HomeMy WebLinkAboutENCLAVE AT REDWOOD - PDP210004 - DOCUMENT MARKUPS - ROUND 3 - DRAINAGE REPORT
PRELIMINARY DRAINAGE REPORT
FOR
ENCLAVE AT REDWOOD
Fort Collins, CO 80524
Date: August 18, 2021
Revised: N/A
Prepared for:
9555 South Kingston Court
Denver, CO 80112
Phone: 303-488-0061
Prepared by:
Mark A. West, P.E., C.F.M.
1120 Lincoln Street, Suite 1000
Denver, CO 80203
Ph.: 303-623-6300, Fax: 303-623-6311
Harris Kocher Smith Project No. 201013
Enclave at Redwood
Table of Contents
August 18, 2021
TABLE OF CONTENTS
I. GENERAL LOCATION AND DESCRIPTION ................................................................................................... 4
A. Site Location ....................................................................................................................................... 4
B. Description of Property ...................................................................................................................... 4
II. MASTER DRAINAGE BASIN DESCRIPTION .................................................................................................. 5
A. Floodplain........................................................................................................................................... 5
B. Master Drainage Basin ....................................................................................................................... 5
III. DRAINAGE DESIGN CRITERIA ................................................................................................................. 6
A. Regulations and Previous Studies ...................................................................................................... 6
B. Four-Step Process .............................................................................................................................. 6
C. Hydrologic Design Criteria ................................................................................................................. 7
D. Hydraulic Design Criteria .................................................................................................................... 8
E. Variances ............................................................................................................................................ 9
F. Erosion Control .................................................................................................................................. 9
IV. PROPOSED DRAINAGE SYSTEMS ............................................................................................................ 9
A. General Concept ................................................................................................................................ 9
B. Proposed Basins ............................................................................................................................... 10
C. Specific Details ................................................................................................................................. 11
V. CONCLUSIONS ......................................................................................................................................... 16
A. Compliance with Standards ............................................................................................................. 16
B. Drainage Concept ............................................................................................................................. 16
VI. REFERENCES ......................................................................................................................................... 17
VII. APPENDICES ......................................................................................................................................... 18
Appendix A – Vicinity Map, FIRM, Geotechnical Engineering Report ....................................................... A
Appendix B - Portions from Relevant Previous Studies ............................................................................ B
Appendix C – Subcatchment Hydrology .................................................................................................... C
Appendix D – Detention Storage Sizing ..................................................................................................... D
Appendix E – SWMM Model and Results .................................................................................................. E
Appendix F – Low Impact Development ................................................................................................... F
Appendix G – Drainage Map ..................................................................................................................... G
Enclave at Redwood
Engineer Certification
August 18, 2021
Engineer Certification
I hereby attest that this report for the preliminary drainage design for the Enclave at Redwood was
prepared by me or under my direct supervision, in accordance with the provisions of the Fort
Collins Stormwater Criteria Manual. I understand that the City of Fort Collins does not and will not
assume liability for drainage facilities designed by others.
__________________________________ _______________________
Mark A. West, P.E., C.F.M. Date
State Of Colorado Registration No. 38561
On Behalf of Harris Kocher Smith
Enclave at Redwood
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August 18, 2021
I. GENERAL LOCATION AND DESCRIPTION
A. Site Location
The proposed Enclave at Redwood development (hereinafter referred to as “Site”) is located in the
southeast ¼ of Section 1, Township 7 north, Range 69 west of the sixth Principal Meridian, City of Fort
Collins, County of Larimer, State of Colorado. The Site is bounded by Redwood Street to the west and
East Suniga Road to the south. Adjacent developments include the Meadows at Redwood PUD Phase 1
(Redwood Meadows) to the west, Northfield to the east and southeast, and Evergreen Park 3rd
(Evergreen) to the north. Lake Canal exists as an irrigation ditch adjacent to the Site to the east and
southeast, and an unnamed drainage ditch lies adjacent to the Site to the north. The Site will connect to
Redwood St to the southwest, Lupine Dr in Redwood Meadows to the west, Steeley Dr in Northfield to
the southeast, and Collamer Dr in Northfield to the east. See Appendix A for the Vicinity Map. The Site
exists entirely within the Dry Creek master drainage basin.
B. Description of Property
The Site is approximately 27.47 acres of undeveloped land. The undeveloped land is made up of various
native trees and grasses. The Site generally slopes from west to east at grades between roughly 0.3%
and 1% towards Lake Canal. Lake Canal is an irrigation ditch and discharging stormwater from the
developed Site to this ditch will require approval from the ditch ownership. Given that the Site
historically flows to this ditch, it is currently assumed that as long as developed flow entering the ditch is
kept to a minimum, as much as is practical, this approval will not be out of reach. Redwood Pond is an
existing regional pond to the northwest of the Site, it historically outfalls east into the unnamed
drainage ditch that exists north of the Site. The proposed development of the Site will feature mutli-
family housing, public and private roadways, walks, parks, a clubhouse with pool, landscaped areas,
three on-Site detention ponds, low-impact-development systems, and the regrading of the Redwood
Pond.
Cole Garner Geotechnical has prepared a Geotechnical Engineering Report for Enclave at Redwood
Residential Development on August 11, 2021. This report details existing soils information for the Site.
Ten borings were drilled on-Site to examine subsurface conditions. Materials encountered in these
borings generally included fine to coarse sand with silt and gravel overlying claystone bedrock.
Sedimentary claystone bedrock was encountered in nine of ten borings at depth between 17 and 23 feet
below Site grades and extending to the bottom of the borings. Groundwater was encountered in all
borings at depths ranging from 8 to 17 feet below existing Site grades. Due to shallow groundwater
concerns, construction dewatering may be required, and underdrains may be requested by the City of
Fort Collins. Please see Appendix A for a copy of the Geotechnical Engineering Report.
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II. MASTER DRAINAGE BASIN DESCRIPTION
A. Floodplain
The Site is shown on the Federal Emergency Agency’s (FEMA) Flood Insurance Rate Map (FIRM)
Community Panel Numbers 08069C0977G and 08069C0981G, effective June 17, 2008. Lake Canal is
partially located within Zone X (0.2% annual chance flood hazard) and therefore the Site is partially
within this zone. Because the development of the Site will not involve altering of Lake Canal, no
floodplain modifications are necessary. Please refer to Appendix A for more details.
B. Master Drainage Basin
The Site exists in the Dry Creek Master Drainage Basin, but according to the City of Fort Collins, the Dry
Creek Master Drainage Plan is outdated and not specific to the Site. The North East College Corridor
Outfall Design Report prepared by Ayres Associates in August 2009 (NECCO) will be utilized to impose
drainage conditions on the Site instead of the Dry Creek Master Drainage Plan. Please see Appendix B
for relevant portions from the NECCO report. This report details the design and construction of a
regional pond that conveys a large amount of runoff from the Dry Creek basin in a new storm main
beneath E Suniga St running from west to east. This report also contains design plans to revise the
existing Redwood Pond’s grading and change its outfall location from the unnamed drainage ditch north
of the Site to the NECCO storm main beneath E Suniga St south of the Site. The regrading of the
Redwood pond never took place and therefore the outlet south to the NECCO storm main never was
constructed either. This storm main south of the Site includes a stub to the Site that will act as the
ultimate outfall of the developed Site.
The basins from the NECCO report that were analyzed for this report include basins 113, 213, 313, 413,
and 812. The Site exists in basins 113 and 313, which the NECCO report calls for future on-Site detention
of these basins with a 100-year release rate of 0.2 cubic-feet-per-second (CFS) per acres. Basin 213
consists of the Redwood Meadows development and calls for future water quality but no detention.
Basin 812 consists of the Redwood Pond, which the report states already provides water quality for all
basins historically tributary to the pond. Basin 413 exists just south of the Site and E Suniga Rd and was
only included in the analysis of the Site drainage because runoff from this basin enters the NECCO storm
main at the same point as the stub connecting to our Site. This basin will not feature detention or water
quality treatment. From the NECCO report’s SWMM model, the combined release rate of the Redwood
Pond (15.96 CFS), basin 213 undetained (72.65 CFS), and the future detained release from basins 113
and 313 (6.92 CFS) is 95.53 CFS conveyed to the stub provided form the NECCO mainline to the Site.
Based on this maximum release rate it is determined that as long as the 100-year release rates from the
proposed on-Site ponds and the Redwood pond meet criteria of 1.0 CFS/ac and do not exceed 95.53 CFS
when combined with flow from basin 213, the constraint of releasing only 0.2 CFS/ac will no longer
apply. This is an assumption for this preliminary drainage design of the Site and may be altered with
future revisions to this drainage report.
Also provided for the development of drainage for the Site was an as-built conditions SWMM model
featuring the constructed NECCO storm main under E Suniga St and the regional pond designed in the
NECCO report. This master model showed that basins 113, 313, 213, 413, 812, and a few others not
mentioned were captured and detained by the Redwood Pond. All of these basins besides 812 grades
away from the Redwood Pond. Another issue was that the NECCO storm main stub to the Site had the
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wrong invert elevation based on land survey conducted by Harris Kocher Smith for the purposes of Site
development. These two issues lead to the development of the Site specific SWMM model that’s output
hydrograph can be input into the master SWMM model provided. Information taken from the master
SWMM model include a tailwater time series at the connection of the stub and the NECCO main, and a
100-year inflow time series of all the existing areas tributary to the Redwood pond upstream. Please see
Appendix E for input time series of the proposed SWMM model. The proposed SWMM output will be
inserted into the provided master SWMM plan for a future revision to this drainage report, once the
proposed SWMM is more fleshed out.
III. DRAINAGE DESIGN CRITERIA
A. Regulations and Previous Studies
The principal design criteria used for this report was the Fort Collins Stormwater Criteria Manual, revised
December 2018 (FCSCM). In addition, Mile High Flood District’s (MHFD) Urban Storm Drainage Criteria
Manual, Volumes 1-3 revised August 2018, September 2017, and January 2021 respectively, was used
(USDCM). The NECCO report from above also provides a certain level of drainage criteria for the Site
that must be met.
B. Four-Step Process
a. Step 1: Employ Runoff Reduction Practices
The Site will feature numerous open areas dedicated for low-impact-development (LID), parks, and
above ground detention. Wherever possible, vegetated swales were used to convey runoff from
impervious areas to LID implements and roof drains were used to convey flow to these swales or directly
to LID implements.
b. Step 2: Implement BMPs That Provide a WQCV with Slow Release
There are ten different LID systems incorporated on-Site: four sand filters, five rain gardens, and one set
of isolator rows for underground infiltration. These ten implements are utilized to treat 75% of newly
added impervious area with the development of the Site per the FCSCM. The remainder of Site Water
Quality Capture Volume (WQCV) will be treated in the three on-Site ponds and revised Redwood Pond.
c. Step 3: Stabilize Streams
The Site will be far upstream of Dry Creek and the NECCO report has already considered developed
release rates from the Site entering Dry Creek. There will be no need to provide stream stabilization of
Dry Creek with the development of the Site.
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d. Step 4: Implement Site Specific and Other Source Control BMPs
Because the Site is not a new development of redevelopment of an industrial or commercial zone this
step is not applicable to the Site.
C. Hydrologic Design Criteria
Design Frequencies – For the Site, the minor storm will be the 2-year and the major storm will be the
100-year. For detention volume and release rates, the WQCV and 100-year storm events have been
analyzed.
Rainfall Data - Fort Collins varies by runoff calculation method. For the purpose of this preliminary
drainage report, only SWMM was used in calculating runoff. Below is Table 4.1-4 which presents rainfall
intensity values for use in SWMM modelling.
Hydrologic Computation Method – As stated above, only EPA SWMM 5.1 was used to calculate runoff
for this preliminary drainage design. As of now the basins tributary to each of the ponds have not been
broken into smaller sub-basins for the purpose of inlet calculations. If needed in future iterations of this
report, the rational method may be utilized. SWMM uses basin imperviousness, slope, area, and width
along with several default parameters to calculate runoff. SWMM inputs can be found in Appendix E as
well as Section IV of this report. The rational method uses weighted “C” coefficients based upon surface
type and a time of concentration to determine runoff. Fort Collins rational method IDF table and “C”
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coefficient table can be found in Appendix C along with SWMM subcatchment default parameters and
imperviousness by surface type tables. A summary of SWMM results for each subcatchment can be
found in Appendix C as well. The rational method “C” coefficient and water quality event rainfall
intensities were utilized in the Modified FAA Procedure to calculate the WQCV for the isolator rows. It
should be noted that imperviousness values used were rounded up from exact calculated values for this
preliminary drainage design.
Detention Volume Computation Method – For project sizes greater than 20 acres, SWMM is required for
detention calculation. SWMM storage nodes are used to represent the ponds and require storage curves
(depth-area), SWMM then computes a volume based on these tables. SWMM outlet links are used to
represent a ponds outlet structure and require rating curves (depth-release). These rating and storage
curves are based on pond design and stage-storage-discharge tables that can be found in Appendix D.
These tables feature orifice and weir sizing and elevation information that computes the release rates
for each stage. SWMM 5.1 accounts for WQCV depth in storage nodes before the design storm, so
SWMM calculated volume includes WQCV. See Appendix E for output results of storage nodes.
Water Quality Control Volume Calculation – For the sand filters and rain gardens, the MHFD spreadsheet
“UD-BMP v3.07” released April 2018 was used to calculate WQCV. This spreadsheet calculates the
WQCV based on a 12-hour drain time. Fort Collins requires 120% of the WQCV to be treated by LID, so
the calculated values from UD-BMP were overridden to meet those requirements. Stage-Storage tables
were provided for nearly all of the sand filters and rain gardens to confirm the volume provided from
the UD-BMP spreadsheet. See Appendix F for the UD-BMP sheets and stage-storage tables for every
sand filter and rain garden proposed. For the isolator rows underground infiltration, a spreadsheet
received from Advanced Drainage Systems (ADS) was used to determine the number of chambers
needed based on product choice, chamber volume, and required WQCV. For the sizing of the system,
the WQCV calculated from UD-BMP’s extended detention basin tab was compared to the WQCV
calculated from the modified FAA procedure and the volume that required the greater number of
chambers was used. Please see Appendix F for the calculation of WQCV and chambers required for the
isolator rows.
D. Hydraulic Design Criteria
Detention Pond Infrastructure Sizing - Express Hydraflow will be used to calculate vegetated swale and
emergency overflow spillway capacities for the Site. UD-BMP will be used to size trickle channels,
forebays, and forebay notches for the ponds. As stated above, pond rating curves will be developed
using stage-storage-discharge tables but the MHFD spreadsheet “SDI_Design_Data_v1.08.xlsm” released
January 2017 will be used to confirm that pond release rates meet Colorado drain time criteria.
Spillways have been sized and included in Appendix D of this report as they pertain to SWMM modelling.
All other items mentioned above will be included in a later version of this report.
Street Capacity Calculation – Street sections upstream of proposed inlets will be analyzed using the
MHFD spreadsheet “MHFD-Inlet v5.01” released April 2021. These calculations will be presented in a
later version of this report.
Storm Sewer Sizing – Inlet sizing will be performed using the MHFD-Inlet spreadsheet. For the
preliminary layout of the storm network inlet sizes were estimated based on contributing area. Inlet
Type 16 curb and valley inlets, Type R curb inlets, and Type C and D areas inlets were utilized for this
Site. Storm pipe sizing and vertical alignment will be analyzed using Bentley StormCAD CONNECT.
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StormCAD computes losses and velocities and produces accurate HGLs and EGLs to prove the storm
sewer network works hydraulically. Inlet sizing calculations and StormCAD pipe sizing calculations will be
provided in a later version of this report.
E. Variances
The drainage design for the Site will require a variance from release rate restrictions imposed in the
NECCO report. This report dictates that on-Site detention ponds must feature a 100-year release rate of
0.2 CFS/ac. In order to reduce size and footprint of above ground detention ponds on-Site, 100-year
release rates equaling 1.0 CFS/ac has been utilized given that the combination of all flows leaving the
Site via the NECCO line be less than 95.53 CFS as detailed above. Note this variance is assumed to be
feasible for this preliminary drainage design and this assumption could change with later revisions to
this report.
F. Erosion Control
The following drainage design will comply with all erosion control criteria put forth in the USDCM,
FCSCM, and NECCO Report. All erosion control materials will be provided with the final drainage report.
IV. PROPOSED DRAINAGE SYSTEMS
A. General Concept
The Site includes revisions to the Redwood Pond to the northwest including regrading and design of a
new outlet, trickle channel, forebay, and spillway. According to National Wetlands Inventory (NWI) data,
there is a wetlands area existing in the current Redwood Pond. The final drainage report for this Site will
further address these wetlands. There are three proposed on-Site detention ponds featuring some level
of water quality treatment. These proposed ponds will treat flows up to the 100-year storm event and
will feature at least 1.0 foot of freeboard above the overflow spillway design depth. All four of these
ponds will release to a proposed storm line that travels from north to south and connects to the stub
provided per the NECCO report. Basin 213 containing the Redwood Meadows neighborhood needs to be
conveyed to this proposed line and will be captured by proposed inlets on-Site and routed to this line
undetained. Further breakdown of the ponds and a summary table can be found below.
The Site will feature ten LID systems: four sand filters, five rain gardens, and underground isolator rows.
These LID features are designed to treat at least 75% of the Site’s impervious areas for the WQCV.
Runoff will be routed to these systems via roof drains, sheet flow, storm sewer pipes, vegetated swales,
and landscaped drains. Flows above the WQCV are designed to overflow via spillway to the nearest
drainage system that can capture them. The sand filters and rain gardens will feature slotted
underdrains that capture treated flows and send them to the nearest storm sewer system or directly to
a pond. Further breakdown of the LID areas and a summary table can be found below.
For the SWMM analysis included in this preliminary drainage design report only seven subcatchments
were used, and the only storm line included was the NECCO stub extension line that receives the outfall
from all four ponds and runoff from basins 213 and 413. This is a high-level analysis of the Site’s
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drainage and the SWMM subcatchments will be broken into sub-basins for the final drainage report.
Below is a breakdown of the seven subcatchments as well as a table featuring their SWMM input
parameters. Also found below is a table summarizing SWMM model results.
See Appendix C for summary of subcatchments and SWMM input parameters. See Appendix D for stage-
storage-discharge tables for ponds and spillway calculations. See Appendix E for SWMM schematic,
input report and output results. See Appendix F for LID calculations, summary table, and exhibit showing
areas and locations of facilities. See Appendix G for drainage map showing pond locations, basins,
drainage patterns, and storm layout.
B. Proposed Basins
Below is Table 1 featuring a summary of all basin input parameters for SWMM and other calculated
parameters.
Table 1:
Pond Subcatchment Summary Table
Basin Area
(ac)
Imperviousness
(%)
Width
(ft)
Slope
(%)
2-Year
Peak
Runoff
(CFS)
100-
Year
Peak
Runoff
(CFS)
113 9.80 75 276 0.60 7.42 47.74
213 7.82 80 336 1.00 8.86 52.18
313-1 4.10 60 193 0.75 3.98 22.49
313-2 8.00 50 272 0.50 5.29 32.54
313-3 5.57 55 220 0.80 4.81 27.59
413 2.19 5 162 1.00 0.32 3.57
812 3.75 2 265 0.50 0.22 4.31
Basin 113 lies on-Site and encompasses the northern portion of the Site. This basin is consisting of multi-
family housing, walks, roadways, alleys, open space, a rain garden and a sand filter. Runoff from this
basin will be conveyed to the Redwood Pond in the northwest. This drainage pattern is opposite to the
historic drainage pattern, but Site flows cannot be freely released into Lake Canal and so this historic
drainage pattern no longer applies. It is assumed some portions of this basin will contribute runoff north
off-Site undetained into the unnamed drainage ditch, but all of this basin is tributary to the pond for the
purpose of this preliminary drainage analysis.
Basin 213 lies off-Site to the west of the Site. This basin consists of the Redwood Meadows development
featuring walks, single-family homes, roadways, and open spaces. Runoff from this basin will not need
detained but will need to be conveyed to the NECCO stub for the Site. Two inlets are proposed in at the
connection of Lupine Dr and the Site to collect all runoff from this basin and route it directly to the
NECCO storm line. This basins imperviousness was assumed to be 80% for conservative analysis but may
decrease to a more exact value for the Final Drainage Report.
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Basin 313-1 lies on-Site and encompasses the southwestern portion of the Site. This basin consists of
multi-family housing, walks, roadways, alleys, open space, two rain gardens, a sand filter, and South
Pond 1. Runoff from this basin will be conveyed to South Pond 1 to the south. It is assumed that some
potions of this basin will flow off-Site undetained into basin 213 or Redwood St, but all of this basin is
tributary to the pond for the purpose of this preliminary drainage analysis.
Basin 313-2 lies on-Site and encompasses the eastern portion and border of the Site. This basin consists
of multi-family housing, walks, roadways, alleys, open space, a sand filter, isolator row chambers,
portions of Lake Canal irrigation ditch and South Pond 2. Runoff from this basin will be conveyed to
South Pond 2 to the southwest. It is assumed that some potions of this basin will flow off-Site
undetained into Lake Canal or the Northfield development, but all of this basin is tributary to the pond
for the purpose of this preliminary drainage analysis.
Basin 313-3 lies on-Site and encompasses the middle portion of the Site. This basin consists of multi-
family housing, walks, roadways, alleys, open space, a sand filter, two rain gardens, and South Pond 3.
Runoff from this basin will be conveyed to South Pond 3 in the middle of the basin. The area South
Pond 3 take up will also be utilized as an open space park.
Basin 413 lies off-Site south of E Suniga St. This basin will remain undeveloped and consists entirely of
open space. Runoff from this basin will sheet flow to a ditch where it is collected by a stub off of the
NECCO main line under E Suniga St and conveyed to the same point as the stub that extends north to
the Site.
Basin 812 lies off-Site to the northwest of the Site. This basin consists of the existing Redwood Pond to
be redesigned. Runoff from this basin will be collected by the Redwood Pond.
C. Specific Details
The Redwood Pond’s redesign is driven by pond bottom elevation requirements and not required
volume. In order to properly route flow from the Site to this pond the bottom was deepened by 3.5 feet,
while tying into existing contours to ensure the berm elevation stayed the same and trees adjacent to
the pond could remain intact. Basin 812 and 113 are tributary to this pond as well as multiple areas off-
Site upstream. The pond was analyzed in SWMM and was found to have a 100-year volume of 6.484
acre-feet (ac-ft) with a water surface elevation (WSEL) of 4957.94 ft. The 100-year release rate for this
pond is 14.76 CFS, which is less than the 15.96 CFS release rate calculated in the NECCO report. The
spillway crest elevation is set above the 100-year WSEL at 4958.00 ft, with a design depth of 1.56 ft and
a width of 10 ft. The berm of this pond is at 4962.00, giving a freeboard of 2.44 ft. Any overflow from
this pond will pass over the spillway and into the unnamed drainage ditch to the northeast of the pond.
This ditch is tributary to Dry Creek and is the location of the historic outfall of the pond. This pond
outfalls to the NECCO storm main stub extension before routing into the NECCO storm main.
South Pond 1 is located in the southwestern portion of the Site and features a depth of 7.67 ft and a
pond bottom elevation of 4952.71 ft. Basin 313-1 (4.10 ac) is tributary to this pond. The pond was
analyzed in SWMM and was found to have a 100-year volume of 0.596 ac-ft with a WSEL of 4958.91 ft.
The 100-year release rate for this pond is 3.50 CFS, which is less than the 4.10 CFS release rate per the
1.0 CFS/ac criteria. The spillway crest elevation is set above the 100-year WSEL at 4959.00 ft, with a
design depth of 0.38 ft and a width of 30 ft. The berm of this pond is at 4960.38, giving a freeboard of
1.0 ft. Any overflow from this pond will pass over the spillway and into E Suniga St where it will be
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collected by inlets and routed into the NECCO storm main under E Suniga St. This pond outfalls to the
NECCO storm main stub extension before routing into the NECCO storm main.
South Pond 2 is located in the southwestern portion of the Site and features a depth of 11.0 ft and a
pond bottom elevation of 4948.00 ft. Basin 313-2 (8.00 ac) is tributary to this pond. The pond was
analyzed in SWMM and was found to have a 100-year volume of 0.872 ac-ft with a WSEL of 4953.73 ft.
The 100-year release rate for this pond is 6.71 CFS, which is less than the 8.00 CFS release rate per the
1.0 CFS/ac criteria. The spillway crest elevation is set above the 100-year WSEL at 4954.00 ft, with a
design depth of 1.14 ft and a width of 5 ft. The berm of this pond is at 4959.00, giving a freeboard of
3.86 ft. Any overflow from this pond will pass over the spillway and into E Suniga St where it will be
collected by inlets and routed into the NECCO storm main under E Suniga St. This pond outfalls to the
NECCO storm main stub extension before routing into the NECCO storm main.
South Pond 3 is located in the middle portion of the Site and features a depth of 6.35 ft and a pond
bottom elevation of 4954.00 ft. Basin 313-2 (5.57 ac) is tributary to this pond. The pond was analyzed in
SWMM and was found to have a 100-year volume of 0.750 ac-ft with a WSEL of 4957.75 ft. The 100-year
release rate for this pond is 4.27 CFS, which is less than the 5.57 CFS release rate per the 1.0 CFS/ac
criteria. The spillway crest elevation is set above the 100-year WSEL at 4958.00 ft, with a design depth of
0.80 ft and a width of 10 ft. The berm of this pond is at 4960.35 ft, giving a freeboard of 1.55 ft. Any
overflow from this pond will pass over the spillway and into the alley to the south of the pond where it
will be collected by an inlet in sump that conveys water to South Pond 2. This pond outfalls to the
NECCO storm main stub extension before routing into the NECCO storm main.
See below for Table 2 showing a summary of pond results from SWMM. The WQCV presented in this
table does not account for the WQCV already treated in LID systems because the LID systems are not
present in the SWMM model, therefore SWMM accounts for the WQCV depth as if there were no LID
systems on-Site.
Table 2:
Detention Pond Summary Table
Pond
100-
Year
Volume
(cu-ft)
100-
Year
Volume
(ac-ft)
100-Year
WSEL
WQCV
(cu-ft)
WQCV
WSEL
Max
Release
Rate (CFS)
Redwood 282,444 6.484 4957.94 *10,672.2 4954.78 14.76
South 1 25,962 0.596 4958.91 3,528.36 4955.30 3.50
South 2 38,001 0.872 4953.73 6,011.28 4950.08 6.71
South 3 32,651 0.750 4957.75 4,443.12 4955.77 4.27
*WQCV for Redwood Pond from On-Site Contributing Area Only
The total release of all the ponds combined with the release from basins 213 and 413 sum up to 84.99
CFS, which is less than the maximum release rate at the NECCO stub of 95.53 CFS.
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Sand Filter 1 is located in the southwest portion of the Site. This sand filter has 4:1 side slopes, a WQCV
depth of 1.5 ft and a total depth of 2.0 ft. Runoff will enter this sand filter via storm sewer pipe,
landscape drains, and roof drains. Wherever concentrated runoff enters this basin will be a concrete
impact basin to protect against scour. The slotted underdrain for this basin will connect to a manhole
just south of the basin, and flow above the WQCV will overflow directly to South Pond 1 adjacent to the
sand filter to the east.
Rain Garden 1 is located in the southwest portion of the Site, just south of the Redwood Meadows
development. This rain garden has vertical walls on all sides and a 2’ buffer from the closest sidewalks.
This rain garden has dimensions of 10’ by 50’ with a WQCV depth of 12”. Runoff will enter this basin via
chase drain at a concrete impact basin. The slotted underdrain will connect to the NECCO stub extension
line east of the rain garden. Flow above the WQCV will overflow south into the roadway to be collected
by the nearest inlet downstream.
Rain Garden 2 is located in the southwest portion of the Site, sandwiched between a roadway to the
north and two buildings to the south. This rain garden has 4:1 side slopes, a WQCV depth of 6” and a
total depth of 1.5’. Runoff will enter this rain garden via chase drain or vegetated swale. Runoff will
enter the vegetated swale via sheet flow and roof drains. The basin will feature an impact basin for the
chase drain and riprap for the swale to protect against scour. The slotted underdrain will connect to the
NECCO stub extension line north of the rain garden. Flow above the WQCV will overflow north into the
roadway to be collected by the nearest inlet downstream.
Sand Filter 2 is located in the southwest portion of the Site. This sand filter has 4:1 side slopes, a WQCV
depth of 1.0 ft and a total depth of 2.0 ft. Runoff will enter this sand filter via sheet flow or a vegetated
swale. Runoff will enter the vegetated swale via chase drain or roof drains, where the swale enters the
sand filter will be riprap to protect against scour. The slotted underdrain for this basin will connect to an
inlet just south of the basin, and flow above the WQCV will overflow north into a different vegetated
swale that conveys flow to the nearest roadway where it is collected by the inlet the underdrain
connects into.
The isolator rows are proposed underneath sidewalk in the northeast portion of the Site. The sizing
process is detailed above in Section III. The result is 25 chambers of ADS SC-310 StormTech chambers
that provide a total system volume of 775 cubic-feet and a combined release rate of 0.39 CFS. This
product was chosen because of its low height being useful to connect to the shallow storm in this area
of the Site. The outfall from these chambers will connect to a storm main that runs along Lake Canal and
into South Pond 2 to the southwest.
Sand Filter 3 is located in the middle portion of the Site. This sand filter has 4:1 side slopes, a WQCV
depth of 1.4 ft and a total depth of 1.9 ft. Runoff will enter this sand filter via sheet flow from the alley
adjacent or roof drains. Wherever concentrated runoff enters this basin will be a concrete impact basin
to protect against scour, and wherever sheet flow enters this basin will be riprap to protect against
scour. The slotted underdrain for this basin will connect to South Pond 3 just east of the basin, and flow
above the WQCV will overflow directly to South Pond 3.
Rain Garden 3 is located in the middle portion of the Site. This rain garden has 8:1 side slopes, a WQCV
depth of 12” and a total depth of 2.0 ft. Runoff will enter this rain garden via storm sewer pipe,
landscape drains, and roof drains. Wherever concentrated runoff enters this basin will be a concrete
Enclave at Redwood
Page 14
August 18, 2021
impact basin to protect against scour. The slotted underdrain for this basin will connect directly to South
Pond 3 to the southwest, and flow above the WQCV will overflow directly to South Pond 3.
Rain Garden 4 is located in the northwest portion of the Site. This rain garden has 4:1 side slopes, a
WQCV depth of 12” and a total depth of 4.25 ft. Runoff will enter this rain garden at three points via
storm sewer. Wherever concentrated runoff enters this basin will be a concrete impact basin to protect
against scour. The slotted underdrain for this basin will connect to an area inlet southwest of the rain
garden. Flow above the WQCV will overflow northwest into a vegetated swale that conveys flow
southeast to the aforementioned area inlet. This area inlet outfalls to the Redwood Pond.
Rain Garden 5 is located in the northeast portion of the Site, sandwiched between a roadway to the
south and buildings to the north. This rain garden has 8:1 side slopes, a WQCV depth of 9” and a total
depth of 1.5’. Runoff will enter this rain garden via chase drain or vegetated swale. Runoff will enter the
vegetated swale via sheet flow and roof drains. The basin will feature an impact basin for the chase
drain and riprap for the swale to protect against scour. The slotted underdrain will connect to a manhole
in the roadway to the northeast. Flow above the WQCV will overflow northeast into the roadway to be
collected by the nearest inlet downstream.
Sand Filter 4 is located in the northeast portion of the Site. This sand filter has 4:1 side slopes, a WQCV
depth of 1.75 ft and a total depth of 3.75 ft. Runoff will enter this sand filter via storm sewer pipe,
landscape drains, and roof drains. Wherever concentrated runoff enters this basin will be a concrete
impact basin to protect against scour. The slotted underdrain for this basin will connect to a manhole in
the roadway west of the basin, and flow above the WQCV will overflow west into the roadway where it
is collected by the next inlet downstream.
Table 3 below features a summary of the LID systems for the Site including tributary area,
imperviousness, impervious area, calculated WQCV, and provide WQCV.
Table 3:
Table 4 below features a summary of the SWMM results for nodes. Table 5 below features a summary of
the SWMM results for links. See Appendix E for a schematic calling out the nodes summarized below.
Enclave at Redwood
Page 15
August 18, 2021
Table 4:
SWMM Node Results Summary Table
Node Type Maximum
Depth (ft)
Maximum
HGL (ft)
Maximum Total
Inflow (CFS)
A2_1 JUNCTION 2.74 4949.62 53.79
A2_2 JUNCTION 3.11 4950.26 51.46
A2_3 JUNCTION 2.7 4951.09 50.3
A2_4 JUNCTION 2.97 4951.78 50.5
A2_5 JUNCTION 3.01 4952.42 50.78
A2_6 JUNCTION 3.04 4953.04 52.05
A2_7 JUNCTION 2.53 4954.19 51.41
A2_8 JUNCTION 2.84 4955.04 52.18
A2_9 JUNCTION 2.34 4955.06 14.76
A2_10 JUNCTION 1.75 4955.08 14.76
NECCO_Stub OUTFALL 2.1 4948.64 57.35
Pond_Redwood STORAGE 4.44 4957.94 58.4
Pond_South_1 STORAGE 6.2 4958.91 22.49
Pond_South_2 STORAGE 5.73 4953.73 32.54
Pond_South_3 STORAGE 3.75 4957.75 27.59
Table 5:
SWMM Link Results Summary Table
Link Type Maximum
Flow (CFS)
Maximum
Velocity
(fps)
Maximum/Full
Flow
Maximum/Full
Depth
A2_1 CONDUIT 53.8 6.76 0.54 0.61
A2_2 CONDUIT 51.27 5.4 0.57 0.71
A2_3 CONDUIT 49.97 5.37 0.55 0.7
A2_4 CONDUIT 50.3 5.61 0.56 0.68
A2_5 CONDUIT 50.5 5.27 0.56 0.72
A2_6 CONDUIT 50.78 5.24 0.56 0.73
A2_7 CONDUIT 50.89 5.82 0.56 0.67
A2_8 CONDUIT 51.41 6.07 0.57 0.65
A2_9 CONDUIT 14.76 4.95 0.57 0.97
A2_10 CONDUIT 14.76 4.62 0.57 0.78
Redwood_Outlet DUMMY 14.76
South1_Outlet DUMMY 3.5
South2_Outlet DUMMY 6.71
South3_Outlet DUMMY 4.27
Enclave at Redwood
Page 16
August 18, 2021
V. CONCLUSIONS
A. Compliance with Standards
The drainage design is prepared in compliance with the FCSCM and the USDCM Volumes 1, 2, and 3.
As stated before, the NECCO report details a pond release of 0.2 CFS/ac, but given that the proposed
ponds release less than the value calculated in the NECCO report, utilizing a release of less than 15.96
CFS for the Redwood Pond and less than 1.0 CFS/ac for the three on-Site ponds is sufficient.
B. Drainage Concept
The drainage design as described herein; if properly constructed and maintained, will effectively control
stormwater runoff in accordance with the criteria. The proposed development will provide water quality
and detention for the 100-year event with associated outlets and spillway overflow. This drainage design
for the proposed development does not have any impact on the NECCO report design. The drainage
design for this Site will have no adverse effects to any upstream or downstream adjacent properties.
Enclave at Redwood
Page 17
August 18, 2021
VI. REFERENCES
1. National Flood Hazard Layer FIRMette, City of Fort Collins, Colorado, Map No. 08069C0977G,
Map No. 08069C0981G, June 17, 2008, Federal Emergency Management Agency.
2. Geotechnical Engineering Report for Enclave at Redwood Residential Development, Cole Garner
Geotechnical, August 11, 2021.
3. Fort Collins Stormwater Criteria Manual, City of Fort Collins, revised December 2018.
4. Urban Storm Drainage Criteria Manual Vol. 1, Mile High Flood District, revised August 2018.
5. Urban Storm Drainage Criteria Manual Vol. 2, Mile High Flood District, revised September 2017.
6. Urban Storm Drainage Criteria Manual Vol. 3, Mile High Flood District, revised November 2010
with updates from January 2021.
7. North East College Corridor Outfall (NECCO) Design Report, Ayres Associates, August 2009.
Enclave at Redwood
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August 18, 2021
VII. APPENDICES
Appendix A Vicinity Map, FIRM, Geotechnical Engineering Report
Appendix B Portions from Relevant Previous Studies
Appendix C Subcatchment Hydrology
Appendix D Detention Storage Sizing
Appendix E SWMM Model and Results
Appendix F Low Impact Development
Appendix G Drainage Map
4.4 Ayres Associates
Figure 4.1. Proposed storm infrastructure.
4.13 Ayres Associates
The Regional Detention Pond has been sized for additional flows generated from the un-
developed portion of the Old Town North development. A storm sewer was not designed to
convey the flows north to the proposed Regional Detention Pond. When this storm sewer is
designed, special considerations must be given to the GWET and NEWT waterline
crossings. These clearances are very critical.
Laterals C6A and C6B are located just west of the regional detention pond along re-aligned
Vine Drive. Lateral C5A is at the intersection of Blondel and Future Vine Drive. This inlet
will reduce flows from Blondel Street. Laterals C4A and C4B are placed just east of College
Avenue and the future re-aligned Vine Drive.
The inlets located within the re-aligned Vine Drive were designed for the 10-year minor storm
event and the 100-year major storm event being handled within the road, based on current
COFC criteria.
Table 4.5 shows Storm Line C1 and C2, 10- and 100-Year Inlet Summary.
Table 4.5. Storm Line C1 and C2, 10- and 100-Year Inlet Summary.
ID Number
and Type Condition
10- / 100-Year
Flow to Inlet
(cfs)
10- / 100-Year
Capture %
10- / 100-Year
Carry-Over (cfs)
Carry-Over
Location
Inlet
C6A
3-Type 13
Combination On-Grade 5 / 11 90 / 83 1/ 2 A5A
Inlet
C6B
3-Type 13
Combination On-Grade 5 / 11 87/ 77 1 / 3 A5B
Inlet
C5A
1-Type 13
Combination On-Grade 5 / 5 100 - -
Inlet
C4A
2-Type 13
Combination Sump 4.5 / 10 100 - -
Inlet
C4B
3-Type 13
Combination Sump 10 / 27 100 - -
Inlet
C3A 3-Type C Orifice/Weir 24 / 60 - - -
A headwall with wing walls and scour stop are incorporated into the outlet of Storm Line C1
and C2. Scour stop was sized by the manufactured. Storm Line C1 will discharge
approximately 202 cfs to the regional pond during a 100-year storm. Storm Line C2 will
discharge approximately 50 cfs into the regional pond during a 100-year storm.
EPA SWMM Hydraulic Modeling
EPA SWMM was used to model the capacity of the parallel Storm Sewer Lines C1 and C2.
The model results show the energy grade line of the system to be below the flowline of the
inlets, therefore the inlets will function properly during a 10- and 100-year event.
4.4 Redwood Pond
The existing Redwood Pond currently outfalls into existing drainage swale that conveys the
storm water to the Evergreen East Pond. The swale and downstream system does not meet
City of Fort Collins drainage criteria for the 100-year event. The improvements to the pond
included re-grading the pond to outlet at the southeast corner of the site. This outlet will
ultimately discharge into the proposed storm sewer in re-aligned Vine Drive. The re-graded
pond has the same footprint and top of berm elevation as the existing pond. A water quality
4.14 Ayres Associates
outlet structure is proposed prior to discharging into the proposed storm sewer system. A 4-
foot concrete trickle pan is incorporated into the pond bottom to direct low flows due to
minimal slope across the pond. The overall footprint encompasses approximately 3.5 acres
at an approximate depth of 6 ft. The required 100-year detention volume is 7.70 ac-ft with
an additional 2.16 ac-ft of storage below the detention storage for water quality. Table 4.6
shows the major characteristics of Redwood Pond.
Table 4.6. Redwood Pond.
Description
Elevation
(ft)
Pond Volume
(ac-ft)
Pond Depth
(ft)
Pond Invert 4953.10 --- ---
Outlet Structure
Elevation 4955.12 2.16 2.02
100-year WSEL 4957.75 9.86 4.65
Spillway into
Pond 4958.39 11.94 5.29
Top of Pond 4959 14.00 5.90
WSEL = Water Surface Elevation
The outlet structure for the redwood pond is a combination of two Type C inlets in series.
The inlets will be separated by a steel plate with an orifice opening sized to provide a 40-
hour drain time for the water quality portion of the pond. One of the frequent maintenance
issues associated with water quality structures is the clogging of the orifice plates. For this
reason we are proposing to install an 18F snout oil-water debris separator inside the first
Type C inlet over the water quality plate. The snout should prevent the orifice plate from
clogging. The second Type C inlet will be set at the water quality elevation and the outlet
pipe from the inlet controls the release rate from the pond.
Currently an undersized storm sewer system conveys flow from inlets along Redwood Street
into Redwood Pond. The series of existing inlets surcharge during the minor events creating
localized flooding at the intersection of Redwood Street and Conifer Street. The existing
inlets and storm sewer do not have the capacity for the 100-year runoff from the surrounding
basins. An adequate storm system would require large diameter storm pipe and deep inlets
to prevent surcharging. Due to existing water and sanitary conflicts in the area, and a
controlled invert into Redwood Pond, the only storm sewer system that could fit within the
existing constraints would be multiple shallow 12-inch diameter storm sewer culverts. These
storm sewer culverts would still not alleviate the minor and major storm flooding problem.
To mitigate the flooding, the design incorporates a 90-foot overflow weir graded into the
pond embankment. The 100-year storm event (approximately 85 cfs) will overtop the curb at
an elevation of approximately 4958.39 and be conveyed to the pond through the weir. Minor
event flows will continue to be captured by the existing inlets located north of the pond. This
design can be looked at in more detail during the final design process.
EPA SWMM Hydraulic Modeling
The proposed weir into Redwood Pond was modeled using a weir in EPA SWMM. The
transverse weir option was chosen with a discharge coefficient of 3.0. During a 100-year
event the 90 foot weir will convey approximately 80 cfs at a max depth of 0.45 ft.
4.15 Ayres Associates
The detention pond was given an initial depth of 2.02 ft to account for water quality storage.
The pond outlet structure was modeled using a combination of a low flow orifice and
overflow weir. A side calculation was performed to verify the accuracy of the generated EPA
SWMM rating curve out of the pond. Based on the calculations the discharge from the pond
is being controlled by the outflow pipe. The model results matched the side calculations (see
Appendix F for Redwood Pond Calculations).
4.5 East Vine Diversion Channel / Regional Detention Pond Outfall
The future East Vine Diversion Channel will serve as the outfall to the North East College
Corridor Outfall drainage system.
Table 4.7 represents the tailwater rating curve that was used in the EPA SWMM model for
the downstream boundary condition (Storm Line A1) at the future East Vine Diversion
Channel. This rating curve is based on a conceptual design of the future channel and was
taken from the preliminary HEC-RAS model of the East Vine Diversion Channel from the
DC3 project (Dry Creek Connection Channel).
Table 4.7. Dry Creek Tailwater Rating Curve.
Time (hr) Tailwater (ft)
100-year Storm Event
0.0 4934.24
0.3 4939.31
1 4937.88
1.19 4938.29
2 4937.31
3 4936.94
4 4936.86
5 4936.79
6 4936.42
4.5.1 Storm Line A1
Storm Line A1 is a major component of the proposed storm drainage system east of
Redwood Street. Storm Line A1 starts at the outlet of the regional pond and continues east
along re-aligned Vine Drive. This system will outfall into the future East Vine Diversion
Channel. Storm Line A1 will collect flow from the Redwood Pond outlet (Storm Line A2), the
Old Town North detention pond (Storm Line A3), and the Green briar Outfall (Storm Line
A4). The storm line consists of a storm sewer ranging in diameter from 48- to 54-inch RCP,
4 ft x 7 ft and 4 ft x 12 ft RCBC, contains 4 storm laterals, a total of 25-Type 13 combination
inlets, snout oil-water debris separators for water quality, and a crossing at Lake Canal.
Box culverts are required at the downstream end of the Storm Line A1 due to cover
constraints. The box culvert is mainly located so the outside edge of the box coincides with
the outside edge of the 6-foot sidewalk of the future re-aligned Vine Drive.
4.17 Ayres Associates
The 60-inch GWET and 42-inch NEWT water lines were also in conflict with the proposed
storm sewer in several locations along Storm Line A1, primarily at the storm lateral locations.
As mentioned previously, Storm Line A1 system is constrained by the upstream and
downstream inverts. The laterals are constrained by the hydraulics of the system.
Discussions between the City of Fort Collins and the City of Greeley occurred during the
initial design stages. The engineers from both sides offered alternatives and made
accommodations where possible. An agreement was made to allow the Greeley waterlines
to be within 6 inches (outside diameter) of the proposed storm sewer. Manhole risers were
incorporated into the storm sewer design where the hydraulics permitted to allow shallow
storm laterals. The construction drawings reflect GWET and NEWT waterline elevations as
Boyle Engineering submitted to the City of Fort Collins in May 2008.
The potential for future development of the land north of the re-aligned Vine Drive required
Ayres to look at the feasibility of future utilities. After investigating potential sanitary sewer
access locations, it was determined that any development in this area would not be able to
have basements. The sanitary sewer on the east side of Redwood Street posed the
greatest concern. The proposed storm sewer and Greeley waterlines create a utility barrier
along re-aligned Vine Drive. Existing sanitary sewer is currently located in Alta Vista, Lemay
Avenue, and Redwood Street. The sewers located in Lemay Avenue and Redwood Street
do not pose a connection problem, however, the sewer located on the west side of Alta Vista
would required to run at minimum grade and drop manholes would be needed to tie into this
system. Other areas west and east of Redwood should not have issues connecting to
existing utilities.
EPA SWMM Hydraulic Modeling
EPA SWMM was used to model the capacity of the Storm Sewer Lines A1. The proposed
design shows the energy grade line of the system to be underground at the inlets, therefore
the inlets will function properly during a 10- and 100-year event.
4.5.2 Storm Line A2
Storm Line A2 outlets Redwood Pond, continues south and connects into Storm Line A1.
This system consists of storm sewer ranging in diameter from 15- to 48-inch RCP, contains
1 storm lateral and a total of 3-Type C inlets and one snout oil-water debris separator for
water quality.
Storm lateral A6 is placed near the cul-de sac just south of Lupine Drive. Inlet A6 will
replace an existing 12-inch pipe that currently conveys flows north to the existing channel
where Redwood Pond outfalls. Runoff (approximately 73 cfs) from the development will be
re-directed into the proposed system.
EPA SWMM Hydraulic Modeling
EPA SWMM was used to model the capacity of Storm Sewer Lines A2. The analysis shows
the energy grade line of the system to be below the flowline of the inlets, therefore the inlets
will function properly during a 10- and 100-year event.
Project Name:Enclave at RedwoodProject No:201013Date:08/16/21Revised:Design by:JMRNECCO Pond 308 (Redwood Pond Regrading)Preliminary Outlet Structure DesignStage-Storage-DischargeFormulas:orifice formula: Q = CoAo(2gH)0.5, with Co = 0.60 weir formula: Q = CdLH1.5 with Cd = 2.8low-flow outletmajor-flow outlet*overflow spillwayWidth (ft):0.14.510Invert Elevation:4953.549574958w.s. elevOutlet Orifice flowOutlet Weir flowOverflow Weir flowTotal Discharge QArea Area incrementalaccumulated Vol.(cfs)(cfs)(cfs)(cfs)(sf)(acre)volume (ac-ft)(ac-ft)4953.5- - - - - 0.00- - 4954.00 - - 0.10 5,344 0.120.020 0.020 4955.01 - - 0.51 33,375 0.770.398 0.419 4956.01 - - 1.11 83,756 1.921.301 1.720 4957.02 - - 1.83 110,689 2.542.225 3.945 4958.03 13 - 15.65 116,111 2.672.603 6.548 4958.253 18 4 24.30 117,548 2.700.671 7.218 4959.04 36 28 67.24 122,328 2.812.065 9.283 4960.04 65 79 148.75 129,103 2.962.886 12.169 4961.05 101 145 250.81 140,319 3.223.092 15.260 4962.05 141 224 369.78 149,037 3.423.321 18.581
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Monday, Aug 16 2021
Redwood Pond Regrade Spillw ay
Trapezoidal Weir
Crest = Sharp
Bottom Length (ft)= 10.00
Total Depth (ft)= 4.00
Side Slope (z:1)= 4.00
Calculations
W eir Coeff. Cw = 3.10
Compute by:Known Q
Known Q (cfs)= 89.89
Highlighted
Depth (ft)= 1.56
Q (cfs)= 89.89
Area (sqft)= 25.33
Velocity (ft/s)= 3.55
Top W idth (ft)= 22.48
0 5 10 15 20 25 30 35 40 45 50 55
Depth (ft)Depth (ft)Redwood Pond Regrade Spillway
-1.00 -1.00
0.00 0.00
1.00 1.00
2.00 2.00
3.00 3.00
4.00 4.00
5.00 5.00
Length (ft)Weir W .S.
Project Name:Enclave at RedwoodProject No:201013Date:08/16/21Revised:Design by:JMRProposed On-Site South Pond 1Preliminary Outlet Structure DesignStage-Storage-DischargeFormulas:orifice formula: Q = CoAo(2gH)0.5, with Co = 0.60 weir formula: Q = CdLH1.5 with Cd = 2.8low-flow outletmajor-flow outlet*overflow spillwayWidth (ft):0.05230Invert Elevation:4952.714958.54959w.s. elevOutlet Orifice flowOutlet Weir flowOverflow Weir flowTotal Discharge QArea Area incrementalaccumulated Vol.(cfs)(cfs)(cfs)(cfs)(sf)(acre)volume (ac-ft)(ac-ft)4952.71- - - - - 0.00- - 4953.00 - - 0.02 28 0.000.000 0.000 4954.00 - - 0.21 1,289 0.030.012 0.012 4955.00 - - 0.49 2,749 0.060.045 0.057 4956.01 - - 0.84 4,313 0.100.080 0.137 4957.01 - - 1.46 5,981 0.140.118 0.255 4958.02 - - 1.75 7,752 0.180.157 0.412 4958.52 - - 1.87 8,677 0.200.094 0.506 4959.02 2 - 3.97 9,627 0.220.105 0.611 4960.02 10 84 96.50 11,606 0.270.243 0.855 4960.382 14 136 152.90 12,385 0.280.105 0.959
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Monday, Aug 16 2021
Southern Pond 1 Spillw ay
Trapezoidal Weir
Crest = Sharp
Bottom Length (ft)= 30.00
Total Depth (ft)= 1.38
Side Slope (z:1)= 4.00
Calculations
W eir Coeff. Cw = 3.10
Compute by:Known Q
Known Q (cfs)= 22.49
Highlighted
Depth (ft)= 0.38
Q (cfs)= 22.49
Area (sqft)= 11.98
Velocity (ft/s)= 1.88
Top W idth (ft)= 33.04
0 5 10 15 20 25 30 35 40 45 50 55
Depth (ft)Depth (ft)Southern Pond 1 Spillway
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
1.50 1.50
2.00 2.00
Length (ft)Weir W .S.
Project Name:Enclave at RedwoodProject No:201013Date:08/16/21Revised:Design by:JMRProposed On-Site South Pond 2Preliminary Outlet Structure DesignStage-Storage-DischargeFormulas:orifice formula: Q = CoAo(2gH)0.5, with Co = 0.60 weir formula: Q = CdLH1.5 with Cd = 2.8low-flow outletmajor-flow outlet*overflow spillwayWidth (ft):0.12.255Invert Elevation:49484953.254954w.s. elevOutlet Orifice flowOutlet Weir flowOverflow Weir flowTotal Discharge QArea Area incrementalaccumulated Vol.(cfs)(cfs)(cfs)(cfs)(sf)(acre)volume (ac-ft)(ac-ft)4948.0- - - - 967 0.02- - 4949.00 - - 0.28 2,838 0.070.042 0.042 4950.01 - - 0.79 4,753 0.110.086 0.128 4951.01 - - 1.45 6,757 0.160.131 0.259 4952.03 - - 2.72 8,880 0.200.179 0.438 4953.03 - - 3.34 11,089 0.250.229 0.667 4954.04 4 - 7.94 13,401 0.310.281 0.948 4955.04 15 14 32.89 15,818 0.360.335 1.283 4956.05 29 40 73.05 18,339 0.420.392 1.675 4957.05 46 73 123.59 20,964 0.480.451 2.125 4958.05 65 112 182.67 23,693 0.540.512 2.638 4959.06 87 157 249.17 26,527 0.610.576 3.214
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Monday, Aug 16 2021
South Pond 2 Spillway
Trapezoidal Weir
Crest = Sharp
Bottom Length (ft)= 5.00
Total Depth (ft)= 5.00
Side Slope (z:1)= 4.00
Calculations
W eir Coeff. Cw = 3.10
Compute by:Known Q
Known Q (cfs)= 32.54
Highlighted
Depth (ft)= 1.14
Q (cfs)= 32.54
Area (sqft)= 10.90
Velocity (ft/s)= 2.99
Top W idth (ft)= 14.12
0 5 10 15 20 25 30 35 40 45 50 55
Depth (ft)Depth (ft)South Pond 2 Spillway
-1.00 -1.00
0.00 0.00
1.00 1.00
2.00 2.00
3.00 3.00
4.00 4.00
5.00 5.00
6.00 6.00
Length (ft)Weir W .S.
Project Name:Enclave at RedwoodProject No:201013Date:08/16/21Revised:Design by:JMRProposed On-Site South Pond 3Preliminary Outlet Structure DesignStage-Storage-DischargeFormulas:orifice formula: Q = CoAo(2gH)0.5, with Co = 0.60 weir formula: Q = CdLH1.5 with Cd = 2.8low-flow outletmajor-flow outlet*overflow spillwayWidth (ft):0.12.510Invert Elevation:49544957.54958w.s. elevOutlet Orifice flowOutlet Weir flowOverflow Weir flowTotal Discharge QArea Area incrementalaccumulated Vol.(cfs)(cfs)(cfs)(cfs)(sf)(acre)volume (ac-ft)(ac-ft)4954.0- - - - - 0.00- - 4955.00 - - 0.28 2,052 0.050.016 0.016 4956.01 - - 0.79 8,468 0.190.112 0.128 4957.01 - - 1.45 16,012 0.370.276 0.405 4958.03 2 - 5.20 23,305 0.540.449 0.853 4959.03 13 28 44.20 32,118 0.740.633 1.487 4960.04 28 79 110.72 37,716 0.870.801 2.287 4960.354 34 101 138.57 39,688 0.910.311 2.598
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Monday, Aug 16 2021
Southern Pond 3 Spillw ay
Trapezoidal Weir
Crest = Sharp
Bottom Length (ft)= 10.00
Total Depth (ft)= 2.35
Side Slope (z:1)= 4.00
Calculations
W eir Coeff. Cw = 3.10
Compute by:Known Q
Known Q (cfs)= 27.59
Highlighted
Depth (ft)= 0.80
Q (cfs)= 27.59
Area (sqft)= 10.56
Velocity (ft/s)= 2.61
Top W idth (ft)= 16.40
0 5 10 15 20 25 30 35 40
Depth (ft)Depth (ft)Southern Pond 3 Spillway
-1.00 -1.00
0.00 0.00
1.00 1.00
2.00 2.00
3.00 3.00
Length (ft)Weir W .S.
PROPOSED LID CALCULATIONSProject:Enclave at RedwoodDate:8/16/2021Date Revised:N/ABy:JMRBasinArea (sq-ft)Calculated Imperviousness (%)*Utilized Imperviousness (%)Impervious Area (sq-ft)WQCV (12-Hour Drain Time) (cu-ft)120% WQCV (cu-ft)LID TreatmentProvided Volume in LID (cu-ft)SF-145180.4377.0677.0634816.04936.241123.49Sand Filter 11169.42RG-118460.7379.6379.6314700.28401.25481.50Rain Garden 1500RG-224628.8252.835513011.41361.92434.30Rain Garden 2578.17SF-242449.22455019102.15583.68700.42Sand Filter 2730.16IR-112819.7174.39759536.58353.5**N/AIsolator Rows775SF-323788.1280.4680.4619139.92525.25630.30Sand Filter 3632.29RG-3126071.3575.9675.9695763.802560.973073.16Rain Garden 33939.05RG-4278156.7473.1575203471.665554.446665.33Rain Garden 46807.94RG-535335.8738.234513508.90454.53545.44Rain Garden 5663.64SF-4115137.7972.227583152.512299.162758.99Sand Filter 42988.58Total LID Treated Area722028.7870.11506203.24Total Site Area1196645.4756.13671677.10Standard WQCV Treatment Area474616.6934.86165451.3875.36%Total LID Treated Impervious Area/Total Site Impervious Area (Must be > 75%)LOW IMPACT DEVELOPMENT SUMMARY TABLE**Isolator Rows were not sized using a 12-Hour Drain Time or 120% WQCV, rather by comparing MHFD EDB WQCV to Modified FAA Method WQCV*In some cases imperviousness was rounded up to be more conservative in sizing LID facilities, exact imperviousness was used to prove 75% of new impervious areas treated by LID
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1.Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia =50.0 %
(100% if all paved and roofed areas upstream of sand filter)
B) Tributary Area's Imperviousness Ratio (i = Ia/100)i =0.500
C) Water Quality Capture Volume (WQCV) Based on 12-hour Drain Time WQCV =0.17 watershed inches
WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including sand filter area)Area =42,449 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV =cu ft
VW QCV = WQCV / 12 * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER =cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER =700.42 cu ft
(Only if a different WQCV Design Volume is desired)
2.Basin Geometry
A) WQCV Depth DWQCV =1.0 ft
B) Sand Filter Side Slopes (Horizontal distance per unit vertical, Z =4.00 ft / ft
4:1 or flatter preferred). Use "0" if sand filter has vertical walls.
C) Minimum Filter Area (Flat Surface Area)AMin =265 sq ft
D) Actual Filter Area AActual =354 sq ft
E) Volume Provided VT =730 cu ft
3.Filter Material
4.Underdrain System
A) Are underdrains provided?1
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y =1.71 ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 =700 cu ft
iii) Orifice Diameter, 3/8" Minimum DO =5/8 in
Enclave at Redwood - Sand Filter 2
Fort Collins, CO
Design Procedure Form: Sand Filter (SF)
JMR
HKS
August 17, 2021
UD-BMP (Version 3.07, March 2018)
Choose One
Choose One
18" CDOT Class B or C Filter Material
Other (Explain):
YES
NO
Sand Filter 2.xlsm, SF 8/17/2021, 9:08 AM
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5.Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
6.Inlet / Outlet Works
A) Describe the type of energy dissipation at inlet points and means of
conveying flows in excess of the WQCV through the outlet
Notes:
Design Procedure Form: Sand Filter (SF)
JMR
HKS
August 17, 2021
Enclave at Redwood - Sand Filter 2
Fort Collins, CO
A vegetated swale will enter the Sand Filter at a riprap rundown.
Flow above the WQCV will overflow via spillway the roadway to be collected
by inlets.
Choose One
YES NO
Sand Filter 2.xlsm, SF 8/17/2021, 9:08 AM
Project Name:Enclave at RedwoodProject No:201013Date:08/17/21Revised:Design by:JMRSand Filter 2Stage-StorageWSELArea Area IncrementalAccumulatedAccumulated(sf)(acre)Volume (ac-ft)Volume (ac-ft)Volume (ac-ft)4959.0354.06 0.0081- - - 4959.5733.25 0.01680.0061 0.0061 266.14 4960.01,137.57 0.02610.0107 0.0168 730.16 WQCV Top4960.51,567.03 0.03600.0155 0.0322 1,403.45 4961.02,021.61 0.04640.0205 0.0528 2,298.20
Sheet 1 of 3Designer:Company:Date:Project:Location:1.Basin Storage VolumeA) Effective Imperviousness of Tributary Area, IaIa =75.0%B) Tributary Area's Imperviousness Ratio (i = Ia / 100 )i =0.750C) Contributing Watershed AreaArea = 0.294 acD) For Watersheds Outside of the Denver Region, Depth of Averaged6 = in Runoff Producing StormE) Design Concept (Select EURV when also designing for flood control)1F) Design Volume (WQCV) Based on 40-hour Drain TimeVDESIGN=0.007346 ac-ft (VDESIGN = (1.0 * (0.91 * i3 - 1.19 * i2 + 0.78 * i) / 12 * Area )Design Procedure Form: Extended Detention Basin (EDB)Enclave at Redwood - Isolator Rows WQCV Option 1HKSAugust 17, 2021Fort Collins, COJMRUD-BMP (Version 3.07, March 2018)Choose OneExcess Urban Runoff Volume (EURV)Water Quality Capture Volume (WQCV)Isolator Rows MHFD WQCV.xlsm, EDB8/17/2021, 9:13 AM
ISOLATOR ROW VOLUME CALCULATION: MODIFIED FAA METHOD FROM FORT COLLINS IDF
Project #201013
Project Name Enclave at Redwood
Designer JMR
Date 8/17/2021
0.804
0.2943
0.0157
Time (min)WQCV Intensity (1/2 of 2-Year
Intensity) (in/hr)
Volume In
(cu-ft)Out Factor Adjusted
Release (CFS)
Volume Out
(cu-ft)
Storage
(cu-ft)
5 1.425 101.154 1.00 0.0157 4.710 96.44
10 1.105 156.877 1.00 0.0157 9.420 147.46
15 0.935 199.113 1.00 0.0157 14.130 184.98
20 0.805 228.572 0.95 0.0149 17.898 210.67
25 0.715 253.772 0.86 0.0135 20.253 233.52
30 0.65 276.842 0.80 0.0126 22.608 254.23
35 0.585 290.684 0.76 0.0119 25.057 265.63
40 0.535 303.816 0.73 0.0115 27.506 276.31
45 0.495 316.239 0.70 0.0110 29.673 286.57
50 0.46 326.532 0.68 0.0107 32.028 294.50
55 0.435 339.664 0.66 0.0104 34.195 305.47
60 0.41 349.247 0.65 0.0102 36.738 312.51
65 0.39 359.895 0.64 0.0100 39.187 320.71
70 0.365 362.734 0.63 0.0099 41.542 321.19
75 0.35 372.672 0.62 0.0097 43.803 328.87
80 0.33 374.802 0.61 0.0096 45.970 328.83
85 0.32 386.159 0.61 0.0096 48.843 337.32
90 0.305 389.709 0.60 0.0094 50.868 338.84
95 0.29 391.128 0.59 0.0093 52.799 338.33
100 0.28 397.517 0.59 0.0093 55.578 341.94
105 0.27 402.486 0.59 0.0093 58.357 344.13
110 0.26 406.035 0.58 0.0091 60.100 345.94
115 0.255 416.328 0.58 0.0091 62.831 353.50
120 0.245 417.393 0.58 0.0091 65.563 351.83
353.50
Qout (From ADS maximum chamber flow rate)(CFS)
FAA Method Table
Maximum Storage Represents Required WQCV
C (From Fort Collins Storm Criteria Manual)
Area (ac)
ABC DEFGHIJKLMNVAULT IDTOTAL REQUIRED WQCV(CF)WQ FLOW (CFS)CHAMBER TYPERELEASE RATE/CHAMBER(CFS)CHAMBER VOLUME(CF)CHAMBER VOLUME + AGGREGATE(CF)NUMBER OF CHAMBERSCOMBINED RELEASE RATE WQCV METHOD(CFS)FAA REQUIRED STORAGE VOLUME (CF)NUMBER OF CHAMBERS (FAA)TOTAL VOLUME (CHAMBERS ONLY)(CF)COMBINED RELEASE RATE FAA METHOD (CFS)TOTAL NUMBER OF CHAMBERS REQUIREDTOTAL SYSTEM VOLUME (CF)3200SC-3100.01614.731.0110.17353.5253680.3925775CONFIRMCONFIRMAMHFD Extended Detention Basin Spreadsheet WQCV * 0.8 (Drain Time Coefficient for allowable 12-hr drain time)B1/2 of the 2-year developed flow rate for the basin being sizedCChamber type based on site constraintsDFlwo rate thru the bottom of the isolator row chamber which is equal to the area of the bottom of the chamber multiplied by the flow rate per unit area (see calculations below)EVolume within chamber only, not accounting for void spaces in surrounding aggregate (standard spec)FVolume includes chamber and void space (40%) in surrounding aggregate , per chamber unit. (standard spec)GNumber of chambers required to provide full WQCV within total installed system, include aggregate = A / FHRelease rate per chamber times number of chambers = D * G IFAA calc based on Flow, WQ and Total Release RateJNumber of chambers required to provide required FAA staorage volume stored within the chamber only (no aggregate stone) = I / EKVolume provided in chambers only (no aggregate storage). This must meet or excheed the required FAA storage volume. Greater of G or J * E LRelease rate per chamber times number of chambers = D * J MTotal number of chambers (Greater of J or G)NTotal system volume. Includes number of chambers plus aggregate. This must meet or exceed required WQCVMC-4500MC-3500SC-740SC-310SC-160LP60.045.030.016.012.0100.077.051.034.025.048.386.085.485.485.433.546.030.220.214.8106.5109.985.914.76.9162.6175.074.931.015.0CHAMBER FLOW RATE CONVERSION (GPM/SF TO CFS)0.35GPM/SF1 CF=7.48052GAL1 GALLON=0.133681CF 1 GPM=0.002228CFS*FLOW RATE BASED ON 1/2 OF NOV 07 QMAX IN FIGURE 17 OF UNH TESTING REPORTMC-4500MC-3500SC-740SC-310SC-160LP0.0260.0360.0240.0160.012(COLUMN D)FLOW RATE / CHAMBER (CFS)STORMTECH CHAMBER VOLUME AND RELEASE RATE CALCULATIONSFLOOR AREA (SF)CHAMBER VOLUME (CF)CHAMBER/AGGREGATE VOLUME (CF)STORMTECH CHAMBER DATAFLOW RATE*CHAMBER FLOW RATECHAMBER DIMENSIONSWIDTH (IN)INSTALLED LENGTH (IN)HEIGHT (IN)
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1.Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia =80.5 %
(100% if all paved and roofed areas upstream of sand filter)
B) Tributary Area's Imperviousness Ratio (i = Ia/100)i =0.805
C) Water Quality Capture Volume (WQCV) Based on 12-hour Drain Time WQCV =0.26 watershed inches
WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including sand filter area)Area =23,788 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV =cu ft
VW QCV = WQCV / 12 * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER =cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER =630.30 cu ft
(Only if a different WQCV Design Volume is desired)
2.Basin Geometry
A) WQCV Depth DWQCV =1.4 ft
B) Sand Filter Side Slopes (Horizontal distance per unit vertical, Z =4.00 ft / ft
4:1 or flatter preferred). Use "0" if sand filter has vertical walls.
C) Minimum Filter Area (Flat Surface Area)AMin =239 sq ft
D) Actual Filter Area AActual =242 sq ft
E) Volume Provided VT =632 cu ft
3.Filter Material
4.Underdrain System
A) Are underdrains provided?1
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y =1.71 ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 =630 cu ft
iii) Orifice Diameter, 3/8" Minimum DO =5/8 in
Enclave at Redwood - Sand Filter 3
Fort Collins, CO
Design Procedure Form: Sand Filter (SF)
JMR
HKS
August 17, 2021
UD-BMP (Version 3.07, March 2018)
Choose One
Choose One
18" CDOT Class B or C Filter Material
Other (Explain):
YES
NO
Sand Filter 3.xlsm, SF 8/17/2021, 9:36 AM
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5.Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
PROVIDE A 30 MIL (MIN) PVC GEOMEMBRANE PER TABLE
SF-4 WITH SEPARATOR FABRIC (PER TABLE SF-3) ABOVE IT.
PROVIDE SEPARATOR FABRIC BELOW THE GEOMEMBRANE
AS WELL IF SUBGRADE IS ANGULAR OR COULD OTHERWISE
PUNCTURE THE GEOMEMBRANE.
6.Inlet / Outlet Works
A) Describe the type of energy dissipation at inlet points and means of
conveying flows in excess of the WQCV through the outlet
Notes:
Runoff will sheet flow or be conveyed via roof drain to the Sanf Filter.
Riprap on the slopes will provide scour protection.
Flows above the WQCV will overflow into South Pond 3 over a vertical wall.
Design Procedure Form: Sand Filter (SF)
JMR
HKS
August 17, 2021
Enclave at Redwood - Sand Filter 3
Fort Collins, CO
Choose One
YES NO
Sand Filter 3.xlsm, SF 8/17/2021, 9:36 AM