HomeMy WebLinkAbout2013-01-15 Planning Board Supplemental Materials (15) Eggleston Environmental
32 Old Framingham Rd Unit 29 Sudbury MA 01776 tel 508.259.1137 fax 866.820.7840
January 7, 2013
North Andover Planning Board
1600 Osgood Street
North Andover, MA 01845
Attn: Judy Tymon, Town Planner
RE: 1018 Osgood Street
Stormwater Management Review
Dear Ms. Tymon and Board Members:
I am writing this letter in follow-up to my November 14, 2012 review letter on the above-
referenced project. Since that time I have received and reviewed the December 7, 2012
revised plans and Stormwater Management Report submitted by MHF Design
Consultants. I also participated in a meeting with town staff and the applicants on
November 19, 2012 to discuss my comments and the issues on the site.
The design revisions reflect a decrease in overall impervious area on the site and provide
stormwater recharge through two proposed infiltration systems, one in the area draining
to the onsite wetland and one in the front portion of the site, in the Watershed Protection
District. The proposed Stormceptor unit has been eliminated, as water quality treatment
of runoff flows is to be provided through the oil/grit chambers and infiltration systems.
The revised drainage analysis reflects these design revisions and uses the more
conservative Rawls rate for modeling of exfiltration. I note that the revised Stormwater
Report also includes data from four additional test pits conducted on the site in November
2012.
My comments on the revised submittal are outlined below:
1. At our meeting on 11/19, we discussed the need for distributing recharge across
the project site, and particularly within the Watershed Protection District. I
indicated to MHF that even if they could not provide a two-foot separation to the
high groundwater elevation, an infiltration system within the WPD should still be
able to provide effective recharge and filtering of flow under most groundwater
conditions. With the additional test pit data, however, a clearer picture of the
groundwater conditions on the site emerges, and I question whether any
subsurface infiltration system can be used to effectively recharge or treat flow
within the WPD. Seven of the eight test pits conducted on the site (four in August
2012 and another four in November 2012) indicate that the high groundwater
elevation is approximately three to four feet below existing grade across the site.
Evidence of groundwater was not seen in the eighth test pit, however based on the
other test pits the ESHGW was estimated to also be four feet below grade. Thus,
the ESHGW elevation across most of the project site ranges from elevation 144 to
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January 7, 2013
140. Infiltration System #1 (near the wetland) is located in a portion of the site
where the proposed ground elevation will be raised. The bottom of the stone in
the system would only be 1.2 feet above the ESHGW, but the system should be
able to function adequately most of the time. Proposed Infiltration System #2,
however, is located in a cut portion of the site, where the proposed surface grades
will be lowered by approximately three feet. The entire subsurface infiltration
system, with a bottom invert of 136.67, would therefore be several feet below the
ESHGW elevation in that area, and likely to be in groundwater most of the year.
2. Even without the groundwater issue, the proposed plan only attenuates the rate of
runoff flow leaving the site, not the volume. The volume of flow discharged to
Osgood Street under post-development conditions would more than twice the
existing runoff volume under every design storm condition, and could exacerbate
any downstream flooding.
3. The proposed design also does not provide sufficient dead storage volume in the
infiltration systems to treat even a ½-inch water quality volume. The TSS
calculations use the storage volume in the Oil/Grit Chambers to reduce the water
quality volume in the infiltration systems. This is an invalid assumption for two
reasons. First, the oil/grit chambers are solid structures and are designed to
operate full and to remain full between storms. Second, in order to provide the
treatment necessary to get the 80% TSS removal credit in the infiltration systems
the entire water quality volume must be infiltrated.
4. It is not clear what the basis is for the saturated thicknesses of 129 and 133 ft used
in the mounding calculations. As indicated in my previous comments, saturated
thickness should be the difference between ESHGW and the bottom of the
aquifer, probably something in the range of 10 to 15 feet. The model may call for
an elevation, but it also needs the saturated thickness of the aquifer as this is part
of what determines how quickly the additional groundwater flow is dispersed.
These elevations are not even consistent with the ESHGW on the site.
5. On Sheet 8, the detail for the Snout Oil/Water separator should be
clarified/relabeled to indicate that it is the outlet hood for the catchbasins, so as
not to confuse it with the detail for the larger oil/water separator chambers. I
would actually suggest that the larger chambers be called out as oil-grit chambers.
The detail for the Stormceptor units should also be removed from the plan.
6. An application for a Watershed Special Permit, including a written certification
by a professional engineer stating that the project will not cause any significant
degradation in the quality or quantity of water in or entering Lake Cochichewick
is still needed.
Once again, I appreciate the opportunity to assist the North Andover Planning Board with
the review of this project, and hope that this information is suitable for your needs. Please
1018 Osgood Street, Stormwater Review 3
January 7, 2013
feel free to contact me if you or the applicants have any questions regarding the issues
addressed herein.
Sincerely,
EGGLESTON ENVIRONMENTAL
Lisa D. Eggleston, P.E.
C: Jennifer Hughes, Conservation Coordinator