2005-10-21_Carver Response Letter 10-20
October 21, 2005
Paul Moline
Carver County Water Management
600 East 4th Street
Chaska, MN 55318
Re: County Comments, 10-14-05
Chanhassen West Business Park
Dear Paul,
This letter is in response to the memo dated 10/14/05 from Carver County Water Management in
regards to the Chanhassen West Business Park project. Changes have been made to the plans
based on comments in the memo, and our response is as follows. A general note on the plan set
– the SWPPP has been separated from the rest of the plans, and now contains three sheets:
narrative, erosion control plan (a carbon copy of the grading plan), & erosion control details.
Stormwater Issues
1. Downstream capacity of the southwest wetland (Outlot A) is needed.
➢ The southwest wetland was originally a pond designed and built by the City of
Chaska, and has since grown and taken on wetland characteristics. City
engineer Bill Monk was able to find the original design data for the pond, and
supplied us with the following information:
• Pond name: EC-P41
• 50 acre direct tributary
• 4.8 acre area
• 14.0 ac-ft of storage
• NWL = 935.0
• HWL = 940.0
• Outflow = 15 cfs
Without additional topography, this is the best information we will be able to
provide. We looked into obtaining aerial topography, but it was either too old or
not accurate enough for our purposes. Given the large amount of storage
currently available and the fact that we are expanding the wetland in addition to
reducing runoff rates, we feel that the capacity should be sufficient.
2. Rainwater Gardens/Infiltration - Details need to be amended to address:
• Remove the sock from the draintile in the rainwater garden typical detail. The
sock is not needed as the trench itself is wrapped with geotextile.
• The draintile in the typical detail is shown as extending only about 1/3 the
distance of the bottom of the rainwater garden. The draintile should extend the
entire distance of the bottom of the rainwater garden.
- The typical detail has been changed to reflect these comments
• Elevations of the trench bottom, layers of rock, and sand / compost / organic mix,
invert of the draintile in relation to the outlet invert elevation and NWL / HWL of
each system and stormwater basins is needed.
- The rims of the raingarden outlets and the elevations of the bottoms of
the raingardens have been called out on the grading plan. The thickness
of the rock layers and soil mixes are the same on every raingarden, and
since the typical detail specifies a depth below the trench bottom, the
elevations of these layers should be easy to calculate during
construction. The raingardens will have no normal water level, as they
should be dry most of the time, and the high water levels will be the
same as the rim elevation of the outlet. The draintile lengths, slopes,
and invert elevations at the outlet structures have been called out on the
utility plan.
• Detail of filtration shelf needs to be specific to the site application. The detail
used is a general template from Carver County meant to be a guide and not for
construction.
- A filtration shelf cross-section has been added to the details sheet, and
the template from Carver County removed from the plans. The draintile
in the shelf and its connection to the outlet structure are shown on the
storm sewer plans.
Erosion Control:
1. It is unclear when the temporary sediment outlets will be used and when the permanent
outlets will be used. In the plan it states that IF the permanent outlet is not installed a
temporary outlet will be needed. Once the permanent outlet is installed, the pond can no
longer be used as a temporary sediment basin. The temporary perforated outlets must
be installed with the excavation of the temporary basins. The basins must be excavated
prior to exposing upslope areas which shed water to the basin locations. Plans sheets
should be amended to reflect this.
➢ The note on the grading plan has been revised to reflect the fact that the pond
can no longer be used as a temporary basin once the permanent outlet has been
installed. The SWPPP has also been modified to address the temporary outlets
and when they should be installed. A detail of the temporary basin construction
has also been added to the details sheets.
Thank you for continuing to work with us on this project; we appreciate your time and effort, and
look forward to the resolution of comments on this project. Please contact me at 952-847-9608 or
jaredj@schoellmadson.com if you have any other questions or comments.
Sincerely,
Jared Jones, E.I.T.
Schoell & Madson, Inc.
CC: Aaron Mlynek, Carver SWCD
Kent Torve, Wenck Associates