HomeMy WebLinkAboutApplication Files_82693_pg0444 Stormtech®Infiltration System
The Stormtech®underground infiltration system will be located along the eastern
portion of the studio development. Test Pit data in this area(see Appendix B)
indicates a fine textured sandy loam mantle over a coarser textured sand. Soil
samples analyzed from these test pits indicate permeability rates between 60 and 92
inches per hour at a depth of three feet below existing grade. Monitoring wells were
set in these test pits and a seasonal high groundwater table(SHGWT)adjustment
factor was applied.The SHGWT varied from elevation 43.0 to 45.0. In consideration
of this data,the Infiltration System has been designed with greater than a three-foot
separation to seasonal high groundwater or bottom at elevation 52.0. The hydrologic
model of this system includes a safety factor of 2(per the"2004 Connecticut
Stormwater Quality Manual,"Chapter 11 Section 11-P3 that reads the"recommended
�. design infiltration rate is equal to one-half the field-measured infiltration rate(i.e.,
safety factor of 2)")and resultant infiltration rate of 29 inches per hour.
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Biofiltration Basins
The biofiltration basins(or rain gardens)manage and treat stormwater runoff using a
conditioned planting soil bed and planting materials to filter runoff stored within a
shallow depression.Typically,the system consists of a pretreatment grass filter strip,
a stone diaphragm,a shallow ponding area,a surface organic layer of mulch,'a
planting soil bed,plant material,a gravel underdrain system and an overflow drain.
' There are,however,several bio£iltration basins associated with this project that
feature slight design modifications,as local conditions dictate. pull details of each
basin's design can be seen on the Site Details and Planting Plan.
Vegetation in a biofiltration basin serves to filter and transpire runoff—improving.
water quality and reducing runoff quantity—and the root systems can enhance
infiltration.The soil medium filters out pollutants and allows storage and infiltration
of stormwater runoff;and the infiltration bed provides additional volume control.
These will be inspected biannually for the first year and annually thereafter for
sediment buildup,erosion,vegetative conditions,etc.Details are included in the
Long Term Operations and Maintenance checklist in Appendix C.
Infiltration Basins
Infiltration Basin 1 will be located to the northwest of the development,and will
accept stormwater from the adjacent access drive and northerly abutting properties.
Stormwater runoff from the access roadway and easterly abutter will flow through a
grassed swale and then treated in a sediment torcha y before entering the basal.
Stormwater runoff from the westerly abutter will not be treated through a sediment
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