HomeMy WebLinkAbout2620 W. ELIZABETH ST., SINGLE-FAMILY DETACHED - PDP - PDP160037 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPRELIMINARY DRAINAGE REPORT
Elizabeth Street Farms
Fort Collins, Colorado
Prepared for:
High Plains Builders, LLC
P.O. Box 2796
Gillette, Wyoming 82717
Phone: (307) 257-2352
Prepared by:
Galloway & Company, Inc.
3760 East 15th Street, Suite 202
Loveland, Colorado 80538
Phone: (970) 800-3300
Contact: Herman Feissner, PE
Original Preparation: November 2nd, 2016
1
TABLE OF CONTENTS
TABLE OF CONTENTS.......................................................................................................................... 1
I. CERTIFICATIONS .......................................................................................................................... 2
II. GENERAL LOCATION AND DESCRIPTION .................................................................................. 5
III. DRAINAGE BASINS AND SUB-BASINS ........................................................................................ 7
IV. DRAINAGE DESIGN CRITERIA ..................................................................................................... 7
V. DRAINAGE FACILITY DESIGN .................................................................................................... 12
VI. EROSION AND SEDIMENT CONTROL MEASURES ................................................................... 14
VII. CONCLUSIONS ............................................................................................................................ 15
VIII. REFERENCES .............................................................................................................................. 16
APPENDIX A - REFERENCE MATERIALS
VICINITY MAP
NRCS SOILS MAP
FEMA FIRMETTE
CITY OF FORT COLLINS FLOOD MAPPING
APPENDIX B - HYDROLOGY CALCULATIONS
EXISTING RUNOFF CALCULATIONS
PROPOSED RUNOFF CALCULATIONS
APPENDIX C - HYDRAULIC CALCULATIONS
SWALE CAPACITY CALCULATIONS
PRELIMINARY DETENTION POND SIZING
APPENDIX D – DRAINAGE MAPS
EXISTING CONDITION DRAINAGE MAP
DEVELOPED CONDITION DRAINAGE MAP
2
I. CERTIFICATIONS
CERTIFICATION OF ENGINEER
“I hereby certify that this report for the final drainage design of Elizabeth Street Farms was
prepared by me (or under my direct supervision) in accordance with the provisions of the Fort
Collins Stormwater Criteria Manual for the owners thereof.”
______________________________________
Herman Feissner, PE
Registered Professional Engineer
State Of Colorado No. 38066
For and on behalf of Galloway & Company, Inc.
CERTIFICATION OF OWNER
“High Plains Builders, LLC hereby certifies that the drainage facilities for the Elizabeth Street
Farms be constructed according to the design presented in this report. We understand that the
City of Fort Collins does not and will not assume liability for drainage facilities designed and/or
certified by our engineer. We also understand that the City of Fort Collins relies on the
representation of others to establish that drainage facilities are designed and constructed in
compliance with City of Fort Collins guidelines, standards, or specifications. Review by the City
of Fort Collins can therefore in no way limit or diminish any liability, which we or any other party
may have with respect to the design or construction of such facilities.”
____________________________________
High Plains Builders, LLC
Attest:
___________________________________
(Name of Responsible Party)
__________________________________
Notary Public
__________________________________
Authorized Signature
`
5
II. GENERAL LOCATION AND DESCRIPTION
LOCATION
Elizabeth Street Farms (hereafter referred to as “the site” or “project site”) is located at 2620
West Elizabeth Street. The site is bounded on north by single-family residential development
and Pear Street right-of-way (existing cul-de-sac); on the south by West Elizabeth Street; and
on the east and west by single-family residential. More specifically, the site is Tract A of West
Plum Street – a Planned Unit Development situate in the north half of Section 16, Township 7
north, Range 69 west of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State
of Colorado. Refer to Appendix A for a vicinity map.
DESCRIPTION OF PROPERTY
The project site consists of ±2.56 acres. It is largely vacant and undeveloped. Previously the
site was used for agriculture. An existing two-story brick house, which the developer plans to
restore, is located in the southeast region of the site. An existing irrigation lateral, which is still
in use, crosses the site from west to east. The existing grades average between 0.50% and
1.00%. Locally steeper grades exist along the irrigation lateral and around the existing house.
The existing runoff flows from northwest to southeast. An existing swale along the north edge
of West Elizabeth Street captures existing runoff and conveys it east.
There are no major drainage ways passing through the project site.
According to the USDA NRCS Web Soil Survey, ‘Altvan-Satanta loams, 0 to 3 percent slopes’
covers roughly one-third of the project site. The remaining two-thirds of the site is ‘Altvan-
Satanta loams, 3 to 9 percent slopes’. These soils are associated with Hydrologic Soil Group
(HSG) ‘B’. HSG ‘B’ soils have a moderate infiltration rate when thoroughly wet. These consist
chiefly of moderately deep or deep, moderately well drained or well drained soils that have
moderately fine texture to moderately coarse texture. These soils have a moderate rate of
water transmission. Refer to Appendix A for additional soils information.
Hollingsworth Associates, Inc. conducted a subsurface study the project site. The results of the
preliminary geotechnical investigation are summarized in Subsurface Study for Ten Lots West
Elizabeth Subdivision Fort Collins, Colorado (Job No.: 16-469 | Dated: October 12, 2016). The
`
6
subsurface conditions across the lots were quite uniform, as indicated by exploratory borings
B-1 through B-5, and consisted of 12 inches of topsoil, 2 to 11 feet of medium stiff to stiff
sandy clay, which occasionally graded into clayey sand, and 2 feet to 5 feet of firm to medium
hard, interlayered and integrated weathered claystone/sandstone overlaying hard to very hard
claystone for the depth drilled, 21 feet.
The site is situated east of the Pleasant Valley and Lake Canal. An operational west-to-east
irrigation lateral flows across the project site. According to the ditch rider, Ed Wendel, five
people currently receive water from the lateral.
Elizabeth Street Farms will develop in a single phase as single-family residential. The project
includes the development of ten (10) single-family lots, street improvements, wet and dry utility
infrastructure and landscaping. A single point of access will be provided from Pear Street.
`
7
III. DRAINAGE BASINS AND SUB-BASINS
MAJOR BASIN DESCRIPTION
The project site is located in the Canal Importation drainage basin. According to the City of
Fort Collins website (www.fcgov.com), this basin “spans nearly five square miles in west-central
Fort Collins. Three major irrigation canals traverse the basin from north to south and impact the
drainage in the basin. The canals can intercept runoff traveling through the basin and transport
it out of the basin, and they also spill runoff into the basin when their capacity is exceeded”.
SUB- BASIN DESCRIPTION
The project site generally conforms to the input parameters of SWMM subbasin 9 in the Canal
Importation drainage basin. SWMM subbasin 9 is 60% impervious. It generally drains to
Elizabeth Street and then east. The proposed site will be ±50% impervious. The runoff
ultimately drains into the Red Fox Meadows neighborhood natural area.
The project site is shown on FEMA Map Numbers 08069C0960F and 08069C0978G (refer to
Appendix A for FEMA Firmettes). Neither map shows the project impacted by an existing
floodplain/floodway. Refer to Appendix A for a copy of each Firmette and a copy of the City of
Fort Collins flood map.
`
8
IV. DRAINAGE DESIGN CRITERIA
REGULATIONS
This preliminary drainage design presented herein was prepared in accordance with the Fort
Collins Amendments to the Urban Drainage and Flood Control District Criteria Manual (i.e.,
Urban Drainage and Flood Control District Urban Storm Drainage Criteria Manuals Volumes 1,
2 and 3 [Manual]). Together, the requirements are referred to as the Fort Collins Stormwater
Criteria Manual [FCSCM].
DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
The project site is encumbered by several constraints; we developed a strategy for
implementing ‘The Four-Step Process’ for stormwater quality management to the maximum
extent practicable. Each step is listed below along with a brief narrative describing the
implementation strategy.
Step 1 - Employ runoff reduction practices
The developed runoff from the A, B and C series of basins flow through vegetated swales
before entering the on-site detention facility. The UDFCD defines a Grass Swale as “Densely
vegetated drainage way with low-pitched side slopes that collects and slowly conveys runoff.
The design of the longitudinal slope and cross-section size forces the flow to be slow and
shallow, thereby facilitating sedimentation while limiting erosion.” The proposed vegetated
swales will have low longitudinal slopes of ±0.50%. It is designed to convey 2-year storm event
runoff in a slow (i.e., <1 ft/sec) and shallow manner (i.e., normal depth <1 foot). This design
encourages settling and infiltration.
Of the newly added impervious area, ±0.97 acre is tributary to vegetated swales. Impervious
areas have been disconnected to the maximum extent practicable.
Step 2 - Implement BMPs that provide a Water Quality Capture Volume (WQCV)
The proposed BMPs do not provide WQCV because of site constraints. Refer to
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS FOR a list.
Step 3 - Stabilize drainageways
`
9
Planting within the vegetated swales will stabilize the drainage way and prevent erosion during
storm events exceeding the 2-year recurrence level.
Step 4 - Implement site specific and other source control BMPS
Site specific considerations such as material storage and other site operations are addressed
in the Stormwater Management Plan (SWMP).
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS
Several constraints drove the design presented here.
· The outfall from the proposed on-site detention pond is the flowline of West Elizabeth
Street, elevation ±5107.41.
· The developer wants to keep the existing house. This influences grading design.
· The closest storm drain system is located more than one-half mile east along Elizabeth
Street.
· The site is generally flat. The existing grades average 0.50% to 1.00%. Locally steeper
grades exist around the structures and existing irrigation lateral.
· The project site is bounded by existing development.
· The existing soils are unsuitable for infiltrating stormwater runoff. He boring logs show
clay underlain by claystone/sandstone and claystone. Considering the expansive
nature of the subgrade soils, the subsurface study recommended that residences be
founded on drilled piers. We did consider LID systems with underdrains that flow into
drywells but feel this is inconsistent with the findings of the subsurface study.
HYDROLOGICAL CRITERIA
For urban catchments that are not complex and are generally 160 acres or less in size, it is
acceptable that the design storm runoff be analyzed using the Rational Method. The Rational
Method is often used when only the peak flow rate or total volume of runoff is needed (e.g.,
storm sewer sizing or simple detention basin sizing). The Rational Method was used to
estimate the peak flow at each design point. Routing calculations (i.e., time attenuation) that
aggregate the basins draining to a specific design point are include in the Rational Method
calculations in Appendix B.
`
10
The Rational Method is based on the Rational Formula:
Q = CiA
Where:
Q = the maximum rate of runoff, cfs
C = a runoff coefficient that is the ratio between the runoff volume from an area and the
average rate of rainfall depth over a given duration for that area
i = average intensity of rainfall in inches per hour for a duration equal to the Time of
Concentration (Tc)
A = area, acres
The one-hour rainfall Intensity-Duration-Frequency tables for use with the Rational Method of
runoff analysis are provided in Table RA-7 and Table RA-8 (refer to Appendix B).
The 2-year and 100-year storm events serve as the basis for the drainage system design. The
2-year storm is considered the minor storm event. It has a fifty percent probability of
exceedance during any given year. The 100-year storm is considered the major storm event. It
has a one percent probability of exceedance during any given year.
The 2-year drainage system, at a minimum, must be designed to transport runoff from the 2-
year recurrence interval storm event with minimal disruption to the urban environment. The
100-year drainage system, as a minimum, must be designed to convey runoff from the 100-
year recurrence interval flood to minimize life hazards and health, damage to structures, and
interruption to traffic and services.
HYDRAULIC CRITERIA
Street Capacity Analysis
The street section generally drains from the crown, across the travel lane and into roadside
swales. The proposed street section is bounded by a 9” wide concrete ribbon along each side
of the asphalt.
Storm Drain and Swale Capacity Analysis
`
11
The storm drain system is limited to short culverts and swales that convey runoff underneath
driveways and along the internal streets. The hydraulic analysis will be completed using
Bentley FlowMaster. Refer to Appendix C for general swale calculations.
`
12
V. DRAINAGE FACILITY DESIGN
GENERAL CONCEPT
This preliminary design presents the design of a system for collecting and conveying
developed runoff from the proposed development at Elizabeth Street Farms on-site detention
pond.
The on-site developed runoff in each tributary begins as overland flow from the residential lot
areas (i.e., roof area, concrete hardscape and landscaping). This runoff then flows from the
lots to the swale sections along the backs and fronts of the lots. These swales drain into on-
site detention pond.
SPECIFIC DETAILS
On-Site Detention Pond
The proposed on-site detention pond was sized with the Modified FAA Method. The site
constraints limited the volume of the on-site detention pond to 11,578 ft3. The proposed
release rate from the detention pond is 0.85 cfs. This exceeds the calculated 2-year release
rate of ±0.40 cfs for basin EX5. There does not appear to be any remaining areas that can
develop in SWMM subbasin 9, except for this site, and the site is located at the far west end of
the basin. Considering this, the proposed increase in release rate will have a negligible effect
on the basin during the 100-year storm event. The pond will discharge to the flowline in West
Elizabeth Street.
A, B and C Basins
These basins comprise approximately 1.91 acres. The developed runoff from the roof areas
will flow overland through landscaping and into vegetated swales along the private internal
streets. Flow in the swales will drain into the on-site detention pond.
D Basin
This basin includes future West Elizabeth Street right-of-way and existing West Elizabeth
Street improvements. The runoff will continue to flow east along Elizabeth Street.
`
13
E Basin
This basin includes private street improvements and portions of two lots. It will drain to Pear
Street undetained. The proposed flows should have a negligible impact on the existing
drainage system in this area.
`
14
VI. EROSION AND SEDIMENT CONTROL MEASURES
A General Permit for Stormwater Discharge Associated with Construction Activities issued by
the Colorado Department of Public Health and Environment (CDPHE), Water Quality Control
Division (WQCD), will be acquired for the site. A Stormwater Management Plan (SWMP) will
be prepared and is presented under separate cover. It will identify the Best Management
Practices (BMPs) which, when implemented, will meet the requirements of the General Permit.
`
15
VII. CONCLUSIONS
COMPLIANCE WITH STANDARDS
The design presented in this preliminary drainage report for Elizabeth Street Farms has been
prepared in accordance with the design standards and guidelines presented in the Fort Collins
Stormwater Criteria Manual.
VARIANCES
No variances are being requested with the proposed improvements described herein.
DRAINAGE CONCEPT
The proposed Elizabeth Street Farms storm drainage improvements should provide adequate
protection for the developed site. The proposed drainage design for the site should not
negatively impact the existing downstream storm drainage system.
`
16
VII. REFERENCES
1. Fort Collins Stormwater Criteria Manual (Addendum to the Urban Storm Drainage
Criteria Manuals Volumes 1, 2 and 3), prepared by City of Fort Collins.
2. Urban Drainage and Flood Control District, Drainage Criteria Manual Volumes 1 and 2,
prepared by Wright-McLaughlin Engineers, dated June 2001 (revised April 2008), and
the Volume 3, prepared by Wright-McLaughlin Engineers, dated September 1992 and
revised November 2010.
PRELIMINARY DRAINAGE REPORT
Elizabeth Street Farms
Fort Collins, Colorado
Prepared for:
High Plains Builders, LLC
P.O. Box 2796
Gillette, Wyoming 82717
Phone: (307) 257-2352
Prepared by:
Galloway & Company, Inc.
3760 East 15th Street, Suite 202
Loveland, Colorado 80538
Phone: (970) 800-3300
Contact: Herman Feissner, PE
Original Preparation: November 2nd, 2016
1
TABLE OF CONTENTS
TABLE OF CONTENTS.......................................................................................................................... 1
I. CERTIFICATIONS .......................................................................................................................... 2
II. GENERAL LOCATION AND DESCRIPTION .................................................................................. 5
III. DRAINAGE BASINS AND SUB-BASINS ........................................................................................ 7
IV. DRAINAGE DESIGN CRITERIA ..................................................................................................... 7
V. DRAINAGE FACILITY DESIGN .................................................................................................... 12
VI. EROSION AND SEDIMENT CONTROL MEASURES ................................................................... 14
VII. CONCLUSIONS ............................................................................................................................ 15
VIII. REFERENCES .............................................................................................................................. 16
APPENDIX A - REFERENCE MATERIALS
VICINITY MAP
NRCS SOILS MAP
FEMA FIRMETTE
CITY OF FORT COLLINS FLOOD MAPPING
APPENDIX B - HYDROLOGY CALCULATIONS
EXISTING RUNOFF CALCULATIONS
PROPOSED RUNOFF CALCULATIONS
APPENDIX C - HYDRAULIC CALCULATIONS
SWALE CAPACITY CALCULATIONS
PRELIMINARY DETENTION POND SIZING
APPENDIX D – DRAINAGE MAPS
EXISTING CONDITION DRAINAGE MAP
DEVELOPED CONDITION DRAINAGE MAP
2
I. CERTIFICATIONS
CERTIFICATION OF ENGINEER
“I hereby certify that this report for the final drainage design of Elizabeth Street Farms was
prepared by me (or under my direct supervision) in accordance with the provisions of the Fort
Collins Stormwater Criteria Manual for the owners thereof.”
______________________________________
Herman Feissner, PE
Registered Professional Engineer
State Of Colorado No. 38066
For and on behalf of Galloway & Company, Inc.
CERTIFICATION OF OWNER
“High Plains Builders, LLC hereby certifies that the drainage facilities for the Elizabeth Street
Farms be constructed according to the design presented in this report. We understand that the
City of Fort Collins does not and will not assume liability for drainage facilities designed and/or
certified by our engineer. We also understand that the City of Fort Collins relies on the
representation of others to establish that drainage facilities are designed and constructed in
compliance with City of Fort Collins guidelines, standards, or specifications. Review by the City
of Fort Collins can therefore in no way limit or diminish any liability, which we or any other party
may have with respect to the design or construction of such facilities.”
____________________________________
High Plains Builders, LLC
Attest:
___________________________________
(Name of Responsible Party)
__________________________________
Notary Public
__________________________________
Authorized Signature
`
5
II. GENERAL LOCATION AND DESCRIPTION
LOCATION
Elizabeth Street Farms (hereafter referred to as “the site” or “project site”) is located at 2620
West Elizabeth Street. The site is bounded on north by single-family residential development
and Pear Street right-of-way (existing cul-de-sac); on the south by West Elizabeth Street; and
on the east and west by single-family residential. More specifically, the site is Tract A of West
Plum Street – a Planned Unit Development situate in the north half of Section 16, Township 7
north, Range 69 west of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State
of Colorado. Refer to Appendix A for a vicinity map.
DESCRIPTION OF PROPERTY
The project site consists of ±2.56 acres. It is largely vacant and undeveloped. Previously the
site was used for agriculture. An existing two-story brick house, which the developer plans to
restore, is located in the southeast region of the site. An existing irrigation lateral, which is still
in use, crosses the site from west to east. The existing grades average between 0.50% and
1.00%. Locally steeper grades exist along the irrigation lateral and around the existing house.
The existing runoff flows from northwest to southeast. An existing swale along the north edge
of West Elizabeth Street captures existing runoff and conveys it east.
There are no major drainage ways passing through the project site.
According to the USDA NRCS Web Soil Survey, ‘Altvan-Satanta loams, 0 to 3 percent slopes’
covers roughly one-third of the project site. The remaining two-thirds of the site is ‘Altvan-
Satanta loams, 3 to 9 percent slopes’. These soils are associated with Hydrologic Soil Group
(HSG) ‘B’. HSG ‘B’ soils have a moderate infiltration rate when thoroughly wet. These consist
chiefly of moderately deep or deep, moderately well drained or well drained soils that have
moderately fine texture to moderately coarse texture. These soils have a moderate rate of
water transmission. Refer to Appendix A for additional soils information.
Hollingsworth Associates, Inc. conducted a subsurface study the project site. The results of the
preliminary geotechnical investigation are summarized in Subsurface Study for Ten Lots West
Elizabeth Subdivision Fort Collins, Colorado (Job No.: 16-469 | Dated: October 12, 2016). The
`
6
subsurface conditions across the lots were quite uniform, as indicated by exploratory borings
B-1 through B-5, and consisted of 12 inches of topsoil, 2 to 11 feet of medium stiff to stiff
sandy clay, which occasionally graded into clayey sand, and 2 feet to 5 feet of firm to medium
hard, interlayered and integrated weathered claystone/sandstone overlaying hard to very hard
claystone for the depth drilled, 21 feet.
The site is situated east of the Pleasant Valley and Lake Canal. An operational west-to-east
irrigation lateral flows across the project site. According to the ditch rider, Ed Wendel, five
people currently receive water from the lateral.
Elizabeth Street Farms will develop in a single phase as single-family residential. The project
includes the development of ten (10) single-family lots, street improvements, wet and dry utility
infrastructure and landscaping. A single point of access will be provided from Pear Street.
`
7
III. DRAINAGE BASINS AND SUB-BASINS
MAJOR BASIN DESCRIPTION
The project site is located in the Canal Importation drainage basin. According to the City of
Fort Collins website (www.fcgov.com), this basin “spans nearly five square miles in west-central
Fort Collins. Three major irrigation canals traverse the basin from north to south and impact the
drainage in the basin. The canals can intercept runoff traveling through the basin and transport
it out of the basin, and they also spill runoff into the basin when their capacity is exceeded”.
SUB- BASIN DESCRIPTION
The project site generally conforms to the input parameters of SWMM subbasin 9 in the Canal
Importation drainage basin. SWMM subbasin 9 is 60% impervious. It generally drains to
Elizabeth Street and then east. The proposed site will be ±50% impervious. The runoff
ultimately drains into the Red Fox Meadows neighborhood natural area.
The project site is shown on FEMA Map Numbers 08069C0960F and 08069C0978G (refer to
Appendix A for FEMA Firmettes). Neither map shows the project impacted by an existing
floodplain/floodway. Refer to Appendix A for a copy of each Firmette and a copy of the City of
Fort Collins flood map.
`
8
IV. DRAINAGE DESIGN CRITERIA
REGULATIONS
This preliminary drainage design presented herein was prepared in accordance with the Fort
Collins Amendments to the Urban Drainage and Flood Control District Criteria Manual (i.e.,
Urban Drainage and Flood Control District Urban Storm Drainage Criteria Manuals Volumes 1,
2 and 3 [Manual]). Together, the requirements are referred to as the Fort Collins Stormwater
Criteria Manual [FCSCM].
DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
The project site is encumbered by several constraints; we developed a strategy for
implementing ‘The Four-Step Process’ for stormwater quality management to the maximum
extent practicable. Each step is listed below along with a brief narrative describing the
implementation strategy.
Step 1 - Employ runoff reduction practices
The developed runoff from the A, B and C series of basins flow through vegetated swales
before entering the on-site detention facility. The UDFCD defines a Grass Swale as “Densely
vegetated drainage way with low-pitched side slopes that collects and slowly conveys runoff.
The design of the longitudinal slope and cross-section size forces the flow to be slow and
shallow, thereby facilitating sedimentation while limiting erosion.” The proposed vegetated
swales will have low longitudinal slopes of ±0.50%. It is designed to convey 2-year storm event
runoff in a slow (i.e., <1 ft/sec) and shallow manner (i.e., normal depth <1 foot). This design
encourages settling and infiltration.
Of the newly added impervious area, ±0.97 acre is tributary to vegetated swales. Impervious
areas have been disconnected to the maximum extent practicable.
Step 2 - Implement BMPs that provide a Water Quality Capture Volume (WQCV)
The proposed BMPs do not provide WQCV because of site constraints. Refer to
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS FOR a list.
Step 3 - Stabilize drainageways
`
9
Planting within the vegetated swales will stabilize the drainage way and prevent erosion during
storm events exceeding the 2-year recurrence level.
Step 4 - Implement site specific and other source control BMPS
Site specific considerations such as material storage and other site operations are addressed
in the Stormwater Management Plan (SWMP).
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS
Several constraints drove the design presented here.
· The outfall from the proposed on-site detention pond is the flowline of West Elizabeth
Street, elevation ±5107.41.
· The developer wants to keep the existing house. This influences grading design.
· The closest storm drain system is located more than one-half mile east along Elizabeth
Street.
· The site is generally flat. The existing grades average 0.50% to 1.00%. Locally steeper
grades exist around the structures and existing irrigation lateral.
· The project site is bounded by existing development.
· The existing soils are unsuitable for infiltrating stormwater runoff. He boring logs show
clay underlain by claystone/sandstone and claystone. Considering the expansive
nature of the subgrade soils, the subsurface study recommended that residences be
founded on drilled piers. We did consider LID systems with underdrains that flow into
drywells but feel this is inconsistent with the findings of the subsurface study.
HYDROLOGICAL CRITERIA
For urban catchments that are not complex and are generally 160 acres or less in size, it is
acceptable that the design storm runoff be analyzed using the Rational Method. The Rational
Method is often used when only the peak flow rate or total volume of runoff is needed (e.g.,
storm sewer sizing or simple detention basin sizing). The Rational Method was used to
estimate the peak flow at each design point. Routing calculations (i.e., time attenuation) that
aggregate the basins draining to a specific design point are include in the Rational Method
calculations in Appendix B.
`
10
The Rational Method is based on the Rational Formula:
Q = CiA
Where:
Q = the maximum rate of runoff, cfs
C = a runoff coefficient that is the ratio between the runoff volume from an area and the
average rate of rainfall depth over a given duration for that area
i = average intensity of rainfall in inches per hour for a duration equal to the Time of
Concentration (Tc)
A = area, acres
The one-hour rainfall Intensity-Duration-Frequency tables for use with the Rational Method of
runoff analysis are provided in Table RA-7 and Table RA-8 (refer to Appendix B).
The 2-year and 100-year storm events serve as the basis for the drainage system design. The
2-year storm is considered the minor storm event. It has a fifty percent probability of
exceedance during any given year. The 100-year storm is considered the major storm event. It
has a one percent probability of exceedance during any given year.
The 2-year drainage system, at a minimum, must be designed to transport runoff from the 2-
year recurrence interval storm event with minimal disruption to the urban environment. The
100-year drainage system, as a minimum, must be designed to convey runoff from the 100-
year recurrence interval flood to minimize life hazards and health, damage to structures, and
interruption to traffic and services.
HYDRAULIC CRITERIA
Street Capacity Analysis
The street section generally drains from the crown, across the travel lane and into roadside
swales. The proposed street section is bounded by a 9” wide concrete ribbon along each side
of the asphalt.
Storm Drain and Swale Capacity Analysis
`
11
The storm drain system is limited to short culverts and swales that convey runoff underneath
driveways and along the internal streets. The hydraulic analysis will be completed using
Bentley FlowMaster. Refer to Appendix C for general swale calculations.
`
12
V. DRAINAGE FACILITY DESIGN
GENERAL CONCEPT
This preliminary design presents the design of a system for collecting and conveying
developed runoff from the proposed development at Elizabeth Street Farms on-site detention
pond.
The on-site developed runoff in each tributary begins as overland flow from the residential lot
areas (i.e., roof area, concrete hardscape and landscaping). This runoff then flows from the
lots to the swale sections along the backs and fronts of the lots. These swales drain into on-
site detention pond.
SPECIFIC DETAILS
On-Site Detention Pond
The proposed on-site detention pond was sized with the Modified FAA Method. The site
constraints limited the volume of the on-site detention pond to 11,578 ft3. The proposed
release rate from the detention pond is 0.85 cfs. This exceeds the calculated 2-year release
rate of ±0.40 cfs for basin EX5. There does not appear to be any remaining areas that can
develop in SWMM subbasin 9, except for this site, and the site is located at the far west end of
the basin. Considering this, the proposed increase in release rate will have a negligible effect
on the basin during the 100-year storm event. The pond will discharge to the flowline in West
Elizabeth Street.
A, B and C Basins
These basins comprise approximately 1.91 acres. The developed runoff from the roof areas
will flow overland through landscaping and into vegetated swales along the private internal
streets. Flow in the swales will drain into the on-site detention pond.
D Basin
This basin includes future West Elizabeth Street right-of-way and existing West Elizabeth
Street improvements. The runoff will continue to flow east along Elizabeth Street.
`
13
E Basin
This basin includes private street improvements and portions of two lots. It will drain to Pear
Street undetained. The proposed flows should have a negligible impact on the existing
drainage system in this area.
`
14
VI. EROSION AND SEDIMENT CONTROL MEASURES
A General Permit for Stormwater Discharge Associated with Construction Activities issued by
the Colorado Department of Public Health and Environment (CDPHE), Water Quality Control
Division (WQCD), will be acquired for the site. A Stormwater Management Plan (SWMP) will
be prepared and is presented under separate cover. It will identify the Best Management
Practices (BMPs) which, when implemented, will meet the requirements of the General Permit.
`
15
VII. CONCLUSIONS
COMPLIANCE WITH STANDARDS
The design presented in this preliminary drainage report for Elizabeth Street Farms has been
prepared in accordance with the design standards and guidelines presented in the Fort Collins
Stormwater Criteria Manual.
VARIANCES
No variances are being requested with the proposed improvements described herein.
DRAINAGE CONCEPT
The proposed Elizabeth Street Farms storm drainage improvements should provide adequate
protection for the developed site. The proposed drainage design for the site should not
negatively impact the existing downstream storm drainage system.
`
16
VII. REFERENCES
1. Fort Collins Stormwater Criteria Manual (Addendum to the Urban Storm Drainage
Criteria Manuals Volumes 1, 2 and 3), prepared by City of Fort Collins.
2. Urban Drainage and Flood Control District, Drainage Criteria Manual Volumes 1 and 2,
prepared by Wright-McLaughlin Engineers, dated June 2001 (revised April 2008), and
the Volume 3, prepared by Wright-McLaughlin Engineers, dated September 1992 and
revised November 2010.
PRELIMINARY DRAINAGE REPORT
Elizabeth Street Farms
Fort Collins, Colorado
Prepared for:
High Plains Builders, LLC
P.O. Box 2796
Gillette, Wyoming 82717
Phone: (307) 257-2352
Prepared by:
Galloway & Company, Inc.
3760 East 15th Street, Suite 202
Loveland, Colorado 80538
Phone: (970) 800-3300
Contact: Herman Feissner, PE
Original Preparation: November 2nd, 2016
1
TABLE OF CONTENTS
TABLE OF CONTENTS.......................................................................................................................... 1
I. CERTIFICATIONS .......................................................................................................................... 2
II. GENERAL LOCATION AND DESCRIPTION .................................................................................. 5
III. DRAINAGE BASINS AND SUB-BASINS ........................................................................................ 7
IV. DRAINAGE DESIGN CRITERIA ..................................................................................................... 7
V. DRAINAGE FACILITY DESIGN .................................................................................................... 12
VI. EROSION AND SEDIMENT CONTROL MEASURES ................................................................... 14
VII. CONCLUSIONS ............................................................................................................................ 15
VIII. REFERENCES .............................................................................................................................. 16
APPENDIX A - REFERENCE MATERIALS
VICINITY MAP
NRCS SOILS MAP
FEMA FIRMETTE
CITY OF FORT COLLINS FLOOD MAPPING
APPENDIX B - HYDROLOGY CALCULATIONS
EXISTING RUNOFF CALCULATIONS
PROPOSED RUNOFF CALCULATIONS
APPENDIX C - HYDRAULIC CALCULATIONS
SWALE CAPACITY CALCULATIONS
PRELIMINARY DETENTION POND SIZING
APPENDIX D – DRAINAGE MAPS
EXISTING CONDITION DRAINAGE MAP
DEVELOPED CONDITION DRAINAGE MAP
2
I. CERTIFICATIONS
CERTIFICATION OF ENGINEER
“I hereby certify that this report for the final drainage design of Elizabeth Street Farms was
prepared by me (or under my direct supervision) in accordance with the provisions of the Fort
Collins Stormwater Criteria Manual for the owners thereof.”
______________________________________
Herman Feissner, PE
Registered Professional Engineer
State Of Colorado No. 38066
For and on behalf of Galloway & Company, Inc.
CERTIFICATION OF OWNER
“High Plains Builders, LLC hereby certifies that the drainage facilities for the Elizabeth Street
Farms be constructed according to the design presented in this report. We understand that the
City of Fort Collins does not and will not assume liability for drainage facilities designed and/or
certified by our engineer. We also understand that the City of Fort Collins relies on the
representation of others to establish that drainage facilities are designed and constructed in
compliance with City of Fort Collins guidelines, standards, or specifications. Review by the City
of Fort Collins can therefore in no way limit or diminish any liability, which we or any other party
may have with respect to the design or construction of such facilities.”
____________________________________
High Plains Builders, LLC
Attest:
___________________________________
(Name of Responsible Party)
__________________________________
Notary Public
__________________________________
Authorized Signature
`
5
II. GENERAL LOCATION AND DESCRIPTION
LOCATION
Elizabeth Street Farms (hereafter referred to as “the site” or “project site”) is located at 2620
West Elizabeth Street. The site is bounded on north by single-family residential development
and Pear Street right-of-way (existing cul-de-sac); on the south by West Elizabeth Street; and
on the east and west by single-family residential. More specifically, the site is Tract A of West
Plum Street – a Planned Unit Development situate in the north half of Section 16, Township 7
north, Range 69 west of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State
of Colorado. Refer to Appendix A for a vicinity map.
DESCRIPTION OF PROPERTY
The project site consists of ±2.56 acres. It is largely vacant and undeveloped. Previously the
site was used for agriculture. An existing two-story brick house, which the developer plans to
restore, is located in the southeast region of the site. An existing irrigation lateral, which is still
in use, crosses the site from west to east. The existing grades average between 0.50% and
1.00%. Locally steeper grades exist along the irrigation lateral and around the existing house.
The existing runoff flows from northwest to southeast. An existing swale along the north edge
of West Elizabeth Street captures existing runoff and conveys it east.
There are no major drainage ways passing through the project site.
According to the USDA NRCS Web Soil Survey, ‘Altvan-Satanta loams, 0 to 3 percent slopes’
covers roughly one-third of the project site. The remaining two-thirds of the site is ‘Altvan-
Satanta loams, 3 to 9 percent slopes’. These soils are associated with Hydrologic Soil Group
(HSG) ‘B’. HSG ‘B’ soils have a moderate infiltration rate when thoroughly wet. These consist
chiefly of moderately deep or deep, moderately well drained or well drained soils that have
moderately fine texture to moderately coarse texture. These soils have a moderate rate of
water transmission. Refer to Appendix A for additional soils information.
Hollingsworth Associates, Inc. conducted a subsurface study the project site. The results of the
preliminary geotechnical investigation are summarized in Subsurface Study for Ten Lots West
Elizabeth Subdivision Fort Collins, Colorado (Job No.: 16-469 | Dated: October 12, 2016). The
`
6
subsurface conditions across the lots were quite uniform, as indicated by exploratory borings
B-1 through B-5, and consisted of 12 inches of topsoil, 2 to 11 feet of medium stiff to stiff
sandy clay, which occasionally graded into clayey sand, and 2 feet to 5 feet of firm to medium
hard, interlayered and integrated weathered claystone/sandstone overlaying hard to very hard
claystone for the depth drilled, 21 feet.
The site is situated east of the Pleasant Valley and Lake Canal. An operational west-to-east
irrigation lateral flows across the project site. According to the ditch rider, Ed Wendel, five
people currently receive water from the lateral.
Elizabeth Street Farms will develop in a single phase as single-family residential. The project
includes the development of ten (10) single-family lots, street improvements, wet and dry utility
infrastructure and landscaping. A single point of access will be provided from Pear Street.
`
7
III. DRAINAGE BASINS AND SUB-BASINS
MAJOR BASIN DESCRIPTION
The project site is located in the Canal Importation drainage basin. According to the City of
Fort Collins website (www.fcgov.com), this basin “spans nearly five square miles in west-central
Fort Collins. Three major irrigation canals traverse the basin from north to south and impact the
drainage in the basin. The canals can intercept runoff traveling through the basin and transport
it out of the basin, and they also spill runoff into the basin when their capacity is exceeded”.
SUB- BASIN DESCRIPTION
The project site generally conforms to the input parameters of SWMM subbasin 9 in the Canal
Importation drainage basin. SWMM subbasin 9 is 60% impervious. It generally drains to
Elizabeth Street and then east. The proposed site will be ±50% impervious. The runoff
ultimately drains into the Red Fox Meadows neighborhood natural area.
The project site is shown on FEMA Map Numbers 08069C0960F and 08069C0978G (refer to
Appendix A for FEMA Firmettes). Neither map shows the project impacted by an existing
floodplain/floodway. Refer to Appendix A for a copy of each Firmette and a copy of the City of
Fort Collins flood map.
`
8
IV. DRAINAGE DESIGN CRITERIA
REGULATIONS
This preliminary drainage design presented herein was prepared in accordance with the Fort
Collins Amendments to the Urban Drainage and Flood Control District Criteria Manual (i.e.,
Urban Drainage and Flood Control District Urban Storm Drainage Criteria Manuals Volumes 1,
2 and 3 [Manual]). Together, the requirements are referred to as the Fort Collins Stormwater
Criteria Manual [FCSCM].
DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
The project site is encumbered by several constraints; we developed a strategy for
implementing ‘The Four-Step Process’ for stormwater quality management to the maximum
extent practicable. Each step is listed below along with a brief narrative describing the
implementation strategy.
Step 1 - Employ runoff reduction practices
The developed runoff from the A, B and C series of basins flow through vegetated swales
before entering the on-site detention facility. The UDFCD defines a Grass Swale as “Densely
vegetated drainage way with low-pitched side slopes that collects and slowly conveys runoff.
The design of the longitudinal slope and cross-section size forces the flow to be slow and
shallow, thereby facilitating sedimentation while limiting erosion.” The proposed vegetated
swales will have low longitudinal slopes of ±0.50%. It is designed to convey 2-year storm event
runoff in a slow (i.e., <1 ft/sec) and shallow manner (i.e., normal depth <1 foot). This design
encourages settling and infiltration.
Of the newly added impervious area, ±0.97 acre is tributary to vegetated swales. Impervious
areas have been disconnected to the maximum extent practicable.
Step 2 - Implement BMPs that provide a Water Quality Capture Volume (WQCV)
The proposed BMPs do not provide WQCV because of site constraints. Refer to
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS FOR a list.
Step 3 - Stabilize drainageways
`
9
Planting within the vegetated swales will stabilize the drainage way and prevent erosion during
storm events exceeding the 2-year recurrence level.
Step 4 - Implement site specific and other source control BMPS
Site specific considerations such as material storage and other site operations are addressed
in the Stormwater Management Plan (SWMP).
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS
Several constraints drove the design presented here.
· The outfall from the proposed on-site detention pond is the flowline of West Elizabeth
Street, elevation ±5107.41.
· The developer wants to keep the existing house. This influences grading design.
· The closest storm drain system is located more than one-half mile east along Elizabeth
Street.
· The site is generally flat. The existing grades average 0.50% to 1.00%. Locally steeper
grades exist around the structures and existing irrigation lateral.
· The project site is bounded by existing development.
· The existing soils are unsuitable for infiltrating stormwater runoff. He boring logs show
clay underlain by claystone/sandstone and claystone. Considering the expansive
nature of the subgrade soils, the subsurface study recommended that residences be
founded on drilled piers. We did consider LID systems with underdrains that flow into
drywells but feel this is inconsistent with the findings of the subsurface study.
HYDROLOGICAL CRITERIA
For urban catchments that are not complex and are generally 160 acres or less in size, it is
acceptable that the design storm runoff be analyzed using the Rational Method. The Rational
Method is often used when only the peak flow rate or total volume of runoff is needed (e.g.,
storm sewer sizing or simple detention basin sizing). The Rational Method was used to
estimate the peak flow at each design point. Routing calculations (i.e., time attenuation) that
aggregate the basins draining to a specific design point are include in the Rational Method
calculations in Appendix B.
`
10
The Rational Method is based on the Rational Formula:
Q = CiA
Where:
Q = the maximum rate of runoff, cfs
C = a runoff coefficient that is the ratio between the runoff volume from an area and the
average rate of rainfall depth over a given duration for that area
i = average intensity of rainfall in inches per hour for a duration equal to the Time of
Concentration (Tc)
A = area, acres
The one-hour rainfall Intensity-Duration-Frequency tables for use with the Rational Method of
runoff analysis are provided in Table RA-7 and Table RA-8 (refer to Appendix B).
The 2-year and 100-year storm events serve as the basis for the drainage system design. The
2-year storm is considered the minor storm event. It has a fifty percent probability of
exceedance during any given year. The 100-year storm is considered the major storm event. It
has a one percent probability of exceedance during any given year.
The 2-year drainage system, at a minimum, must be designed to transport runoff from the 2-
year recurrence interval storm event with minimal disruption to the urban environment. The
100-year drainage system, as a minimum, must be designed to convey runoff from the 100-
year recurrence interval flood to minimize life hazards and health, damage to structures, and
interruption to traffic and services.
HYDRAULIC CRITERIA
Street Capacity Analysis
The street section generally drains from the crown, across the travel lane and into roadside
swales. The proposed street section is bounded by a 9” wide concrete ribbon along each side
of the asphalt.
Storm Drain and Swale Capacity Analysis
`
11
The storm drain system is limited to short culverts and swales that convey runoff underneath
driveways and along the internal streets. The hydraulic analysis will be completed using
Bentley FlowMaster. Refer to Appendix C for general swale calculations.
`
12
V. DRAINAGE FACILITY DESIGN
GENERAL CONCEPT
This preliminary design presents the design of a system for collecting and conveying
developed runoff from the proposed development at Elizabeth Street Farms on-site detention
pond.
The on-site developed runoff in each tributary begins as overland flow from the residential lot
areas (i.e., roof area, concrete hardscape and landscaping). This runoff then flows from the
lots to the swale sections along the backs and fronts of the lots. These swales drain into on-
site detention pond.
SPECIFIC DETAILS
On-Site Detention Pond
The proposed on-site detention pond was sized with the Modified FAA Method. The site
constraints limited the volume of the on-site detention pond to 11,578 ft3. The proposed
release rate from the detention pond is 0.85 cfs. This exceeds the calculated 2-year release
rate of ±0.40 cfs for basin EX5. There does not appear to be any remaining areas that can
develop in SWMM subbasin 9, except for this site, and the site is located at the far west end of
the basin. Considering this, the proposed increase in release rate will have a negligible effect
on the basin during the 100-year storm event. The pond will discharge to the flowline in West
Elizabeth Street.
A, B and C Basins
These basins comprise approximately 1.91 acres. The developed runoff from the roof areas
will flow overland through landscaping and into vegetated swales along the private internal
streets. Flow in the swales will drain into the on-site detention pond.
D Basin
This basin includes future West Elizabeth Street right-of-way and existing West Elizabeth
Street improvements. The runoff will continue to flow east along Elizabeth Street.
`
13
E Basin
This basin includes private street improvements and portions of two lots. It will drain to Pear
Street undetained. The proposed flows should have a negligible impact on the existing
drainage system in this area.
`
14
VI. EROSION AND SEDIMENT CONTROL MEASURES
A General Permit for Stormwater Discharge Associated with Construction Activities issued by
the Colorado Department of Public Health and Environment (CDPHE), Water Quality Control
Division (WQCD), will be acquired for the site. A Stormwater Management Plan (SWMP) will
be prepared and is presented under separate cover. It will identify the Best Management
Practices (BMPs) which, when implemented, will meet the requirements of the General Permit.
`
15
VII. CONCLUSIONS
COMPLIANCE WITH STANDARDS
The design presented in this preliminary drainage report for Elizabeth Street Farms has been
prepared in accordance with the design standards and guidelines presented in the Fort Collins
Stormwater Criteria Manual.
VARIANCES
No variances are being requested with the proposed improvements described herein.
DRAINAGE CONCEPT
The proposed Elizabeth Street Farms storm drainage improvements should provide adequate
protection for the developed site. The proposed drainage design for the site should not
negatively impact the existing downstream storm drainage system.
`
16
VII. REFERENCES
1. Fort Collins Stormwater Criteria Manual (Addendum to the Urban Storm Drainage
Criteria Manuals Volumes 1, 2 and 3), prepared by City of Fort Collins.
2. Urban Drainage and Flood Control District, Drainage Criteria Manual Volumes 1 and 2,
prepared by Wright-McLaughlin Engineers, dated June 2001 (revised April 2008), and
the Volume 3, prepared by Wright-McLaughlin Engineers, dated September 1992 and
revised November 2010.
PRELIMINARY DRAINAGE REPORT
Elizabeth Street Farms
Fort Collins, Colorado
Prepared for:
High Plains Builders, LLC
P.O. Box 2796
Gillette, Wyoming 82717
Phone: (307) 257-2352
Prepared by:
Galloway & Company, Inc.
3760 East 15th Street, Suite 202
Loveland, Colorado 80538
Phone: (970) 800-3300
Contact: Herman Feissner, PE
Original Preparation: November 2nd, 2016
1
TABLE OF CONTENTS
TABLE OF CONTENTS.......................................................................................................................... 1
I. CERTIFICATIONS .......................................................................................................................... 2
II. GENERAL LOCATION AND DESCRIPTION .................................................................................. 5
III. DRAINAGE BASINS AND SUB-BASINS ........................................................................................ 7
IV. DRAINAGE DESIGN CRITERIA ..................................................................................................... 7
V. DRAINAGE FACILITY DESIGN .................................................................................................... 12
VI. EROSION AND SEDIMENT CONTROL MEASURES ................................................................... 14
VII. CONCLUSIONS ............................................................................................................................ 15
VIII. REFERENCES .............................................................................................................................. 16
APPENDIX A - REFERENCE MATERIALS
VICINITY MAP
NRCS SOILS MAP
FEMA FIRMETTE
CITY OF FORT COLLINS FLOOD MAPPING
APPENDIX B - HYDROLOGY CALCULATIONS
EXISTING RUNOFF CALCULATIONS
PROPOSED RUNOFF CALCULATIONS
APPENDIX C - HYDRAULIC CALCULATIONS
SWALE CAPACITY CALCULATIONS
PRELIMINARY DETENTION POND SIZING
APPENDIX D – DRAINAGE MAPS
EXISTING CONDITION DRAINAGE MAP
DEVELOPED CONDITION DRAINAGE MAP
2
I. CERTIFICATIONS
CERTIFICATION OF ENGINEER
“I hereby certify that this report for the final drainage design of Elizabeth Street Farms was
prepared by me (or under my direct supervision) in accordance with the provisions of the Fort
Collins Stormwater Criteria Manual for the owners thereof.”
______________________________________
Herman Feissner, PE
Registered Professional Engineer
State Of Colorado No. 38066
For and on behalf of Galloway & Company, Inc.
CERTIFICATION OF OWNER
“High Plains Builders, LLC hereby certifies that the drainage facilities for the Elizabeth Street
Farms be constructed according to the design presented in this report. We understand that the
City of Fort Collins does not and will not assume liability for drainage facilities designed and/or
certified by our engineer. We also understand that the City of Fort Collins relies on the
representation of others to establish that drainage facilities are designed and constructed in
compliance with City of Fort Collins guidelines, standards, or specifications. Review by the City
of Fort Collins can therefore in no way limit or diminish any liability, which we or any other party
may have with respect to the design or construction of such facilities.”
____________________________________
High Plains Builders, LLC
Attest:
___________________________________
(Name of Responsible Party)
__________________________________
Notary Public
__________________________________
Authorized Signature
`
5
II. GENERAL LOCATION AND DESCRIPTION
LOCATION
Elizabeth Street Farms (hereafter referred to as “the site” or “project site”) is located at 2620
West Elizabeth Street. The site is bounded on north by single-family residential development
and Pear Street right-of-way (existing cul-de-sac); on the south by West Elizabeth Street; and
on the east and west by single-family residential. More specifically, the site is Tract A of West
Plum Street – a Planned Unit Development situate in the north half of Section 16, Township 7
north, Range 69 west of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State
of Colorado. Refer to Appendix A for a vicinity map.
DESCRIPTION OF PROPERTY
The project site consists of ±2.56 acres. It is largely vacant and undeveloped. Previously the
site was used for agriculture. An existing two-story brick house, which the developer plans to
restore, is located in the southeast region of the site. An existing irrigation lateral, which is still
in use, crosses the site from west to east. The existing grades average between 0.50% and
1.00%. Locally steeper grades exist along the irrigation lateral and around the existing house.
The existing runoff flows from northwest to southeast. An existing swale along the north edge
of West Elizabeth Street captures existing runoff and conveys it east.
There are no major drainage ways passing through the project site.
According to the USDA NRCS Web Soil Survey, ‘Altvan-Satanta loams, 0 to 3 percent slopes’
covers roughly one-third of the project site. The remaining two-thirds of the site is ‘Altvan-
Satanta loams, 3 to 9 percent slopes’. These soils are associated with Hydrologic Soil Group
(HSG) ‘B’. HSG ‘B’ soils have a moderate infiltration rate when thoroughly wet. These consist
chiefly of moderately deep or deep, moderately well drained or well drained soils that have
moderately fine texture to moderately coarse texture. These soils have a moderate rate of
water transmission. Refer to Appendix A for additional soils information.
Hollingsworth Associates, Inc. conducted a subsurface study the project site. The results of the
preliminary geotechnical investigation are summarized in Subsurface Study for Ten Lots West
Elizabeth Subdivision Fort Collins, Colorado (Job No.: 16-469 | Dated: October 12, 2016). The
`
6
subsurface conditions across the lots were quite uniform, as indicated by exploratory borings
B-1 through B-5, and consisted of 12 inches of topsoil, 2 to 11 feet of medium stiff to stiff
sandy clay, which occasionally graded into clayey sand, and 2 feet to 5 feet of firm to medium
hard, interlayered and integrated weathered claystone/sandstone overlaying hard to very hard
claystone for the depth drilled, 21 feet.
The site is situated east of the Pleasant Valley and Lake Canal. An operational west-to-east
irrigation lateral flows across the project site. According to the ditch rider, Ed Wendel, five
people currently receive water from the lateral.
Elizabeth Street Farms will develop in a single phase as single-family residential. The project
includes the development of ten (10) single-family lots, street improvements, wet and dry utility
infrastructure and landscaping. A single point of access will be provided from Pear Street.
`
7
III. DRAINAGE BASINS AND SUB-BASINS
MAJOR BASIN DESCRIPTION
The project site is located in the Canal Importation drainage basin. According to the City of
Fort Collins website (www.fcgov.com), this basin “spans nearly five square miles in west-central
Fort Collins. Three major irrigation canals traverse the basin from north to south and impact the
drainage in the basin. The canals can intercept runoff traveling through the basin and transport
it out of the basin, and they also spill runoff into the basin when their capacity is exceeded”.
SUB- BASIN DESCRIPTION
The project site generally conforms to the input parameters of SWMM subbasin 9 in the Canal
Importation drainage basin. SWMM subbasin 9 is 60% impervious. It generally drains to
Elizabeth Street and then east. The proposed site will be ±50% impervious. The runoff
ultimately drains into the Red Fox Meadows neighborhood natural area.
The project site is shown on FEMA Map Numbers 08069C0960F and 08069C0978G (refer to
Appendix A for FEMA Firmettes). Neither map shows the project impacted by an existing
floodplain/floodway. Refer to Appendix A for a copy of each Firmette and a copy of the City of
Fort Collins flood map.
`
8
IV. DRAINAGE DESIGN CRITERIA
REGULATIONS
This preliminary drainage design presented herein was prepared in accordance with the Fort
Collins Amendments to the Urban Drainage and Flood Control District Criteria Manual (i.e.,
Urban Drainage and Flood Control District Urban Storm Drainage Criteria Manuals Volumes 1,
2 and 3 [Manual]). Together, the requirements are referred to as the Fort Collins Stormwater
Criteria Manual [FCSCM].
DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
The project site is encumbered by several constraints; we developed a strategy for
implementing ‘The Four-Step Process’ for stormwater quality management to the maximum
extent practicable. Each step is listed below along with a brief narrative describing the
implementation strategy.
Step 1 - Employ runoff reduction practices
The developed runoff from the A, B and C series of basins flow through vegetated swales
before entering the on-site detention facility. The UDFCD defines a Grass Swale as “Densely
vegetated drainage way with low-pitched side slopes that collects and slowly conveys runoff.
The design of the longitudinal slope and cross-section size forces the flow to be slow and
shallow, thereby facilitating sedimentation while limiting erosion.” The proposed vegetated
swales will have low longitudinal slopes of ±0.50%. It is designed to convey 2-year storm event
runoff in a slow (i.e., <1 ft/sec) and shallow manner (i.e., normal depth <1 foot). This design
encourages settling and infiltration.
Of the newly added impervious area, ±0.97 acre is tributary to vegetated swales. Impervious
areas have been disconnected to the maximum extent practicable.
Step 2 - Implement BMPs that provide a Water Quality Capture Volume (WQCV)
The proposed BMPs do not provide WQCV because of site constraints. Refer to
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS FOR a list.
Step 3 - Stabilize drainageways
`
9
Planting within the vegetated swales will stabilize the drainage way and prevent erosion during
storm events exceeding the 2-year recurrence level.
Step 4 - Implement site specific and other source control BMPS
Site specific considerations such as material storage and other site operations are addressed
in the Stormwater Management Plan (SWMP).
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS
Several constraints drove the design presented here.
· The outfall from the proposed on-site detention pond is the flowline of West Elizabeth
Street, elevation ±5107.41.
· The developer wants to keep the existing house. This influences grading design.
· The closest storm drain system is located more than one-half mile east along Elizabeth
Street.
· The site is generally flat. The existing grades average 0.50% to 1.00%. Locally steeper
grades exist around the structures and existing irrigation lateral.
· The project site is bounded by existing development.
· The existing soils are unsuitable for infiltrating stormwater runoff. He boring logs show
clay underlain by claystone/sandstone and claystone. Considering the expansive
nature of the subgrade soils, the subsurface study recommended that residences be
founded on drilled piers. We did consider LID systems with underdrains that flow into
drywells but feel this is inconsistent with the findings of the subsurface study.
HYDROLOGICAL CRITERIA
For urban catchments that are not complex and are generally 160 acres or less in size, it is
acceptable that the design storm runoff be analyzed using the Rational Method. The Rational
Method is often used when only the peak flow rate or total volume of runoff is needed (e.g.,
storm sewer sizing or simple detention basin sizing). The Rational Method was used to
estimate the peak flow at each design point. Routing calculations (i.e., time attenuation) that
aggregate the basins draining to a specific design point are include in the Rational Method
calculations in Appendix B.
`
10
The Rational Method is based on the Rational Formula:
Q = CiA
Where:
Q = the maximum rate of runoff, cfs
C = a runoff coefficient that is the ratio between the runoff volume from an area and the
average rate of rainfall depth over a given duration for that area
i = average intensity of rainfall in inches per hour for a duration equal to the Time of
Concentration (Tc)
A = area, acres
The one-hour rainfall Intensity-Duration-Frequency tables for use with the Rational Method of
runoff analysis are provided in Table RA-7 and Table RA-8 (refer to Appendix B).
The 2-year and 100-year storm events serve as the basis for the drainage system design. The
2-year storm is considered the minor storm event. It has a fifty percent probability of
exceedance during any given year. The 100-year storm is considered the major storm event. It
has a one percent probability of exceedance during any given year.
The 2-year drainage system, at a minimum, must be designed to transport runoff from the 2-
year recurrence interval storm event with minimal disruption to the urban environment. The
100-year drainage system, as a minimum, must be designed to convey runoff from the 100-
year recurrence interval flood to minimize life hazards and health, damage to structures, and
interruption to traffic and services.
HYDRAULIC CRITERIA
Street Capacity Analysis
The street section generally drains from the crown, across the travel lane and into roadside
swales. The proposed street section is bounded by a 9” wide concrete ribbon along each side
of the asphalt.
Storm Drain and Swale Capacity Analysis
`
11
The storm drain system is limited to short culverts and swales that convey runoff underneath
driveways and along the internal streets. The hydraulic analysis will be completed using
Bentley FlowMaster. Refer to Appendix C for general swale calculations.
`
12
V. DRAINAGE FACILITY DESIGN
GENERAL CONCEPT
This preliminary design presents the design of a system for collecting and conveying
developed runoff from the proposed development at Elizabeth Street Farms on-site detention
pond.
The on-site developed runoff in each tributary begins as overland flow from the residential lot
areas (i.e., roof area, concrete hardscape and landscaping). This runoff then flows from the
lots to the swale sections along the backs and fronts of the lots. These swales drain into on-
site detention pond.
SPECIFIC DETAILS
On-Site Detention Pond
The proposed on-site detention pond was sized with the Modified FAA Method. The site
constraints limited the volume of the on-site detention pond to 11,578 ft3. The proposed
release rate from the detention pond is 0.85 cfs. This exceeds the calculated 2-year release
rate of ±0.40 cfs for basin EX5. There does not appear to be any remaining areas that can
develop in SWMM subbasin 9, except for this site, and the site is located at the far west end of
the basin. Considering this, the proposed increase in release rate will have a negligible effect
on the basin during the 100-year storm event. The pond will discharge to the flowline in West
Elizabeth Street.
A, B and C Basins
These basins comprise approximately 1.91 acres. The developed runoff from the roof areas
will flow overland through landscaping and into vegetated swales along the private internal
streets. Flow in the swales will drain into the on-site detention pond.
D Basin
This basin includes future West Elizabeth Street right-of-way and existing West Elizabeth
Street improvements. The runoff will continue to flow east along Elizabeth Street.
`
13
E Basin
This basin includes private street improvements and portions of two lots. It will drain to Pear
Street undetained. The proposed flows should have a negligible impact on the existing
drainage system in this area.
`
14
VI. EROSION AND SEDIMENT CONTROL MEASURES
A General Permit for Stormwater Discharge Associated with Construction Activities issued by
the Colorado Department of Public Health and Environment (CDPHE), Water Quality Control
Division (WQCD), will be acquired for the site. A Stormwater Management Plan (SWMP) will
be prepared and is presented under separate cover. It will identify the Best Management
Practices (BMPs) which, when implemented, will meet the requirements of the General Permit.
`
15
VII. CONCLUSIONS
COMPLIANCE WITH STANDARDS
The design presented in this preliminary drainage report for Elizabeth Street Farms has been
prepared in accordance with the design standards and guidelines presented in the Fort Collins
Stormwater Criteria Manual.
VARIANCES
No variances are being requested with the proposed improvements described herein.
DRAINAGE CONCEPT
The proposed Elizabeth Street Farms storm drainage improvements should provide adequate
protection for the developed site. The proposed drainage design for the site should not
negatively impact the existing downstream storm drainage system.
`
16
VII. REFERENCES
1. Fort Collins Stormwater Criteria Manual (Addendum to the Urban Storm Drainage
Criteria Manuals Volumes 1, 2 and 3), prepared by City of Fort Collins.
2. Urban Drainage and Flood Control District, Drainage Criteria Manual Volumes 1 and 2,
prepared by Wright-McLaughlin Engineers, dated June 2001 (revised April 2008), and
the Volume 3, prepared by Wright-McLaughlin Engineers, dated September 1992 and
revised November 2010.
`
APPENDIX A
REFERENCE MATERIAL
`
VICINITY MAP
Vicinity Map
Not to Scale
West Elizabeth Street
Rocky Road
Project Site
Overland Trail
Timber Lane
Pearl Street
Orchard Place
`
NRCS SOILS MAP
Hydrologic Soil Group—Larimer County Area, Colorado
(2620 West Elizabeth Street)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
10/25/2016
Page 1 of 4
4491570 4491580 4491590 4491600 4491610 4491620 4491630 4491640 4491650 4491660 4491670
4491570 4491580 4491590 4491600 4491610 4491620 4491630 4491640 4491650 4491660 4491670
489340 489350 489360 489370 489380 489390 489400 489410 489420 489430 489440 489450 489460 489470 489480 489490
489340 489350 489360 489370 489380 489390 489400 489410 489420 489430 489440 489450 489460 489470 489480 489490
40° 34' 32'' N
105° 7' 33'' W
40° 34' 32'' N
105° 7' 26'' W
40° 34' 29'' N
105° 7' 33'' W
40° 34' 29'' N
105° 7' 26'' W
N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 35 70 140 210
Feet
0 10 20 40 60
Meters
Map Scale: 1:765 if printed on A landscape (11" x 8.5") sheet.
Warning: Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at 1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil line
placement. The maps do not show the small areas of contrasting
soils that could have been shown at a more detailed scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL: http://websoilsurvey.nrcs.usda.gov
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more accurate
calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as of
the version date(s) listed below.
Soil Survey Area: Larimer County Area, Colorado
Hydrologic Soil Group
Hydrologic Soil Group— Summary by Map Unit — Larimer County Area, Colorado (CO644)
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
3 Altvan-Satanta loams, 0
to 3 percent slopes
B 0.8 31.6%
4 Altvan-Satanta loams, 3
to 9 percent slopes
B 1.7 68.4%
Totals for Area of Interest 2.6 100.0%
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive precipitation
from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly
wet. These consist mainly of deep, well drained to excessively drained sands or
gravelly sands. These soils have a high rate of water transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well drained
soils that have moderately fine texture to moderately coarse texture. These soils
have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of water
transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay layer
at or near the surface, and soils that are shallow over nearly impervious material.
These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in their
natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
Hydrologic Soil Group—Larimer County Area, Colorado 2620 West Elizabeth Street
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
10/25/2016
Page 3 of 4
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Hydrologic Soil Group—Larimer County Area, Colorado 2620 West Elizabeth Street
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
10/25/2016
Page 4 of 4
`
FEMA FIRMETTE
`
CITY OF FORT COLLINS FLOOD
MAPPING
1,128
188.0
2620 West Elizabeth Street
Fort Collins, Colorado
This map is a user generated static output from the City of Fort Collins FCMaps
Internet mapping site and is for reference only. Data layers that appear on this
map may or may not be accurate, current, or otherwise reliable.
City of Fort Collins - GIS
143.0
1:
WGS_1984_Web_Mercator_Auxiliary_Sphere
0 71.50 143.0 Feet
Notes
Legend
857
FEMA Floodplain
FEMA High Risk - Floodway
FEMA High Risk - 100 Year
FEMA Moderate Risk - 100 / 500 Year
FEMA Map Panel
City Floodplains
City High Risk - Floodway
City High Risk - 100 Year
City Moderate Risk - 100 Year
City Limits
`
APPENDIX B
HYDROLOGY CALCULATIONS
`
EXISTING RUNOFF CALCULATIONS
Subdivision: Elizabeth Street Farms Project Name: 2620 West Elizabeth Street
Location: CO, Fort Collins Project No.:
Calculated By: H. Feissner
Checked By: H. Feissner
INPUT User Input Date: 11/1/16
INPUT User Input Single-Family Alley-Loaded SF Duplex/AttachedMulti-Family
95% 5% 62.5% 70% 75% 80%
EX1 0.05 50 0.00 0 20 0.05 20 95 0.00 0 0.20
EX2 0.10 50 0.00 0 20 0.0998235 20 95 0.00 0 0.20
EX3 0.18 50 0.00 0 20 0.1802596 20 95 0.00 0 0.20
EX4 0.76 50 0.05 3 20 0.6972378 18 95 0.01 1 0.23
EX5 1.26 50 0.00 0 20 1.2228005 19 95 0.04 3 0.22
Σ Basins 2.35
COMPOSITE RUNOFF COEFFICIENTS
Area (ac) Area
Weighted
Gravel (e.g., driveway) Undeveloped, Heavy Soil: Flat <2%
Area Weighted Runoff
Coefficient Area (ac)
HPB01.01
Area
Weighted
C2
Basin ID Total Area (ac) Runoff
Coefficient Area (ac)
Existing Roof and Concrete (e.g., slab)
Runoff
Coefficient
Page 1 of 1 11/1/2016
Subdivision: Elizabeth Street Farms Project Name: 2620 West Elizabeth Street
Location: CO, Fort Collins Project No.:
Calculated By: H. Feissner
Checked By: H. Feissner
Date: 11/1/16
1 2 3 4 5 6 7 8 9 10 13 14 16 17 18 19 20 21 22 23 24
BASIN D.A. Hydrologic C2 C
5 C100
U.S. D.S. L Calculated S Ti | 2-Year T
i | 100-Year L Calculated Cv
VEL. Tt COMP. T
c | 2-Year COMP. Tc | 100-Year
TOTAL Urbanized Tc T
c | 2-Year Tc | 100-Year
ID (AC) Soils Group Cf=1.00 C
f=1.00 Cf
=1.25 (FT) Slope, % (%) (MIN) (MIN) (FT) Slope, % (FPS) (MIN) (MIN) (MIN) LENGTH(FT) (MIN) (MIN) (MIN)
EX1 0.05 C 0.20 0.20 0.25 12.26 10.94 10 13.20 2.00 2.3 2.1 0 0.00 15 0.0 0.0 2.3 2.1 10 10.1 5.0 5.0
EX2 0.10 C 0.20 0.20 0.25 9.14 7.84 50 2.60 2.00 8.7 8.2 0 0.00 15 0.0 0.0 8.7 8.2 50 10.3 8.7 8.2
EX3 0.18 C 0.20 0.20 0.25 12.19 10.83 10 13.60 2.00 2.2 2.1 0 0.00 15 0.0 0.0 2.2 2.1 10 10.1 5.0 5.0
EX4 0.76 C 0.23 0.23 0.29 11.30 7.84 304 1.14 2.00 27.1 25.3 0 0.00 15 0.0 0.0 27.1 25.3 304 11.7 27.1 25.3
EX5 1.26 C 0.22 0.22 0.28 12.56 9.14 342 1.00 2.00 30.4 28.5 0 0.00 15 0.0 0.0 30.4 28.5 342 11.9 30.4 28.5
NOTES:
Ti = (1.87*(1.1 - CC
f)*(L)^0.5)/((S)^0.33), S in % Cv
Tt=L/60V (Velocity From Fig. 501) 2.5
Velocity V=Cv*S^0.5, S in ft/ft 5
Tc Check = 10+L/180 7
For Urbanized basins a minimum Tc of 5.0 minutes is required. 10
For non-urbanized basins a minimum Tc of 10.0 minutes is required 15
20
FG Elevations
Paved Areas and Shallow Paved Swales
DATA
Type of Land Surface
Heavy Meadow
Tillage/Field
Short Pasture and Lawns
Nearly Bare Ground
Grassed Waterway
INITIAL/OVERLAND FINAL
(Ti)
TRAVEL TIME
(Tt) (URBANIZED BASINS)
STANDARD FORM SF-2
TIME OF CONCENTRATION
SUB-BASIN Tc CHECK
HPB01.01
Page 1 of 1 11/1/2016
Project Name: 2620 West Elizabeth Street
Subdivision: Elizabeth Street Farms Project No.: HPB01.01
Location: CO, Fort Collins Calculated By: H. Feissner
Design Storm: Checked By: H. Feissner
Date:
TRAVEL TIME
STREET
Design Point
Basin ID
Area (Ac)
Runoff Coeff. | C 2
Tc | 2-Year (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Tc (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Slope (%)
Street Flow (cfs)
Design Flow (cfs)
Slope (%)
Pipe Size (inches)
Length (ft)
Velocity (fps)
Tt (min)
REMARKS
EX1 EX1 0.05 0.20 5.0 0.01 2.86 0.0
EX2 EX2 0.10 0.20 8.7 0.02 2.39 0.0
EX3 EX3 0.18 0.20 5.0 0.04 2.86 0.1
EX4 EX4 0.76 0.23 27.1 0.18 1.36 0.2
EX5 EX5 1.26 0.22 30.4 0.28 1.27 0.4
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE
STANDARD FORM SF-3
STORM DRAINAGE SYSTEM DESIGN
(RATIONAL METHOD PROCEDURE)
2-Year
11/1/16
Page 1 of 1 11/1/2016
STANDARD FORM SF-3
STORM DRAINAGE SYSTEM DESIGN
(RATIONAL METHOD PROCEDURE)
Project Name: 2620 West Elizabeth Street
Subdivision: Elizabeth Street Farms Project No.: HPB01.01
Location: CO, Fort Collins Calculated By: H. Feissner
Design Storm: Checked By: H. Feissner
Date:
TRAVEL TIME
STREET
Design Point
Basin ID
Area (Ac)
Runoff Coeff. | C 100
Tc | 100-Year (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Tc (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Slope (%)
Street Flow (cfs)
Design Flow (cfs)
Slope (%)
Pipe Size (inches)
Length (ft)
Velocity (fps)
Tt (min)
REMARKS
EX1 EX1 0.05 0.25 5.0 0.01 9.98 0.1
EX2 EX2 0.10 0.25 8.2 0.02 8.52 0.2
EX3 EX3 0.18 0.25 5.0 0.05 9.98 0.4
EX4 EX4 0.76 0.29 25.3 0.22 4.93 1.1
EX5 EX5 1.26 0.28 28.5 0.35 4.59 1.6
100-Year
DIRECT RUNOFF TOTAL RUNOFF STREET
11/1/16
PIPE
Page 1 of 1 11/1/2016
`
PROPOSED RUNOFF
CALCULATIONS
41
Table RO-11
Rational Method Runoff Coefficients for Composite Analysis
Character of Surface Runoff Coefficient
Streets, Parking Lots,
Drives:
Asphalt 0.95
Concrete 0.95
Gravel 0.5
Roofs 0.95
Recycled Asphalt 0.8
Lawns, Sandy Soil:
Flat <2% 0.1
Average 2 to 7% 0.15
Steep >7% 0.2
Lawns, Heavy Soil:
Flat <2% 0.2
Average 2 to 7% 0.25
Steep >7% 0.35
(4) A new Section 2.9 is added, to read as follows:
2.9 Composite Runoff Coefficient
Drainage sub-basins are frequently composed of land that has multiple surfaces or zoning
classifications. In such cases a composite runoff coefficient must be calculated for any
given drainage sub-basin.
The composite runoff coefficient is obtained using the following formula:
( )
t
n
i
i i
A
C A
C
∑
= = 1
*
(RO-8)
Where: C = Composite Runoff Coefficient
Ci = Runoff Coefficient for Specific Area (Ai)
Ai = Area of Surface with Runoff Coefficient of Ci, acres or feet2
n = Number of different surfaces to be considered
At = Total Area over which C is applicable, acres or feet2
(5) A new Section 2.10 is added, to read as follows:
42
2.10 Runoff Coefficient Adjustment for Infrequent Storms
The runoff coefficients provided in tables RO-10 and RO-11 are appropriate for use with
the 2-year storm event. For storms with higher intensities, an adjustment of the runoff
coefficient is required due to the lessening amount of infiltration, depression retention,
evapo-transpiration and other losses that have a proportionally smaller effect on storm
runoff. This adjustment is applied to the composite runoff coefficient.
These frequency adjustment factors are found in Table RO-12.
Table RO-12
Rational Method Runoff Coefficients for Composite Analysis
Storm Return Period
(years)
Frequency Factor
Cf
2 to 10
11 to 25
26 to 50
51 to 100
1.00
1.10
1.20
1.25
Note: The product of C times Cf cannot exceed the value of 1, in the cases where it does a value of
1 must be used
(6) Section 3.1 is deleted in its entirety.
(7) Section 3.2 is deleted in its entirety.
(8) Section 3.3 is deleted in its entirety.
(9) A new Section 4.3 is added, to read as follows:
4.3 Computer Modeling Practices
(a) For circumstances requiring computer modeling, the design storm hydrographs must
be determined using the Stormwater Management Model (SWMM). Basin and
conveyance element parameters must be computed based on the physical characteristics
of the site.
(b) Refer to the SWMM Users’ Manual for appropriate modeling methodology, practices
and development. The Users’ Manual can be found on the Environmental Protection
Agency (EPA) website (http://www.epa.gov/ednnrmrl/models/swmm/index.htm).
(c) It is the responsibility of the design engineer to verify that all of the models used in
the design meet all current City criteria and regulations.
4.3.1 Surface Storage, Resistance Factors, and Infiltration
Table RO-13 provides values for surface storage for pervious and impervious surfaces
and the infiltration rates to be used with SWMM. Table RO-13 also lists the appropriate
infiltration decay rate, zero detention depth and resistance factors, or Manning’s “n”
values, for pervious and impervious surfaces to be used for SWMM modeling in the city
of Fort Collins.
Subdivision: Elizabeth Street Farms Project Name: 2620 West Elizabeth Street
Location: CO, Fort Collins Project No.:
Calculated By: H. Feissner
Checked By: H. Feissner
INPUT User Input Date: 11/1/16
INPUT User Input Single-Family Alley-Loaded SF Duplex/AttachedMulti-Family
95% 5% 65% 70% 75% 80%
A1 0.23 95 0.00 0 20 0.05 4 60 0.18 47 0.52
A2 0.32 95 0.10 30 20 0.04 2 60 0.18 34 0.66
B1 0.24 95 0.04 18 20 0.06 5 60 0.13 33 0.56
B2 0.46 95 0.13 27 20 0.07 3 60 0.26 34 0.64
C1 0.30 95 0.00 0 20 0.07 5 60 0.22 45 0.50
D1 0.49 95 0.29 57 20 0.19 8 60 0.00 0 0.65
E1 0.17 95 0.11 62 20 0.00 0 60 0.06 20 0.82
DP01 0.37 95 0.05 13 20 0.32 17 60 0.00 0 0.30
Σ Basins 2.56
ΣDetention Pond Tributary Area 1.91
Area
Weighted
C2
Basin ID Total Area (ac) Runoff
Coefficient Area (ac)
Lots (e.g., Single-Family, Alley Loaded)
Runoff
Coefficient
COMPOSITE RUNOFF COEFFICIENTS
Area (ac) Area
Weighted
Asphalt + Concrete Walks Lawns, Heavy Soil: Flat <2%
Area Weighted Runoff
Coefficient Area (ac)
HPB01.01
Page 1 of 1 11/1/2016
Subdivision: Elizabeth Street Farms Project Name: 2620 West Elizabeth Street
Location: CO, Fort Collins Project No.:
Calculated By: H. Feissner
Checked By: H. Feissner
Date: 11/1/16
1 2 3 4 5 6 7 8 9 10 13 14 16 17 18 19 20 21 22 23 24
BASIN D.A. Hydrologic C2 C
5 C100
L S Ti | 2-Year T
i | 100-Year L Calculated Cv
VEL. Tt COMP. T
c | 2-Year COMP. Tc | 100-Year
TOTAL Urbanized Tc T
c | 2-Year Tc | 100-Year
ID (AC) Soils Group Cf=1.00 C
f=1.00 Cf
=1.25 (FT) (%) (MIN) (MIN) (FT) Slope, % (FPS) (MIN) (MIN) (MIN) LENGTH(FT) (MIN) (MIN) (MIN)
A1 0.23 C 0.52 0.52 0.65 49 2.00 6.1 4.7 164 0.48 15 1.0 2.6 8.7 7.3 213 11.2 8.7 7.3
A2 0.32 C 0.66 0.66 0.83 53 2.00 4.7 2.9 161 0.50 15 1.1 2.5 7.2 5.5 214 11.2 7.2 5.5
B1 0.24 C 0.56 0.56 0.70 41 2.00 5.2 3.8 170 0.51 15 1.1 2.6 7.8 6.5 211 11.2 7.8 6.5
B2 0.46 C 0.64 0.64 0.80 56 2.00 5.1 3.3 223 0.50 15 1.1 3.5 8.6 6.8 279 11.6 8.6 6.8
C1 0.30 C 0.50 0.50 0.62 49 2.00 6.2 4.9 224 0.50 15 1.1 3.5 9.8 8.5 273 11.5 9.8 8.5
D1 0.49 C 0.65 0.65 0.81 30 2.00 3.6 2.3 382 0.87 20 1.9 3.4 7.1 5.7 412 12.3 7.1 5.7
E1 0.17 C 0.82 0.82 1.00 44 2.00 2.7 1.0 76 1.46 20 2.4 0.5 3.3 1.5 120 10.7 5.0 5.0
DP01 0.37 C 0.30 0.30 0.38 13 8.50 2.6 2.4 228 0.51 15 1.1 3.5 6.2 5.9 241 11.3 6.2 5.9
NOTES:
Ti = (1.87*(1.1 - CC
f)*(L)^0.5)/((S)^0.33), S in % Cv
Tt=L/60V (Velocity From Fig. 501) 2.5
Velocity V=Cv*S^0.5, S in ft/ft 5
Tc Check = 10+L/180 7
For Urbanized basins a minimum Tc of 5.0 minutes is required. 10
For non-urbanized basins a minimum Tc of 10.0 minutes is required 15
20
STANDARD FORM SF-2
TIME OF CONCENTRATION
SUB-BASIN Tc CHECK
HPB01.01
INITIAL/OVERLAND FINAL
(Ti)
TRAVEL TIME
(Tt) (URBANIZED BASINS)
Paved Areas and Shallow Paved Swales
DATA
Type of Land Surface
Heavy Meadow
Tillage/Field
Short Pasture and Lawns
Nearly Bare Ground
Grassed Waterway
Page 1 of 1 11/1/2016
36
RAINFALL INTENSITY-DURATION-FREQUENCY CURVE
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
0.00 10.00 20.00 30.00 40.00 50.00 60.00
STORM DURATION (minutes)
RAINFALL INTENSITY (inches/hour)
2-Year Storm 10-Year Storm 100-Year Storm
Figure RA-16 City of Fort Collins Rainfall Intensity-Duration-Frequency Curves
(13) Section 5.0 is deleted in its entirety.
(14) Section 6.0 is deleted in its entirety.
(15) Section 7.0 is deleted in its entirety.
(16) Section 7.1 is deleted in its entirety.
(17) Section 7.2 is deleted in its entirety.
(18) Section 7.3 is deleted in its entirety.
(19) Section 8.0 is deleted in its entirety.
(20) Table RA-1 is deleted in its entirety.
Project Name: 2620 West Elizabeth Street
Subdivision: Elizabeth Street Farms Project No.: HPB01.01
Location: CO, Fort Collins Calculated By: H. Feissner
Design Storm: Checked By: H. Feissner
Date:
TRAVEL TIME
STREET
Design Point
Basin ID
Area (Ac)
Runoff Coeff. | C 2
Tc | 2-Year (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Tc (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Slope (%)
Street Flow (cfs)
Design Flow (cfs)
Slope (%)
Pipe Size (inches)
Length (ft)
Velocity (fps)
Tt (min)
REMARKS
A1 A1 0.23 0.52 8.7 0.12 2.39 0.3 Lot runoff drains to a vegetated swale
A2 A2 0.32 0.66 7.2 0.21 2.55 0.5
B1 B1 0.24 0.56 7.8 0.13 2.49 0.3 Lot runoff drains to a vegetated swale
B2 B2 0.46 0.64 8.6 0.29 2.40 0.7 Lot runoff drains to a vegetated swale
C1 C1 0.30 0.50 9.8 0.15 2.28 0.3 Lot runoff drains to a vegetated swale
D1 D1 0.49 0.65 7.1 0.32 2.57 0.8 Right-of-Way runoff drains to West Elizabeth Street C&G
E1 E1 0.17 0.82 5.0 0.14 2.86 0.4 Lot and private street runoff drains to Pear Street
DP01 DP01 0.37 0.30 6.2 0.11 2.69 0.3 On-Site Detention Pond | Outfalls to flowline of West Elizabeth Street
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE
STANDARD FORM SF-3
STORM DRAINAGE SYSTEM DESIGN
(RATIONAL METHOD PROCEDURE)
2-Year
11/1/16
Page 1 of 1 11/1/2016
STANDARD FORM SF-3
STORM DRAINAGE SYSTEM DESIGN
(RATIONAL METHOD PROCEDURE)
Project Name: 2620 West Elizabeth Street
Subdivision: Elizabeth Street Farms Project No.: HPB01.01
Location: CO, Fort Collins Calculated By: H. Feissner
Design Storm: Checked By: H. Feissner
Date:
TRAVEL TIME
STREET
Design Point
Basin ID
Area (Ac)
Runoff Coeff. | C 100
Tc | 100-Year (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Tc (min)
C*A (Ac)
I (in/hr)
Q (cfs)
Slope (%)
Street Flow (cfs)
Design Flow (cfs)
Slope (%)
Pipe Size (inches)
Length (ft)
Velocity (fps)
Tt (min)
REMARKS
A1 A1 0.23 0.65 7.3 0.15 8.85 1.3 Lot runoff drains to a vegetated swale
A2 A2 0.32 0.83 5.5 0.27 9.74 2.6 Lot and private street runoff drains to a vegetated swale
B1 B1 0.24 0.70 6.5 0.16 9.24 1.5 Lot runoff drains to a vegetated swale
B2 B2 0.46 0.80 6.8 0.37 9.08 3.3 Lot runoff drains to a vegetated swale
C1 C1 0.30 0.62 8.5 0.18 8.41 1.6 Lot runoff drains to a vegetated swale
D1 D1 0.49 0.81 5.7 0.40 9.60 3.8 Right-of-Way runoff drains to West Elizabeth Street C&G
E1 E1 0.17 1.00 5.0 0.17 9.98 1.7 Lot and private street runoff drains to Pear Street
DP01 DP01 0.37 0.38 5.9 0.14 9.51 1.3 On-Site Detention Pond | Outfalls to flowline of West Elizabeth Street
PIPE
100-Year
DIRECT RUNOFF TOTAL RUNOFF STREET
11/1/16
Page 1 of 1 11/1/2016
`
APPENDIX C
HYDRAULIC CALCULATIONS
`
SWALE CAPACITY CALCULATIONS
Project Description
Friction Method Manning Formula
Solve For Discharge
Input Data
Roughness Coefficient 0.035
Channel Slope 0.005 ft/ft
Normal Depth 1.00 ft
Left Side Slope 4.00 ft/ft (H:V)
Right Side Slope 3.00 ft/ft (H:V)
Normal Depth (ft) Discharge (ft³/s) Velocity (ft/s) Flow Area (ft²) Wetted Perimeter (ft) Top Width (ft)
0.00 0.00 0.00 0.00
0.10 0.01 0.40 0.04 0.73 0.70
0.20 0.09 0.63 0.14 1.46 1.40
0.30 0.26 0.83 0.32 2.19 2.10
0.40 0.56 1.00 0.56 2.91 2.80
0.50 1.02 1.16 0.88 3.64 3.50
0.60 1.65 1.31 1.26 4.37 4.20
0.70 2.49 1.45 1.72 5.10 4.90
0.80 3.55 1.59 2.24 5.83 5.60
0.90 4.87 1.72 2.84 6.56 6.30
1.00 6.45 1.84 3.50 7.29 7.00
Vegetated Swale
11/1/2016 9:49:52 AM
Bentley Systems, Inc. Haestad Methods Solution BentleCyenter FlowMaster V8i (SELECTseries 1) [08.11.01.03]
27 Siemons Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 Page 1 of 1
`
PRELIMINARY DETENTION POND
SIZING
Project:
Basin ID:
Design Information (Input): Design Information (Input):
Catchment Drainage Imperviousness Ia = 51 percent Catchment Drainage Imperviousness I
a = 51 percent
Catchment Drainage Area A = 1.91 acres Catchment Drainage Area A = 1.91 acres
Predevelopment NRCS Soil Group Type = D A, B, C, or D Predevelopment NRCS Soil Group Type = D A, B, C, or D
Return Period for Detention Control T = 2 years (2, 5, 10, 25, 50, or 100) Return Period for Detention Control T = 100 years (2, 5, 10, 25, 50, or 100)
Time of Concentration of Watershed Tc = 10 minutes Time of Concentration of Watershed Tc = 10 minutes
Allowable Unit Release Rate q = 0.38 cfs/acre Allowable Unit Release Rate q = 0.45 cfs/acre
One-hour Precipitation P1 = 0.82 inches One-hour Precipitation P
1 = 2.86 inches
Design Rainfall IDF Formula i = C1* P1
/(C2+Tc
)^C3 Design Rainfall IDF Formula i = C1
* P1/(C2
+Tc)^C3
Coefficient One C1 = 28.50 Coefficient One C
1 = 28.50
Coefficient Two C2 = 10 Coefficient Two C
2 = 10
Coefficient Three C3 = 0.789 Coefficient Three C
3 = 0.789
Determination of Average Outflow from the Basin (Calculated): Determination of Average Outflow from the Basin (Calculated):
Runoff Coefficient C = 0.34 Runoff Coefficient C = 0.61
Inflow Peak Runoff Qp-in = 1.43 cfs Inflow Peak Runoff Qp-in = 8.93 cfs
Allowable Peak Outflow Rate Qp-out = 0.73 cfs Allowable Peak Outflow Rate Qp-out = 0.85 cfs
Mod. FAA Minor Storage Volume = 611 cubic feet Mod. FAA Major Storage Volume = 11,513 cubic feet
Mod. FAA Minor Storage Volume = 0.014 acre-ft Mod. FAA Major Storage Volume = 0.264 acre-ft
20 <- Enter Rainfall Duration Incremental Increase Value Here (e.g. 5 for 5-Minutes)
Rainfall Rainfall Inflow Adjustment Average Outflow Storage Rainfall Rainfall Inflow Adjustment Average Outflow Storage
Duration Intensity Volume Factor Outflow Volume Volume Duration Intensity Volume Factor Outflow Volume Volume
minutes inches / hr acre-feet "m" cfs acre-feet acre-feet minutes inches / hr acre-feet "m" cfs acre-feet acre-feet
(input) (output) (output) (output) (output) (output) (output) (input) (output) (output) (output) (output) (output) (output)
5 2.76 0.012 1.00 0.73 0.005 0.007 5 9.62 0.077 1.00 0.85 0.006 0.071
25 1.41 0.032 0.70 0.51 0.018 0.014 25 4.93 0.198 0.70 0.59 0.020 0.177
45 0.99 0.040 0.61 0.45 0.028 0.012 45 3.45 0.249 0.61 0.52 0.032 0.217
65 0.77 0.045 0.58 0.42 0.038 0.007 65 2.70 0.282 0.58 0.49 0.044 0.238
85 0.64 0.049 0.56 0.41 0.048 0.001 85 2.24 0.306 0.56 0.47 0.056 0.250
105 0.55 0.052 0.55 0.40 0.058 -0.006 105 1.93 0.325 0.55 0.47 0.067 0.258
125 0.49 0.054 0.54 0.39 0.068 -0.013 125 1.70 0.341 0.54 0.46 0.079 0.262
145 0.44 0.057 0.53 0.39 0.078 -0.021 145 1.52 0.355 0.53 0.45 0.091 0.264
165 0.40 0.059 0.53 0.39 0.088 -0.029 165 1.39 0.367 0.53 0.45 0.102 0.264
185 0.36 0.060 0.53 0.38 0.098 -0.038 185 1.27 0.378 0.53 0.45 0.114 0.263
205 0.34 0.062 0.52 0.38 0.108 -0.046 205 1.18 0.387 0.52 0.45 0.126 0.261
225 0.31 0.063 0.52 0.38 0.118 -0.055 225 1.10 0.396 0.52 0.44 0.138 0.259
245 0.30 0.065 0.52 0.38 0.128 -0.063 245 1.03 0.405 0.52 0.44 0.149 0.255
265 0.28 0.066 0.52 0.38 0.138 -0.072 265 0.97 0.412 0.52 0.44 0.161 0.251
285 0.26 0.067 0.52 0.38 0.148 -0.081 285 0.92 0.420 0.52 0.44 0.173 0.247
305 0.25 0.068 0.52 0.38 0.158 -0.090 305 0.87 0.426 0.52 0.44 0.184 0.242
325 0.24 0.069 0.52 0.38 0.168 -0.099 325 0.83 0.433 0.52 0.44 0.196 0.237
345 0.23 0.070 0.51 0.38 0.178 -0.108 345 0.79 0.439 0.51 0.44 0.208 0.231
365 0.22 0.071 0.51 0.37 0.188 -0.117 365 0.76 0.445 0.51 0.44 0.220 0.225
385 0.21 0.072 0.51 0.37 0.198 -0.127 385 0.73 0.450 0.51 0.44 0.231 0.219
405 0.20 0.073 0.51 0.37 0.209 -0.136 405 0.70 0.455 0.51 0.44 0.243 0.213
425 0.19 0.074 0.51 0.37 0.219 -0.145 425 0.68 0.461 0.51 0.43 0.255 0.206
445 0.19 0.074 0.51 0.37 0.229 -0.154 445 0.65 0.465 0.51 0.43 0.266 0.199
465 0.18 0.075 0.51 0.37 0.239 -0.164 465 0.63 0.470 0.51 0.43 0.278 0.192
485 0.17 0.076 0.51 0.37 0.249 -0.173 485 0.61 0.475 0.51 0.43 0.290 0.185
505 0.17 0.077 0.51 0.37 0.259 -0.182 505 0.59 0.479 0.51 0.43 0.301 0.178
Project:
Basin ID:
UDFCD DETENTION BASIN VOLUME ESTIMATING WORKBOOK Version 2.35, Released January 2015
DETENTION VOLUME BY THE MODIFIED FAA METHOD
Elizabeth Street Farms
DP01
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 200 400 600 800 1000 1200 1400
Volume (acre-feet)
Duration (Minutes)
Inflow and Outflow Volumes vs. Rainfall Duration
Minor Storm Inflow Volume Minor Storm Outflow Volume Minor Storm Storage Volume Major Storm Inflow Volume Major Storm Outflow Volume Major Storm Storage Volume
HPB01.01_UD-Detention_v2.35.xls, Modified FAA 11/1/2016, 10:05 AM
Project Number: HPB01.01
Project Location: Fort Collins, Colorado Date: 11/1/2016
Calculations By: H. Feissner
Pond Description: DP01
User Input Cell: Blue Text
Design Point: A1
Design Storm: WQCV
Required Volume: 0 ft3 5107.60 ft
Design Storm: 10-year
Required Volume: 0 ft3 5107.60 ft
Design Storm: 100-year
Required Volume: 11513 ft3 5109.59 ft
Contour
Elevation
Contour
Area Depth
Incremental
Volume
Cummulative
Volume
Incremental
Volume
Cummulative
Volume
Cummulative
Volume
ft ft2 ft ft3 ft3 ft3 ft3 ac-ft
5107.60 0 0 0 0 0 0 0.000
5107.80 164 0.20 16 16 11 11 0.000
5108.00 1248 0.20 141 158 124 136 0.003
5108.20 3337 0.20 459 616 442 577 0.013
5108.40 5585 0.20 892 1508 883 1460 0.034
5108.60 6793 0.20 1238 2746 1236 2696 0.062
5108.80 7522 0.20 1431 4178 1431 4127 0.095
5109.00 8156 0.20 1568 5745 1567 5694 0.131
5109.20 9107 0.20 1726 7472 1725 7419 0.170
5109.40 10345 0.20 1945 9417 1944 9363 0.215
5109.60 11823 0.20 2217 11634 2215 11578 0.266
Detention Pond Stage-Storage Calculations
Average End Area Method:
Required Volume Water Surface Elevation
Conic Volume Method:
100-year
5108
5108
5109
5109
5110
5110
0 2000 4000 6000 8000 10000 12000 14000
Contour Elevation, ft
Cummulative Volume, ft3
Stage - Storage | Site
`
APPENDIX D
DRAINAGE MAPS
525 0.16 0.077 0.51 0.37 0.269 -0.192 525 0.57 0.483 0.51 0.43 0.313 0.170
545 0.16 0.078 0.51 0.37 0.279 -0.201 545 0.56 0.487 0.51 0.43 0.325 0.162
565 0.16 0.079 0.51 0.37 0.289 -0.210 565 0.54 0.491 0.51 0.43 0.337 0.155
585 0.15 0.079 0.51 0.37 0.299 -0.220 585 0.53 0.495 0.51 0.43 0.348 0.147
605 0.15 0.080 0.51 0.37 0.309 -0.229 605 0.51 0.499 0.51 0.43 0.360 0.139
625 0.14 0.080 0.51 0.37 0.319 -0.239 625 0.50 0.502 0.51 0.43 0.372 0.131
645 0.14 0.081 0.51 0.37 0.329 -0.248 645 0.49 0.506 0.51 0.43 0.383 0.123
665 0.14 0.081 0.51 0.37 0.339 -0.258 665 0.48 0.509 0.51 0.43 0.395 0.114
685 0.13 0.082 0.51 0.37 0.349 -0.267 685 0.47 0.513 0.51 0.43 0.407 0.106
705 0.13 0.082 0.51 0.37 0.359 -0.277 705 0.46 0.516 0.51 0.43 0.419 0.098
725 0.13 0.083 0.51 0.37 0.369 -0.286 725 0.45 0.519 0.51 0.43 0.430 0.089
745 0.13 0.084 0.51 0.37 0.379 -0.296 745 0.44 0.523 0.51 0.43 0.442 0.081
765 0.12 0.084 0.51 0.37 0.389 -0.305 765 0.43 0.526 0.51 0.43 0.454 0.072
785 0.12 0.084 0.51 0.37 0.399 -0.315 785 0.42 0.529 0.51 0.43 0.465 0.063
805 0.12 0.085 0.51 0.37 0.410 -0.325 805 0.41 0.532 0.51 0.43 0.477 0.054
825 0.12 0.085 0.51 0.37 0.420 -0.334 825 0.40 0.534 0.51 0.43 0.489 0.046
845 0.11 0.086 0.51 0.37 0.430 -0.344 845 0.40 0.537 0.51 0.43 0.500 0.037
865 0.11 0.086 0.51 0.37 0.440 -0.353 865 0.39 0.540 0.51 0.43 0.512 0.028
885 0.11 0.087 0.51 0.37 0.450 -0.363 885 0.38 0.543 0.51 0.43 0.524 0.019
905 0.11 0.087 0.51 0.37 0.460 -0.373 905 0.38 0.545 0.51 0.43 0.536 0.010
925 0.11 0.088 0.51 0.37 0.470 -0.382 925 0.37 0.548 0.51 0.43 0.547 0.001
945 0.10 0.088 0.51 0.37 0.480 -0.392 945 0.36 0.551 0.51 0.43 0.559 -0.008
965 0.10 0.088 0.51 0.37 0.490 -0.402 965 0.36 0.553 0.51 0.43 0.571 -0.018
985 0.10 0.089 0.51 0.37 0.500 -0.411 985 0.35 0.556 0.51 0.43 0.582 -0.027
1005 0.10 0.089 0.50 0.37 0.510 -0.421 1005 0.35 0.558 0.50 0.43 0.594 -0.036
1025 0.10 0.090 0.50 0.37 0.520 -0.431 1025 0.34 0.560 0.50 0.43 0.606 -0.045
1045 0.10 0.090 0.50 0.37 0.530 -0.440 1045 0.34 0.563 0.50 0.43 0.618 -0.055
1065 0.09 0.090 0.50 0.37 0.540 -0.450 1065 0.33 0.565 0.50 0.43 0.629 -0.064
1085 0.09 0.091 0.50 0.37 0.550 -0.460 1085 0.33 0.568 0.50 0.43 0.641 -0.073
1105 0.09 0.091 0.50 0.37 0.560 -0.469 1105 0.32 0.570 0.50 0.43 0.653 -0.083
1125 0.09 0.091 0.50 0.37 0.570 -0.479 1125 0.32 0.572 0.50 0.43 0.664 -0.092
1145 0.09 0.092 0.50 0.37 0.580 -0.489 1145 0.31 0.574 0.50 0.43 0.676 -0.102
1165 0.09 0.092 0.50 0.37 0.590 -0.498 1165 0.31 0.576 0.50 0.43 0.688 -0.111
1185 0.09 0.092 0.50 0.37 0.600 -0.508 1185 0.30 0.579 0.50 0.43 0.700 -0.121
1205 0.09 0.093 0.50 0.37 0.611 -0.518 1205 0.30 0.581 0.50 0.43 0.711 -0.131
Mod. FAA Minor Storage Volume (cubic ft.) = 611 Mod. FAA Major Storage Volume (cubic ft.) = 11,513
Mod. FAA Minor Storage Volume (acre-ft.) = 0.0140 Mod. FAA Major Storage Volume (acre-ft.) = 0.2643
Determination of MAJOR Detention Volume Using Modified FAA Method
(For catchments less than 160 acres only. For larger catchments, use hydrograph routing method)
(NOTE: for catchments larger than 90 acres, CUHP hydrograph and routing are recommended)
UDFCD DETENTION BASIN VOLUME ESTIMATING WORKBOOK Version 2.35, Released January 2015
Determination of MINOR Detention Volume Using Modified FAA Method
DETENTION VOLUME BY THE MODIFIED FAA METHOD
Elizabeth Street Farms
DP01
HPB01.01_UD-Detention_v2.35.xls, Modified FAA 11/1/2016, 10:05 AM
Survey Area Data: Version 10, Sep 22, 2015
Soil map units are labeled (as space allows) for map scales 1:50,000
or larger.
Date(s) aerial images were photographed: Apr 22, 2011—Apr 28,
2011
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor shifting
of map unit boundaries may be evident.
Hydrologic Soil Group—Larimer County Area, Colorado
(2620 West Elizabeth Street)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
10/25/2016
Page 2 of 4