HomeMy WebLinkAboutBEEBE CHRISTIAN SCHOOL - FDP - FDP170011 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTDrainage Report
for
BeBee Christian School/Church
2103 Nancy Gray Avenue
Fort Collins, CO 80525
Prepared by: Mike Beach, P.E.
Project #:16‐026‐001
Dated: March 14, 2017
5255 Ronald Reagan Blvd, Suite 210 Johnstown, CO 80534
(303) 322‐6480 EMAIL: mbeach@ridgetopeng.com
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Section Page
1. Project Engineer’s Certification ........................................................................................ 3
2. Project Location & Overview ............................................................................................ 4
3. Drainage Design Criteria ................................................................................................... 5
4. Hydraulic Calculations ....................................................................................................... 6
5. Facility Design .................................................................................................................... 7
6. Conclusion ......................................................................................................................... 8
7. References ......................................................................................................................... 8
Appendix A – Site Vicinity Map
Appendix B – Flood Insurance Rate Map
Appendix C – NRCS Soils Map
Appendix D – Hydrologic Criteria
Fort Collins Rainfall Intensity Curve
Fort Collins Rainfall Intensity Table
Table RO‐3 – Recommended Percentage Imperviousness Values
Appendix E – Hydraulic Calculations
Runoff Coefficients & Imperviousness
Time of Concentration ‐ SF‐2
Rational Method Runoff Flows – SF‐3 (2‐Year Event)
Rational Method Runoff Flows – SF‐3 (10‐Year Event)
Rational Method Runoff Flows – SF‐3 (100‐Year Event)
Detention Volume by Modified FAA Method
Rain Garden Calculations (both Subbasins)
LID Summary
Drain Times
Appendix F – Maps
Drainage Plan/Subbasin Map
Appendix G – References
Table of Contents
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Section 1: PROJECT ENGINEER’S CERTIFICATION
I hereby attest that this Final Drainage Report for the design of stormwater management
facilities for BeBee Christian School on 2103 Nancy Gray Ave. was prepared by me, or under my
direct supervision, in accordance with the provisions of the City of Fort Collins Storm Drainage
Design Criteria and Construction Standards for the responsible parties thereof.
__________________________
Mike Beach, P.E.
Registered Professional Engineer
State of Colorado No. 45088
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Section 2: Project Location & Overview
Location
The sites address is 2103 Nancy Gray Avenue, Fort Collins, CO; which is described as a portion
of Outlot A, Spring Creek Farms North, City of Fort Collins, County of Larimer, State of Colorado.
For Recording purposes: Part of the Southeast Quarter of Section 19, TP 7 N, R 68 W of the 6th
PM.
Located along the north side of Nancy Gray Ave. and east of South Timberline Road, the site is
described as above and contains 3.326 acres of undeveloped vacant land. (See Appendix A –
Site Vicinity Map)
Existing Condition
The existing site is covered by native vegetation and slopes predominately towards South
Timberline Road at approximately 0.5% slope. Stormwater runoff collected into an existing
stormwater detention pond along the eastern side of the site. Currently the detained water is
released at 2.0 cfs via an existing outlet structure and conveyed via piping to “Storm 2” as
delineated in previous storm reports. Storm 2 connects to Sidehill Filing 2 Storm system. The
Sidehill report requires the outlet be contained to a 1.1 cfs release rate and it appears existing
pond has not been updated since its original construction, therefore, as part of this project the
pond is being redesigned to accommodate the slower release rate.
Proposed Condition
The intent of the development is to construct a new building of approximately 26,200 SF,
asphalt concrete parking, curb and gutter, playground area containing grass and gravel
material, and landscaping. It has been determined by the survey that the existing pond does
not have sufficient detention capacity for this project. Existing pond size of 0.866 ac‐ft is will be
slightly regraded along the west side to increase overall pond volume to meet the minimum
required.
Floodplain
FEMA Floodplain mapping in this neighborhood is located on map‐panel 08069C1000F,
Effective December 18, 2006, (Not Printed), however it appears the subject property is located
in Zone X, which is outside the 100‐year floodplain. Flood Insurance Rate Map not available for
printing.
Existing Soils
The Natural Resource Conservation Service mapped the historic soils on the site as part of the
Nunn clay loam. The Hydrologic Class is Type ‘C’ Soils. (Appendix C – NRCS Soils Map)
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Section 3: Drainage Design Criteria
Reference Manuals
Storm drainage design criteria is defined in the City of Fort Collins “Storm Drainage Design
Criteria and Construction Standards Manual (FCSDCM) and Amendments, which references to
the “Urban Drainage and Flood Control District” (UDFCD) and the “Urban Storm Drainage
Criteria Manual” (USDCM), Volumes 1, 2, and 3, dated 2008 with revisions (the Manual), are
used and referenced in this report.
Low Impact Development (LID) Requirements
The proposed development is required to meet the City’s current Land Use Code, which
includes:
‐ 50% of the newly added or modified impervious area must be treated by LID
techniques and 25% of new paved areas must be pervious.
‐ 75% of all newly added or modified impervious area must be treated by LID
techniques.
Four Step Process
The City of Fort Collins has adopted the “Four Step Process” that is recommended in Volume 3
of the USDCM for selecting BMP’s in urban redevelopment areas. The goal of this process is to
minimize adverse impacts on receiving waters. The following describe how each step of the
process are incorporated in the proposed development:
Step 1 – Employ Runoff Reduction Practices
By selecting a site that is undeveloped that currently does not have water quality treatment,
the impacts of the proposed development will be less due to the addition of the water quality
capture volume included. The project proposes to install a rain garden with underdrains to aid
in conveyance and infiltration opportunities which will reduce the effects of the added
impervious area from the development of the site.
Step 2 – Water Quality Capture Volume (WQCV)
In order to reduce the sediment load and other pollutants leaving the site, it is important to
provide WQCV to minimize the downstream impacts. This project proposes to provide rain
garden prior to the stormwater entering the detention pond, both located on the site. These
facilities are sized to release at or below the historic runoff rates.
Step 3 – Stabilize Drainageways
Since this site is in close proximity to Poudre River it is important to take this Step into
consideration. By providing stormwater detention and water quality and lowering the historic
runoff and providing a metered outflow the project impacts to the drainageway are essentially
reduced. The majority of the drainageway leaving the site is already stabilized.
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Step 4 – Implement Site Specific and Other Source Control BMP’s
The proposed project will contain the following BMP’s:
Rain Garden: All runoff from the site will be directed into and through the proposed
BMP facilities prior to leaving the site. Rain gardens utilize native plants that act as
sponges to reduce the amount of stormwater runoff and absorb contaminants.
Maintenance: In order for BMP’s to remain effective proper maintenance needs are
required. Scheduled visual inspections and routine repair after larger storm events are
critical in the maintenance and effectiveness of the installed BMP’s. The sites owner will
be advised of these practices during installation in order to maximize the efficiency of
the installed BMP’s.
Section 4: Hydraulic Calculations
Criteria
Runoff criteria for the proposed sites was analyzed for the 2‐YR, 10‐YR, and 100‐YR storm
events. The Rational Method is being used for the design of the proposed improvements.
Rainfall Intensities used in the proposed design were determined as a function of the Time of
Concertation for each subbasin. The values used were determined by the City of Fort Collins
Rainfall Intensity/Duration/Frequency curve (Figure RA‐16 and Table RA‐7 (see Appendix D).
Imperviousness
Values from Table RO‐11 (Fort Collins Amendment to the USDCM) were used in determining
the sites imperviousness. The overall proposed site improvements and landscaping equate to a
total onsite proposed Imperviousness of 46%, while the total basin reaching the pond is 53%.
Time of Concentration & Runoff Coefficient
The onsite area was separated into three (3) subbasins with slopes varying between 0.5% to
1.5%. The calculated Time of Concentration (TC) for each subbasin are listed in Appendix E. See
Basin Map in Appendix F for more information.
WQCV
The UD‐BMP_v3.03 (UDFCD) spreadsheet was used to calculated the required Rain Garden
water quality capture volume. See Appendix E for design information.
The existing detention pond design also included a volume allotted for the WQCV. Per the
previous report by Stantec, a volume of 0.033 ac‐ft is required for WQCV. This report calculates
a required WQCV for a 40 hour release to be 0.046 ac‐ft. This volume is accounted for within
the regraded pond and the volume calculations. The previous pond design did not allow for an
extended WQ drain time, so therefore a new orifice plate is being designed and included with
this project. The revised pond will outlet the WQCV over a 40‐hour period as allowed by the
Manual.
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Detention
The existing detention pond is already constructed and was designed by Stantec with an
allowable release rate of 1.1 cfs. The design was also discussed in the “Interim Timberline Road
Widening – Drake to Prospect” and “Spring Creek Farms North Filing No. 2”. See existing report
information in Appendix G.
The detention facility Drain Time Spreadsheet and Stage Storage relationship spreadsheet have
been calculated and is included within Appendix E of this report. The calculated drain time for
99% of the inflow volume of the pond with the proposed project and off‐site area included is
approximately 23 hours for the 100‐year event, which is below the maximum allowed drain
time.
Section 5: Facility Design
The proposed improvements will increase the amount of impervious area; however, measures
have been included in the design to minimize the amount of stormwater released from the site.
Per previous design reports, specifically the Spring Creek Farms North, the outlet structure is
designed to release at 1.1 cfs. The existing water quality plate in the existing pond will need to
be replaced to meet current requirements and drain times. The plate will be designed with a
single 1‐3/16” diameter orifice to release the WQCV at 0.045 cfs, and a single 2‐5/16” diameter
orifice above the WQCV to release the 100‐year event at 1.1 cfs. Per our report, the
predeveloped 2‐year event release rate is 2.44 cfs per for the entire 3.32‐acre site (see
Appendix E for historic calcs). The runoff will sheet flow until a concentrated point and then be
conveyed via trickle channel or gutters and then deposited into the water quality device and/or
detention pond.
The site was broken into three developed subbasins for water quality purposes; Subbasin 1.0
(0.856 acres), Subbasin 2.0 (1.859 acres), and Subbasin Pond (0.608 acres). OS‐7 and PS‐1 (as
described below) was also modeled and included in the detention calculations. See Appendix E
for breakdown of the different area types for each subbasin and Appendix F for the Subbasin
Map.
Subbasin 1.0 contains half of the proposed building and the parking and paved areas south of
the building. The runoff will be conveyed to the east side of the building and outlet into a
proposed rain garden. The rain garden will outlet to the modified detention pond.
Subbasin 2.0 contains half of the building, the parking and paved areas to the west of the
building, and the playground area. All areas drain to the north and east. Water Quality for this
subbasin will be provided via a rain garden. The rain garden will outlet to the modified
detention pond.
Pond Basin is that area within the existing Drainage Easement on the east side of the site. The
existing detention pond was already sized for a release of 1.1 cfs, however current regulations
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require slight modifications to the steel orifice plate to meet drain times. Currently it has one
3.5” diameter orifice plate that will be replaced with a new plate containing a single 1‐3/16”
diameter orifice to release the WQCV at 0.045 cfs, and a single 2‐5/16” diameter orifice above
the WQCV to release the 100‐year event at 1.1 cfs.. The existing pond was modeled with the
proposed project and the 100‐year WSEL’s is indicated on the Drainage Plan in Appendix F.
OS‐7 – This off‐site basin is the area of Nancy Gray and a portion of Timberline that drain to the
existing detention pond. Full design calculations are included in order to determine the
required detention volume is being provided within the existing pond.
PS‐1 – This off‐site basin is the northeast portion of the Police Services site that drains to the
existing type R in Nancy Gray that is conveyed to the existing detention pond on‐site.
Since there is only one detention pond modeled with this report we have provided a summary
of the proposed design below. See supporting calculations within the Appendix of this report.
Area
(ac)
Sub 1.0 0.86 1.33 2.32 6.21 0.013 ‐ ‐ ‐ ‐ ‐
Sub 2.0 1.86 2.19 3.73 10.20 0.028 ‐ ‐ ‐ ‐ ‐
Pond 0.61 0.17 0.29 0.74 ‐ ‐ ‐ ‐ ‐
OS‐7 1.24 1.81 3.85 11.07 ‐ ‐ ‐ ‐ ‐ ‐
PS‐1 0.88 1.19 2.05 5.79 ‐ ‐ ‐ ‐ ‐ ‐
Total Site 3.32 3.28 5.63 14.48 ‐ ‐ ‐ ‐ ‐ ‐
Total Untreated 2.73 3.15 5.37 14.58 0.046 0.046 ‐ ‐ ‐ ‐
Total Basin 5.44 5.77 10.02 25.62 ‐ ‐ 1.168 1.031 1.213 1.1
Release
Rate (cfs)
2‐Year
Flow (cfs)
100‐Year
Flow (cfs)
WQCV
(ac‐ft)
100‐Year
Vol. (ac‐ft)
Detention
Req'd (ac‐ft)
Tot. Volume
Provided (ac‐ft)
10‐Year
Flow (cfs)
WQCV
Req'd
Section 6: Conclusions
All computations that have been reviewed or completed within this report are in compliance
with the City of Fort Collins Storm Drainage Design Criteria and Construction Standards manual
and the Urban Drainage and Flood Control District Urban Storm Drainage Criteria Manual
Volumes 1 &2.
The proposed drainage concepts presented in this report and on the construction plans
adequately provide for stormwater detention and water quality treatment of impervious areas
by releasing the 100‐Year storm event at the required allowable release rate of 1.1 cfs, which is
less than the Predeveloped 2‐Year runoff rates. The Water Quality design will benefit the site
by reducing the initial discharge from the site and removal of any sediments or other pollutants
generated by the developed site. As well, with the modified steel outlet plate, water quality is
now being provided for the offsite basins that drain to the existing pond.
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Section 7: References
The Natural Resource Conservation Service Soil Survey for Larimer County and the WSS
website.
FEMA FIRM map‐panel 08069C1000F Effective 12/18/2006. – Not Printed.
“City of Fort Collins Stormwater Criteria Manual” dated December 2011
Urban Drainage and Flood Control District Urban Storm Drainage Criteria Manual
(UDFCD), Volumes 1, 2, and 3, dated 2008 with revisions (the Manual).
Drainage Report: Police Services Building
Drainage Report – Spring Creek Farms North
Drainage Report – Timberline Road Widening
Drainage Report – Sidehill Filing 2 – Stormwater Pipe Reduction
w Appendix A
Site Vicinity Map
Appendix B
Flood Insurance Rate Map
(Not available to Print)
Appendix C
NRCS Soils Map
Appendix D
Hydrologic Criteria
Appendix E
Hydraulic Calculations
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Appendix F
Maps
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Appendix G
References