Water Resources Coordinator Memo 06-30-2011Memorandum
TO: Angie Kairies, Planner
FROM: Terrance Jeffery, Water Resources Coordinator
DATE: June 28, 2011
RE: Southview Design request for variance from §20 -615
10036 Trails End Road
The lot in question currently has 25% hard cover which is the most hardcover allowed by
code for single family residential lots. The applicant desires to add a sport court totaling
681 square feet in area which would bring the total hard cover on the lot to 29.3%
which exceeds the allowable hardcover area.
Pervious Surface
The applicant contends that the proposed sport court would be pervious and would
allow for more infiltration than would occur if the area was left as traditional lawn. To
support this position, the applicant has provided a study prepared, at the request of
Sport Court, Inc. by the Utah Water Research Laboratory at Utah State University. This
report has been attached to this memorandum. In summary the study finds that the
Hydraulic Conductivity, or the rate at which water passes through the SportTile is
greater than gravel, sand and other soil types. This indicates that the sport tile would
not be the limiting factor for the infiltration rates in the area but rather that the
underlying materials would limit infiltration. The authors draw the same conclusion on
page 7.
The soils in this area are Kilkenny- Lester Loams. These soils are in the Hydrologic Group
C which has a high potential for runoff and can have an infiltration rate as low as 0.05
inches per hour. The seasonally high water table is within three (3) feet of the surface
and the soil has approximately 40% clay by volume. These soils compact readily to the
point where they are virtually impervious. The Installation Instructions SporteaseT°"
prepared by Sport Court, and provided by the applicant, instructs on page 2 of 6 that
after excavation, the subgrade shall be compacted. It then goes on to state in the next
section that the base material should be compacted as well. This will effectively render
the sport court area impervious; negating any benefit derived from the hydraulic
conductivity of the SportBase TM.
Purpose
When stormsewer systems are designed, it is inherent that certain assumptions are
made in order to model anticipated runoff volumes and rates. One assumption is that
no lot will have greater than 25% hardcover. If several individual lots exceed this 25% the stormsewer
infrastructure becomes inadequate for the new conditions and localized flooding, infrastructure
damage, erosion, stream degradation and other deleterious effects may result. Currently Chanhassen
City Code does not recognize any alternative hard surface materials. If we were to allow alternate
pervious surfaces a number of issues including site design, engineering of soils and materials,
construction observation, long term maintenance and long term preservation. Even with the resolution
of these issues it is important to note why the Minnesota Pollution Control Agency and numerous other
agencies are considering alternative hard surface materials with a high hydraulic conductivity.
The agency shall develop performance standards, design standards, or other tools to
enable and promote the implementation of low - impact development and other storm
water management techniques. For the purposes of this section, "low- impact
development" means an approach to storm water management that mimic's a site's
natural hydrology as the landscape is developed. Using low- impact development
approach, storm water is managed on -site and the rate and volume of predevelopment
storm water reaching receiving waters is unchanged. The calculation of predevelopment
hydrology is based on native soil and vegetation. - Minnesota Statutes 2009, section
115.03, subdivision 5c
The intent is to reduce runoff by mimicking the hydrology and hydraulics of the natural environment,
not to allow for the maximization of allowed hardcover on a lot of record and then further increase
hardscaping. Not only are there implications to surface water management, there are also aesthetic and
financial implications that accompany the loss of green space.
Other Considerations
Significant efforts were made during the development of Settler's West to protect the bluff areas by
controlling the rate and volume of runoff and maintaining existing drainage conditions. Sensitive areas
were put into protective outlots and discharge points from the stormsewer conveyance system were
designed and constructed with extraordinary stabilization and energy dissipation methods. As this area
ultimately drains to the bluffs along the Hennepin County Regional Rails to Trails System, any potential
increase in runoff rate or volume could potentially result in erosion along the bluff.
Conclusion and Recommendation
While it is evident that the actual SportBase tiles are pervious, the construction method and the
existing site conditions would not result in a truly pervious system but rather in a highly compacted clay
subgrade with less hydraulic conductivity than currently exists. Even in the event that proper
engineering and construction could provide for adequate infiltration, the Chanhassen City Code does not
make allowances for alternate pervious hard cover. If as we move forward, the City considers allowing
certain pervious surfaces, it will be important to consider the reason behind limiting hard cover and the
implications and limitations of any such system. Until such a time as these issues are resolved and City
Code allows for some use of alternate pervious systems in certain redevelopment or hardship
circumstances, I must recommend the request for variance be denied.
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