HomeMy WebLinkAboutSP-20-019 - Supplemental - 0057 Hinesburg Road (58)Site Plan Review Comments
The Stormwater Section (City) has reviewed the “Green Mountain Habitat for Humanity- 57 Hinesburg Rd” site plan prepared by Lakeside Environmental Group, LLC, dated Feb. 12, 2020. We
would like to offer the following comments:
The proposed project is located in the Potash Brook watershed. This watershed is listed as stormwater impaired by the State of Vermont Department of Environmental Conservation (DEC).
The project proposes to create 10,420 square feet of impervious area on the parcel. This results in less than 1 total acre of impervious area on the parcel.
The VT ANR Natural Resource Atlas wetland advisory layer shows possible Class II wetland located 65 feet to the west of the lot. Has a delineation been performed to assure that there
is not a wetland/ buffer impact?
Lakeside observed the same wetland mapping and examined the land 50 feet to the west of the site and did not observe any hydrologic or vegetative indicators of wetland soils, so no formal
delineation was performed.
Considering the potential nearby wetlands and close proximity to Tributary 3 of Potash Brook, has the applicant performed infiltration tests in the location of the proposed “Stormwater
Infiltration Basin”? Based on information from recent projects in this area, the City is aware that the water table is quite high in this area- potentially making infiltration difficult.
Lakeside agrees that the area is expected to have a seasonal high water table. The Duane/Deerfield soils are typically expected to experience high groundwater during the spring and
late fall, and occasionally during the months in between after an extended period of precipitation. During periods of normal precipitation, the groundwater level can drop significantly
in these soils.
For stormwater design purposes, Lakeside assumed a SHWT of approximately 311 feet, one foot below the lowest, nearby ground elevation (see blue shaded area at elev. 312). As the proposed
basin has a bottom elevation of 311 feet, the basin should allow infiltration during most times of the year.
What size storm is the Infiltration Basin designed for? As currently designed, the Infiltration Basin overflow will send water directly into the perimeter fence and then onto the paved
area of the loading docks for Commerce Square. This design is not ideal due to the potential impacts to adjoining property. Could the applicant configure the overflow to drain out the
western side of the basin?
The basin has not be designed to a particular storm event as the ¼ acre impervious area does not trigger any stormwater regulations (city or state).
The project site currently drains onto the Commerce Square site, not to the western side of the basin. This is indicated by the 313 foot controur line highlighted in blue. The western
end of the property is, in effect, a shallow berm above elev. 313 (see red dashed line). The applicant does not have the legal right to discharge onto the adjoining property in that
area.
The existing drainage flows onto the rear of the Commerce Square site and then flows westerly toward the mapped wetland area referred to earlier (see blue arrows).
The applicant should include a note within the plans about the maintenance needs of the infiltration basin.
A maintenance note has been added.
Silt fence or other EPSC measures should be in place along the western perimeter of the lot.
For the reason explained under item 5 above, silt fence along the western property line would serve no purpose and has not been added to the plan.
Due to site constraints, considerations should be noted for vehicle parking, materials and soil stockpiles during construction.
Excess soil from foundation and basin excavation will be removed from the site, not stockpiled. See construction sequence note 8.
Once rough grading is achieved, the base stone for the parking lot will be installed to provide for stable construction parking. See construction sequence note 16.
Thank you for the opportunity to comment.
-Emmalee