2005-09-22_SWCD response letter
September 22, 2005
Paul Moline
Carver County Water Management
600 East 4th Street
Chaska, MN 55318
Re: SWCD Comments
Chanhassen West Business Park
Dear Paul,
This letter is in response to the memo dated 9/19/05 from Carver County Water Management in
regards to the Chanhassen West Business Park project in Chaska. Changes have been made to
the plans based on comments in the memo, and our response is as follows:
Stormwater Issues
1. The calculations for existing conditions should be resubmitted and show the effects of
ponding in the northeast wetland. The submitted calculations use sheet flow runoff from
35.74 acres to the southwest wetland, which is not representative of the Existing
Drainage figure. The increase in runoff may have impacts on downstream and water
quality treatment on site.
➢ The drainage figures and HydroCAD model have been revised to more
accurately model where runoff from the site ends up. Please refer to the
attached, updated storm report.
2. Water Quality calculations need to be submitted for review.
➢ The pond has been designed to NURP standards for water quality, the
calculations of which can be found in the revised drainage calculations under the
NURP rainfall event. In summary, 3.817 af (166,269 cf) of runoff enters the pond
during the 2.5” rainfall, and the pond has 176,459 cf of dead pool storage
available. Calculations on the sizing of raingardens to accommodate runoff from
the median rainfall event in the Twin Cities area have also been submitted.
Please refer to the attached exhibit.
3. Proposed CNs for greenspace should be raised to C type soils to account for the affect of
compaction.
➢ The drainage calculations have been revised to reflect this change.
4. Downstream capacity of the southwest wetland (Outlot A) should be submitted.
➢ The survey provided to us does not contain enough information to determine the
capacity of this wetland. In addition to a lack of topographical information from
the interior of the wetland, the western boundary of the wetland is not located on
the property.
5. The outlet of the northwest wetland (Outlot B) should be submitted.
➢ Information on the inlet from the northwest wetland to the storm sewer pipe along
the western boundary has been added to the plans.
6. Rainwater Gardens/Infiltration - Additional details are needed including:
➢ Documentation should be submitted for the volume provided for infiltration.
Several basins are noted on the plans, but no calculations are provided
• Calculations on the sizing of raingardens to accommodate runoff from
the median rainfall event in the Twin Cities are included in an attached
exhibit and spreadsheet. In general, the raingardens are sized using the
assumption that roof drains will not be used, and that roof runoff will flow
across the parking lot. Raingardens are proposed in as many locations
as possible, but site conditions and landscape requirements restricted
the size and number of raingardens, and as such, they are only
proposed where practical. In some cases, the raingardens do not
provide enough storage for runoff from the median rainfall event, but it
was our opinion that some treatment is better than none at all.
➢ Soils used (sand fill or rock fill, 50% clean sand, 25% loamy topsoil, 25%
compost)
• A note has been added to the Landscape plan which specifies that this
type of soil should be used for rain garden construction.
➢ Cross-sections
• A typical rain garden cross-section has been provided on the details
sheet.
➢ Erosion control blanket and type
➢ Timing of installation (after contributing area is stabilized).
➢ To reduce initial degradation of basins, It is recommended plugs, shrubs and
grasses are used in addition to the ‘rainwater garden seed mix’.
• A note has been added to the Landscape plan which details these
items.
Erosion Control:
1. Stable EOF’s are needed for the proposed pond on site. The EOF should be clearly
labeled on the plan and a detail is needed. The EOF can be stabilized with a turf re -
enforcement mat or fabric and riprap.
➢ A EOF has been added to the plans, and a detail has been added to the grading
plan. It shall be stabilized with a turf reinforcement mat.
2. Erosion control blanket should be specified in the swale from the FES to the wetland
along the west boundary of the site. The blanket specified should adequately protect the
area from designed velocity and depth of flow. The blanket and seed in the swale should
be installed within 24 hours of culvert installation. All blankets on the plan should be
shown as a shaded area.
➢ The swale on the west side of the property has been removed and replaced with
storm sewer, so an erosion control blanket will no longer be necessary in that
area.
3. Erosion control blanket is recommended for the pond slopes from around 952 to 942
contours. All blanket on the plan should be shown as a shaded area.
➢ An erosion control blanket has been added to pond slopes and designated by a
hatch pattern at all locations on the site.
4. Erosion note number 5 should include 2 tons of mulch per acre with seed for stabilization;
seed alone is not considered stabilization.
➢ Note #10 on the SWPPP has been changed to reflect the addition of 2 tons of
mulch per acre in addition to seeding for final stabilization.
5. All curb cuts need riprap and fabric or sod installed within 24 hours of curb installation.
➢ All curb cuts will have 10 CY of Class IV riprap added, and a note has been
added to the grading plan and SWPPP.
Erosion Control:
1. Temporary sediment basins are needed prior to disturbing upslope areas. The areas of
temporary sediment basins should be labeled on the plan. A temporary outlet is needed
for the pond; details should be provided. A temporary outlet could consist of a perforated
riser and rock cone. Temporary basins are needed in the area of the proposed
permanent stormwater pond; the SE corner of the site prior to discharging to the culvert
under Galpin.
➢ Temporary sedimentation basins have been added to the plan in the locations
requested, and are clearly labeled. The primary basin (the stormwater pond) will
have a temporary outlet, and a detail has been provided on the details sheet.
2. The Wimco inlet control detail proposed will work for all curbside inlets. Any and all area
inlets or drop inlets in paved areas will need alternate controls / Wimco details. The
engineer should check on alternate designs for Wimco-type inlet controls to fit the various
type of inlets.
➢ A Wimco insert detail for circular manholes and catch basins has been added to
the details sheet.
3. Additional inlet controls may be needed for adjacent inlets on Galpin and Street A.
➢ A note has been added to the grading plan instructing the contractor to install
Wimco-type catch basin inserts on all proposed catch basins on Galpin Court
(formerly Street A) and on Galpin Blvd.
4. Silt fence will be needed around Outlot A along the east side between the pond the
wetland.
5. Silt fence needed for Lot 5, Outlot B.
6. Silt fence needed north of berm in Lot 6 as there is a FES taking water north off site.
7. Silt fence needed south of Lot 1.
➢ Silt fence has been added in these locations
8. Each building site will need perimeter controls once the streets are paved and rock exits
will be needed as each lot is built-out.
➢ A note has been added to the grading plan detailing this requirement, and since
each building will be developed and approved separately, it is likely that this
requirement will be met at the time the buildings are built-out.
9. Type 1 and Type 2 silt fence locations need to be specified on the plan. Type 2 should
be installed around all wetland areas and in the SE corner of the site to protect the culvert
under Galpin. The SF should be extended along the South side to close the gap in the
SF.
➢ The two types of silt fence have been added to the plans in the areas requested.
SWPPP Review:
- Does the SWPPP Narrative: Identify the entity responsible for long term O&M?
o The Owners Association has been specified on the SWPPP
- Do plan sheets identify: Locations and types of sediment basins?
o Sediment basin locations have been specified on the plans
- Are Temporary Sedimentation basins:
o Adequately sized?
▪ The major sedimentation basin on the site, the stormwater pond, is
adequately sized to handle runoff from the entire site. The other
sedimentation basins are small and will only treat a limited amount of
water, but are oversized for the amount of area that wil lbe draining
to them.
o Designed to prevent short-circuiting?
▪ The sedimentation basin locations should prevent short-circuiting
o Are outlets designed to remove floating debris?
o Are outlets designed to allow complete drawdown?
▪ The temporary outlet is detailed on the details sheet, and will be able
to remove floating debris and allow complete drawdown if installed.
o Do outlets have energy dissipation?
o Have a stabilized emergency spillway?
▪ The SWPPP instructs the contractor to provide energy dissipation
and a stabilized emergency spillway.
- Does the wet sedimentation basin have a stabilized emergency overflow?
o The pond has a emergency overflow stabilized with a turn reinforcement mat,
and is detailed on the grading plan.
- Are wetted perimeters of ditches stabilized within 200 feet of surface water?
o A note has been added to the SWPPP. There are no swales on the site as of
the most current plan revision
- Are sediment control practices established on down gradient perimeters?
o Silt fence has been added to close any gaps on down gradients
- Do stockpiles have sediment control and placed in areas away from surface waters?
o A note has been added to the SWPPP instructing sediment control around
any stockpiles the contractor may use.
If you have any questions or wish to discuss any of these responses, please feel free to contact
me at 952-847-9608.
Sincerely,
Jared Jones, E.I.T.
Schoell & Madson, Inc.
CC: Pat Minger, Minger Construction
Aaron Mlynek, Carver SWCD
Bob Generous, City of Chanhassen
Lori Haak, City of Chanhassen