Stormwater Summary
HYDROLOGY REPORT
FOR
BOULDER COVE
Chanhassen, Minnesota
February 13, 2014
Otto Project No. 14-0105
Prepared for:
Lennar Corporation
Prepared by:
9 WEST DIVISION STREET - BUFFALO, MN 55313 - (763) 682-4727 - FAX (763) 682-3522
Volume Control Narrative
BOULDER COVE
Lennar Corp.
The existing soils on this property have been determined to be a limiting factor in
meeting the one inch volume abstraction requirement. Braun Intertec’s soil borings
indicate type D soils throughout the site (refer to attached Geotechnical Evaluation
Report). Two of the borings show a layer of type B soils above type D soils; One in the
location of the proposed wet pond and the other near the proposed filtration basin.
In comparison to the 2005 multi-family development approval, this single-family plan is
showing the proposed street pulled further south to expand the northern basin area.
The intent was to provide infiltration in the north-central portion of the site where the
type B soil was found in boring #5.
In October of 2013, an additional test pit was taken in the proposed basin to determine
the feasibility of infiltration. The results indicate that the infiltration rate would be on the
order of 0.06 inches per hour for the area. (refer to attached letter by Braun Intertec
dated October 31, 2013)
Once it became evident that infiltration on the site is infeasible, Lennar stepped up their
landscaping design in an effort to provide additional volume abstraction. They are
proposing 124 new trees in addition to other plantings throughout the site. They are
also preserving the existing berm and trees along the north side of the site.
We are proposing a filtration basin and wet pond to meet the phosphorus calculations.
BOULDER COVE
Lennar Corporation
Otto Project No. 14-0105
February 12, 2014
Volume Abstraction
1" Volume Abstraction = 1" x 3.4 acres = 12,342 CF
Minimum Volume Abstraction required = 0.5" x 3.4 acres = 6,171 CF
Proposed Filtration Basin
Volume Below Overflow = 13,687 CF
Proposed Volume Abstraction = 13,687 CF ÷ 2 = 7,178 CF
Tree Preservation
AV = 53,713 SF preserved x 0.10 x 1" = 448 CF
Planting of New Trees
SpeciesNo.
Avg %
Interceptor
Maple368
Linden712
Oak610
Evergreen4410
Ornamental3110
AV = (0.5)(%I)(Canopy Area) x 1 inch
Per City Code: 1 Tree = 1,089 SF Canopy
AV = 536 CF
Total Proposed AV = 8,162 CF (0.66" of Rainfall Over Site)
BOULDER COVE
Lennar Corporation
Otto Project No. 13-0425
February 3, 2014
Phosphorus Calculations (per Appendix L of MN Stormwater Manual )
L = PRvCA(0.2)
Rv = 0.05 + 0.009I
I = % Impervious
Lpre =26 (0.05 + 0.009 x 9.72)(0.3)(13.38)(0.2)=2.87lb/yr
Lpost =26 (0.05 + 0.009 x 25.41)(0.3)(13.38)(0.2)=5.82lb/yr
Removals:LR = (Lpost)(BMPre)(%DA served)
LR bio:(5.82)(0.5)(0.30) = 0.87 lb/yr
LR wet pond:(5.82-0.87)(0.5)(0.89) = 2.20 lb/yr
Total Lpost =5.82-0.87-2.20 =2.75lb/yr
SUMMARY
Runoff (cfs)
· AREA TO NORTH 2 Year 10 Year 100 Year
Existing 2.80 6.03 9.81
Proposed 2.71 4.92 8.25
· AREA TO WEST 2 Year 10 Year 100 Year
Existing 6.42 12.00 23.65
Proposed 2.61 6.78 21.41