HomeMy WebLinkAboutFile Documents.1230 Snowbunny Ln.0001.2020 (10).BCHO
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GLENWOOD SPRINGS 118 West Sixth St, Suite 200 | Glenwood Springs, CO 81601 | 970.945.1004
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February 4, 2020
Mr. Steve Novy
Green Line Architects
65 North 4th Street, Suite 5
Carbondale, Colorado, 81623
RE: 1230 Snowbunny Lane, Parcel 273512207004- Drainage Memo for Replacing Drywell
Dear Steve:
This report addresses Civil Engineering aspects of the proposed development of the Valentine Residence
along Snowbunny Lane in the City of Aspen and is intended to meet the review requirements for the City
of Aspen Engineering Department. The applicant is seeking an addendum to their change order approval
with a minor adjustment for the replacement and construction of a drywell that was thought to be in working
order. An addendum drawing showing the location of the replaced drywell is attached.
Site Drainage
In terms of overall drainage patterns, the site slopes generally from southwest to northeast. Due to the
location of the house, the drainage patterns have been split into two major catchments, east of the house
and that which is west of the house. Detention requirements are being handled by soil berms at the low
spots of the east and west sides of the house. Water quality is being treated by a combination of
permeable pavers, tree credits, and grass buffers. An existing drywell in the SW basin was being utilized
for water quality treatment. This drywell was being fed by a trench drain at the bottom of the driveway at
the garage interface. Additionally, a portion of the roof runoff in the SW basin is directed to the drywell.
Upon further investigation, the existing drywell was found to be inadequate and inoperable in its current
state. It is likely that sediment build up and improper maintenance has led to its failure. The drywell has
been replaced in the same location with a new precast concrete drywell with dimensions 10’ deep and 4’
wide. The use of a drywell has been recommended by HP Kumar based on a percolation test done at the
site on September 24, 2018.
The proposed drywell possesses the ability to store runoff in excess of 125 cubic feet. Without a
controlled release, the drywell must be sized for 1.5 times the WQCV and the entire storm runoff
volume, per COA URMP. For this basin, the required WQCV is 21.8 cubic feet with a required drywell
size of roughly 33 cubic feet. Basin 6 has an impervious area of 1133 square feet contributing to the
drywell. Multiplying this area by the one-hour storm duration requires the drywell to be 116 cubic feet of
storage, less than the proposed drywell storage volume. The proposed drywell is oversized based on
minimum drywell size criteria set forth by the URMP. Considerations of the timeline of this project,
found it easier to install a standard drywell rather than submit for a formal variance. This treatment
volume still does not include the gravel pack surrounding the exterior of the drywell.
02/10/2020
Impervious area of
1133sf x 1.23 (design
storm runoff) =
1,394cf of storage. Is
there a typo here?
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GLENWOOD SPRINGS 118 West Sixth St, Suite 200 | Glenwood Springs, CO 81601 | 970.945.1004
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Considering the small amount of water to be treated, the drywell will rely on percolation into the ground
for release of collected stormwater. In the unlikely event the drywell fills up, a grated manhole lid will
allow the runoff to overflow and continue downhill towards the detention berm. With this design, the
intention to allow proper operation of the drywell can be experienced for many years to come.
Maintenance for the drywell has been included at the bottom of this memo.
We hope this report is enough for the Change Order process for the development of this site. Please feel
free to contact us if we may provide further information or clarification.
Respectfully,
Rick L Barth, P.E.
Senior Engineer
Civil Services Sector
SGM
02/10/2020
02/10/2020
This drywell detail
does not meet COA
URMP drywell
standards. Chapter 8
requires a two
chamber drywell if
WQCV is being
treated by this BMP.
Provide a plan that
calls out elevations of
tie ins to the drywell,
sump and rim
elevations, etc. All
pertinent information
for asbuilt review.
02/10/2020
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