HomeMy WebLinkAboutFAIRWAY APARTMENTS - PDP210005 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT
Fairway Lane Apartments – Fort Collins, CO
Preliminary Drainage Report
Project No. 20065
Jensen LaPlante Development
1603 Oakridge Drive, Suite #101
Fort Collins, CO 80525
Submittal Date: 03/17/2021
March 17, 2021
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Preliminary Drainage and Erosion Control Report for
FAIRWAY LANE APARTMENTS
Dear Staff:
Sanderson Stewart is pleased to submit this Preliminary Drainage and Erosion Control Report for your
review. This report accompanies the Project Development Plan submittal for the proposed Fairway Lane
Apartments.
This report has been prepared in accordance to Fort Collins Stormwater Criteria Manual (FCSCM) and
serves to document the stormwater impacts associated with the proposed 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,
SANDERSON STEWART
Charles Sonnier, PE
Senior Engineer/Project Manager
Fairway Lane Apartments March 17, 2021 Page 1
Project No. 20065
March 17, 2021
Project No. 20065
PRELIMINARY (PDP) STORMWATER MANAGEMENT REPORT
FOR
FAIRWAY LANE APARTMENTS
FORT COLLINS, COLORADO
I. Overview Narrative
The proposed project is a multi-family development that will include the construction of eight new
apartment buildings, five garages, a clubhouse with amenities and supporting infrastructure
improvements. The purpose of this report is to identify the required stormwater management and water
quality facilities for the developed site. The design standards governing this project are the Fort Collins
Stormwater Criteria Manual (FCSCM), Dec 2018 and the Mile High Flood District’s Urban Storm
Drainage Criteria Manual (USDCM), Volume 1-3.
II. General Project Location
The proposed development is approximately 10.4 acres located at the northwest corner of the South
College Avenue and West Fairway Lane intersection. The project site associated with this report
encompasses existing Parcel Nos. 9602100006, 9602100012, 9602100015, and 9602109002. More
specifically, the project site lies within the northeast corner of Section 3, Township 6 North, Range 69
West, latitude 40.5188, longitude -104.5.0785. The site is generally bounded by the Fort Collins South
Transit Center/Fossil Boulevard on the west, the Spradley Barr Ford car dealership on the north,
College Avenue on the east and Fairway Lane on the south. Surrounding properties include: the Spradley
Barr Ford car dealership adjacent to the property on the north side, Woodley’s Fine Furniture,
Worldpac, and Florida Tile on the south side of Fairway Lane Road, and VCA Fort Collins Animal
Hospital, and Fossil Ridge Animal Hospital east of South College Avenue. The existing project site is
undeveloped and has several mature trees with the ground cover consisting of small shrubs and grasses.
Refer to Appendix A for a Vicinity map.
III. Existing Site Information
Major Basin Description
This property is located within the City of Fort Collins Fossil Creek Drainage Basin that outfalls to the
Poudre River drainage basin which is a major tributary to the South Platte River.
Sub-Basin Description
The site is mostly flat to minimally sloped. Generally, runoff from the site sheet flows to the east and
south to the existing detention pond, travels through an existing 15” RCP pipe to a water quality facility
on the Woodley’s property south of Fairway Lane. The flow is then conveyed through a curb chase onto
College Avenue where it is picked up by curb inlets approximately 700 feet south and 1400 feet south of
the Woodley’s property, and ultimately discharges into Fossil Creek. Flow then conveys southeast
approximately 2.5 miles to the Fossil Creek Reservoir and then another 0.5 miles from the Fossil Creek
Reservoir Outlet to the Cache La Poudre River.
Fairway Lane Apartments March 17, 2021 Page 2
Project No. 20065
This project site sits within the FEMA Zone X, Area of Minimal flood Hazard, per FEMA FIRM Maps
08069C1000F, dated December 19, 2006.
According to the NRCS website, the site consists of primarily Nunn clay loam, 1 to 3 percent slopes,
and Fort Collins loam, 0 to 3 percent slopes, both classified as Type C soils. The soils have an erodibility
factor of 0.28 to 0.43 which suggests a moderate susceptibility to sheet and rill erosion by runoff.
Refer to Appendix A for soil data and floodplain map.
IV. Project Description
The proposed project will consist of eight new multi-family buildings, five garages, and one clubhouse.
Proposed utility improvements will consist of storm sewer, sanitary sewer, and waterline improvements.
Surface improvements will include driveway, parking lot, landscaped areas and concrete sidewalk
surrounding the buildings. Off-site improvements will mainly consist of a sidewalk, a driveway
connection, and an emergency access connection on the north side of the site connecting to the adjacent
property. Refer to Appendix A for Site Plan.
V. Drainage Design Criteria
A. Regulations/Development Criteria
The design standards governing this project are the Fort Collins Stormwater Criteria Manual
(FCSCM), Dec 2018 and the Mile High Flood District’s Urban Storm Drainage Criteria Manual
(USDCM), Volume 1-3.
B. Four-Step Process
The overall stormwater management strategy to be employed with the proposed project will
utilize the “Four Step Process” to minimize adverse impacts of urbanization on receiving
waters. The following is a description of how the proposed development will incorporated
each step.
Step 1 – Employ Runoff Reduction Practices
Several techniques will be utilized with the proposed development to facilitate the reduction of
runoff peaks, volumes, and pollutant loads by implementing multiple Low Impact
Development (LID) strategies. Proposed techniques will include providing landscaped islands
and buffer areas throughout the site to reduce the overall impervious area, to minimize directly
connected impervious areas (MDCIA) and to grade the site such that runoff is routed over a
longer distance to increase the time of concentration.
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; however, urban
development of this intensity will still generate stormwater runoff that will require additional
BMPs and water quality. The majority of stormwater runoff from the site will ultimately be
treated by the proposed detention pond with an associated outlet structure on the southeast
corner of the property. The detention pond will act as a sedimentation basin by allowing for
the sediment, fines and pollution suspended in the stormwater to settle out before being
released downstream.
Fairway Lane Apartments March 17, 2021 Page 3
Project No. 20065
Step 3 – Stabilize Streams
There are no major wet drainage conveyances within the subject property.
Step 4 – Implement Site Specific and Other Source Control BMPs
The proposed project will improve upon site specific source controls compared to historic
conditions: The proposed development will provide water quality treatment measures; thus,
eliminating sources of potential pollution previously left exposed to weathering and runoff
processes. The site-specific measure for this project that will provide a higher level of water
quality treatment (LID) will consist of permeable paver systems located throughout the site.
C. Hydrological Criteria
1. 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 proposed development. Tabulated data contained in Table 3.4-1
in the FCSCM has been utilized for Rational Method runoff calculations. Refer to
Appendix B for criteria used.
2. The Rational Method will be utilized to compute stormwater runoff utilizing
coefficients and calculation methods contained in Chapter 5, Section 3.2 of the
FCSCM.
3. Three separate design storms will be utilized to address distinct drainage scenarios.
The first design storm event analyzed was 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
comparison purposes only, was the 10-year event.
4. The project site falls within the Fossil Creek Drainage Basin. According to the City
of Fort Collins, the developed release rate from the stie must be a maximum 0.2
cfs/acre.
5. No other assumptions or calculation methods have been used with this
development that are not referenced by current City of Fort Collins criteria.
6. Per recent meetings with City Stormwater staff, it was discussed that the preferred
outfall would be to utilize the existing Weberg PUD outfall pipe that discharges to
the south across the Woodly’s parking lot that ultimately collects in a water quality
pond near the southeast corner of the parking lot and discharges to the South
College Avenue right-of-way. Most, if not all, of the existing outfall pipe across the
Woodly’s parking lot lies within an existing access and drainage easement.
Coordination with Woodly’s Furniture store ownership and Water Utilities
Development Review will be required regarding discharging to this pipe. Refer to
Appendix E for excerpt of Weberg PUD Drainage Report and Plat.
D. Hydraulic Criteria
1. All drainage and LID facilities proposed with the project are designed in accordance
with criteria outlined in the FCSCM and/or the USDCM. Refer to Appendix B for
Rational Method calculation.
Fairway Lane Apartments March 17, 2021 Page 4
Project No. 20065
E. Modifications of Criteria
1. The proposed development is not requesting any modifications to criteria at this
time.
VI. Proposed Drainage Facilities
A. General Concept
The intent of the proposed design is to safely and adequately convey the minor and major
design storm given the site’s existing constraints that were previously mentioned.
1. Stormwater from the project site will be collected into a proposed pond at the
southeast corner of the site. This will serve as detention and provide standard water
quality treatment for the contributing basins totaling approximately 11.4-acres.
2. A more detailed description of the projects sub-basins and drainage patterns is
provided below.
B. Specific Details
The proposed storm drainage system will collect runoff via sheet flow, curb and gutter and
drainage pans and convey to proposed area and curb inlets. The storm drain piping will then
convey the collected stormwater to the proposed detention pond on the southeast corner of the
property. Three total outfall pipes are proposed to discharge into the proposed detention pond.
The basins contributory to the proposed detention pond and the off-site basins are described
below.
1. There are five (5) total sub-basins analyzed for this project. Three (3) basins will be
collected in the proposed storm system and detention pond while the two (2)
remaining basins consist of areas that cannot be captured and detained but instead
will flow to South College Avenue. These basins are further described below.
Basin Nos. 1 and 2: These sub-basins total to approximately 9.9 acres and are
proposed to be collected in various storm inlets throughout the site via sheet flow, curb
and gutter and drainage pans. The flow will then be piped to discharge points into the
detention pond at the two design points, 1 and 2, associated with the two basins. These
basins generally are comprised of rooftop, driveway and parking lot, sidewalk, and
landscaping. The percent composite imperviousness for these basins is approximately
61%.
Basin 3: This sub-basin totals to approximately 0.89 acres and consists of the southeast
side of the transit center, the east half of Fossil Blvd, and the north side of Fairway
Lane. The runoff is proposed to be collected in a storm inlet just to the east of the
proposed access off Fossil Blvd. The flow will then be piped directly into the proposed
detention pond at design point 3. The percent composite imperviousness of this basin
is approximately 53%.
Basin OS-1: This basin consists of a 0.27-acre area that cannot be collected and
conveyed to the proposed detention pond. OS-1 is located along the east side of the
site and will generally sheet flow east to the South College Avenue right-of-way. The
percent composite imperviousness of this basin is approximately 67% with an
approximate discharge rate of 0.54 cfs (2-year) and 2.3 cfs (100-year).
Fairway Lane Apartments March 17, 2021 Page 5
Project No. 20065
Basin OS-2: Basin OS-2 is a small 0.12-acre strip of the north half of Fossil Blvd that
currently flows into the existing detention basin for Lot 2 of the Weberg PUD. The on-
site grading of the proposed detention pond would prevent this basin from being
directed into the proposed detention pond and is therefore proposed to flow east to the
South College Avenue right-of-way and then south to the same point that the flow
from this basin historically discharges to. The percent composite imperviousness of this
basin is approximately 92% with an approximate discharge rate of 0.30 cfs (2-year) and
1.2 cfs (100-year).
Refer to Appendix C for the Drainage Basin Map
2. A curb cut currently exists at the southeast edge of the Spradley Barr Ford Dealership
property to accommodate a future road to the south from the Spradley Barr property.
Stormwater currently flows into the project site at this location and over a riprap
rundown. This project proposes to re-establish the curb line at this location as the
future roadway will no longer be constructed. As a result, stormwater on the Spradley
Barr property will continue to flow east to the South College Avenue right-of-way
rather than flow onto the project site.
3. Standard water quality and LID treatment will be served by the detention facility at
the southeast portion of the site and permeable paver systems located throughout the
site.
4. Final design details, and construction documentation shall be provided to the City of
Fort Collins for review prior to Final Development Plan approval.
5. Stormwater facility Standard Operating Procedures (SOP) will be provided by the
City of Fort Collins in the Development Agreement, as required.
C. Low Impact Development (LID)
The LID requirement option for this project will treat at least 50% of the new and/or
modified impervious areas. Proposed on-site LID treatment intended to meet this
requirement will include permeable paver systems located throughout the site. Refer to
Appendix D for permeable paver LID Exhibit with calculations.
D. Detention Basin Design
The proposed detention basin is located at the southeast corner of the site. The basin is designed to
store the required runoff from the proposed site and attenuate the flows to at or below the developed
maximum release rate for the Fossil Creek Drainage Basin of 0.2 cfs/acre. The detention basin will also
be designed to treat and release the WQCV with a 40-hour drain time.
The detention pond’s outlet structure will be designed with an orifice plate to slowly discharge the water
quality volume while the structure’s horizontal and vertical openings will attenuate the flows to at or
below the maximum 0.2 cfs/acre release rate. An emergency spillway will be designed to pass the
developed 100-year event to the Fairway Lane right-of-way.
The control structure will utilize the existing 15-inch RCP outfall pipe that will convey runoff south
under Fairway lane, through the Weberg property, and outfall to the College Avenue Right-Of-Way.
Refer to Table 1 below for proposed detention basin input parameters and storage volume results.
Fairway Lane Apartments March 17, 2021 Page 6
Project No. 20065
Table 1. Proposed Detention Basin Summary Table
Pond ID
Contributing
Area
(Acres)
Allowed
Release
Rate @
0.2
cfs/acre
Required
100-yr
Detention
Volume
(ac-ft)
Provided
100-yr
Detention
Volume
(ac-ft)
100-yr
Water
Surface
Elevation
(ft)
SE
Pond 11.42 2.28 2.47 2.47 4997.65
VII. Variance Requests (if necessary)
The proposed development is not requesting any variances at this time.
VIII. Erosion Control
Erosion and sediment control will be achieved through construction control measure to include various
BMPs selected from the FCSCM such as silt fencing, vehicle tracking devices, inlet/outlet protection
and curb protection, concrete washout, vehicle tracking control and stabilized staging areas. BMP
locations will be shown in the corresponding Utility Plans associated with this report. A separate
Erosion Control Report and Erosion Control Plans will be provided at Final Development Plan
submittal.
IX. Conclusion
The proposed drainage system design will provide safe conveyance of the minor and major storm events
while providing increased water quality treatment compared to the existing condition.
The City of Fort Collins Stormwater Criteria Manual has been used to establish the criteria for the
developed condition stormwater management plan within the project area.
A. Compliance with Standards
1. The drainage design proposed with the proposed project complies with the City of Fort
Collins’ Stormwater Criteria Manual.
2. The drainage design proposed with this project complies with requirements for the
Fossil Creek Drainage Basin.
3. The drainage plan and stormwater management measures proposed with the proposed
development are compliant with all applicable State and Federal regulations governing
stormwater discharge.
4. The erosion control materials that will be submitted with the final plans will comply
with the requirements set forth by the City of Fort Collins’ Stormwater Criteria Manual
and the Urban Storm Drainage Criteria Manual.
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Project No. 20065
B. Drainage Concept
1. The drainage design proposed with this project will effectively limit any potential
flooding and/or damage associated with runoff quantities while effectively treating
runoff through water quality mitigation features.
2. Standard water quality and LID treatment will be provided by such measures as grass
buffers, grassed swales, proposed detention pond, and permeable pavers.
REFERENCES
1. City of Fort Collins, December 2018, Fort Collins Stormwater Criteria Manual.
2. Mile High Flood District, 2001 (November 2010 with some sections updated April 2018),
Urban Storm Drainage Criteria Manual, Volume 3, Stormwater Quality.
3. United States Department of Agriculture Natural Resources Conservation Service; Web Soil
Survey; accessed on-line September 2019.
4. Federal Emergency Management Agency; FEMA Flood Map Service Center; accessed on-line
September 2019.
APPENDICES
1. Appendix A - Site Information
a. Vicinity Map
b. Site Plan
c. NRCS Soils Data
d. FEMA Maps
2. Appendix B – Hydrologic Calculations
a. Storm Criteria
b. Detention Basin Volume Calculations
3. Appendix C – Drainage Basin Map
a. Drainage Basin Map
4. Appendix D – LID Exhibit
a. LID Exhibit with Permeable Paver Calculations
5. Appendix E - Relevant Drainage Reports
a. Pages from the Weberg PUD Drainage Report
APPENDIX A Fairway Lane Apartments – Fort Collins, CO
20065
ProjectSite
BLDG
2
BLDG
9
BLDG
1
BLDG
7
BLDG
6
BLDG
5
BLDG
4
BLDG
3
LOT 1SOUTH TRANSIT CENTER SUBDIVISIONLOT 1
SPRADLEY BARR FORD SUBDIVISION
LOT 1
WEBERG, P.U.D.SOUTH COLLEGE AVENUE (SH 287)FOSSIL BLVDLOT 1
GEORGE T. SANDERS CO. P.U.D.
REC. NO. 19960013545
WEST FAIRWAY LANE
0 50
SCALE: 1" = 50'
1002550
PRELIMINARY DEVELOPMENT PLANNORTH
FILE:PROJECT NO:CAD:QUALITY ASSURANCE:DRAWING HISTORYDATE DESCRIPTIONSITE PLANFAIRWAY LANE APARTMENTSCOLLEGE AVE & FAIRWAY LANEFORT COLLINS, COLORADOC3.0 CJS20065_SITE_PROD.DWGERB3/17/211ST PDP SUBMITTAL--------------200065 PLAN NOTES
1 NEW ASPHALT PAVING
NEW CONCRETE WALK
NEW ACCESSIBILITY RAMP
NEW 4" PAINTED WHITE STRIPE AT 3' O.C. (TYP.)
NEW PAINTED INTERNATIONAL DISABLED SYMBOL (TYP.)
NEW 4" PAINTED WHITE PARKING STRIPE (TYP.)
NEW HIGH VOLUME DRIVE TYPE IV CONCRETE DRIVE APPROACH
PER LCUASS STANDARDS
12" WHITE STOP BAR
STOP SIGN (R1-1)
NEW TRASH PAD AND ENCLOSURE (SEE ARCH. FOR DETAILS)
2
3
4
5
6
7
8
9
10
NEW BIKE RACKS11
POOL AND AMENITY AREA12
TRANSFORMER PAD
PARK SPACE/PLAYGROUND
TRICKLE CHANNEL
OUTLET STRUCTURE & RIPRAP PROTECTION
13
14
15
16
SECONDARY EMERGENCY ACCESS WITH EMERGENCY ACCESS GATE AND
ROLL-OVER CURB17
LANDSCAPE WALL FOR TREE PROTECTION18
NEW CURB AND GUTTER (TYP.)19
DOG PARK20
NON-ENGINEERED RETAINING WALL (VARYING HEIGHT - 1 FT TO 2.5 FT)21
DOGHOUSES WITH FIRE RISER (TYP.)22
LIGHT POLES23
PERGOLA AND BBQ GRILLS24
PEDESTRIAN HANDICAP RAMPS AND CROSSWALK (TYP.)25
SIDEWALK CHASE AND FLUME (TO MATCH EXISTING WIDTH)26
CITY OF FORT COLLINS, COLORADO
UTILITY PLAN APPROVAL
APPROVED:
CHECKED BY:
CHECKED BY:
CHECKED BY:
CHECKED BY:
CHECKED BY:
CITY ENGINEER DATE
WATER & WASTEWATER UTILITY DATE
STORMWATER UTILITY DATE
PARKS & RECREATION DATE
TRAFFIC ENGINEER DATE
ENVIRONMENTAL PLANNER DATE
STANDARD 90 DEGREE PARKING SPACE WITH 2' OVERHANG (TYP.)27
SIDEWALK ANGLED PARKING SPACE WITH 2' OVERHANG (TYP.)28
STANDARD COMPACT PARKING SPACES (TYP.)29
PARALLEL PARKING SPACES (TYP.)30
NOTES:
1.EXISTING UNDERGROUND INSTALLATIONS & PRIVATE UTILITIES SHOWN ARE INDICATED
ACCORDING TO THE BEST INFORMATION AVAILABLE TO THE ENGINEER. THE ENGINEER DOES NOT
GUARANTEE THE ACCURACY OF SUCH INFORMATION. SERVICE LINES (WATER, POWER, GAS,
STORM, SEWER, TELEPHONE & TELEVISION) MAY NOT BE STRAIGHT LINES OR AS INDICATED ON
THE PLANS. STATE LAW REQUIRES CONTRACTOR TO CALL ALL UTILITY COMPANIES BEFORE
EXCAVATION FOR EXACT LOCATIONS.
2.UNLESS OTHERWISE SPECIFIED, ALL CONSTRUCTION LAYOUT AND STAKING SHALL BE PERFORMED
UNDER THE RESPONSIBLE CHARGE OF A LAND SURVEYOR LICENSED IN THE STATE WHERE THE
PROJECT IS LOCATED AND BY A PARTY CHIEF OR ENGINEERING TECHNICIAN EXPERIENCED IN
CONSTRUCTION LAYOUT AND STAKING TECHNIQUES AS ARE REQUIRED BY THE SPECIFIC TYPE OF
WORK BEING PERFORMED.
FIRE DEPARTMENT CONNECTION (FDC)31
Detention Basin Area
9
Custom Soil Resource Report
Soil Map
448516044852004485240448528044853204485360448540044854404485480448516044852004485240448528044853204485360448540044854404485480493240 493280 493320 493360 493400 493440 493480
493240 493280 493320 493360 493400 493440 493480
40° 31' 12'' N 105° 4' 47'' W40° 31' 12'' N105° 4' 36'' W40° 31' 1'' N
105° 4' 47'' W40° 31' 1'' N
105° 4' 36'' WN
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 50 100 200 300
Feet
0 20 40 80 120
Meters
Map Scale: 1:1,650 if printed on A portrait (8.5" x 11") sheet.
Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Map Unit Polygons
Soil Map Unit Lines
Soil Map Unit Points
Special Point Features
Blowout
Borrow Pit
Clay Spot
Closed Depression
Gravel Pit
Gravelly Spot
Landfill
Lava Flow
Marsh or swamp
Mine or Quarry
Miscellaneous Water
Perennial Water
Rock Outcrop
Saline Spot
Sandy Spot
Severely Eroded Spot
Sinkhole
Slide or Slip
Sodic Spot
Spoil Area
Stony Spot
Very Stony Spot
Wet Spot
Other
Special Line Features
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Larimer County Area, Colorado
Survey Area Data: Version 15, Jun 9, 2020
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Aug 11, 2018—Aug
12, 2018
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Custom Soil Resource Report
10
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
35 Fort Collins loam, 0 to 3 percent
slopes
3.4 26.8%
74 Nunn clay loam, 1 to 3 percent
slopes
9.4 73.2%
Totals for Area of Interest 12.9 100.0%
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
given area, the contrasting minor components are identified in the map unit
descriptions along with some characteristics of each. A few areas of minor
components may not have been observed, and consequently they are not
mentioned in the descriptions, especially where the pattern was so complex that it
was impractical to make enough observations to identify all the soils and
miscellaneous areas on the landscape.
The presence of minor components in a map unit in no way diminishes the
usefulness or accuracy of the data. The objective of mapping is not to delineate
pure taxonomic classes but rather to separate the landscape into landforms or
landform segments that have similar use and management requirements. The
delineation of such segments on the map provides sufficient information for the
development of resource plans. If intensive use of small areas is planned, however,
Custom Soil Resource Report
11
onsite investigation is needed to define and locate the soils and miscellaneous
areas.
An identifying symbol precedes the map unit name in the map unit descriptions.
Each description includes general facts about the unit and gives important soil
properties and qualities.
Soils that have profiles that are almost alike make up a soil series. Except for
differences in texture of the surface layer, all the soils of a series have major
horizons that are similar in composition, thickness, and arrangement.
Soils of one series can differ in texture of the surface layer, slope, stoniness,
salinity, degree of erosion, and other characteristics that affect their use. On the
basis of such differences, a soil series is divided into soil phases. Most of the areas
shown on the detailed soil maps are phases of soil series. The name of a soil phase
commonly indicates a feature that affects use or management. For example, Alpha
silt loam, 0 to 2 percent slopes, is a phase of the Alpha series.
Some map units are made up of two or more major soils or miscellaneous areas.
These map units are complexes, associations, or undifferentiated groups.
A complex consists of two or more soils or miscellaneous areas in such an intricate
pattern or in such small areas that they cannot be shown separately on the maps.
The pattern and proportion of the soils or miscellaneous areas are somewhat similar
in all areas. Alpha-Beta complex, 0 to 6 percent slopes, is an example.
An association is made up of two or more geographically associated soils or
miscellaneous areas that are shown as one unit on the maps. Because of present
or anticipated uses of the map units in the survey area, it was not considered
practical or necessary to map the soils or miscellaneous areas separately. The
pattern and relative proportion of the soils or miscellaneous areas are somewhat
similar. Alpha-Beta association, 0 to 2 percent slopes, is an example.
An undifferentiated group is made up of two or more soils or miscellaneous areas
that could be mapped individually but are mapped as one unit because similar
interpretations can be made for use and management. The pattern and proportion
of the soils or miscellaneous areas in a mapped area are not uniform. An area can
be made up of only one of the major soils or miscellaneous areas, or it can be made
up of all of them. Alpha and Beta soils, 0 to 2 percent slopes, is an example.
Some surveys include miscellaneous areas. Such areas have little or no soil
material and support little or no vegetation. Rock outcrop is an example.
Custom Soil Resource Report
12
Larimer County Area, Colorado
35—Fort Collins loam, 0 to 3 percent slopes
Map Unit Setting
National map unit symbol: 2tlnc
Elevation: 4,020 to 6,730 feet
Mean annual precipitation: 14 to 16 inches
Mean annual air temperature: 46 to 48 degrees F
Frost-free period: 135 to 160 days
Farmland classification: Prime farmland if irrigated
Map Unit Composition
Fort collins and similar soils:85 percent
Minor components:15 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Fort Collins
Setting
Landform:Stream terraces, interfluves
Landform position (three-dimensional):Interfluve, tread
Down-slope shape:Linear
Across-slope shape:Linear
Parent material:Pleistocene or older alluvium and/or eolian deposits
Typical profile
Ap - 0 to 4 inches: loam
Bt1 - 4 to 9 inches: clay loam
Bt2 - 9 to 16 inches: clay loam
Bk1 - 16 to 29 inches: loam
Bk2 - 29 to 80 inches: loam
Properties and qualities
Slope:0 to 3 percent
Depth to restrictive feature:More than 80 inches
Drainage class:Well drained
Runoff class: Low
Capacity of the most limiting layer to transmit water (Ksat):Moderately high to high
(0.20 to 2.00 in/hr)
Depth to water table:More than 80 inches
Frequency of flooding:None
Frequency of ponding:None
Calcium carbonate, maximum content:12 percent
Maximum salinity:Nonsaline to very slightly saline (0.1 to 2.0 mmhos/cm)
Available water capacity:High (about 9.1 inches)
Interpretive groups
Land capability classification (irrigated): 3e
Land capability classification (nonirrigated): 3e
Hydrologic Soil Group: C
Ecological site: R067BY002CO - Loamy Plains
Hydric soil rating: No
Custom Soil Resource Report
13
Minor Components
Nunn
Percent of map unit:10 percent
Landform:Stream terraces
Landform position (three-dimensional):Tread
Down-slope shape:Linear
Across-slope shape:Linear
Ecological site:R067BY002CO - Loamy Plains
Hydric soil rating: No
Vona
Percent of map unit:5 percent
Landform:Interfluves
Landform position (three-dimensional):Side slope, interfluve
Down-slope shape:Linear
Across-slope shape:Linear
Ecological site:R067BY024CO - Sandy Plains
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
Btk - 13 to 25 inches: clay loam
Bk1 - 25 to 38 inches: clay loam
Bk2 - 38 to 80 inches: clay loam
Custom Soil Resource Report
14
Properties and qualities
Slope:1 to 3 percent
Depth to restrictive feature:More than 80 inches
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 content:7 percent
Maximum salinity:Nonsaline to very slightly saline (0.1 to 2.0 mmhos/cm)
Sodium adsorption ratio, maximum:0.5
Available water capacity:High (about 9.9 inches)
Interpretive groups
Land capability classification (irrigated): 2e
Land capability classification (nonirrigated): 3e
Hydrologic Soil Group: C
Ecological site: R067BY042CO - Clayey Plains
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:R067BY042CO - Clayey Plains
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:R067BY002CO - Loamy Plains
Hydric soil rating: No
Custom Soil Resource Report
15
Soil Information for All Uses
Soil Properties and Qualities
The Soil Properties and Qualities section includes various soil properties and
qualities displayed as thematic maps with a summary table for the soil map units in
the selected area of interest. A single value or rating for each map unit is generated
by aggregating the interpretive ratings of individual map unit components. This
aggregation process is defined for each property or quality.
Soil Erosion Factors
Soil Erosion Factors are soil properties and interpretations used in evaluating the
soil for potential erosion. Example soil erosion factors can include K factor for the
whole soil or on a rock free basis, T factor, wind erodibility group and wind erodibility
index.
K Factor, Whole Soil
Erosion factor K indicates the susceptibility of a soil to sheet and rill erosion by
water. Factor K is one of six factors used in the Universal Soil Loss Equation
(USLE) and the Revised Universal Soil Loss Equation (RUSLE) to predict the
average annual rate of soil loss by sheet and rill erosion in tons per acre per year.
The estimates are based primarily on percentage of silt, sand, and organic matter
and on soil structure and saturated hydraulic conductivity (Ksat). Values of K range
from 0.02 to 0.69. Other factors being equal, the higher the value, the more
susceptible the soil is to sheet and rill erosion by water.
"Erosion factor Kw (whole soil)" indicates the erodibility of the whole soil. The
estimates are modified by the presence of rock fragments.
16
Table—Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
35 Fort Collins loam, 0 to 3
percent slopes
C 3.4 26.8%
74 Nunn clay loam, 1 to 3
percent slopes
C 9.4 73.2%
Totals for Area of Interest 12.9 100.0%
Rating Options—Hydrologic Soil Group
Aggregation Method: Dominant Condition
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Custom Soil Resource Report
23
National Flood Hazard Layer FIRMette
0 500 1,000 1,500 2,000250
Feet
Ü
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOOD
HAZARD AREAS
Without Base Flood Elevation (BFE)
Zone A, V, A99
With BFE or DepthZone AE, AO, AH, VE, AR
Regulatory Floodway
0.2% Annual Chance Flood Hazard, Areas
of 1% annual chance flood with average
depth less than one foot or with drainage
areas of less than one square mileZone X
Future Conditions 1% Annual
Chance Flood HazardZone X
Area with Reduced Flood Risk due to
Levee. See Notes.Zone X
Area with Flood Risk due to LeveeZone D
NO SCREEN Area of Minimal Flood Hazard Zone X
Area of Undetermined Flood HazardZone D
Channel, Culvert, or Storm Sewer
Levee, Dike, or Floodwall
Cross Sections with 1% Annual Chance
17.5 Water Surface Elevation
Coastal Transect
Coastal Transect Baseline
Profile Baseline
Hydrographic Feature
Base Flood Elevation Line (BFE)
Effective LOMRs
Limit of Study
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The basemap shown complies with FEMA's basemap
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The flood hazard information is derived directly from the
authoritative NFHL web services provided by FEMA. This map
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become superseded by new data over time.
This map image is void if the one or more of the following map
elements do not appear: basemap imagery, flood zone labels,
legend, scale bar, map creation date, community identifiers,
FIRM panel number, and FIRM effective date. Map images for
unmapped and unmodernized areas cannot be used for
regulatory purposes.
Legend
OTHER AREAS OF
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MAP PANELS
8
B 20.2
The pin displayed on the map is an approximate
point selected by the user and does not represent
an authoritative property location.
1:6,000
105°5'1"W 40°31'18"N
105°4'23"W 40°30'51"N
Basemap: USGS National Map: Orthoimagery: Data refreshed October, 2020
Project Site
APPENDIX B Fairway Lane Apartments – Fort Collins, CO
20065
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
2.0 Runoff Methodologies
3.1 Rational Formula
Page 3
2.0 Runoff Methodologies
There are two runoff analysis methodologies that are approved by the City: the Rational Method and
the Stormwater Management Model (SWMM). In general, the chosen methodology should follow the
basin size limitations listed in Table 2.0-1 below. SWMM must also be used to assess the performance of
multiple detention basins in parallel or in series in a particular watershed. The City is the determining
authority with respect to the appropriate methodology to use under uncertain circumstances. Please
note that the Colorado Urban Hydrograph Procedure (CUHP) is not allowed to be utilized for hydrology
analysis for Fort Collins area projects because this procedure is calibrated using Denver/Boulder rainfall
data.
Table 2.0-1: Runoff Calculation Method
Project Size Runoff Calculation Method
< 5 acres Rational Method Required
5-20 acres Rational Method or SWMM Accepted
≥ 20 acres SWMM Required
Reference: Drainage Report submittal requirements must be prepared in accordance with
the criteria set forth in Chapter 2: Development Submittal Requirements.
3.0 Rational Method
3.1 Rational Formula
The methodology and theory behind the Rational Method is not covered in this Manual as this subject is
well described in many hydrology reference books. However, the Rational Method procedure is
generally provided in the following sections. Runoff coefficient calculations, rainfall data, and the time of
concentration formula are specific to the City and are included below.
The Rational Formula is represented by the following equation:
𝐐𝐐=𝐂𝐂𝐂𝐂𝐂𝐂 Equation 5-1
Where: Q = Peak Rate of Runoff, cfs
C = Runoff Coefficient, dimensionless
I = Rainfall Intensity, in/hr
A = Area of the Basin or Sub-basin, acres
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.2 Runoff Coefficients
Page 4
3.2 Runoff Coefficients
Runoff coefficients used for the Rational Method are determined based on either overall land use or
surface type across the drainage area. For Overall Drainage Plan (ODP) submittals, when surface types
may not yet be known, land use shall be used to estimate flow rates and volumes. Table 3.2-1 lists the
runoff coefficients for common types of land uses in the City.
Table 3.2-1. Zoning Classification - Runoff Coefficients
Land Use Runoff Coefficient (C)
Residential
Urban Estate 0.30
Low Density 0.55
Medium Density 0.65
High Density 0.85
Commercial
Commercial 0.85
Industrial 0.95
Undeveloped
Open Lands, Transition 0.20
Greenbelts, Agriculture 0.20
Reference: For further guidance regarding zoning classifications, refer to the Land Use
Code, Article 4.
For a Project Development Plan (PDP) or Final Plan (FP) submittals, runoff coefficients must be based on
the proposed land surface types. Since the actual runoff coefficients may be different from those
specified in Table 3.2-1, Table 3.2-2 lists coefficients for the specific types of land surfaces.
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.2 Runoff Coefficients
Page 5
Table 3.2-2. Surface Type - Runoff Coefficients
Surface Type Runoff Coefficients
Hardscape or Hard Surface
Asphalt, Concrete 0.95
Rooftop 0.95
Recycled Asphalt 0.80
Gravel 0.50
Pavers 0.50
Landscape or Pervious Surface
Lawns, Sandy Soil, Flat Slope < 2% 0.10
Lawns, Sandy Soil, Avg Slope 2-7% 0.15
Lawns, Sandy Soil, Steep Slope >7% 0.20
Lawns, Clayey Soil, Flat Slope < 2% 0.20
Lawns, Clayey Soil, Avg Slope 2-7% 0.25
Lawns, Clayey Soil, Steep Slope >7% 0.35
3.2.1 Composite Runoff Coefficients
Drainage sub-basins are frequently composed of land that has multiple surface types or zoning
classifications. In such cases a composite runoff coefficient must be calculated for any given drainage
sub-basin.
The composite runoff coefficient is obtained using the following formula:
( )
t
n
i
ii
A
xAC
C
∑
==1 Equation 5-2
Where: C = Composite Runoff Coefficient
Ci = Runoff Coefficient for Specific Area (Ai), dimensionless
Ai = Area of Surface with Runoff Coefficient of Ci, acres or square feet
n = Number of different surfaces to be considered
At = Total Area over which C is applicable, acres or square feet
3.2.2 Runoff Coefficient Frequency Adjustment Factor
The runoff coefficients provided in Table 3.2-1 and Table 3.2-2 are appropriate for use with the 2-year
storm event. For any analysis of storms with higher intensities, an adjustment of the runoff coefficient is
required due to the lessening amount of infiltration, depression retention, evapotranspiration and other
losses that have a proportionally smaller effect on high-intensity storm runoff. This adjustment is
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.3 Time of Concentration
Page 6
applied to the composite runoff coefficient. These frequency adjustment factors, Cf, are found in Table
3.2-3.
Table 3.2-3. Frequency Adjustment Factors
Storm Return Period
(years)
Frequency Adjustment
Factor (Cf)
2, 5, 10 1.00
25 1.10
50 1.20
100 1.25
3.3 Time of Concentration
3.3.1 Overall Equation
The next step to approximate runoff using the Rational Method is to estimate the Time of
Concentration, Tc, or the time for water to flow from the most remote part of the drainage sub-basin to
the design point under consideration.
The Time of Concentration is represented by the following equation:
𝐓𝐓𝐜𝐜=𝐓𝐓𝐢𝐢+𝐓𝐓𝐭𝐭 Equation 5-3
Where: Tc = Total Time of Concentration, minutes
Ti = Initial or Overland Flow Time of Concentration, minutes
Tt = Channelized Flow in Swale, Gutter or Pipe, minutes
3.3.2 Overland Flow Time
Overland flow, Ti, can be determined by the following equation:
𝐓𝐓𝐢𝐢=𝟏𝟏.𝟖𝟖𝟖𝟖(𝟏𝟏.𝟏𝟏−𝐂𝐂𝐂𝐂𝐂𝐂𝐟𝐟)√𝐋𝐋√𝐒𝐒𝟑𝟑 Equation 3.3-2
Where: C = Runoff Coefficient, dimensionless
Cf = Frequency Adjustment Factor, dimensionless
L = Length of Overland Flow, feet
S = Slope, percent
CXCF
PRODUCT OF CXCF
CANNOT EXCEED THE
VALUE OF 1
OVERLAND FLOW LENGTH
L=200’ MAX IN DEVELOPED AREAS
L=500’ MAX IN UNDEVELOPED
AREAS
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.4 Intensity-Duration-Frequency Curves for Rational Method
Page 7
3.3.3 Channelized Flow Time
Travel time in a swale, gutter or storm pipe is considered “channelized” or “concentrated” flow and can
be estimated using the Manning’s Equation:
𝐕𝐕= 𝟏𝟏.𝟒𝟒𝟒𝟒𝐧𝐧 𝐑𝐑𝟐𝟐/𝟑𝟑𝐒𝐒𝟏𝟏/𝟐𝟐 Equation 5-4
Where: V = Velocity, feet/second
n = Roughness Coefficient, dimensionless
R = Hydraulic Radius, feet (Hydraulic Radius = area / wetted perimeter, feet)
S = Longitudinal Slope, feet/feet
And:
𝐓𝐓𝐭𝐭=𝐋𝐋𝐕𝐕𝐂𝐂𝐕𝐕𝐕𝐕 Equation 5-5
3.3.4 Total Time of Concentration
A minimum Tc of 5 minutes is required. The maximum Tc
allowed for the most upstream design point shall be
calculated using the following equation:
𝐓𝐓𝐜𝐜=𝐋𝐋𝟏𝟏𝟖𝟖𝐕𝐕+𝟏𝟏𝐕𝐕 Equation 3.3-5
The Total Time of Concentration, Tc, is the lesser of the
values of Tc calculated using Tc = Ti + Tt or the equation
listed above.
3.4 Intensity-Duration-Frequency Curves for Rational Method
The two-hour rainfall Intensity-Duration-Frequency curves for use with the Rational Method is provided
in Table 3.4-1 and Figure 3.4-1.
TC
• A MINIMUM TC OF 5
MINUTES IS REQUIRED IN
ALL CASES.
• A MAXIMUM TC OF 5
MINUTES IS TYPICAL FOR
SMALLER, URBAN PROJECTS.
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.4 Intensity-Duration-Frequency Curves for Rational Method
Page 8
Table 3.4-1. IDF Table for Rational Method
Duration
(min)
Intensity
2-year
(in/hr)
Intensity
10-year
(in/hr)
Intensity
100-year
(in/hr)
Duration
(min)
Intensity
2-year
(in/hr)
Intensity
10-year
(in/hr)
Intensity
100-year
(in/hr)
5 2.85 4.87 9.95
39 1.09 1.86 3.8
6 2.67 4.56 9.31
40 1.07 1.83 3.74
7 2.52 4.31 8.80
41 1.05 1.80 3.68
8 2.40 4.10 8.38
42 1.04 1.77 3.62
9 2.30 3.93 8.03
43 1.02 1.74 3.56
10 2.21 3.78 7.72
44 1.01 1.72 3.51
11 2.13 3.63 7.42
45 0.99 1.69 3.46
12 2.05 3.50 7.16
46 0.98 1.67 3.41
13 1.98 3.39 6.92
47 0.96 1.64 3.36
14 1.92 3.29 6.71
48 0.95 1.62 3.31
15 1.87 3.19 6.52
49 0.94 1.6 3.27
16 1.81 3.08 6.30
50 0.92 1.58 3.23
17 1.75 2.99 6.10
51 0.91 1.56 3.18
18 1.70 2.90 5.92
52 0.9 1.54 3.14
19 1.65 2.82 5.75
53 0.89 1.52 3.10
20 1.61 2.74 5.60
54 0.88 1.50 3.07
21 1.56 2.67 5.46
55 0.87 1.48 3.03
22 1.53 2.61 5.32
56 0.86 1.47 2.99
23 1.49 2.55 5.20
57 0.85 1.45 2.96
24 1.46 2.49 5.09
58 0.84 1.43 2.92
25 1.43 2.44 4.98
59 0.83 1.42 2.89
26 1.4 2.39 4.87
60 0.82 1.4 2.86
27 1.37 2.34 4.78
65 0.78 1.32 2.71
28 1.34 2.29 4.69
70 0.73 1.25 2.59
29 1.32 2.25 4.60
75 0.70 1.19 2.48
30 1.30 2.21 4.52
80 0.66 1.14 2.38
31 1.27 2.16 4.42
85 0.64 1.09 2.29
32 1.24 2.12 4.33
90 0.61 1.05 2.21
33 1.22 2.08 4.24
95 0.58 1.01 2.13
34 1.19 2.04 4.16
100 0.56 0.97 2.06
35 1.17 2.00 4.08
105 0.54 0.94 2.00
36 1.15 1.96 4.01
110 0.52 0.91 1.94
37 1.16 1.93 3.93
115 0.51 0.88 1.88
38 1.11 1.89 3.87
120 0.49 0.86 1.84
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.4 Intensity-Duration-Frequency Curves for Rational Method
Page 9
Figure 3.4-1. Rainfall IDF Curve – Fort Collins
FORT COLLINS STORMWATER CRITERIA MANUAL Detention (Ch. 6)
2.0 Water Quantity Detention
2.2 SWMM
Page 6
Table 2.1-1: Detention Calculation Method
Project Size * Detention Calculation Method
< 5 acres Modified FAA Required
5-20 acres Modified FAA or SWMM Accepted
≥20 acres SWMM Required
*Project Size must include any offsite runoff that is tributary to the subject site
Note about the UDFCD Manual: Because of the Master Drainage Plans detention
requirements, the City does not allow for detention basins to be designed with the “full-
spectrum detention” method that is described in the UDFCD Manual.
2.2 SWMM
For project sites equal to or greater than 20 acres, the use of a Stormwater Management Model
(SWMM) is required.
If there are upstream detention facilities within the watershed that contribute and route runoff into the
site being designed, hydrograph routing methods must be employed to allow for the upstream facilities
to be included in the overall SWMM model.
Reference: The theory and methodology for reservoir routing is not covered in this Manual
as this subject is well described in many hydrology reference books.
2.3 Modified FAA Procedure
The Modified FAA Procedure (1966) detention sizing method as modified by Guo (1999a), provides a
reasonable estimate of volume requirements for detention facilities. This method provides sizing for one
level of peak control only and not for multi-stage control facilities.
The input required for this Modified FAA volume calculation procedure includes:
A = area of the catchment tributary to the detention facility (acres)
C = runoff coefficient
Qout = allowable maximum release rate from the detention facility (cfs)
Tc = time of concentration for the tributary catchment (minutes)
I = rainfall intensity (inches/hour) at the site taken from Chapter 4: Hydrology Standards, for the
relevant return frequency storms
The calculations are best set up in a tabular (spreadsheet) form with each 5-minute increment in
duration being entered in rows and the following variables being entered, or calculated, in each column:
1) Storm Duration Time, T (minutes), up to 120 minutes
FORT COLLINS STORMWATER CRITERIA MANUAL Detention (Ch. 6)
2.0 Water Quantity Detention
2.4 Detention Basin Volume
Page 7
2) Rainfall Intensity, I (inches per hour)
3) Inflow volume, Vi (cubic feet), calculated as the cumulative volume at the given storm
duration using the equation:
Vi = CIA (60T) Equation 6-1
4) Calculated outflow volume, Vo, (cubic feet), given the maximum allowable release rate,
Qout (cfs), over the duration T:
Vo= Qout (60 T) Equation 6-2
5) Required detention volume, Vs (cubic feet), calculated using the equation:
Vs = Vi – Vo Equation 6-3
The value of Vs increases with time, reaches a maximum value, and then starts to decrease. The
maximum value of Vs is the required detention volume for the detention facility.
Note about UDFCD Manual: Please note that the UDFCD excel-based spreadsheets are not
allowed to be used to calculate required detention volumes because they utilize Denver
region rainfall data. The Design Engineer will be required to establish their own
spreadsheet for calculating basin volume requirements based on the Modified FAA
Procedure documented above using Fort Collins IDF curves.
2.4 Detention Basin Volume
2.4.1 Stage-Storage
A relationship between the water surface elevation and detention basin volume, commonly referred to
as a “stage-storage” curve, needs to be developed. This relationship, in conjunction with the “stage-
discharge” relationship will provide the required detention volume. An initial detention basin design
must be created and a “stage-storage” curve developed that corresponds to the design.
The available detention volume shall be based on the following formula:
𝐕𝐕=𝐃𝐃𝟑𝟑�𝐀𝐀+𝐁𝐁+√𝐀𝐀𝐁𝐁� Equation 6-4
Where V = Volume between two contours, ft3
D = Depth between contours, feet
A = Area of bottom contour, ft2
B = Area of top contour, ft2
Fairway Lane ApartmentsFort Collins, ColoradoLAND USE AND IMPERVIOUSNESS3/16/202120065Surface TypeRunoff Coefficients% ImperviousAsphalt0.95100%2-Year Cf1.00Concrete 0.95 90%10-Year Cf1.00Rooftop0.9590%100-Year Cf1.25Gravel0.5040%Clayey Soil Landscape < 2%0.202%Clayey soil Landscape 2-7%0.252%BASIN IMPERVIOUS CALCSBasins Conveying to On-Site Detention Basin to SoutheastTotal Area, A Total Area, A2-Year 10-Year 100-Year %Basin Description sf Acres Asphalt Rooftop Concrete Landscape Composite Composite Composite Composite IA Soil TypeDesign PointArea (sf) Area (sf) Area (sf) Area (sf) Runoff Coefficient Runoff Coefficient Runoff Coefficient Impervious, I (A,B,C,D)1 B-1 424,2749.74165,780 95,353 0 163,141 0 0.66 0.66 0.8360% 5.85C2 B-2 8,2760.197,400 0 0 876 0 0.87 0.87 1.0090% 0.17C3 B-3 38,7680.8920,170 0 0 18,598 0 0.59 0.59 0.7453% 0.47CO1 OS-1 11,7610.277,800 0 0 3,961 0 0.70 0.70 0.8767% 0.18CO2OS-25,2270.124,8000042700.890.891.0092%0.11CGravel Area (sf)Fort Collins Rational Calculations_2-10-100-Yr_.xlsxComposite 100-Yr RunoffCoefficient = 0.82(contributory to detention)
0.82
2.28
11.42
Time (min)Intensity
(In/hr)Qin (cfs)Vin (ft3)Qout (cfs)Vout (ft3)
Volume Detained
(ft3)
Volume Detained
(acre-ft)
5 9.95 93.18 27953 2.28 685 27268 0.63
10 7.72 72.29 43376 2.28 1370 42006 0.96
15 6.52 61.06 54950 2.28 2056 52895 1.21
20 5.6 52.44 62929 2.28 2741 60188 1.38
25 4.98 46.63 69952 2.28 3426 66526 1.53
30 4.52 42.33 76189 2.28 4111 72078 1.65
35 4.08 38.21 80234 2.28 4796 75438 1.73
40 3.74 35.02 84055 2.28 5482 78573 1.80
45 3.46 32.40 87482 2.28 6167 81315 1.87
50 3.23 30.25 90741 2.28 6852 83889 1.93
55 3.03 28.37 93635 2.28 7537 86097 1.98
60 2.86 26.78 96416 2.28 8222 88193 2.02
65 2.71 25.38 98972 2.28 8908 90065 2.07
70 2.59 24.25 101866 2.28 9593 92273 2.12
75 2.48 23.22 104507 2.28 10278 94229 2.16
80 2.38 22.29 106979 2.28 10963 96016 2.20
85 2.29 21.44 109367 2.28 11648 97718 2.24
90 2.21 20.70 111755 2.28 12334 99421 2.28
95 2.13 19.95 113693 2.28 13019 100674 2.31
100 2.06 19.29 115744 2.28 13704 102040 2.34
105 2.00 18.73 117991 2.28 14389 103602 2.38
110 1.94 18.17 119902 2.28 15074 104827 2.41
115 1.88 17.61 121475 2.28 15760 105715 2.43
120 1.84 17.23 124060 2.28 16445 107615 2.47
2.47
Detention requirements for Detention Pond for Major Storm (100-Yr)
Modified FAA Method
Total Detention @ 120 mins (Acre-ft) =
Runoff Coefficient
Allowed Release Rate (cfs)
Area (Acres)
APPENDIX C Fairway Lane Apartments – Fort Collins, CO
20065
9.74
B-1
0.66
0.83
0.89
B-3
0.59
0.74
0.19
B-2
0.87
1.00
0.12
OS-2
0.89
1.00
0.27
OS-1
0.70
0.87
0 50
SCALE: 1" = 50'
1002550 PRELIMINARY DEVELOPMENT PLANNORTH
NOTE:
-EXISTING UNDERGROUND INSTALLATIONS & PRIVATE UTILITIES SHOWN ARE INDICATED ACCORDING TO THE BEST INFORMATION AVAILABLE TO THE ENGINEER. THE
ENGINEER DOES NOT GUARANTEE THE ACCURACY OF SUCH INFORMATION. SERVICE LINES (WATER, POWER, GAS, STORM, SEWER, TELEPHONE & TELEVISION) MAY NOT BE
STRAIGHT LINES OR AS INDICATED ON THE PLANS. STATE LAW REQUIRES CONTRACTOR TO CALL ALL UTILITY COMPANIES BEFORE EXCAVATION FOR EXACT LOCATIONS.
-UNLESS OTHERWISE SPECIFIED, ALL CONSTRUCTION LAYOUT AND STAKING SHALL BE PERFORMED UNDER THE RESPONSIBLE CHARGE OF A LAND SURVEYOR LICENSED IN
THE STATE WHERE THE PROJECT IS LOCATED AND BY A PARTY CHIEF OR ENGINEERING TECHNICIAN EXPERIENCED IN CONSTRUCTION LAYOUT AND STAKING TECHNIQUES AS
ARE REQUIRED BY THE SPECIFIC TYPE OF WORK BEING PERFORMED.FILE:PROJECT NO:CAD:QUALITY ASSURANCE:DRAWING HISTORYDATE DESCRIPTION----DRAINAGE PLANFAIRWAY LANE APARTMENTSCOLLEGE AVE & FAIRWAY LANEFORT COLLINS, COLORADOC6.0 CJS20065_DRAIN_PROD.DWGERB3/17/211ST PDP SUBMITTAL--------------200065C2
C100
BASIN #
AREA
C2 = 2 YR RUNOFF COEFFICIENT
C100 = 100 YR RUNOFF COEFFICIENT
Detention Basin
Storage = 2.47 Ac-Ft
APPENDIX D Fairway Lane Apartments – Fort Collins, CO
20065
BLDG
2
BLDG
9
BLDG
1
BLDG
7
BLDG
6
BLDG
5
BLDG
4
BLDG
3
LID TREATMENT EXHIBIT
NORTH
50 25 0
SCALE:1" = 50'
50 100
PROPOSED PERMEABLE PAVERS
IMPERVIOUS SURFACES
CALCULATED WITHIN THESE
AREAS AND CONTRIBUTORY
TO PAVER SECTIONS
Paver Area
Description Paver Area (sf)Contributing
Impervious Area (sf)
Run-On Ratio
(.33 Min Allowed)
P1 1,551 3,448 0.45
P2 1,577 3,399 0.46
P3 3,177 7,355 0.43
P4 3,341 7,335 0.46
P5 2,471 4,927 0.50
P6 2,512 6,327 0.40
P7 1874 5676 0.33
P8 1408 4330 0.33
P9 14082 38461 0.37
P10 3649 7437 0.49
P11 2256 6940 0.33
Totals 37,898 95,635 0.40
NEW OR MODIFIED IMPERVIOUS AREAS 265,152 SF
TARGET TREATMENT PERCENTAGE 50%
MINIMUM AREA TO BE TREATED BY LID MEASURES 132,576 SF
TOTAL NEW PAVED DRIVEABLE SURFACE 104,707
TARGET PERMEABLE DRIVEABLE SURFACE 25%
PERMEABLE DRIVEABLE SURFACE 37,898
PERCENT OF TOTAL DRIVEABLE SURFACE BEING PERMEABLE PAVERS 36.2%
ONSITE AREA CONTRIBUTORY TO PAVERS 133,533 SF
PERCENT TOTAL PROJECT AREA TREATED 50.4%
PROJECT SUMMARY
PERMEABLE PAVER SYSTEM
= 26,177 sf
(including paver sections)
APPENDIX E Fairway Lane Apartments – Fort Collins, CO
20065