2023-11-02_Storwmater Memo
November 2, 2023
The proposed Galpin Industrial Project in Chanhassen is Lot 3 of Chanhassen’s PUD Site #5,
Chanhassen West Business Park. As part of this development, which was permitted in 2005, a regional
stormwater pond with a filtration shelf was installed. In addition, several rain gardens for the individual
sites were designed for the individual proposed lots. The intent of the proposed stormwater design is to
provide equivalent stormwater treatment to what was proposed in the 2005 PUD. Documents relating to
the original design and capacity of the existing stormwater facilities have been included with this memo.
STORMWATER DESIGN CRITERIA
The existing site was designed in 2005 with the following criteria:
1. Rate control for the 2- (2.80”), 10- (4.10”), and 100-year (5.90”) TP 40 Rainfall events.
a. Discharge rates for the full PUD were analyzed to each major runoff location (South
Wetland, Northwest Wetland, and Galpin Blvd Culvert)
2. A water quality treatment volume (Filtration Volume) of 0.306 inches over the impervious area
a. Treatment Volume (ft^3) = Impervious Area (sf) x 0.34/12 (inches) x 0.9
3. A wet pond designed to NURP standards, with a dead storage containing a volume equivalent to
the runoff from a 2.5” rainfall event.
Lot 3 contained two rain gardens, Rain Garden 6 and Rain Garden 7, which were designed to provide
water quality treatment for the proposed site. Rain Garden 6 was a shared rain garden which was
installed with the development of Lot 2, and had a design volume of 854 cf. Rain Garden 7 was designed
on the eastern edge of the property and had a design storage volume of 714 cf.
The existing filtration bench of the existing NURP Pond in combination with the on-site rain gardens
as designed met the approved stormwater requirements for the development.
PROPOSED STORMWATER DESGIN CRITERIA
The proposed grading plan ties down to Rain Garden 6 per the original designed conditions. The
proposed design also includes two proposed rain gardens along the northeastern edge of the property
line. The northern rain garden has a bottom of 959.50 and an outlet of 961.00, with a storage volume of
271 cf. The southern proposed rain garden has a bottom of 958.9 and an outlet of 960.17 with a storage
volume of 453 cf. These two basins combined provided 724 cf of storage, which exceeds the designed
storage volume of Rain Garden 7.
The proposed development does not exceed the designed imperviousness maximum of the site set
forth in the approved PUD. Therefore, the site will continue to meet the permitted stormwater
requirements through the existing BMPs and the construction of the new rain gardens per the approved
plans.
Sincerely,
Emily Castanias, PE