HomeMy WebLinkAboutSP-20-019 - Supplemental - 0057 Hinesburg Road (62)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:
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 existing grading allows for sheet flow over pervious surfaces to the northwestern corner of the property. This is considerably different than sheet flow from impervious surfaces
being captured in an infiltration basin and then overflowing from a single location onto the adjacent property. The applicant is encouraged to do some modeling to determine what level
storm event will trigger an overflow. The applicant should be encouraged to demonstrate that their proposed practice will not impact the adjoining property in a detrimental manner.
Additionally, the determination of surface flow on the adjoining property appears to be inaccurate. There is a catch basin located in the corner of the paved surface of the Pomerleau
property, approximately in the location of the highlighted blue 312’ elevation. The flow from that surrounding area is captured by that catch basin.
The historic flows from the project site onto Pomerleau historically flowed west. The catch basin installed by Pomerleau intercepted those flows, but the storm drain still flows west.
We have modeled the stormwater for the 25 year storm and increased the height of the basin by one foot so that the POST development flows to Pomerleau equal the PRE development flows.
The modeling by Watershed Consulting Associates is attached.
We used the infiltration rate of 6 inches per hour for the Adams and Deerfield Soils as listed by the NRCS.
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.
These construction notes don’t appear to be with this submittal. Please be sure to include the construction notes on the applicable sheets.
The construction notes are on sheet 2.0.
Thank you for the opportunity to comment.
-Emmalee