HomeMy WebLinkAboutEngineer's Drainage Memo 07.09.2020hd
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255 Maroon Drive Drainage Memo
07/09/2020
Existing Conditions:
The undeveloped property described as 255 Maroon Drive is located within Garfield County just
south of Glenwood Springs, CO. The site, Property ID #218527416035, is in the northwest
corner of the Four Mile Ranch Subdivision, at the top of a ridge and at the highest point of the
neighborhood. There are no slopes draining onto the existing site. To the north and south are
developed lots, with the Right -Of -Way to the southwest. A City of Glenwood water tank is
located to the northern corner of the property within an easement. Open space, trail, and utility
easements are located on the lot to the west nearest the Right -Of -Way. Access and drainage
easements follow the northern and southern property lines. The site generally slopes to the
southwest towards the right of way, excluding the slopes that grade to the north and south to
accommodate existing driveways. The driveways are located within the access easements and
serve the existing residences with steep grades (over 10 percent) up to the homes.
Proposed Conditions:
A residence is proposed on the site that is shown in the civil set. It has a 2900 square foot
footprint and includes a 3-car garage in the northern portion. A deck is located to the south of
the structure. The site would be accessed using the existing driveway to the south, which is to
be shared with the neighbor. Drainage from the structure will be collected in a series of inlets,
downspouts, and pipes, which convey the runoff to the roadside swale along the driveway. The
driveway is pitched towards the swale, so the driveway will be collected in the system as well.
From there the runoff will flow offsite into the HOA Right -Of -Way and into the subdivision's
drainage infrastructure. The HOA has designated detention ponds for the developed lots to
utilize. All existing and currently being constructed sites are using the community ponds for their
stormwater. No offsite basins exist as the site is at the high point of a ridge.
Drainage Analysis:
Peak flows of the site were calculated using the Rational Method to determine the impact of the
proposed project on the existing conditions and verify capacity of the proposed infrastructure.
This method uses the analyzed basin's area, soil type, percentage of imperviousness, time of
concentration, and the historic storm data to determine the maximum flow rate from a specified
storm event. For smaller basins with a time of concentration of less than five minutes, a time of
concentration of five minutes is assumed — when smaller times of concentration are used in the
Rational Method the results tend to be inaccurate. The one -hour rainfall events used for this
analysis are for a 25-year and a 100-year storm event. The rainfall depths (Pi) are historical
data collected by the NOAA historic storm database and are 1.05 inches for a one -hour 25-year
and 1.3 inches for a 1-hour 100-year storm event. Below is a table summarizing the calculations
to determine the predeveloped and developed conditions for both events.
Birdhouse Designs LLC (970) 404-1144
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Designs
100 Year 1 Hour Peak Discharge Calculations
Rainfall depth (P1)
1.3
Note: For basins with a flow length of less than 500 feet, a Time of Concentration is assumed at 5 minute
These Calculations are also assuming a NRCS Hydrologic Soil Class B.
Return Period (years)
100
Predeveloped Conditions
Basin #
Total Area
Impervious Area
Percent Impervious
C Value
Time of Concentration
Intensity
Peak Discharge
(ft2)
(ft2)
(%)
From Table
(Td)
1=88.8P11(10+Td)1 o5z
QMax (ft3lsec)
1
7742.00
0.00
0.00 %
0.350
5
6.69
0.42
Developed Conditions
Basin #
Total Area
Impervious Area
Percent Impervious
C Value
Time of Concentration
Intensity
Peak Discharge
(ft)
(ft)
(%)
From Table
(Td)
1=88.8P1/(10+Td)1052
QM (ft3/sec)
1
7742.00
6762.00
87.34%
0.750
5
6.69
0.89
25 Year 1 Hour Peak Discharge Calculations
Rainfall depth (P1)
1.05
Note: For basins with a flow length of less than 500 feet, a Time of Concentration is assumed at 5 minutes.
These Calculations are also assuming a NRCS Hydrologic Soil Class B.
Return Period (Years)
25
Predeveloped Conditions
Basin #
Total Area
Impervious Area
Percent Impervious
C Value
Time of Concentration
Intensity
Peak Discharge
A (ft)
/{ (ft2)
/j/At (%)
From Table
(Td)
I=88.8P1/(10+Td)1 052
QMa% (ft3/sec)
1
7742.00
0.00
0.00 %
0.370
5
5.40
0.36
Developed Conditions
Basin #
Total Area
Impervious Area
Percent Impervious
C Value
Time of Concentration
Intensity
Peak Discharge
(ft2)
/{ (2)
P,/A, (%)
From Table
(Td)
1=88.8P1/(10+Td)1 052
QMax (ft3/sec)
1
7742.00
6762.00
87.34%
0.749
5
5.40
0.72
The onsite dra'nage conveyance structures proposed with this project have been designed to
have capacity for a 1-hour 100-year storm event. The basin has been divided into sub -basins,
which were analyzed for each inlet and pipe. Spreadsheets showing these calculations and
supplemental information can be provided upon request to Birdhouse Designs.
The Four Mile Ranch Homeowner's Association requests that the stormwater is released into
the Subdivision's detention ponds. This is to minimize disturbance on the site and reduce visible
infrastructure. Although no proposed detention is being sized, Birdhouse Designs has run an
analysis showing how much capacity is needed in the community detention pond for the
proposed residence, assuming a 1-hour 25-year event to meet Garfield County requirements.
Below is a spreadsheet displaying the maximum detention required using the Modified FAA
Method.
Birdhouse Designs LLC (970) 404-1144
nouse
Designs
25 Year Storage Calculations- Basin 1
C
0.749
A
7742.00
ft2
Tc
5
Minutes
Oa
0.355
ft3
P1 (100)
1.05
Rainfall Duration (minutes)
Intensity (inches/Hour)
Volume In ft3
Volume Out ft3
Volume Difference ft3
(Td)
I=88.8P1/(10+Td)1 °52
Vi=(1/720)*C*I*Td*A
Vo 30(1+Tc/Td)*Qa*T
Vd=V;-Vo
5
5.40
217.41 106.52
110.89
7
4.73
266.83 127.83
139.00
9
4.21
305.18 149.13
156.05
11
3.79
335.73 170.44
165.29
13
3.44
360.56 191.74
168.81
15
3.15
381.09 213.05
168.04
17
2.91
398.31 234.35
163.96
19
2.70
412.93 255.65
157.28
21
2.52
425.47 276.96
148.52
23
2.36
436.33 298.26
138.07
25
2.21
445.81 319.57
126.24
27
2.09
454.13 340.87
113.26
29
1.98
461.49 362.18
99.32
31
1.87
468.04 383.48
84.55
33
1.78
473.88 404.79
69.10
35
1.70
479.13 426.09
53.04
Maximum Difference
168.81
If there are any comments regarding the stormwater analysis performed for the site, feel free to
contact me directly.
Jay Engstrom
Owner, P.E.
Birdhouse Designs LLC
Jay@birdhouse-designs.com
(970) 404 - 1144
Birdhouse Designs LLC (970) 404-1144