HomeMy WebLinkAboutPLATTE RIVER POWER AUTHORITY CAMPUS - PDP - PDP170040 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT1501 Academy Ct. Ste. 203 Fort Collins, Colorado 80524 (970) 530-4044
Preliminary Drainage Report
Platte River Power
Authority HQ Campus
Fort Collins, CO
Prepared for:
Platte River Power Authority
2000 East Horsetooth Road
Fort Collins, CO 80525
November 14, 2017
Drainage Report
Platte River Power Authority HQ Campus
Fort Collins, CO
i U16014-DrainageReport-Preliminary.doc
November 14, 2017
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Platte River Power Authority HQ Campus
Fort Collins, Colorado
Project Number: U16014
Dear Staff:
United Civil Design Group, LLC. is pleased to submit this Preliminary Drainage Report for the
Platte River Power Authority Headquarters Campus site in Fort Collins, Colorado. In general,
this report serves to document the stormwater impacts associated with the proposed
redevelopment of the existing campus.
We understand that review by the City of Fort Collins is to assure general compliance with
standardized criteria contained in the Fort Collins Stormwater Criteria Manual (FCSCM). This
report was prepared in compliance with technical criteria set forth in both the Urban Drainage
and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual and the Fort Collins
Stormwater Criteria Manual.
If you should have any questions or comments as you review this report, please feel free to
contact us at your convenience.
Sincerely,
United Civil Design Group
Kevin Brazelton, PE
Engineering Manager
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TABLE OF CONTENTS
PAGE
I. General Location and Description ........................................................................................... 1
A. Location ............................................................................................................................ 1
B. Description of Property ................................................................................................... 1
C. Description of Development............................................................................................ 2
II. Drainage Basins and Sub-Basins .............................................................................................. 3
A. Major Basin Description .................................................................................................. 3
B. Sub-Basin Description ...................................................................................................... 3
III. Drainage Design Criteria ......................................................................................................... 3
A. Regulations ...................................................................................................................... 3
B. Directly Connected Impervious Area (DCIA) ................................................................... 3
C. Development Criteria Reference and Constraints .......................................................... 4
D. Hydrological Criteria ........................................................................................................ 5
E. Hydraulic Criteria ............................................................................................................. 6
F. Floodplain Regulations .................................................................................................... 6
G. Modifications of Criteria .................................................................................................. 6
IV. Drainage Facility Design .......................................................................................................... 6
A. General Concept .............................................................................................................. 6
B. Specific Details ................................................................................................................. 9
V. Sediment/Erosion Control ..................................................................................................... 15
VI. Conclusions ........................................................................................................................... 15
A. Compliance with Standards ........................................................................................... 15
B. Drainage Concept .......................................................................................................... 15
C. Stormwater Quality ....................................................................................................... 15
VII. References ......................................................................................................................... 16
APPENDICES
APPENDIX A – Hydrology Computations
APPENDIX B – Detention and Water Quality Calculations
APPENDIX C – Hydraulic Computations
C.1 – Storm Pipe Calculations
C.2 – Inlet Sizing Calculations (Reserved for Final Submittal)
C.3 – Swale and Sidewalk Chase Calculations (Reserved for Final Submittal)
C.3 – Riprap Sizing Calculations (Reserved for Final Submittal)
APPENDIX D – Referenced Materials
APPENDIX E – Drainage Exhibits
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VICINITY MAP
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I. General Location and Description
A. Location
The Platte River Power Authority Headquarters Campus (referred herein as ”the
site” or “the Campus”) is a parcel of land situated in the Southeast Quarter of
Section 30, Township 7 North, Range 68 West of the 6th
Principal Meridian to
the City of Fort Collins, Larimer County, Colorado. More specifically, the site is
located at the northwest corner of Horsetooth Road and Timberline Road and is
bounded by East Horsetooth Road to the south, Timberline Road to the east,
existing commercial property and Danfield Court to the north, and Union Pacific
Railroad right-of-way to the west.
B. Description of Property
The campus has an existing platted area of 16.7-acres and has acted as the
headquarters for Platte River Power Authority since it was first constructed in
the late 1970s.
The following is a summary of the existing conditions:
Ground Cover - The site currently exists as commercial property and is
currently covered with various buildings, pavements and open areas.
Grades – The majority of the site slopes towards one of three (3) drainage
outfalls. In general, the slopes within each outfall basin range between 0 to
5 percent, with some landscaped berm areas reaching near 20%.
Soil Type - According to the United States Department of Agriculture (USDA)
Natural Resources Conservation Service (NRCS) Soil Survey, the site consists
of Nunn clay loam (Hydrologic Soil Group C).
Utilities – The campus includes an extensive network of existing utilities
including mains and services for water, sanitary sewer, natural gas, electric,
telephone and data lines. These mains and services are located within the
property limits of the campus along with mains located within the rights-of-
ways for Horsetooth Road, Timberline Road and Danfield Court.
Detention Ponds and Storm Sewer – The campus currently has three (3)
drainage outfalls available to accept stormwater flows from the site. These
include the existing wet pond which currently acts as an irrigation and
stormwater retention facility for the campus, an existing 12-inch storm
sewer located at the east side of the campus along Timberline Road, and the
curb and gutter at the Danfield Court cul-de-sac. There are also two (2)
other internal detention areas that drain to the 12-inch storm sewer outfall.
The campus also has limited internal storm drains varying in size from 4-inch
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to 15-inch. Of note, there is an existing storm inlet and drain located on the
north side of Horsetooth Road that collects stormwater from the roadway
and conveys it into the existing wet pond on the south side of the property.
This existing system will require to be maintained and/or replaced with the
construction of the proposed project.
Drainageways – No known externally owned drainageways currently exist
within the proposed development limits.
Irrigation - City records indicate that there is an existing irrigation pipe
located along the southern property boundary, running from west to east.
Only portions of this irrigation system have been found or identified with
utility locates or site survey but from the information available it is believed
that this system consists of a 20-inch concrete pipe that is located just north
of the existing curb and gutter along Horsetooth Road. The existing onsite
wet pond also serves as an irrigation system and is supplied by an existing
irrigation well located on the campus.
Floodplains – The campus is not located within either a City or FEMA
floodplain.
C. Description of Development
The planned campus redevelopment is planned to be constructed phases.
Ultimately, the campus redevelopment will consist of the following
improvements:
• The demolition of most of the existing buildings and much of the existing
pavements throughout the campus.
• The construction of seven (7) proposed commercial buildings including
the main headquarters building and various other support buildings.
• The construction of an internal roadway network with associated parking
areas.
• The construction of utilities in support of the campus including sanitary,
water and storm drainage.
• The construction of detention and water quality facilities located
throughout the site with the main detention area located in the
northeast portion of the site.
• The reconfiguration and relining of the existing wet pond.
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II. Drainage Basins and Sub-Basins
A. Major Basin Description
The site is located within the Foothills drainage basin within the City of Fort
Collins Master Drainage Plan. Stormwater from this basin adjacent to the site is
collected within streets and storm sewer systems and ultimately discharged into
the Fossil Creek Reservoir Inlet Ditch.
B. Sub-Basin Description
Historically, the property has been utilized as the main headquarters for Platte
River Power Authority. The campus has grown over the decades but is currently
largely developed containing a variety of existing buildings, utility infrastructure
systems and open spaces. Stormwater from the campus is collected in one of
three (3) basins that either convey water after detention to public infrastructure
or retain the stormwater onsite. The proposed project will maintain the historic
drainage patterns to the extent feasible, but will eliminate the stormwater
retention aspect of the site and reroute that stormwater to the main detention
facility to be provided on the site.
III. Drainage Design Criteria
A. Regulations
The design criteria for this study are directly from the City of Fort Collins Storm
Drainage Design Criteria and Construction Standards Manual and the Urban
Storm Drainage Criteria Manuals Volumes 2, and 3 (referred to herein as
USDCM).
B. Directly Connected Impervious Area (DCIA)
With the adoption of the USDCM, the City has also adopted the “Four Step
Process” that is recommended in Volume 3 of the USDCM in selecting structural
BMPs for the redeveloping urban areas. The following portions of this summary
describe each step and how it has been utilized for this project:
Step 1 – Employ Runoff Reduction Practices
The objective of this step is to reduce runoff peaks and volumes and to employ
the technique of “minimizing directly connected impervious areas” (MDCIA).
This project accomplishes this by:
• Routing the roof and pavement flows through bioswales, underground
detention treatment/infiltration facilities and vegetated swales to increase
time of concentration, promote infiltration and provide water quality.
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Step 2 – Provide Water Quality Capture Volume (WQCV)
The objective of providing WQCV is to reduce the sediment load and other
pollutants that exit the site. For this project WQCV is provided within
underground treatment facilities and a campus waterquality pond.
Step 3 – Stabilize Drainageways
The site is not adjacent or near a major drainageway; however, this project will
pay stormwater development and stormwater utility fees which the City uses, in
part, to maintain the stability of the City drainageway systems.
Step 4 – Consider Need for Site Specific and Source Control BMPs
Site specific and source control BMPs are generally considered for large
industrial and commercial sites. The redevelopment of the existing campus will
include multiple site specific and source controls, including:
• Covered storage areas for vehicles and materials
• A warehouse for materials storage
• A vehicular maintenance building with appropriate internal waste collection
systems
• Dedicated maintenance personnel providing landscape maintenance and
snow and ice management
C. Development Criteria Reference and Constraints
The campus has most recently been included within the Drainage Study entitled,
“Platte River Power Authority Maintenance Facility”, prepared by RBD
Engineering Consultants Inc., and dated March 1984. With the proposed
complete redevelopment of the site and the age of the previous studies, this
report relies very little on previous drainage information and studies prepared
for the site.
Four (4) drainage constraints for the campus have been identified and include
the following:
Existing Storm Outfall
The existing 12-inch storm sewer outfall for the site conveys stormwater north
down Timberline Road. Utilizing invert and hydraulic grade line information
provided within the construction plans entitled, “Collindale Industrial Park Storm
Sewer Extension”, prepared by Meline & Irelan Consulting Engineers and dated
February 1995, the existing system has been analyzed to determine the
maximum allowable release rate from the campus to this existing 12-inch
outfall. Refer to Appendix C for the hydraulic analysis of the existing storm drain
and Appendix D for the information used to create the hydraulic model.
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Utilizing the mentioned information, it was determined that the release rate
from the main detention area will be limited to 3.15 cfs.
Danfield Court Outfall
Storm sewer does not exist at the west end of Danfield Court; therefore, any
drainage outfall at the north property line will be required to tie into the curb
and gutter of the street or construct offsite storm sewer. The current design
does not include offsite storm sewer and the elevation of the existing curb and
gutter in relation to site grades limits the feasibility of providing a significant
amount of detention or water quality. To limit the impact of these constraints,
the design proposes to maintain the existing parking lot with its’ current parking
lot detention system. Per information provided by City personnel, maintaining
the existing parking lot and detention system will be acceptable with the
condition that no new impervious area is added to this existing outfall and only
minor improvements are made to the existing parking area.
Horsetooth Road Drainage
Approximately 1-acre of Horsetooth Road drains to an existing inlet located
immediately west of the south entrance to the existing site. Drainage collected
via the existing inlet is conveyed to the existing wet pond and retained onsite.
The proposed design will alter the design of this existing system but will
required to collect, treat, and convey the Horsetooth Road runoff through the
proposed redeveloped campus.
Existing Offsite Storm Sewer
The existing property north of the main campus (formerly the “LSI property”)
has existing storm sewer along the west and south perimeters of that property.
Documentation of the design of these storm sewers is not available; therefore, it
has been assumed that the offsite system has only been designed for the
existing property and impervious areas that currently drain to the existing storm
inlets. The proposed design will be required to limit the area draining to the
offsite storm sewer system and ensure that existing flowrates to the system are
not exceeded.
D. Hydrological Criteria
In accordance with the Fort Collins policy, a minor and major storm for the Fort
Collins area is identified as the 2-year and 100-year storms, respectively. A
major storm for the Fort Collins area has a recurrence interval of 100 years and
has a peak intensity of 9.95 in/hr.
These storms have been used as a basis for planning and system design. The
peak flow rates for design points have been calculated based on the Rational
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Method as described in the USDCM with storm duration set equal to the time of
concentration for each sub-basin. This method was used to analyze the
developed runoff from the 2-year (minor) and the 100-year (major) storm
events. The Rational Method is widely accepted for drainage design involving
small drainage areas (less than 160 acres) and short time of concentrations. The
Rational Method is ideal for storm sewer sizing and small detention pond sizing
(for tributary areas no larger than 90 to 160 acres). Runoff coefficients were
assumed based on impervious area and soil type and are given in Appendices.
E. Hydraulic Criteria
The redeveloped campus will convey runoff to the proposed pond(s) via swales,
gutters, concrete pans, and pipes. The FCSCM and USDCM were referenced for
all hydraulic calculations. In addition, the following computer programs were
utilized:
• Hydraflow Storm Sewers Extension for AutoCAD Civil 3D
• Hydraflow Express Extension for AutoCAD Civil3D
• UD-Inlet by UDFCD
F. Floodplain Regulations
The campus is not located within either a City or FEMA floodplain.
G. Modifications of Criteria
No modifications to the City drainage criteria or codes are being requested.
IV. Drainage Facility Design
A. General Concept
The project includes the redevelopment of a vast majority of the existing
campus including the headquarters (HQ) building, support buildings and
supporting roadway, parking and utility infrastructure. In general, the site is
being graded to maintain existing drainage patterns, where feasible, and will
collect stormwater runoff via swales, pans and storm sewer and direct them to
the proposed detention and water quality pond located at the east side of the
site. The pond will discharge to the existing 12-inch storm sewer located along
the west edge of Timberline Road.
All referenced tables, charts, formulas, etc. are included in the Appendices. The
area, time of concentration, and runoff of each proposed sub-basin is
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summarized in Appendix A. The project site was divided into multiple
developed sub-basins as explained further below:
Basin A
Basin A consists of Basins A1-A13 which are designed to be collected via a
series of swales, pans and storm drains that will discharge to the proposed
detention pond at the east side of the property. Basin A will contain the
majority of the proposed improvements and will ultimately consist of
rooftop, sidewalk, roadway, and landscaped areas. Specifics to each sub-
basin within Basin A include the following:
Basin A1 consists of approximately 3.5 acres and includes the proposed
detention pond and portions of the proposed paving improvements around
the HQ building. Approximately 8,500 square feet (sf) of future building is
also included within the hydrology and detention pond calculations. A pond
outlet and emergency overflow will be provided at the northeast end of the
pond that will both be directed towards Timberline Road. Refer to Appendix
A for additional information.
Basins A2-A5 consist of approximately 2.3 acres and includes a portion of the
proposed HQ and the employee parking areas for the project north of HQ.
These basins are collected with proposed storm sewer and conveyed to the
proposed detention pond in Basin A1.
Basins A6-A11 consist of approximately 3.0 acres and includes the
maintenance yard facility north of HQ and the employee parking areas.
These basins are collected with proposed storm sewer and conveyed to the
proposed detention pond in Basin A1.
Basin A12 consists of approximately 1.5 acres and includes minimal
proposed improvements with this project. The area is planned for future
expansion of an unknown type. For storm sewer calculations the basin has
been assumed to have a percent imperviousness of 90%. Due to the
unknown nature of the future improvements, Basin A12 will be required to
provide detention, water quality and LID improvements either within Basin
A12 or within Basin A1 with the future improvements. Refer to Appendix A
for additional information.
Basin A13 consists of approximately 0.25 acres and includes the southern
portion of the HQ building. The basin will be collected with a proposed roof
drain system and conveyed to the detention pond in Basin A1.
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Basin B
Basin B includes Basins B1-B2 which consist of approximately 1.9 acres south
of HQ and will include the proposed visitor parking area and landscaping.
The basins are designed to be collected via a bioswale and storm drain that
will discharge to the proposed detention pond in Basin A1. Approximately
11,000 (sf) of future paving is also included within the hydrology and
detention pond calculations to accommodate for future visitor parking on
the west end of Basin B2. Refer to Appendix A for additional information.
Basin C
Basin C includes Basin C1 which consists of approximately 1.2 acres south HQ
and primarily includes the proposed wet pond and landscaping. Detention
will be provided for Basin C1 over the permanent water surface of the
proposed wet pond. A pond outlet and emergency overflow will be provided
at the east end of the pond that will both be directed towards Basin A1.
Offsite Basins
The site includes five (5) existing offsite basins that are being maintained
with the proposed redevelopment. Specifics to each offsite basin include
the following:
Basin OS1 consists of approximately 0.7 acres of onsite property that drains
offsite towards the north and east ultimately draining to Timberline Road.
The area of this basin has increased from historic conditions but due to the
removal of pavement area within the basin the percent-imperviousness has
been reduced. Ultimately, the runoff generated from this basin has
increased slightly with the proposed conditions (approximately 0.5 cfs in the
100-year event); however, the additional flow is not anticipated to
negatively impact the street capacity of Timberline Road or downstream
infrastructure.
Basin OS2 consists of approximately 1.2 acres of Horsetooth Road and onsite
property that drain to an existing inlet located just west of the Horsetooth
Road entrance into the property. Drainage from this inlet is currently
conveyed into the existing wet pond located south of HQ. The
redevelopment is raising the permanent water surface of the wet pond by
approximately 3-feet; therefore, Basin OS2 will be collected with a proposed
storm drain system and routed around the proposed wet pond to the
detention pond in Basin A1 where flows from the basin will be treated and
detained prior to being released from the pond.
Basin OS3 consists of approximately 0.4 acres of onsite property that drains
offsite to the adjacent commercial property east and north of the
redevelopment (formerly the “LSI property”). This basin was originally
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owned by, and was designed to drain towards, the offsite property but has
been recently purchased by Platte River Power Authority. In the existing
conditions Basin OS3 includes approximately 2.2 acres or property. The
proposed redevelopment alters the drainage patterns to minimize the size of
Basin OS3; and therefore, the amount of drainage that flows offsite. Much
of the existing basin is now included within the onsite Basins A7-A11. Refer
to the Drainage Plans and Appendix A for additional information.
Basin OS4 consists of approximately 1.3 acres of onsite property that drains
to Danfield Court. The basin currently exists as a parking lot that includes a
small amount of parking lot detention prior to being released to Danfield
Court. This existing parking lot is remaining largely unmodified with the
proposed redevelopment; therefore, per communications with City
personnel, no drainage improvements are being made to the basin.
Basins OS5 consists of approximately 0.5 acres of landscaped area that
historically drains west offsite and onto Union Pacific Railroad property. The
proposed redevelopment is proposing to maintain the existing drainage
patterns, size and percent imperviousness of this basin.
B. Specific Details
Detention
The proposed redevelopment includes two (2) extended detention ponds for
the proposed improvements. These ponds are located within Basins A1 and
C1 and are shown within this report as Pond A and Pond C.
Pond A includes detention and waterquality for all of Basins A, B and OS2.
The release rate from the pond is limited by the existing 12-inch storm sewer
outfall for the site which conveys stormwater north down Timberline Road.
As previously mentioned, the release rate from Pond A was determined to
be 3.15 cfs by analyzing the existing 12-inch storm sewer system. Refer to
Appendices C and D for additional information. Also, note that neither
detention or water quality are provided for the future conditions of basin
A12. Due to the unknown nature of the future improvements, Basin A12 will
be required to provide detention, water quality and LID improvements
either within Basin A12 or within Basin A1 with the future improvements.
Refer to Appendix A for additional information. A pond outlet structure and
emergency overflow will be provided at the northeast end of the pond that
will both be directed towards Timberline Road.
Pond C includes only Basin C1 and will provide detention over the
permanent water surface of the proposed wet pond. The pond is designed
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to require less than 6-inches of ponding from the permanent water surface
to the 100-year water surface. A pond outlet and emergency overflow will
be provided at the east end of the pond that will both be directed towards
Basin A1.
A summary of both detention ponds are as follows:
Pond
Designation
Detention
Volume
Required
(ac-ft)
Detention
Volume
Provided
(ac-ft)
100-year
WSEL
Pond
Release
Rate
(cfs)
WQCV
Required
(ac-ft)
WQCV
Provided
(ac-ft)
Water
Quality
WSEL
Top of
Pond
Elevation
Pond A 2.77 2.88 4952.00 3.15 0.26 0.30 4949.60 4953.00
Pond C 0.32 0.35 4955.50 0.50 0.02 0.02 4955.05 4956.50
Stormwater Quality
Several long-term stormwater quality measures have been selected for the
site that will provide treatment of stormwater prior to it being discharged
from the site. For this project the following measures have been included:
Grass Swales - The soft bottom/overbank areas of the swales allow for
infiltration to occur as well as including a flow path across a vegetated
surface slowing the conveyance rate of storm water runoff while also
removing sediment and other pollutants. These have been utilized to
convey and treat stormwater prior to entering the storm sewer systems or
Pond A.
Grass Open Areas/Buffers – Grass open areas and/or grass buffers allow
for infiltration to occur as well as including a flow path across a vegetated
surface slowing the conveyance rate of storm water runoff while also
removing sediment and other pollutants. These have been utilized to
convey and treat stormwater prior to entering the storm sewer systems or
Ponds A and C.
Bioswales – Bioswales remove sediment and pollution from stormwater
runoff via infiltration through an engineered sand media. This slows the
conveyance rate of storm water runoff, reduces runoff and provides a flow
path across a vegetated surface. These have been utilized to convey and
treat stormwater prior to entering the storm sewer system and Pond A.
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utilized to collect, store and treat storm water runoff prior to it entering
Pond A.
Wet Pond – A wet pond provides water quality by acting as a permanent
sedimentation basin which collects and stores the runoff prior to
discharging to the pond outlet structure. This removes sediment from the
flows entirely which settle to the bottom of the pond. The existing onsite
wet pond is being redeveloped as part of this project and will collect and
treat only those areas that drain immediately to the wet pond itself.
Extended Detention Basin – Extended detention basins, in conjunction
with the use of a waterquality plate, collect, store and slowly release minor
storm events. This allows runoff to pond, infiltrate into the grass bottom
of the pond, and settle out particulates prior to being discharged from the
site. An extended detention basin has been selected as a highly effective
last treatment option prior to discharging runoff from the site.
Low Impact Development (LID)
In December of 2015, Fort Collins City Council adopted the revised Low
Impact Development (LID) policy and criteria which requires developments
within City limits to meet certain enhanced stormwater treatment
requirements in addition to more standard treatment techniques. The
proposed development will be required to meet the newly adopted LID
criteria which requires the following:
• Treat no less than 75% of any newly added impervious area using one or
a combination of LID techniques.
• Treat no less than 50% of any newly added impervious area using one or
a combination of LID techniques when at least 25% of any newly added
pavement is provided with permeable pavement.
The following measures are being implemented with this proposed
development:
Underground Detention – Basins A3-A11 (with 144,320 sf of proposed
impervious area) will be routed through an underground detention system
located north of HQ. The underground detention system will have an
overall volume of approximately 3,900 cubic feet which exceeds the
minimum design requirements for the area. Refer to Appendix B for
additional information.
Bioswales – Basins B1-B2 (with 28,300 sf of proposed impervious area) will
be routed through a bioswale (with a surface area of 3,035 sf). Basin B-2
will include future parking area of approximately 11,000 sf. Once the
future parking area is constructed, the bioswale will also require extension
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to provide LID treatment of the future parking lot expansion. With future
construction, the bioswale within Basins B1 and B2 will treat approximately
39,260 sf of impervious area.
Wet Pond – Basin C1 (with 5,730 sf of proposed impervious area) will drain
directly into the wet pond within the basin. The wet pond will act as a
permanent sedimentation basin and will remove sediment and pollution
from storm water runoff prior to releasing it downstream.
Future LID Treatment – At the end of construction, Basin A12 will be left as
open space but is planned for future expansion. With the future
development of the basin, it is estimated that a minimum of 17,500 sf of
future impervious area will require to be treated to maintain a total
treatment for the campus of 75%.
In total, the development is proposing to collect and treat 206,809 sf (75%)
of the overall 277,324 sf of impervious area developed (including both
proposed and future developments) with this project. In addition, the site
is also being designed with other various open landscaped areas and
vegetated swales and buffers to provide additional water quality in
addition to the areas being treated with LID techniques.
Standard Operating Procedures (SOPs)
In order for physical stormwater Best Management Practices (BMPs) to be
effective, proper maintenance is essential. Maintenance includes both
routinely scheduled activities, as well as non-routine repairs that may be
required after large storms, or as a result of other unforeseen problems.
Standard Operating Procedures should clearly identify BMP maintenance
responsibility. BMP maintenance is typically the responsibility of the entity
owning the BMP.
Identifying who is responsible for maintenance of BMPs and ensuring that an
adequate budget is allocated for maintenance is critical to the long-term
success of BMPs. Maintenance responsibility may be assigned either publicly
or privately. For this project, the privately owned BMPs including vegetative
swales and bioswales, underground detention, and the extended detention
pond are to be maintained by the property owner.
Storm Sewers
Multiple storm sewer systems are being proposed with the redevelopment
of the campus that include storm sewer mains, roof drains and underdrains.
All storm sewer on the property is expected to be private. The system that
collects and conveys the public storm water from Horsetooth Road will be
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included within a drainage easement. Due to the complexities of the
proposed site and constraints of the existing site, it is currently anticipated
that all storm sewer mains and roof drains will be sized to accommodate the
100-year storm event. Further sizing and hydraulic information will be
provided during Final Compliance design.
Wet Pond
As previously mentioned, the existing wet pond on the site is also being
redeveloped/reconfigured with the project. As part of the reconfiguration,
the following design elements are being provided:
• A 12-foot depth is being provided in the pond to allow for increased
water quality and to allow for the use of a geothermal mechanical
system for HQ. Related to these elements, the elevation difference
between the finished floor of HQ and the permanent water surface of
the pond has been minimized to accommodate a deeper pond while
minimizing the impacts to existing groundwater. The existing pond sits
approximately 6-feet below the existing building. The current design
has set the proposed permanent lake water surface 3-feet below the
proposed finished floor.
• The amount of surface drainage conveyed to the pond is being
minimized to limit the amount of storage required above the
permanent water surface due to storm runoff.
• The pond is being provided with an emergency overflow and outlet.
Currently the pond does not have an emergency overflow or any
means of draining the lake other than through irrigation. The
proposed design has located both an emergency overflow and outlet
structure on the east side of the lake.
Phasing
As mentioned previously, the redevelopment of the campus will be
accomplished in phases due to the requirement that the existing
Engineering/Operations (E/O) and Headquarters (HQ) buildings remain fully
operational during the construction of the proposed HQ building. Only when
the proposed HQ is fully operational and in use will the existing E/O and HQ
buildings be demolished. Due to this, it is currently expected that two major
phases of construction will be required. The initial phase of construction will
construct much of the proposed drainage infrastructure apart from the
southern portion of Pond A as this is where the existing HQ building is
located. The final phase of construction will construct the remainder of
Pond A. Through initial communication with City personnel, it is currently
anticipated that the Stormwater Department will issue a full CO for the
proposed HQ building with an escrow provision as required.
Drainage Report
Platte River Power Authority HQ Campus
Fort Collins, CO
14 U16014-DrainageReport-Preliminary.doc
Drainage Summaries
The following is a basin summary table for existing and proposed conditions:
Existing Basins
Basin Design
Pt.
Area %I Runoff
Coefficients
Peak
Discharge
acre C2 C100 Q
2 Q100
cfs cfs
EX1 EX1 3.75 48.6% 0.59 0.74 5.14 22.42
EX2 EX2 2.38 32.3% 0.47 0.59 2.23 9.73
EX3 EX3 2.19 80.4% 0.83 1.00 5.16 21.75
EX4 EX4 2.33 18.9% 0.38 0.48 1.88 8.20
EX5 EX5 1.35 12.5% 0.32 0.40 0.91 3.98
OS-1 OS-1 0.31 20.5% 0.38 0.47 0.27 1.17
OS-2A OS-2A 0.33 26.6% 0.43 0.54 0.38 1.65
OS-2B OS-2B 0.97 77.0% 0.80 1.00 1.94 8.47
OS-3 OS-3 2.21 20.1% 0.38 0.48 1.62 7.09
OS-4 OS-4 1.29 73.3% 0.76 0.95 2.17 9.47
OS-5 OS-5 0.54 2.0% 0.25 0.31 0.30 1.31
Totals 22.00 95.25
Proposed Basins
Basin Design
Pt.
Area Area Runoff
Coefficients
Peak
Discharge
acre acre C2 C100 Q
2 Q100
cfs cfs
A-1 A-1 3.46 17.5% 0.37 0.46 2.82 12.32
A-2 A-2 0.94 0.81 0.82 1.00 1.85 7.85
A-3 A-3 0.37 0.90 0.95 1.00 1.01 3.71
A-4 A-4 0.58 0.91 0.90 1.00 1.16 4.48
A-5 A-5 0.40 0.85 0.84 1.00 0.96 3.96
A-6 A-6 0.31 0.65 0.75 0.94 0.58 2.54
A-7 A-7 0.32 0.53 0.66 0.82 0.45 1.96
A-8 A-8 0.10 0.72 0.81 1.00 0.22 0.95
A-9 A-9 0.71 0.90 0.95 1.00 1.94 7.11
A-10 A-10 0.86 0.72 0.80 1.00 1.48 6.39
A-11 A-11 0.65 0.26 0.44 0.55 0.71 3.12
A-12 A-12 1.53 0.02 0.25 0.31 1.02 4.45
A-12
(Future)
A-12
(Future) 1.53 0.90 0.88 1.00 3.60 14.25
A-13 A-13 0.26 0.90 0.95 1.00 0.72 2.63
B-1 B-1 0.46 0.51 0.62 0.78 0.61 2.66
B-2 B-2 1.43 0.46 0.57 0.71 1.88 8.21
C-1 C-1 1.19 0.12 0.33 0.41 1.04 4.52
OS-1 OS-1 0.68 0.02 0.25 0.31 0.39 1.70
OS-2A OS-2A 0.21 0.41 0.54 0.68 0.30 1.29
Drainage Report
Platte River Power Authority HQ Campus
Fort Collins, CO
15 U16014-DrainageReport-Preliminary.doc
V. Sediment/Erosion Control
Erosion control, both temporary and permanent, is a vital part of any development
project. For this project, the site disturbance will be over 1-acre; therefore, a
comprehensive Erosion Control Report will be completed under a separate cover during
the Final Compliance phase of the project. The Erosion Control Report will be utilized
for site construction activities and will be prepared in accordance with City and
Colorado Department of Public Health and Environment requirements.
VI. Conclusions
A. Compliance with Standards
Storm drainage designs for the Platte River Power Authority Headquarters
campus have typically followed the guidelines provided by the Fort Collins
Stormwater Criteria Manual and/or the Urban Storm Drainage Criteria Manual.
In addition, the site drainage has been designed to meet City requirements in
terms of detention, waterquality and Low Impact Development standards.
B. Drainage Concept
The drainage system has been designed to convey the runoff to the designated
design points and the existing public infrastructure in an effective, safe manner.
No negative impacts are anticipated to the City of Fort Collins Master Drainage
Plan or to downstream properties or infrastructure due to the proposed
improvements.
C. Stormwater Quality
Several long-term stormwater quality measures have been selected for the site
that will provide treatment of stormwater prior it to being discharged from the
site. For this site this includes grass swales, grass open areas/buffers, a
bioswale, underground detention and an extended detention basin.
Drainage Report
Platte River Power Authority HQ Campus
Fort Collins, CO
16 U16014-DrainageReport-Preliminary.doc
VII. References
1. Fort Collins Stormwater Criteria Manual, City of Fort Collins, as adopted in 2011.
2. Urban Storm Drainage Criteria Manual, Urban Drainage and Flood Control District,
Wright Water Engineers, Inc., Denver, Colorado, Revised April 2008.
3. Larimer County Urban Area Street Standards, Larimer County, Colorado, Repealed
and Reenacted, Effective April 1, 2007.
4. Geotechnical Engineering Report – PRPA New Headquarters Campus, Terracon
Consultants, Inc., October 30, 2017.
5. Storm Drainage Study for Platte River Power Authority Maintenance Facility, RBD
Engineering Consultants, March 1984.
6. Collindale Industrial Park Storm Sewer Extension Construction Plans, Meline &
Irelan, Inc., Revised May 1995.
7. Collindale Industrial Park Construction Plans, James H. Stewart & Associates, Revised
October 1978.
8. Natural Resources Conservation Service Web Soil Survey at
websoilsurvey.nrcs.usda.gov/app
APPENDIX A
Hydrology Computations
Runoff Coefficients and % Impervious
Platte River Power Authority Campus, Fort Collins, CO
Basin Design Pt. Composite NRCS Soil
Total Total Roof
(1) Asphalt Concrete
(1)
Gravel
(1)
Lawns
(1) Imperviousness Type C
2 C100
%I = 90% %I = 100% %I = 90% %I =40% %I=2% (%I)
acres sf sf sf sf sf sf
EX1 EX1 3.75 163,553 19,370 51,627 8,627 2,670 81,259 48.6% C/D 0.59 0.74
EX2 EX2 2.38 103,485 2,883 22,033 7,506 1,767 69,296 32.3% C/D 0.47 0.59
EX3 EX3 2.19 95,233 17,683 54,408 6,104 1,029 16,009 80.4% C/D 0.83 1.00
EX4 EX4 2.33 101,325 9,012 5,208 4,316 810 81,979 18.9% C/D 0.38 0.48
EX5 EX5 1.35 58,652 16,194 42,458 12.5% C/D 0.32 0.40
OS-1 OS-1 0.31 13,409 928 343 3,331 8,807 20.5% C/D 0.38 0.47
OS-2A OS-2A 0.33 14,302 2,539 1,177 10,586 26.6% C/D 0.43 0.54
OS-2B OS-2B 0.97 42,132 26,583 6,282 9,267 77.0% C/D 0.80 1.00
OS-3 OS-3 2.21 96,344 14,084 4,110 78,150 20.1% C/D 0.38 0.48
OS-4 OS-4 1.29 56,148 38,984 2,061 15,103 73.3% C/D 0.76 0.95
OS-5 OS-5 0.54 23,668 23,668 2.0% C/D 0.25 0.31
17.64 768,253 48,948 216,394 40,526 25,801 436,584 41.1% C/D 0.54 0.67
Notes:
(1) Recommended % Imperviousness Values from USDCM Vol 1 - Ch 5 - Table RO-3. Increased Lawns from 0% to 2% Impervious.
(2) Runoff C is based on %I, UDFCD Eq RO-7 and correction factors in Table RO-4.
Composite Runoff Coefficients
Areas (2)
Existing Basins
Existing Total
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Runoff Coefficients and % Impervious
Platte River Power Authority Campus, Fort Collins, CO
Existing Basins
Basin Design Pt. Composite NRCS Soil
Total Total Roof
(1) Asphalt Concrete
(1)
Gravel
(1)
Lawns
(1) Imperviousness Type C
2 C100
acres sf %I = 90% %I = 100% %I = 90% %I =40% %I=2% (%I)
sf sf sf sf sf
A-1 A-1 3.46 150,739 11,029 8,701 5,865 - 125,144 17.5% C/D 0.37 0.46
A-2 A-2 0.94 40,805 - 26,691 6,712 - 7,403 80.6% C/D 0.82 1.00
A-3 A-3 0.37 16,242 16,242 - - - - 90.0% C/D 0.95 1.00
A-4 A-4 0.58 25,276 2,825 16,621 4,116 - 1,714 90.6% C/D 0.90 1.00
A-5 A-5 0.40 17,344 - 13,831 905 - 2,608 84.7% C/D 0.84 1.00
A-6 A-6 0.31 13,391 117 - 9,482 - 3,791 65.1% C/D 0.75 0.94
A-7 A-7 0.32 14,010 6,334 - 1,841 - 5,835 53.4% C/D 0.66 0.82
A-8 A-8 0.10 4,429 - 3,539 - 889 72.3% C/D 0.81 1.00
A-9 A-9 0.71 31,121 5,826 - 25,295 - (0) 90.0% C/D 0.95 1.00
A-10 A-10 0.86 37,508 5,872 - 23,853 - 7,783 71.7% C/D 0.80 1.00
A-11 A-11 0.65 28,184 7,569 - - - 20,615 25.6% C/D 0.44 0.55
A-12 A-12 1.53 66,694 - - - - 66,694 2.0% C/D 0.25 0.31
A-12 (Future) A-12 (Future) 1.53 66,694 - 60,000 - - 6,694 90.2% C/D 0.88 1.00
A-13 A-13 0.26 11,517 11,517 - - - - 90.0% C/D 0.95 1.00
A-TOTAL A-TOTAL 10.50 457,259 67,332 65,845 81,608 - 242,474 44.8% C/D 0.58 0.72
B-1 B-1 0.46 20,135 - 5,907 4,744 - 9,484 51.5% C/D 0.62 0.78
B-2 B-2 1.43 62,469 - 25,335 3,277 - 33,857 46.4% C/D 0.57 0.71
B-TOTAL B-TOTAL 1.90 82,604 - 31,242 8,021 - 43,341 47.6% C/D 0.58 0.73
C-1 C-1 1.19 51,708 - - 5,726 - 45,982 11.7% C/D 0.33 0.41
C-TOTAL C-TOTAL 1.19 51,708 - - 5,726 - 45,982 11.7% C/D 0.33 0.41
OS-1 OS-1 0.68 29,533 29,533 2.0% C/D 0.25 0.31
OS-2A OS-2A 0.21 8,940 2,539 1,177 5,224 41.4% C/D 0.54 0.68
OS-2B OS-2B 0.97 42,132 26,583 6,282 9,267 77.0% C/D 0.80 1.00
OS-3 OS-3 0.41 17,716 6,871 1,027 9,818 45.1% C/D 0.56 0.70
OS-4 OS-4 1.26 54,693 38,984 2,061 13,648 75.2% C/D 0.78 0.97
OS-5 OS-5 0.54 23,668 23,668 2.0% C/D 0.25 0.31
OS-TOTAL OS-TOTAL 4.06 176,682 - 74,977 10,547 - 91,158 48.8% C/D 0.59 0.74
17.64 768,253 67,332 172,063 105,902 - 422,955 43.8% C/D 0.56 0.71
Notes:
(1) Recommended % Imperviousness Values from USDCM Vol 1 - Ch 5 - Table RO-3. Increased Lawns from 0% to 2% Impervious.
(2) Runoff C is based on %I, UDFCD Eq RO-7 and correction factors in Table RO-4.
(3) Basin A-1 includes approximately 8,500 sf of future building. Basin B-2 includes approximately 11,000 sf of future parking area.
(4) Basin A-12 (Future) is included for storm sewer sizing only and is not included within "A-TOTAL" above. Detention and LID requirements will be required within Basin A-12 with future
development.
Proposed Total
Proposed Basins
Areas Composite Runoff Coefficients
(2)
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Time of Concentration
Platte River Power Authority Campus, Fort Collins, CO
Basin Design Pt. Area
Initial Ground C5
(1) Length Slope t
i
(2) Length Slope C
v Velocity
(3)
tt
(4) Check t
c? Total Length t
c
(5)
acres Type ft % min ft % fps min min Urban? ft min min
EX1 EX1 3.75 Lawn 0.09 35 3.8% 6.9 390 2.3% 20
3.0 2.1 9.1 Yes 425 12 9.1
EX2 EX2 2.38 Lawn 0.09 275 0.6% 35.8 190 1.0% 7
0.7 4.5 40.3 Yes 465 13 12.6
EX3 EX3 2.19 Paved 0.90 20 1.2% 1.5 415 1.0% 20
2.0 3.5 5.0 Yes 435 12 5.0
EX4 EX4 2.33 Lawn 0.09 50 1.0% 12.9 205 1.1% 7
0.7 4.7 17.5 Yes 255 11 11.4
EX5 EX5 1.35 Lawn 0.09 105 1.1% 18.1 130 0.1% 7
0.2 9.3 27.4 Yes 235 11 11.3
OS-1 OS-1 0.31 Lawn 0.09 70 4.4% 9.4 20 4.6% 20
4.3 0.1 9.4 Yes 90 11 9.4
OS-2A OS-2A 0.33 Lawn 0.09 30 5.9% 5.6 60 1.2% 20
2.2 0.5 6.0 Yes 90 11 6.0
OS-2B OS-2B 0.97 Paved 0.90 50 2.0% 2.0 490 0.7% 20
1.6 5.1 7.1 Yes 540 13 7.1
OS-3 OS-3 2.21 Lawn 0.09 30 10.0% 4.7 610 0.4% 7
0.4 23.0 27.6 Yes 640 14 13.6
OS-4 OS-4 1.29 Lawn 0.09 60 5.0% 8.3 250 1.1% 20
2.1 2.0 10.3 Yes 310 12 10.3
OS-5 OS-5 0.54 Lawn 0.09 30 10.0% 4.7 200 0.7% 7
0.6 5.7 10.4 Yes 230 11 10.4
Notes:
(1) C5 based initial ground type and Table RO-5
(2)ti = [0.395(1.1-C5)L1/2]/S1/3, S= slope in %, L=length of overland flow (400' max)
(3)V=CvS0.5, S=watercourse slope in ft/ft, UDFCD Equation RO-4
(4)tt=L/(V*60 sec/min)
(5)tc check (for urban or developed areas only) = total length/180 + 10
(6)min tc = 5 min
Existing Basins
ti+t
t Final
tc
(6)
Initial Overland Flow Time (t tc Check for Urbanized Basins
i) Travel/Channelized Time of Flow (tt
)
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Time of Concentration
Platte River Power Authority Campus, Fort Collins, CO
Existing Basins
Basin Design Pt. Area
Initial Ground C5
(1) Length Slope t
i
(2) Length Slope C
v Velocity
(3)
tt
(4) Check t
c? Total Length t
c
(5)
acres Type ft % min ft % fps min min Urban? ft min min
A-1 A-1 3.46 Paved 0.90 235 1.5% 4.8 465 0.5% 20
1.4 5.5 10.3 Yes 700 14 10.3
A-2 A-2 0.94 Lawn 0.09 10 2.0% 4.6 305 0.5% 20
1.4 3.6 8.2 Yes 315 12 8.2
A-3 A-3 0.37 Roof 0.73 50 12.5% 2.1 15 30.0% 20
11.0 0.0 2.1 Yes 65 10 5.0
A-4 A-4 0.58 Lawn 0.09 36 2.0% 8.7 242 2.0% 20
2.8 1.4 10.1 Yes 278 12 10.1
A-5 A-5 0.40 Paved 0.90 50 2.0% 2.0 133 1.5% 20
2.4 0.9 2.9 Yes 183 11 5.0
A-6 A-6 0.31 Lawn 0.09 17 2.0% 6.0 123 2.0% 20
2.8 0.7 6.7 Yes 140 11 6.7
A-7 A-7 0.32 Lawn 0.09 47 1.5% 10.9 200 1.0% 15
1.5 2.2 13.2 Yes 247 11 11.4
A-8 A-8 0.10 Lawn 0.09 15 2.0% 5.6 50 2.0% 20
2.8 0.3 5.9 Yes 65 10 5.9
A-9 A-9 0.71 Roof 0.73 25 12.5% 1.5 210 0.5% 20
1.4 2.5 3.9 Yes 235 11 5.0
A-10 A-10 0.86 Lawn 0.09 63 4.0% 9.2 200 1.5% 20
2.4 1.4 10.5 Yes 263 11 10.5
A-11 A-11 0.65 Lawn 0.09 20 7.0% 4.3 190 0.5% 15
1.1 3.0 7.3 Yes 210 11 7.3
A-12 A-12 1.53 Lawn 0.09 10 2.0% 4.6 205 2.0% 20
2.8 1.2 5.8 Yes 215 11 5.8
A-12 (Future) A-12 (Future) 1.53 Lawn 0.09 10 2.0% 4.6 205 2.0% 20
2.8 1.2 5.8 Yes 215 11 5.8
A-13 A-13 0.26 Roof 0.73 72 12.5% 2.5 15 30.0% 20
11.0 0.0 2.5 Yes 87 10 5.0
A-TOTAL A-TOTAL 10.50 Lawn 0.09 51 1.5% 11.4 985 0.4% 20
1.3 13.0 24.4 Yes 1036 16 15.8
B-1 B-1 0.46 Lawn 0.09 70 2.0% 12.1 40 2.0% 20
2.8 0.2 12.4 Yes 110 11 10.6
B-2 B-2 1.43 Lawn 0.09 50 25.0% 4.5 290 0.5% 15
1.1 4.6 9.0 Yes 340 12 9.0
B-TOTAL B-TOTAL 1.90 Lawn 0.09 50 25.0% 4.5 1350 0.3% 20
1.0 22.5 27.0 Yes 1400 18 17.8
C-1 C-1 1.19 Lawn 0.09 16 2.0% 5.8 75 10.0% 15
4.7 0.3 6.1 Yes 91 11 6.1
C-TOTAL C-TOTAL 1.19 Roof 0.09 16 2.0% 5.8 75 10.0% 15
4.7 0.3 6.1 Yes 91 11 6.1
OS-1 OS-1 0.68 Lawn 0.09 70 4.4% 9.4 20 4.6% 20
4.3 0.1 9.4 Yes 90 11 9.4
OS-2A OS-2A 0.21 Lawn 0.09 30 5.9% 5.6 60 1.2% 20
2.2 0.5 6.0 Yes 90 11 6.0
Rational Method Peak Runoff
Platte River Power Authority Campus, Fort Collins, CO
Basin Design Pt. Area Final
acre tc
(6) C
2 C5
C100 I
2 I5
I100 Q
2 Q5
Q100
min in/hr in/hr in/hr cfs cfs cfs
EX1 EX1 3.75 9 0.59 0.59 0.74 2.30 3.26 8.03 5.14 7.28 22.42
EX2 EX2 2.38 13 0.47 0.47 0.59 1.98 2.79 6.92 2.23 3.14 9.73
EX3 EX3 2.19 5 0.83 0.83 1.00 2.85 3.97 9.95 5.16 7.18 21.75
EX4 EX4 2.33 11 0.38 0.38 0.48 2.13 2.98 7.42 1.88 2.64 8.20
EX5 EX5 1.35 11 0.32 0.32 0.40 2.13 2.98 7.42 0.91 1.28 3.98
OS-1 OS-1 0.31 9 0.38 0.38 0.47 2.30 3.26 8.03 0.27 0.38 1.17
OS-2A OS-2A 0.33 6 0.43 0.43 0.54 2.67 3.79 9.31 0.38 0.54 1.65
OS-2B OS-2B 0.97 7 0.80 0.80 1.00 2.52 3.61 8.80 1.94 2.78 8.47
OS-3 OS-3 2.21 14 0.38 0.38 0.48 1.92 2.70 6.71 1.62 2.28 7.09
OS-4 OS-4 1.29 10 0.76 0.76 0.95 2.21 3.08 7.72 2.17 3.02 9.47
OS-5 OS-5 0.54 10 0.25 0.25 0.31 2.21 3.08 7.72 0.30 0.42 1.31
22.00 68.54 95.25
Existing Basins
Totals
Runoff Coefficients Rainfall Intensity Peak Discharge
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Rational Method Peak Runoff
Platte River Power Authority Campus, Fort Collins, CO
Existing Basins
Basin Design Pt. Area Final
acre tc
(6) C
2 C5
C100 I
2 I5
I100 Q
2 Q5
Q100
min in/hr in/hr in/hr cfs cfs cfs
A-1 A-1 3.46 10.0 0.37 0.37 0.46 2.21 3.08 7.72 2.82 3.93 12.32
A-2 A-2 0.94 8.0 0.82 0.82 1.00 2.40 3.44 8.38 1.85 2.65 7.85
A-3 A-3 0.37 5.0 0.95 0.95 1.00 2.85 3.97 9.95 1.01 1.41 3.71
A-4 A-4 0.58 10.0 0.90 0.90 1.00 2.21 3.08 7.72 1.16 1.61 4.48
A-5 A-5 0.40 5.0 0.84 0.84 1.00 2.85 3.97 9.95 0.96 1.34 3.96
A-6 A-6 0.31 7.0 0.75 0.75 0.94 2.52 3.61 8.80 0.58 0.84 2.54
A-7 A-7 0.32 11.0 0.66 0.66 0.82 2.13 2.98 7.42 0.45 0.63 1.96
A-8 A-8 0.10 6.0 0.81 0.81 1.00 2.67 3.79 9.31 0.22 0.31 0.95
A-9 A-9 0.71 5.0 0.95 0.95 1.00 2.85 3.97 9.95 1.93 2.69 7.11
A-10 A-10 0.86 11.0 0.80 0.80 1.00 2.13 2.98 7.42 1.48 2.07 6.39
A-11 A-11 0.65 7.0 0.44 0.44 0.55 2.52 3.61 8.80 0.71 1.02 3.12
A-12 A-12 1.53 6.0 0.25 0.25 0.31 2.67 3.79 9.31 1.02 1.45 4.45
A-12 (Future) A-12 (Future) 1.53 6.0 0.88 0.88 1.00 2.67 3.79 9.31 3.60 5.11 14.25
A-13 A-13 0.26 5.0 0.95 0.95 1.00 2.85 3.97 9.95 0.72 1.00 2.63
A-TOTAL A-TOTAL 10.50 16.0 0.58 0.58 0.72 1.81 2.53 6.30 11.00 15.35 47.85
B-1 B-1 0.46 11.0 0.62 0.62 0.78 2.13 2.98 7.42 0.61 0.86 2.66
B-2 B-2 1.43 9.0 0.57 0.57 0.71 2.30 3.26 8.03 1.88 2.67 8.21
B-TOTAL B-TOTAL 1.90 18.0 0.58 0.58 0.73 1.70 2.38 5.92 1.88 2.63 8.18
C-1 C-1 1.19 6.0 0.33 0.33 0.41 2.67 3.79 9.31 1.04 1.47 4.52
C-TOTAL C-TOTAL 1.19 6.0 0.33 0.33 0.41 2.67 3.79 9.31 1.04 1.47 4.52
OS-1 OS-1 0.68 9.0 0.25 0.25 0.31 2.30 3.26 8.03 0.39 0.55 1.70
OS-2A OS-2A 0.21 6.0 0.54 0.54 0.68 2.67 3.79 9.31 0.30 0.42 1.29
OS-2B OS-2B 0.97 7.0 0.80 0.80 1.00 2.52 3.61 8.80 1.94 2.78 8.47
OS-3 OS-3 0.41 12.0 0.56 0.56 0.70 2.05 2.89 7.16 0.47 0.66 2.05
OS-4 OS-4 1.26 10.0 0.78 0.78 0.97 2.21 3.08 7.72 2.15 3.00 9.39
OS-5 OS-5 0.54 10.0 0.25 0.25 0.31 2.21 3.08 7.72 0.30 0.42 1.31
Totals 23.99 33.78 101.08
Proposed Basins
Runoff Coefficients Rainfall Intensity Peak Discharge
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
APPENDIX B
Detention and Water Quality Calculations
Dention Pond Volume (FAA Method)
Platte River Power Authority Campus, Fort Collins, CO
POND ID: POND A
100 Year Storm Into Detention Facility
Area = 590,935 square feet
Area = 13.57 acres
C 100 = 0.74
Q 100 = 56.02 cfs
Release Rate Out of Pond
Q OUT = 3.15 cfs
Unit Flow = 0.23 cfs/acre
T C = 15.8 minutes
Notes:
1. Release Rate limited by the existing 12" outfall capacity in Timberline Road.
2. Pond area includes all of Basins A, B and OS-2 but excludes "Basin A-12 (Future)".
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T) Intensity (I) Qin= ∑C100*Area*I Vi=(Qin*T*60) Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 9.95 100.3 30,081 1.00 3.15 945 29,136
10 7.72 77.8 46,678 1.00 3.15 1,890 44,788
15 6.52 65.7 59,134 1.00 3.15 2,835 56,299
20 5.60 56.4 67,719 0.89 2.82 3,379 64,341
25 4.98 50.2 75,277 0.82 2.57 3,851 71,426
30 4.52 45.5 81,989 0.76 2.40 4,324 77,665
35 4.08 41.1 86,342 0.73 2.28 4,796 81,546
40 3.74 37.7 90,454 0.70 2.20 5,269 85,185
45 3.46 34.9 94,142 0.68 2.13 5,741 88,401
50 3.23 32.5 97,649 0.66 2.07 6,214 91,435
55 3.03 30.5 100,763 0.64 2.03 6,686 94,077
60 2.86 28.8 103,756 0.63 1.99 7,159 96,597
70 2.59 26.1 109,621 0.61 1.93 8,104 101,517
80 2.38 24.0 115,123 0.60 1.89 9,049 106,074
90 2.21 22.3 120,263 0.59 1.85 9,994 110,269
100 2.06 20.8 124,555 0.58 1.82 10,939 113,617
110 1.94 19.5 129,030 0.57 1.80 11,884 117,146
120 1.84 18.5 133,504 0.57 1.78 12,829 120,675
Required Detention Volume
V 100 = 120,675 cubic feet
V 100 = 2.77 acre-ft
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Dention Pond Volume (FAA Method)
Platte River Power Authority Campus, Fort Collins, CO
POND ID: POND C
100 Year Storm Into Detention Facility
Area = 51,708 square feet
Area = 1.19 acres
C 100 = 1.00
Q 100 = 4.52 cfs
Release Rate Out of Pond
Q OUT = 0.50 cfs
Unit Flow = 0.42 cfs/acre
T C = 6.1 minutes
Notes:
1. Release Rate limited to maximize detention volume but keep maximum ponding to less than 6".
2. For Pond C Calculations, C100 has been assumed to be 1.00 assuming that the pond water surface is at the maximum elevation
and all water that falls on the wet pond will pass through the outlet .
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T) Intensity (I) Qin= ∑C100*Area*I Vi=(Qin*T*60) Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 9.95 11.8 3,543 1.00 0.50 150 3,393
10 7.72 9.2 5,498 0.80 0.40 241 5,257
15 6.52 7.7 6,966 0.70 0.35 316 6,650
20 5.60 6.6 7,977 0.65 0.33 391 7,586
25 4.98 5.9 8,867 0.62 0.31 466 8,401
30 4.52 5.4 9,658 0.60 0.30 541 9,117
35 4.08 4.8 10,171 0.59 0.29 616 9,555
40 3.74 4.4 10,655 0.58 0.29 691 9,964
45 3.46 4.1 11,089 0.57 0.28 766 10,323
50 3.23 3.8 11,503 0.56 0.28 841 10,662
55 3.03 3.6 11,869 0.56 0.28 916 10,953
60 2.86 3.4 12,222 0.55 0.28 991 11,231
70 2.59 3.1 12,913 0.54 0.27 1,141 11,772
80 2.38 2.8 13,561 0.54 0.27 1,291 12,270
90 2.21 2.6 14,166 0.53 0.27 1,441 12,725
100 2.06 2.4 14,672 0.53 0.27 1,591 13,081
110 1.94 2.3 15,199 0.53 0.26 1,741 13,458
120 1.84 2.2 15,726 0.53 0.26 1,891 13,835
Required Detention Volume
V 100 = 13,835 cubic feet
V 100 = 0.32 acre-ft
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Water Quality
Platte River Power Authority Campus, Fort Collins, CO
Required Onsite Water Quality Capture Volume
Basin Area Area Imperviousness Watershed WQCV*
(Extended Detention)
(sf) (acres) (%) (inches) (cf)
A-TOTAL 457,259 10.497 72% 0.29 13,080
B-TOTAL 82,604 1.896 73% 0.29 2,383
C-TOTAL 51,708 1.187 41% 0.18 943
TOTAL REQUIRED 591,571 13.581 70% 0.27 16,183
Less WQ Provided by Underground Detention 3,895
Less WQ Provided in Wet Pond 943
Total WQ Provided in Pond A 11,346
* based on 40 hr storage and additional 20% for extended detention
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Water Quality - Low Impact Development
Platte River Power Authority Campus, Fort Collins, CO
Required Onsite Water Quality Capture Volume For Underground Detention
Basin Area Area Imperviousness Watershed WQCV*
(Extended Detention)
(sf) (acres) (%) (inches) (cf)
Basin A South - Underground Detention LID
A-3 16,242 0.37 90% 0.321 435
A-4 25,276 0.58 91% 0.325 685
A-5 17,344 0.40 85% 0.288 417
A-12 66,694 1.53 2% 0.012 67
125,555 2.88 43% 0.149 1,564
Basin A North - Underground Detention LID
A-6 13,391 0.31 65% 0.204 227
A-7 14,010 0.32 53% 0.172 201
A-8 4,429 0.10 72% 0.229 84
A-9 31,121 0.71 90% 0.321 833
A-10 37,508 0.86 72% 0.226 708
A-11 28,184 0.65 26% 0.110 258
128,642 2.95 63% 0.198 2,127
Total To Underground Detention 3,691
* Based on 12 hr storage forunderground detention
** Basin A-12 Future will be required to provide LID treatment at time of future development.
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Dention Pond Volume (FAA Method)
Platte River Power Authority Campus, Fort Collins, CO
POND ID: Basin A South - Underground Detention LID
WQ Basin Characateristics
Area = 125,555 square feet
Area = 2.88 acres
C 2 = 0.48
Q in = 2.07 cfs
WQ Release Rate Determined from Number of Chambers and Chamber Release Rate
Q OUT = 0.52 cfs
Unit Flow = 0.18 cfs/acre
T C = 15.8 minutes
Notes:
1. Release rate out of pond based on number of chambers multiplied by release rate per chamber.
2. Rainfall intensities approximated as one-half the 2-yr peak runoff rate.
3. C2 utilized for water quality event detention calculation.
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T) Intensity (I) Qin= ∑C100*Area*I Vi=(Qin*T*60) Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 1.425 2.0 591 1.00 0.52 156 436
10 1.105 1.5 917 1.00 0.52 311 606
15 0.935 1.3 1,164 1.00 0.52 467 697
20 0.805 1.1 1,336 0.89 0.46 557 780
25 0.715 1.0 1,484 0.82 0.42 634 849
30 0.650 0.9 1,619 0.76 0.40 712 906
35 0.585 0.8 1,700 0.73 0.38 790 910
40 0.535 0.7 1,776 0.70 0.36 868 909
45 0.495 0.7 1,849 0.68 0.35 946 903
50 0.460 0.6 1,909 0.66 0.34 1,024 886
55 0.435 0.6 1,986 0.64 0.33 1,101 885
60 0.410 0.6 2,042 0.63 0.33 1,179 863
65 0.385 0.5 2,077 0.62 0.32 1,257 820
70 0.365 0.5 2,121 0.61 0.32 1,335 786
75 0.345 0.5 2,148 0.61 0.31 1,413 735
80 0.330 0.5 2,192 0.60 0.31 1,491 701
85 0.315 0.4 2,223 0.59 0.31 1,568 654
90 0.305 0.4 2,279 0.59 0.30 1,646 632
95 0.290 0.4 2,287 0.58 0.30 1,724 563
100 0.280 0.4 2,324 0.58 0.30 1,802 522
105 0.270 0.4 2,353 0.58 0.30 1,880 474
110 0.260 0.4 2,374 0.57 0.30 1,958 417
115 0.255 0.4 2,434 0.57 0.29 2,035 399
120 0.245 0.3 2,441 0.57 0.29 2,113 327
Required Detention Volume
V 100 = 910 cubic feet
V 100 = 0.02 acre-ft
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Dention Pond Volume (FAA Method)
Platte River Power Authority Campus, Fort Collins, CO
POND ID: Basin A North - Underground Detention LID
WQ Basin Characateristics
Area = 128,642 square feet
Area = 2.95 acres
C 2 = 0.67
Q in = 2.69 cfs
WQ Release Rate Determined from Number of Chambers and Chamber Release Rate
Q OUT = 0.71 cfs
Unit Flow = 0.24 cfs/acre
T C = 15.8 minutes
Notes:
1. Release rate out of pond based on number of chambers multiplied by release rate per chamber.
2. Rainfall intensities approximated as one-half the 2-yr peak runoff rate.
3. C2 utilized for water quality event detention calculation.
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T) Intensity (I) Qin= ∑C100*Area*I Vi=(Qin*T*60) Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 1.425 2.8 846 1.00 0.71 212 634
10 1.105 2.2 1,312 1.00 0.71 425 887
15 0.935 1.9 1,665 1.00 0.71 637 1,028
20 0.805 1.6 1,911 0.89 0.63 759 1,152
25 0.715 1.4 2,122 0.82 0.58 865 1,257
30 0.650 1.3 2,315 0.76 0.54 971 1,344
35 0.585 1.2 2,431 0.73 0.51 1,077 1,353
40 0.535 1.1 2,541 0.70 0.49 1,184 1,357
45 0.495 1.0 2,644 0.68 0.48 1,290 1,355
50 0.460 0.9 2,731 0.66 0.47 1,396 1,335
55 0.435 0.9 2,840 0.64 0.46 1,502 1,338
60 0.410 0.8 2,920 0.63 0.45 1,608 1,312
65 0.385 0.8 2,971 0.62 0.44 1,714 1,257
70 0.365 0.7 3,033 0.61 0.43 1,820 1,213
75 0.345 0.7 3,072 0.61 0.43 1,926 1,145
80 0.330 0.7 3,134 0.60 0.42 2,033 1,102
85 0.315 0.6 3,179 0.59 0.42 2,139 1,040
90 0.305 0.6 3,259 0.59 0.42 2,245 1,014
95 0.290 0.6 3,271 0.58 0.41 2,351 920
100 0.280 0.6 3,324 0.58 0.41 2,457 867
105 0.270 0.5 3,366 0.58 0.41 2,563 802
110 0.260 0.5 3,395 0.57 0.40 2,669 726
115 0.255 0.5 3,481 0.57 0.40 2,776 706
120 0.245 0.5 3,490 0.57 0.40 2,882 609
Required Detention Volume
V 100 = 1,357 cubic feet
V 100 = 0.03 acre-ft
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Underground Detention
Platte River Power Authority Campus, Fort Collins, CO
Required Underground Detention/WQ Volume
Chamber Total Required Detention/ Chamber Chamber Unit Chamber *Installed Min. Min. Min. Req'd Design No. of Provided Provided Total
ID Detention/WQ WQ Type Release Rate
c Unit Chamber Unit No. of WQ Isolator Row Control
i Chambers WQ Isolator Installed
Volume
a
Inflow
b
Volume
d Volume inc. Chambers
f Release Volume by Provided Release Row System
Aggregate
e
Rate
g
FAA Method
h
Rate
j
Volume
k
Volume
l
(cf) (cfs) (cfs) (cf) (cf) (sf) (cfs) (cf) (cfs) (cf) (cf)
Basin A South 1564 2.07 SC-740 0.024 45.90 74.90 21.00 0.50 910 WQCV 22 0.52 1,010 1,648
Basin A North 2127 2.69 SC-740 0.024 45.90 74.90 29.00 0.68 1,357 WQCV 30 0.71 1,377 2,247
TOTAL 3691 4.76 SC-740 0.024 45.90 74.90 50.00 1.18 2,267 WQCV 52 1.23 2,387 3,895
*Volume assumes minimum aggregate depth below chamber.
a. Total required WQCV calculated per 12-hr drain time.
b. WQ inflow is approximated as one-half the 2-yr peak runoff rate.
c. Release rate per chamber, limited by flow through geotextile with accumulated sediment.
d. Volume within chamber only, not accounting for void spaces in surrounding aggregate. The Isolator Row(s) are sized per this unit volume.
e. Volume includes chamber and void spaces (40%) in surrounding aggregate, per chamber unit. The total system WQCV is sized per this unit volume.
f. Number of chambers required to provide full WQCV within total installed system, including aggregate.
g. Release rate per chamber times number of chambers. This is used at the 'outlet control' for the FAA calculations.
h. Minimum 'chamber-only' volume to ensure dirty water is fully contained within Isolator Row.
i. Determines whether the design is controlled by the Isolator Row volume or WQCV.
j. Release rate per chamber times number of chambers provided. This is the approximate controlled discharge from the WQ event.
k. Volume provided in chambers only (no aggregate storage). This number must meet or exceed the required FAA storage volume.
l. System volume includes total number of chambers, plus surrounding aggregate. This number must meet or exceed the required 12-hr WQCV.
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
Low Impact Development Summary
Platte River Power Authority Campus, Fort Collins, CO
Basin Proposed
Impervious Area
LID Treatment Area Treated % of Site
Treated
(sf) (acres) (sf) (%)
A-1 25,596 None (1) - 0%
A-2 33,402 None - 0%
A-3 16,242 Underground Detention 16,242 6%
A-4 23,562 Underground Detention 23,562 8%
A-5 14,736 Underground Detention 14,736 5%
A-6 9,600 Underground Detention 9,600 3%
A-7 8,175 Underground Detention 8,175 3%
A-8 3,539 Underground Detention 3,539 1%
A-9 31,121 Underground Detention 31,121 11%
A-10 29,725 Underground Detention 29,725 11%
A-11 7,569 Underground Detention 7,569 3%
A-12 (Future) 17,500 Future LID Treatment (2) 17,500 6%
A-13 11,517 None - 0%
B-1 10,651 Bioswale 10,651 4%
B-2 28,612 Bioswale (3) 28,612 10%
C-1 5,726 Wet Pond 5,726 2%
Total Site 277,273 206,758 75%
(1) Basin A-1 includes approximately 8,500 sf of future building area not being treated
(2) Minimum required future LID treatment for basin A-12
(3) Basin B-2 includes 11,000 sf of future treated area by the future extension of the proposed bioswale
Low Impact Development (LID) Treated Areas
Calculations by: KRB
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
APPENDIX C
Hydraulic Calculations
APPENDIX D
Referenced Materials
United States
Department of
Agriculture
A product of the National
Cooperative Soil Survey,
a joint effort of the United
States Department of
Agriculture and other
Federal agencies, State
agencies including the
Agricultural Experiment
Stations, and local
participants
Custom Soil Resource
Report for
Larimer County
Area, Colorado
Platte River Power Authority
Natural
Resources
Conservation
Service
February 23, 2017
9
Custom Soil Resource Report
Soil Map
4486900 4487100 4487300 4487500 4487700 4487900 4488100 4488300 4488500
4486900 4487100 4487300 4487500 4487700 4487900 4488100 4488300 4488500
495900 496100 496300 496500 496700 496900 497100
495900 496100 496300 496500 496700 496900 497100
40° 32' 50'' N
105° 2' 55'' W
40° 32' 50'' N
105° 1' 59'' W
40° 31' 54'' N
105° 2' 55'' W
40° 31' 54'' N
105° 1' 59'' W
N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 400 800 1600 2400
Feet
0 100 200 400 600
Meters
Map Scale: 1:8,480 if printed on A portrait (8.5" x 11") sheet.
Soil Map may not be valid at this scale.
Map Unit Legend
Larimer County Area, Colorado (CO644)
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
6 Aquepts, ponded 3.5 1.0%
34 Fort Collins loam, 0 to 1 percent
slopes
8.6 2.4%
35 Fort Collins loam, 0 to 3 percent
slopes
4.2 1.2%
36 Fort Collins loam, 3 to 5 percent
slopes
5.7 1.6%
73 Nunn clay loam, 0 to 1 percent
slopes
180.2 50.8%
74 Nunn clay loam, 1 to 3 percent
slopes
131.4 37.1%
75 Nunn clay loam, 3 to 5 percent
slopes
13.3 3.8%
76 Nunn clay loam, wet, 1 to 3
percent slopes
7.6 2.2%
Totals for Area of Interest 354.6 100.0%
Map Unit Descriptions
The map units delineated on the detailed soil maps in a soil survey represent the
soils or miscellaneous areas in the survey area. The map unit descriptions, along
with the maps, can be used to determine the composition and properties of a unit.
A map unit delineation on a soil map represents an area dominated by one or more
major kinds of soil or miscellaneous areas. A map unit is identified and named
according to the taxonomic classification of the dominant soils. Within a taxonomic
class there are precisely defined limits for the properties of the soils. On the
landscape, however, the soils are natural phenomena, and they have the
characteristic variability of all natural phenomena. Thus, the range of some
observed properties may extend beyond the limits defined for a taxonomic class.
Areas of soils of a single taxonomic class rarely, if ever, can be mapped without
including areas of other taxonomic classes. Consequently, every map unit is made
up of the soils or miscellaneous areas for which it is named and some minor
components that belong to taxonomic classes other than those of the major soils.
Most minor soils have properties similar to those of the dominant soil or soils in the
map unit, and thus they do not affect use and management. These are called
noncontrasting, or similar, components. They may or may not be mentioned in a
particular map unit description. Other minor components, however, have properties
and behavioral characteristics divergent enough to affect use or to require different
management. These are called contrasting, or dissimilar, components. They
generally are in small areas and could not be mapped separately because of the
scale used. Some small areas of strongly contrasting soils or miscellaneous areas
are identified by a special symbol on the maps. If included in the database for a
Custom Soil Resource Report
11
73—Nunn clay loam, 0 to 1 percent slopes
Map Unit Setting
National map unit symbol: 2tlng
Elevation: 4,100 to 5,700 feet
Mean annual precipitation: 14 to 15 inches
Mean annual air temperature: 48 to 52 degrees F
Frost-free period: 135 to 152 days
Farmland classification: Prime farmland if irrigated
Map Unit Composition
Nunn and similar soils: 85 percent
Minor components: 15 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Nunn
Setting
Landform: Terraces
Landform position (three-dimensional): Tread
Down-slope shape: Linear
Across-slope shape: Linear
Parent material: Pleistocene aged alluvium and/or eolian deposits
Typical profile
Ap - 0 to 6 inches: clay loam
Bt1 - 6 to 10 inches: clay loam
Bt2 - 10 to 26 inches: clay loam
Btk - 26 to 31 inches: clay loam
Bk1 - 31 to 47 inches: loam
Bk2 - 47 to 80 inches: loam
Properties and qualities
Slope: 0 to 1 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: Medium
Capacity of the most limiting layer to transmit water (Ksat): Moderately low to
moderately high (0.06 to 0.20 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Calcium carbonate, maximum in profile: 7 percent
Salinity, maximum in profile: Nonsaline (0.1 to 1.0 mmhos/cm)
Sodium adsorption ratio, maximum in profile: 0.5
Available water storage in profile: High (about 9.1 inches)
Interpretive groups
Land capability classification (irrigated): 3e
Land capability classification (nonirrigated): 4e
Custom Soil Resource Report
18
Hydrologic Soil Group: C
Ecological site: Clayey Plains (R067BY042CO)
Hydric soil rating: No
Minor Components
Heldt
Percent of map unit: 10 percent
Landform: Terraces
Landform position (three-dimensional): Tread
Down-slope shape: Linear
Across-slope shape: Linear
Ecological site: Clayey Plains (R067BY042CO)
Hydric soil rating: No
Wages
Percent of map unit: 5 percent
Landform: Terraces
Landform position (three-dimensional): Tread
Down-slope shape: Linear
Across-slope shape: Linear
Ecological site: Loamy Plains (R067BY002CO)
Hydric soil rating: No
74—Nunn clay loam, 1 to 3 percent slopes
Map Unit Setting
National map unit symbol: 2tlpl
Elevation: 3,900 to 5,840 feet
Mean annual precipitation: 13 to 17 inches
Mean annual air temperature: 50 to 54 degrees F
Frost-free period: 135 to 160 days
Farmland classification: Prime farmland if irrigated
Map Unit Composition
Nunn and similar soils: 85 percent
Minor components: 15 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Nunn
Setting
Landform: Terraces
Landform position (three-dimensional): Tread
Down-slope shape: Linear
Across-slope shape: Linear
Parent material: Pleistocene aged alluvium and/or eolian deposits
Typical profile
Ap - 0 to 9 inches: clay loam
Bt - 9 to 13 inches: clay loam
Custom Soil Resource Report
19
Btk - 13 to 25 inches: clay loam
Bk1 - 25 to 38 inches: clay loam
Bk2 - 38 to 80 inches: clay loam
Properties and qualities
Slope: 1 to 3 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: Medium
Capacity of the most limiting layer to transmit water (Ksat): Moderately low to
moderately high (0.06 to 0.20 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Calcium carbonate, maximum in profile: 7 percent
Salinity, maximum in profile: Nonsaline to very slightly saline (0.1 to 2.0
mmhos/cm)
Sodium adsorption ratio, maximum in profile: 0.5
Available water storage in profile: High (about 9.9 inches)
Interpretive groups
Land capability classification (irrigated): 2e
Land capability classification (nonirrigated): 3e
Hydrologic Soil Group: C
Ecological site: Clayey Plains (R067BY042CO)
Hydric soil rating: No
Minor Components
Heldt
Percent of map unit: 10 percent
Landform: Terraces
Landform position (three-dimensional): Tread
Down-slope shape: Linear
Across-slope shape: Linear
Ecological site: Clayey Plains (R067BY042CO)
Hydric soil rating: No
Satanta
Percent of map unit: 5 percent
Landform: Terraces
Landform position (three-dimensional): Tread
Down-slope shape: Linear
Across-slope shape: Linear
Ecological site: Loamy Plains (R067BY002CO)
Hydric soil rating: No
75—Nunn clay loam, 3 to 5 percent slopes
Map Unit Setting
National map unit symbol: 2tlpm
Custom Soil Resource Report
20
Table—Hydrologic Soil Group (Platte River Power Authority)
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
6 Aquepts, ponded A/D 3.5 1.0%
34 Fort Collins loam, 0 to 1
percent slopes
B 8.6 2.4%
35 Fort Collins loam, 0 to 3
percent slopes
C 4.2 1.2%
36 Fort Collins loam, 3 to 5
percent slopes
B 5.7 1.6%
73 Nunn clay loam, 0 to 1
percent slopes
C 180.2 50.8%
74 Nunn clay loam, 1 to 3
percent slopes
C 131.4 37.1%
75 Nunn clay loam, 3 to 5
percent slopes
C 13.3 3.8%
76 Nunn clay loam, wet, 1 to
3 percent slopes
C 7.6 2.2%
Totals for Area of Interest 354.6 100.0%
Rating Options—Hydrologic Soil Group (Platte River Power
Authority)
Aggregation Method: Dominant Condition
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Custom Soil Resource Report
28
References
American Association of State Highway and Transportation Officials (AASHTO).
2004. Standard specifications for transportation materials and methods of sampling
and testing. 24th edition.
American Society for Testing and Materials (ASTM). 2005. Standard classification of
soils for engineering purposes. ASTM Standard D2487-00.
Cowardin, L.M., V. Carter, F.C. Golet, and E.T. LaRoe. 1979. Classification of
wetlands and deep-water habitats of the United States. U.S. Fish and Wildlife
Service FWS/OBS-79/31.
Federal Register. July 13, 1994. Changes in hydric soils of the United States.
Federal Register. September 18, 2002. Hydric soils of the United States.
Hurt, G.W., and L.M. Vasilas, editors. Version 6.0, 2006. Field indicators of hydric
soils in the United States.
National Research Council. 1995. Wetlands: Characteristics and boundaries.
Soil Survey Division Staff. 1993. Soil survey manual. Soil Conservation Service.
U.S. Department of Agriculture Handbook 18. http://www.nrcs.usda.gov/wps/portal/
nrcs/detail/national/soils/?cid=nrcs142p2_054262
Soil Survey Staff. 1999. Soil taxonomy: A basic system of soil classification for
making and interpreting soil surveys. 2nd edition. Natural Resources Conservation
Service, U.S. Department of Agriculture Handbook 436. http://
www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?cid=nrcs142p2_053577
Soil Survey Staff. 2010. Keys to soil taxonomy. 11th edition. U.S. Department of
Agriculture, Natural Resources Conservation Service. http://
www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?cid=nrcs142p2_053580
Tiner, R.W., Jr. 1985. Wetlands of Delaware. U.S. Fish and Wildlife Service and
Delaware Department of Natural Resources and Environmental Control, Wetlands
Section.
United States Army Corps of Engineers, Environmental Laboratory. 1987. Corps of
Engineers wetlands delineation manual. Waterways Experiment Station Technical
Report Y-87-1.
United States Department of Agriculture, Natural Resources Conservation Service.
National forestry manual. http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/
home/?cid=nrcs142p2_053374
United States Department of Agriculture, Natural Resources Conservation Service.
National range and pasture handbook. http://www.nrcs.usda.gov/wps/portal/nrcs/
detail/national/landuse/rangepasture/?cid=stelprdb1043084
29
United States Department of Agriculture, Natural Resources Conservation Service.
National soil survey handbook, title 430-VI. http://www.nrcs.usda.gov/wps/portal/
nrcs/detail/soils/scientists/?cid=nrcs142p2_054242
United States Department of Agriculture, Natural Resources Conservation Service.
2006. Land resource regions and major land resource areas of the United States,
the Caribbean, and the Pacific Basin. U.S. Department of Agriculture Handbook
296. http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?
cid=nrcs142p2_053624
United States Department of Agriculture, Soil Conservation Service. 1961. Land
capability classification. U.S. Department of Agriculture Handbook 210. http://
www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_052290.pdf
Custom Soil Resource Report
30
This unofficial copy was downloaded on Nov-02-2016 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact Engineering Office 281 North College Fort Collins, CO 80521 USA
This unofficial copy was downloaded on Nov-02-2016 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact Engineering Office 281 North College Fort Collins, CO 80521 USA
This unofficial copy was downloaded on Nov-02-2016 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact Engineering Office 281 North College Fort Collins, CO 80521 USA
APPENDIX E
Drainage Exhibits
SOUTHEAST CORNER
S30, T.7N., R68W
50' RIGHT-OF-WAY
R.O.W.
WIDTH
VARIES
ACCESS EASEMENT
WIDTH VARIES
60' RIGHT-OF-WAY
107.5' RIGHT-OF-WAY
50' RIGHT-OF-WAY
R.O.W.
WIDTH VARIES
2.21
1.29
2.38
3.75
2.33
1.35
2.19
0.31
0.33
0.97
0.54
EX1
EX4
OS1
EX2
EX3
EX5
OS2a
OS3
OS4
OS2b
OS5
EAST HORSETOOTH ROAD
SOUTH TIMBERLINE ROAD
DANFIELD COURT
UNION PACIFIC RAILROAD
OFFSITE PROPERTY OWNED BY:
2001 DANFIELD LLC
EXISTING E/O
BUILDING
EXISTING HQ
EXISTING WORKOUT SPACE
EXISTING MICROWAVE
BUILDING
EXISTING POOL
CAR GARAGE
EXISTING
CARPENTER
SHOP
EXISTING POND
EXISTING DETENTION
EXISTING
DETENTION
EXISTING DETENTION
EXISTING STORM DRAINING
HORSETOOTH ROAD
EXISTING STORM
SOUTHEAST CORNER
S30, T.7N., R68W
50' RIGHT-OF-WAY
R.O.W.
WIDTH
VARIES
60' RIGHT-OF-WAY
107.5' RIGHT-OF-WAY
50' RIGHT-OF-WAY
R.O.W.
WIDTH VARIES
A1
C1
OS1
OS2a
OS3
OS4
OS2b
A2
A3
A13
B1
B2
A5 A4
A6
A7
A9
A10
A11
A8
0.41
1.26
3.46
1.19
0.68
0.21
0.97
0.37 0.26
0.94
0.46
1.43
0.40 0.58
A12
1.53
0.31
0.86 0.71 0.32
0.65
0.10
0.54
OS5
DETENTION
POND
DETENTION
POND
POND OUTLET STRUCTURE
POND OUTLET STRUCTURE
EAST HORSETOOTH ROAD
SOUTH TIMBERLINE ROAD
DANFIELD COURT
UNION PACIFIC RAILROAD
OFFSITE PROPERTY OWNED BY:
2001 DANFIELD LLC
HQ BUILDING
MICROWAVE
BUILDING
MAINTENANCE SHOP
/ TRUCK WASH
POOL CAR
WAREHOUSE
SUBSTATION GARAGE
FLEET PARKING GARAGE A
PROPOSED
POND
EXISTING E&O
BUILDING
EXISTING
HEADQUARTERS
FLEET PARKING
GARAGE B
BIOSWALE W/
UNDERDRAIN
POTENTIAL FUTURE
PARKING AREA
POTENTIAL FUTURE BIOSWALE
UNDERGROUND
DETENTION
EXISTING STORM
DRAINING BASIN OS3
PLATTE RIVER POWER AUTHORITY 11/14/17
U16014
1" = 60'
1" = N/A
PLATTE RIVER POWER AUTHORITY HQ
NORTH
11/8/2017 10:51:06 AM
P:\U16014 - PRPA CAMPUS\CADD\CP\CX.XX_PROPOSED DRAINAGE PLAN.DWG
CX.XX_PROPOSED DRAINAGE PLAN
KRB NKS
C6.00
26
DRAINAGE PLAN
0 60' 120'
SCALE: 1" = 60'
30'
PREPARED FOR:
JOB NUMBER
SHEET NUMBER
OF SHEETS
DATE SUBMITTED:
The engineer preparing these plans will not be responsible
for, or liable for, unauthorized changes to or uses of these
plans. All changes to the plans must be in writing and
must be approved by the preparer of these plans.
NO. BY DATE
CAUTION
REVISIONS:
VERTICAL:
HORIZONTAL:
SCALE
PRELIMINARY
PLANS
NOT FOR CONSTRUCTION
1501 Academy Ct.
Ste. 203
Fort Collins, CO 80524
(970) 530-4044
www.unitedcivil.com
Civil Engineering &
Consulting
PROJ. MGR:
DRAWING NAME:
PATH:
DATE: TIME: DESIGNER:
UNITED CIVIL
Design Group LLC
NOTES
1. EXISTING UNDERGROUND AND OVERHEAD PUBLIC AND PRIVATE UTILITIES
AS SHOWN ARE INDICATED ACCORDING TO THE BEST INFORMATION
AVAILABLE TO THE ENGINEER.
2. VEGETATIVE SWALES, BIOSWALES, UNDERGROUND DETENTION AND THE
WATERQUALITY AND DETENTION POND SHALL BE MAINTAINED BY THE
PROPERTY OWNER AND FOLLOW APPLICABLE STANDARD OPERATING
PROCEDURES AS SHOWN IN DEVELOPMENT AGREEMENT.
5124
5124
PROPOSED 1' CONTOUR
EXISTING 1' CONTOUR
STORM SEWER
STORM INLET
LEGEND
EXISTING CURB & GUTTER
PROPOSED CURB & GUTTER
PROPOSED FLOWLINE
LIMITS OF DISTURBANCE
EXISTING PROPERTY LINE
EXISTING R.O.W LINE
EXISTING EASEMENT
BASIN BOUNDARY
SWALE SECTION
D-101
XXXX
BASIN DESIGNATION
BASIN AREA (ACRE)
2 - YR RUNOFF COEFF.
100 - YR RUNOFF COEFF.
DESIGN POINT
FLOW DIRECTION
5125
5125
PROPOSED 5' CONTOUR
EXISTING 5' CONTOUR
A A
DRAINAGE SUMMARY
POND SUMMARY
LOW IMPACT DEVELOPMENT TREATMENT SUMMARY
Basin
Proposed
Impervious
Area
LID Treatment Area Treated % of Site
Treated
(sf) (acres) (sf) (%)
A-1 25,596 None 0 0%
A-2 33,402 None 0 0%
A-3 16,242 Underground Detention 16,242 6%
A-4 23,562 Underground Detention 23,562 8%
A-5 14,736 Underground Detention 14,736 5%
A-6 9,600 Underground Detention 9,600 3%
A-7 8,175 Underground Detention 8,175 3%
A-8 3,539 Underground Detention 3,539 1%
A-9 31,121 Underground Detention 31,121 11%
A-10 29,725 Underground Detention 29,725 11%
A-11 7,569 Underground Detention 7,569 3%
A-12 (Future) 17,500 Future LID Treatment 17,500 6%
A-13 11,517 None 0 0%
B-1 10,651 Bioswale 10,651 4%
B-2 28,612 Bioswale 28,612 10%
C-1 5,726 Wet Pond 5,726 2%
Total Site 277,273 206,758 75%
Proposed Basins
Basin Area %I C2 C100 Q2 Q100
(acre) (cfs) (cfs)
A-1 3.46 18% 0.37 0.46 2.82 12.32
A-2 0.94 81% 0.82 1.00 1.85 7.85
A-3 0.37 90% 0.95 1.00 1.01 3.71
A-4 0.58 91% 0.90 1.00 1.16 4.48
A-5 0.40 85% 0.84 1.00 0.96 3.96
A-6 0.31 65% 0.75 0.94 0.58 2.54
A-7 0.32 53% 0.66 0.82 0.45 1.96
A-8 0.10 72% 0.81 1.00 0.22 0.95
A-9 0.71 90% 0.95 1.00 1.94 7.11
A-10 0.86 72% 0.80 1.00 1.48 6.39
A-11 0.65 26% 0.44 0.55 0.71 3.12
A-12 1.53 2% 0.25 0.31 1.02 4.45
A-13 0.26 90% 0.95 1.00 0.72 2.63
A-TOTAL 10.50 45% 0.58 0.72 11.00 47.85
B-1 0.46 51% 0.62 0.78 0.61 2.66
B-2 1.43 46% 0.57 0.71 1.88 8.21
B-TOTAL 1.90 48% 0.58 0.73 1.88 8.18
C-1 1.19 12% 0.33 0.41 1.04 4.52
C-TOTAL 1.19 12% 0.33 0.41 1.04 4.52
OS-1 0.68 2% 0.25 0.31 0.39 1.70
OS-2A 0.21 41% 0.54 0.68 0.30 1.29
OS-2B 0.97 77% 0.80 1.00 1.94 8.47
OS-3 0.41 45% 0.56 0.70 0.47 2.05
OS-4 1.26 75% 0.78 0.97 2.15 9.39
OS-5 0.54 2% 0.25 0.31 0.30 1.31
Pond A Pond C
WQCV REQUIRED 0.26 AC-FT 0.02 AC-FT
WQCV PROVIDED 0.30 AC-FT 0.02 AC-FT
DETENTION VOLUME REQUIRED 2.77 AC-FT 0.32 AC-FT
DETENTION VOLUME PROVIDED 2.88 AC-FT 0.35 AC-FT
WQCV WATER SURFACE ELEVATION 4949.60 4955.05
100-YEAR WATER SURFACE ELEVATION 4,952.00 4,955.50
TOP OF POND ELEVATION 4,953.00 4,956.50
DRAINING BASIN OS3
PLATTE RIVER POWER AUTHORITY 11/14/17
U16014
1" = 60'
1" = N/A
PLATTE RIVER POWER AUTHORITY HQ
NORTH
11/8/2017 10:11:32 AM
P:\U16014 - PRPA CAMPUS\CADD\CP\CX.XX_EXISTING DRAINAGE PLAN.DWG
CX.XX_EXISTING DRAINAGE PLAN
KRB NKS
EX-DR
1
EXISTING DRAINAGE PLAN
0 60' 120'
SCALE: 1" = 60'
30'
PREPARED FOR:
JOB NUMBER
SHEET NUMBER
OF SHEETS
DATE SUBMITTED:
The engineer preparing these plans will not be responsible
for, or liable for, unauthorized changes to or uses of these
plans. All changes to the plans must be in writing and
must be approved by the preparer of these plans.
NO. BY DATE
CAUTION
REVISIONS:
VERTICAL:
HORIZONTAL:
SCALE
PRELIMINARY
PLANS
NOT FOR CONSTRUCTION
1501 Academy Ct.
Ste. 203
Fort Collins, CO 80524
(970) 530-4044
www.unitedcivil.com
Civil Engineering &
Consulting
PROJ. MGR:
DRAWING NAME:
PATH:
DATE: TIME: DESIGNER:
UNITED CIVIL
Design Group LLC
NOTES
1. EXISTING UNDERGROUND AND OVERHEAD PUBLIC AND PRIVATE UTILITIES
AS SHOWN ARE INDICATED ACCORDING TO THE BEST INFORMATION
AVAILABLE TO THE ENGINEER.
2. VEGETATIVE SWALES, BIOSWALES, UNDERGROUND DETENTION AND THE
WATERQUALITY AND DETENTION POND SHALL BE MAINTAINED BY THE
PROPERTY OWNER AND FOLLOW APPLICABLE STANDARD OPERATING
PROCEDURES AS SHOWN IN DEVELOPMENT AGREEMENT.
5124
5124
PROPOSED 1' CONTOUR
EXISTING 1' CONTOUR
STORM SEWER
STORM INLET
LEGEND
EXISTING CURB & GUTTER
PROPOSED CURB & GUTTER
PROPOSED FLOWLINE
LIMITS OF DISTURBANCE
EXISTING PROPERTY LINE
EXISTING R.O.W LINE
EXISTING EASEMENT
BASIN BOUNDARY
SWALE SECTION
D-101
XXXX
BASIN DESIGNATION
BASIN AREA (ACRE)
2 - YR RUNOFF COEFF.
100 - YR RUNOFF COEFF.
DESIGN POINT
FLOW DIRECTION
5125
5125
PROPOSED 5' CONTOUR
EXISTING 5' CONTOUR
A A
DRAINAGE SUMMARY
Existing Basins
Basin Area %I C2
C100 Q
2 Q100
(acre) (cfs) (cfs)
EX1 3.75 49% 0.59 0.74 5.14 22.42
EX2 2.38 32% 0.47 0.59 2.23 9.73
EX3 2.19 80% 0.83 1.00 5.16 21.75
EX4 2.33 19% 0.38 0.48 1.88 8.20
EX5 1.35 12% 0.32 0.40 0.91 3.98
OS-1 0.31 20% 0.38 0.47 0.27 1.17
OS-2A 0.33 27% 0.43 0.54 0.38 1.65
OS-2B 0.97 77% 0.80 1.00 1.94 8.47
OS-3 2.21 20% 0.38 0.48 1.62 7.09
OS-4 1.29 73% 0.76 0.95 2.17 9.47
OS-5 0.54 2% 0.25 0.31 0.30 1.31
OS-2B OS-2B 0.97 Paved 0.90 50 2.0% 2.0 490 0.7% 20
1.6 5.1 7.1 Yes 540 13 7.1
OS-3 OS-3 0.41 Lawn 0.09 140 1.5% 18.9 165 0.7% 20
1.7 1.6 20.5 Yes 305 12 11.7
OS-4 OS-4 1.26 Lawn 0.09 60 5.0% 8.3 250 1.1% 20
2.1 2.0 10.3 Yes 310 12 10.3
OS-5 OS-5 0.54 Lawn 0.09 30 10.0% 4.7 200 0.7% 7
0.6 5.7 10.4 Yes 230 11 10.4
Notes:
(1) C5 based initial ground type and Table RO-5
(2)ti = [0.395(1.1-C5)L1/2]/S1/3, S= slope in %, L=length of overland flow (400' max)
(3)V=CvS0.5, S=watercourse slope in ft/ft, UDFCD Equation RO-4
(4)tt=L/(V*60 sec/min)
(5)tc check (for urban or developed areas only) = total length/180 + 10
(6)min tc = 5 min
Proposed Basins
Initial Overland Flow Time (ti) Travel/Channelized Time of Flow (t
t) ti
+tt t
c Check for Urbanized Basins Final
tc
(6)
Date: 11/7/2017 P:\U16014 - PRPA Campus\Reports\Drainage\Calculations\U16014-Drain Calcs.xlsm
OS-2B OS-2B 0.97 0.77 0.80 1.00 1.94 8.47
OS-3 OS-3 0.41 0.45 0.56 0.70 0.47 2.05
OS-4 OS-4 1.26 0.75 0.78 0.97 2.15 9.39
OS-5 OS-5 0.54 0.02 0.25 0.31 0.30 1.31
Totals 68.21 101.08
Underground Detention – Underground detention diverts storm water
runoff during the water quality event and detains the flows underground.
This provides for the removal of sediment and other pollutants from the
runoff while also promoting infiltration. Underground detention has been