HomeMy WebLinkAboutDrainage Reports - 06/11/2025OVERALL DRAINAGE REPORT
19 OLD TOWN SQUARE #238 | FORT COLLINS, CO 80524 | 970-530-4044 | www.unitedcivil.com
H ERITAGE C HRISTIAN A CADEMY
Industrial Business Park International PUD
Fort Collins, CO
Prepared for:
Heritage Christian Academy
2506 Zurich Drive #1
Fort Collins, CO 80524
Date:
February 19, 2025
City of Fort Collins Approved Plans
Approved by:
Date:
Dan Mogen
06/11/2025
ODP240001
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February 19, 2025
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Heritage Christian Academy Improvements
Fort Collins, Colorado
Project Number: U24018
Dear Staff:
United Civil Design Group, LLC. is pleased to submit this Overall Drainage Report for the Heritage Christian Academy site in
Fort Collins, Colorado. In general, this report serves to document the stormwater impacts associated with future
improvements related to the existing property and the planned site. The site was previously analyzed by Lamp Rynearson
Associates in March 2018. The current condition of the site appears to reflect the design established by Lamp Rynearson
Associates, approved through the Larimer County development review process. The March 2018 Final Drainage Report for
the Industrial Business Park International PUD (referred herein as “The March 2018 Final Drainage Report) is referenced with
this ODP report.
We understand that review by the City of Fort Collins is to assure general compliance with criteria established with the March
2018 Final Drainage Report, and standardized criteria contained in the Fort Collins Stormwater Criteria Manual. This report
was prepared in compliance with technical criteria set forth in the Fort Collins Stormwater Criteria Manual.
If you should have any questions or comments as you review this report, please feel free to contact us at your convenience.
Sincerely,
United Civil Design Group
Colton Beck, PE
Project Manager
2.19.2025
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TABLE OF CONTENTS
I. General Location and Description ......................................................................................................................1
A. Location and Project Description ................................................................................................. 1
B. Description of Property ................................................................................................................ 2
C. Floodplains ................................................................................................................................... 3
II. Drainage Basins and Sub-Basins .........................................................................................................................3
A. Major Basin Description ............................................................................................................... 3
B. Sub-Basin Description .................................................................................................................. 3
III. Drainage Design Criteria ....................................................................................................................................3
A. Regulations ................................................................................................................................... 3
B. Directly Connected Impervious Area (DCIA) ................................................................................ 3
C. Hydrological Criteria ..................................................................................................................... 4
D. Hydraulic Criteria.......................................................................................................................... 4
E. Modifications of Criteria .............................................................................................................. 4
IV. Drainage Facility Design .....................................................................................................................................5
A. General Concept ........................................................................................................................... 5
B. Specific Details ............................................................................................................................. 5
V. Erosion Control ...................................................................................................................................................8
VI. Conclusions ........................................................................................................................................................8
A. Compliance with Standards ......................................................................................................... 8
B. Drainage Concept ......................................................................................................................... 8
C. Stormwater Quality ...................................................................................................................... 8
VII. References ......................................................................................................................................................9
APPENDICES
APPENDIX A – Hydrology Calculations
APPENDIX B – Hydraulic Calculations
B.1 – Low Impact Development Calculations (Reserved for future submittal)
B.2 – Water Quality Calculations
B.3 – Detention Computations
B.4 – Inlet Sizing Calculations (Reserved for future submittal)
B.5 – Storm Pipe Calculations (Reserved for future submittal)
B.6 – Curb Channel Calculations (Reserved for future submittal)
B.7 – Weir Calculations (Reserved for future submittal)
APPENDIX C – Referenced Materials
APPENDIX D – Drainage Plan
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I.GENERAL LOCATION AND DESCRIPTION
A.LOCATION AND PROJECT DESCRIPTION
The Heritage Christian Academy site (referred herein as “the site”) exists as a portion of the Industrial Business Park
International PUD, located in the northwest and southwest quarters of Section 8, T7N, R68W of the 6th P.M., City of Fort
Collins, Larimer County, Colorado. The entirety of the property, consisting of approximately 20.1-acres, is located west of
South Timberline Road and east of Mexico Way. The east side of the school property currently exists as a school building with
adjacent parking and athletic field; the west side exists in an overlot graded condition. The future Heritage Christian Academy
site improvements are limited to 13.6-acres of disturbed area west and southwest of the existing school facility.
Dry Creek, the ultimate discharge location for stormwater within the Industrial Business Park International PUD, is located
south of the development. The existing site improvements (i.e., infrastructure east of Munich Way) drain stormwater
primarily to the south by way of surface drainage to drainage swales constructed along International Blvd. Future
improvements (i.e., west of Munich Way) are intended to drain to an existing detention pond (i.e., Pond B) constructed in
2020. Pond B is constructed to release stormwater to an existing drainage swale system in International Blvd and ultimately
to Dry Creek.
Below is an aerial map depicting the vicinity of the site. Dry Creek exists to the south, and the Timbervine Subdivision borders
the site to the north. Other nearby subdivisions are represented below.
FIGURE 1:SITE VICINITY MAP
The site improvements will ultimately include the construction of new education and athletic facilities with associated
landscaping, walks, and parking lots. This drainage report presents the overall drainage plan for the development. In general,
this report serves to provide an analysis of the drainage impacts associated with the development of site as it relates to
existing and future drainage facilities on-site. The project is currently in the ODP stage; additional design information will be
provided with further site design (i.e., PDP and FDP applications).
Mexico Way
Munich Way
Project
Site
Pond B
Timbervine Subdivision Dry Creek
Subdivision
East Ridge
Subdivision
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B.DESCRIPTION OF PROPERTY
The property east of Munich Way exists in a fully developed condition. The school facility east of the Munich Way exists with
concrete and asphalt pavement, sidewalks, rooftop, and landscaping. In its existing school facility and associated impervious
areas drain stormwater by means of sheet flow, concrete pans, curb and gutter. The existing site ultimately drains off-site to
drainage swales along International Blvd.
Below are summaries of key components of the school facility in its existing conditions.
Land Use - The site’s current land use is commercial/industrial.
Ground Cover - The site exists as a school development with concrete and asphalt pavement, sidewalk, rooftop, and
surrounding grass landscaping. The majority of landscaping is specific to a maintained athletic field.
Existing Topography – The site slopes in a multitude of directions away from the existing on-site building; however,
runoff ultimately drains south down Zurich Drive to a system of drainage swales along International Blvd.
Grades – In general, the existing site is sloped to the east and south at approximately 0.5% to 1.0%.
Soil Type - The USDA’s Web Soil Survey shows that the eastern portion of the property consists of both “Type B” and
“Type C” soils, namely Flouvaquents (nearly level) and Loveland Clay Loam (0% to 1% slopes).
Utilities – The following dry utility lines run along the south side of the site: gas, electric, fiber optic. Water mains
and sanitary sewer are also present to the south of the school facility in Zurich Drive.
Drainage Features and Storm Sewer – The eastern portion of the campus is adjacent to the Lake Canal - stormwater
does not appear to be conveyed to this canal. A series of storm sewer and drainage swales exist at the downstream
end of the business park along International Blvd – this infrastructure conveys the eastern portion of the Industrial
Business Park International PUD. A 30” outfall exists at the downstream end of the mentioned drainage features.
Refer to the Drainage Plan for visual context.
The property west of Munich Way exists in a partially developed, overlot-graded condition. Apart from the completion of
connecting roadways and underground utilities to support the Industrial Business Park International PUD, the remainder of
the private land remains largely undeveloped. The existing land is currently graded to drain southerly to an existing detention
pond, namely Pond B.
Below are summaries of key components of the western side in its existing conditions.
Land Use - The site’s current land use is commercial/industrial.
Ground Cover - The site exists in an overlot grading condition.
Existing Topography – The site generally slopes to the south to an existing detention pond (Pond B).
Grades – In general, the western portion of the site is sloped the south at approximately 0.5% to 2.0%.
Soil Type - The USDA’s Web Soil Survey shows that the eastern portion of the property consists of both “Type B” and
“Type C” soils, namely Flouvaquents (nearly level), Loveland Clay Loam (0% to 1% slopes), and Table Mountain Loam
(0% to 1% slopes). The on-site soils provide moderate infiltration and are suitable for development.
Utilities – The following dry utility lines run along the perimeter of the site: gas, electric, cable TV, fiber optic. Water
mains and sanitary sewer are also present in the recently constructed roadways to support the Industrial Business
Park International PUD.
Drainage Features and Storm Sewer – A detention pond (Pond B) exists on-site to support the development of the
school site. This detention pond exists with an outlet structure and 18” outfall pipe that drains to an existing drainage
swale in International Blvd south of the project site (refer to Drainage Plan for visual context). In addition to the
controlled release from Pond B, stormwater within the Industrial Business Park International PUD drains to the
mentioned drainage swale and ultimately to a 30” RCP outfall.
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C.FLOODPLAINS
The existing site is in the vicinity of the Dry
Creek Floodplain, which is a FEMA-
designated 100-year floodplain and
floodway. The existing site, being outside
the bounds of the Dry Creek FEMA
floodplain, is located in an area with minimal
flood risk. The FEMA FIRM Panel # is
08069C0983H effective 5/2/2012. The
current FEMA FIRM Map is included in the
appendices.
FIGURE 2:FLOODPLAIN MAP
II.DRAINAGE BASINS AND SUB-BASINS
A.MAJOR BASIN DESCRIPTION
The existing site is located within the Dry Creek drainage basin. The site drains downstream within the Dry Creek Basin and is
ultimately conveyed to the Cache La Poudre River. No known master planning improvements are associated with or adjacent
to the site.
B.SUB-BASIN DESCRIPTION
A portion of the school property exists within the Industrial Business Park International PUD plans completed by Lamp
Rynearson & Associates. The March 2018 Final Drainage Report, including the associated Drainage Plan, is provided in the
appendices. The project area exists within Basins B1, B2, and B3 of the drainage design. These basins are designed to convey
stormwater to the existing, downstream Pond B within Basin B1. Characteristics of these planned basins are further described
under this cover.
III.DRAINAGE DESIGN CRITERIA
A.REGULATIONS
The design criteria for this study are directly and primarily from the City of Fort Collins Stormwater Criteria Manual
(referred to herein as FCSCM). Mile High Flood District Criteria Manuals Volumes 2, and 3 (referred to herein as MHFD) are
also referred to with the drainage design.
B.DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
The “Four Step Process” that is recommended with the FCSCM in selecting structural BMPs for redeveloping urban areas.
The following portions of this summary describe each step and how it has been utilized for this project:
Step 1 – Employ Runoff Reduction Practices
The objective of this step is to reduce runoff peaks and volumes and to employ the technique of “minimizing directly
connected impervious areas” (MDCIA). This project accomplishes this by:
Project
Area
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Routing the roof and pavement flows through bioretention facilities and vegetated buffers to increase the time of
concentration, promote infiltration and provide water quality.
Step 2 – Provide Water Quality Capture Volume (WQCV)
The objective of providing WQCV is to reduce the sediment load and other pollutants that exit the site. For this project
WQCV is provided within the existing water quality and detention facility. Low Impact Development (LID) is also to be
provided with site improvements.
Step 3 – Stabilize Drainageways
The site is upstream of Dry Creek and the use of LID will help slow runoff from the site and benefit the stabilization of the
Dry Creek drainageway. In addition, this project will pay stormwater development and stormwater utility fees which the
City uses, in part, to maintain the stability of the City drainageway systems.
Step 4 – Consider Need for Site Specific and Source Control BMPs
Site specific and source control BMPs are generally considered for large industrial and commercial sites. The
redevelopment of the existing site will include multiple site-specific and source controls, including dedicated maintenance
personnel providing landscape maintenance and snow and ice management. Other site-specific and source controls are to
be better known and addressed with future PDP and FDP submittals.
C.HYDROLOGICAL CRITERIA
City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, provided by Figure RA-16 of the Fort Collins Stormwater
Criteria Manual, are utilized for all hydrologic computations related to the site in its existing/historic and future conditions.
Since this site is relatively small and does not have complex drainage basins, the peak flow rates for design points have been
calculated based on the Rational Method as described in the City of Fort Collins Stormwater Criteria Manual (FCSCM) with
storm intensity correlating with the time of concentration for each sub-basin. This method was used to analyze the developed
runoff from the 2-year (minor) and the 100-year (major) storm events. The Rational Method is widely accepted for drainage
design involving small drainage areas (less than 20-acres) and short time of concentrations. Runoff coefficients are calculated
from a composite imperviousness based on surface type. The HCA site appears to have been assigned assumed future
impervious values with the March 2018 Final Drainage Report by way of Land Use (i.e., Light & Heavy Industrial), accounting
for impervious values ranging between 80% - 90%. Areas at the south end of the site, where detention area was accounted
for, had an assumed imperviousness of 40%. Similar assumptions found in the March 2018 Final Drainage Report are provided
with this ODP report as FCSCM provides similar impervious values for Commercial and Industrial land uses (80% - 90%).
Detailed, site-specific runoff coefficient calculations will be provided by way of surface type with PDP and FDP submittals as
the site layout develops.
D.HYDRAULIC CRITERIA
The developed site will convey runoff to existing design points via swales, concrete pans, and pipes. The City of Fort Collins
Stormwater Criteria Manual (FCSCM) are referenced for all hydraulic calculations.
Drainage conveyance facility capacities ultimately proposed with the development project, including an extended detention
pond, shall be designed in accordance with criteria outlined primarily in the FCSCM or the Mile High Flood District’s Criteria
Manual where necessary and not covered by FCSCM.
E.MODIFICATIONS OF CRITERIA
The original design of the Industrial Business Park International PUD was completed under design standards per Larimer
County. With the annexation of the property in the City of Fort Collins, the constructed drainage system will be further
analyzed in relation to the FCSCM with the inclusion of Low Impact Development requirements. There are currently no
variance requests proposed with the future stormwater design of the site.
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IV.DRAINAGE FACILITY DESIGN
A.GENERAL CONCEPT
Developed runoff will be designed to largely maintain planned drainage patterns. Existing conveyance methods include sheet
flow, concrete pans, curb and gutter, inlets, and storm sewer that ultimately drain south to Pond B for water quality treatment
and detention storage. Per City standards, water quality and low impact development (LID) will ultimately be proposed with
the project to mitigate the impervious areas that are being modified with the development.
B.SPECIFIC DETAILS
Hydrology
Site improvements intend to adhere to the drainage design established by the March 2018 Final Drainage Report and the
FCSCM. The entirety of the school property is to be annexed into the City of Fort Collins, however, the project site is limited
to approximately 14.57-acres related to the partially developed area of the industrial business park, namely drainage basins
B1, B2, and B3 of the March 2018 Final Drainage Report. The table below summarizes the hydrologic impact associated with
the site improvements relative to the planned conditions provided in the March 2018 Final Drainage Report. Refer to the
Drainage Plan, hydrology calculations, and references attached for additional information.
TABLE 1 -HYDROLOGIC SUMMARY
Project Area
Planned Proposed
Overall Area (acre) 14.6 15.3
% Imperviousness 72.0% 72.0% (1)
(1)The proposed imperviousness is assumed at 72% for ODP purposes;
hydrology and impervious calculations will be analyzed with future PDP
and FDP submittals as the site plan progresses.
A discrepancy in area between the planned basin area and proposed basin area is due to the physical area that drains to the
existing Pond B. Based on existing topography along Zurich Drive and the western side of the school facility, it appears that
more area exists within Basin B than originally assumed. This minor discrepancy is not an indictment on the March 2018 Final
Drainage Report and calculations, nor is the larger drainage area considered a concern, but this actuality will be further
evaluated with the overall site release on future PDP and FDP submittals.
On-site Basins
The following basins provide drainage delineations for the site in its improved condition. Refer to Appendix A for hydrology
computations and Appendix B for calculations related to Water Quality, Low Impact Development, and other hydraulic
features.
Basin B
Sub-drainage basins B1-B3 of the March 2018 Final Drainage Report represent areas where runoff is captured and conveyed
to Pond B. For purposes of this Overall Drainage Report, the mentioned basins are combined into one basin (i.e., Basin B).
This overall basin consists of roofs, concrete and asphalt paving, and landscaping. This basin will be sub-divided into several
basins with future submittals.
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Stormwater Quality
Stormwater quality is required to be provided for the new/planned impervious area on the site. The existing Pond B was
designed with 0.48 acre-ft of volume for extended detention purposes – this volume was calculated per MHFD computations
assuming a composite imperviousness of 72.0% and an additional 20% factor for additional capacity for ODP purposes. Below
is a minimum required WQCV calculation per the FCSCM.
WQCV = a(0.91i3 – 1.19i2 + 0.78i)
WQCV = 1.0 x (0.91(0.72)3 – 1.19(0.72)2 + 0.78(0.72))
WQCV = 0.28
V = (WQCV12) x A x 1.2
V = (0.2812) x 15.3 x 1.2
V = 0.44 acre-ft
The original WQCV design of Pond appears to be adequate for City of Fort Collins standards; however, based on a recent
topographic survey, the constructed pond appears to exist with insufficient volume per the March 2018 Final Drainage Report.
The current WQCV capacity is 0.25 acre-ft. To meet requirements per City of Fort Collins, one of the following options, or
perhaps a combination of the following options, will need to be considered with the development of the site:
1)Replace/reconstruct the outlet structure with additional water quality depth.
2)Re-grade the existing pond to generate additional volume.
3)Reduce the required standard WQCV to 50% by implementing LID methods to treat 50% of the site.
4)Reduce the required standard WQCV to 25% by implementing LID methods to treat 75% of the site.
Regarding WQCV and LID, the intention of this ODP report is to affirm adherence to FCSCM. The provided options will be
addressed and selected with future PDP and FDP submittals.
Low Impact Development (LID)
In December of 2015, Fort Collins City Council adopted the revised Low Impact Development (LID) policy and criteria which
requires developments within City limits to meet certain enhanced stormwater treatment requirements in addition to more
standard treatment techniques. The proposed development will be required to meet the newly adopted LID criteria which
requires the following:
-Treat no less than 75% of any newly added impervious area using one or a combination of LID techniques.
-Treat no less than 50% of any newly added impervious area using one or a combination of LID techniques when at
least 25% of any newly added pavement is provided with permeable pavement.
Detention
Detention is required to be provided for the new/planned impervious area on the site. The existing Pond B was designed
with 3.3 acre-ft of required 100-year detention volume; the pond was constructed with a capacity of 4.7 acre-ft. The existing
Pond B was designed assuming a composite imperviousness of 72.0% utilizing the Modified FAA Method. The 100-year release
rate (2.91-cfs) was designed in accordance with criteria established for development within the Dry Creek Basin (i.e., 0.20
cfs/acre) and the area (14.57-acres) assumed with the March 2018 Final Drainage Report.
The nature of the existing detention pond and outlet structure are recognized as design constraints for the school site
improvements; however, modifications to the outlet structure orifice plate may be necessary for 100-year release purposes.
The existing Pond B was designed to detain 14.57-acres of developed area, though it appears that additional developed land
does drain to Pond B. An additional volume of 0.58 acre-ft is anticipated from what was originally assumed (3.3-acre-ft).
Despite this modification to the design of Pond B, the existing pond was constructed with extra capacity up to 4.70 acre-ft.
Therefore, provided potential modifications to the 100-year orifice plate, the existing Pond B was sufficiently sized to meet
required detention volume requirements per the FCSCM. On the following page is a 100-year detention calculation per the
FCSCM at a duration of 120-minutes. Refer to Appendix A and Appendix B for new calculations per FCSCM.
WQCV = Water Quality Capture Volume, watershed inches
a = 1.0 for 40-hr drain times
i = Percent Imperviousness
V = Required WQCV (acre-ft)
A = Tributary catchment area (acres)
1.2 = Additional 20% of Required Storage
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Vi = CIA (60T)
Vi = 0.94 x 1.84 inhr x 15.29 acres x 60(120min)
Vi = 4.36 acre-ft
Vo = Qout (60T)
Vo = 2.91cfs x 60(120min)
Vo = 0.48 acre-ft
Vs = Vi – Vo
Vs = 3.88 acre-ft
To right is a summary of design elements related
to existing Pond B.
Emergency Spillway
The existing Pond B was designed with a 68-ft long weir, 6-inches in depth, equipped to convey 76.47-cfs. Provided City of
Fort Collins runoff coefficient calculations, initial runoff computations provided under this ODP Report indicate that the
required 100-year release through the spillway may increase from what was originally calculated in the March 2018 Final
Drainage Report. In the event that a higher total runoff value is conveyed to the existing Pond B, the emergency weir may
require reconstruction. The reconstruction of the emergency spillway may also be necessary based on other design elements
with an improved site. The sufficiency of the previously designed and constructed emergency spillway will be verified with
future PDP and FDP submittals.
Standard Operating Procedures (SOPs)
In order for physical stormwater Best Management Practices (BMPs) to be effective, proper maintenance is essential.
Maintenance includes both routinely scheduled activities, as well as non-routine repairs that may be required after large
storms, or as a result of other unforeseen problems. Standard Operating Procedures should clearly identify BMP maintenance
responsibility. BMP maintenance is typically the responsibility of the entity owning the BMP.
Identifying who is responsible for maintenance of BMPs and ensuring that an adequate budget is allocated for maintenance
is critical to the long-term success of BMPs. Maintenance responsibility may be assigned either publicly or privately. For this
project, the privately owned BMPs including Pond B, grass swales, and any installed LID features, are to be maintained by the
property owner.
Storm Sewer
Multiple storm sewers and roof drains will be designed with future site improvements. All storm sewers will be private and
are typically sized to accommodate the flows from the 100-year storm event. Hydraulic computations of these systems will
be provided in Appendix B with future PDP and FDP submittals.
Inlets
Multiple inlets will be designed with future site improvements. All proposed storm sewers will be private and are typically
sized to accommodate the flows from the 100-year storm event. Hydraulic computations of these systems will be provided
in Appendix B with future PDP & FDP submittals.
TABLE 2 –POND B SUMMARY
Pond B
Existing WQCV (ac-ft) 0.25
Required WQCV (ac-ft) 0.44
Required V100 (ac-ft) 3.88
VMAX (ac-ft) 4.70
Bottom of Pond (Elev) 4928.0
WQCV (Elev) 4929.6
Required V100 (Elev) 4932.2
VMAX (Elev) 4932.7
Emergency Spillway (Elev) 4932.7
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V.EROSION CONTROL
Erosion control, both temporary and permanent, is a vital part of any development project. For this project, the site
disturbance is greater than 1 acre; therefore, a CDPHE Stormwater Management Plan (SWMP) will be required.
Comprehensive erosion control measures are included with the site improvements. Refer to the Utility Plans for additional
information. At a minimum, the following temporary BMP’s will be installed and maintained to control on-site erosion and
prevent sediment from traveling off-site during construction:
Silt Fence – a woven synthetic fabric that filters runoff. The silt fence is a temporary barrier that is placed at the base
of a disturbed area.
Vehicle Tracking Control – a stabilized stone pad located at points of ingress and egress on a construction site. The
stone pad is designed to reduce the amount of mud transported onto public roads by construction traffic.
Inlet Protection – acts as a sediment filter. It is a temporary BMP and requires proper installation and maintenance
to ensure their performance.
Straw Wattles – wattles act as a sediment filter in swales around inlets. They are a temporary BMP and require
proper installation and maintenance to ensure their performance.
The contractor shall store all construction materials and equipment and shall provide maintenance and fueling of equipment
in confined areas on-site from which runoff will be contained and filtered. Temporary Best Management Practices (BMP’s)
will be inspected by the contractor at a minimum of once every two weeks and after each significant storm event.
VI.CONCLUSIONS
A.COMPLIANCE WITH STANDARDS
Storm drainage calculations have followed the guidelines provided primarily by the City of Fort Collins Stormwater Criteria
Manual. Mile High Flood District Criteria Manuals Volumes 1, 2 and 3, and the March 2018 Final Drainage Report are also
adhered to as necessary where the FCSCM does not cover.
B.DRAINAGE CONCEPT
To meet City of Fort Collins stormwater criteria, the existing drainage system may require modifications to Pond B. The overall
size and capacity of Pond B was designed and constructed with additional capacity necessary to meet City of Fort Collins
requirements; however, several design features related to both water quality and detention will require verification with
future improvements.
C.STORMWATER QUALITY
Multiple long-term stormwater quality measures will be necessary on-site to provide treatment of stormwater prior to it
being discharged from the site. For this site this includes extended detention and will ultimately include LID techniques
throughout the site.
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VII.REFERENCES
1.City of Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, December 2018.
2.Mile High Flood District Criteria Manual Volume 1 and 2, Mile High Flood District, Denver, Colorado, January 2016.
3.Final Drainage Report, Lamp Rynearson & Associates, Fort Collins, Colorado, March 2018.
4.Natural Resources Conservation Service Web Soil Survey at: websoilsurvey.nrcs.usda.gov/app
5.Flood Insurance Rate Map, FEMA, Panel 08069C0983H, https://hazards.fema.gov/femaportal/
6.Stormwater Discharges Associated with Construction Activity, Stormwater Management Plan Preparation Guides, State
of Colorado, www.colorado.com
A PPENDIX A
H YDROLOGY C ALCULATIONS
RUNOFF COEFFICIENTS AND % IMPERVIOUS
Heritage Christian Academy, Fort Collins, CO
Basin Design Pt.Composite
Total Total Roof (1)Asphalt &Recycled Gravel & Lawns(1)Imperviousness C2 C100
Concrete(1)Asphalt(1)Pavers(1)
%I = 90%%I = 100%%I = 80%%I =40%%I=2%(%I)
C=0.95 C=0.95 C=0.80 C=0.50 C=0.25
acres sf sf sf sf sf sf
B1 B1 5.91 257,440 231,181 26,259 90.0%0.74 0.85
B2 B2 4.27 186,001 148,057 37,944 80.0%0.64 0.80
B3 B3 4.39 191,228 74,197 117,032 40.0%0.29 0.61
Total B1 14.57 634,669 -453,434 --181,235 72.0%
Notes:
(1) Recommended % Imperviousness Values per Table 4.1-3 Surface Type - Percent Impervious in Fort Collins Stormwater Criteria Manual
(2) Runoff C is based Table 3.2-2. Surface Type - Runoff Coefficients and Table 3.2-3. Frequency Adjustment Factors in Fort Collins Stormwater Criteria Manual
(3) Value per March 2018 Final Drainage Report, Lamp Rynearson & Associates
Planned Basins
Composite Runoff Coefficients (3)Areas
(3)
(3)
(3)
(3)
(3)
(3)
(3)
(3)
(3)
Date: 1/21/2025 C:\United Civil Dropbox\Projects\U24018 - Heritage Christian Academy\Reports\Drainage\Calculations\Hydrology-Fort Collins
TIME OF CONCENTRATION (2-YR)
Heritage Christian Academy, Fort Collins, CO
Basin Design Pt.Area
CXCF(1)Length Slope Ti
(2)Slope Length n R Velocity (3)Tt
(4)Tc max
(5)
acres ft %min %ft fps min min min min
B1 B1 5.91 13.0
B2 B2 4.27 13.9
B3 B3 4.39 28.1
Notes:
(1) C=CX*CF is less than or equal to 1.0 (Cf = 1.0)
(2) ti = [1.87(1.1-CXCF)L1/2]/S1/3, S= slope in %, L=length of overland flow (200' max urban, 500' max rural)
(3) V=(1.49/n)R2/3S1/2, S = slope in ft/ft, FCSCM Equation 5-4
(4) tt=L/(V*60 sec/min)
(5) Maximum tc = total length/180 + 10
(6) Minimum tc = 5 min
(7) Value per March 2018 Final Drainage Report, Lamp Rynearson & Associates
Travel/Channelized Time of Flow (Tt)
Planned Basins
Ti+Tt Final Tc
(6)
Overland Flow (Ti)
(7)
(7)
(7)
Date: 1/21/2025 C:\United Civil Dropbox\Projects\U24018 - Heritage Christian Academy\Reports\Drainage\Calculations\Hydrology-Fort Collins
TIME OF CONCENTRATION (100-YR)
Heritage Christian Academy, Fort Collins, CO
Basin Design Pt.Area
CXCF(1)Length Slope Ti
(2)Slope Length n R Velocity (3)Tt
(4)Tc max
(5)
acres ft %min %ft fps min min min min
B1 B1 5.91 13.0
B2 B2 4.27 13.9
B3 B3 4.39 28.1
Notes:
(1) C=CX*CF is less than or equal to 1.0 (Cf = 1.25)
(2) ti = [1.87(1.1-CXCF)L1/2]/S1/3, S= slope in %, L=length of overland flow (200' max urban, 500' max rural)
(3) V=(1.49/n)R2/3S1/2, S = slope in ft/ft, FCSCM Equation 5-4
(4) tt=L/(V*60 sec/min)
(5) Maximum tc = total length/180 + 10
(6) Minimum tc = 5 min
(7) Value per March 2018 Final Drainage Report, Lamp Rynearson & Associates
Planned Basins
Overland Flow (Ti)Travel/Channelized Time of Flow (Tt)
Ti+Tt Final Tc
(6)
(7)
(7)
(7)
Date: 1/21/2025 C:\United Civil Dropbox\Projects\U24018 - Heritage Christian Academy\Reports\Drainage\Calculations\Hydrology-Fort Collins
RATIONAL METHOD PEAK RUNOFF
Heritage Christian Academy, Fort Collins, CO
Basin Design Pt.Contributing Area 2-Year 100-Year
Basins acre tc tc C2 C100 I2 I100 Q2 Q100
min min in/hr in/hr cfs cfs
B1 B1 B1 5.91 13 13 0.74 0.85 1.98 6.92 8.66 34.76
B2 B2 B2 4.27 14 14 0.64 0.80 1.92 6.71 5.25 22.92
B3 B3 B3 4.39 28 28 0.29 0.61 1.34 4.69 1.71 12.56
Planned Basins
Peak DischargeRainfall IntensityRunoff Coefficients
Date: 1/21/2025 C:\United Civil Dropbox\Projects\U24018 - Heritage Christian Academy\Reports\Drainage\Calculations\Hydrology-Fort Collins
A PPENDIX B
H YDRAULIC C ALCULATIONS
WATER QUALITY
Heritage Christian Academy, Fort Collins, CO
Required Water Quality Capture Volume
Basin Area Area Imperviousness Watershed WQCV WQ Treatment
(sf)(acres)(%)(inches)(cf)Method
B 665,977 15.289 72%0.28 18,929 Extended Detention
(1) Water quality by way of extended detetion provided based on 40-hour storage
(2) Water Quality design to include LID methods per City of Fort Collins crtieria; Required Extended Detention WQCV to be reduced with PDP and FDP calculations.
(3) Assumed Imperviousness per original 2018 report. Site Imperviousness to be analyzed with PDP and FDP calculations.
Date: 1/21/2025 C:\United Civil Dropbox\Projects\U24018 - Heritage Christian Academy\Reports\Drainage\Calculations\Hydrology-Fort Collins
DETENTION POND VOLUME (FAA Method)
Heritage Christian Academy, Fort Collins, CO
POND ID:Pond B
100 Year Storm Into Detention Facility
Area =665,977 square feet
Area =15.29 acres
C 2 = 0.75
C 100 = 0.94
Release Rate Out of Pond
Q OUT = 2.91 cfs
Notes:
1. Pond area includes all of Basin B.
2. C100 value shown is a weighted average of the C values per City of Fort Collins criteria.
3. Release rate per the march 2018 Final Drainage Report, Lamp Rynearson & Associates
Detention Volume Calculations
Rainfall Rainfall Inflow Rate Inflow Volume Adjustment
Factor
Average Outflow Volume Required
Duration (T)Intensity (I)Qin= ∑C100*Area*I Vi=(Qin*T*60)Factor Outflow Rate Vo=Qav*T *60 Storage Volume
m= 0.5(1 + Tc/T) Qav = m*Qout Vs=Vi-Vo
min in/hr cfs ft 3 cfs ft 3 ft 3
5 9.95 142.6 42,773 1.00 2.91 873 41,900
10 7.72 110.6 66,374 1.00 2.91 1,746 64,628
15 6.52 93.4 84,085 1.00 2.91 2,619 81,466
20 5.60 80.2 96,293 1.00 2.91 3,492 92,801
25 4.98 71.4 107,040 1.00 2.91 4,365 102,675
30 4.52 64.8 116,584 1.00 2.91 5,238 111,346
35 4.08 58.5 122,774 1.00 2.91 6,111 116,663
40 3.74 53.6 128,620 1.00 2.91 6,984 121,636
45 3.46 49.6 133,865 1.00 2.91 7,857 126,008
50 3.23 46.3 138,852 1.00 2.91 8,730 130,122
55 3.03 43.4 143,279 1.00 2.91 9,603 133,676
60 2.86 41.0 147,535 1.00 2.91 10,476 137,059
70 2.59 37.1 155,875 1.00 2.91 12,222 143,653
80 2.38 34.1 163,699 1.00 2.91 13,968 149,731
90 2.21 31.7 171,007 1.00 2.91 15,714 155,293
100 2.06 29.5 177,111 1.00 2.91 17,460 159,651
110 1.94 27.8 183,473 1.00 2.91 19,206 164,267
120 1.84 26.4 189,835 1.00 2.91 20,952 168,883
Required Detention Volume
V 100 = 168,883 cubic feet
V 100 = 3.88 acre-ft
V MAX = 4.70 acre-ft
Date: 11/7/2024 C:\United Civil Dropbox\Projects\U24018 - Heritage Christian Academy\Reports\Drainage\Calculations\Hydrology-Fort Collins
A PPENDIX C
R EFERENCED M ATERIALS
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
Jurisdiction Boundary
Digital Data Available
No Digital Data Available
Unmapped
This map complies with FEMA's standards for the use of
digital flood maps if it is not void as described below.
The basemap shown complies with FEMA's basemap
accuracy standards
The flood hazard information is derived directly from the
authoritative NFHL web services provided by FEMA. This map
was exported on 7/24/2024 at 11:03 AM and does not
reflect changes or amendments subsequent to this date and
time. The NFHL and effective information may change or
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
FLOOD HAZARD
OTHER AREAS
GENERAL
STRUCTURES
OTHER
FEATURES
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°2'27"W 40°35'35"N
105°1'50"W 40°35'8"N
Basemap Imagery Source: USGS National Map 2023
BEGINNING OF REFERENCED
TIMBERVINE FINAL DRAINAGE REPORT
END OF REFERENCED
TIMBERVINE FINAL DRAINAGE REPORT
END OF MARCH 2018 FINAL DRAINAGE REPORT
A PPENDIX D
D RAINAGE P LAN