HomeMy WebLinkAboutHOMESTEAD AT CLARENDON HILLS - FDP200032 - - DRAINAGE REPORT (2)
September 30, 2020
DRAINAGE LETTER AND LID REPORT
HOMESTEAD AT CLARENDON HILLS
Fort Collins, Colorado
Prepared for:
Kenneth R. Mitchell
Mosaic Real Estate LLC
1021 Nightingale Drive
Fort Collins, Colorado. 80525
Prepared by:
301 North Mason Street, Suite 100
Fort Collins, Colorado 80524
Phone: 970.221.4158
www.northernengineering.com
Project Number: 1013-008
Drainage Letter and LID Report – Homestead at Clarendon Hills
Date: September 30, 2020
Project: Homestead at Clarendon Hills Project No. 1013-008
Preliminary Development Plan (PDP)
Fort Collins, Colorado
Attn: City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
To whom it may concern:
This letter serves to address the stormwater impacts of the proposed project known as “Homestead at
Clarendon Hills”. The site is located in southwest Fort Collins bounded to the north by Front Range
Community College, to the east by existing Clarendon Hills Subdivision, to the south by Clarendon
Hills Drive and Langdale Drive, and to the west by South Shields Street. The project is part of the
Drainage Report for Clarendon Hills by Stuart and Associates written in 1986. The proposed
development consists of 8 low-density single-family lots and a detention area. The project site also
contains an existing irrigation ditch along the north property line.
The project was included in the drainage report for Clarendon Hills Subdivision. Therefore, this letter
intends to show conformance with the approved drainage design, including general drainage patterns
and imperviousness. This letter also documents how the project meets new Low Impact Development
requirements implemented City of Fort Collins since the approval of the original project.
Original Drainage Patterns
The original Clarendon Hill drainage design divided the subdivision into 3 basins. The current project
site is split by two of these basins - Drainage Basin A draining to the north, and Drainage Basin B
draining to the south. Drainage Basin B flows into Clarendon Hills Drive and Langdale Drive and
travels via gutter and pan flow south along Clarendon Hills Drive. The flow is then collected by inlets
at Clarendon Hills Drive and Hinsdale Drive and conveyed to a series of existing detention ponds.
Drainage Basin A generally flows to the northeast to an existing irrigation ditch on the property. A
small undisturbed area north of the irrigation ditch will flow into the Front Range Community College
drainage system. From there, it flows through existing drainage ways east to Mail Creek, and
eventually to Fossil Creek. An Existing Drainage Exhibit and excerpts from the original report are
provided in the appendices for reference.
Page | 3
Proposed Drainage Patterns
The Homestead at Clarendon Hills project was divided into seven proposed drainage basins that
essentially follow the established drainage patterns, as well as the drainage patterns approved with
the Drainage Report for Clarendon Hills.
Basin A encompasses the rear of the lots and open space between the rear lot lines and the existing
irrigation ditch. The only proposed improvements with Basin A are landscaping in the rear of the lots.
Flows from the basin will drain north into the irrigation ditch. Basin A is consistent with Basin A from
the Drainage Report for Clarendon Hills.
Basin B will roughly match the existing area draining south to the existing Clarendon Hills ponds.
Proposed improvements will include single-family homes, the front and side of the lots, driveways,
walks, and detention area. Basin B is further subdivided into Basins B1, B2, B3, and B4. Basins B1 and
B3 consists of the open space at the west and east ends that will discharge into the Clarendon Hills and
Langdon Drives curb and gutter. Basins B2 consists of Lots 1 through 8 building envelopes, driveways
and detention pond. The project was graded to allow the building envelopes to drain to south, and
small swales and curb and gutter in the shared driveway will convey drainage towards the detention
pond. Basin B4 consists of the landscaping along the front of the pond that is incapable of draining
into the detention pond. The release rate from the detention pond is reduced to accommodate the
undetained releases from Basins B1, B3, and B4. An exhibit calculating the release rate is included in
the appendices.
The percent impervious for the lots was developed by assuming a typical building footprint of 3,250
square feet. The building footprint was placed along the rear building envelope to maximize the
length of the driveway. This resulted in 49% imperviousness on a 7,982 sq. ft. lot. The area of the
driveway was adjusted on the other lots to maintain a 49% imperviousness on the other lots. Along
with the shared driveway, this resulted in a percent imperviousness of 51% in Basin B3. An exhibit is
included in the appendices.
Additionally, Basin OS1 and OS2 are on the east and the north sides of the site to quantify flows to the
Front Range Community College campus. Basin OS1 consists of the side and rear of Lot 8 and
adjacent open space. Basin OS2 is a small sliver of open space on the north side of the irrigation ditch
that drains directly to the Front Range Community College campus.
Development of Existing Runoff Coefficient
The drainage for Clarendon Hills Subdivision was originally analyzed in the Drainage Report:
Clarendon Hills: Fort Collins, Colorado prepared by Stuart and Associates in 1986 as part of the approval
process for Clarendon Hills 1st Filing. Homestead for Clarendon Hills is in the northwest corner of the
original drainage master plan for Clarendon Hills 1st Filing. In the approved original drainage report
for Clarendon Hills (1st Filing), the project site was planned as office and multi-family with a
developed runoff coefficient of C=0.50.
According to the approved drainage report for Clarendon Hills 4th Filing, prepared by Land
Development Services and dated April 9, 1991, a church site of 8.27 acres in the middle of the
Clarendon Hills 4th Filing was assumed to be an offsite undeveloped drainage basin with a developed
Page | 4
runoff coefficient of C=0.25. According to the approved drainage report for Clarendon Hills 6th Filing,
prepared by Land Development Services and dated January 8, 1992, the original developer of
Clarendon Hills swapped the northwest corner of Clarendon Hills (including Homestead for
Clarendon Hills) for the church site in Clarendon Hills 4th Filing. The old church site was then
developed as Clarendon Hills 6th Filing with runoff coefficients from 0.28 to 0.70.
Because of the subsequent drainage studies and land swaps, we assumed an existing runoff coefficient
of C=0.25 for Homestead at Clarendon Hills. A runoff coefficient of 0.25 is consistent with the
following design considerations:
• The Fort Collins Stormwater Criteria Manual for lawns with clayey soil and an average slope of
2% to 7% requires a runoff coefficient of C = 0.25. The existing slopes throughout the site is
between 2% to 7% with clayey soil.
• The drainage report for Clarendon Hills 4th Filing assumed a runoff coefficient C=0.25 for the
church site, and the church site was subsequently part of a land swap for the northwest corner
of the Clarendon Hills Subdivision.
Detention
Per the Clarendon Hills 1st Filing Drainage Report, “The ponds were designed using a mass diagram
that required 10.46 acre-feet to detain the 100-year developed storm and release at a 2-year historic
rate…”. Assuming a historic runoff coefficient of 0.25, detention from Basin B was provided as long as
the overall flow at Design Point B does not exceed 1.9 cfs. Because of the increased impervious area
within Basin B, detention is required within Basin B2 to reduce the post-developed flow rates at
Design Point B to equal or less than 1.9 cfs. Since Basins B1, B3, and B4 will be released into the
surrounding curb and gutter undetained, the detention pond in Basin B2 is designed to release the
100-year event at 0.3 cfs. A release rate of 1.2 cfs is required to maintain a 100-year flow rate of 1.9 cfs
at Design Point B. (See the included flowrate calculations included in the appendices.)
Inlets
By limiting the 100-year flowrate at Design Point B, the flowrate at the existing inlets will match or
decrease from the projected flowrates in the previously approved Clarendon Hills drainage reports.
This results in matching or decreasing runoff to the existing inlets that were designed and approved
with the earlier project, so no additional inlet calculations have been provided as a part of this report.
Water Quality/LID Conformance
According to Section 6.0 of the City of Fort Collins Stormwater Manual, “50% site treated with ‘standard’
water quality plus 50% site treated with LID (including pavers) equals 100% of site-treated (requirements met),
or 25% site treated with ‘standard’ water quality plus 75% treated with LID equals 100% of site treated
(requirements met)”, (page 14). This project proposes to treat 90.5% of the new impervious areas
through two rain gardens as LID treatment. The detention pond will provide an additional 1,187 cubic
feet of water quality treatment for 90.5% of new impervious area. However, due to grading
constraints along the east side of the project and requirements to provide a maintenance access for th e
ditch company, a gravel pedestrian path (3,754 sq. ft. of new impervious area) will not be treated by
either the rain gardens or the detention pond. An exhibit showing the area of LID treatment is
included in the appendices.
Page | 5
Although exact impervious area is unavailable at this preliminary stage, the building envelopes will
drain towards the rain gardens and the detention pond. The high point along each lot line is set at the
rear of the building envelopes to maximize as much of the lot draining towards the south. Space
limitations on the detention pond and rain gardens, natural buffer offsets from the ditch, and fire
accessibility requirements restrict the drainage towards the front. Even with the grading restrictions,
the proposed flowrate into the Mail Creek Basin is 2.2 cfs less than the existing flowrate.
Erosion and Sediment Control
During construction, the Contractor will follow the appropriate and applicable City of Fort Collins
standards for erosion and sediment control. Since more than 10,000 sf will be disturbed as a part of
this project, a comprehensive Stormwater Management Plan will be prepared for this project at final
design. Post construction water quality and erosion control will be achieved by a fully established and
stabilized site. All areas disturbed during construction will receive permanent hardscape, landscape,
or building structure.
Floodplains
There are no regulatory floodplains associated with the project.
Conclusions
The proposed grading concept matches the original drainage patterns. The site matches the designed
imperviousness draining to the existing Clarendon Hills curb and gutter. Onsite detention will be
provided for the increased imperviousness of the area draining to towards the existing Clarendon
Hills ponds, and onsite detention will be required to maintain historic flow rates within Clarendon
Hills. Stormwater quality has been provided and meets the city requirements for Low Impact
Development treatment. Therefore, it is my professional opinion that Homestead at Clarendon Hills
satisfies all applicable stormwater criteria and will effectively limit potential damage associated with
its stormwater runoff.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Frederick S. Wegert, PE
Project Engineer
Project:
Calculations By:
#####Date:
Length
(ft)
Elev
Up
Elev
Down
Slope
(%)
Ti
2-Yr
(min)
Ti
10-Yr
(min)
Ti
100-Yr
(min)
Length
(ft)
Elev
Up
Elev
Down
Slope
(%)
Velocity
(ft/s)
Tt
(min)
Length
(ft)
Elev
Up
Elev
Down
Slope
(%)
Velocity
(ft/s)
Tt
(min)
Comp.
Tc 2-Yr
(min)
Max.
Tc 2-Yr
(min)
Tc
2-Yr
(min)
Comp.
Tc 10-Yr
(min)
Max.
Tc 10-Yr
(min)
Tc
10-Yr
(min)
Comp.
Tc 100-Yr
(min)
Max.
Tc 100-Yr
(min)
Tc
100-Yr
(min)
ex a EX A 65 93.83 88.55 8.12%6.4 6.4 5.9 N/A N/A N/A 665 88.55 85.49 0.46%1.02 10.89 17.27 14.06 14.06 17.27 14.06 14.06 16.80 14.06 16.80
b EX B 30 93.00 91.75 4.17%5.4 5.4 5.0 340 91.75 84.93 2.01 2.83 2.00 N/A N/A N/A 7.41 12.06 7.41 7.41 12.06 7.41 7.01 12.06 7.01
b EX OS1 96 93.00 89.90 3.23%10.5 10.5 9.8 N/A N/A N/A 98 89.90 86.13 3.85%2.94 0.56 11.09 11.08 11.08 11.09 11.08 11.08 10.32 11.08 10.32
b EX OS2 45 92.23 88.81 7.60%5.4 5.4 5.0 N/A N/A N/A N/A N/A N/A 5.42 10.25 5.42 5.42 10.25 5.42 5.02 10.25 5.02
ex a North 65 93.83 88.55 8.12%6.4 6.4 5.9 N/A N/A N/A 665 88.55 85.49 0.46%1.02 10.89 17.27 14.06 14.06 17.27 14.06 14.06 16.80 14.06 16.80
Time of Concentration
Combined Basins
Rational Equation: Q = CiA (Equation 6-1 per MHFD)
Design
Point Basin
Overland Flow Channelized Flow Swale Flow
Velocity (Swale Flow), V = 15·S ½
EXISTING TIME OF CONCENTRATION COMPUTATIONS (MHFD)
Overland Flow, Time of Concentration:
Homestead at Clarendon
Gutter/Swale Flow, Time of Concentration:F. Wegert
Tt = L / 60V (Equation 6-4 per MHFD)October 29, 2020
Tc = Ti + Tt (Equation 6-2 per MHFD)
Intensity, i (per Table 3.4-1 of the Fort Collins Stormwater Manual)
Velocity (Gutter Flow), V = 20·S ½
(Equation 6-4 per MHFD)
(Equation 3.3-2 per Fort Collins
}
𝑆𝑖=1.87 1.1 −𝐶∗𝐶𝑓𝐿
𝑆ൗ13
Notes:
1)Add 4900 to all elevations.
Page 2 of 4
Tc2 Tc10 Tc100 C2 C10 C100
I2
(in/hr)
I10
(in/hr)
I100
(in/hr)
Q2
(cfs)
Q10
(cfs)
Q100
(cfs)
ex a EX A 2.382 14.06 14.06 16.80 0.25 0.25 0.31 1.92 3.29 6.20 1.1 2.0 4.6
b EX B 0.684 7.41 7.41 7.01 0.25 0.25 0.31 2.52 4.31 8.80 0.4 0.7 1.9
b EX OS1 0.147 11.08 11.08 10.32 0.25 0.25 0.31 2.13 3.63 7.72 0.1 0.1 0.4
b EX OS2 0.064 5.42 5.42 5.02 0.25 0.25 0.31 2.85 4.87 9.95 0.0 0.1 0.2
ex a North 2.594 14.06 14.06 16.80 0.25 0.25 0.31 1.92 3.29 6.20 1.25 2.13 5.03
Combined Basins
Velocity (Swale Flow), V = 15·S ½
Rational Equation: Q = CiA (Equation 6-1 per MHFD)
Design
Point Basin Area
(acres)
Tc (Min)Runoff C Intensity Flow
Velocity (Gutter Flow), V = 20·S ½
EXISTING DIRECT RUNOFF COMPUTATIONS (MHFD)
Overland Flow, Time of Concentration:
Project:Homestead at Clarendon
Gutter/Swale Flow, Time of Concentration:Calculations By:F. Wegert
Tt = L / 60V (Equation 6-4 per MHFD)Date:October 29, 2020
Tc = Ti + Tt (Equation 6-2 per MHFD)
Intensity, I from Fig. 3.4.1 Fort Collins Stormwater Manual
(Equation 6-4 per MHFD)}
𝑆𝑖=1.87 1.1 −𝐶∗𝐶𝑓𝐿
𝑆ൗ13
(Equation 3.3-2 per Fort Collins
Stormwater Manual)
Page 3 of 4
Length
(ft)
Elev
Up
Elev
Down
Slope
(%)
Ti
2-Yr
(min)
Ti
10-Yr
(min)
Ti
100-Yr
(min)
Length
(ft)
Elev
Up
Elev
Down
Slope
(%)
Velocity
(ft/s)
Tt
(min)
Length
(ft)
Elev
Up
Elev
Down
Slope
(%)
Velocity
(ft/s)
Tt
(min)
Comp.
Tc 2-Yr
(min)
Max.
Tc 2-Yr
(min)
Tc
2-Yr
(min)
Comp.
Tc 10-Yr
(min)
Max.
Tc 10-Yr
(min)
Tc
10-Yr
(min)
Comp.
Tc 100-Yr
(min)
Max.
Tc 100-Yr
(min)
Tc
100-Yr
(min)
a A 60 92.89 89.01 6.47%6.7 6.7 6.2 N/A N/A N/A 705 89.01 85.49 0.50%1.06 11.09 17.78 14.25 14.25 17.78 14.25 14.25 17.32 14.25 17.32
b B1 40 93.97 92.33 4.10%6.0 6.0 5.5 326 92.33 84.93 2.27 3.01 1.80 N/A N/A N/A 7.80 12.03 7.80 7.80 12.03 7.80 7.27 12.03 7.27
b B2 120 94.10 91.77 1.94%8.2 8.2 5.7 375 91.77 84.93 1.82 2.70 2.31 N/A N/A N/A 10.47 12.75 10.47 10.47 12.75 10.47 7.99 12.75 7.99
b B3 40 92.18 91.54 1.60%9.1 9.1 8.6 366 91.54 84.93 1.81 2.69 2.27 N/A N/A N/A 11.37 12.26 11.37 11.37 12.26 11.37 10.86 12.26 10.86
b B4 20 91.89 90.41 7.40%3.6 3.6 3.4 167 90.41 84.93 3.28 3.62 0.77 N/A N/A N/A 4.42 11.04 5.00 4.42 11.04 5.00 4.15 11.04 5.00
os1 OS1 35 92.28 91.47 2.31%5.4 5.4 4.4 N/A N/A N/A 134 91.47 86.13 3.99%2.99 0.75 6.12 10.94 6.12 6.12 10.94 6.12 5.16 10.94 5.16
os2 OS2 25 92.23 90.55 6.72%4.2 4.2 3.9 N/A N/A N/A N/A N/A N/A 4.21 10.14 5.00 4.21 10.14 5.00 3.90 10.14 5.00
a North 60 92.89 89.01 6.47%6.6 6.6 6.1 N/A N/A N/A 705 89.01 85.49 0.50%1.06 11.09 17.69 14.25 14.25 17.69 14.25 14.25 17.21 14.25 17.21
b South 95 94.23 92.41 1.92%7.7 7.7 5.6 350 92.41 84.93 2.14 2.92 2.00 N/A N/A N/A 9.67 12.47 9.67 9.67 12.47 9.67 7.55 12.47 7.55
Rational Equation: Q = CiA (Equation 6-1 per MHFD)
Velocity (Swale Flow), V = 15·S ½
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS (MHFD)
Overland Flow, Time of Concentration:
Homestead at Clarendon
Gutter/Swale Flow, Time of Concentration:F. Wegert
Tt = L / 60V (Equation 6-4 per MHFD)October 29, 2020
Tc = Ti + Tt (Equation 6-2 per MHFD)
Velocity (Gutter Flow), V = 20·S ½
Project:
Calculations By:
Date:
Intensity, i (per Table 3.4-1 of the Fort Collins Stormwater Manual)
Time of Concentration
Combined Basins
Design
Point Basin
Overland Flow Channelized Flow Swale Flow
(Equation 6-4 per MHFD)
(Equation 3.3-2 per Fort Collins
}
𝑆𝑖=1.87 1.1 −𝐶∗𝐶𝑓𝐿
𝑆ൗ13
Notes:
1)Add 4900 to all elevations.
Page 2 of 3