HomeMy WebLinkAboutReports - Drainage - 09/03/2025
FINAL DRAINAGE REPORT
Toyota Pedersen
Fort Collins, Colorado
Prepared for:
Pedersen Properties
4455 S Mason Street
Fort Collins, CO 80525
Prepared by:
Kimley-Horn and Associates, Inc.
3325 South Timberline Road - Suite 130
Fort Collins, Colorado 80525
(970) 822-7911
Project #: 296073000
Prepared: September 3, 2025
kimley-horn.com 3325 S Timberline Rd, Suite 130, Fort Collins, CO 80525 970-822-7911
September 3, 2025
City of Fort Collins
Stormwater Engineering
281 N. College Ave.
Fort Collins, CO 80524
RE: Toyota Pedersen
Final Drainage Report
Dear Reviewer:
Kimley-Horn and Associates, Inc. is pleased to submit this Final Drainage Report for your review
as part of the Project Development Plan (PDP) Major Amendment (MJA) submittal for the above
referenced project.
This report and attached drainage plans have been prepared in accordance with the Fort Collins
Stormwater Criteria Manual (“FCSCM”) and the latest Mile High Flood District Urban Storm
Drainage Criteria Manual (“USDCM”). These documents serve to document stormwater impacts
associated with the proposed Toyota Pedersen Project. We understand the review by the City of
Fort Collins is to ensure general compliance with standardized criteria contained in the FCSCM
and USDCM.
Please contact us with any questions or concerns.
Thank You,
KIMLEY-HORN AND ASSOCIATES, INC.
Ramsey Pickard, P.E.
Final Drainage and Erosion Control Report
Toyota Pedersen – Fort Collins, Colorado
TABLE OF CONTENTS
I. GENERAL LOCATION AND EXISITING SITE INFORMATION ............................... 1
II. MASTER DRAINAGE BASIN DESCRIPTION ...................................................... 2
IV. FLOODPLAIN INFORMATION ............................................................................. 3
V. PROJECT DESCRIPTION .................................................................................... 3
VII. PROPOSED DRAINAGE FACILITIES .................................................................. 5
VIII. DRAINAGE DESIGN CRITERIA ........................................................................ 7
IX. VARIANCE REQUESTS ....................................................................................... 8
X. EROSION CONTROL ........................................................................................... 8
XI. CONCLUSION ...................................................................................................... 8
X. REFERENCES ........................................................................................................... 9
APPENDIX A – REFERENCED CRITERIA ..............................................................................
APPENDIX B – HYDROLOGIC CALCULATIONS AND EXHIBITS ................................................
APPENDIX C – HYDRAULIC CALCULATIONS ........................................................................
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I. GENERAL LOCATION AND EXISTING SITE INFORMATION
Toyota Pedersen (the “Project”) is in the southeast quarter of Section 35, Township 07
North, Range 69 West, of the Sixth Principal Meridian, City of Fort Collins, Larimer County,
State of Colorado.
The property is bounded by South Mason Street to the West, Kensington Drive to the
South, a Target department store to the North, and South College Avenue to the East. A
Vicinity Map is shown below in Figure 1.
Figure 1: Vicinity Map
Surrounding properties include an existing department store to the east and north, car
dealership to the west, and commercial retail developments to the south.
The property currently consists of an existing car dealership and a paved parking lot. The
Project site is located within the General Commercial (CG) Zone District.
The Project is located within the Mail Creek Basin, see copy of the City of Fort Collins
Drainage Basins image included in Appendix A. The master basin is discussed in more
detail below.
The existing site is split into two on-site basins. The east basin drains from the south to
the northeast to storm inlets located on-site, ultimately draining to the storm sewer system
along S College Ave. The western basin drains from south to north to storm inlets located
on-site, ultimately draining to a storm sewer system located along S College Ave via the
Target parcel within an existing drainage easement . While they are split into two basins
on site, both storm lines ultimately drain to the storm system along S College Ave. The
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existing parking lot generally slopes from south to north with slopes ranging from 0 to 4
percent. There are no known existing irrigation facilities within the site.
A Natural Resource Conservation Service (NRCS) Web Soil Survey for the project area
was obtained to determine the soil characteristics of the site. The results of this study
show that much of the site consists of hydrologic soil group (HSG ) Type B with soil that
includes Altvan-Santana Loam. The northeast corner of the site consists of hydrologic
soil group (HSG) Type C with soils that include Nunn Clay soil. Therefore, HSG Type C
soils were assumed for the entirety of the site for hydrologic calculations. A copy of the
Custom Soil Resource Report is provided in Appendix A.
A Geotechnical Engineering Report dated August 25, 2025, was prepared by Cole Garner
Geotechnical. There are no known significant geologic features on this site. Groundwater
was encountered at depths ranging from about 18 to 20 feet below the existing ground
surface in the boring test holes, and at about 14 feet below existing site grade at Boring
No. 6. Groundwater levels will not likely affect planned development at this site. Additional
information for the Geotechnical report can be found within Appendix A.
II. MASTER DRAINAGE BASIN DESCRIPTION
As noted above, the project is in the City of Fort Collins Mail Creek Master Drainage Basin.
This basin has a maximum allowable release rate of the 2-yr historic discharge rate. A
map of the Mail Creek Basin is included in Appendix A. When improvements are proposed
to an existing developed site and there is an increase in impervious area greater than
1,000 square feet, additional onsite detention is required for runoff from new impervious
surfaces. The area of imperviousness is decreasing; therefore, no detention is required.
The Water Quality and low impact development (LID) requirements for the Mail Creek
Basin follows FCSCM, which are outlined in the Drainage Design Criteria section of the
report. To our knowledge, no master drainage reports exist for the Project site. The Project
proposes the removal of a portion of the existing building and paved parking lot. There are
no known irrigation facilities that are influenced by the local drainage.
The project is designed to provide LID and water quality treatment for the proposed
improvements. The drainage is planned to follow historic drainage patterns, where it will
connect to a storm sewer located within S. College Avenue through two sub-basins onsite.
III. EXISTING SITE DRAINAGE
Historically, the site drains from south to north, towards existing private storm inlets onsite,
ultimately draining to storm sewer along S. College Avenue. The existing sub-basin
descriptions are included below. The rational calculations within Appendix B include
areas, imperviousness, and other applicable information. There are two existing detention
ponds located onsite east of South Mason Street. Both ponds will be re-graded to drain
to the proposed site.
Sub-basin EX-A:
The eastern portion of the site, Sub-basin EX-A, slopes east from 1% to 5% and drains
north. The drainage follows the curb and gutter and valley pans within the site as it enters
private storm inlets. Flows ultimately drain to the public storm sewer located along S.
College Avenue. Basin EX-A is comprised of Type C soils as classified by the NRCS.
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There are two existing ponds in this basin. They currently only take drainage from their
area, and there is no master drainage report to know the design intent. There is no
observed outlet in the northern pond, so storm sewer will be added in the ultimate
condition. The southern pond outfalls to an existing 12” pipe in a headwall. The flow out
of the ponds currently is not restricted, and these ponds are not observed to be used as
detention.
Sub-basin EX-B:
The western portion of the site, Sub-basin EX-B, slopes east from 1% to 5% and drains
north. The drainage follows the curb and gutter and valley pans within the site as it enters
private storm inlets. Flows ultimately drain to a private storm system north of the site which
then flows in the public storm sewer located along S. College Avenue . Basin EX-B is
comprised of Type B and Type C soils as classified by the NRCS.
Sub-basin OS1:
The perimeter of the site, consisting of landscaped areas, Sub-basin OS1, slopes towards
the curb and gutter along S. College Avenue, located to the east of the Project. Basin OS1
is comprised of Type B soils as classified by the NRCS.
IV. FLOODPLAIN INFORMATION
The Toyota Pedersen site is located on the Federal Emergency Management Agency
(FEMA) Flood Insurance Rate Map (FIRM) number 08069C1000F dated December 19,
2006, and lies within Zone X. Zone X is defined as areas of minimal flood hazard and
determined to be outside the 0.2% annual chance floodplain. A copy of the FEMA
FIRMette is included in Appendix A. Additionally; the Project is not located within any City
of Fort Collins floodplains. A copy of the City of Fort Collins Flood Map is included in
Appendix A.
V. PROJECT DESCRIPTION
The Project is proposing to develop the +/- 5.02-acre site to include a building addition,
parking lot improvements, and stormwater improvements. Note that the existing site
consists almost entirely of impervious areas. The project will provide LID and water quality
treatment for the proposed improvements.
VI. PROPOSED DRAINAGE BASINS
The proposed sub-basin descriptions are included below. The rational calculations within
Appendix B include areas, imperviousness, and other applicable information.
Sub-basin 1A:
Sub-basin 1A is in the northeastern corner of the site and consists of concrete sidewalks,
asphalt parking lot, and landscaping improvements . The drainage is anticipated to flow
north where it will be collected and routed to a Type R storm inlet located at design point
1A. The drainage will be conveyed into an underground LID system before discharging
into the existing public storm sewer within S. College Avenue.
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Sub-basin 2A:
Sub-basin 2A is in the central part of the site and consists of an existing building and
building additions, the drainage is anticipated to flow into roof drains and into the private
storm sewer system. The drainage will be conveyed into an underground LID system
before discharging into the existing public storm sewer within S. College Avenue.
Sub-basin 3A:
Sub-basin 3A is in the southeastern corner of the site and consists of concrete sidewalks,
asphalt parking lot, and landscaping improvements . The drainage is anticipated to flow
south where it will be collected and routed to a Type R storm inlet located at design point
3A. The drainage will be conveyed into an underground LID system before discharging
into the existing public storm sewer within S. College Avenue.
Sub-basin 1B:
Sub-basin 1B is in the northwestern corner of the site and consists of asphalt parking lot
and landscaping improvements. The drainage is anticipated to flow north where it will be
collected and routed to a Type R storm inlet located at design point 1B. The drainage will
be conveyed into an underground LID system before discharging into the existing public
storm sewer north of the site.
Sub-basin 2B:
Sub-basin 2B is in the southwestern corner of the site and consists of asphalt parking lot
and landscaping improvements. The drainage is anticipated to flow to a low point in the
center of the basin where it will be collected in a Type R storm inlet located at design point
2B. The drainage will be conveyed into an underground LID system before discharging
into the existing public storm sewer north of the site.
Sub-basin 3B:
Sub-basin 3B is in the southern portion of the site and consists of concrete sidewalks,
asphalt parking lot, and landscaping improvements. The drainage is anticipated to flow to
a low point in the center of the basin where it will be collected in a Type R storm inlet
located at design point 3B. The drainage will be conveyed into an underground LID system
before discharging into the existing public storm sewer north of the site.
Sub-basin OS-1:
Sub-basin OS-1 is located around the perimeter of the site and consists of landscaping
improvements. The site is also proposing sidewalk improvements along College Avenue
and Kensington Drive. The drainage is conveyed to the curb and gutter of College Avenue
and Kensington Road. There are no public storm inlets in Kensington Road. The drainage
is collected within existing public storm inlets along S. College Ave.
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VII. PROPOSED DRAINAGE FACILITIES
Proposed Site Description
Site grading is designed to convey the proposed parking lot drainage to proposed storm
inlets that discharge into the underground storm chamber system (“underground system”)
via private storm sewer. The Project proposes using ADS MC-3500 Stormtech chambers,
and the proposed configuration is included in Appendix B.
Proposed Detention Facilities
As noted above, the Mail Creek Basin limits the allowable release rate to the 2-yr historic
discharge rate. The City of Fort Collins policy also allows for existing impervious areas to
be grandfathered into the allowable release rate when determining the required detention
for a project. In the case of this project, the existing site is 85% impervious, and the final
development will see a reduction in overall imperviousness to 82% impervious. As a result,
no detention is required. Existing and proposed impervious exhibits were used to calculate
the impervious areas for each condition. The drainage plans showing existing and
proposed conditions are included within Appendix B. This reduction is summarized in
Table 1 below.
Area Summary
Project Area 5.02 AC
Existing Impervious Area 4.31 AC
Proposed Impervious Area 4.10 AC
Change in Impervious Area 0.21 AC Reduction
Table 1: Impervious Area Summary
Proposed LID and Water Quality Treatment
This LID system for the eastern Basin A will also provide water quality treatment for the
basin. The underground systems outflow discharges to a public storm line along S College
Avenue to the east of the Site.
This LID system for the western Basin B will also provide water quality treatment for the
basin. The underground systems outflow discharges to a private storm line to the north of
the site, ultimately draining to the storm sewer along S College Avenue to the east of the
Site.
The hydraulic calculations for the storm sewer, inlet, and the overflow inlet structure weir
calculation are included Appendix C.
Minor flows and major flows within the proposed site will be conveyed via overland flow
and concentrated flow via curb and gutter prior to entering storm inlets located throughout
the site.
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LID is required to treat 75% of all newly added or modified impervious area based on
FCSCM Section 2.3.7. As shown below, these minimums are being exceeded, and the
basins are treating 100% of the site in total. These impervious areas are depicted within
the Proposed Impervious Exhibit as a part of Appendix B. Note that the eastern and
western basins will route to separate underground systems that will serve as LID treatment
and provide water quality treatment.
The Water Quality Capture Volume (WQCV) for the developed site was calculated using
equation 7-1 in the FCSCM. A 12-hour drain time based on Equation 7-1 and Table 5.1-1
was used to determine the minimum required WQCV for the area draining to the
underground system. Table 2 shows the required and provided water quality and LID
volumes for the proposed site. See calculations included in Appendix B.
Water Quality and LID Values (Eastern Basin A)
Impervious Area for Proposed Improvements 103,673 SF
Required LID Impervious Area 77,755 SF
Provided LID Impervious Area 103,673 SF
Required Water Quality Volume 3,496 CF
Provided Water Quality Volume 3,906 CF
Water Quality and LID Values (Western Basin B)
Impervious Area for Proposed Improvements 73,923 SF
Required LID Impervious Area 56,192 SF
Provided LID Impervious Area 73,923 SF
Required Water Quality Volume 2,390 CF
Provided Water Quality Volume 2,836 CF
Table 2: Water Quality and LID Summary
The existing site consisted of predominately impervious surfaces that received no
detention or water quality prior to leaving the site and entering the existing storm inlets
within S. College Avenue. As a result, the drainage runoff for the proposed perimeter
sidewalk outside of the proposed right-of-way will not be attenuated but will receive water
quality treatment through the vegetative buffer along the parkway before entering the
existing storm inlets within S. College Avenue. There are two existing detention ponds
west of the existing parking lot. The existing impervious area doesn’t appear to drain to
the existing ponds before leaving the site.
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The underground system and isolator rows were sized according to the FCSTM and the
associated calculations are included within Appendix B. Additionally, isolator rows will be
installed where the private storm enters the underground system.
A drainage easement encompassing the entire footprint of the underground system and
private storm lines will be dedicated to the City of Fort Collins and is shown on the Plat.
The proposed underground system is placed in a location that is accessible for inspections
and maintenance.
VIII. DRAINAGE DESIGN CRITERIA
The Project was designed to conform to the requirements outlined in the Fort Collins
Stormwater Criteria Manual (FCSCM) and the latest Mile High Flood District (MHFD)
Urban Storm Drainage Criteria Manual (USDCM).
A Four Step Process was implemented for the drainage design and protection of receiving
water bodies:
Step 1 - Runoff Reduction Practices
Runoff is routed through vegetated buffers via sheet flow wherever reasonably possible
to increase time of concentration and promote infiltration. By Minimizing Directly
Connected Impervious Areas (MDCIA), peak runoff volumes and pollutant loads are
reduced. Infiltration is promoted in the existing ponds to the west and underground LID
systems.
Step 2 – Implement Best Management Practices (BMPs) to treat the WQCV
A LID underground system is proposed that will provide treatment for the WQCV with slow
release and/or infiltration.
Step 3 – Stabilizing Streams
Stream stabilization was considered but not implemented due to open channels being
minimized for site accessibility.
Step 4 – Implementing Site Specific and Other Source Control BMPs
Site specific controls that will be implemented include locating material storage away from
storm drainage facilities.
The Rational Method was used for all subbasins with areas less than 90 acres. Per the
FCSCM, the storm frequencies used to analyze the drainage design were the 2 -year and
the 100-year storms. The FCSCM Tables 3.2-1, 3.2-2, and 3.2-3 were utilized to determine
the stormwater runoff coefficients. Rainfall intensities used for the rational calculations
were obtained from Table 3.4 -1 in the FCSCM. Rainfall depths are shown in Table 3
below.
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One-Hour Rainfall Depths
2 YR 0.82 inches
100 YR 2.86 inches
Table 3: One-hour Rainfall Depths Summary
Hydraulic calculations have been included within Appendix C. These calculations include
analysis of the storm sewer analysis, street capacity, inlet sizing, and the overflow inlet
structure.
IX. VARIANCE REQUESTS
No variances are requested currently.
X. EROSION CONTROL
During construction, temporary erosion and sediment control practices will be used to limit
soil erosion and migration of sediment off site. An erosion control report has been included
with the Final Drainage Report.
XI. CONCLUSION
The Toyota Pedersen site is designed to conform to the criteria in the FCSCM and the
USDCM. The proposed underground system provides LID and is designed to treat the
water quality for the proposed improvements within the Project.
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X. REFERENCES
City of Fort Collins Flood Maps, City of Fort Collins GIS, Accessed November 5, 2024, at
<https://gisweb.fcgov.com/HTML5Viewer/Index.html?viewer=FCMaps&LayerTheme=flo
odplains>
Custom Soil Resource Report, Natural Resources Conservation Service, United States
Department of Agriculture. Web Soil Survey. November 5, 2024.
Fort Collins Stormwater Criteria Manual, City of Fort Collins, December 2018.
National Flood Hazard Layer Firmette , Federal Emergency Management Agency;
Accessed November 5, 2024.
Urban Storm Drainage Criteria Manual, Volumes 1 -3, Mile High Flood District, Updated
March 2024.
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Toyota Pedersen – Fort Collins, Colorado
Appendix A – Referenced Criteria
Final Drainage and Erosion Control Report
Toyota Pedersen – Fort Collins, Colorado
Appendix B – Hydrologic Calculations and Exhibits
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Toyota Pedersen – Fort Collins, Colorado
Appendix C – Hydraulic Calculations