HomeMy WebLinkAboutDrainage Reports - 07/29/2022Final Drainage Report
For
H-25 Multi Family
Harmony I-25 Subdivision,Fort Collins,CO
City of Fort Collins Approved Plans
Approved by:
Date:
November 5, 2021
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Final Drainage and Erosion Control Report for
H-25 MULTI-FAMILY, HARMONY I-25 SUBDIVISION
Dear Staff:
Sanderson Stewart is pleased to submit this Final Drainage and Erosion Control Report for your review.
This report accompanies the Project Development Plan submittal for the proposed H-25 Multi-Family
development.
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
H-25 Multi-Family November 2021 Page 1
Project No. 19059.02
November 5, 2021
Project No. 19059.02
I. Overview Narrative
The proposed project is a multi-family development that will include the construction of 12 new multi-
plex buildings, 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 Urban Storm Drainage Criteria Manual
(USDCM), Volume 1-3.
II. General Project Location
The overall Harmony & Strauss Cabin development is a 266-acre site located at the southwest quadrant
of Harmony Road and Interstate 25. The project site associated with this report consists of
approximately 15.7-acres within Parcel No. 8603000028 (approximately 38 acres) that is located within
the overall 266-acre development. More specifically, the project site lies within the southeast corner of
Harmony Road and Strauss Cabin Road in the west half of Section 3, Township 6 North, Range 68
West, latitude 40.5228, longitude 104.9983. The site is generally bounded by Strauss Cabin Road on the
west, Harmony Road on the north, Harmony Gardens landscape nursery on the east and Weitzel Pond
on the south. Surrounding properties include: the Arapaho Bend Natural Area and the Harmony Road
Transportation Transfer Center on the north side of Harmony Road, Island Lake Marina, and the Eagle
View Natural Area on the south side of Kechter Road, the Chandler property (now under Nine Bridges,
LLC ownership) west of Strauss Cabin Road, the Mountain Life Church and the Budurus properties
located along Strauss Cabin Road further to the south. East of the property is the I-25 interstate and the
Town of Timnath. Refer to Appendix A for a Vicinity map.
III. Existing Site Information
Major Basin Description
This property is located within the Cache la Poudre watershed and City of Fort Collins 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 floodway channel and travels south through a series of existing large inactive gravel
pits that have essentially become retention ponds for stormwater. This drainage way outlets into two
culverts under Kechter Road, approximately a mile south from the project site and is assumed to be the
historic stormwater release location for this area. Flow then conveys south of Kechter Road and into
H-25 Multi-Family November 2021 Page 2
Project No. 19059.02
Island Lake then flows due east under I-25 and then south east approximately a half mile to Fossil Creek
Reservoir Outlet and then another half mile onto the Cache La Poudre River.
guidance that is being completed concurrently with this report. The intent of the modeling effort is to
remove the portion of the project area from the existing floodplain that lies outside of the currently
approved LOMR-F area and also allow residential buildings to be constructed within this area.
Ultimately, it is anticipated that the proposed improvements will be located outside of the half-foot
Poudre River floodway, with the exception of a few improvements that will require a No-Rise
Certificate. These improvements include a dog park, a nature trail and a water main that will connect to
an existing transmission water main located within the floodway.
The Fossil Creek Reservoir Inlet Ditch (FCRID) and the Boxelder Ditch are located along the western
side of the overall site with the FCRID being further to the west. The Boxelder Ditch crosses Strauss
Cabin Road and enters
This project site sits within the FEMA regulated floodway and floodplain, per FEMA FIRM Maps
08069C0994F & 08069C1013F, dated December 19, 2006.
According to the NRCS website, the site consists mainly of Caruso clay loam, 0 to 1 percent slope with a
classification of Type C soils. This soil type has an erodibility factor of 0.32 which suggests a moderate
susceptibility to sheet and rill erosion by runoff. Refer to Appendix A for soil classification information.
Refer to Appendix A for soil data and floodplain map.
IV. Project Description
The proposed project will consist of 12 new multi-family buildings with 290 total units. 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
emergence access connection all along the east side of Strauss Cabin Road.
V. Drainage Design Criteria
A. Regulations/Development Criteria
The design standards governing this project are the Fort Collins Stormwater Criteria Manual
(USDCM), Volume 1-3.
B. Four-Step Process
The overall stormwater management strategy employed with the proposed project utilizes
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. Proposed
techniques include providing landscaped islands and buffer areas throughout the site to reduce
H-25 Multi-Family November 2021 Page 3
Project No. 19059.02
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 reduce 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 existing Weitzel gravel pond to the south of the property. This large existing
gravel pond will act as a sedimentation basin by allowing for the sediment, fines and
pollution suspended in the stormwater to settle out into the existing groundwater fed ponds.
Step 3 Stabilize Streams
There are no major wet drainage conveyances within the subject property, however, there is
a dog park and some nature trail proposed within the adjacent half-foot Poudre River
floodway that is dry year around. Proposed stabilization to this area will mainly be a result of
the construction activity and will consist of temporary seeding with mulching and hydro-
mulch. Permanent BMP measures will include permanent seeding and plantings, as needed.
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; thus, eliminating
sources of potential pollution previously left exposed to weathering and runoff processes.
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
has been utilized for Rational Method runoff calculations. Refer to Appendix B for
criteria used.
2. The Rational Method has been employed to compute stormwater runoff utilizing
coefficients and calculation methods contained in Chapter 5, Section 3.2 of the
FCSCM.
3. Three separate design storms have been utilized to address distinct drainage
scenarios. The first design storm event analyzed was
Storm, which has a 2-year recurrence interval. The second event considered is the
-year recurrence interval. The third storm
computed, for comparison purposes only, was the 10-year event.
4. No other assumptions or calculation methods have been used with this
development that are not referenced by current City of Fort Collins criteria.
5. Per recent meetings with City Stormwater staff and per the approved ODP drainage
report, the existing gravel pond adjacent and to the south of the project site will
serve as detention, provide standard water quality treatment, and will meet LID
requirements. This will be the case until such time that future phasing of the 266-
H-25 Multi-Family November 2021 Page 4
Project No. 19059.02
acre property is developed and a regional detention pond is constructed. Refer to
Appendix A for the ODP plan and Appendix D for excerpts from the associated
ODP drainage report.
D. Hydraulic Criteria
1. All drainage 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.
2. The LOMR modeling effort as outlined in Section III above will be completed
concurrently with the development plan process and will follow the City of Fort
Collins and FEMA requirements.
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 constraints and off-site flooding concerns by the City that
were previously mentioned.
1. Stormwater from the project site will be discharged into the existing Weitzel gravel
pond to the south. This will serve as detention, provide standard water quality, and
LID treatment until the overall 266- acre property develops further to the south and
a regional detention pond is constructed.
2. Reservation of space is provided along Harmony Road for a potential future drainage
channel that will accommodate and route the half-foot rise Poudre River floodway
flows from the north around the project development site. The future drainage
channel will require a hydraulic model through the CLOMR/LOMR process. This
work is anticipated in the future to allow for other project sites within the 266-acre
property to develop.
3. 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 south and east and to existing Weitzel gravel pond
adjacent to the south of the subject property. This pond is intended to provide detention,
standard water quality and LID treatment for the proposed projects site.
1. There are twenty-two (22) total sub-basins for the project site. Eighteen (18) of these
will be collected in the proposed storm system and the other four (4) are located on
the fringes of the site and will mainly flow historically. A more detailed description of
H-25 Multi-Family November 2021 Page 5
Project No. 19059.02
these sub-basins is described as follows:
Basin Nos. 1 through 18: These basins total to approximately 9.8 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 the south and discharged
into the existing Weitzel gravel pond. These basins generally are comprised of rooftop,
driveway and parking lot, sidewalk, and landscaping. The percent composite
imperviousness for all of these basins is approximately 74%.
OS-1 and OS-4: These sub-basins total to approximately 0.7 acres and will collect
flows from the back half of two building roof-tops and landscaping and discharge to
adjacent Strauss Cabin Road. The flow from OS-1 travels along the east curb and gutter
and south to an existing Type R inlet. Runoff from OS-4 travels north along the east
curb and gutter to an existing Type R inlet also. Flow from these inlets convey south
through a piped system and discharge approximately a half mile to the south of the
project site into the existing Weitzel gravel pond. The percent composite
imperviousness for both of these basins is approximately 36%.
Basin OS-2: This 4-acre sub-basin will collect flows from the back half of two building
roof-tops, landscaping, and undisturbed ground and convey to south to existing Weitzel
gravel pond via grassed swales and overland flow. This basin has a percent
imperviousness of approximately 6%.
Basin OS-3: This 3-acre sub-basin will collect flows from Harmony Road runoff, the
landscaping and undisturbed ground and convey to the east and then south via existing
drainage channels and low-lying areas to existing Weitzel gravel pond. This basin has a
percent imperviousness of approximately 2%.
Refer to Appendix D for the Drainage Basin Map
2. Rational Method calculations were performed for each sub-basin to calculate inlet
and pipe sizing. The UD Inlet Version 4.06 spreadsheets
were used to calculate inlet sizing to accept the 2-year and 100-year design storm
events for the combination inlets. Refer to Appendix C for print outs of the UD Inlet
spreadsheets.
3. 100-year flow rates within each sub-basin calculated by the Rational Method were
assumed to be contributory to the sub-surface storm drain system. Hydraflow Storm
Sewers Extension for Autodesk Civil 3D Version 2019.2 was used to determine pipe
sizes and slopes that would allow the hydraulic grade lines (HGLs) to remain below
proposed finish grade. Refer to Appendix C for plan and profiles of storm drain
system
4. Standard water quality and LID treatment will be served by the existing Weitzel
gravel pond to which the runoff from the project site will discharge into.
5. Final design details, and construction documentation shall be provided to the City of
Fort Collins for review prior to Final Development Plan approval.
6. Stormwater facility Standard Operating Procedures (SOP) will be provided by the
City of Fort Collins in the Development Agreement.
H-25 Multi-Family November 2021 Page 6
Project No. 19059.02
C. Floodplain Management
The project site sits within the FEMA regulated floodway and floodplain, per FEMA FIRM
Maps 08069C0994F & 08069C1013F, dated December 19, 2006. More specifically, flood zone
designations within this project site are as follows:
Flood Zone AE, FLOODWAY AREA: Flood Zone AE is determined to be a Special
Flood Hazard Area subject to inundation by the 1% annual chance flood. The 1%
annual chance flood has a 1% chance of being equaled or exceeded in any given year.
Zone AE is where base flood elevations have been determined. Floodway areas in zone
AE show the location of the channel of the stream plus any adjacent floodplain areas
that must be kept free of encroachment so that the 1% annual chance flood can be
carried without substantial increases in flood heights.
Flood Zone AE, 100-YEAR FLOODPLAIN: Flood Zone AE is determined to be a
Special Flood Hazard Area subject to inundation by the 1% annual chance flood. The
1% annual chance flood has a 1% chance of being equaled or exceeded in any given
year. Zone AE is where base flood elevations have been determined.
Flood Zone X, Shaded: Areas of 0.2% annual chance flood.
Flood Zone X, Unshaded: Areas determined to be outside the 0.2% annual chance
floodplain.
There exists an approved LOMR-F (Case # 00-08-175A, effective date 12-12-2000)
across a portion of the project site.
There exists an approved CLOMR-F (Case # 00-08-182C) across a portion of the
project site. Refer to Appendix F for CLOMR-F documents.
There is also an approved LOMR (Case # 14-08-0580P, effective date 12-15-2014) that
corrected the floodway and floodplain lines to align with the original floodplain work
map. Information provided in this LOMR was taken into consideration when
determining the project limits.
Chapter 10, Flood Prevention and Protection, of the Fort Collins Municipal Code
provided guidance and limitations for building and development within the floodway
and floodplain. Some of the fundamental flood protection requirements for this project
site include:
o Designing and constructing buildings with a lowest finished floor (FF) being
as shown on the FEMA
FIRM map for the area;
o Building foundation types anticipated for the buildings on this site include slab-
on-grade or crawl space. (Basements are not anticipated for buildings at this
project site.) If crawl space foundation type is utilized then they must be built
in accordance with the requirements of the City of Fort Collins Municipal
Code, safety and venting requirements outlined in Section 10-39, and specific
standards for crawl spaces outlined in Section 40;
H-25 Multi-Family November 2021 Page 7
Project No. 19059.02
o Regulatory Floodproo
o Structures built within the floodplain (or flood fringe) are required to have an
Emergency Preparedness Plan (ERPP);
o No floatable materials are allowed unless they are properly anchored.
o Floodplain Use Permits will be required to be submitted for any construction
located within designated floodplain and/or floodway.
o Elevation or floodproofing certificates will be required before Certificate of
Occupancy is issued for any building.
D. Low Impact Development (LID)
The LID requirement for this project is treating at least 75% of the new and/or modified
impervious areas which translates to approximately 373,262 square feet of area to be treated.
Approximately 396,832 square feet (or 80% of the of the new and/or modified impervious area)
will to the outfall that discharges into the existing Weitzel gravel pond to the south where
standard water quality and LID level treatments will occur.
1. Existing Weitzel Gravel Pond
The Weitzel Pits are simply inactive gravel mine pits that have become ponds due to
the water table elevation in the area. The west pit (denoted as Ex. Gravel Pond #1 in
the approved ODP), the largest of the Weitzel Pits, will be utilized as a detention and
water quality basin for stormwater runoff from the project site. Refer to Appendix E
for pages from the approved ODP drainage report.
VII. Variance Requests
LID Requirement: For the H-25 project, LID treatment will be provided through the existing gravel
pond to the south. The next development within the ODP, however, will have to account for H-25
Also, the approved Harmony & I-25 ODP contains a note that requires LID facilities be provided to
treat the WQCV specified in the approved drainage report for H-23 (The Wyatt).
VIII. Erosion Control
Erosion and sediment control will be achieved through construction control measures 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 are shown in the corresponding Utility Plans associated with this report. A separate Erosion
Control Report and Erosion Control Plans have also been prepared and associated with this final
drainage report. See Appendix D for Erosion Control Plan.
H-25 Multi-Family November 2021 Page 8
Project No. 19059.02
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 a
developed condition runoff plan within the H-25 Multi Family project area. This FDP drainage design
report has highlighted the following items:
Project site development is outside of the effective half-foot Poudre River floodway
boundary, with the exception of a few improvements that will require a No-Rise Certificate.
Reservation of space is provided along Harmony Road for a potential future drainage
channel that will accommodate and route the offsite half-foot rise Poudre River floodway flows
from the north around the project development site.
Stormwater detention, standard water quality and LID treatment are provided within the
existing Weitzel gravel pond.
A. Compliance with Standards
1. The drainage design proposed with the proposed project complies with the City of Fort
2. The drainage design proposed with this project complies with requirements for Cache
La Poudre 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 submitted with the final plans comply with the
Urban Storm Drainage Criteria Manual.
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. Water quality will be treated through grass buffers, grassed swales and existing gravel
pond.
H-25 Multi-Family November 2021 Page 9
Project No. 19059.02
REFERENCES
1. City of Fort Collins, December 2018,
2. Mile High Flood District, 2001 (November 2010 with some sections updated April 2018),
3. United States Department of Agriculture Natural Resources Conservation Service;
accessed on-line September 2019.
4. Federal Emergency Management Agency; accessed on-line
September 2019.
APPENDICES
1. Appendix A - Site Information
a. Vicinity Map
b. ODP Plan
c. Overall Drainage Plan
d. NRCS Soils Data
e. FEMA Maps
2. Appendix B Rational Method Calculations
a. Storm Criteria
b. Rational Method Calculations
3. Appendix C Storm System Calculations
a. Inlet Calculations
b. Storm Drain Calculations
4. Appendix D Drainage Basin Map
a. Drainage Basin Map
b. Erosion Control Plan
5. Appendix E ODP Drainage Report
a. Pages from the ODP Drainage Report
6. Appendix F CLOMR-F Documents
a. CLOMR-F Case No. Document
b. LOMR/CLOMR-F Exhibit
H-25 Multi-Family Fort Collins,CO
2-LANE ARTERIAL
60' EXISTING ROW
H-25 Multi-Family Fort Collins,CO
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:
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:
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:
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:
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:
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:
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
H-25 Multi-Family Fort Collins,CO
H-25 Multi-Family Fort Collins,CO
H-25 Multi-Family Fort Collins,CO
Harmony and I-25 ODP Drainage Report
Page 4
Figure 2. Gravel Pit Information
Harmony and I-25 ODP Drainage Report
Page 6
Figure 3. Outfall to the Poudre River
Historic stormwater runoff rates from this site have been calculated based upon the site as it
was prior to mining activities.
Harmony and I-25 ODP Drainage Report
Page 7
3.0 Developed Conditions Plan
3.1 Design Criteria
The overall drainage plan presented in this report has been developed in accordance with the
City of Fort Collins Stormwater Criteria Manual and the City of Fort Collins submittal
requirements for an ODP. The drainage plan presented is based upon the allowable developed
stormwater release rates prescribed for the Poudre River drainage basin, that being the 2-year
historic rate; and an integrated stormwater management program that will incorporate “Low
Impact Development” (LID) principals into the design.
The purpose of this developed condition analysis is basically to establish the maximum runoff
rates from the “Limits of Development” within the ODP and to determine the 100-year detention
volume required. The existing Weitzel Pits, now groundwater fed ponds, will be utilized as
detention and water quality basins for stormwater runoff from this site. Specifically, the ponds
shown as Ponds #1 and #2 in Figure 2, or as the area shown as the detention basin on the
Grading and Drainage Plan, is the planned area for detention.
Developed runoff analysis for the ODP is based on runoff coefficients for zoning classifications.
For this project, the zoning classification is H-C (Harmony Corridor), and the minor storm runoff
coefficient is 0.95, and 1.0 for the major storm.
The allowable release from the ODP site is 9.8 cfs (2-yr historic release rate). The 100-year
release from the undeveloped areas adjacent to the ODP site (east of the Limits of
Development) is 57.2 cfs. This amount will be treated as a pass-through flow. The resulting
amount of stormwater passing from the detention basin will be approximately 67 cfs (9.8 +
57.2). Stormwater release from the detention basin will need to be provided through a controlled
release structure, likely to be a spillway that would need to be built below the historic ground
surface so as not to fill in or affect the floodway.
In addition, there is an offsite basin to the west of Strauss Cabin Road that currently drains via
the Boxelder Ditch. A conveyance will be provided to this property to convey at least the 2-yr
historic flows (allowed release after development) thru the ODP site. (100-year conveyance of
this flow will be provided if development on that parcel does not occur first.) This flow will be
considered a pass through flow and will be added to the allowed release from the detention
pond.
The required amount of stormwater detention for the full development of the land use parcels in
the “Limits of Development” area is 16.2 ac-ft plus an additional 20% (City criteria), resulting in
19.4 ac-ft. The area of the Weitzel Pond #1 that is outside of the “Limits of Development” and
Weitzel Pond #2 will constitute the detention basin for this project. The approximate water
surface area for these two ponds combined is approximately 18 acres, resulting in
approximately 1.1’ of stormwater storage depth above the groundwater elevation. The detention
water surface elevation is still below the Base Flood Elevations as published on the current
FEMA maps.
rates from the “Limits of Development” within the ODP and to determine the 100-year detention
volume required. The existing Weitzel Pits, now groundwater fed ponds, will be utilized as
detention and water quality basins for stormwater runoff from this site. Specifically, the ponds
volume required. The existing Weitzel Pits, now groundwater fed ponds, will be utilized as
H-25 Multi-Family Fort Collins,CO