HomeMy WebLinkAboutFORTY-THREE PRIME - FDP210019 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORT
FINAL DRAINAGE REPORT
FORTY-THREE PRIME
FORT COLLINS, COLORADO
OCTOBER 20, 2021
NORTHERNENGINEERING.COM
970.221.4158
FORT COLLINS
GREELEY
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FORT COLLINS | GREELEY COVER LETTER
October 20, 2021
City of Fort Collins
Stormwater Utility
700 Wood Street
Loveland, CO 80537
RE: FINAL DRAINAGE REPORT FOR
FORTY-THREE PRIME
Dear Staff:
Northern Engineering is pleased to submit this Final Drainage Report for your review. This report accompanies
the combined Final Plan submittal for the proposed Forty-Three Prime.
This report has been prepared in accordance with the Fort Collins Stormwater Criteria Manual (FCSCM) and
serves to document the stormwater impacts associated with the proposed Forty-Three Prime multifamily
project. We understand that review by the City of Fort Collins 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.
Frederick S. Wegert, PE
Project Engineer
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FORT COLLINS | GREELEY TABLE OF CONTENTS
TABLE OF CONTENTS
I. GENERAL LOCATION AND DESCRIPTION .......................................................... 1
II. DRAIN BASINS AND SUB-BASINS ..................................................................... 3
III. DRAINAGE DESIGN CRITERIA .......................................................................... 4
IV. DRAINAGE FACILITY DESIGN ........................................................................... 6
V. CONCLUSIONS .............................................................................................. 8
VI. REFERENCES ................................................................................................ 9
TABLES AND FIGURES
FIGURE 1 – VICINITY MAP ...................................... ERROR! BOOKMARK NOT DEFINED.
FIGURE 2 – AERIAL PHOTOGRAPH ........................ ERROR! BOOKMARK NOT DEFINED.
FIGURE 3 – FEMA FIRMETTE (MAP NUMBER 08069C0994F) ............................................3
APPENDICES
APPENDIX A – HYDROLOGIC COMPUTATIONS
APPENDIX B – HYDRAULIC CALCULATIONS
APPENDIX C –WATER QUALITY/LID COMPUTATIONS
APPENDIX D – EROSION CONTROL REPORT
APPENDIX E – USDA SOILS REPORT
APPENDIX F – FEMA FIRMETTE
APPENDIX G – EXCERPTS FROM PREVIOUS REPORTS
MAP POCKET
DR1 – DRAINAGE EXHIBIT
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GENERAL LOCATION AND DESCRIPTION
A. LOCATION
Vicinity Map
The Forty-Three Primeproject site is located in the northeast quarter of Section 13, Township 6
North, Range 69 West of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State
of Colorado. The project site is Tract A of the Provincetowne P.U.D. Filing 2, which was
approved in 2001.
The project site (refer to Figure 1) is bordered to the north by East Trilby Road; to the south by
Candlewood Drive; to the east by Brittany Drive and to the west by Autumn Ridge Drive.
A regional detention pond for Provincetowne Subdivision is located on the east quarter of the
site.
B. DESCRIPTION OF PROPERTY
The Forty-Three Primeare comprised of ±4.87acres.
The site is currently occupied by a small asphalt parking lot and native grasses.
Figure 1 – Vicinity Map
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The existing groundcover consists of native grasses. A regional detention pond with
established vegetation is on the east quarter of the site.
The original Provincetowne drainage design divided the site into five (5) basins – Basins 212,
220, 221, 225 and 406. In general, all of the basins drained from south to north, where they
were intercepted by a swale along the north boundary. The exception was Basin 212, which
drained west to Autumn Ridge Drive and then collected by an inlet located on Trilby. Once
captured, the flows from all of the basins were conveyed to the northeast corner of site and
detained in Detention Pond 306 in the Provincetowne drainage design. Drainage will then be
conveyed via storm sewer to Detention Pond 307 in the Provincetown e drainage design. The
Provincetowne drainage design ultimately discharges into Fossil Creek. An Existing Drainage
Exhibit has been provided at the back of this report for reference, along with excerpts from the
original report in the appendix.
According to the United States Department of Agriculture (USDA) Natural Resources
Conservation Service (NRCS) Soil Survey website:
http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx), the site consists of Cushman
fine sandy loam (Hydrologic Soil Group C), Kim loam (Hydrologic Soil Group B), and Renohill
clay loam (Hydrologic Soil Group D).
There is 36” storm sewer along the north property line and a regional detention pond for
Provincetowne Subdivision within the project site.
The proposed development will consist of five multifamily buildings and one clubhouse. Other
proposed improvements include: a new parking lot, sidewalks and landscaping.
The proposed land use is multifamily attached. This is use is permitted with administrative
review in the Low-Density Mixed-Use District (LMN).
Figure 2 – Aerial Photograph
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C. FLOODPLAIN
The subject property is within FEMA regulatory floodplain for Fossil Creek. In particular, the
project site is not located within Area of Minimal Flood Hazard, Zone X, per Map Number
08069C1200F (Effective date: December 19, 2006). The project site is not located within a City of
Fort Collins regulated floodplain.
DRAIN BASINS AND SUB-BASINS
A. MAJOR BASIN DESCRIPTION
Forty-Three Prime is within the City of Fort Collins Fossil Creek major drainage basin.
Specifically, the project site is situated in the western half of this major drainage basin. The
Fossil Creek drainage basin extends along the south end of Fort Collins, from the foothills
across Interstate 25 past County Road 5. It encompasses 32 square miles in the city of Fort
Collins and Larimer County. Historically, the basin consisted of agricultural land, but the
basin has experienced significant development in the recent past. Runoff from the major
drainage basin drains to Fossil Creek Reservoir.
Figure 3 – FEMA Firmette (Map Number 08069C1200F)
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B. SUB-BASIN DESCRIPTION
The project site was included in the drainage study for Provincetowne P.U.D. Filing by Manhard
Consulting and dated March 22, 2001.
The outfall for the project site is Detention Pond 306 (on the eastern third of the site) of the
Provincetowne drainage study.
The existing subject site can be defined with five (5) sub-basin that encompasses the entire
project site.
The existing site runoff generally drains from south to north to be collected by either a
drainage swale or the Trilby Road curb and gutter and discharging into Detention Pond 306.
Existing storm sewer from Provincetowne Filing 3 conveys stormwater along the north side of
the site and into Detention Pond 306. Detention Pond 306 is on the eastern third of the site.
DRAINAGE DESIGN CRITERIA
A. ORIGINAL PROVISIONS
There are no optional provisions outside of the FCSCM proposed with Forty-Three Prime.
Off-site drainage from Provincetowne Subdivision passes through the site and into a
regional detention pond (Detention Pond 306) on the east side of the site.
B. STORMWATER MANAGEMENT STRATEGY
The overall stormwater management strategy employed with Forty-Three Prime 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. The first consideration taken in trying to
reduce the stormwater impacts of this development is the site selection itself. By choosing
an already developed site with public storm sewer currently in place, the burden is
significantly less than developing a vacant parcel absent of any infrastructure.
The Forty-Three Prime aims to reduce runoff peaks, volumes and pollutant loads from
frequently occurring storm events (i.e., water quality (i.e., 80th percentile) and 2-year storm
events) by implementing Low Impact Development (LID) strategies. Wherever practical,
runoff will be routed across landscaped areas or through rain gardens. These LID practices
reduce the overall amount of impervious area, while at the same time Minimizing Directly
Connected Impervious Areas (MDCIA). The combined LID/MDCIA techniques will be
implemented, where practical, throughout the development, thereby slowing runoff and
increasing opportunities for infiltration.
Step 2 – Implement BMPs that Provide a Water Quality Capture Volume (WQCV) with
Slow Release. The efforts taken in Step 1 will help to minimize excess runoff from frequently
occurring storm events; however, urban development of this intensity will still have
stormwater runoff leaving the site. The primary water quality treatment will occur in the
underground vaults located at the south end of the project.
Step 3 – Stabilize Drainageways. As stated in Section I.B.5, above, the site is in the Fossil
Creek Major Basin, however no changes to the channel are proposed with this project. While
this step may not seem applicable to Forty-Three Prime, the proposed project indirectly
helps achieve stabilized drainageways, nonetheless. Once again, site selection has a positive
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effect on stream stabilization. By developing an infill site with existing stormwater
infrastructure, combined with LID and MDCIA strategies, the likelihood of bed and bank
erosion is reduced. 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.
C. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS
The subject property is not part of any Overall Development Plan (ODP) drainage study or
similar “development/project” drainage master plan.
The site plan is constrained on by a public street on all sides. A regional detention pond is on
the east third of the site.
D. HYDROLOGICAL CRITERIA
The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure 3.4-
1 of the FCSCM, serve as the source for all hydrologic computations associated with The Forty-
Three Primedevelopment. Tabulated data contained in Table 3.4-1 has been utilized for
Rational Method runoff calculations.
The Rational Method has been employed to compute stormwater runoff utilizing coefficients
contained in Table 3.2-2 of the FCSCM.
The Rational Formula-based Federal Aviation Administration (FAA) procedure has not been
utilized for detention storage calculations since detention is not required for the project.
Two 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.
E. HYDRAULIC CRITERIA
The drainage facilities proposed with The Forty-Three Prime project are designed in
accordance with criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control
District’s (UDFCD) Urban Storm Drainage Criteria Manual.
F. FLOODPLAIN REGULATIONS COMPLIANCE
As previously mentioned, this project within an Area of Minimal Flood Hazard, Zone X, per
FEMA.
G. MODIFICATIONS OF CRITERIA
No formal modifications are requested at this time.
H. CONFORMANCE WITH WATER QUALITY TREATMENT CRITERIA
City Code requires that 100% of runoff from a project site receive some sort of water quality
treatment. This project proposes to provide water quality treatment using rain gardens at
east of the parking lot and along the north property line. Rain gardens are considered a LID
treatment method. Due to the physical constraints associated with an infill project of this
nature and the prohibition of providing water quality facilities within the public right-of-way,
there are some small, narrow areas around the perimeter of the project that cannot be
captured. The uncaptured areas tend to be narrow strips of concrete flatwork that link the
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building entrances to the public sidewalks as well as small planter beds between the
building and public sidewalks or property lines. However, 100% of the site will discharge,
either through internal storm sewer or public curb and gutter, into the existing regional
detention pond.
I. CONFORMANCE WITH LOW IMPACT DEVELOPMENT (LID)
Although stormwater quantity detention is not required, stormwater quality will be
addressed by permanent Best Management Practices (BMPs) and Low Impact Development
(LID) requirements. City LID requirements specify that either 75% of all newly added
impermeable areas receive water quality treatment from a LID facility OR 50% of new
pavements be treated by a LID methoud along with 25% of drivable surfaces being
permeable pavers. This project proposes to treat at least 75% of the new impervious areas
through a combination of a large rain garden on the east side of the project site.
The large rain garden will treat 78,285 square-feet of impervious area on the site. Three
small rain gardens along the north property line will treat 12,093 square feet. The total
treated impervious area is 90,377 square feet. This is 93.32% of the total impervious area
within the project area, which exceeds the amount of LID treatment required by City Code.
The large rain garden will be designed as a single feature that will treat the majority of the
parking lot, concrete flatwork and rooftops by conveying flows from the parking area into
the rain garden. Two 12” pipe will allow both sides of the large rain garden to equalize. The
facility will have 12” of ponding, at which point stormwater will flow out the east side of the
facility, down a reinforced rundown and into the existing detention pond. A LID Treatment
Exhibit is provided with this report detailing the treatment areas and methods.
DRAINAGE FACILITY DESIGN
A. GENERAL CONCEPT
The main objective of The Forty-Three Primedrainage design is to maintain existing drainage
patterns, while not adversely impacting adjacent properties.
A 36” storm sewer from Provincetowne Subdivision conveys storm water along the north
property line and into a regional detention pond on the east third of the site.
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.
Drainage for the project site has been analyzed using seven (7) drainage sub-basins,
designated as sub-basins OS1, A, B1 – B4, and C. The drainage patterns anticipated for the
basins are further described below.
Sub-Basin OS1
Sub-Basin OS1 encompasses the west edge of the site that drains directly into the Autumn
Ridge Drive curb and gutter. Runoff from Sub-Basin OS1 will collect with the Autumn Ridge
curb and gutter, flow into the Trilby Road curb and gutter, and into an inlet at the
intersection of Trilby Road and Brittany Drive. The inlet discharges into the regional
detention pond (Pond 306) along the east third of the site. Pond 306 discharges into the
Provincetowne storm sewer and detention system and ultimately into Fossil Creek.
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Sub-Basin A
Sub-Basin A encompasses the parking lot and the large rain garden. Runoff will sheet flow
across the parking lot to be collected by onsite curb and gutter. The curb and gutter will
convey the stormwater to a 7.5 ft wide sidewalk culvert. The sidewalk culvert will discharge
the runoff into the rain garden. Two 12-inch pipes are located within the rain garden to
convey stormwater past a firepit and into the other half of the rain garden. The 12-inch
pipes will equalize the flow between the two halves of the large rain garden. Stormwater
will overtop the east wall of the rain garden, travel down a turf reinforced spillway, and into
the Pond 306. Pond 306 discharges into the Provincetowne storm sewer and detention
system and ultimately into Fossil Creek.
Sub-Basins B1 – B4
Sub-Basins B1 to B4 consist of the apartment buildings and surrounding landscaping. A
storm drain system is provided to collect as much roof runoff as possible and convey it to
the large rain garden described in Sub-Basin A. Runoff will then overtop the east wall of the
rain garden, travel down a turf reinforced spillway, and into Pond 306. Pond 306 discharges
into the Provincetowne storm sewer and detention system and ultimately into Fossil Creek.
Sub-Basin C
Sub-Basin C consist of that area of the site that drains directly into Pond 306. Roof runoff
along the north buildings (Buildings A, B, C, and D) will sheet flow into a drainage swale
along the north property line where three small rain gardens will provide water quality
treatment. The three rain gardens will discharge into an existing 36” storm sewer, and the
storm sewer discharges into Pond 306. Pond 306 discharges into the Provincetowne storm
sewer and detention system and ultimately into Fossil Creek.
A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the end of this report.
B. SPECIFIC DETAILS
Stormwater detention for this site is provided by the regional detention pond located on site
(Pond 306). While the drainage patterns of the proposed project generally conform with the
previously approved drainage design, the drainage basins themselves do not have the same
extents or areas. Since a direct comparison cannot be made between the original and
proposed basins, a weighted percent imperviousness was developed. Using this approach, we
find the original drainage plan had a Composite Percent Imperviousness of 42.6% (C100=0.56)
while the proposed project will have a Composite Percent Imperviousness of 39.5% (C100=0.44).
This is an overall imperviousness decrease of 3.1%, which will result in decreased runoff from
the project than the original report anticipated.
As shown in the analysis of overall site imperviousness, the proposed project will decrease the
imperviousness of the project site when compared to the originally approved project, thereby
decreasing the runoff from the project site. This decrease in imperviousness and runoff also
results in a decrease in the required detention volume identified in the original Provincetowne
report. Since detention was accounted for in the original drainage report and the detention
required by this project is less than originally assumed, no additional detention is required as a
part of this project, and no changes to the existing facilities are proposed. The FAA method was
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not used to size the on-site detention volume for quantity detention since detention is not
required.
Similar to the analysis performed for detention, all areas draining to existing inlets have a
decrease in imperviousness and area from what was projected in the original Provinc etowne
report. This results in decreased runoff to the existing inlets that were designed and approved
with the earlier project, so no additional inlet calculations from the proposed infrastructure is
provided as a part of this report.
The majority of the water quality for Forty-Three Prime is provided by the onsite rain gardens
shown in Table 1. Basin OS 1 and portions of Basin C, due to grading constraints and proximity
to the existing detention pond (Pond 306), wil not drain towards the onsite rain gardens.
Instead, these basins will drain into Detention Pond 306. As mentioned above, Basin OS1 will
drain into existing curb and gutter along Autumn Ridge Drive and Trilby Road where a Type-R
inlet will collect and direct drainage into Pond 306. The northern area of Basin C, including
runoff from the north half of Buildings A, B, and C, will be treated by Rain Gardens C1, C2,and
C3. However, the eastern half of Basin C, including Pond 306, drains directly into Pond 306.
As mentioned above, water quality for 93.32% of the impervious area of the site is provided
within one of the rain gardens. Required water quality volume, per City of Fort Collins criteria,
for the area of Basins C and OS1 bypassing the rain gardens is 493 cu. ft. According to the
Provincetowne Subdivision Filing 2 Drainage Report, Pond 306 was constructed to provide
2.74 ac-ft of detention. While designed and constructed prior to Fort Collins water quality
requirments, Pond 306 will detain stormwater to allow sedimentation to occur. Furthermore,
according to the Provincetowne Subdivision Filing 2 Drainage Report, Pond 306 will discharge
into Pond 307 where additional water quality treatment can occur.
Description
Required
Volume (cu. ft)
Design Volume
(cu. ft.)
Large Double Rain Gardens 1457 1904
Rain Garden C1 122 171
Rain Garden C2 152 200
Rain Garden C3 122 197
Table 1: Onsite Water Quality Volumes
CONCLUSIONS
A. COMPLIANCE WITH STANDARDS
The design elements comply without variation and meet all LID requirements.
The drainage design proposed with The Forty-Three Prime complies with the City of Fort
Collins Master Drainage Plan for Fossi Creek and the Provincetowne P.U.D. Filing 2 Drainage
Report.
The site is located with an Area of Minimal Flood Hazard, Zone X, per FEMA.
The drainage plan and stormwater management measures proposed with The Forty-Three
Prime project are compliant with all applicable State and Federal regulations governing
stormwater discharge.
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B. DRAINAGE CONCEPT
The proposed grading concept closely matches the original drainage patterns and decreases
overall site imperviousness and additional detention is proven to be unnecessary. Stormwater
quality has been provided and meets the city requirements for Low Impact Development
treatment. Therefore, it is my professional opinion that Forty-Three Prime satisfies all
applicable stormwater criteria and will effectively limit potential damage associated with its
stormwater runoff.
The drainage design proposed with this project will effectively limit potential damage
associated with its stormwater runoff.
The Forty-Three Prime development will not impact the Master Drainage Plan
recommendations for the McClelland major drainage basin.
REFERENCES
1. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by Ordinance
No. 159, 2018, and referenced in Section 26-500 of the City of Fort Collins Municipal Code.
2. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation Service,
United States Department of Agriculture.
3. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control District,
Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008.
4. Final Drainage and Erosion Control Report for Provincetowne PUD Filing Two, March 22, 2001,
Manhard Consulting, Ltd.
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APPENDIX A
HYDROLOGIC COMPUTATIONS
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APPENDIX B
HYDRAULIC CALCULATIONS
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FORT COLLINS | GREELEY APPENDIX
APPENDIX C
WATER QUALITY/LID COMPUTATIONS
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FORT COLLINS | GREELEY APPENDIX
APPENDIX D
EROSION CONTROL REPORT
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FORT COLLINS | GREELEY EROSION CONTROL REPORT
EROSION CONTROL REPORT
A comprehensive Erosion and Sediment Control Plan (along with associated details) has been included with the
final construction drawings. It should be noted; however, any such Erosion and Sediment Control Plan serves
only as a general guide to the Contractor. Staging and/or phasing of the BMPs depicted, and additional or
different BMPs from those included may be necessary during construction, or as required by the authorities
having jurisdiction.
It shall be the responsibility of the Contractor to ensure erosion control measures are properly maintained and
followed. The Erosion and Sediment Control Plan is intended to be a living document, constantly adapting to
site conditions and needs. The Contractor shall update the location of BMPs as they are installed, removed, or
modified in conjunction with construction activities. It is imperative to appropriately reflect the current site
conditions at all times.
The Erosion and Sediment Control Plan shall address both temporary measures to be implemented during
construction, as well as permanent erosion control protection. Best Management Practices from the Volume 3,
Chapter 7 – Construction BMPs will be utilized. Measures may include, but are not limited to, silt fencing and/or
wattles along the disturbed perimeter, gutter protection in the adjacent roadways, and inlet protection at
existing and proposed storm inlets. Vehicle tracking control pads, spill containment and clean-up procedures,
designated concrete washout areas, dumpsters, and job site restrooms shall also be provided by the Contractor.
Grading and Erosion Control Notes can be found on Sheet CS2 of the Utility Plans. The Final Utility Plans will also
contain a full-size Erosion Control Plan as well as a separate sheet dedicated to Erosion Control Details. In
addition to this report and the referenced plan sheets, the Contractor shall be aware of, and adhere to, the
applicable requirements outlined in any existing Development Agreement(s) of record, as well as the
Development Agreement, to be recorded prior to issuance of the Development Construction Permit. Also, the
Site Contractor for this project may be required to secure a Stormwater Construction General Permit from the
Colorado Department of Public Health and Environment (CDPHE), Water Quality Control Division – Stormwater
Program, before commencing any earth disturbing activities. Prior to securing said permit, the Site Contractor
shall develop a comprehensive Storm Water Management Plan (SWMP) pursuant to CDPHE requirements and
guidelines. The SWMP will further describe and document the ongoing activities, inspections, and maintenance
of construction BMPs.
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APPENDIX E
USDA SOILS REPORT
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FORT COLLINS | GREELEY APPENDIX
APPENDIX F
FEMA FIRMETTE
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FORT COLLINS | GREELEY APPENDIX
APPENDIX G
EXCERPTS FROM PREVIOUS REPORTS
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FORT COLLINS | GREELEY APPENDIX
MAP POCKET
DR1 – DRAINAGE EXHIBIT