Deer Haven Stormwater Management Review WSB
\\wsbgvfiles01.wsbeng.local\projects\015820-000\Admin\Docs\Deer Haven Subdivision\015820-000 Deer Haven Stormwater Management Review.docx 701 XENIA AVENUE S | SUITE 300 | MINNEAPOLIS, MN | 55416 | 763.541.4800 | WSBENG.COM
Memorandum
To: Mr. Matt Unmacht, City of Chanhassen
From: Jeff Sandberg, PE
Foster Otten, EIT
Date: November 3, 2020
Re: Deer Haven Stormwater Management Review
WSB Project No. 015820-000
The following documents were submitted by Loucks on October 20, 2020 and were reviewed
against the City of Chanhassen and Minnehaha Creek Watershed District (MCWD) requirements:
• Full Plan Set.pdf, prepared by Loucks, dated 10/16/2020
• Hydrology Report.pdf, prepared by Loucks, dated 10/13/2020
• Final Plat 10-15-20.pdf, prepared by Loucks, dated 10/15/2020
We recommend that the following comments be addressed as the project design moves forward:
General
1. MCWD has notified the City that the development triggers MCWD Stormwater
Management rules because the parcel is being split into 3 or more lots and increasing
impervious surface. Applicant will be required to obtain a permit through MCWD.
Water Quality
1. Page 1 of the hydrology report states “Soils information is assumed to be mostly D soils,
based on the area and the wetland to the north. Filtration will be used to reduce runoff
rates, and volumes.” Soil borings should be provided to confirm the web soil survey data is
correct and to confirm infiltration is not feasible . If soil borings show infiltration is feasible,
infiltration should be used to meet stormwater management requirements. This would also
allow for smaller basins.
a. If infiltration cannot be used, an abstraction analysis must be provided according to
MCWD Stormwater Management Rule 3(c)(2)(i).
2. Applicant should include information on how pretreatment will be provided for the proposed
biofiltration basins in the hydrology report (i.e. vegetated filter strips).
Rate Control
1. Outlet pipe slopes in HydroCAD should be updated to be consistent with what is shown in
the plans.
2. If a biofiltration basin continues to be proposed, applicant should route exfiltration
downstream instead of it being discarded in HydroCAD.
Volume Control
1. Based on the impervious areas included in the hydrology report and a ccording to MCWD
Stormwater Rule 5(c), (and shown in Table 4 in Section 11) the site:
a. is greater than 1 acre
b. is disturbing less than 40% of the site
Deer Haven Addition
November 3, 2020
Page 2
\\wsbgvfiles01.wsbeng.local\projects\015820-000\Admin\Docs\Deer Haven Subdivision\015820-000 Deer Haven Stormwater Management Review.docx
c. is increasing the existing impervious surface by more than 50%
Therefore, the MCWD rules state that the entire site’s impervious surface (18,142 sf) must
be accounted for in calculations for water quality, rate control, and volume control.
Currently the applicant is providing calculations for only the additional impervious surface.
Applicant should confirm proposed impervious area as noted previously and confirm which
MCWD requirements apply based on Table 4 in Section 11 of MCWD Rule N.
2. Water quality volume credit is given at a 0.5 rate for filtration basins according to MCWD
Volume Abstraction Credit Schedule (Appendix A of MCWD Rule N). With the 0.5 factor
applied the proposed basins do not provide the required water quality volume. Applicant
should update the subsequent water quality volume calculations with the 0.5 credit rate
applied and provide updated HydroCAD stage/storage data (shown at the outlet elevation
to confirm water quality volume) for both ponds.
Filtration Basin Design
1. If a biofiltration basin continues to be used applicant should update the MIDS calculations
to reflect a filtration basin with underdrain to get accurate modeling results.
2. Provide an operation and maintenance plan that identifies the maintenance schedule and
responsible party. This should include information on how the system will be cleaned out.
Flood Elevation
1. The City of Chanhassen requires low floor elevations to maintain at least 3 feet of
freeboard to adjacent ponding areas HWL and floodplains. Per the submitted report, the
NW filtration basin (2P) has a 100-year HWL of 1026.49’. Applicant should include the low
opening and low floor elevations for the proposed buildings and make sure the elevation
allows for 3 feet of freeboard above the basins.
Civil Plans
1. Applicant is showing grading and utility work north of the parcel boundary . Applicant must
show easements on plans for all improvements outside of property limits.
2. Easements must be shown for on-site storm sewer and around ponding areas if public.
3. Operations and Maintenance Plan to include maintenance schedule and entity responsible
for maintenance must be provided for any private utility or pond.
4. Proposed draintile should be shown in the plans tying to the overflow structure. Applicant
should include draintile size, slope, inverts and cleanout locations.
5. Update detail to reflect filtration bioretention basin instead of infiltration basin (depending
on outcome of soil borings).
6. Provide details for silt fence and rock construction entrance.
7. FES must have energy dissipation with appropriately sized and quantity of rip rap. Sho w
rip rap detail on the plans.
8. The proposed area included in SWPPP note #3 site data in the full plan set doesn’t match
the proposed impervious area listed in the hydrology report. Applicant should confirm
impervious areas and update plans/hydrology report as needed.
9. Emergency overflow elevations and location should be included on the plans for both
basins.
10. Storm sewer must be 12” minimum per city requirements.
11. Add inlet protection to the eastern catch basin on Yosemite Avenue near the construction
entrance.
12. Two lines of silt fence should be placed adjacent to the wetland.
13. The NW basin is shown within 15’ of the adjacent building. Applicant may want to consider
adjusting the design to provide further separation between the pond and structure.
We request that the applicant address each of the comments above and provide a written response
by restating each comment and stating how each comment was addressed and include supporting
documentation. Please feel free to reach out with any questions.