HomeMy WebLinkAboutLINCOLN EAST CONVENIENCE CENTER PUD, LOT 1 - FINAL - 40-94E - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTVICINITY MAP
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storage tank slab have been elevated to at least 18 inches above the base flood
elevation of 40.0 feet. The impact of the development will be studied in greater
detail during Floodplain Use Permit Review Process.
It has brought to our attention that City Criteria requires that new developments detain
adjacent half streets, more specifically, the adjacent half streets of Lincoln & Lemay. The
applicant is requesting that the Diamond Shamrock development be excused from
detaining the adjacent half -streets based on the following:
A. Currently, the adjacent half streets of Lincoln and Lemay Avenues are both
located within the 100 year floodplain according to Flood Insurance Rate Maps
Community Panel No. 080101-0179 D dated March 18, 1996 and Community Panel
No. 080102-0004 C dated March 18, 1996. As mentioned earlier in this report, the
elevation of the site has been lifted two to five feet from the existing grades to meet
Fort Collins Flood Prevention and Protection requirements, and ensure safety from
the 100 year floodplain. As a result of the increased grade elevations of the site,
detaining of the adjacent half -streets within the site has been made extremely
difficult.
B. The applicant is currently controlling the off -site developed flows (Basins OS-1
and OS-2) from the adjacent half -streets of both the northerly and easterly private
access drives.
C. As part of the conditional approval of the Preliminary PUD, the applicant is
participating with the City to provide considerable improvements to the existing
drainage conditions along Lincoln Avenue with the installation of the 43 inch by 63
inch H.E.R.C.P. to control the projected flows of approximately 128.1 cfs.
It is determined from the City Reference Manual and Wind Erodibility Zone Map that
the wind erodibility zone is moderate for this site. The potential exists for moderate
erosion during construction. Immediately after overlot grading, gravel mulch is to be
placed in areas that are to be paved, excluding building envelopes. Permanent sod or
seeding with mulch is to be applied immediately after final grading of landscaped areas
calling for grass. Calculations supporting erosion control measures may be found in
Appendix B.
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empty into the proposed 43 inch by 63 inch H.E.R.C.P. (horizontal elliptical
reinforced concrete pipe) along Lincoln Avenue. As a condition of approval for
the Lincoln East Preliminary PUD, the applicant is participating with the City in
the installation of a 128.1 cfs capacity storm sewer along Lincoln Avenue to
handle proposed flows as indicated by the City for the Lincoln/Lemay
intersection. This aforementioned 43 inch by 63 inch H.E.R.C.P. will be
extended along the Lincoln Avenue right-of-way from the existing 12 inch clay
pipe under Lemay Avenue to the roadside ditch beyond the east property line of
the site. Calculations supporting the required flows of the proposed 43 inch by
63 inch H.E.R.C.P. storm sewer along Lincoln Avenue are provided in the
Appendix of this report.
It is the understanding of the applicant that Lemay Avenue will be improved by
the City in the future. Since this area has experienced poor drainage in the past,
the rise in the site's finished grades will cause ponding in the small barrow ditch
along Lemay Avenue until those improvements are completed.
This development and the installation of the storm sewer to Lincoln Avenue will
greatly improve the current drainage conditions at this intersection. In addition,
the property to the east will no longer receive the historical flows of 0.30 and
1.09 cfs from this property since the developed flows will be safely routed into
the roadside ditch along Lincoln Avenue.
The minimum required detention volume of 10303 cubic feet for the Diamond
Shamrock property has been calculated using the 2 year historical flow per the
City of Fort Collins Storm Drainage Design Criteria. The allowable 2 year
historical release rate from the Lincoln East PUD is 0.59 cfs. The detention
pond provided for the Diamond Shamrock development has been limited to a
release rate of 0.49 cfs since 0.10 cfs of undetained runoff is previously released
from basin A-2.
The controlled release from the detention pond will be regulated by an outlet
structure which will be provided with an orifice plate over the outlet pipe. The
Diamond Shamrock detention area will be limited to the previously mentioned
0.49 cfs release rate by a 2 5/8 inch diameter opening. In case of emergency,
where this controlled outlet system may fail or plug, water may increase beyond
the 100 year water surface of 39.4 feet to the elevation of approximately 40.9,
and release into the Lincoln Avenue right-of-way through the southerly curb cut
into the Lincoln East PUD.
A Floodplain Use Permit from the City of Fort Collins will be required prior to
development of this site. In order to meet requirements for the City of Fort
Collins Flood Prevention and Protection, the site buildings and underground
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The remaining 0.70 acres has been identified as Basin B. This basin is composed
of the northern and eastern areas of the property and releases the largely
unconcentrated flows of 0.27 and 0.97 cfs, respectively, into the adjacent
property along the east property line. Offsite flows also contribute to this basin
from the north of a small area of 0.07 acres consisting of 0.03 and 0.12 cfs. This
combined area eventually empties into the roadside ditch along Lincoln Avenue.
The site is divided into two different basins. Basin A-1 is the majority of the
development and basin A-2 consists of a small undetained basin located in the
southeasterly corner of the site. Two offsite basins have been examined with this
study to help determine the impact of this development and control the developed
flows being released into the large roadside ditch along Lincoln Avenue. The
elevation of the site has been lifted two to five feet with this development to meet
Fort Collins Flood Prevention and Protection requirements .for minimum floor
elevations of eighteen inches above the 100 year FEMA floodplain base flood
elevation of 40.0 (provided on the Grading, Drainage and Erosion Control Plan).
Furthermore, the future development of North Lemay Avenue and Lincoln
Avenue was considered in the design grades to provide smooth transition of the
access points for the interim and future conditions.
Basins A-1, OS-1, and OS-2 are 1.518 acres of landscaping, paving, and roof
area that combine for the 2 and 100 year runoff of 3.09 and 10.04 cfs,
respectively. These basins are all detained within two separate ponds provided
for the development. The developed release from these basins are controlled
through a series of storm sewers which release into the detention pond area. The
detention pond is designed with an outlet structure that limits the release of the
development to historical flows of the 2 year storm. The 15 inch R.C.P. outfall
from the outlet structure will be connected to a proposed 30 inch storm sewer to
be constructed by the Fort Collins Housing Authority for the San Cristo PUD
development along the east edge of the private access drive near the east property
line. Specific details and additional information regarding the detention design is
provided later in this report and on the Grading, Drainage and Erosion Control
Plan.
Basin A-2 consists of 0.047 undetained acres of landscaping and paving which
combine for the 2 and 100 year runoff of 0.03 and 0.10 cfs. Basin A-2 is a very
small basin located in the southeasterly comer of the site that conveys runoff to a
proposed storm sewer inlet located within Lincoln Avenue.
Flows generated by the development of the adjacent one-half of Lincoln Avenue
will be captured by a proposed storm sewer inlet on the north side of Lincoln
Avenue. This captured runoff will be directed into a 15 inch R.C.P. that will
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The Lincoln East, P.U.D. is bounded by North Lemay Avenue on the west, East
Lincoln Avenue on the south, and undeveloped land on the north and east, owned by
Webster Associates. It is located in the NW 1/4 of Section 32, Township 7 north, Range
68 west of the Sixth Principle Meridian, City of Fort Collins, Larimer County, Colorado.
The Lincoln East, P.U.D. contains 1.41 acres and is presently single family
residences and agricultural land. The topography of the site generally slopes from west to
east at approximately 0-0.8 percent. The property is situated in the fringe area (zone A-4)
of the 100 year designated flood hazard area according to the Flood Insurance Rate Map
Community Panel No. 080101-0179 C dated March 18, 1996.
FEW 17-TIT .141
This storm drainage and erosion control study has been prepared using the City of
Fort Collins Storm Drainage Design Criteria, dated May, 1984 and the City of
Fort Collins Erosion Control Reference Manual, dated January, 1991.
. • • - . -..
The Rational Method for determining surface runoff was used for the project site.
The 2 year and 100 year storm even criteria were used in calculating runoff
flows. Times of concentration were calculated using the overland flow equation
and estimates of average flow velocity. Composite runoff coefficients were
calculated for individual site basins and the overall site. Calculations are included
in the Appendix A to this report.
Historically, the land for this site drains to the southeast and eventually into the
large roadside ditch along the north R.O.W. of Lincoln Avenue. The entire site
has been divided into two major basins, A and B.
Basin A, 0.72 acres, is located adjacent to the right-of-way of both Lincoln and
Lemay. Offsite flows from the adjacent one-half of Lemay flow into the site and
travel southeasterly combining with the offsite flows from the Lincoln right-of-
way and have a release for the combined 2 and 100 year runoff of 1.55 and 4.87
cfs, respectively, into the large roadside ditch along Lincoln Avenue. The land
along the east R.O.W. of Lemay Avenue drains poorly and has received
occasional ponding.
Table of Contents
Description 1ae
I. General Location and Description 1
II. Drainage Design Criteria
A. Regulations 1
B. Hydrological Criteria 1
III. Drainage Facility Design
A. General Concept and Offsite Influences - Historic Basins 1
B. Developed Site Drainage Basins 2
C. Detention 3
IV. Erosion Control Study 4
Vicinity Map A.1
Rainfall Intensity Duration Curve A•2
Estimate of Average Flow Velocity A.3
Runoff Computations: 2 Year Developed A.4
Runoff Computations: 100 Year Developed A.5
Composite Runoff Coefficients for Developed Basins:
2 Year
A.6
100 Year
A.7
Runoff Computations: 2 Year Historic
A•8
Runoff Computations: 100 Year Historic
A.9
Composite Runoff Coefficients for Historic Basins
A.10
100 Yr. Required Detention Volume for Lot 1, Diamond Shamrock
A.11
Detention Pond Capacity for Diamond Shamrock
A.12
Orifice Sizing Calculations for Diamond Shamrock
A.13
Capacity of Storm Sewer for Lincoln/Lemay Intersection Discharge
A.14
Rainfall Performance Standards for Fort Collins B.1
Rainfall Performance Standard Evaluation B.2
Effectiveness Calculations B.3 - B.6
Construction Schedule B.7
Erosion Control Cost Estimate B•g
Galloway, Romero & Associates
Design Engineering Planning
Final Drainage and Erosion
Control Study
for
Diamond Shamrock, Inc.
Lincoln East, P.U.D.
at
North Lemay Avenue & East Lincoln
Avenue
Fort Collins, Colorado
April, 1"7
Prepared for:
Diamond Shamrock, Inc.
520 East 56th Avenue
Denver, Colorado 80216
(303) 297-9777
Attn: Hugh Green
Prepared by:
Galloway, Romero & Associates
5350 DTC Parkway
Englewood, Colorado 80111
(303) 770-8884
Attn: Carl T. Schmidtlein
Galloway, Romero & Associates
Design Engineering Planning
6360 DTC Parkway
Englewood, CO 80111
(303) 770-8884
FAX: (303) 770-3636
April 25, 1997
This final storm drainage and erosion control study for Lincoln East, P.U.D. was
prepared using the City of Fort Collins Storm Drainage Design Criteria, dated May, 1984
and the City of Fort Collins Erosion Control Reference Manual, dated January, 1991.
The results of this study are reported in the enclosed report and utility plans.
David L. Jones, P.E.
Galloway, Romero & Associates
5350 DTC Parkway
Englewood, CO 80111
(303) 770-8884
RE
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01 30254
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