HomeMy WebLinkAboutHOMESTEAD AT CLARENDON HILLS - PDP190007 - DOCUMENT MARKUPS - ROUND 4 - DRAINAGE REPORTAugust 14, 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: August 14, 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. All proposed roof drains will drain south to the proposed detention area.
Basin B is further subdivided into Basins B1, B2, B3, and B4. Basins B1 and B3 consists of the two lots
(Lots 1 and 8) at the west and east ends, and Basins B1 and B3 will discharge into the Clarendon Hills
and Langdon Drives curb and gutter. Basin B2 consists of the Lots 2 through 7 and driveways and
discharge into 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.
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
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.
Page | 4
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 0.3 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
City LID requirements specify that 50% of all single-family areas and 75% of all new impervious area
receive water quality treatment from a LID facility. This project proposes to treat 79.8% of the new
impervious areas through two rain gardens and a grass buffer. The rain gardens will also provide
water quality treatment for the project site. A vegetated buffer will be located between the rear lot
lines and the irrigation ditch. The ditch access road limits the available native land to be used as a
vegetated buffer. An exhibit showing the area of LID treatment is included in the appendices.
Although exact impervious area is unavailable at this preliminary stage, the majority of the residential
area will drain towards the rain gardens or across the vegetated buffer. The front 2/3rds of the lot will
be graded to drain south towards the rain gardens. The rear 1/3rd of the lot will be graded to drain
towards the vegetated buffer. The high point along each lot line is set to maximize as much of the lot
that drains towards the rain gardens as possible. 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 0.8 cfs less than the existing flowrate.
Page | 5
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:
C2*Cf C10*Cf C100*Cf
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)
Tc
2-Yr
(min)
Tc
10-Yr
(min)
Tc
100-Yr
(min)
ex a EX A 0.25 0.25 0.31 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% 0.10 108.93 14.06 14.06 14.06
ex b EX B 0.25 0.25 0.31 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 7.41 7.01
ex os1 EX OS1 0.25 0.25 0.31 195 93.00 86.13 3.52% 14.6 14.6 13.5 N/A N/A N/A N/A N/A N/A 11.08 11.08 11.08
ex os2 EX OS2 0.25 0.25 0.31 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 5.42 5.02
ex a North 0.25 0.25 0.31 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% 0.10 108.93 14.06 14.06 14.06
Gutter/Swale Flow, Time of Concentration:
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 14.06 0.25 0.25 0.31 1.92 3.29 6.71 1.1 2.0 5.0
ex 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
ex os1 EX OS1 0.147 11.08 11.08 11.08 0.25 0.25 0.31 2.13 3.63 7.42 0.1 0.1 0.3
ex os2 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 14.06 0.25 0.25 0.31 1.92 3.29 6.71 1.2 2.1 5.4
Homestead at Clarendon
F. Wegert
August 5, 2020
Project:
Gutter/Swale Flow, Time of Concentration: Calculations By:
Date:
Combined and Routed Basins
HISTORIC DIRECT RUNOFF COMPUTATIONS
Intensity Flow
Overland Flow, Time of Concentration:
Intensity, I from Fig. RA-2
Rational Equation: Q = CiA (Equation 6-1 per MHFD)
Design
Point
Basin
Area
(acres)
Tc (Min) Runoff C
Tt = L / 60V (Equation 6-4 per MHFD)
Tc = Ti + Tt (Equation 6-2 per MHFD)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
(Equation 6-4 per MHFD)
(Equation 6-3 per MHFD)
}
𝑇𝑖 =
1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿
𝑆 ൗ
1
3
Page 3 of 3
Character of Surface
Runoff
Coefficient1
Percent
Impervious1
0.95 100%
0.95 90%
0.50 40%
0.55 50%
0.20 2%
0.25 2%
Basin ID
Basin Area
(sq.ft.)
Basin Area
(acres)
Asphalt,
Concrete (acres)
Rooftop (acres) Gravel (acres)
Residential: Low
Density (acres)
Undeveloped:
Greenbelts,
Agriculture
(acres)
Lawns, Clayey
Soil, Avg Slope 2-
7% (acres)
Percent
Impervious
C2*Cf
Cf = 1.00
C5*Cf
Cf = 1.00
C10*Cf
Cf = 1.00
C100*Cf
Cf = 1.25
A 67,789 1.556 0.000 0.000 0.176 0.338 1.043 0.000 17% 0.31 0.31 0.31 0.39
B1 7,111 0.163 0.000 0.000 0.012 0.123 0.000 0.029 41% 0.49 0.49 0.49 0.62
B2 52,848 1.213 0.149 0.000 0.000 0.902 0.000 0.162 50% 0.56 0.56 0.56 0.70
B3 3,897 0.089 0.000 0.000 0.000 0.082 0.000 0.007 46% 0.53 0.53 0.53 0.66
B4 3,034 0.070 2% 0.25 0.25 0.25 0.31
OS1 5,313 0.122 0.000 0.000 0.028 0.044 0.050 0.000 28% 0.40 0.40 0.40 0.49
OS2 2,806 0.064 2% 0.25 0.25 0.25 0.31
North 75,907 1.743 0.000 0.000 0.203 0.382 1.157 0.000 17% 0.31 0.31 0.31 0.39
South 66,891 1.536 0.149 0.000 0.012 1.107 0.000 0.268 46% 0.54 0.54 0.54 0.67
DEVELOPED RUNOFF COEFFICIENT CALCULATIONS
Asphalt, Concrete
Rooftop
Gravel
Residential: Low Density
Homestead at Clarendon
F. Wegert
August 5, 2020
Project:
Calculations By:
Date:
Undeveloped: Greenbelts, Agriculture Composite Runoff Coefficient2
1) Runoff coefficients per Tables 3.2-1 & 3.2 of the FCSM. Percent impervious per Tables 4.1-2 & 4.1-3 of the FCSM.
Project:
Calculations By:
Date:
C2*Cf C10*Cf C100*Cf
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)
Tc
2-Yr
(min)
Tc
10-Yr
(min)
Tc
100-Yr
(min)
a A 0.31 0.31 0.39 60 92.89 89.01 6.47% 6.1 6.1 5.5 N/A N/A N/A 705 89.01 85.49 0.50% 0.11 110.86 14.25 14.25 14.25
b B1 0.49 0.49 0.62 40 93.97 92.33 4.10% 4.5 4.5 3.6 326 92.33 84.93 2.27 3.01 1.80 N/A N/A N/A 6.28 6.28 5.37
b B2 0.56 0.56 0.70 95 94.23 92.41 1.92% 7.9 7.9 5.9 350 92.41 84.93 2.14 2.92 2.00 N/A N/A N/A 9.93 9.93 7.88
b B3 0.53 0.53 0.66 60 93.10 91.71 2.32% 6.3 6.3 4.8 305 91.71 84.93 2.22 2.98 1.70 N/A N/A N/A 7.99 7.99 6.55
b B4 0.25 0.25 0.31 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 5.00 5.00 5.00
os1 OS1 0.40 0.40 0.49 160 93.83 93.10 0.46% 21.7 21.7 18.6 N/A N/A N/A N/A N/A N/A 10.89 10.89 18.62
Tc2 Tc10 Tc100 C2 C10 C100
I2
(in/hr)
I10
(in/hr)
I100
(in/hr)
Q2
(cfs)
Q10
(cfs)
Q100
(cfs)
a A 1.556 14.25 14.25 14.25 0.31 0.31 0.39 1.92 3.29 6.71 0.9 1.6 4.0
b B1 0.163 6.28 6.28 5.37 0.49 0.49 0.62 2.67 4.56 9.31 0.2 0.4 0.9
b B2 1.213 9.93 9.93 7.88 0.56 0.56 0.70 2.26 3.86 7.88 1.5 2.6 6.7
b B3 0.089 7.99 7.99 6.55 0.53 0.53 0.66 2.46 4.21 8.59 0.1 0.2 0.5
b B4 0.070 5.00 5.00 5.00 0.25 0.25 0.31 2.85 4.87 9.95 0.0 0.1 0.2
os1 OS1 0.122 10.89 10.89 18.62 0.40 0.40 0.49 2.17 3.71 7.57 0.1 0.2 0.5
os2 OS2 0.064 5.00 5.00 5.00 0.25 0.25 0.31 2.85 4.87 9.95 0.0 0.1 0.2
a North 1.743 14.25 14.25 116.38 0.31 0.31 0.39 1.92 3.29 6.71 1.0 1.8 4.6
b South 1.536 10.27 10.27 8.30 0.54 0.54 0.67 2.21 3.78 7.72 1.8 3.1 7.9
Gutter/Swale Flow, Time of Concentration:
Project:
Calculations By:
Date:
Homestead at Clarendon
F. Wegert
August 5, 2020
Combined and Routed Basins
DEVELOPED DIRECT RUNOFF COMPUTATIONS
Intensity Flow
Overland Flow, Time of Concentration:
Intensity, I from Fig. RA-2
Rational Equation: Q = CiA (Equation 6-1 per MHFD)
Design
Point
Basin
Area
(acres)
Tc (Min) Runoff C
Tt = L / 60V (Equation 6-4 per MHFD)
Tc = Ti + Tt (Equation 6-2 per MHFD)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
(Equation 6-4 per MHFD)
(Equation 6-3 per MHFD)
}
𝑇𝑖 =
1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿
𝑆 ൗ
1
3
Page 3 of 3
Project Number:
Project Name:
Project Location:
Basin Notes
Historic Basin EX A 5.0 cfs
Historic Basin EX OS1 0.3 cfs
Historic Basin EX OS2 0.2 cfs
Total Q100 to the North 5.5 cfs
Proposed Basin A 4.0 cfs
Proposed Basin OS1 0.5 cfs
Proposed Basin OS2 0.2 cfs
Total Q100 to the North 4.7 cfs
Change in Q100 to the North 0.8 cfs Results in a decrease for the 100-year storm.
Basin Notes
Historic Basin EX B 1.9 cfs
Total Q100 to the South 1.9 cfs
Proposed Basin B1 0.9 cfs
Proposed Basin B3 0.5 cfs
Proposed Basin B4 0.2 cfs
Detention Pond Release Rate 0.3 cfs
Pond release rate determined by subtracting Basin B1, B3, and
B4 from Basin EX B.
Total Q100 to the South 1.9 cfs
Change in Q100 to the South 0.0 cfs No change for the 100-year storm.
Q100
Historic vs. Developed Flowrates to the North
Historic vs. Developed Flowrates to the South
HISTORIC VS. DEVELOPED FLOWRATES
1013-008
Homestead at Clarendon
Fort Collins, Colorado
Q100
Project:
Calc. By:
Date:
1.213 <-- INPUT from impervious calcs
50.00 <-- INPUT from impervious calcs
0.5000 <-- CALCULATED
12 hours <-- from MHFD Vol. 3 Table 3-2
0.8 <-- from MHFD Vol. 3 Table 3-2
0.165 <-- MHFD Vol. 3 Equation 3-1
0.017 <-- MHFD Vol. 3 Equation 3-3
727 <-- MHFD Vol. 3 Equation 3-3
** <-- INPUT from stage-storage table
** <-- CALCULATED from Figure EDB-3
dia (in) = ** <-- INPUT from Figure 5
n = ** <-- INPUT from Figure 5
t (in) = ** <-- INPUT from Figure 5
number of rows = ** <-- from WQ Depth and row spacing
August 5, 2020
WQCV (ft3) =
WATER QUALITY POND DESIGN CALCULATIONS
Primary Pond
Required Storage & Outlet Works
Homestead at Clarendon
F. Wegert
Circular Perforation Sizing
**To be completed at final design
Basin Area =
Basin Percent Imperviousness =
Basin Imperviousness Ratio =
WQCV (watershed inches) =
WQCV (ac-ft) =
WQ Depth (ft) =
Area Required Per Row, a (in2) =
Drain Time Coefficient =
Drain Time =
Project:
Date:
Bottom Elevation: 5088.6 364 ft
3
Overflow Elevation: 5089.6 316 ft
3
Crest of Pond Elev.: 5090.0 5089.70
Maximum
Elevation
Minimum
Elevation
cu. ft. acre ft cu. ft. acre ft
5088.6 N/A 0.0 0 0.0 0.00 0.0 0.00
5089.0 5088.6 358.8 0.4 71.8 0.00 71.8 0.00
5089.5 5089.0 431.8 0.5 197.7 0.00 269.4 0.01
5090.0 5089.5 509.8 0.5 235.4 0.01 504.8 0.01
Volume at Overflow:
Project Number:
Project Location:
Calculations By:
Pond No.:
West Rain Garden Stage Storage Curve
Contour Contour
Surface Area
(ft
2
)
Depth
Incremental Volume Cummalitive Volume
Pond Stage Storage Curve
1013-008
Fort Collins, Colorado
F. Wegert
West Rain Garden
Elev at Design Volume:
Homestead at Clarendon
8/5/2020
Pond Outlet and Volume Data
Design Volume:
8/4/2020 5:50 PM
P:\1013-008\Drainage\WatQual\
1013-008_Stage-Storage West.xlsx\Pond
Project: Homestead at Clarendon Calc. By:
Location: Fort Collins, Colorado Date:
42,675 sf
75%
32,006 sf
26,137 sf
7,897 sf
34,034 sf
79.8%
Total Treatment Area
Percent Total Project Area Treated
08/05/20
Onsite LID Treatment
F. Wegert
Vegetated Buffers
Total New Impervious Area
Project Summary
Target Treatment Percentage
Minimum Area to be Treated by LID measures
LID Treatment
Rain Gardens
VAULT
ELEC
E ELEC
E
VAULT
ELEC
CONTROL
IRR
CONTROL
IRR
CONTROL
IRCRONTROL
IRR
CONTROL
IRR
CONTROL
IRR CONTROL
IRR CONTROL
IRR
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Project:
Calc. By:
Date:
2.0% per City of Fort Collins SWCM
5.0% per City of Fort Collins SWCM
10.0% per City of Fort Collins SWCM
2.5%
8.3%
14 ft per City of Fort Collins SWCM
14 ft to 48 ft
25 ft
14 ft to 52 ft
42 ft
15,794 ft2 Entire Residential Area
24,698 ft2
1.56
Min. Allowable Cross-Slope
Max. Allowable Cross-Slope for LID
Max. Allowable Cross-Slope
VEGETATED BUFFER
Basin A
Design Data & Analysis
Vegetated Buffer Area
Run-on Ratio
Homestead at Clarendon
F. Wegert
August 5, 2020
Design Width (S <= 10.0%)
Average Design Width (S <= 10.0%)
Min. Design Cross-Slope (Smin)
Max. Design Cross-Slope (Smax)
Min. Allowable Width
Design Width (S <= 5.0%)
Average Design Width (S <= 5.0%)
Treatment Area
Note:
Existing coverage consists of well established native grasses with over 95% coverage. As much existing
vegetation and grading will be preserved as feasibly possible. Disturbed areas within vegetated buffer
will occur along rear lot lines, and this work is necessary to daylight lot grades into existing topography.
Any disturbed areas will be replaced with a native grass seed mixture and landscaping in accordance
with the erosion control and landscaping plans.
Average slopes are around 2.5% towards the south and east and gradually get steeper towards the north
and west. An existing irrigation ditch along the north edge of the grass buffer impacts the slopes along
the north edge of the grass buffer. The steeper slopes, greater than 5.0%, are generally along the
irrigation ditch.
Some minor grading will be required in the grass buffer to daylight proposed lawn grades into existing
grades. However, any disturbed areas within the grass buffer will be revegetated per the erosion control
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CLARENDON HILLS DRIVE
LANGDALE DRIVE
VEGETATED BUFFER
RAIN GARDEN
RAIN GARDEN
HOMESTEAD AT CLARENDON HILLS
SHEET NO:
P:\1013-008\DWG\DRNG\1013-008_LID.DWG
301 N. Howes Street, Suite 100
Fort Collins, Colorado 80521
E NGINEER ING
N O R T H E RN
PHONE: 970.221.4158
www.northernengineering.com
DRAWING REFERENCE:
LID TREATMENT EXHIBIT F. Wegert
1 in = 60 ft
August 5, 2020 LID-1
DRAWN BY:
SCALE:
ISSUED:
( IN FEET )
0
1 INCH = 60 FEET
60 60
LEGEND
RAIN GARDEN TREATMENT AREA
VEGETATED BUFFER
VEGETATED BUFFER
TREATMENT AREA
UNTREATED AREA
NOTES:
1. EXCEPT FOR LOTS 1 AND 8, UNTREATED AREA CONSISTS
OF LANDSCAPING AND LAWNS.
2. RAIN GARDEN TREATMENT AREA CONSISTS OF
APPROXIMATELY SOUTH 2
3 OF LOT, ROOFS, DRIVEWAYS,
AND SHARED DRIVEWAY FOR LOTS 2 - 7.
3. APPROXIMATELY NORTH 1
3 OF LOTS WILL DRAIN TOWARDS
VEGETATED BUFFER AREA BETWEEN REAR LOT LINES AND
IRRIGATION DITCH.
Block 23 / Morningstar LID Summary
Project Summary
Total New Impervious Area 42,675 sf
Target Treatment Percentage 75%
Minimum Area to be Treated by LID measures 32,006 sf
Rain Gardens
Total Rain Garden Treatment Area 26,137 sf
Vegetated Buffers
Total Vegetated Buffer Treatment Area 7,897 sf
Total Treatment Area 34,034 sf
Percent Total Project Area Treated 79.8%
os2 OS2 0.25 0.25 0.31 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 5.00 5.00 5.00
a North 0.31 0.31 0.39 60 92.89 89.01 6.47% 6.1 6.1 5.5 N/A N/A N/A 705 89.01 85.49 0.50% 0.11 110.86 14.25 14.25 116.38
b South 0.54 0.54 0.67 95 94.23 92.41 1.92% 8.3 8.3 6.3 350 92.41 84.93 2.14 2.92 2.00 N/A N/A N/A 10.27 10.27 8.30
Overland Flow, Time of Concentration:
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Channelized Flow Swale Flow
Design
Point
Basin(s)
Tt = L / 60V (Equation 6-4 per MHFD)
Tc = Ti + Tt (Equation 6-2 per MHFD)
Overland Flow Time of Concentration
Homestead at Clarendon
F. Wegert
August 5, 2020
Gutter/Swale Flow, Time of Concentration:
Combined and Routed Basins
Intensity, i (per Table 3.4-1 of the Fort Collins Stormwater Manual)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
Rational Equation: Q = CiA (Equation 6-1 per MHFD)
(Equation 6-4 per MHFD)
(Equation 3.3-2 per Fort Collins
Stormwater Manual)
}
𝑇𝑖 =
1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿
𝑆 ൗ
1
3
Page 2 of 3
Lawns, Clayey Soil > 7%: C = 0.35, Percent Impervious = 2%
Combined and Routed Basins
2) Composite Runoff Coefficient adjusted per Table 3.2-3 of the Fort Collins
Stormwater Manual (FCSM).
Lawns, Clayey Soil, Avg Slope 2-7%
Open Space (undeveloped): C = 0.25, Percent Impervious = 2%
USDA SOIL TYPE: B
Page 1 of 3
Combined and Routed Basins
Intensity, i (per Table 3.4-1 of the Fort Collins Stormwater Manual)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
Rational Equation: Q = CiA (Equation 6-1 per MHFD)
Overland Flow, Time of Concentration:
HISTORIC TIME OF CONCENTRATION COMPUTATIONS
Channelized Flow Swale Flow
Design
Point
Basin(s)
Tt = L / 60V (Equation 6-4 per MHFD)
Tc = Ti + Tt (Equation 6-2 per MHFD)
Overland Flow Time of Concentration
Homestead at Clarendon
F. Wegert
August 5, 2020
(Equation 6-4 per MHFD)
(Equation 3.3-2 per Fort Collins
Stormwater Manual)
}
𝑇𝑖 =
1.87 1.1 − 𝐶 ∗ 𝐶𝑓 𝐿
𝑆 ൗ
1
3
Page 2 of 3