HomeMy WebLinkAboutDrainage Report435 Brookie
Drainage Report
March 26,2024
Existing Conditions
The property described as 435 Brookie is located in Garfield County outside of Carbondale,
Colorado, The site is designated as Lot 10 of the Roaring Fork Mesa at Aspen Glen. The site
borders lots with developed residences on the north, west, and south. To the east of the property
is Brookie Right-Of-Way.
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Shown above is an image of the Garfield County GIS Map
displaying the location of 435 Brookie
The lot is currently undeveloped, with a general grade of the lot sloping to the east at around 5-
10 percent. Brookie provides access to the site, while also servicing other residences to the cul-
de-sac to the south. An electric transformer is located in the northeast corner of the property. Two
communications pedestal are located on the site, with one at the southeast corner and the other
at the northeast corner. lt is assumed that an existing water service stub-out has already been
installed and is located along the eastern property line. lt is also assumed that an existing sewer
service tee is in the northeast corner of the property. Both of these tie-ins will require location and
depth verification prior to construction.
Crysial River Civil l,l.C Page iL of l;
Proposed Conditions
The proposed structure is placed in the center of the lot and consists of a two-story residence and
an attached garage. A concrete driveway will enter the property off Brookie on the eastern
property line and approach the garage to the west. A proposed paver walkway exits off the
driveway and runs north to the entryway patio. On the western side of the residence is a concrete
patio, a crusher fines patio, and a landscaping planter. The patio areas are surrounded by a
concrete retaining wall to transition into the existing slopes. A stairway runs further west from the
crusher fines patio up to a lawn area above the planter.
The stormwater runoff created by proposed conditions will be collected by inlets, downspouts,
and french drains, which all convey the stormwater to a drywell located south of the driveway. A
trench drain collects the runoff from the driveway along the property line. All roof areas are
collected by downspouts, which will tie into the proposed pipe network to be conveyed to the
drywell.
Drainage Analysis
Multiple inlets, drains, downspouts, and french drains are proposed to collect runoff from the
proposed impervious and pervious areas, as well as areas that slope towards the structure. The
entirety of the proposed residence will be conveyed to the proposed drywell via a pipe network.
All impervious areas are collected by this system. The drywell has been sized to release through
an overflow at the predeveloped flow rate of a 25-year 1-hour storm event.
This drainage analysis uses procedures that have been standardized by the Mile High Flood
District's Urban Storm Drainage Criteria Manual, or the USDCM. Peak flows of the site have been
calculated using the Rational Method to determine the impacts the proposed project has on the
existing conditions, along with verifying the capacity of the proposed infrastructure. This method
uses the analyzed basin's area, soil type (assumed soil type B), percentage of imperviousness,
time of concentration, and the historic storm data from NOAA Atlas 14 to determine the maximum
flow rate from a specified storm event. The Rational Method uses the equation below to determine
the Maximum Flow Rate:
Q=ClA
Where,
- O is the peak stormwater runoff rate, in cubic feet per second
- C is a runoff coefficient of the drainage area
- I is the storm intensity, in inches per hour
- A is the area of the drainage basin, in acres
The Runoff Coefficient, C, is determined using table 6-5 from the USDCM. To determine the storm
intensity, l, Equation 2.1from the City of Aspen's Urban Runoff Management Plan is being utilized,
as it is a neighboring jurisdiction that has more similar historical data in comparison to the
USDCM. 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
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Rational Method, the results tend to be inaccurate, as per the USDCM. The rainfall event used
for this analysis is for a 1-hour 25-year storm event. Below is a table summarizing the calculations
to determine the predeveloped and developed conditions for the onsite basins.
The onsite drainage conveyance structures proposed with this project have been designed to
have capacity for a 1-hour 10O-year storm event. The basins have 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 from Crystal River Civil.
Garfield County requires stormwater runoff from a development site to not exceed more than the
historic runoff. For this design, CRC is proposing using historic data from a 1-hour 25-year event.
Detention is necessary to offset the increased flows from proposed impervious areas to be
released at the historic rate. An analysis has been performed using the FAA Method to determine
this necessary detention volume. See table below displaying the calculations for the maximum
difference between the stormwater volume entering the system and volume leaving the system
for Basin 1.
0.992 B
88.8P.l:+' (10 + Td)' 052 Q=ClAt
Note: For basins with a flow length of less than 500 feet, a Time of Concentration is assumed at 5 minutes. These calculations are assuming a NRCS Hydrologic Soil Class B.
Rainlall depth values derived from NOAA Atlas 14 data. lntensity equation has been derived lrom the Aspen area NOAA Atlas 14 IDF Curue.
6778 0 0.00%0.240 5 5.10 0.'19
Condltions
6778.00 3871.00 57.110k 0.600 5 5.10 0.48
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Crystal River Civil LLC 974.5r0.5312 Page 3 of 5
1 25-lear 1-Hour
0.992 0.6
5 6778.00
100.19
q =ufocrroe
OO.Oril--' (10 + To;r'osz
Vo = 3o(1+fttUra V=ViVo
Analysis Uses the Federal Aviation Association Stormwater Management lVethod. By Calculating the Difference in the Volumes, the
Required for Detention is Determined.
10 3.77 212.89 85.73 127.17
11 3.58 222.47 91.44 131.02
12 3.41 231.10 97.16 133.94
13 3.25 238.92 102.87 136.05
3.11 246.03 108.59 137.4414
15 2.98 252.52 114.30 138.22
16 2.86 258.47 120.02 138.45
2.75 263.94 125.73 138.2017
18 2.65 268.97 131.45 137.53
2.55 273.63 137.16 136.4619
138.45
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To meet this required storage, the storm system will convey runoff to the drywell, with an overflow
that releases along the eastern property line. The proposed 145 cubic feet of storage meets the
requirement of a 1-hour 25-year event, and the overflow will release stormwater at a historic flow
rate. Sizing for the drywell is shown in the table below.
To ensure that the drywell overflow is meeting the predeveloped flow rate, the Bernoulli Equation
was used to determine the size of hole to drill in the orifice plate. Below is a summary of the
calculation used. This 1.32-inch orifice plate is to be installed in the drywell at the overflow pipe,
where it can easily be accessed for maintenance.
d-\=tug).u":t'4rdaE, ),*nfr5,l
/Nam6)0,lamI d hm
I 4 100.53 M.77 145,30 lS-,46
Crystal River Civil LLC 974.5K).5312 Page 4 of 5
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Note: This calculation uses the Bernoulli Equation to determine the historic, pre-
flow rate
For location, sizing, and all information regarding the stormwater design please see the civil
grading and drainage plan. lf there are any comments regarding the stormwater analysis
performed for the site, feel free to contact me directly.
Jay Engstrom
Principal, P.E.
Crystal River Civil LLC
Jav@Crvstal l.com
(970) 404 - 1144
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Crystal River Civil LLC 970.510.53L2 Page 5 of 5