HomeMy WebLinkAboutFile Documents.1050 Matchless Dr.0113-2020-BRES (18) P.O.Box 3901 YARNELL CONSULTING & 229 Midland Ave.
Eagle,Colorado 81631 Basalt,Colorado 81621
CIVIL DESIGN, LLC
(970) 323-7008 . 1/3/2020
Hailey Guglielmo
City of Aspen Engineering Department
201 N.Mill Street,Suite 101
Aspen,Colorado 81611
Hailey.Guglielmo@CityOfAspen.com
Subject: 1040&1050 Matchless Drive (Lots 2 and 3,Alpine Acres Subdivision)
Variance Request
Hailey:
Last fall,we discussed the construction of detached garages on the existing asphalt parking lot for
the Racquet Club Condominium Association at 1040&1050 Matchless Drive(Lots 2 and 3,Alpine
Acres Subdivision). In accordance with the city's Urban Runoff Management Plan (URMP),
redevelopment within this major drainage basin requires full detention and water quality
improvements for all new impervious area within the limits of the project.
There are several constraints on the project. First,the finished floor elevations of the existing
buildings are too low which does not permit optimal positive drainage away from the buildings.
Second,the site includes several large,mature trees that limit the opportunity for regrading.
Finally,there is no storm drainage infrastructure on or immediately adjacent to the site such that all
runoff is conveyed overland.
Combined,these constraints do not permit an adequate footprint or elevation for construction of a
dry or wet detention basin,or a bio-retention or sand filter facility that meets the city of Aspen
URMP.We met in your office September 18, 2019 to review these constraints which you
understood and then agreed to permitting below-grade detention for this project.The next
challenge was to determine the means by which to provide the below-grade detention.
A fully-contained vault was deemed not optimal due to lack of a downstream storm sewer system
that could be employed to drain the vault.This would have required a pump and creative approach
to discharge the force main onto grade in a sheet-flow condition.
In my opinion,drywells that employ a precast concrete manhole structure are a last resort. First,
the relatively small footprint translates to a small area for promoting infiltration. Second,they are
challenging to maintain because they require full removal of the structure,replacement of the
medium beneath it,and then replacement of the structure.Third,they are inefficient in the quantity
of volume they hold. For example,a 4-foot-diameter structure only holds 12.5 cubic feet per foot of
depth.When a site needs to detain over 600 cubic feet of volume,a manhole is not an efficient way
to do it.
The third option we evaluated was a below-grade chamber system. Several companies manufacture
comparable products that all work similarly and could be interchanged depending on pricing. In
RECEIVE[
1040 & 1050 Matchless Drive (Lots 2 And 3,Alpine Acres Subdivision) Pageg5 1 1 2 0 2 0
ASPEN
BUILDING DEPARTMENT
P.O. Box 3901 YARNELL CONSULTING & 229 Midland Ave.
Eagle, Colorado 81631 Basalt, Colorado 81621
CIVIL DESIGN, LLC
(970) 323-7008 • 1/3/2020
this case,we evaluated Storm Tech by Advanced Drainage Systems.This appears to be an optimal
solution for this site for the following reasons:
1. Stormwater is infiltrated beneath the full footprint of the system; therefore,a large area
relative to a drywell.
2. No pump is required as part of the system.
3. Filter fabric is placed on top of the gravel medium to intercept particulates that would
otherwise plug the gravel.These particulates are easily removed via vacuuming which is
part of regular maintenance.
Since below-grade chambers are not listed as an approved storage volume device, I am hereby
requesting a variance for this project.The following reflect the"Variances"procedure identified
within Section 29.01.060 of the City of Aspen Municipal Code.
1. Identify the criteria from which the applicant seeks a variance.
Due to the constraints being imposed on this project site,I am seeking a variance from URMP
Section 5.3 which limits the types of detention basins to those listed.
2. Provide a written justification outlining why the criterion cannot be met.
As stated previously, the types of detention basins listed in the URMP Section 5.3 have been
evaluated and considered for use on this site. Unfortunately, they were all deemed to be
incompatible with all the constraints of the site.
3. Identify an alternate criteria or standard that is proposed to comply with the intent of the
criteria from which the variance is sought.
In general, the goal of the detention basin is to intercept surface runoff and release it at no
more than the historic rate for a given site.A below-grade chamber system appropriately
placed to intercept drainage from the site and store it within the void space until it can leach
into the surrounding soils is a very similar approach but more effective than a drywell where
water is stored in the structure and infiltrates into the soils beneath the structure itself
4. Provide supporting documentation, including necessary calculations.
We are including detention volume calculations for the project site,plus the storage volume
calculations for the chamber system in the drainage report being submitted as part of the
building permit submission for the project.
5. Provide an explanation of any potential adverse impacts of the proposed variance to
adjacent landowners.
1 do not foresee adverse impacts of construction of a below-grade chamber system for adjacent
landowners. The system should intercept and infiltrate the 100-year detention volume on the
subject property, and it is proposed as far as possible from building foundations to minimize
any impacts thereto.Storm events exceeding the available volume of the chamber system
should bubble out of an overflow structure and continue along historic drainage paths within
Cemetery Lane right-of-way-not to any adjacent,private properties.
1040 & 1050 Matchless Drive (Lots 2 And 3,Alpine Acres Subdivision) Pageg, 112 02 0
ASPEN
BUILDING DEPARTMENT
P.O. Box 3901 YARNELL CONSULTING & 229 Midland Ave.
Eagle, Colorado 81631 Basalt, Colorado 81621
CIVIL DESIGN, LLC
(970) 323-7008 • 1/3/2020
I appreciate your understanding of the difficult constraints imposed by this site and your
willingness to consider the acceptability of a below-grade chamber system to be constructed as a
detention basin.
Do let me know if you have any questions.
Sincerely,
,\Vv
Justin J.Yarnell, PE
Colorado PE Number 47241
11 rt`_
i-i
1040 & 1050 Matchless Drive (Lots 2 And 3,Alpine Acres Subdivision) Paged , 112 02 0
ASPEN
BUILDING DEPARTMENT