WSB Galpin Development Initial Stormwater Review 1-2-19 wsb
Memorandum
To: Paul Oehme. PE
From: Jeff Sandberg. PE.WSB
Kendra Fallon. EIT. WSB
Date: January 2. 2019
Re. Galpin Development Initial Stormwater Review
WSB Project No. 013315-000
The following documents were submitted by Pioneer Engineering in early December 2018 and
were reviewed against the City of Chanhassen (City) requirements:
° • 01-118100-Proposed-(11-20-2018).p8c
• 01-118100-Proposed-(11-20-2018) (1).p8c
w • 116037-Atlas 14-MN 24-hr SO: undated
cip
3 • 116037-Atlas 14-MN: undated !}
• Final Galpin Feasibility Report (PW176a): dated August 2018 SV ^�
• Galpin Existing 11-20-18
• Galpin Proposed 11-20-18 S ,
• Galpin Site Preliminary Plat and PUD— 1st Submittal 12-05-2018 5
• Lennar Galpin Blvd Tree List. undated
• MNR Galpin Blvd MnRAM Memo 7-23-18
• RPBCWDSnowmelt_10d_100yr: undated
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• Stormwater Report 11-07-18
• Wetland Delineation Report_updated: dated September 27. 2017
Z The comments are only preliminary comments and do not constitute a full review of the submitted
materials. Additional comments may be generated as a full review is conducted.
0_
a General
1. Wetland permitting is required due to the proximity and proposed impacts to wetlands
onsite. A wetland permit ap lic tion has not yet been received by the City from the
applicant.
2. Wetland review will include ensuring hydrology is maintained to all wetlands to be
o preserved as well as review of proposed stormwater impacts.
3. Storm sewer sizing calculations should be provided to confirm storm sewer is sized to
convey the 10-year storm event.
n 4. An NPDES permit and accompanying storm water pollution prevention plan (SWPPP) will
be required prior to the start of construction.
5. An operations and maintenance plan for the proposed stormwater management system
will be required prior to approval.
6. Provide infiltration test results per MPCA Requirements in the location of each proposed
infiltration area.
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Galpin Development Initial Stormwater Review
January 2, 2019
Page 2
Preliminary Plans
1. Show all existing storm sewer and other water resource related features in plans.
2. Adjust and show all easements over the Metropolitan Council Environmental Services'
sanitary sewer on the preliminary and final plat.
3. Clearly indicate what storm sewer will be private and what will be public. All public storm
sewer will be required to be shown in profile view. Applicant should confirm there are no
conflicts with the watermain or sanitary sewer throughout the site.
4. The following comments pertain to all proposed ponds that include an infiltration bench
(Basins 100. 200 and 300):
a. Infiltration test results will be required in the location of all proposed infiltration
benches and infiltration basins. Infiltration test results have not been submitted in
the location of proposed Basin 100. If filtration is to be used, the applicant will
need to provide a specification for amended soils.
b. The exfiltration above the piped outlet elevation should be included in the
HydroCAD model and the exfiltration rate should correspond to what is presented
in the infiltration test results.
c. The proposed infiltration benches will be required to meet the 48-hour drawdown
requirement.
d. Outlets from the pond should be moved out of the permanent pond area and
should be moved away from the inlet to minimize the risk of short circuiting.
5. The proposed piped outlet being modeled for Basin 400 is not shown in the plans.
6. Access routes for all proposed stormwater basins are required for maintenance
purposes. Applicant should callout access locations for all proposed stormwater basins.
7. A defined riprap EOF spillway will be required for all stormwater basins per details
provided on Sheet 33. Applicant should include location and elevation of all EOF
spillways on the storm sewer plans.
HydroCAD Model
1. The soil borings provided show that there are clay soils throughout the site. Modeling
should be updated to reflect the D soils present.
kfrit/ 2. Proposed and existing HydroCAD�dels show be modeling the same area. There is
6N roughly 120 acres included in the existing conditions model that is not included in the
' i proposed conditions model. All offsite drainage should be included in the models.
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in
3. Existing and proposed conditions drainage area maps should be updated to show the
location and boundaries of all subcatchments included in the models.
tilli 4. Time of concentrations should be calculated in HydroCAD and not directly entered to
confirm accuracy. Provide supporting calculations for all_directly entered times of
concentrations.
5. From the drainage area maps, it appears that P-LlJ 2-6 (P.E. Edit the existing model
corresponds to Pond 500P in the proposed conditions mo- e-t. e modeled storage for
the wetland differs between the existing and proposed conditions models but no wetland
impacts are shown on Sheet 37 for this wetland. Applicant should update the models so
that they have the same storage modeled for the wetland.
a. There is a proposed piped outlet from this wetland that is over 4 feet lower than
the current natural spillway outlet. The applicant will be required to show that this
does not change the hydrology of the existing wetland as part of the wetland
permit requirements.
6. There are numerous inconsistencies between what is shown in the plans o SIP .r
the outlets of the stormwater basins and what is being modeled in HydroCA' -Of
Applicant will need to update the models so that they are representative of wh. I •••-ing
shown in the plans.
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Galpin Development Initial Stormwater Review
January 2. 2019
Page 3
P8 Model
1. The same total area should be modeled in P8 as in HydroCAD.
2. The model should be run for at least 50 years and should include the most recent
precipitation data included in the precipita ib rftlt-te get accurate removal efficiencies.
3. The modeling of the proposed stormwater ponds with filtration benches is incorrect and is
overestimating the removal efficiencies of the basins. The applicant should update the
model for the stormwater basins in the following way:
'a. The design infiltration rate should be added to the flood pool section of the
,stormwater pond devices where applicable. This will model infiltration above the
outlet elevation. The design infiltration rate should match what is being modeled
%'� in HydroCAD.
J b. ,The infiltration basins should be removed from the model (except for Device
I ' 600i).
. Applicant should confirm total phosphorus and total suspended solids removal
requirements are still being met after the model has been updated.
4. Watersheds 201 and 600 are only modeling.the impervious area. The pervious area in
these watersheds should be adde .te e model.
5. Watershed 300 in the 8 has 5° impervious while the corresponding HydroCAD
subwatershed has 5° mpervious modeled. Applicant should update the models to be
consistent with on other and representative of the plans.
Stormwater Management Plan
1. The total areas listed in the table in Section III.A of the Stormwater Management Plan are
inconsistent with the total area cal ed out in the plans and the total area being modeled.
Applicant should include the-en-tire site in-flie shown it-the'table.
We request that the applicant respond by restating each comment and stating how each
comment was addressed. Please reach out with any questions.
K/0133150001From-To/Data\013315-000 Galpn Development Intel Stormwater Reoew 010218 ooc.