HomeMy WebLinkAboutFile Documents.203 E Hallam St.0174.2018 (19).ARBK1
Drainage Report
203 EAST HALLAM ST.
ASPEN, CO
August 16, 2018
Prepared by
Richard Goulding, P.E.
Roaring Fork Engineering
592 Highway 133
Carbondale, CO 81623
1/7/2019
Reviewed by Engineering
02/12/2019 12:23:35 PM
"It should be known that this review shall not
relieve the applicant of their responsibility to
comply with the requirements of the City of
Aspen. The review and approval by the City is
offered only to assist the applicant's
understanding of the applicable Engineering
requirements." The issuance of a permit based
on construction documents and other data shall
not prevent the City of Aspen from requiring the
correction of errors in the construction
documents and other data.
2
Drainage Report
203 EAST HALLAM ST.
ASPEN, CO
I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 203 E. HALLAM ST. WAS
PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE
CITY OF ASPEN URBAN RUNOFF MANAGEMENT PLAN AND APPROVED VARIANCES AND
EXCEPTIONS LISTED THERETO. I UNDERSTAND THAT IT IS THE POLICY OF THE CITY OF ASPEN
THAT THE CITY OF ASPEN DOES NOT AND WILL NOT ASSUME LIABILITY FOR DRAINAGE FACILITIES
DESIGNED BY OTHERS.
RICHARD GOULDING, P.E.
RFE Project # 2018-40
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Table of Contents
1.0 General .................................................................................................................................................... 4
1.1 Existing Site ......................................................................................................................................... 4
1.2 Proposed Conditions ........................................................................................................................... 4
2.0 Drainage Basins and Sub‐basins .............................................................................................................. 5
2.1 Drainage Basins and Storage ............................................................................................................... 5
3.0 Peak Flows ............................................................................................................................................... 6
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1.0 General
1.1 Existing Site
The following report is an evaluation of the property at 203 E. Hallam St. This report highlights the
changes to the property since the date of the previously submitted drainage report of July 14, 2015.
Since this date, the property has changed ownership. This report discusses the changes to the
landscaped areas of the residence and the addition of a rooftop deck.
The property is located in Northern Aspen, or the “West End”, on the southeast corner of E. Hallam St.
and N. Aspen St. The parcel is a 6,000 square foot lot with an existing 3,291 square foot single family
residence. The property is accessed from the 20‐foot alley right of way on the south side of the
residence. The property is relatively flat, with a gentle slope to the northwest. A large impervious patio
is located on the northwest side of the residence. Electrical, phone, television cable, and gas are located
in the alleyway, while water and sewer are located in the E. Hallam St. right of way.
A Geotechnical Report was completed on January 8, 2015, by HP Geotech, which identified in soil
consisting of gravel and cobble. A percolation test was also conducted on January 6, 2015 with results
varying from 0.3 minute per inch (MPI) to a max percolation rate of 0.5 MPI. This maximum reading was
taken after 42 minutes.
1.2 Proposed Conditions
The original project, completed in 2015, was classified as a ‘Major Project’ as per Table 1.1 of the City of
Aspen’s (COA) Urban Runoff Management Plan (URMP). This classification was the result of the
proposed development exceeding 1000 square feet (sf) and disturbing an area of approximately 5,000
sf, which is over 50% of the site. The current project consists of changes to the landscape plan and as
such is only classified as a Minor Project per comments provided via email from the City of Aspen on July
20, 2018. The intent of this report is to demonstrate compliance with the requirements of the URMP.
The Low Impact Design (LID) Principles in the introduction of the manual were used as a guide
throughout the design process.
The proposed development consists of changes to the patio areas on the southwest and eastern sides of
the property, and the addition of a rooftop deck. The existing patio/lawn area on the southwest side of
the residence will be demolished and a Dasso Deck will be installed. This area will include a firepit
surrounded by seating along with a platform for a landscape feature. Access panels to the existing
drywells have been incorporated into the design of this side of the residence. All existing trees will be
salvaged and replanted on the property. New concrete stepping stones will be installed to provide a
walkway from the south side of the property to the sidewalk on the western side.
On the eastern side of the property, the existing hardscape will be demolished and replaced. A new hot
tub, and raised planter boxes will also be installed. Also, on the east side of the residence, the existing
softscape will be removed and replaced with a dog run made of artificial turf. Documentation for the
dog run product (K9 Grass) can be found in the Landscape plans.
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Finally, a rooftop deck will be installed on the existing roof. The deck will be accessed through a staircase
and a hatch as shown on the Architectural drawings. The footprint of the deck is over existing structure
so the amount of impervious area is not increased. Other efforts have been made to break up
continuous impervious area by using spaced pavers for walkways the allow for infiltration between
them. The new decking and dog run turf is also pervious.
2.0 Drainage Basins and Sub‐basins
The site was divided into two major drainage basins. A Drainage Exhibit is provided within the Civil Permit
Set. The sub‐basins were created to calculate the concentrated flow from each impervious area, including
patios, decks and roofs. The impervious area of each basin was used to test the existing infrastructure for
capacity.
2.1 Drainage Basins and Storage
Basin 1 is 3,829 square feet (sf), 100% impervious, and consists the entire roof area, the north and west
patios, and the western walkways. Runoff from this basin will be collected and conveyed to a drywell,
which has capacity for the Total Detention of this basin. The Total Detention is calculated below.
The drywell has adequate capacity. Calculations are shown below.
Basin 2 is 260 sf and is 100% impervious. It is located on the eastern side of the residence and consists of
the new hot tub, dog run and planters. A new trash enclosure is also proposed. The runoff from Basin 2
is diverted north and into a level spreader. This level spreader is a nine‐foot‐long perforated 4” diameter
PVC pipe bedded in screened rock. This level spreader disperses runoff into the lawn and eventually into
the curb and gutter of East Hallam St. if the flows are to become large enough. The level spreader is
existing and was approved by the COA during the Certificate of Occupancy process of the previous
development of this property. The Water Quality Capture Volume (WQVC) calculation for this basin is
shown below.
Full Detention Storage
Basin Total Area Impervious Area Impervious Full Detention Depth Factor of Safety Required Storage BMP
(ft2)(ft2) (%) (in) F.O.S. (ft
3)
1 3829.00 3829.00 100.00% 1.23 1 392
Exiting Drywell
Drywell Storage
Drywell Basins Diameter Storage Depth Perforated Depth Internal Volume External (18" of Screened Rock) Volume Total Capacity Required Capacity
(Name) (#) D (ft) H (ft) P (ft) π*H*(D/2)
2) (ft3) 0.3*π*P*((D/2)+1.5)
2 ‐ (D/2)2) (ft3)(ft3)(ft3)
Existing Drywell 1 6 10.7 4 303 113 416 392
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For the previous building permit this level spreader was said to handle a 981‐sf tributary basin that was
46.92% impervious. The level spreader was installed per the approved design with the perforated pipe
being backfilled with screened rock. The 41.9 cf screened rock bed is 9 feet long 16 inches wide and 3.5
feet deep. This surrounding screened rock provides 12.6 cubic feet of storage. There is also 10 feet from
the level spreader to the edge of the property.
3.0 Peak Flows
The FAA Rational Method was used to calculate the peak flows for each basin for the 100‐year storm
event. The below equation was derived by the City of Aspen to estimate the Intensity for the Aspen
area.
I = 88.8P1/(10+Td )1.052
Td = Time of Concentration (minutes)
P1= 1‐hour Rainfall Depth (in)
I= Rainfall intensity (inches per hour)
Actual Td on the site is significantly less than 5 minutes and equations used to calculate rainfall intensity
are only valid for a Time of Concentration of greater than 5 minutes therefore, the smallest valid Time of
Concentration (5min.) value was used. The 1‐hour Rainfall depth (P1) was determined from NOAA Atlas
14 Volume 8 as 1.23 inches for the 100‐year event.
The soil and surface type along with the square footage was then used to calculate the peak flow using
the following equation.
Qp= Peak Discharge (cfs)
C= Runoff Coefficient (Unitless)
I= Rainfall intensity (inches per hour)
A= Area (Acres)
Peak flow values were used to test the capacity of the proposed conveyance structures, such as inlets
and piping. The tables below contain the peak flows for developed and undeveloped conditions for a
100‐year storm.
WQCV
WQCV
Required
Basin Total Area Impervious Area Impervious WQCV Tbl. Val. Volume BMP
(ft2)(ft2) (%) (in) (ft
3)
2 260.00 260.00 100.00% 0.255 5.5
Existing Level Spreader
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The conveyance system’s capacity has not been reduced from the existing conditions previously
approved for the development. The total 100‐year peak flows of the site have not increased as a result
of these landscape and grading improvements.
100 Year Peak Discharge Developed Calculations
1 Hour(P 1)1.23
Return Period 100
Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft
2)(ft2)(%)From Table (Td) I=88.8P 1/(10+Td)1.052 (ft3/sec)
1 3829.00 3829.00 100.00% 0.950 5 6.33 0.528
2 260.00 260.00 100.00% 0.950 5 6.33 0.036
100 Year Peak Discharge Pre Development Calculations
1 Hour(P 1)1.23
Return Period 100
Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft
2)(ft2)(%)From Table (Td) I=88.8P 1/(10+Td)1.052 (ft3/sec)
1 3829.00 0.00 0.00% 0.350 5 6.33 0.195
2 260.00 0.00 0.00% 0.350 5 6.33 0.013
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