HomeMy WebLinkAboutSTANFORD SENIOR LIVING - FDP210017 - REPORTS - DRAINAGE REPORT (2)
FINAL DRAINAGE REPORT
STANFORD SENIOR LIVING
Fort Collins, Colorado
March 14, 2023
Prepared for:
ESL Fort Collins LLC
1550 Market Street, Suite 200
Denver, CO 80202
Prepared by:
301 North Howes Street, Suite 100
Fort Collins, Colorado 80521
Phone: 970.221.4158 Fax: 970.221.4159
www.northernengineering.com
Project Number: 1530-002
This Drainage Report is consciously provided as a PDF.
Please consider the environment before printing this document in its entirety.
When a hard copy is absolutely necessary, we recommend double-sided printing.
March 15, 2023
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Final Drainage Report for
Stanford Senior Living
Dear Staff:
Northern Engineering is pleased to submit this Final Drainage Report for your review. This report
accompanies the Final Development Review submittal for the proposed Stanford Senior Living.
This report has been prepared in accordance with the Fort Collins Stormwater Criteria Manual
(FCSCM) and Urban Storm Drainage Criteria Manual. This report serves to document the
stormwater impacts associated with the proposed Stanford Senior Living project. We understand
that review by the City is to assure general compliance with standardized criteria contained in the
FCSCM.
If you should have any questions as you review this report, please feel free to contact us.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Blaine Mathisen, PE
Project Engineer
Stanford Senior Living
Final Drainage Report
TABLE OF CONTENTS
I. GENERAL LOCATION AND DESCRIPTION ................................................................... 1
A. Location ............................................................................................................................................. 1
B. Description of Property ..................................................................................................................... 2
C. Floodplain.......................................................................................................................................... 3
II. DRAINAGE BASINS AND SUB-BASINS ....................................................................... 5
A. Major Basin Description .................................................................................................................... 5
B. Sub-Basin Description ....................................................................................................................... 5
III. DRAINAGE DESIGN CRITERIA ................................................................................... 6
A. Regulations........................................................................................................................................ 6
B. Four Step Process .............................................................................................................................. 6
C. Development Criteria Reference and Constraints ............................................................................ 7
D. Hydrological Criteria ......................................................................................................................... 7
E. Hydraulic Criteria .............................................................................................................................. 7
F. Floodplain Regulations Compliance .................................................................................................. 8
G. Modifications of Criteria ................................................................................................................... 8
IV. DRAINAGE FACILITY DESIGN .................................................................................... 8
A. General Concept ............................................................................................................................... 8
B. Specific Details ................................................................................................................................ 10
V. CONCLUSIONS ...................................................................................................... 11
A. Compliance with Standards ............................................................................................................ 11
B. Drainage Concept ............................................................................................................................ 12
References ....................................................................................................................... 13
APPENDICES:
APPENDIX A – Hydrologic Computations
APPENDIX B – Hydraulic Computations
B.1 – Storm Sewers
B.2 – Inlets
B.3 – Detention Facilities
APPENDIX C – Water Quality Design Computations
APPENDIX D – Erosion Control Report
APPENDIX E – Additional References
Stanford Senior Living
Final Drainage Report
LIST OF TABLES AND FIGURES:
Figure 1 - Vicinity Map ........................................................................................................ 1
Figure 2 – Aerial Photograph ................................................................................................ 3
Figure 3 – Existing FEMA Floodplains .................................................................................... 4
Figure 4 – Existing City Floodplains ....................................................................................... 4
Table 1 - Pond Summary ................................................................................................... 11
Table 2 - Chamber Count Summary ..................................................................................... 11
MAP POCKET:
Existing Drainage Exhibit
DR1 - Drainage Exhibit
Stanford Senior Living
Final Drainage Report 1
I. GENERAL LOCATION AND DESCRIPTION
A. Location
1. Vicinity Map
Figure 1 - Vicinity Map
2. Stanford Senior Living project is located in the southwest quarter of Section 25,
Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins,
County of Larimer, State of Colorado.
3. The project site is located at the southwest corner of the Stanford Road and Monroe
Drive intersection.
4. Currently the site is vacant on the north side of the lot with native grasses and
patches of gravel. The south half of the project site is currently an asphalt parking lot
with landscaped medians that the Marriot Hotel is currently utilizing for guest parking.
The undeveloped portion of the site drains via overland flow at an average grade of
3.50% to the south towards the existing parking lot. In general, the site conveys
stormwater runoff from the north to the south where it is collected by a customized
inlet in the southeast corner of the existing parking lot.
5. In the existing condition most of the site is being captured by a custom inlet in the
southeast corner of the parking lot that was designed per Strachan Subdivision 3rd
Filing. A small portion of the existing drive entrance aisle is being conveyed via curb
and gutter to existing inlets at the northwest corner of the Stanford Road and
Horsetooth Road intersection. Both the custom inlet in the parking lot and inlets at
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Final Drainage Report 2
the intersection tie into existing 30”x36” HERCP pipes. From there the runoff is
routed via the HERCP pipes into Warren Lake which is located at the southeast corner
of the Stanford Road and Horsetooth Road intersection.
6. “Stanford Senior Living is in the Foothills Drainage Basin Master Plan that was
prepared by Resource Consultants, Inc. and dated July 1980. The Strachan
Subdivision Third Filing was developed in 1980 and was divided into two basins.
The Southerly Basin, which contains 17.9 acres and drains to the corner of
Horsetooth Road and Stanford Road. Mitchell and Co. were the developers, and they
paid the Warren Lake Company for storm water detention rights in Warren Lake for
Strachan Subdivision 1st, 2nd, and 3rd Filings. These filings correspond to the area
between Monroe Drive and Horsetooth Road and between College Avenue and
Stanford Road. They also developed the Strachan Continental Subdivision at the
southwest corner of Horsetooth Road and J.F.K Parkway. Its detention was also paid
for in Warren Lake.” – Final Drainage Report for J.F.K. Office P.U.D
7. According to the Final Drainage Report for J.F.K. Office P.U.D the HERCP pipes in the
southeast corner of the parking lot were sized to carry the 10-year event (48 cfs) for
the Strachan Subdivision while the 100-year event overtopped Horsetooth Road and
sheet flowed to Warren Lake. Due to the report being 40 years old it is important that
the Stanford Senior Living project does not adversely impact downstream
infrastructure but is still allowed to utilize the Warren Lake Company agreement to
some degree. Therefore, the Stanford Senior Living project will detain the difference
between the historic 10-year runoff and the proposed 100-year runoff. By utilizing
this approach, the Stanford Senior Living center will be reducing the historic flows to
the existing inlet at the corner of the parking lot. Additionally, the downstream
infrastructure will not be adversely impacted, and the Stanford Senior Living center
will no longer contribute to the 100-year flows that currently overtop Horsetooth Road
during the major event.
B. Description of Property
1. The Stanford Senior Living project has a total acreage of 3.79 acres. However, the
Stanford Senior Living project only plans on developing 2.50 acres and leaving the
existing parking lot in place. The existing parking lot (1.29 acres) historically sheet
flows to the southeast to the custom inlet box as previously described. Therefore,
Stanford Senior Living will only be detaining and treating 2.50 acres. The proposed
flows to the existing inlet will be reduced because the Stanford Senior Living Center is
detaining the difference between the 10-year historic and 100-year developed. The
10-year historic rate is also a conservative release because it assumed that there is no
pervious area currently on the 2.50 acres that is proposed with this development.
However, that is not the case as there is existing hardscape on the property. These
areas include a drive aisle and some gravel roads that are currently being undetained
and untreated. Please refer to Appendix A for the runoff calculations.
Stanford Senior Living
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Figure 2 – Aerial Photograph
2. The Stanford Senior Living project will consist of a new assisted living and memory
care center. It will be a four-story building with underground parking. There is an
existing parking lot that is currently being used by the Marriot Hotel for overflow guest
parking which will now serve as the Stanford Senior Living parking lot.
3. According to the United States Department of Agriculture (USDA) Natural Resources
Conservation Service (NRCS) Soil Survey, 100 percent of the site consists of Nunn
clay loam, which fails into Hydrologic Soil Group C
4. No irrigation facilities or major drainageways are within the property limits.
5. The project site is within a General Commercial District (C-G) Zoning District. The
proposed use is permitted within the zone district.
C. Floodplain
1. The subject property is not located in a FEMA or City regulatory floodplain.
2. The FEMA Panel 08069C0987G illustrates that this project site is not near any FEMA
delineated regulatory floodplain.
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Figure 3 – Existing FEMA Floodplains
Figure 4 – Existing City Floodplains
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II. DRAINAGE BASINS AND SUB-BASINS
A. Major Basin Description
1. The Stanford Senior Living project is in the Foothills Basin which is centrally located
in Fort Collins. The basin is mostly developed with commercial development along
College Avenue and mixed-use residential in the remainder of the basin. The basin
drains from west to east through open channels or a storm sewer system to the Fossil
Creek Reservoir.
B. Sub-Basin Description
1. The property historically drains from the northwest to the southeast corner of the
existing parking lot via overland flow. The existing parking lot was developed as a part
of the Strachan Subdivision 3rd Filing and is currently being used by the Marriot Hotel
and Merrill Lynch Wealth Management. As previously described, there is a custom
inlet in the southeast corner of the parking lot that has three 30”x36” HERCP that
currently routes the 10-year storm event east across Stanford Road to a junction box
that then routes the runoff south across Horsetooth Road where it is ultimately
discharged into Warren Lake. The 100-year event has historically overtopped
Horsetooth Road and overland flowed south to Warren Lake. There are two on-grade
inlets at the northwest corner of the intersection of Horsetooth Road and Stanford
Road that collects additional flows. These additional flows are then routed to the
existing 30”x36” HERCP pipes crossing Stanford Road.
2. A historic rational calculation was performed for the 2.50 acres that will be getting
disturbed during the construction of the Stanford Senior Living project. According the
FCSCM Chapter 6 developments are only responsible for detaining any newly installed
impervious areas. Since the parking lot is existing, and downstream of the new
development, the Stanford Senior Living project will not be detaining or treating any
runoff that has historically drained to the custom inlet at the southeast corner of the
parking lot. Additionally, the Stanford Senior Living project is maintaining the existing
drainage patterns within Stanford Road and Monroe Drive. Therefore, no runoff
generated on the adjacent streets will be routed through the proposed detention and
water quality facility.
3. Following FCSCM requirements for rational calculations the 10-year runoff generated
by the existing 2.50 acres is 2.23 cfs. Therefore, Stanford Senior Living can have a
max release rate of 2.23 cfs. Traditionally, Fort Collins requires the 2-year event
dictate the max release rate but since there is a historic agreement in place with
Warren Lake Company to detain all runoff from Strachan Subdivision Third Filing this
new development can release at a higher release rate than current FCSCM
requirements. By releasing at the historic 10-year event it ensures that downstream
infrastructure is not adversely impacted while also allowing the developers to use their
agreement with Warren Lake Company to some extent. See Section IV.A.4 below for a
more detailed description of the project’s proposed drainage patterns.
4. No offsite drainage is being routed through the property.
5. Please refer to the Appendix for additional information about the existing conditions.
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III. DRAINAGE DESIGN CRITERIA
A. Regulations
There are optional provisions outside of the FCSCM proposed with the Stanford Senior
Living project. Stanford Senior Living project will be detaining the difference between the
historic 10-year event and proposed 100-year event. This deviates from the standard
historic 2-year and proposed 100-year difference for detention requirements set forth in
the FCSCM.
B. Four Step Process
The overall stormwater management strategy employed with Stanford Senior Living project
utilizes the “Four Step Process” to minimize adverse impacts of urbanization on receiving
waters. The following is a description of how the proposed development has incorporated
each step.
Step 1 – Employ Runoff Reduction Practices
Several techniques have been utilized with the proposed development to facilitate the
reduction of runoff peaks, volumes, and pollutant loads as the site is developed from the
current use by implementing multiple Low-Impact Development (LID) strategies including:
Providing vegetated open areas along the north, south, east and west portion of the
site to reduce the overall impervious area and to minimize directly connected
impervious areas (MDCIA).
Routing runoff through the drain rock within the underground detention facility to
increase time of concentration, promote infiltration, and provides water quality via the
isolator rows.
Step 2 – Implement BMPs That Provide a Water Quality Capture Volume (WQCV) with
Slow Release
The efforts taken in Step 1 will facilitate the reduction of runoff and provide the necessary
BMPs required for water quality. All runoff that is captured on site will be routed through
isolator rows within Stormtech chambers. The isolator rows will remove sediment and
other pollutants through filtration.
Step 3 – Stabilize Drainageways
As stated in Section I.B.4, above, there are no major drainageways in or near the subject
site. While this step may not seem applicable to Stanford Senior Living, the proposed
project indirectly helps achieve stabilized drainageways nonetheless. Once again, site
selection has a positive effect on stream stabilization. By detaining the difference between
the 10-year and 100-year events there will be less stress on the downstream
infrastructure. Combining detention with LID reduces the likelihood of bed and bank
erosion within Warren Lake. Furthermore, this project will pay one-time stormwater
development fees, as well as ongoing monthly stormwater utility fees, both of which help
achieve citywide drainageway stability.
Step 4 – Implement Site Specific and Other Source Control BMPs.
This step typically applies to industrial and commercial developments and is not
necessarily applicable for this project. However, a localized trash collection system that is
stored externally to the building and in a sump condition will reduce the potential impacts
of garbage making its way downstream.
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Final Drainage Report 7
C. Development Criteria Reference and Constraints
1. There are existing drainage studies for this area. They were completed as part of the
Strachan Continental Subdivision as well as the J.F.K Office P.U.D. These reports
can be found in the Appendix and serve as the basis for the 10-year release rate. Only
the pertinent information was pulled from these reports and highlighted. For the
complete reports please refer to the Fort Collins erecords website.
2. The subject property is an "in-fill" development project as the property is surrounded
by currently developed properties. As such, several constraints have been identified
during this analysis that will impact the proposed drainage system including:
Existing elevations along the northern and eastern right-of-ways make it difficult to
capture all the flow along these edges. However, it should be noted that the areas
that are freely discharging to either Monroe Drive or Stanford Road are associated
with the vegetated parkways. Therefore, runoff impacts are minor, and it still
maintains the historic conveyance path for these areas.
As previously mentioned, overall drainage patterns within the Master Drainage
Plan for Strachan Continental Subdivision will be maintained. The conveyance
path that runoff is using is the same as it historically has been. There are issues
with the Strachan Continental master plan as it is no longer compliant with Fort
Collins codes and standards. However, at the time of this report there are no
known issues with this existing master drainage plan. The city has not identified
the intersection of Horsetooth Road and Stanford Road as a problem intersection
and there are currently no future plans to revitalize this intersection. However, the
Stanford Senior Living Center is proposing detaining and treating 2.50 acres of
runoff in order to help alleviate some of the downstream infrastructure. For
additional information on detention and water quality please see Section IV.B of
this report.
D. Hydrological Criteria
1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in
Figure RA-16 of the FCSCM, serve as the source for all hydrologic computations
associated with this development. Tabulated data contained in Table RA-7 has been
utilized for Rational Method runoff calculations.
2. The Rational Method has been employed to compute stormwater runoff utilizing
coefficients contained in Tables RO-11 and RO-12 of the FCSCM.
3. The Rational Formula-based Modified Federal Aviation Administration (FAA)
procedure has been utilized for detention storage calculations.
4. Three separate design storms have been utilized to address distinct drainage
scenarios. The first event analyzed is the “Minor,” or “Initial” Storm, which has a 2-
year recurrence interval. The second event considered is the “Major Storm,” which
has a 100-year recurrence interval. The third storm computed, for detention
requirements, is the 10-year recurrence interval.
5. No other assumptions or calculation methods have been used with this development
that are not referenced by current City of Fort Collins criteria.
E. Hydraulic Criteria
1. As previously noted, the subject property historically drains to the custom inlet at the
southeast corner of the existing parking lot. From there, the runoff is routed via a
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Final Drainage Report 8
series of elliptical pipes to Warren Lake where it has been historically detained and
treated for water quality.
2. All drainage facilities proposed with the Stanford Senior Living project are designed in
accordance with criteria outlined in the FCSCM and/or the Mile High Flood Control
District’s Urban Storm Drainage Criteria Manual.
3. As stated in Section I.C.1, above, the subject property is not located within any
regulatory floodplain.
4. Stanford Senior Living project does not propose to modify any natural drainageways.
F. Floodplain Regulations Compliance
1. As previously mentioned, all structures are located outside of any FEMA 100-year or
City floodplain, and thus are not subject to any floodplain regulations.
G. Modifications of Criteria
1. The proposed Stanford Senior Living development is requesting a modification at this
time. As previously stated, the original Strachan Thrid Filing included this area as part
of the agreement with Warren Lake Company to detain and provide water quality
treatment for developed stormwater runoff. However, to meet current FCSCM
requirements Stanford Senior Living can no longer allow the 100-year event to overtop
Horsetooth Road. Therefore, to meet current FCSCM criteria while also utilizing the
Warren Lake agreement to the fullest extent possible Stanford Senior Living will be
detaining the difference between the historic 10-year event and proposed 100-year
event.
IV. DRAINAGE FACILITY DESIGN
A. General Concept
1. The main objectives of Stanford Senior Living drainage design are to improve the
surrounding stormwater infrastructure while also maintaining historic drainage
patterns.
2. As previously mentioned, there are no off-site flows draining onto the existing
property.
3. A list of tables and figures used within this report can be found in the Table of
Contents at the front of the document. The tables and figures are located within the
sections to which the content best applies.
4. The Stanford Senior Living project is composed of nine major drainage basins,
designated as Basins A, B, C, D, E, F, G, H, and OS. The drainage patterns for each
major basin are further described below. Basin H has 12 minor drainage basins
associated with the breakdown of the roofline.
Basin A
Basin A represents the area in the northwest corner of the site. It consists of a small
portion of concrete walk and patio but most of the area is landscaping. Runoff from
this basin will enter the storm sewer and be conveyed to the Stormtech chambers.
Stanford Senior Living
Final Drainage Report 9
Basin B
Basin B encompasses a small area near the trash enclosure. Runoff generated in
Basin B will enter the storm sewer system via an area inlet and be conveyed to the
Stormtech chambers.
Basin C
Basin C is associated with the memory care garden near the northeast corner of the
site. Runoff in Basin C will be collected via area inlets and be routed via storm sewer
to the Stormtech chambers.
Basin D
Basin D consists of a small amount of concrete walk and the rest being landscaping
with a swale conveying stormwater runoff to an area drain near the drive entrance of
the site. Runoff from Basin D will be conveyed to the Stormtech chambers via a storm
sewer.
Basin E
Basin E is associated with the area in front of the building. This area consists of
concrete walks, asphalt drive aisles, and landscaped islands. Runoff from Basin E is
collected via a series of area drains. Once runoff is collected in the area drains the
runoff is routed to the Stormtech chambers via the storm sewer.
Basin F
Basin F is associated with the drive entrance that will flow east and be captured by an
on-grade inlet. Runoff will then be routed to the Stormtech chambers via the storm
sewer.
Basin G
Basin G is a swath of land along southwest boundary near the Marriot Hotel. Runoff
generated in Basin G will flow along the property line in a landscape swale and then
be routed to an area inlet that is located at the entrance to the garage. From there the
runoff will be conveyed via the storm sewer to the Stormtech chambers.
Basin H (Basins H1-H12)
Basin H encompasses the entire building and consists of 12 minor basins. Runoff
from Basins H1-H11 will be collected in roof leaders and conveyed to the Stormtech
chambers via the storm sewer. There is also an internal courtyard (Basin H12) in the
center of the building which will generate a small amount of runoff. Runoff from the
courtyard will be collected by area drains which will then be conveyed via storm sewer
that runs through the building and connects to the external storm sewer that runs
adjacent to the building. All runoff from Basin H will be routed to the Stormtech
chambers.
Stanford Senior Living
Final Drainage Report 10
Basin OS
Basin OS is a small amount of landscaping and concrete area that cannot be captured
on site because of topographic constraints. Runoff from Basin OS (0.48 cfs) sheet
flows north offsite to Monroe Drive where it then flows east towards Stanford Road.
Once in Stanford Road it flows south to the inlet at the northwest corner of the
Horsetooth Road and Stanford Road intersection. Because Basin OS is releasing
undetained, the Stanford Senior Living project will include the undetained flow as part
of the allowable release rate, and thus will throttle the release from the Stormtech
chambers. By reducing the overall release rate by the undetained flows from Basin OS
(0.48 cfs), that ensures that Stanford Senior Living is in fact releasing no more than
the historic 10-yr release rate of 2.23 cfs and all downstream infrastructure is not
adversely impacted.
A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the end of
this report.
B. Specific Details
1. The release rate for the proposed development was established by calculating the 10-
year historic runoff rate of the area proposed for development minus the 100-year
developed flow from Basin OS as previously discussed. As mentioned in section III.G
this is a modification to the FCSCM. By utilizing a reduced historic 10-year peak
runoff the Stanford Senior Living project will maintain downstream infrastructure while
also utilizing their agreement with Warren Lake Company to detain this site’s runoff.
2. The Stanford Senior Living project area will no longer be contributing to the spill that
overtops Horsetooth Road during the major event because the project is detaining the
difference between the historic 10-year and developed 100-year runoff.
3. These release rates were utilized in the FAA method for design of the pond. (Refer to
Appendix B for these calculations).
4. Detention Pond Calculations
Pond
Calculations were completed for the underground pond, based on the proposed land
use characteristics of Basins A, B, C, D, E, F, G, and H with a release rate of 1.75
cfs, indicates a detention volume of 19,271 cu. ft. The 1.75 cfs was calculated by
taking the historic 10-yr runoff rate of 2.23 cfs and subtracting Basin OS 100-yr
runoff rate of 0.48 (2.23-0.48=1.75 cfs). Based on Stormtech specifications in
order to achieve 19,271 cu. ft. of detention it will require 103 MC-3500 chambers.
However, based on the Stormtech system layout Stanford Senior Living will be
providing 20,002 cu. ft. of detention.
5. Water Quality Pond Calculations
Pond
The Stanford Senior Living project will be providing 100% of their water quality
capture volume in the form of isolator rows within the Stormtech system. Isolator
rows qualify as a BMP measure and therefore 100% of the water quality capture
volume will be provided in the form of low impact development (LID). Isolator rows
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Final Drainage Report 11
are designed to retain sediment and filter out the first flush of any storm event. At the
end of each isolator row is a Nyloplast Basin that allows for routine and easy
maintenance so that the entire system can be maintained for its design life. Basin OS
(0.15 acres) cannot be captured but water quality is still be provided for in Warren
Lake per the offsite detention/water quality agreement with the Warren Lake
Company, as it historically has been.
Following FCSCM requirements yields a WQCV of 2,272 cu. ft. Utilizing available
chamber sizing and infiltration data provided by Stormtech requires 18 additional
MC-3500 chambers to meet the required WQCV.
6. Pond Summary
Pond
The Stanford Senior Living project will be utilizing 121 MC-3500 Stormtech
chambers to fulfill their stormwater detention and treatment requirements. Stanford
Senior Living will be providing 100% WQCV using LID measures.
Required
Detention
(cu. ft.)
Provided
Detention
(cu. ft.)
Required
WQCV
(cu. ft.)
Provided
WQCV
(cu. ft.)
Required
Total
Volume
(cu. ft.)
Provided
Volume
(cu. ft.)
19,271 20,002 2,272 2,275 21,543 22,277
Table 1 - Pond Summary
MC-3500 Chamber Counts
Detention WQCV Total
103 18 121
Table 2 - Chamber Count Summary
V. CONCLUSIONS
A. Compliance with Standards
1. The drainage design proposed with Stanford Senior Living project complies with the
City of Fort Collins’ Stormwater Criteria Manual.
2. The drainage design proposed with Stanford Senior Living project complies with the
City of Fort Collins’ Master Drainage Plan for the Foothills Basin.
3. The drainage design proposed with Stanford Senior Living project complies with the
Master Drainage Plan for Strachan Continental Subdivision.
4. There are no regulatory floodplains associated with the Stanford Senior Living
development.
5. The drainage plan and stormwater management measures proposed with the Stanford
Senior Living development are compliant with all applicable State and Federal
regulations governing stormwater discharge.
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B. Drainage Concept
1. The drainage design proposed with this project will effectively limit potential damage
associated with its stormwater runoff. The drainage design proposed with this project
will be utilizing the agreement with the Warren Lake Company to some extent.
2. The Stanford Senior Living project will detain the difference between the historic 10-
year event and the developed 100-year event.
3. The Stanford Senior Living project will be providing 100% WQCV using an LID
measure.
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Final Drainage Report 13
References
1. City of Fort Collins Landscape Design Guidelines for Stormwater and Detention Facilities,
November 5, 2009, BHA Design, Inc. with City of Fort Collins Utility Services.
2. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by Ordinance No.
159, 2018, and referenced in Section 26-500 (c) of the City of Fort Collins Municipal Code.
3. Larimer County Urban Area Street Standards, Adopted January 2, 2001, Repealed and
Reenacted, Effective October 1, 2002, Repealed and Reenacted, Effective April 1, 2007.
4. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation
Service, United States Department of Agriculture.
5. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control
District, Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008.
6. Final Drainage Report For J.F.K Office P.U.D, January 9, 1995, Vaught-Frye Architects
7. Strachan Continental Subdivision Storm Drainage Report, July 3, 1981, James H. Stewart &
Associates, INC
APPENDIX A
HYDROLOGIC COMPUTATIONS
CHARACTER OF SURFACE1: Percentage
Impervious
2-yr Runoff
Coefficient
10-yr Runoff
Coefficient
100-yr Runoff
Coefficient
Developed
Asphalt .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………100%0.95 0.95 1.00
Concrete .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………100%0.95 0.95 1.00
Rooftop .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………90%0.95 0.95 1.00
Gravel .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………40%0.50 0.50 0.63
Pavers .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………40%0.50 0.50 0.63
Landscape or Pervious Surface
Playgrounds .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………25%0.35 0.35 0.44
Lawns Clayey Soil .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………2%0.25 0.25 0.31
Lawns Sandy Soil .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………2%0.15 0.15 0.19
Notes:
Basin ID Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Rooftop
(ac)
Area of
Gravel
(ac)
Area of
Pavers
(ac)
Area of
Playgrounds
(ac)
Area of Lawns
(ac)
Composite
% Imperv.
2-year
Composite Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
H 2.50 0.00 0.00 0.00 0.00 0.00 0.00 2.50 2%25%25%31%
HISTORIC BASIN % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
2) Runoff Coefficients are taken from the Fort Collins Stormwater Criteria Manual, Chapter 3. Table 3.2-1 and 3.2-2
1) Percentage impervious taken from the Fort Collins Stormwater Criteria Manual, Chapter 5, Table 4.1-2 and Table 4.1-3
Overland Flow, Time of Concentration:
Channelized Flow, Time of Concentration:
Total Time of Concentration :
T c is the lesser of the values of Tc calculated using T c = T i + T t
C2 C100
Length,
L
(ft)
Slope,
S
(%)
Ti2 Ti100
Length,
L
(ft)
Slope,
S
(%)
Roughness
Coefficient
Assumed
Hydraulic
Radius
Velocity,
V
(ft/s)
Tt
(min)Tc (Eq. 3.3-5) Tc2 = Ti +Tt Tc100 = Ti +Tt Tc2 Tc100
h H 0.25 0.31 310 2.75%20.0 18.6 0 N/A N/A N/A N/A N/A 11.7 20.0 18.6 11.7 11.7
HISTORIC TIME OF CONCENTRATION
Channelized Flow
Design
Point Basin
Overland Flow Time of Concentration
Frequency Adjustment Factor:
(Equation 3.3-2 FCSCM)
(Equation 5-5 FCSCM)
(Equation 5-4 FCSCM)
(Equation 3.3-5 FCSCM)
Table 3.2-3 FCSCM
Therefore Tc2=Tc10
Rational Method Equation:
Rainfall Intensity:
h H 2.50 11.7 11.7 0.25 0.31 2.09 3.57 7.29 1.31 2.23 5.65
Tc100
(min)
Intensity,
i2
(in/hr)
Intensity,
i100
(in/hr)
HISTORIC RUNOFF COMPUTATIONS
Design
Point Basin(s)Area, A
(acres)
Tc2
(min)
Flow,
Q2
(cfs)
Flow,
Q100
(cfs)
C2 C100
IDF Table for Rational Method - Table 3.4-1 FCSCM
Intensity,
i10
(in/hr)
Flow,
Q10
(cfs)
AiCCQf
BASIN
TOTAL
AREA
(acres)
Tc2
(min)
Tc100
(min)
C2 C100
Q2
(cfs)
Q10
(cfs)
Q100
(cfs)
H 2.50 11.7 11.7 0.25 0.31 1.31 2.23 5.65
Rational Flow Summary | Historic Basin Flow Rates
6/8/202110:28 AM P:\1530-002\Drainage\Hydrology\1530-002_Historical Rational Calcs (FCSCM).xlsx\Summary Tables
CHARACTER OF SURFACE1:
Percentage
Impervious
2-yr Runoff
Coefficient
10-yr Runoff
Coefficient
100-yr Runoff
Coefficient
Developed
Asphalt .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………100%0.95 0.95 1.00
Concrete .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………100%0.95 0.95 1.00
Rooftop .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………90%0.95 0.95 1.00
Gravel .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………40%0.50 0.50 0.63
Pavers .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………40%0.50 0.50 0.63
Landscape or Pervious Surface
Playgrounds .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………25%0.35 0.35 0.44
Lawns Clayey Soil .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………2%0.25 0.25 0.31
Lawns Sandy Soil .…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..……………………………………………….…………………………..………………………………………………2%0.15 0.15 0.19
Notes:
Basin ID Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Rooftop
(ac)
Area of
Gravel
(ac)
Area of
Pavers
(ac)
Area of
Playgrounds
(ac)
Area of Lawns
(ac)
Composite
% Imperv.
2-year
Composite Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
A 0.12 0.00 0.02 0.00 0.00 0.00 0.00 0.11 17%0.36 0.36 0.45
B 0.05 0.00 0.04 0.00 0.00 0.00 0.00 0.01 82%0.82 0.82 1.00
C 0.09 0.00 0.09 0.00 0.00 0.00 0.00 0.00 100%0.95 0.95 1.00
D 0.41 0.00 0.07 0.00 0.00 0.00 0.00 0.34 19%0.37 0.37 0.46
E 0.27 0.21 0.02 0.00 0.00 0.00 0.00 0.05 84%0.83 0.83 1.00
F 0.05 0.03 0.01 0.00 0.00 0.00 0.00 0.01 79%0.80 0.80 1.00
G 0.19 0.03 0.02 0.00 0.00 0.00 0.00 0.15 26%0.42 0.42 0.53
H1 0.05 0.00 0.00 0.05 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H2 0.11 0.00 0.00 0.11 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H3 0.05 0.00 0.00 0.05 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H4 0.11 0.00 0.00 0.11 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H5 0.05 0.00 0.00 0.05 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H6 0.04 0.00 0.00 0.04 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H7 0.04 0.00 0.00 0.04 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H8 0.06 0.00 0.00 0.06 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H9 0.19 0.00 0.00 0.19 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H10 0.19 0.00 0.00 0.19 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H11 0.09 0.00 0.00 0.09 0.00 0.00 0.00 0.00 90%0.95 0.95 1.00
H12 0.16 0.00 0.15 0.00 0.00 0.00 0.00 0.02 90%0.88 0.88 1.00
OS 0.15 0.00 0.00 0.00 0.00 0.00 0.00 0.14 5%0.27 0.27 0.34
Basins A-OS 2.50 0.27 0.42 0.99 0.00 0.00 0.00 0.82 64% 0.72 0.72 0.90
DEVELOPED BASIN % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
2) Runoff Coefficients are taken from the Fort Collins Stormwater Criteria Manual, Chapter 3. Table 3.2-1 and 3.2-2
1) Percentage impervious taken from the Fort Collins Stormwater Criteria Manual, Chapter 5, Table 4.1-2 and Table 4.1-3
Combined Basins
Overland Flow, Time of Concentration:
Channelized Flow, Time of Concentration:
Total Time of Concentration :
T c is the lesser of the values of Tc calculated using T c = T i + T t
C2 C100
Length,
L
(ft)
Slope,
S
(%)
Ti2 Ti100
Length,
L
(ft)
Slope,
S
(%)
Roughness
Coefficient
Assumed
Hydraulic
Radius
Velocity,
V
(ft/s)
Tt
(min)Tc (Eq. 3.3-5) Tc2 = Ti +Tt Tc100 = Ti +Tt Tc2 Tc100
a A 0.36 0.45 50 11.00%4.4 3.9 0 N/A N/A N/A N/A N/A 10.3 4.4 3.9 5.0 5.0
b B 0.82 1.00 25 5.20%1.5 0.5 0 N/A N/A N/A N/A N/A 10.1 1.5 0.5 5.0 5.0
c C 0.95 1.00 30 4.00%1.0 0.6 0 N/A N/A N/A N/A N/A 10.2 1.0 0.6 5.0 5.0
d D 0.37 0.46 20 15.00%2.5 2.2 260 2.23%0.025 0.45 5.23 0.8 11.6 3.3 3.0 5.0 5.0
e E 0.83 1.00 37 4.76%1.8 0.7 19 0.50%0.013 0.15 2.29 0.1 10.3 2.0 0.8 5.0 5.0
f F 0.80 1.00 25 2.24%2.1 0.7 35 3.43%0.013 0.15 5.99 0.1 10.3 2.2 0.8 5.0 5.0
g G 0.42 0.53 25 12.00%2.8 2.3 75 5.00%0.025 0.45 7.83 0.2 10.6 2.9 2.5 5.0 5.0
h1 H1 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h2 H2 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h3 H3 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h4 H4 0.95 1.00 20 0.67%1.4 1.0 0 N/A N/A N/A N/A N/A 10.1 1.4 1.0 5.0 5.0
h5 H5 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h6 H6 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h7 H7 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h8 H8 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h9 H9 0.95 1.00 30 0.67%1.8 1.2 0 N/A N/A N/A N/A N/A 10.2 1.8 1.2 5.0 5.0
h10 H10 0.95 1.00 30 0.67%1.8 1.2 0 N/A N/A N/A N/A N/A 10.2 1.8 1.2 5.0 5.0
h11 H11 0.95 1.00 15 0.67%1.2 0.8 0 N/A N/A N/A N/A N/A 10.1 1.2 0.8 5.0 5.0
h12 H12 0.88 1.00 25 0.67%2.3 1.1 0 N/A N/A N/A N/A N/A 10.1 2.3 1.1 5.0 5.0
os OS 0.27 0.34 10 21.00%1.8 1.6 0 N/A N/A N/A N/A N/A 10.1 1.8 1.6 5.0 5.0
Stormtech Basins A-OS 0.72 0.90 20 2.25% 2.4 1.3 15 1.20% 1.30% 15.00% 0.35 0.7 10.2 3.1 2.0 5.0 5.0
Combined Basins
DEVELOPED DIRECT TIME OF CONCENTRATION
Channelized Flow
Design
Point Basin
Overland Flow Time of Concentration
Frequency Adjustment Factor:
(Equation 3.3-2 FCSCM)
(Equation 5-5 FCSCM)
(Equation 5-4 FCSCM)
(Equation 3.3-5 FCSCM)
Table 3.2-3 FCSCM
Therefore Tc2=Tc10
Rational Method Equation:
Rainfall Intensity:
a A 0.12 5.0 5.0 0.36 0.45 2.85 4.87 9.95 0.13 0.22 0.56
b B 0.05 5.0 5.0 0.82 1.00 2.85 2.85 9.95 0.12 0.12 0.53
c C 0.09 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.24 0.24 0.90
d D 0.41 5.0 5.0 0.37 0.46 2.85 2.85 9.95 0.43 0.43 1.90
e E 0.27 5.0 5.0 0.83 1.00 2.85 2.85 9.95 0.65 0.65 2.73
f F 0.05 5.0 5.0 0.80 1.00 2.85 2.85 9.95 0.11 0.11 0.50
g G 0.19 5.0 5.0 0.42 0.53 2.85 2.85 9.95 0.23 0.23 1.01
h1 H1 0.05 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.14 0.14 0.51
h2 H2 0.11 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.28 0.28 1.05
h3 H3 0.05 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.13 0.13 0.48
h4 H4 0.11 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.29 0.29 1.08
h5 H5 0.05 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.14 0.14 0.50
h6 H6 0.04 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.12 0.12 0.45
h7 H7 0.04 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.12 0.12 0.44
h8 H8 0.06 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.17 0.17 0.61
h9 H9 0.19 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.53 0.53 1.93
h10 H10 0.19 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.53 0.53 1.94
h11 H11 0.09 5.0 5.0 0.95 1.00 2.85 2.85 9.95 0.24 0.24 0.87
h12 H12 0.16 5.0 5.0 0.88 1.00 2.85 2.85 9.95 0.41 0.41 1.63
os OS 0.15 5.0 5.0 0.27 0.34 2.85 2.85 9.95 0.11 0.11 0.49
Stormtech Basins A-OS 2.50 5.0 5.0 0.72 0.90 2.85 2.85 9.95 5.13 6.42 22.41
DEVELOPED RUNOFF COMPUTATIONS
Design
Point Basin(s)Area, A
(acres)
Tc2
(min)
Flow,
Q2
(cfs)
Flow,
Q100
(cfs)
C2 C100
IDF Table for Rational Method - Table 3.4-1 FCSCM
Intensity,
i10
(in/hr)
Flow,
Q10
(cfs)
Combined Basins
Tc100
(min)
Intensity,
i2
(in/hr)
Intensity,
i100
(in/hr)
()()()AiCCQf=
BASIN
TOTAL
AREA
(acres)
Tc2
(min)
Tc100
(min)
C2 C100
Q2
(cfs)
Q100
(cfs)
A 0.12 5.0 5.0 0.36 0.45 0.13 0.56
B 0.05 5.0 5.0 0.82 1.00 0.12 0.53
C 0.09 5.0 5.0 0.95 1.00 0.24 0.90
D 0.41 5.0 5.0 0.37 0.46 0.43 1.90
E 0.27 5.0 5.0 0.83 1.00 0.65 2.73
F 0.05 5.0 5.0 0.80 1.00 0.11 0.50
G 0.19 5.0 5.0 0.42 0.53 0.23 1.01
H1 0.05 5.0 5.0 0.95 1.00 0.14 0.51
H2 0.11 5.0 5.0 0.95 1.00 0.28 1.05
H3 0.05 5.0 5.0 0.95 1.00 0.13 0.48
H4 0.11 5.0 5.0 0.95 1.00 0.29 1.08
H5 0.05 5.0 5.0 0.95 1.00 0.14 0.50
H6 0.04 5.0 5.0 0.95 1.00 0.12 0.45
H7 0.04 5.0 5.0 0.95 1.00 0.12 0.44
H8 0.06 5.0 5.0 0.95 1.00 0.17 0.61
H9 0.19 5.0 5.0 0.95 1.00 0.53 1.93
H10 0.19 5.0 5.0 0.95 1.00 0.53 1.94
H11 0.09 5.0 5.0 0.95 1.00 0.24 0.87
H12 0.16 5.0 5.0 0.88 1.00 0.41 1.63
OS 0.15 5.0 5.0 0.27 0.34 0.11 0.49
Basins A-OS 2.50 5.0 5.0 0.72 0.90 5.13 22.41
Rational Flow Summary | Developed Basin Flow Rates
Combined Basins
4/18/202311:17 AM P:\1530-002\Drainage\Hydrology\1530-002_Proposed Rational Calcs (FCSCM).xlsx\Summary Tables
APPENDIX B
HYDRAULIC COMPUTATIONS
B.1 – Storm Sewers
B.2 – Inlets
B.3 – Detention Facilities
APPENDIX B.1
STORM SEWERS
STORM LINE A - OUTFALL