HomeMy WebLinkAboutTHE OVERLOOK CONVENIENCE CENTER AT ARAPAHOE FARM PUD - PRELIMINARY - 55-87N - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT 1
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PRELIMINARY DRAINAGE STUDY
FOR THE OVERLOOK AT ARAPAHOE FARMS P.U.D.
FORT COLLINS, COLORADO
November 25, 1996
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Prepared for:
Overlook Development Company
4714 Valley Ridge Court
Fort Collins, Colorado 80526
Prepared by:
RBD, Inc. Engineering Consultants
"A Division of the Sear-Brown Group"
209 South Meldrum Street
Fort Collins, Colorado 80521
(970) 482-5922
RBD Job No. 747-001
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INC.
' Rap
Engineering Consultants
A division of The Sear-Brown Group
209 S.Meldrum
' Fort Collins,Colorado 80521
970/482-5922
' November 25, 1996
Mr. Basil Hamdan
Mr. Matt Fater
City of Fort Collins
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Utility Services, Stormwater
235 Matthews
Fort Collins, Colorado 80522
RE: PreliminaryDrainage Studyfor the Overlook at Arapahoe Farms P.U.D.
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Gentlemen:
We are pleased to submit to you, for your review, this Preliminary Drainage Study for the
Overlook at Arapahoe Farms P.U.D. All computations within this report have been completed
in compliance with the City of Fort Collins Storm Drainage Design Criteria.
We appreciate your time and consideration in reviewing this submittal. Please call if you have
any questions or concerns regarding the content of this report. We would be pleased to meet with
I you regarding any comments you have after completing your review.
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Respectfully,
RBD Inc. Engineering Consultants
Prepared by: Reviewed by:
(NOT AVAILABLE)
Perry bot, E.I.T. Kevin Gingery, P.E.
Project Engineer Water Resources Project Manager
cc: 747-001(B) Calculations & Notes
' Dave Pietenpol, Overlook Development Company
H:\USER\CIVIL\PROJECTS\ARAPAHOE.OVL\DRAINAGE\drainrep.pre
Denver 303/458-5526
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ITABLE OF CONTENTS
IDESCRIPTION PAGE
I. GENERAL LOCATION AND DESCRIPTION
I A. LOCATION 1
B. DESCRIPTION OF PROPERTY 1
II. DRAINAGE BASINS AND SUB-BASINS
A. MAJOR BASIN DESCRIPTION 1
B. SUB=BASIN DESCRIPTION 1
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III. DRAINAGE DESIGN CRITERIA
REGULATIONS 2
IA.B. DEVELOPMENT CRITERIA REFERENCE,
CONSTRAINTS, AND COMPARISON 3
C. HYDROLOGICAL CRITERIA 3
D. HYDRAULIC CRITERIA 3 .
E. VARIANCES FROM CRITERIA 3
IIV. DRAINAGE FACILITY DESIGN
A. GENERAL CONCEPT 3
IB. SPECIFIC DETAILS 4
V. STORM WATER QUALITY
IA. GENERAL CONCEPT 5
VI. EROSION CONTROL
I A. GENERAL CONCEPT 5
I VII. CONCLUSIONS
A. COMPLIANCE WITH STANDARDS 5
B. DRAINAGE CONCEPT 5
IREFERENCES 7
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VICINITY MAP APPENDIX 1
DRAINAGE VICINITY MAP 3
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HYDROLOGY CALCULATIONS 5
EXCERPTS FROM: "FINAL DRAINAGE STUDY FOR
ARAPAHOE FARMS TOWNHOMES P.U.D." 11
ISUPPORTING CHARTS, TABLES AND FIGURES 14
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PRELIMINARY DRAINAGE STUDY
FOR THE OVERLOOK AT ARAPAHOE P.U.D.
FORT COLLINS, COLORADO
I. GENERAL LOCATION AND DESCRIPTION
A. Location
This site is located within the Overlook/Gates development in Fort Collins, just off
Harmony Road at the entrance to the overall development. The project is bounded
to the northeast by the proposed Harmony Road, to the northwest by the back lots
of the Overlook at Woodridge, First Filing, and to the south by the existing
Harmony Road.
More precisely the proposed development is located in the Southwest Quarter of
' Section 34, Township 7 North, Range 69 West of the 6th Principal Meridian, City
of Fort Collins, Larimer County, Colorado. The project site is shown on the
Vicinity Map in the Appendix.
1 B. Property
Description of P
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The Overlook at Arapahoe P.U.D. site contains approximately 4.69 acres and is
generally undeveloped at the present. The existing site is presently open ground
covered with native grasses. The site slopes gradually to the southeast at.
approximately 2.0% to 3.0% and historically drains to an existing roadside ditch
running along the south boundary of the site.
I H. DRAINAGE BASINS AND SUB-BASINS
A. Major Basin Description
The project site is located in the McClellands/Mail Creek Basin. It is more
particularly detailed in the "Preliminary Master Drainage Study for Woodridge",
December 1991, RBD Inc.
B. Sub-Basin Description
For the most part, this site is delineated as Basin 7C on the "Preliminary Grading
and Drainage Plan" for Woodridge, by RBD Inc. A portion of that plan is
included in the Appendix under the "Drainage Vicinity Map" section.
The site is split by the proposed new Seneca Street, which runs northeast to
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southwest in the eastern section of the site. As mentioned before, there is an
existing roadside ditch running along the south property line of the site, but this
will be removed with this development.
Of the five internal sub-basins, Basin 10 is the largest and drains entirely to a
grated manhole located west of the intersection of Harmony Road and Seneca
Street. This basin includes the back lot areas of Lots 1 through 8, The Overlook
at Woodridge P.U.D., First Filing and has therefore slightly increased the area
originally proposed for drainage to the inlet. This condition with the back lots was
pre-existing, and was not planned for in the "Preliminary Grading and Drainage
Plan" for Woodridge or on the "Drainage and Erosion Control Plan" for the
Overlook at Woodridge, First Filing. The manhole draining this basin then
discharges to a proposed 30" RCP storm drain which was previously designed with
Arapahoe Farms Townhomes P.U.D. Basin 20 consists of the proposed new
Seneca Street and its associated right-of-way. This basin drains into Harmony
Road, towards a proposed 20' curb inlet, which is in-line with the aforementioned
30" RCP storm drain. The extent of this inlet's carrying capacity will be discussed
in the "Drainage Facility Design" section. The smaller piece of commercial
property on the east side of Seneca Street, delineated as Basin 30, drains north into
a swale along the northeast limit of the basin, then into another grated manhole,
also draining into the previously designed 30" RCP storm drain.
The other two basins are more or less insignificant, and were delineated simply to
show where some of this property's runoff is headed offsite. Basin 50 consists of
a small portion of Seneca Street, draining offsite into the proposed Harmony Ridge
development. It was necessary to drain this area to the southwest since natural
grades dictate a high point as shown on the Preliminary Grading Plan. Finally,
Basin 60 consists of a small greenbelt along the south property line which will
drain offsite into the proposed Harmony Ridge Development. Due to the minimal
quantity of this runoff, it does not appear that this situation will adversely affect
the development to the south.
Basins 7A and 7B from the Preliminary Grading and Drainage Plan for Woodridge
were incorporated into this design, to show how Harmony Road street flows
compare to before. Basin H2 indicates the most downstream point along Harmony
that this project could plausibly affect. Street flows have been given on the
Preliminary Drainage and Grading Plan.
III. DRAINAGE DESIGN CRITERIA
A. Regulations
The City of Fort Collins Storm Drainage Design Criteria is being used for the
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subject site.
B. Development Criteria Reference, Constraints, and Comparison
The criteria established by the "Preliminary Master Drainage Study for
Woodridge", December 1991 by RBD Inc. and the "Final Drainage and Erosion
Control Study for Arapahoe Farms Townhomes P.U.D.", July 15, 1994 by RBD
Inc. were used for designing the drainage facilities for this site.
1 This site was not planned to require on-site detention. Based on the "Preliminary
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' Master Drainage Study for Woodridge" and the "Final Drainage and Erosion
Control Study for Arapahoe Farms Townhomes P.U.D.", the site drains undetained
into the proposed storm drain beneath Harmony Road.
' C. Hydrological Criteria.
The Rational Method for determining surface runoff was only used for the project
site. The 10-year and 100-year storm event criteria, obtained by the City of Fort
Collins, were used in calculating runoff values. These calculations and criteria are
included in the Appendix.
D. Hydraulic Criteria
All hydraulic calculations within this report have been prepared in accordance with
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the City of Fort Collins Storm Drainage Criteria and are included in the Appendix.
E. Variances from Criteria
No variances from City of Fort Collins Storm Drainage Criteria are being sought
for this project.
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IV. DRAINAGE FACILITY DESIGN
A. General Concept
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The on-site runoff produced by the portion of the site on the west side of Seneca
Street (Basin 10) will drain via on-site swales and curb & gutter to a low point
established west of the intersection of Seneca and Harmony, behind one of the
proposed buildings. At this low point, a manhole with a grated lid will capture
flows and release them into a 21" RCP. This 21" RCP will then enter a proposed
20' on-grade curb inlet located along Harmony Road. This inlet then flows into
the 30" RCP storm drain mentioned earlier.
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The commercial area to the east of Seneca Street (Basin 30) also drains to an inlet,
which discharges to the 30" RCP beneath Harmony Road. The proposed drainage
and grading plan is included in the back pocket of this report.
B. Specific Details
Per the "Preliminary Grading and Drainage Plan" for Woodridge and the "Final
Drainage and Erosion Control Study" for Arapahoe Farms Townhomes P.U.D.,
it was proposed that 36.6 cfs be released from this site during the major storm into
the inlet that basically serves this basin. Even though the area that drains to this
inlet has, in fact, increased, there appears to only be 26.5 cfs draining from Basin
10 and 2.6 cfs draining from Basin 30, during the major storm event.
Conservatively adding these two values from the area which drain to the proposed
30" RCP storm drain reveals 29.1 cfs leaving the site in this pipe. This project's
site plan has decreased the proposed runoff to the downstream regional pond.
The drainage from the Harmony Road 1/2-street width along the northeast property
line of this site was accounted for in the "Preliminary Master Drainage Study for
Woodridge". Basically, all of the Harmony Road runoff coining from upstream of
Seneca Street heads towards the low point that now exists where the 20' curb inlet
will be built in the future (see Drainage Plan, attached). This stormwater then
drains off the road and into a collection area, west of the Old Harmony Road and
Ithe New Harmony Road intersection.
The Harmony Road realignment will be completed prior to this project's
construction, which means that the 20' curb inlet will be installed in its proposed
location, as shown on the Arapahoe Farms Townhomes P.U.D. This curb inlet
was designed to capture a portion of the flows coming from the areas and streets
' upstream of Seneca Street. Per the "Preliminary Master Drainage Study for
Woodridge", this inlet will capture 4.4 cfs from upstream and let 14.9 cfs pass on
down Harmony Road. This condition will not be altered with this set of plans.
On the preliminary plan, Seneca Street south of Harmony was not supposed to
' drain into Harmony Road, but rather, into the pipe beneath Harmony. Based on
the existing grading, it will not be possible to contain Seneca Street so that it will
not drain directly to Harmony Road. Therefore, we are proposing that this
tsegment of Seneca Street drain undetained into Harmony Road. Since
approximately 900' of Harmony Road will be removed with this project, and
replaced with developed areas which will have their runoff routed elsewhere, we
propose that the allowance of the 180' of Seneca Street to continue draining to
Harmony will not exacerbate the existing drainage pattern. The amount of flow
passing from Seneca into Harmony is shown at Design Point 2 (2.6 cfs), and even
' with the existing flows in Harmony Road, the street capacities for stormwater are
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not reached. Along with Seneca Street draining to Harmony Road, there will be
20.5 cfs approaching the previously mentioned 20' curb inlet, and after the inlet
' takes 4.4 cfs, 16.1 cfs flow on down the road. (See Hydrology Section in the
_. Appendix of this report)
' V. STORM WATER QUALITY
A. General Concept
The water quality of storm water runoff must be addressed on all final design
utility plans. The Overlook at Arapahoe Farms P.U.D. is anticipating construction
beginning in 1997. Best Management Practices (BMP's) for the treatment of storm
water runoff will be determined during final design for this project. However,
BMP's may include grass-lined swales, minimized DCIA, and installation of silt
fence and gravel inlet filters.
All construction activities must comply with the State of Colorado permitting
process for Storm water Discharges Associated with Construction Activity. A
Colorado Department of Health NPDES Permit will be required before any
construction grading can begin.
VI. EROSION CONTROL
A. General Concept
During final design for this site, appropriate measures will be designed to control
erosion from the site. These measures to limit erosion will typically include
installation of silt fence, reseeding and mulching all disturbed areas which will not
be paved, and using a gravel mulch on areas which are to be paved.
VII. CONCLUSIONS
A. Compliance with Standards
All computations that have been completed within this report are in compliance
with the City of Fort Collins Erosion Control Reference Manual for construction
Sites and the Storm Drainage Design Criteria Manual.
B. Drainage Concept
The proposed drainage concepts presented in this report and on the construction
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plans adequately provide for the conveyance and release of developed on-site flows
into the existing curb&gutter on Harmony Road and mostly into the proposed 30"
' RCP storm drain. No on-site detention was proposed with this project, but the
amount of stormwater leaving the site has gone down from what was planned for
with Woodridge and Arapahoe Farms Townhomes P.U.D. The existing Detention
Pond downstream at the southeast corner of the Woodridge development has been
designed and built to account for flows released from the Overlook at Arapahoe
P.U.D. site.
If groundwater is encountered at the time of construction a Colorado Department
of Health Construction Dewatering Permit will be required.
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REFERENCES
Ii. Storm Drainage Design Criteria and Construction Standards, by the City of Fort
Collins, Colorado, May 1984, revised January 1991.
2. Erosion Control Reference Manual for Construction Sites, by the City of Fort
Collins, Colorado, January 1991.
3. Preliminary/Master Drainage Study for Woodridge, by RBD, Inc., Fort Collins,
Colorado, December 1991.
4. Final Drainage and Erosion Control Study for Arapahoe Farms Townhomes
P.U.D.,by RBD, Inc., Fort Collins, Colorado, July 15, 1994.
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APPENDIX
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W. HORSETOOTH ROAD
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DR
HARMONY ROAD
PROJECT SITE
VICINITY MAP
SCALE: 1 "=1000'
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1 DRAINAGE VICINITY MAP
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HYDROLOGY CALCULATIONS
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EXCERPTS FROM: "FINAL DRAINAGE STUDY FOR
ARAPAHOE FARMS TOWNHOMES P.U.D.°
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*MOST FREQUENTLY OCCURRING "UNDEVELOPED"
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