood -
Plain".
5
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4-4
DESIGN CRITERIA
LANDMARK ENGINEERS &"AkCHI-TECTS LTD.
2300 W. Eisenhower Boulevard
® Loveland, Colorado
80537
(303) 667-6286
TO �,J"4Q r I k, L
GENTLEMEN:
We are sending you
the following items:
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LETTER OF TRANSMITTAL
ate Job No.
`i : Lei
ttention
E:
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,3 ""Zesss I �—
MAIN OFFICE: 2300 W. Eisenhower Blvd., Loveland, CO 80 3 94286• Fort Collins (303)221-5550
BRANCH OFFICE: 2235 2nd: Avenue, Greeley, 0080631 (303)356-6286*Denver (303) 629- 7124
ST71-3 4/19/79 '
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