HomeMy WebLinkAboutFile Documents.506 W Hallam St.0019.2019 (7).ARBK MINOR DRAINAGE REPORT
FOR
506 WEST HALLAM STREET
CITY OF ASPEN, COLORADO
PARCEL ID: 2735-124-25-007
PREPARED FOR:
HS Building Group LLC
314 Suite B, Aspen Airport Business Center
Aspen, Colorado 81611
PREPARED BY:
High Country Engineering, Inc.
1517 Blake Avenue, Suite 101
Glenwood Springs, CO 81601
(970) 945-8676
November 2, 2018
HCE JOB NUMBER: 2181039.00
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29975
RECEIVED
01/28/2019
ASPEN
BUILDING DEPARTMENT
I. EXISTING CONDITIONS
A. Overall
The site is located at 506 West Hallam in the City of Aspen, Colorado. The property has an
existing residence, covered carports at the northern end of the site and a concrete walkway that
runs south from the front of the residence to West Hallam Street. Landscaped areas include grass
lawns and trees surrounded by mulch. Existing topography contain of approximately 1%-7%
slopes. The gravel carport area of the site currently drains north away from the building as sheet
flow and into the gravel alley. From there, drainage travels east down the alley and into the
North 4th Street. Then the runoff travels north along the west side of North 4th Street until
entering an inlet in the northwest corner of the intersection at West Francis Street and North
Fourth Street. The NRCS soil map for Aspen located in the URMP lists the hydrologic soil group
for the site as type B.
II. PROPOSED CONDITIONS
A. Overall
The proposed project consists of replacing the current gravel around the garage with a heated
concrete surface, and replacing the existing concrete walkway located at the south end of the site
with heated concrete. Drainage improvements are proposed for identified areas.
B. Basin Delineation
The proposed project consists of two delineated basins for the purpose of calculating proposed
detention for water quality. The basins are listed by the proposed construction in those areas,
which include the driveway area, front walkway and R.O.W. walkway.
The Driveway Area basin includes a concrete apron north of the garage and the concrete parking
area west of the garage. Drainage in this basin sheet flows to the north into a proposed trench
drain located just inside the north property line. The runoff is then discharged into the WQCV
drywell located beneath the concrete parking area. The total impervious area has been calculated
as 738.6 SF and 100% impervious, which requires 16.0 CF of WQCV and a drywell WQCV of
24.0 CF.
The Front Walkway basin and R.O.W. Walkway basin are connected by a continuous walkway
that runs from the front door of the residence to West Hallam Street. Drainage from the Front
Walkway basin drains south as sheet flow to an existing low point, and the runoff is captured by
two area inlets approximately 17 feet north of the south property line. The R.O.W. Walkway
basin sheet flows north into the same previously inlets. Once the runoff is captured by the inlets,
stormwater is conveyed into a perforated pipe that runs parallel to the sidewalk. The runoff is
evenly distributed in the perforated pipe and flows through the perforations and 18 inches of
gravel beneath the pipe. WQCV will be stored in the gravel voids as stormwater infiltrates the
underlying soils. The total impervious area for both basins has been calculated as 383.1 SF and
100% impervious, which requires a total of 8.3 CF of WQCV and a drain tile WQCV of 16.6 CF.
RECEIVED
01/28/2019
ASPEN
BUILDING DEPARTMENT
See Table 1 for WQCV calculations.
III. DRAINAGE DESIGN
A. Drainage Issues
The drainage design follows the existing drainage patterns. The walkway areas currently drain to
a low point within the sidewalk without a specific outlet. As for the northern area of the site, a
portion of the gravel alley currently drains onto a small portion the site before continuing north
and east down the alley.
B. Drainage Alterations
A small beveled edge will be installed at the north edge of the existing walkway to prevent the
portion of alley drainage from entering the site to continue down the alley. A trench drain will be
installed just south of the beveled edge that with capture all of the driveway area runoff that will
be conveyed to the proposed drywell for water quality. Due to minimum size requirement in the
URMP, the drywell has the capability to detain a WQCV more than required. If the drywell
overflows, stormwater will backflow out of the trench drain and into the alley, thus beginning to
follow the historic flow path into North 4th Street.
The low point in the front walkway will be replaced by two area inlets that will capture
stormwater to be conveyed into a perforated pipe and gravel detention beneath the walkway.
Runoff will flow through the gravel voids and infiltrate the soils beneath. The gravel reservoir
beneath the walkway has the capability to detain 53.2 CF of stormwater within 30% gravel
voids. If the gravel detention overflows, stormwater will back up out of the area inlets and pond
until sheet flowing east into the existing landscape, as it does historically.
Table 1: WQCV Calculations
IMPERVIOUS AREA PERCENT WQCV(Watershed
_ BASIN AREA(S.F.) AREA(ACRE) (SF) IMPERVIOUS inches) WQCV(CF)
Driveway Area 738.6 0.017 738.6 100.0 0.260 16.0
Front Walkway 284.7 0.007 284.7 100.0 0.260 6.2
R.O.W.Walkway 98.4 0.002 98.4 100.0 0.260 2.1
Driveway Drywell 24.0
Walkway Gravel 16.6
RECEIVED
01/28/2019
ASPEN
BUILDING DEPARTMENT