Final Environmental Impact Report - Whittier Bridge/I-95 ...

Whittier Bridge/I-95 Improvement Project FEIR
Chapter 2.0: Response to Comments on the Environmental Assessment/Draft Environmental Impact Report
the Merrimack River. Additional treatment for the redevelopment portion of the project will be
incorporated to the maximum extent practicable.
No headwalls, splash pads, aprons, or preformed scour holes will be located in land under water,
bordering vegetated wetlands, or salt marsh.
DEP-13: Standard 2 – Peak Rate Control. Peak rate control was evaluated using the macro-approach. As there are
50 separate outfalls, plus stormwater discharges from bridge scuppers, the conservation commissions will need to
evaluate whether selection of the design points avoids disproportionate impacts to anyone wetland or water. No peak
rate control was proposed for 100-year storm to Merrimack River. An evaluation needs to be submitted to the
Newburyport and Amesbury Conservation Commissions that documents that no increased off-site flooding will result
from not attenuating the 100-year storm runoff from the Merrimack River bridges.
Response: The largest storm events that have occurred through an 87-year period were in April
2007 and April 2010. These events indicated that the actual flow rate in the Merrimack River
adjusted for the Whittier Bridge was on the order of 132,000 cubic feet per second. The incremental
increase in flow rate as a result of the 100-year storm would represent less than 0.01% of this total
flow rate in the Merrimack River. Furthermore, due to the overall size of the Merrimack River
Watershed and the resultant time of concentration for the peak discharge to reach the Whittier
Bridge, attenuating the 100-year storm would not provide any benefit to downstream properties.
Extended detention of the 100-year storm could be counterproductive as the delayed discharge
could introduce stormwater runoff into the river at a point in time that that is closer to its peak; under
this scenario, it would be more advantageous to divert the runoff into the river in advance of its peak.
Consequently, not attenuating the 100-year storm will not increase the potential for off-site flooding
to downstream properties. This information has been submitted to the conservation commissions.
DEP-14: The stormwater recharge standard in MassDEP Stormwater Handbook Volume 1, Chapter 1, Footnote 8,
allows recharge using the macro approach for MassDOT highway and bridge projects, provided recharge is directed
to the same sub-watershed. MassDEP generally interprets sub-watershed to be first order systems. MassDEP
identified 9 first order sub-watersheds within the 4.25-mile long project route where no recharge is proposed,
however recharge was proposed within at least the third order watersheds. The first order systems are classified as
vulnerable wetlands, along with vernal pools and public drinking water supplies. MassDEP met with MassDOT to
discuss and requested additional information, and that information is under development. While recharge credit from
extended detention and wet basins is not appropriate as was indicated in the Stormwater Report, MassDOT
consultants indicated proposed swales potentially may be able to be redesigned with check dams to enhance
infiltration. As MassDOT has already filed NOIs with the Newburyport, Amesbury, and Salisbury conservation
commission in advance of completing the MEPA process, MassDEP requests that MassDOT update their
applications, or that the conservation commissions condition the project accordingly. MassDEP request that the
Secretary require that MassDOT address this issue within the scope of their FEIR or in the permit process.
Response: MassDOT has incorporated additional provisions into the stormwater management
design such that some level of recharge will be provided within the nine sub-watersheds identified by
MassDEP. As discussed in coordination meetings with MassDEP, the quantity of recharge that could
be provided within these nine locations is limited by either the physical site characteristics, proximity
to adjacent resource areas, or environmental concerns. A water quality swale will run in parallel with
the highway alignment for the majority of the project limits. Check dams will be included in the
design of the median water quality swale to facilitate additional recharge. This information has been
provided to the conservation commissions with each NOI filing.
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