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47 Spire Ridge Way
Engineering Report and Drainage Analysis
July 19,2022
Existing Conditions
The property described as 47 Spire Ridge Way is located in Garfield County just outside of
Carbondale, Colorado. The site is designated as Property lD #239513405002 and is Lot H8 of
Filing 6 of the Aspen Glen Subdivision. The site borders a developed lot to the north with an
undeveloped lot to the south. The western side of the lot borders a parcel that is utilized by the
golf club. The east of the property borders a cul-de-sac on the Spire Ridge Way Right-Of-Way,
which serves as the main access to the property.
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Site
Aspen Glen
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Shown above is an image of the Garfield County GIS Map
displaying the location of 47 Spire Ridge Way
The 0.55-acre lot is undeveloped, with a general grade sloping to the east at around 5-10 percent.
Communications, telephone, and electric pedestals sit in the 10' wide utility easement on the east
border of the property. The subdivision has community water, and an existing water service with
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Crystal River Civil LLC 970.51_0.5312 Page 1 of 5
a curbstop that has already been installed and is located just west of the road. A sewer stub is
also located on the site within the utility easement, along with an electric transformer, gas, and
commu n ications pedestals.
Proposed Gonditions
A residence is proposed to be constructed in the center of the lot. The garage is located on the
northeast side of the building, with an auto court to the south. On the west side of the residence
are several patios and walkways, which transition into landscaping and into existing grade. A
series of pipes, inlets, and downspouts collect the runoff from the development and conveys the
onsite stormwater to a drywell. The proposed drywell sits just east of the residence below the
autocourt. lt is sized to release predeveloped flow rates and mitigates any stormwater increases
due to proposed impervious areas,
Access
The proposed driveway enters the property off Spire Ridge Way on the east side of the lot. The
12-foot-wide concrete driveway connects into the existing rollover curb and gutter of the cul-de-
sac and approaches the residence to the west. As the driveway nears the residence it widens into
an auto court in front of the attached garage, with additional parking spaces to the south and
adequate space for vehicles to turn around.
Utilities
Water The proposed residence will be tied into the existing community water line that runs under
Spire Ridge Way. An existing curb stop is located on the site and will be utilized for the water
service. The water service will run west from the curb stop and tie into the side of the proposed
residence.
Wastewate,n A sewer service stub is located on the site for development use. This will need to be
verified and located for construction and will be utilized for this residence.
Communications: The existing communication pedestal is located on the eastern side of the
property. The proposed conduit runs west from the pedestal and ties into the southern side of the
residence.
Electric: The existing electric transformer is located on the eastern side of the property. The
proposed conduit runs west from the transformer and ties into to eastern side of the residence.
Drainage Analysis
Multiple inlets, drains, and swales are proposed to collect runoff from the impervious and pervious
areas, as well as areas that slope towards the structure. The entirety of the residence will be
collected by this drainage system as all downspouts are collected. The storm system consists of
a pipe network that conveys the runoff to a drywell located east of the residence. This detention
Crystal Rìver Cir¡il LLC 970.5 10.53 1 2 Page 2 of 5
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drywell has been sized to release through an overflow at the predeveloped flow rate of a25-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 2.1Írom 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
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 basin, which has been
labeled as Basin 1.
The onsite drainage conveyance structures proposed with this project have been designed to
have capacity for a 1-hour 1OO-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 from Crystal River Civil.
Crir5¡¡¡ Rir¡et Civil LLC 970.510 5312
. 88.8Pr
¡ = dı;ïJrıø O"ClArIB
Note: For bas¡ns with a flow length of less than 500 feet, a Time of Concentration is assumed at 5 minutes. These calculat¡ons are assuming a NRCS Hydrologic Soil Class B.
Ra¡nfall depth values der¡ved from NOAq Atlas 14 data. lntensity equatìon has been der¡ved from the Aspen area NOAAAtlas 14 IDF Curve.
Cond¡t¡ons
0.00%0.240 514 0.27'l 0
Conditions
5.14 0.7119521.00 6173.00 u.u%0 630
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Page 3 of 5
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Aspen Glen subdivision requires stormwater runoff from a development site to not to impact
neighboring properties or 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 increased 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 of Basin
1 entering the system and volume leaving the system.
To meet this required storage, the storm system utilizes a drywell with an overflow that releases
into existing landscaping to the north. The proposed 230 cubic feet of storage within the drywell
meets the requirement for the historic release of a 1-hour 25-year event, and an 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 1-9116 inch orifice plate is to be installed on the daylighting overflow pipe,
where it can easily be accessed for maintenance.
{-rystal Rivr:r Civil L LC Page 4 of 5
1 25-vear 1-Hour
1 0.63
5 9521.00
0.27 10
88.8P.t--' (10 + 1o¡r'osz v, =frcrrua
vo = 3o(1*frrutn V-ViVo
Analysis Uses the Federal Aviation Association Stormwater [¿anagement lVlethod. By Calculating the Difference in the Volumes, the
V
Required for Detention is Determined.
l0 3.80 316.53 121.39 195.14
11 361 330.77 129.48 201.28
12 3.44 343.60 137.58 206.03
13 3.28 355.23 145.67 209.56
3.14 365.80 153.76 212.0414
15 3.00 375.46 161.85 213.60
16 2.88 384.30 1 69.95 214.35
17 2.77 392.43 178.04 214.39
18 2.67 399.91 1 86.1 3 213.78
'19 2.57 406.83 194.22 212.61
214.39
v = hrd),u"=r¡1"t$1!r-"q5,1
dtn
5 I 4 176.71 54.1 9
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970 510 5312
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0.62
32.2
6061^93
6060.58
1.35
Note: This calculation uses the Bernoulli Equation to determine the historic, pre-
atd outlet.
eo= rffi'ft n,: lt++f, incrres
0.27 0.05 1.57
For location, sizing, and all information regarding the stormwater design please see the civil
grading and drainage plan. lf there are any comments regard¡ng the stormwater analysis
performed for the site, feelfree to contact me directly.
Jay Engstrom
Principal, P.E.
Crystal River Civil LLC
Jav@Crysta I RiverCivi l. com
(970) 404 - 1144
Crystal Rirrer Civil LLC 970.5 10.53 12 Page 5 of 5