2002-09-26 Review Storm Water Mgmt Practices
~~5~' MEMORANDUM
3535 Vadnais Center Drive, St. Paul, MN 55110-5196
651.490.2000 800.325.2055 651.490.2150 FAX
TO: Pete Carlson, SEH - St. Cloud
FROM: Ron Leaf, Jon French, Todd Udvig, SEH - St. Paul
DATE: September 26, 2002
RE: Review of storm water management practices: Outlets at Albertville -Phase III
SEH No. A-ALBEV0308.00
Storm Water (Lot Coverage Credits)
A review of the site plans submitted to the City of Albertville by JMJ Properties, Inc., dated August 12,
2002, for the Outlets at Albertville, Phase II, determined that the impervious lot coverage for the site is
approximately 71.4%. This exceeds the 50% effective lot coverage threshold allowed under the City's
Shoreland Ordinance. Based on our review of the 16.46-acre site, low impact development (LID)
practices and/or other storm water BMPs that account for a total of 4450 cu. feet of runoff (fora 0.5-inch
rainfall event) are needed to reduce the effective lot coverage to 50%. This assume the actual lot
coverage stays at 71.4%.
Both infiltration and non-infiltration methods could be applicable to the site. The Wright County Soil
Survey Manual defines the subsurface soils as being of the Haden, Dundas, and Nessel soil series. These
underlying soils are associated with permeability rates ranging from 0.2-0.63 inches per hour.
LID practices applicable to this site include depressed parking islands in the southern portion of the south
parking area. Other options include grated-infiltration trenches, rain gardens and/or a grass Swale along
the southern portion of the property. For example, a rain garden located at the southwestern edge of the
site, with a surface area of 2000 sq. feet and a depth of one foot, would treat approximately 2000 cu. feet
of runoff from the southwest parking lot. Drainage from the lot could be conveyed to the rain garden
through acurb-cut section at the south end of the lot. Runoff from larger events would be directed back
into the storm sewer system or into a grass Swale. Either option would ultimately discharge into the
detention pond located on the east side of the site.
The following is a summary of approximate runoff credits for various storm water options at the site:
Credit Needed = 4450 cu.ft.
Rain garden south of Southwest parking lot (curb cut from lot)
Approx. 3200 sq.ft. x 1.5 ft deep
Rain garden northwest corner of Bldg. D
(Approx. 1200 sq.ft. x 1.5 feet deep)
Grass Swale along south/southwest property boundary
(Approx. 250 feet x 1 foot deep -curb cuts from parking lot)
Infiltration trench(es) between parking islands throughout site
located to capture sheet flow (Approx. 50 sq.ft. x 1.5 ft deep each)
Depressed parking lot islands - bio-retention areas
(Approx. 100 sq. ft x 1.5 ft. deep each)
2000 cu.ft.
750 cu.ft.
1250 cu.ft.
100 cu.ft. each
60 cu.ft. each
Short Elliott Hendrickson Inc. Offices located throughout the Upper Midwest Equal Opportunity Employer
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Outlets at Albertville
September 26, 2002
Page 2
Wetlands
The use of rain gardens as credit for wetland mitigation is possible. However, the rain garden design
would have to meet wetland criteria and not receive direct storm water discharge without pretreatment.
Essentially, they would need to create atwo-cell storm water treatment system with the second cell being
the rain garden. The pretreatment would have to be designed and constructed to meet 50 °Io phosphorus
removal as required by WCA. Additionally, .the rain garden would need to be designed for a maximum
24-inch rise in water level for the 10-year critical storm event and the design standards for a replacement
wetlands must be followed. The goal would have to be a palustrine emergent wetland. In reality, it does
not appear that the developer would be able to meet these requirements within the confines of the site,
especially considering the current site layout.
The avoidance of wetland impacts should also be evaluated further. If the site design involved moving or
rearranging a building layout in order to avoid wetland impacts the developer may not have to provide the
additional compensation (i.e., approximately 0.3 acres) since there would be zero or reduced impacts to
one or more of the wetlands. Additionally, credit could be granted for the vegetative restoration of the
wetlands avoided if additional credit is needed. Is all of the parking shown on the site plan necessary to
meet City requirements? Can they reduce the number of parking spaces? Another possibility is for the site
design to have the replacement wetland in the south portion of the site, move the longer building to the
east and avoid wetland impacts. There must be enough watershed to support the avoided wetland as it
currently exists. Also why couldn't the buildings be two story to avoid wetland impacts?
JMF/TU/RBL
Attachments:
Site Maps With LID Practices Options ("Outlet North" and "Outlet South")
o:\wazer\1 pmjecPalbertv\a16ev0308.00\jmjoutletreview.doc