HomeMy WebLinkAbout2013-02-05 Planning Board Supplemental Materials (21) d
_ 44 Stiles Road °Suite One ®Salem, New Hampshire 03079
® — TEL (603) 893-0720 - FAX (603) 893-0733
F Design Consultants, Inc. www.mhfdesign.com
January 28, 2013
North Andover Planning Board
1600 Osgood Street
North Andover, MA 01845
Attn: Judy Tymon, Town Planner
Re: 1018 Osgood Street
JFJ Holdings,LLC
Sub: Response to Eggleston
Environmental Review Comments
Dated January 7, 2013
Dear Ms. Tvmon and Board Members:
On behalf of our client, JFJ Holdings, LLC,please find enclosed a revised set of
plans and supporting documentation regarding the above referenced project. The plans
have been revised to address the comments in the Eggleston Environmental review letter
dated January 7, 2013. Based on those comments, as shown below,we offer the
following responses in bold:
1. At our meeting on 11119, 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 there was less than two feet of separation to the
seasonal high groundwater elevation, an infiltration system within the WPD
should still be able to provide effective recharge and filtering of flow under most
periods of lower groundwater. With the additional test pit data included in this
submission, however, a clearer picture of the groundwater conditions on the site
emerges, and I question whether any below-grade system can be used to
effectively infiltrate storrnwater 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 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 below-grade infiltration system, with a bottom invert of 136.67,
would therefore be several feet lower than the ESHGW elevation in that area, and
likely to be in groundwater most of the year.
ENGINEERS 0 PLANNERS SURVEYORS
Ms.Judy Tymon
January 28,2013
Page 2 of 3
MHF Design Consultants,Inc.
Response: Additional test pits have been performed on 1/28/13 and added to
the Flan Set and Stormwater Deport and the results provide consistent data
with the previously submitted logs. Please note that infiltration system #2
has been raised two additional feet compared to the previous design. In
addition,the results of'fest Pit 9113-4 indicate a depth to eshwt of 6', thereby
providing the required 2' of separation to the eshwt to proposed infiltration
system #1.
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.
Response: We have increased the volume capacity within both infiltration
systems to the maximum extent based on the existing site conditions. In
order to mitigate the Post Development volumes the onsite infiltration
systems would need to be increased in upwards of 5-10x the size of the
currently proposed system and in our opinion is unreasonable and
inconsistent with previous developments approved in'Town.
3. The proposed design also does not provide sufficient dead storage volume in the
infiltration systems to capture and treat even a %2-inch water quality volume. The
TSS calculations submitted include 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 — they have no
storage capacity. Second, the treatment provided in the infiltration system is only
in the flow that is actually captured and infiltrated; the overflow from the system
is not treated. Therefore in order to get the 80% TSS removal credit the entire
water quality volume must be captured and infiltrated.
Response: Storage volume within both infiltration systems has been
increased to treat the 1/2-inch water quality volume.
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 relative elevations as the saturated thickness of
the aquifer is part of what determines how quickly the additional groundwater
flow is dispersed. The elevations used in the analysis are not even consistent with
the ESHGW on the site.
Response: The saturated thickness was taken by subtracting the 10 to 15 feet
from the ground surface elevation at each test pit location in the vicinity o
each infiltration system. This has been revised by subtracting the 10 to 15
feet from the eshwt as suggested.
Ms.Judy TymonEMUSINNumaw NINE=, MW BMW
January 28,2013
Page 3 of 3 _ P
HF Design Consultants, Inc.
5. The detail for the Snout Oil/Water separator on Sheet 8 should be clarified or
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. 1 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.
Response: The detail for the Snout Oil/Water separator has been relabeled
as suggested and the Stormceptor unit details have been removed from the
plan set.
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.
Response: Written certification has been provided as suggested.
Eased on the revised plans and documents,we feel that we have addressed the comments
in both the review letter and the staff meeting.
Please contact our office if you have any comments or questions.
Sincerely,
I) sign Consultants,Inc.
is A Tymula -
Project Manager
F:\Projects\Eng\305311\EE Review--Response 1-28-13.docx
cc: Lisa Eggleston, Eggleston Environmental
Greg Nolan, JFJ Holdings, LLC