2005-09-22_Galpin Lyman Storm Report Revised 9-22-05
September 22, 2005
Paul Moline
Carver County Planning & Zoning
600 East 4th Street
Chaska, MN 55318
Re: Stormwater Management Update
Chanhassen West Business Park
Dear Paul,
We have completed revisions to the drainage areas and calculations for Chanhassen West
Business Park based on comments from Carver County Soil & Water. The existing site
delineation was revised to more accurately model where stormwater runoff ends up, and a new
HydroCAD model was generated. Please refer to the attached drainage exhibits and HydroCAD
report for reference.
I. Existing Site Delineation
The existing site has been divided into four drainage areas, three of which drain to on-site
wetlands, and a fourth which drains to a culvert leading off-site. Area EX1 drains to the
southwest wetland by overland flow, on-site culverts, and swales along the western and
southern edges of the property. Areas EX2 and EX4 drain to the northwest and
northeast wetlands, respectively. It appears that the northwest wetland will overflow to
the east and into the northeast wetland should water levels rise high enough, but more
than likely runoff will simply infiltrate into these two wetlands under average rainfall
events. Area EX3 drains to a culvert under Galpin Boulevard.
II. Proposed Site Delineation
The proposed site has been divided into four drainage areas, and keeps drainage patters
as close to existing conditions as possible. The most significant change to the proposed
site conditions was the elimination of the northeast wetland, along with a few smaller
wetlands that were included in area EX1. The largest drainage area is 1S, which
encompasses the entirety of the impervious surface and a majority of the developed area
on the site, and drains to proposed pond 4P before being discharged to the southwest
wetland. Area 2S is of similar size to area EX2, and drains to the northwest wetland.
Since the northeast wetland will be removed, and with it the northwest wetlands’ natural
overflow area, an equalizer pipe has been installed along the western edge of the
property to connect the northwest wetland to the southwest wetland and provide a route
for overflow should it become necessary. Area 4S drains directly into the southwest
wetland from surrounding green areas on the site, and area 3S continues to drain to the
culvert under Galpin Boulevard.
III. Rate Control
Based on these new delineations, a new rate comparison was performed for the 2, 10,
and 100-year storm events, the results of which are included in the table below.
Rainfall Event Southwest Wetland Northwest Wetland Galpin Blvd Culvert
Existing Proposed Existing Proposed Existing Proposed
2-Year 10.03 10.00 2.33 2.23 0.66 1.20
10-Year 28.64 25.80 7.42 7.09 2.10 2.67
100-Year 60.37 60.22 16.33 15.61 4.62 4.98
Care was taken to match rates as close to existing conditions as possible so as not to
deviate significantly from existing flows entering the wetlands, and runoff rates are less
than proposed conditions for all storm events for runoff entering the wetlands. This has
been accomplished largely with the pond and its outlet control structure (refer to details
sheet in the plan set). The northwest wetland did not contribute any runoff to the
southwest wetland during any storm event based on our model, which took infiltration into
account.
The rates are higher than existing conditions for the Galpin Blvd culvert despite a
reduction in the contributing area. This is due to the fact that the area draining to the
culvert will be landscaped and will have a higher curve number than the existing wooded
area, and therefore will contribute more runoff. There is no impervious area contributing
runoff to the culvert, so water quality should remain equal to existing conditions.
Please refer to the HydroCAD report for a more detailed analysis.
If you have any questions or comments regarding this report, please contact me at 952-847-9608.
Sincerely,
Jared Jones, E.I.T.
Schoell & Madson, Inc.