HomeMy WebLinkAboutFORT COLLINS NISSAN-KIA - PDP210017 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORTPRELIMINARY DRAINAGE REPORT
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Fossil Creek Subdivision
Fort Collins, CO
Prepared for:
Fort Collins Nissan-Kia
5811 S College Ave
Fort Collins, CO 80526
Date:
October 6, 2021
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October 6, 2021
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Fort Collins Nissan-Kia
Fort Collins, Colorado
Project Number: U21004
Dear Staff:
United Civil Design Group, LLC. is pleased to submit this Preliminary Drainage Report for the Fort Collins Nissan-Kia site in
Fort Collins, Colorado. In general, this report serves to document the stormwater impacts associated with the proposed
improvements related to the existing site.
We understand that review by the City of Fort Collins is to assure general compliance with standardized criteria contained in
the Fort Collins Stormwater Criteria Manual (FCSCM). This report was prepared in compliance with technical criteria set forth
in both the Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual and the Fort Collins
Stormwater Criteria Manual.
If you should have any questions or comments as you review this report, please feel free to contact us at your convenience.
Sincerely,
United Civil Design Group
Colton Beck, PE Kevin Brazelton, PE
Project Engineer Principal
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TABLE OF CONTENTS
I. General Location and Description ...................................................................................................................................... 1
A. General Location & Existing Site Information ............................................................................................. 1
B. Description of Property ............................................................................................................................... 2
C. Project Description ..................................................................................................................................... 2
D. Floodplains .................................................................................................................................................. 3
II. Drainage Basins and Sub-Basins ......................................................................................................................................... 3
A. Major Basin Description .............................................................................................................................. 3
B. Sub-Basin Description ................................................................................................................................. 3
III. Drainage Design Criteria ..................................................................................................................................................... 3
A. Regulations ................................................................................................................................................. 3
B. Directly Connected Impervious Area (DCIA) ............................................................................................... 4
C. Hydrological Criteria ................................................................................................................................... 4
D. Hydraulic Criteria ........................................................................................................................................ 4
E. Modifications of Criteria ............................................................................................................................. 5
IV. Drainage Facility Design ..................................................................................................................................................... 5
A. General Concept ......................................................................................................................................... 5
B. Specific Details ............................................................................................................................................ 5
V. Erosion Control ................................................................................................................................................................. 12
VI. Conclusions ...................................................................................................................................................................... 13
A. Compliance with Standards ...................................................................................................................... 13
B. Drainage Concept...................................................................................................................................... 13
C. Stormwater Quality................................................................................................................................... 13
VII. References .................................................................................................................................................................... 14
APPENDICES
APPENDIX A – Hydrology Calculations
APPENDIX B – Hydraulic Calculations
B.1 – Low Impact Development Calculations
B.2 – Water Quality Calculations
B.3 – Detention Pond and Orifice Calculations
B.4 – Inlet Sizing Calculations (Reserved for Final Submittal)
B.5 – Storm Pipe Calculations (Reserved for Final Submittal)
B.6 – Curb Channel Calculations (Reserved for Final Submittal)
APPENDIX C – Referenced Materials
APPENDIX D – Drainage Exhibits
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I.GENERAL LOCATION AND DESCRIPTION
A.GENERAL LOCATION &EXISTING SITE INFORMATION
The Fort Collins Nissan-Kia site (referred herein as “the site”) is located within the Fossil Creek Subdivision, situated in the
northeast quarter of Section 11, T6N, R69W of the 6th P.M., City of Fort Collins, Larimer County, Colorado. The property,
consisting of approximately 17.0 acres, is located immediately west of S College Ave. The site is split into two separate areas
divided by Crestridge St. South of Crestridge St. exists a car dealership to be redesigned and redeveloped. This portion of the
property is bounded by Crestridge St to the north, S College Ave to the east, a school facility and residential land to the west,
and a commercial property to the south. On-site stormwater drains off-site undetained to either S College Ave, Crestridge St,
or off-site to the west.
The northern, undeveloped portion of the site is bounded by Crestridge St to the south, S College Ave to the east, and
undeveloped land to the north and west. Stormwater on this portion of the site largely drains the northwest via sheet flow
to Fossil Creek. A portion of the site adjacently north of Crestridge St drains to the south via sheet flow. All on-site runoff
drains downstream within the City of Fort Collins Fossil Creek master drainage basin.
Storm sewer infrastructure exists on-site south of Crestridge St and drains to the off-site to the east. This storm pipe exists as
an irrigation bypass system. A City of Fort Collins Floodway and an Erosion Buffer are established with Fossil Creek. Also,
existing on-site is an underground irrigation pipe owned by North Louden Ditch Company. This irrigation pipe drains from
south to north through the site, ultimately to infrastructure that drains across S College Ave to the east. The on-site portion
of this ditch lateral is planned to be relocated; however, this relocation is unrelated to the drainage design of the site.
An existing residential subdivision and small commercial property are constructed to drain runoff to Venus Ave, conveyed
through an existing wetland area west of Venus Ave to Lang Gulch and ultimately to Fossil Creek.
FIGURE 1:SITE VICINITY MAP
Smokey St S College AveBueno Dr Venus AveCrestridge St
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B.DESCRIPTION OF PROPERTY
The northern portion of the project site currently exists largely as grass/weed landscape. The southern portion of the site
includes concrete and asphalt pavement, a commercial building, and gravel landscaping. In its historic condition, the site
primarily sheet flows in two general directions: north and south, all conveyed to Fossil Creek.
Below are summaries of key components of the site in its existing conditions.
Land Use - The site’s current land use is commercial.
Ground Cover - The northern portion of the site exists as grass/weed landscape. The weed/grass cover is good (ie.,
heavy or dense cover with nearly all ground surfaces protected by vegetation). The southern portion of the site is
fully developed as consists largely of impervious hardscaping.
Existing Topography – The northern, undeveloped portion of the site is bounded by Crestridge St to the south, S
College Ave to the east, and undeveloped land to the north and west. Stormwater on this portion of the site largely
drains the northwest via sheet flow to Fossil Creek. A portion of the site adjacently north of Crestridge St drains to
the south via sheet flow. All on-site runoff drains downstream within with the City of Fort Collins Fossil Creek master
drainage basin.
Grades – In general, the northern portion of the site slopes northwesterly at approximately 1.0% to 15.0%; the
southern portion of the site is sloped in several directions at approximately 1.0% to 3.0%
Soil Type – The USDA’s Web Soil Survey shows that the site consists of “Type B”, “Type C”, and “Type D” soils. The
Web Soil Survey indicates the site is comprised of Kim Loam (5.0 to 9.0% slopes), Kim Thedalund Loams (3.0 to 15.0%
slopes), Midway Clay Loam (5.0% to 25.0% slopes) and Thedalund Loam (3.0% to 9.0% slopes). The on-site soils
provide moderate infiltration and are suitable for development.
Utilities – The following utilities exist adjacent to the site in Crestridge St, College Avenue and Venus Drive: potable
water, sanitary sewer, natural gas, electric and telecommunications.
Irrigation Facilities – An existing on-site underground irrigation pipe owned by North Louden Ditch Company. This
irrigation pipe drains from south to north through the site, ultimately to infrastructure that drains across S College
Ave to the east.
Drainage Features and Storm Sewer – Storm sewer infrastructure exists on-site south of Crestridge St. North of the
site exists the Fossil Creek. For development purposes, a City of Fort Collins Floodway and an Erosion Buffer are
established with Fossil Creek. The western side of the site drains via sheet flow to the Lang Gulch. This portion of the
Lang Gulch exists immediately upstream of Fossil Creek and is accounted for within the City of Fort Collins Floodway
boundary.
C.PROJECT DESCRIPTION
The proposed Fort Collins Nissan-Kia site improvements are limited to approximately 19.0 acres of disturbance. South of
Crestridge St, the proposed site improvements consist of the demolition of the existing car dealership building and associated
paving, and ultimately the construction of a new building with associated drive lanes and inventory parking lot. North of
Crestridge St, site improvements include the extension of Venus Ave, and the addition of a car dealership building and
associated drive lanes and inventory parking lot. Designed with the proposed commercial buildings and paving include
underground utilities, underground water quality, and underground detention. On-site stormwater south of Crestridge Dr is
designed to be conveyed via storm sewer to proposed drainage facilities west of Venus Ave.
In addition to an existing school facility adjacent to the site at the intersection of the Venus Ave and Crestridge St, the existing
Skyview Subdivision to the south is associated with the site in regard to stormwater runoff. In its existing state, this
development along with the school facility drains approximately 3.7-acres to an existing on-site wetland area. With a
proposed relocation of the mentioned wetlands, runoff is now designed to be conveyed to a proposed on-site detention
facility.
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D.FLOODPLAINS
The proposed development is not in a FEMA
designated floodplain. According to the FEMA
Flood Insurance Rate Map Panel (08069C1000F),
the site is zoned in an area of minimal flood
hazard. The FEMA FIRM Map is included in the
appendix.
West and Northwest of the site exists a “High
Risk – Floodway” boundary along Fossil Creek,
established by the City of Fort Collins. This
boundary is provided on the Drainage Plan (see
Appendix D) and a City of Fort Collins floodplain
map is included in Appendix C.
FIGURE 2:FLOODPLAIN MAP
II.DRAINAGE BASINS AND SUB-BASINS
A.MAJOR BASIN DESCRIPTION
The proposed development is located within the Fossil Creek Basin and will adhere to the Fossil Creek Basin Master Drainage
Plan. In conformance with master plan, the site is designed to release on-site runoff at a rate not to exceed 0.2 cfs/acre.
B.SUB-BASIN DESCRIPTION
Historically the southern portion of the property has been utilized for commercial purposes. The site is designed to operate
in a similar fashion (i.e., commercial), though with significant site improvements. Despite these significant improvements, the
proposed site is largely designed to conform to existing drainage patterns – downstream within the Fossil Creek Basin. In its
existing condition, stormwater on the northern portion of the site is largely conveyed the north to Fossil Creek. The southern
portion of the property drains in a multitude of directions: west to a neighboring property, east to US 287, and north to
Crestridge St. In its improved condition, on-site runoff is intended to drain to multiple downstream underground water quality
and detention facilities by way of sheet flow, swales, pans, inlets, and storm sewer.
III.DRAINAGE DESIGN CRITERIA
A.REGULATIONS
The design criteria for this study are directly from the City of Fort Collins Storm Drainage Design Criteria and Construction
Standards Manual and the Urban Storm Drainage Criteria Manuals Volumes 2, and 3 (referred to herein as USDCM).
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B.DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
With the adoption of the USDCM, the City has also adopted the “Four Step Process” that is recommended in Volume 3 of
the USDCM in selecting structural BMPs for the redeveloping urban areas. The following portions of this summary describe
each step and how it has been utilized for this project:
Step 1 – Employ Runoff Reduction Practices
The objective of this step is to reduce runoff peaks and volumes and to employ the technique of “minimizing directly
connected impervious areas” (MDCIA). This project accomplishes this by:
Routing the roof and pavement flows through a bioretention facility, underground water quality treatment/infiltration
facilities and vegetated swales to increase time of concentration, promote infiltration and provide water quality.
Step 2 – Provide Water Quality Capture Volume (WQCV)
The objective of providing WQCV is to reduce the sediment load and other pollutants that exit the site. For this project
WQCV is provided within underground treatment facilities, a bioretention pond and an extended detention pond.
Step 3 – Stabilize Drainageways
The site exists adjacent to lang Gulch. A storm outfall is proposed immediately upstream of the Lang Gulch; stabilization is
planned with this pipe connection. As a result of the pipe connection, additional stabilizing will be provided via the
elimination of existing runoff from traveling down the sides of the channel. This project will pay stormwater development
and stormwater utility fees which the City uses, in part, to maintain the stability of the City drainageway systems.
Step 4 – Consider Need for Site Specific and Source Control BMPs
Site specific and source control BMPs are generally considered for large industrial and commercial sites. The
redevelopment of the existing site will include multiple site specific and source controls, including:
Covered storage areas for vehicles and materials.
Dedicated vehicle maintenance areas within each build with appropriate internal waste collection systems.
Dedicated maintenance personnel providing landscape maintenance and snow and ice management.
Bioretention water quality control ponds.
C.HYDROLOGICAL CRITERIA
City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, provided by Figure RA-16 of the Fort Collins Stormwater
Criteria Manual, are utilized for all hydrologic computations related to the site in its existing/historic and proposed conditions.
Since this site is relatively small and does not have complex drainage basins, the peak flow rates for design points have been
calculated based on the Rational Method as described in the USDCM and the City of Fort Collins Stormwater Criteria Manual
(FCSCM) with storm duration set equal to the time of concentration for each sub-basin. This method was used to analyze the
developed runoff from the 10-year (minor) and the 100-year (major) storm events. The Rational Method is widely accepted
for drainage design involving small drainage areas (less than 160 acres) and short time of concentrations. Runoff coefficients
are assumed based on impervious area and are given in the Appendices.
D.HYDRAULIC CRITERIA
The developed site will convey runoff to the proposed pond or underground treatment facility via swales, concrete channels,
and pipes. The City of Fort Collins Stormwater Criteria Manual (FCSCM) and USDCM are referenced for all hydraulic
calculations. In addition, the following computer programs are utilized:
Storm Sewer Extension for AutoCAD Civil3D
Hydraflow Express Extension for AutoCAD Civil3D
UD-Inlet by UDFCD
Drainage conveyance facility capacities proposed with the development project, including storm sewer, swales, and inlet
capacities, are designed in accordance with criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control
District’s Urban Storm Drainage Criteria Manual (UDFCD).
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E.MODIFICATIONS OF CRITERIA
The site improvements are proposed with the omission of variances to City of Fort Collins criteria.
IV.DRAINAGE FACILITY DESIGN
A.GENERAL CONCEPT
Developed runoff is designed to be conveyed in a safe and effective manner via swales, concrete channels, and storm sewer
systems/inlets. Stormwater is designed to be released within the Fossil Creek Basin. Runoff that drains off-site to the west
(south of Crestridge St) is heavily reduced with the proposed site. Runoff from the site in its improved condition is primarily
conveyed to underground water quality and detention facilities. Due to grading constraints, not all on-site runoff is fully
detained - these areas are considered in the overall release rate from the site.
B.SPECIFIC DETAILS
Hydrology
As previously stated, hydrology from the developed site is designed to comply with criteria set forth in the City of Fort Collins
Stormwater Criteria Manual. Referenced tables, charts, formulas, etc. are included in the appendices. The area, time of
concentration, and runoff of each proposed sub-basin is summarized in Appendix A. The following information outlines the
basin characteristics and drainage patterns for each basin.
Existing Basins
The following basins provide proposed drainage delineations for the site in its existing condition. Refer to Appendix A for
hydrology computations and Appendix D for visual representation of existing basins.
Basin EX1
Basin EX1 (refer to attached Existing Drainage Map) represents runoff associated with CDOT Right-of-Way in College Ave.
This basin consists of concrete paving, asphalt paving, gravel, and landscaping. Runoff within this basin is conveyed north
along College Ave and ultimately to Fossil Creek.
Basin EX2
Basin EX2 represents on-site runoff that drains to College Ave. This basin consists of concrete paving, asphalt paving, gravel,
and landscaping. Runoff within this basin is conveyed College Ave (i.e., Basin EX1) via sheet flow and concrete curb.
Basin EX3
Basin EX3 represents runoff associated with CDOT Right-of-Way in College Ave. This basin consists of concrete paving, asphalt
paving, gravel, and landscaping. Runoff within this basin is conveyed south along College Ave within the Fossil Creek Basin.
Basin EX4
Basin EX4 represents on-site runoff that drains to College Ave. This basin consists of concrete paving, asphalt paving, gravel,
and landscaping. Runoff within this basin is conveyed to College Ave (i.e., Basin EX3) via sheet flow and concrete curb.
Basin EX5
Basin EX5 represents on-site runoff that drains to Fossil Creek. This basin is to remain undeveloped, consisting of landscaped
area. Runoff within this basin is conveyed to the northwest, designed to drain to off-site to a culvert system associated with
Fossil Creek.
Basin EX6
Basin EX6 represents on-site runoff that drains to the Lang Gulch, and ultimately Fossil Creek. A portion of this basin is
developed and is designed to drain to existing wetlands located around the southwestern region of the site; other
undeveloped areas drain directly to the Lang Gulch via sheet flow. This basin consists of concrete paving, asphalt paving,
gravel, and landscaping.
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Basin OS1
Basin OS1 is a drainage basin associated with and adjacent commercial property and the Skyview Subdivision, located
immediately south of the site. This basin is fully developed and drains on-site via sheet flow along Venus Ave. This basin
accounts for approximately 3.7-acres of off-site drainage.
Basin OS2
Basin OS2 represents off-site runoff that drains to Fossil Creek. This basin is to remain undeveloped, consisting of landscaped
area. Similar to Basin EX5, runoff within this basin is conveyed to the northwest to a culvert system associated with Fossil
Creek.
Proposed Basins
The following basins provide proposed drainage delineations for the site in its improved condition. Refer to Appendix A for
hydrology computations and Appendix B for calculations related to Detention, Water Quality, Low Impact Development, and
other hydraulic features.
Basin A
Sub-basins A1-A16 (see Drainage Plans attached) are generally located around the southern region of the site. These basins
consist of roofs, asphalt and concrete paving, and landscaping. Runoff within these Basins is conveyed to a Bioretention Pond
(Pond A2) via sheet flow, curb, inlets, and storm sewer. Runoff conveyed to the Bioretention Pond is further discharged
southwest to a detention facility (Pond A1). Runoff from Basin A is released to Lang Gulch and ultimately to Fossil Creek.
Basin B
Sub-basins B1-B6 represent on-site drainage basins generally located around the central region of the site. These basins
consist of roofs, asphalt and concrete paving, and landscaping. Runoff within these basins is conveyed to Detention Pond B1
by way of multiple water quality treatment facilities, namely a bioretention pond (i.e., Pond B2) and a wetland channel.
Runoff from Basin B is released to Lang Gulch via Pond A1 and ultimately to Fossil Creek.
Basin C
Sub-basins C1-C4 represent on-site drainage basins generally located around the northern region of the site. These basins
consist of roofs, asphalt and concrete paving, and landscaping. Runoff within Basins C3 and C4 is conveyed to a proposed
underground water quality and detention pond facility (Pond C) via inlets and storm sewer. Basin C1 and C2 (i.e., Venus Ave)
is conveyed to storm sewer in Venus Ave, treated with Snouts within the inlets and released downstream to Fossil Creek.
Basin D
Basin D1 represents CDOT Right-of-Way in College Ave at the Southeast corner of the site. In accordance with established
drainage patterns, this area of College Ave drains south along College Ave, ultimately to Fossil Creek. The basin consists of
concrete pavement, asphalt pavement, and landscaping.
Basin E
Basin E1, similar to Basins D1, represents CDOT Right-of-Way in College Ave; however, this portion of College Ave drains to
the north. In accordance with established drainage patterns, this area of College Ave drains north along College Ave,
ultimately to Fossil Creek. The basin consists of concrete pavement, asphalt pavement, and landscaping.
Basin OS1
Basin OS1 is a drainage basin associated with and adjacent commercial property and the Skyview Subdivision, located
immediately south of the site. This basin is fully developed and drains on-site via sheet flow along Venus Ave. This basin
accounts for approximately 3.7-acres of off-site drainage. In its improved condition, Venus Ave is designed to convey Basin
OS1 to a proposed detention facility (i.e., Pond A1) via storm sewer infrastructure.
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Basin OS2
Basin OS2 represents off-site runoff that drains to Fossil Creek. This basin is to remain similar to its existing condition;
however, an additional 12” culvert system is provided at the downstream Fossil Creek Trail.
Stormwater Quality
I.Bioretention
Bioretention is designed to remove sediment and pollution from stormwater runoff via infiltration through an
engineered sand media. This slows the conveyance rate of storm water runoff, reduces runoff and provides a flow
path across a vegetated surface. Bioretention ponds are proposed with the site improvements and are utilized to
convey and treat stormwater prior to release to downstream proposed detention facilities and ultimately Fossil
Creek.
Bioretention Pond A2 is designed to provide approximately 8,020-sf of bioretention area. Provided 12-inches of
depth, the bioretention pond provides an approximate volume of 6,195-cf – this volume exceeds the required WQCV
for Basin A2 and Basins A4-16 (6,150-cf). Ponding above the initial 12-inches of pond depth is intended for head
purposes to discharge the 100-year event through the outlet structure. Runoff is ultimately conveyed to the adjacent
Detention Pond A1. Below is a summary of Bioretention Pond A2:
Pond Bottom of
Pond Elevation
Bioretention
WSEL
Bioretention Volume
Required
(cf)
Bioretention Volume
Provided
(cf)
Pond A2 5009.00 5010.00 6,150 6,195
Bioretention Pond B2 is designed to provide about 780-sf of bioretention area. Provided 12-inches of depth, the
bioretention pond provided an approximate volume of 1,430-cf – this volume exceeds the required WQCV for Basins
A2-16 (427-cf). Ponding above the initial 12-inches of pond depth is intended for head purposes to discharge the
100-year event through the outlet structure. Below is a summary of the bioretention pond, recognized as Pond B2:
Pond Bottom of
Pond Elevation
Bioretention
WSEL
Bioretention Volume
Required
(cf)
Bioretention Volume
Provided
(cf)
Pond B2 5011.80 5012.80 427 1,430
II.Underground Water Quality
An underground water quality facility is designed to divert storm water runoff during the water quality event and
treat the flows through infiltration prior to being collected within an underdrain system. This provides for the
removal of sediment and other pollutants from the runoff while also promoting infiltration. Underground water
quality chambers are designed to collect and treat storm water runoff from Basins C3-C4 prior to being released to
downstream storm sewer associated with Basin C.
The underground water quality system, designated as “Pond C” is designed with a volume of 3,730-cf which exceeds
the minimum design requirements for the area (2,444-cf). Below is a summary table for water quality related to
Basins C3 and C4. Refer to Appendix B for additional information.
Pond
WQCV Required
(cf)
WQCV Provided
(cf)
Pond C1 2,444 3,730
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III.Extended Detention - WQCV
The proposed site improvements include water quality for off-site Basin OS1. This 3.70-acre area necessitates a
WQCV approximately 2,985-cf. Pond A1 also considers water quality for Basin A3 (i.e., 571-cf). Pond A1 is sufficiently
sized to provide this combined treatment volume of 3,557-cf. Below is a summary table for water quality related to
Basins A3 and OS1. Refer to Appendix B for additional information.
Pond
WQCV Necessary
(cf)
WQCV Provided
(cf)
Pond A1 3,557 4,470
IV.Structural Water Quality Treatment
Snouts are provided with inlets related to Basin C1 and C2; runoff from these basins are designed be released from
the site undetained. These structures are designed to act as temporary water quality features. Proposed immediately
downstream of the mentioned storm infrastructure including snouts (i.e, in series with the structural water quality
treatment) is a grass swale and buffer area. This area is intended to provide additional water quality treatment until
future development is pursued on the neighboring property to the north. With future development of this
downstream, off-site area, the landowner of the neighboring property will be required to replace the temporary
grass swale and buffer area with a permanent water quality feature adequate to treat Basins C1 and C2.
Low Impact Development (LID)
In December of 2015, Fort Collins City Council adopted the revised Low Impact Development (LID) policy and criteria which
requires developments within City limits to meet certain enhanced stormwater treatment requirements in addition to more
standard treatment techniques. The proposed development will be required to meet the newly adopted LID criteria which
requires the following:
-Treat no less than 75% of any newly added impervious area using one or a combination of LID techniques.
-Treat no less than 50% of any newly added impervious area using one or a combination of LID techniques when at
least 25% of any newly added pavement is provided with permeable pavement.
The following measures are implemented with this proposed development:
I.Underground Water Quality
Basins C3-C4, combined for 124,235-sf of proposed impervious area, are designed to be routed through an
underground water quality system located in the northern region of the site. The proposed impervious area treated
through underground water quality includes proposed parking, driveway, and roof areas. The underground water
quality system is designed with an overall capacity of 3,896-cf which exceeds the minimum design requirements for
the area. Refer to Appendix B for additional information.
II.Bioretention
Basins A2-A16, accounting for 210,565-sf of proposed impervious area, are designed to be conveyed to Bioretention
Pond A2. All new or modified impervious area related to Basins A2-A16 is to be treated by Pond A2.
The entirety of Basin B4, accounting for 16,368-sf of newly proposed impervious area, is designed to be conveyed to
and treated by Bioretention Pond B2.
III.Wetland Channel
Basins B1-B3 and B5-B6, accounting for approximately 69,000-sf of newly proposed impervious area, is designed to
drain to and treated by a wetland channel. This channel is proposed within Detention Pond B1.
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Provided that the application of proposed pervious pavers is below 25% of the total of newly added pavement, the site is
required to treat no less than 75% of added impervious area. Provided the LID measures noted above, approximately 76.1%
of the site is treated in its improved condition. Refer to Appendix B for calculations
Detention
I.Pond A1
The design of Pond A1 is intended for extended detention purposes for all ‘A” Basins and Basin OS1. This detention
facility is designed to release captured runoff at a rate of 3.50-cfs in the 100-yr event, significantly reducing the
existing undetained release of Basin OS1 to Lang Gulch and ultimately Fossil Creek. For emergency situations, the
pond is designed to spill stormwater through the emergency overflow structure which is located at the western end
of the detention pond. Water discharged from the emergency overflow will be conveyed directly to the Lang Gulch.
Below is a summary of Pond A1 utilizing the FAA method:
Pond Bottom of
Pond
Detention
WSEL
Top of Pond
Elevation
Detention
Volume Required
(ac-ft)
Detention
Volume Provided
(ac-ft)
100-yr
Release Rate
(cfs)
Pond A1 5002.60 5010.00 5011.00 1.99 2.12 3.50
II.Pond B1
The design of Pond B1 is intended for extended detention purposes for all ‘B” Basins. This detention facility is
designed to release captured runoff at a rate of 2.00-cfs in the 100-yr event. For emergency situations, the pond is
designed to spill stormwater through the emergency overflow structure which is located at the southwestern end
of the detention pond. Water discharged from the emergency overflow will be conveyed directly to the Lang Gulch.
Below is a summary of Pond B1 utilizing the FAA method:
Pond Bottom of
Pond
WQCV
WSEL
Detention
WSEL
Top of Pond
Elevation
Detention
Volume Required
(ac-ft)
Detention
Volume Provided
(ac-ft)
100-yr
Release Rate
(cfs)
Pond B1 5002.70 5005.00 5009.00 5010.00 0.49 0.88 2.00
III.Pond C1
Runoff captured within Basins C3-C4 are routed to underground chambers, designed to collect and detain storm
water runoff prior to being released to Fossil Creek. Stormwater events that exceed the water quality event (see
section Stormwater Quality narrative above) are designed to exceed the capacity of the system’s isolator row and
be further conveyed throughout the underground facility to other chambers for detention purposes. The minimum
100-yr release via underground detention is established by a release rate per chamber to an underdrain – this release
rate is limited by flow through geotextile with accumulated sediment.
Below is a summary of Pond C1 utilizing the FAA method:
Pond
Detention Volume
Required
(ac-ft)
Detention Volume
Provided
(ac-ft)
100-yr
Release Rate
(cfs)
Pond C1 0.55 0.56 2.40
P RELIMINARY D RAINAGE R EPORT
F ORT C OLLINS N ISSAN -K IA
F ORT C OLLINS ,CO
10 U21004_Drainage Report
Drainage Summary
A basin summary table is included below:
Existing/Historic Basins
Basin Area %I C2 C100 Q2 Q100
(acre) (cfs) (cfs)
EX1 2.12 65.0% 0.70 0.87 3.40 14.85
EX2 1.70 60.6% 0.67 0.84 3.05 13.30
EX3 0.41 82.1% 0.82 1.00 0.96 4.12
EX4 0.38 88.9% 0.89 1.00 0.96 3.80
EX5 7.97 2.0% 0.25 0.31 3.21 13.95
EX6 6.44 24.7% 0.41 0.52 5.12 22.35
OS1 3.70 55.6% 0.64 0.80 6.33 27.60
OS2 5.75 2.0% 0.25 0.31 2.37 10.33
Proposed Basins
Basin Area %I C2 C100 Q2 Q100
(acre) (cfs) (cfs)
A1 0.95 2.0% 0.25 0.31 0.57 2.49
A2 0.41 2.0% 0.25 0.31 0.26 1.13
A3 0.45 72.9% 0.76 0.95 0.98 4.27
A4 0.45 86.7% 0.86 1.00 1.10 4.50
A5 0.48 100.0% 0.95 1.00 1.30 4.77
A6 0.14 95.5% 0.92 1.00 0.36 1.37
A7 0.38 62.3% 0.68 0.85 0.74 3.25
A8 0.09 100.0% 0.95 1.00 0.26 0.94
A9 0.18 100.0% 0.95 1.00 0.48 1.76
A10 0.50 75.6% 0.78 0.97 1.11 4.86
A11 0.05 100.0% 0.95 1.00 0.13 0.47
A12 0.81 77.7% 0.79 0.99 1.83 7.97
A13 0.48 90.0% 0.95 1.00 1.31 4.80
A14 0.57 94.2% 0.82 1.00 1.46 5.62
A15 0.74 93.3% 0.90 1.00 1.90 7.36
A16 0.60 90.0% 0.95 1.00 1.63 5.98
B1 0.95 2.0% 0.25 0.31 0.60 2.60
B2 0.99 78.4% 0.80 0.99 2.25 9.81
B3 0.42 100.0% 0.95 1.00 1.13 4.14
B4 0.59 63.3% 0.69 0.86 1.16 5.08
B5 0.22 61.8% 0.68 0.85 0.43 1.88
B6 0.30 77.2% 0.79 0.98 0.68 2.97
C1 0.61 84.9% 0.84 1.00 1.46 6.07
C2 0.83 58.8% 0.66 0.82 1.56 6.81
C3 2.73 91.9% 0.89 1.00 6.94 27.15
C4 0.38 90.0% 0.95 1.00 1.04 3.82
D1 0.49 67.9% 0.72 0.90 1.00 4.37
E1 2.12 77.4% 0.79 0.99 4.47 19.48
OS1 3.70 55.6% 0.64 0.80 6.33 27.60
OS2 5.84 2.0% 0.25 0.31 2.41 10.49
P RELIMINARY D RAINAGE R EPORT
F ORT C OLLINS N ISSAN -K IA
F ORT C OLLINS ,CO
11 U21004_Drainage Report
100-year Event: Proposed Runoff to College Ave
Basin
Designation
Release Rate
(cfs)
Basin D1 4.37
Basin E1 19.48
Proposed Runoff 23.85
Refer to Appendix D for an Existing Drainage Map that provides a visual representation of Existing Basins EX1-EX6 and OS1-
OS2. In the site’s existing condition, nearly 2.1-acres (i.e., Basin EX2 and EX4) of on-site runoff drains undetained to College
Ave. In addition to on-site basins EX2 and EX4, Basins EX1 and EX2 (i.e., College Ave) also contribute runoff to the west half
of College Ave – the entirety of Basins EX1-EX4 account for approximately 36.07-cfs in the 100-year event. In the site’s
improved condition, this 2.1-acre area is captured and conveyed to on-site detention; runoff related to all “A” basins are
detained. Moreover, the improved site is designed to convey 0.35-acres of College Ave runoff (see Basin A10). Despite the
addition of a drive lane to College Ave (i.e., additional impervious area), there is a significant reduction in runoff conveyed by
College Ave in the 100-year Event. The tables below summarize the impacts of the proposed improvements to College Ave
relative to the site’s existing condition.
100-year Event: Existing Runoff to College Ave
In the site’s its improved condition, the total runoff related to College Ave during the 100-year event is significantly reduced.
In relation to the Fossil Creek Basin, the site is limited to a 0.2-cfs/acre release rate in its improved condition – this amounts
to 3.10-cfs over 15.51 acres (i.e., sum of Basins A, B, and C). Provided the 3.7-acre conveyance and detention of off-site runoff
(i.e., Basin OS1), the overall release to Fossil Creek considers the 27.60-cfs that Basin OS1 produces in the 100-year event.
Thus, the total maximum on-site release to Fossil Creek is limited to 30.70-cfs. The proposed drainage design limits the overall
release to Fossil Creek to 20.77-cfs. Although Basins C1 and C2 are released undetained to Fossil Creek, the improved site is
designed to overdetain for Basin OS1 and limit the site’s overall release to Fossil Creek. The tables below summarize the
impacts of the proposed improvements to Fossil Creek relative to the site’s limiting criteria.
100-year Event: Allowed Release to Fossil Creek
Basin
Designation
Release Rate
(cfs)
Basin EX1 14.85
Basin EX2 13.30
Basin EX3 4.12
Basin EX4 3.80
Existing Runoff 36.07
Basin
Designation
Release Rate
(cfs)
Basin OS1 27.60
0.2 cfs/acre 3.10
Total Release 30.70
100-yr Event: Proposed On-Site Release to Fossil Creek
Basin/Pond
Designation
Release Rate
(cfs)
Pond A1 3.50
Pond B1 2.00
Pond C1 2.40
Basin C1 6.07
Basin C2 6.81
Total Release 20.77
P RELIMINARY D RAINAGE R EPORT
F ORT C OLLINS N ISSAN -K IA
F ORT C OLLINS ,CO
12 U21004_Drainage Report
Standard Operating Procedures (SOPs)
In order for physical stormwater Best Management Practices (BMPs) to be effective, proper maintenance is essential.
Maintenance includes both routinely scheduled activities, as well as non-routine repairs that may be required after large
storms, or as a result of other unforeseen problems. Standard Operating Procedures should clearly identify BMP maintenance
responsibility. BMP maintenance is typically the responsibility of the entity owning the BMP.
Identifying who is responsible for maintenance of BMPs and ensuring that an adequate budget is allocated for maintenance
is critical to the long-term success of BMPs. Maintenance responsibility may be assigned either publicly or privately. For this
project, the privately owned BMPs including grass swales, underground water quality and detention, and the bioretention
pond, are to be maintained by the property owner.
Storm Sewers
There are multiple storm sewer networks proposed with the site improvements. Several networks including roof drains,
storm drains, and underground water quality and detention features are proposed ultimately with outfalls to Fossil Creek.
Proposed storm sewer systems are both private and public, designed to accommodate the flows from the 100-year storm
event. Hydraulic computations of these systems will be provided with the initial final compliance submittal.
Inlets
There are multiple inlets proposed with the site improvements, including Nyloplast drain basins and Type R inlets. Type R
Inlets are designed throughout the site on-grade and in sump to capture developed runoff for detention. On-site drain basins
are designed in a sump condition and are designed to convey runoff produced within smaller interior basins to on-site
bioretention and detention facilities. All inlets on the site are sized to provide adequate capacity and convey the 100-year
storm event. Hydraulic computations of these systems will be provided with the initial final compliance submittal.
V.EROSION CONTROL
A separate Erosion Control Report / Storm Water Management Plan (SWMP) will be prepared for the site in accordance with
the Stormwater Discharge Permit for Colorado Department of Public Health and Environment as the site will disturb an area
greater than 1-acre. The Erosion Control Report will be completed during the Final Compliance phase of the project and will
include more detailed information on the sediment and erosion control items for this project. It is intended that the proposed
improvements will comply with Erosion Control Criteria per the FCSCM, and all Erosion Control Materials will be provided
with the Final Drainage Report. At a minimum, the following temporary BMP’s will be installed and maintained to control
on-site erosion and prevent sediment from traveling off-site during construction:
Silt Fence – a woven synthetic fabric that filters runoff. The silt fence is a temporary barrier that is placed at the base
of a disturbed area.
Vehicle Tracking Control – a stabilized stone pad located at points of ingress and egress on a construction site. The
stone pad is designed to reduce the amount of mud transported onto public roads by construction traffic.
Riprap – Riprap will be used downstream of all storm sewer outfalls to control erosion of the receiving channels.
Inlet Protection – acts as a sediment filter. It is a temporary BMP and requires proper installation and maintenance
to ensure their performance.
Straw Wattles – wattles act as a sediment filter in swales around inlets. They are a temporary BMP and require
proper installation and maintenance to ensure their performance.
Slope Protection – Slopes should be terraced using a “tracked” vehicle, run perpendicular to slope to inhibit rill/gulley
erosion.
The contractor shall store all construction materials and equipment and shall provide maintenance and fueling of equipment
in confined areas on-site from which runoff will be contained and filtered. Temporary Best Management Practices (BMP’s)
will be inspected by the contractor at a minimum of once every two weeks and after each significant storm event.
P RELIMINARY D RAINAGE R EPORT
F ORT C OLLINS N ISSAN -K IA
F ORT C OLLINS ,CO
13 U21004_Drainage Report
VI.CONCLUSIONS
A.COMPLIANCE WITH STANDARDS
Storm drainage calculations have followed the guidelines provided by the Urban Storm Drainage Criteria Manuals Volumes
1, 2 and 3 and the City of Fort Collins Stormwater Criteria Manual.
B.DRAINAGE CONCEPT
The drainage system has been designed to convey the runoff to the designated design points and the existing public
infrastructure in an effective, safe manner. No negative impacts are anticipated to the City of Fort Collins Master Drainage
Plan or to downstream properties or infrastructure due to the proposed improvements.
C.STORMWATER QUALITY
Multiple long-term stormwater quality measures have been selected for the site that will provide treatment of stormwater
prior it to being discharged from the site. For this site this includes grass swales, a bioretention pond, and underground water
quality.
P RELIMINARY D RAINAGE R EPORT
F ORT C OLLINS N ISSAN -K IA
F ORT C OLLINS ,CO
14 U21004_Drainage Report
VII.REFERENCES
1.City of Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, November 2017.
2.Urban Storm Drainage Criteria Manual Volume 1 and 2, Urban Drainage and Flood Control District, Denver, Colorado,
June 2001, Revised April 2008.
3.Natural Resources Conservation Service Web Soil Survey at: websoilsurvey.nrcs.usda.gov/app
4.Preliminary Flood Insurance Rate Map, FEMA, Panel 2079E, https://hazards.fema.gov/femaportal/
5.Stormwater Discharges Associated with Construction Activity, Stormwater Management Plan Preparation Guides, State
of Colorado, www.colorado.com
A PPENDIX A
D RAINAGE C ALCULATIONS
RUNOFF COEFFICIENTS AND % IMPERVIOUS
Nissan Dealership, Fort Collins, CO
Basin Design Pt.Composite NRCS Soil
Total Total Roof (1)Asphalt Concrete(1)Gravel(1)Lawns(1)Imperviousness Type C2 C100
acres %I = 90%%I = 100%%I = 100%%I =40%%I=2%(%I)
C=0.95 C=0.95 C=0.95 C=0.50 C=0.25
sf sf sf sf sf sf
EX1 EX1 2.12 92,255 54,030 3,425 4,665 30,135 65.0%C/D 0.70 0.87
EX2 EX2 1.70 74,165 4,185 5,225 32,995 6,200 25,560 60.6%C/D 0.67 0.84
EX3 EX3 0.41 18,045 10,235 2,435 5,375 -82.1%C/D 0.82 1.00
EX4 EX4 0.38 16,625 4,145 10,100 2,380 -88.9%C/D 0.89 1.00
EX5 EX5 7.97 347,140 347,140 2.0%C/D 0.25 0.31
EX6 EX6 6.44 280,470 7,095 15,390 42,290 2,450 213,245 24.7%C/D 0.41 0.52
OS1 OS1 3.70 161,165 18,000 52,870 19,150 71,145 55.6%C/D 0.64 0.80
OS2 OS2 5.75 250,410 250,410 2.0%C/D 0.25 0.31
Basin Design Pt.Composite NRCS Soil
Total Total Roof (1)Asphalt Concrete(1)Pavers(1)Lawns(1)Imperviousness Type C2 C100
acres sf %I = 90%%I = 100%%I = 100%%I =40%%I=2%(%I)
C=0.95 C=0.95 C=0.95 C=0.50 C=0.25
sf sf sf sf sf
A1 A1 0.95 41,480 41,480 2.0%C/D 0.25 0.31
A2 A2 0.41 17,975 17,975 2.0%C/D 0.25 0.31
A3 A3 0.45 19,800 10,510 3,805 5,485 72.9%C/D 0.76 0.95
A4 A4 0.45 19,715 15,515 1,530 2,670 86.7%C/D 0.86 1.00
A5 A5 0.48 20,865 20,865 -100.0%C/D 0.95 1.00
A6 A6 0.20 8,745 5,145 575 3,025 66.1%C/D 0.71 0.88
A7 A7 0.38 16,710 9,235 1,045 6,430 62.3%C/D 0.68 0.85
A8 A8 0.09 4,110 4,110 -100.0%C/D 0.95 1.00
A9 A9 0.18 7,695 7,200 495 -100.0%C/D 0.95 1.00
A10 A10 0.50 21,945 14,915 1,565 5,465 75.6%C/D 0.78 0.97
A11 A11 0.05 2,065 2,065 -100.0%C/D 0.95 1.00
A12 A12 0.81 35,285 26,630 635 8,020 77.7%C/D 0.79 0.99
A13 A13 0.48 21,030 21,030 -90.0%C/D 0.95 1.00
A14 A14 0.57 24,620 23,160 1,460 94.2%C/D 0.91 1.00
A15 A15 0.74 32,215 30,000 2,215 93.3%C/D 0.90 1.00
A16 A16 0.60 26,200 26,200 -90.0%C/D 0.95 1.00
B1 B1 0.95 41,250 41,250 2.0%C/D 0.25 0.31
B2 B2 0.99 43,197 33,665 9,532 78.4%C/D 0.80 0.99
B3 B3 0.42 18,110 18,110 -100.0%C/D 0.95 1.00
B4 B4 0.59 25,857 14,374 1,800 9,683 63.3%C/D 0.69 0.86
B5 B5 0.22 9,748 4,705 1,240 3,803 61.8%C/D 0.68 0.85
B6 B6 0.30 13,195 8,885 1,240 3,070 77.2%C/D 0.79 0.98
-0.00 ----
C1 C1 0.61 26,565 16,845 5,625 4,095 84.9%C/D 0.84 1.00
C2 C2 0.83 36,362 15,795 5,275 15,292 58.8%C/D 0.66 0.82
C3 C3 2.73 118,860 100,000 9,000 9,860 91.9%C/D 0.89 1.00
C4 C4 0.38 16,710 16,710 -90.0%C/D 0.95 1.00
D1 D1 0.49 21,220 12,365 1,900 6,955 67.9%C/D 0.72 0.90
E1 E1 2.12 92,490 59,100 12,035 21,355 77.4%C/D 0.79 0.99
OS1 OS1 3.70 161,165 18,000 52,870 19,150 71,145 55.6%C/D 0.64 0.80
OS2 OS2 5.84 254,355 254,355 2.0%C/D 0.25 0.31
Notes:
(1) Recommended % Imperviousness Values per USDCM Volume 1, Table 6-3.
(2) Runoff C is based on %I and NRCS soil group per UDFCD Volume 1, Table 6-4.
Areas Composite Runoff Coefficients (2)
Composite Runoff Coefficients (2)Areas
Existing Basins
Proposed Basins
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
TIME OF CONCENTRATION
Nissan Dealership, Fort Collins, CO
Basin Design Pt.Area
C2(1)Length Slope ti(2)Length Slope Cv Velocity (3)tt(4)Check tc?Total Length tc(5)
acres ft %min ft %fps min min Urban?ft min min
EX1 EX1 2.12 0.70 50 2.2%4.0 1180 2.3%20 3.0 6.6 10.5 Yes 1230 17 10.5
EX2 EX2 1.70 0.67 160 2.8%7.0 155 1.0%20 2.0 1.3 8.3 Yes 315 12 8.3
EX3 EX3 0.41 0.82 40 2.0%2.6 730 2.5%20 3.2 3.8 6.4 Yes 770 14 6.4
EX4 EX4 0.38 0.89 45 2.0%2.1 165 3.5%20 3.7 0.7 2.8 Yes 210 11 5.0
EX5 EX5 7.97 0.25 500 10.0%16.1 320 2.0%7 1.0 5.4 21.4 No --21.4
EX6 EX6 6.44 0.41 265 1.8%16.6 376 2.6%20 3.2 1.9 18.6 Yes 641 14 13.6
OS1 OS1 3.70 0.64 65 2.0%5.3 475 2.0%20 2.8 2.8 8.1 Yes 540 13 8.1
OS2 OS2 5.75 0.25 380 10.0%14.0 360 2.0%7 1.0 6.1 20.1 No --20.1
Basin Design Pt.Area
C2(1)Length Slope ti(2)Length Slope Cv Velocity (3)tt(4)Check tc?Total Length tc(5)
acres ft %min ft %fps min min Urban?ft min min
A1 A1 0.95 0.25 120 14.0%7.0 160 1.0%15 1.5 1.8 8.8 Yes 280 12 8.8
A2 A2 0.41 0.25 45 12.0%4.5 170 0.5%15 1.1 2.7 7.2 Yes 215 11 7.2
A3 A3 0.45 0.76 40 2.0%3.1 210 1.5%20 2.4 1.4 4.6 Yes 250 11 5.0
A4 A4 0.45 0.86 70 2.6%2.7 170 2.8%20 3.3 0.8 3.5 Yes 240 11 5.0
A5 A5 0.48 0.95 95 3.9%1.7 100 0.6%20 1.5 1.1 2.8 Yes 195 11 5.0
A6 A6 0.20 0.71 65 2.2%4.4 90 2.0%20 2.8 0.5 4.9 Yes 155 11 5.0
A7 A7 0.38 0.68 45 3.0%3.5 70 1.0%20 2.0 0.6 4.1 Yes 115 11 5.0
A8 A8 0.09 0.95 45 2.9%1.3 50 0.6%20 1.5 0.5 1.8 Yes 95 11 5.0
A9 A9 0.18 0.95 45 2.2%1.4 110 0.8%20 1.8 1.0 2.4 Yes 155 11 5.0
A10 A10 0.50 0.78 20 5.0%1.5 210 0.8%20 1.8 2.0 3.5 Yes 230 11 5.0
A11 A11 0.05 0.95 10 2.0%0.7 50 0.5%20 1.4 0.6 1.3 Yes 60 10 5.0
A12 A12 0.81 0.79 65 2.3%3.4 165 1.1%20 2.1 1.3 4.7 Yes 230 11 5.0
A13 A13 0.48 0.95 70 2.0%1.8 100 2.0%20 2.8 0.6 2.4 Yes 170 11 5.0
A14 A14 0.57 0.91 160 3.3%2.9 40 0.5%20 1.4 0.5 3.4 Yes 200 11 5.0
A15 A15 0.74 0.90 160 3.3%3.1 80 0.5%20 1.4 0.9 4.0 Yes 240 11 5.0
A16 A16 0.60 0.95 75 2.0%1.9 60 2.0%20 2.8 0.4 2.2 Yes 135 11 5.0
B1 B1 0.95 0.25 50 20.0%4.0 280 1.2%15 1.6 2.8 6.9 Yes 330 12 6.9
B2 B2 0.99 0.80 215 2.0%6.4 115 0.8%20 1.8 1.1 7.5 Yes 330 12 7.5
B3 B3 0.42 0.95 195 1.6%3.2 15 1.6%20 2.5 0.1 3.3 Yes 210 11 5.0
B4 B4 0.59 0.69 130 2.7%6.1 110 0.5%15 1.1 1.7 7.8 Yes 240 11 7.8
B5 B5 0.22 0.68 95 2.5%5.5 50 4.0%20 4.0 0.2 5.7 Yes 145 11 5.7
B6 B6 0.30 0.79 55 3.7%2.7 40 5.2%20 4.6 0.1 2.9 Yes 95 11 5.0
---------
C1 C1 0.61 0.84 55 2.5%2.6 380 5.0%20 4.5 1.4 4.0 Yes 435 12 5.0
C2 C2 0.83 0.66 60 2.5%4.6 300 5.0%20 4.5 1.1 5.7 Yes 360 12 5.7
C3 C3 2.73 0.89 180 3.5%3.3 300 2.5%20 3.2 1.6 4.9 Yes 480 13 5.0
C4 C4 0.38 0.95 65 2.0%1.7 75 2.0%20 2.8 0.4 2.2 Yes 140 11 5.0
D1 D1 0.49 0.72 40 2.0%3.4 210 2.5%20 3.2 1.1 4.6 Yes 250 11 5.0
E1 E1 2.12 0.79 40 2.0%2.8 945 2.5%20 3.2 5.0 7.8 Yes 985 15 7.8
OS1 OS1 3.70 0.64 65 2.0%5.3 475 2.0%20 2.8 2.8 8.1 Yes 540 13 8.1
OS2 OS2 5.84 0.25 380 10.0%14.0 360 2.0%7 1.0 6.1 20.1 No --20.1
Notes:
(1) C=Cx*Cf
(2)ti = [0.395(1.1-C5)L1/2]/S1/3, S= slope in %, L=length of overland flow (300' max urban, 500' max rural)
(3)V=KS0.5, S=watercourse slope in ft/ft, UDFCD Equation 6-4
(4)tt=L/(V*60 sec/min)
(5)tc check (for urban or developed areas only) = total length/180 + 10
(6)min tc = 5 min
Existing Basins
Proposed Basins
Travel/Channelized Time of Flow (tt)
ti+tt
tc Check for Urbanized Basins
Final tc
(6)
ti+tt Final tc
(6)
tc Check for Urbanized BasinsTravel/Channelized Time of Flow (tt)
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
TIME OF CONCENTRATION
Nissan Dealership, Fort Collins, CO
Basin Design Pt.Area
C100
(1)Length Slope ti
(2)Length Slope Cv Velocity (3)tt
(4)Check tc?Total Length tc
(5)
acres ft %min ft %fps min min Urban?ft min min
EX1 EX1 2.12 0.87 50 2.2%2.2 1180 2.3%20 3.0 6.6 8.8 Yes 1230 17 8.8
EX2 EX2 1.70 0.84 160 2.8%4.2 155 1.0%20 2.0 1.3 5.5 Yes 315 12 5.5
EX3 EX3 0.41 1.00 40 2.0%0.9 730 2.5%20 3.2 3.8 4.8 Yes 770 14 5.0
EX4 EX4 0.38 1.00 45 2.0%1.0 165 3.5%20 3.7 0.7 1.7 Yes 210 11 5.0
EX5 EX5 7.97 0.31 500 10.0%14.9 320 2.0%7 1.0 5.4 20.3 No --20.3
EX6 EX6 6.44 0.52 265 1.8%14.1 376 2.6%20 3.2 1.9 16.0 Yes 641 14 13.6
OS1 OS1 3.70 0.80 65 2.0%3.5 475 2.0%20 2.8 2.8 6.3 Yes 540 13 6.3
OS2 OS2 5.75 0.31 380 10.0%13.0 360 2.0%7 1.0 6.1 19.0 No --19.0
Basin Design Pt.Area
C100
(1)Length Slope ti
(2)Length Slope Cv Velocity (3)tt
(4)Check tc?Total Length tc
(5)
acres ft %min ft %fps min min Urban?ft min min
A1 A1 0.95 0.31 120 14.0%6.5 160 1.0%15 1.5 1.8 8.3 Yes 280 12 8.3
A2 A2 0.41 0.31 45 12.0%4.2 170 0.5%15 1.1 2.7 6.9 Yes 215 11 6.9
A3 A3 0.45 0.95 40 2.0%1.4 210 1.5%20 2.4 1.4 2.8 Yes 250 11 5.0
A4 A4 0.45 1.00 70 2.6%1.1 170 2.8%20 3.3 0.8 1.9 Yes 240 11 5.0
A5 A5 0.48 1.00 95 3.9%1.1 100 0.6%20 1.5 1.1 2.2 Yes 195 11 5.0
A6 A6 0.20 0.88 65 2.2%2.4 90 2.0%20 2.8 0.5 2.9 Yes 155 11 5.0
A7 A7 0.38 0.85 45 3.0%2.1 70 1.0%20 2.0 0.6 2.7 Yes 115 11 5.0
A8 A8 0.09 1.00 45 2.9%0.9 50 0.6%20 1.5 0.5 1.4 Yes 95 11 5.0
A9 A9 0.18 1.00 45 2.2%0.9 110 0.8%20 1.8 1.0 2.0 Yes 155 11 5.0
A10 A10 0.50 0.97 20 5.0%0.6 210 0.8%20 1.8 2.0 2.6 Yes 230 11 5.0
A11 A11 0.05 1.00 10 2.0%0.5 50 0.5%20 1.4 0.6 1.0 Yes 60 10 5.0
A12 A12 0.81 0.99 65 2.3%1.2 165 1.1%20 2.1 1.3 2.5 Yes 230 11 5.0
A13 A13 0.48 1.00 70 2.0%1.2 100 2.0%20 2.8 0.6 1.8 Yes 170 11 5.0
A14 A14 0.57 1.00 160 3.3%1.5 40 0.5%20 1.4 0.5 2.0 Yes 200 11 5.0
A15 A15 0.74 1.00 160 3.3%1.5 80 0.5%20 1.4 0.9 2.5 Yes 240 11 5.0
A16 A16 0.60 1.00 75 2.0%1.2 60 2.0%20 2.8 0.4 1.6 Yes 135 11 5.0
0.00
B1 B1 0.95 0.31 50 20.0%3.7 280 1.2%15 1.6 2.8 6.6 Yes 330 12 6.6
B2 B2 0.99 0.99 215 2.0%2.2 115 0.8%20 1.8 1.1 3.3 Yes 330 12 5.0
B3 B3 0.42 1.00 195 1.6%2.2 15 1.6%20 2.5 0.1 2.3 Yes 210 11 5.0
B4 B4 0.59 0.86 130 2.7%3.6 110 0.5%15 1.1 1.7 5.3 Yes 240 11 5.3
B5 B5 0.22 0.85 95 2.5%3.3 50 4.0%20 4.0 0.2 3.5 Yes 145 11 5.0
B6 B6 0.30 0.98 55 3.7%1.0 40 5.2%20 4.6 0.1 1.2 Yes 95 11 5.0
-0.00 --------
C1 C1 0.61 1.00 55 2.5%1.0 380 5.0%20 4.5 1.4 2.4 Yes 435 12 5.0
C2 C2 0.83 0.82 60 2.5%2.9 300 5.0%20 4.5 1.1 4.0 Yes 360 12 5.0
C3 C3 2.73 1.00 180 3.5%1.6 300 2.5%20 3.2 1.6 3.2 Yes 480 13 5.0
C4 C4 0.38 1.00 65 2.0%1.2 75 2.0%20 2.8 0.4 1.6 Yes 140 11 5.0
0.00
D1 D1 0.49 0.90 40 2.0%1.8 210 2.5%20 3.2 1.1 2.9 Yes 250 11 5.0
E1 E1 2.12 0.99 40 2.0%1.0 940 2.5%20 3.2 5.0 6.0 Yes 980 15 6.0
0.00
OS1 OS1 3.70 0.80 65 2.0%3.5 475 2.0%20 2.8 2.8 6.3 Yes 540 13 6.3
OS2 OS2 5.84 0.31 380 10.0%13.0 360 2.0%7 1.0 6.1 19.0 No --19.0
Notes:
(1) C=Cx*Cf
(2)ti = [0.395(1.1-C5)L1/2]/S1/3, S= slope in %, L=length of overland flow (300' max urban, 500' max rural)
(3)V=KS0.5, S=watercourse slope in ft/ft, UDFCD Equation 6-4
(4)tt=L/(V*60 sec/min)
(5)tc check (for urban or developed areas only) = total length/180 + 10
(6)min tc = 5 min
Existing Basins
Travel/Channelized Time of Flow (tt)
ti+tt
tc Check for Urbanized Basins
Final tc
(6)
Proposed Basins
Travel/Channelized Time of Flow (tt)
ti+tt
tc Check for Urbanized Basins
Final tc
(6)
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
RATIONAL METHOD PEAK RUNOFF
Nissan Dealership, Fort Collins, CO
Basin Design Pt.Area 2-Year 100-Year
acre tc tc C2 C100 I2 I100 Q2 Q100
min min in/hr in/hr cfs cfs
EX1 EX1 2.12 11.0 9.0 0.70 0.87 2.30 8.03 3.40 14.85
EX2 EX2 1.70 8.0 6.0 0.67 0.84 2.67 9.31 3.05 13.30
EX3 EX3 0.41 6.0 5.0 0.82 1.00 2.85 9.95 0.96 4.12
EX4 EX4 0.38 5.0 5.0 0.89 1.00 2.85 9.95 0.96 3.80
EX5 EX5 7.97 21.0 20.0 0.25 0.31 1.61 5.60 3.21 13.95
EX6 EX6 6.44 14.0 14.0 0.41 0.52 1.92 6.71 5.12 22.35
OS1 OS1 3.70 8.0 6.0 0.64 0.80 2.67 9.31 6.33 27.60
OS2 OS2 5.75 20.0 19.0 0.25 0.31 1.65 5.75 2.37 10.33
Basin Design Pt.Area 2-Year 100-Year
acre tc tc C2 C100 I2 I100 Q2 Q100
min min in/hr in/hr cfs cfs
A1 A1 0.95 9.0 8.0 0.25 0.31 2.40 8.38 0.57 2.49
A2 A2 0.41 7.0 7.0 0.25 0.31 2.52 8.80 0.26 1.13
A3 A3 0.45 5.0 5.0 0.76 0.95 2.85 9.95 0.98 4.27
A4 A4 0.45 5.0 5.0 0.86 1.00 2.85 9.95 1.10 4.50
A5 A5 0.48 5.0 5.0 0.95 1.00 2.85 9.95 1.30 4.77
A6 A6 0.20 5.0 5.0 0.71 0.88 2.85 9.95 0.41 1.77
A7 A7 0.38 5.0 5.0 0.68 0.85 2.85 9.95 0.74 3.25
A8 A8 0.09 5.0 5.0 0.95 1.00 2.85 9.95 0.26 0.94
A9 A9 0.18 5.0 5.0 0.95 1.00 2.85 9.95 0.48 1.76
A10 A10 0.50 5.0 5.0 0.78 0.97 2.85 9.95 1.11 4.86
A11 A11 0.05 5.0 5.0 0.95 1.00 2.85 9.95 0.13 0.47
A12 A12 0.81 5.0 5.0 0.79 0.99 2.85 9.95 1.83 7.97
A13 A13 0.48 5.0 5.0 0.95 1.00 2.85 9.95 1.31 4.80
A14 A14 0.57 5.0 5.0 0.91 1.00 2.85 9.95 1.46 5.62
A15 A15 0.74 5.0 5.0 0.90 1.00 2.85 9.95 1.90 7.36
A16 A16 0.60 5.0 5.0 0.95 1.00 2.85 9.95 1.63 5.98
B1 B1 0.95 7.0 7.0 0.25 0.31 2.52 8.80 0.60 2.60
B2 B2 0.99 7.0 5.0 0.80 0.99 2.85 9.95 2.25 9.81
B3 B3 0.42 5.0 5.0 0.95 1.00 2.85 9.95 1.13 4.14
B4 B4 0.59 8.0 5.0 0.69 0.86 2.85 9.95 1.16 5.08
B5 B5 0.22 6.0 5.0 0.68 0.85 2.85 9.95 0.43 1.88
B6 B6 0.30 5.0 5.0 0.79 0.98 2.85 9.95 0.68 2.97
C1 C1 0.61 5.0 5.0 0.84 1.00 2.85 9.95 1.46 6.07
C2 C2 0.83 6.0 5.0 0.66 0.82 2.85 9.95 1.56 6.81
C3 C3 2.73 5.0 5.0 0.89 1.00 2.85 9.95 6.94 27.15
C4 C4 0.38 5.0 5.0 0.95 1.00 2.85 9.95 1.04 3.82
D1 D1 0.49 5.0 5.0 0.72 0.90 2.85 9.95 1.00 4.37
E1 E1 2.12 8.0 6.0 0.79 0.99 2.67 9.31 4.47 19.48
OS1 OS1 3.70 8.0 6.0 0.64 0.80 2.67 9.31 6.33 27.60
OS2 OS2 5.84 20.0 19.0 0.25 0.31 1.65 5.75 2.41 10.49
Existing Basins
Peak Discharge
Proposed Basins
Runoff Coefficients Rainfall Intensity Peak Discharge
Rainfall IntensityRunoff Coefficients
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
A PPENDIX B
H YDRAULIC C ALCULATIONS
LOW IMPACT DEVELOPMENT SUMMARY
Nissan Dealership, Fort Collins, CO
Basin Proposed
Impervious Area
LID Treatment Area Treated % of Site Treated
(sf)(acres)(sf)(%)
A1 830 None 0 0.0%
A2 360 Bioretention 360 0.1%
A3 14,425 None 0 0.0%
A4 17,098 Bioretention 17,098 3.2%
A5 20,865 Bioretention 20,865 3.8%
A6 5,781 Bioretention 5,781 1.1%
A7 10,409 Bioretention 10,409 1.9%
A8 4,110 Bioretention 4,110 0.8%
A9 7,695 Bioretention 7,695 1.4%
A10 11,201 Bioretention 11,201 2.1%
A11 2,065 Bioretention 2,065 0.4%
A12 27,425 Bioretention 27,425 5.1%
A13 18,927 Bioretention 18,927 3.5%
A14 23,189 Bioretention 23,189 4.3%
A15 30,044 Bioretention 30,044 5.5%
A16 23,580 Bioretention 23,580 4.3%
B1 825 Wetland Channel 825 0.2%
B2 33,856 Wetland Channel 33,856 6.2%
B3 18,110 Wetland Channel 18,110 3.3%
B4 16,368 Bioretention 16,368 3.0%
B5 6,021 Wetland Channel 6,021 1.1%
B6 10,186 Wetland Channel 10,186 1.9%
C1 22,552 None 0 0.0%
C2 21,376 None 0 0.0%
C3 109,197 Underground Water Quality 109,197 20.1%
C4 15,039 Underground Water Quality 15,039 2.8%
D1 6,889 None 0 0.0%
E1 53,932 None 0 0.0%
OS1 9,795 None 0 0.0%
Total New/Modified Site 542,149 412,351 76.1%
Low Impact Development (LID) Treated Areas
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
WATER QUALITY
Nissan Dealership, Fort Collins, CO
Required Water Quality Capture Volume
Basin Area Area Imperviousness Watershed WQCV
(sf)(acres)(%)(inches)(cf)
A1 41,480 0.952 2%0.01 42
A2 17,975 0.413 2%0.01 18
A3(3)19,800 0.455 73%0.29 571
A4 19,715 0.453 87%0.30 493
A5 20,865 0.479 100%0.40 696
A6 8,745 0.201 66%0.21 151
A7 16,710 0.384 62%0.20 272
A8 4,110 0.094 100%0.40 137
A9 7,695 0.177 100%0.40 257
A10 21,945 0.504 76%0.24 443
A11 2,065 0.047 100%0.40 69
A12 35,285 0.810 78%0.25 740
A13 21,030 0.483 90%0.32 563
A14 24,620 0.565 94%0.35 721
A15 32,215 0.740 93%0.34 925
A16 26,200 0.601 90%0.32 701
B1 41,250 0.947 2%0.01 42
B2 43,197 0.992 78%0.25 917
B3 18,110 0.416 100%0.40 604
B4 25,857 0.594 63%0.20 427
B5 9,748 0.224 62%0.19 157
B6 13,195 0.303 77%0.25 274
----
C1 26,565 0.610 85%0.36 800
C2 36,362 0.835 59%0.23 703
C3 118,860 2.729 92%0.33 3,311
C4 16,710 0.384 90%0.32 447
D1 21,220 0.487 68%0.21 376
E1 92,490 2.123 77%0.25 1,928
OS1(3)161,165 3.700 56%0.22 2,985
(1)Water quality provided by bioretention and based on 12-hour storage
(2)Water quality provided by underground detention and based on 12-hour storage
(3)based on 40 hr storage and additional 20% for extended detention
(4) Basins C1 & C2 free release from the site
(5) Basins D1 & E1 are related to CDOT R.O.W and are not captured with the site improvemens
(1)
(2)
(2)
(4)
(4)
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
UNDERGROUND DETENTION & WATER QUALITY
Nissan Dealership, Fort Collins, CO
Required Underground Detention/WQ Volume
Chamber Total Required Detention/ Chamber Chamber Chamber *Installed Min.End Cap *Installed Min.Min. Req'd Design No. of No. of Provided Provided Total
ID Detention/WQ WQ Type Unit Unit Chamber Unit No. of Unit End Cap WQ Isolator Row Controli Chambers End Caps Release Isolator Installed
Volumea Inflowb Release Volumed Volume inc.Chambersf Volumed Volume inc. Release Volume by Provided Provided Ratej Row System
Ratec Aggregatee Aggregatee Rateg FAA Methodh Volumek Volumel
(cf)(cfs)(cfs)(cf)(cf)(sf)(cf)(cf)(cfs)(cf)(cfs)(cf)(cf)
Pond C1 (LID/WQ)3,758 3.99 MC-4500 0.028 106.50 166.30 22 39.50 118.60 0.62 2,159 WQCV 22 2 0.62 2,343 3,896
Pond C1 (Detention)24,182 30.97 MC-4500 0.028 146.00 284.90 80 39.50 118.60 2.25 0 WQCV 85 12 2.4 N/A 24,217
*Volume assumes minimum aggregate depth below chamber.
a. Total required WQCV calculated per 12-hr drain time.
b. WQ inflow is approximated as one-half the 2-yr peak runoff rate.
c. Release rate per chamber, limited by flow through geotextile with accumulated sediment.
d. Volume within chamber only, not accounting for void spaces in surrounding aggregate. The Isolator Row(s) are sized per this unit volume.
e. Volume includes chamber and void spaces (40%) in surrounding aggregate, per chamber unit. The total system WQCV is sized per this unit volume.
f. Number of chambers required to provide full WQCV within total installed system, including aggregate.
g. Release rate per chamber times number of chambers. This is used at the 'outlet control' for the FAA calculations.
h. Minimum 'chamber-only' volume to ensure dirty water is fully contained within Isolator Row.
i. Determines whether the design is controlled by the Isolator Row volume or WQCV.
j. Release rate per chamber times number of chambers provided.
k. Volume provided in chambers only (no aggregate storage). This number must meet or exceed the required FAA storage volume.
l. System volume includes total number of chambers, plus surrounding aggregate. This number must meet or exceed the required 12-hr WQCV.
Date: 8/20/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP.xlsm
DETENTION POND VOLUME (FAA Method)
Nissan Dealership, Fort Collins, CO
POND ID:Pond A1
100 Year Storm Into Detention Facility
Area =440,140 square feet
Area =10.10 acres
C 2 = 0.70
C 100 = 0.84
Release Rate Out of Pond
Q OUT = 3.50 cfs
Unit Flow =0.35 cfs/acre
Notes:
1. Pond area includes all "A" Basins & Basin OS1.
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T)Intensity (I)Qin= ∑C100*Area*I Vi=(Qin*T*60)Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 9.95 84.1 25,241 1.00 3.50 1,050 24,191
10 7.72 65.3 39,168 1.00 3.50 2,100 37,068
15 6.52 55.1 49,620 1.00 3.50 3,150 46,470
20 5.60 47.4 56,824 1.00 3.50 4,200 52,624
25 4.98 42.1 63,166 1.00 3.50 5,250 57,916
30 4.52 38.2 68,798 1.00 3.50 6,300 62,498
35 4.08 34.5 72,451 1.00 3.50 7,350 65,101
40 3.74 31.6 75,901 1.00 3.50 8,400 67,501
45 3.46 29.3 78,996 1.00 3.50 9,450 69,546
50 3.23 27.3 81,938 1.00 3.50 10,500 71,438
55 3.03 25.6 84,551 1.00 3.50 11,550 73,001
60 2.86 24.2 87,063 1.00 3.50 12,600 74,463
70 2.59 21.9 91,984 1.00 3.50 14,700 77,284
80 2.38 20.1 96,601 1.00 3.50 16,800 79,801
90 2.21 18.7 100,913 1.00 3.50 18,900 82,013
100 2.06 17.4 104,516 1.00 3.50 21,000 83,516
110 1.94 16.4 108,270 1.00 3.50 23,100 85,170
120 1.84 15.6 112,025 1.00 3.50 25,200 86,825
Required Detention Volume
V 100 = 86,825 cubic feet
V 100 = 1.99 acre-ft
Date: 8/20/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP.xlsm
DETENTION POND VOLUME (FAA Method)
Nissan Dealership, Fort Collins, CO
POND ID:Pond B1
100 Year Storm Into Detention Facility
Area =151,357 square feet
Area =3.47 acres
C 2 = 0.64
C 100 = 0.78
Release Rate Out of Pond
Q OUT = 2.00 cfs
Unit Flow =0.58 cfs/acre
Notes:
1. Pond area includes all "B" Basins.
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T)Intensity (I)Qin= ∑C100*Area*I Vi=(Qin*T*60)Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 9.95 26.8 8,046 1.00 2.00 600 7,446
10 7.72 20.8 12,486 1.00 2.00 1,200 11,286
15 6.52 17.6 15,818 1.00 2.00 1,800 14,018
20 5.60 15.1 18,114 1.00 2.00 2,400 15,714
25 4.98 13.4 20,136 1.00 2.00 3,000 17,136
30 4.52 12.2 21,931 1.00 2.00 3,600 18,331
35 4.08 11.0 23,096 1.00 2.00 4,200 18,896
40 3.74 10.1 24,196 1.00 2.00 4,800 19,396
45 3.46 9.3 25,182 1.00 2.00 5,400 19,782
50 3.23 8.7 26,120 1.00 2.00 6,000 20,120
55 3.03 8.2 26,953 1.00 2.00 6,600 20,353
60 2.86 7.7 27,754 1.00 2.00 7,200 20,554
70 2.59 7.0 29,323 1.00 2.00 8,400 20,923
80 2.38 6.4 30,794 1.00 2.00 9,600 21,194
90 2.21 6.0 32,169 1.00 2.00 10,800 21,369
100 2.06 5.6 33,318 1.00 2.00 12,000 21,318
110 1.94 5.2 34,514 1.00 2.00 13,200 21,314
120 1.84 5.0 35,711 1.00 2.00 14,400 21,311
Required Detention Volume
V 100 = 21,369 cubic feet
V 100 = 0.49 acre-ft
Date: 8/20/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP.xlsm
DETENTION POND VOLUME (FAA Method)
Nissan Dealership, Fort Collins, CO
POND ID:Pond C1 - Underground Detention
100 Year Storm Into Detention Facility
Area =135,570 square feet
Area =3.11 acres
C 2 = 0.90
C 100 = 1.00
Release Rate Out of Pond
Q OUT = 2.40 cfs
Unit Flow =0.77 cfs/acre
Notes:
1. Pond area includes Basins C3 & C4.
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T)Intensity (I)Qin= ∑C100*Area*I Vi=(Qin*T*60)Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 9.95 31.0 9,290 1.00 2.40 720 8,570
10 7.72 24.0 14,416 1.00 2.40 1,440 12,976
15 6.52 20.3 18,263 1.00 2.40 2,160 16,103
20 5.60 17.4 20,914 1.00 2.40 2,880 18,034
25 4.98 15.5 23,249 1.00 2.40 3,600 19,649
30 4.52 14.1 25,321 1.00 2.40 4,320 21,001
35 4.08 12.7 26,666 1.00 2.40 5,040 21,626
40 3.74 11.6 27,936 1.00 2.40 5,760 22,176
45 3.46 10.8 29,075 1.00 2.40 6,480 22,595
50 3.23 10.1 30,158 1.00 2.40 7,200 22,958
55 3.03 9.4 31,119 1.00 2.40 7,920 23,199
60 2.86 8.9 32,044 1.00 2.40 8,640 23,404
70 2.59 8.1 33,855 1.00 2.40 10,080 23,775
80 2.38 7.4 35,554 1.00 2.40 11,520 24,034
90 2.21 6.9 37,142 1.00 2.40 12,960 24,182
100 2.06 6.4 38,468 1.00 2.40 14,400 24,068
110 1.94 6.0 39,849 1.00 2.40 15,840 24,009
120 1.84 5.7 41,231 1.00 2.40 17,280 23,951
Required Detention Volume
V 100 = 24,182 cubic feet
V 100 = 0.56 acre-ft
Date: 8/20/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP.xlsm
DETENTION POND VOLUME (FAA Method)
Nissan Dealership, Fort Collins, CO
POND ID:Pond C1 (Underground Water Quality)
WQ Basin Characateristics
Area =135,570 square feet
Area =3.11 acres
C 2 = 0.90
C 100 = 1.00
WQ Release Rate Determined from Number of Chambers and Chamber Release Rate
Q OUT = 0.62 cfs
Unit Flow =0.20 cfs/acre
Notes:
1. Release rate out of pond based on number of chambers multiplied by release rate per chamber.
2. Rainfall intensities approximated as one-half the 2-yr peak runoff rate.
3. C2 utilized for water quality event detention calculation.
4. Pond is designed to provide WQ for Basins C3 & C4.
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T)Intensity (I)Qin= ∑C100*Area*I Vi=(Qin*T*60)Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 1.425 4.0 1,196 1.00 0.62 186 1,010
10 1.105 3.1 1,855 1.00 0.62 372 1,483
15 0.935 2.6 2,355 1.00 0.62 558 1,797
20 0.805 2.3 2,703 1.00 0.62 743 1,960
25 0.715 2.0 3,001 1.00 0.62 929 2,072
30 0.650 1.8 3,274 1.00 0.62 1,115 2,159
35 0.585 1.6 3,438 1.00 0.62 1,301 2,137
40 0.535 1.5 3,593 1.00 0.62 1,487 2,106
45 0.495 1.4 3,740 1.00 0.62 1,673 2,067
50 0.460 1.3 3,862 1.00 0.62 1,859 2,003
55 0.435 1.2 4,017 1.00 0.62 2,044 1,972
60 0.410 1.1 4,130 1.00 0.62 2,230 1,900
65 0.385 1.1 4,201 1.00 0.62 2,416 1,785
70 0.365 1.0 4,290 1.00 0.62 2,602 1,688
75 0.345 1.0 4,344 1.00 0.62 2,788 1,556
80 0.330 0.9 4,432 1.00 0.62 2,974 1,459
85 0.315 0.9 4,495 1.00 0.62 3,160 1,336
90 0.305 0.9 4,609 1.00 0.62 3,345 1,263
95 0.290 0.8 4,625 1.00 0.62 3,531 1,094
100 0.280 0.8 4,701 1.00 0.62 3,717 984
105 0.270 0.8 4,760 1.00 0.62 3,903 857
110 0.260 0.7 4,802 1.00 0.62 4,089 713
115 0.255 0.7 4,923 1.00 0.62 4,275 649
120 0.245 0.7 4,936 1.00 0.62 4,460 476
Required Detention Volume
V 100 = 2,159 cubic feet
V 100 = 0.05 acre-ft
Date: 8/20/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP.xlsm
DETENTION POND STORAGE AND ORIFICE CALCULATIONS
Nissan Dealership, Fort Collins, CO
POND ID:Detention Pond A1
Stage Storage Calculations
Elevation Area (sf)Depth (ft)
Incremental
Volume (cf)Volume total (cf)
Volume total (ac-
ft)
5,002.60 386.39 0.00 0 0 0.00
5,003.00 2,348.39 0.40 492 492 0.01
5,003.40 5,228.93 0.40 1,478 1,969 0.05
5,003.80 7,339.29 0.40 2,502 4,471 0.10
5,004.20 8,104.02 0.40 3,087 7,558 0.17
5,004.60 8,858.92 0.40 3,391 10,950 0.25
5,005.00 9,622.23 0.40 3,695 14,645 0.34
5,005.40 10,397.70 0.40 4,003 18,648 0.43
5,005.80 11,233.24 0.40 4,325 22,973 0.53
5,006.20 12,212.56 0.40 4,688 27,661 0.64
5,006.60 13,269.49 0.40 5,095 32,756 0.75
5,007.00 14,211.28 0.40 5,495 38,251 0.88
5,007.40 15,134.21 0.40 5,868 44,119 1.01
5,007.80 16,052.55 0.40 6,236 50,355 1.16
5,008.20 17,055.77 0.40 6,621 56,976 1.31
5,008.60 18,160.48 0.40 7,042 64,018 1.47
5,009.00 19,264.83 0.40 7,484 71,502 1.64
5,009.40 20,480.73 0.40 7,948 79,450 1.82
5,009.80 21,831.87 0.40 8,461 87,911 2.02
5,010.00 22,543.68 0.20 4,437 92,349 2.12
Detention Pond Storage & Water Quality
Detention Volume Required =86,825 ft3 Water Quality Volume Required =3,557 ft3
1.99 ac-ft 0.08 ac-ft
Elev81 - Elev80 (VolRequired-Vol80)+Elev80 Elev81 - Elev80 (VolRequired-Vol80)+Elev80
Vol81 - Vo80 Vol81 - Vo80
Required W.S. Elevation = 5009.75 ft Required W.S. Elevation = 5003.68 ft
Provided W.S. Elevation = 5010.00 ft Provided W.S. Elevation = 5003.80 ft
Detention Volume Provided =92,349 ft3 Water Quality Volume Provided =4,471 ft3
2.12 ac-ft 0.10 ac-ft
100-year Depth D = 7.40 ft Water Quality Depth D = 1.20 ft
Top-of-Berm = 5011.00 ft 100-year W.S. Elevation = 5010.00 ft
Freeboard = 1.00 ft Freeboard = 6.20 ft
Orifice Opening and Water Quality Plate
Orifice Flow Rate, Q = CoAo(2gHo)0.5 cfs Area Required per WQ Plate Row 0.56 in2
Area of orifice Ao, ft2 = Q diam of WQ Hole = 0.85 in
Co(2gHo)0.5 diam of WQ Hole (fraction) = 7/8 in
C o = 0.65 Space Between Rows 4 in
Q = Q out of pond = 3.50 cfs Number of Columns 1
gravitational acceleration g = 32.2 ft/sec2 Number of Rows 4
diam = 0.566 ft
Effective head on orifice H o = D-1/2diam =7.12 ft
Area of orifice A o = 0.25 ft2
Area of orifice A o = 36.22 in2
100-Year Orifice Diameter = 6.79 6 13/16 inches
100-Year Detention Volume and Elevations Water Quality Volume and Elevations
W.S. Elevation, ft = W.S. Elevation, ft =
100-Year Orifice Calculation Water Quality Plate Calculation
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
DETENTION POND STORAGE AND ORIFICE CALCULATIONS
Nissan Dealership, Fort Collins, CO
POND ID:Detention Pond B1
Stage Storage Calculations
Elevation Area (sf)Depth (ft)
Incremental
Volume (cf)Volume total (cf)
Volume total (ac-
ft)
5,005.20 101.22 0.00 0 0 0.00
5,005.40 845.29 0.20 83 83 0.00
5,005.60 1,846.91 0.20 263 345 0.01
5,005.80 2,721.64 0.20 454 799 0.02
5,006.00 3,664.58 0.20 636 1,436 0.03
5,006.20 4,762.37 0.20 840 2,276 0.05
5,006.40 6,060.53 0.20 1,080 3,356 0.08
5,006.60 7,766.58 0.20 1,379 4,735 0.11
5,006.80 9,347.25 0.20 1,709 6,444 0.15
5,007.00 10,363.36 0.20 1,970 8,414 0.19
5,007.20 11,322.35 0.20 2,168 10,582 0.24
5,007.40 12,246.48 0.20 2,356 12,938 0.30
5,007.60 13,143.87 0.20 2,539 15,477 0.36
5,007.80 14,022.27 0.20 2,716 18,193 0.42
5,008.00 14,888.24 0.20 2,891 21,083 0.48
5,008.20 15,746.17 0.20 3,063 24,146 0.55
5,008.40 16,600.07 0.20 3,234 27,381 0.63
5,008.60 17,458.93 0.20 3,406 30,786 0.71
5,008.80 18,338.80 0.20 3,579 34,366 0.79
5,009.00 19,235.07 0.20 3,757 38,123 0.88
Detention Pond Storage & Water Quality
Detention Volume Required =21,369 ft3 Water Quality Volume Required =1,994 ft3
0.49 ac-ft 0.05 ac-ft
Elev81 - Elev80 (VolRequired-Vol80)+Elev80 Elev81 - Elev80 (VolRequired-Vol80)+Elev80
Vol81 - Vo80 Vol81 - Vo80
Required W.S. Elevation = 5008.11 ft Required W.S. Elevation = 5006.13 ft
Provided W.S. Elevation = 5009.00 ft Provided W.S. Elevation = 5006.30 ft
Detention Volume Provided =38,123 ft3 Water Quality Volume Provided =2,816 ft3
0.88 ac-ft 0.06 ac-ft
100-year Depth D = 3.80 ft Water Quality Depth D = 1.10 ft
Top-of-Berm = 5010.00 ft 100-year W.S. Elevation = 5009.00 ft
Freeboard = 1.00 ft Freeboard = 2.70 ft
Orifice Opening and Water Quality Plate
Orifice Flow Rate, Q = CoAo(2gHo)0.5 cfs Area Required per WQ Plate Row 0.41 in2
Area of orifice Ao, ft2 = Q diam of WQ Hole = 0.72 in
Co(2gHo)0.5 diam of WQ Hole (fraction) = 3/4 in
C o = 0.65 Space Between Rows 4 in
Q = Q out of pond = 2.00 cfs Number of Columns 1
gravitational acceleration g = 32.2 ft/sec2 Number of Rows 4
diam = 0.509 ft
Effective head on orifice H o = D-1/2diam =3.55 ft
Area of orifice A o = 0.20 ft2
Area of orifice A o = 29.32 in2
100-Year Orifice Diameter = 6.11 6 1/8 inches
100-Year Detention Volume and Elevations Water Quality Volume and Elevations
W.S. Elevation, ft = W.S. Elevation, ft =
100-Year Orifice Calculation Water Quality Plate Calculation
Date: 10/5/2021 C:\United Civil Dropbox\Projects\U21004 - Nissan Dealership\Reports\Drainage\Calculations\U21004-Drain Calcs-PDP
COLLEGE AVE (HWY 287)VENUS AVESKYVIEW SUBDIVISIONZONED RLOS13.70EX12.12EX21.70EX30.41EX40.38EX57.97FOSSIL CREEKS COLLEGE AVER.O.W BOUNDARYEXISTING WETLANDSOS25.75EX66.44PATH:0'150'300'SCALE: 1" = 150'75'C:\UNITED CIVIL DROPBOX\PROJECTS\U21004 - NISSAN DEALERSHIP\CADD\ET\EXISTING DRAINAGE MAP.DWG EXISTING DRAINAGE MAPDRAWING NAME:SHEET SIZE: 11 x 17
FORT COLLINS NISSANEXISTING DRAINAGE MAPDATE:October 5, 2021PREPARED FOR:TCC CORPORATIONJOB NUMBER:U21004NOTE:THIS EXHIBIT WAS PREPARED FOR INFORMATION ONLY AND THE ENGINEER PREPARING THESE PLANS WILL NOT BE RESPONSIBLE FOR UNAUTHORIZED USE OF THEM.SHEETOF11NORTH19 OLD TOWN SQUARE #238 | FORT COLLINS, CO 80524 | (970) 530-4044 | www.unitedcivil.comUNITED CIVILDesign GroupEXISTING MAJOR CONTOUREXISTING MINOR CONTOURXX.XXBASIN DESIGNATIONEXISTING BASINEXISTING BASIN LINELEGENDBASIN AREA (ACRE)