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

Whittier Bridge/I-95 Improvement Project FEIR
Chapter 1.0: Changes Since the Draft Environmental Impact Report and Resolution of Additional Issues
2. During demolition of the existing bridge, the new northbound bridge will be cleared and
cofferdams limited in demolition of the existing bridge piers.
3. During construction of the new I-95 southbound bridge, Pier 1 will be constructed on top of
the existing Pier 1 and will utilize the same cofferdam used for demolition of the existing
piers. The demolition cofferdams for Piers 2, 3 and 4 will be removed prior to construction of
new Piers 2 and 3.
4. The cofferdams will be designed to be overtopped and reduce flow impedance during
periods of high river flow. The crest of the cofferdams are at elevation 6.1 feet NAVD88
which is two feet above Mean High Water and 1.3 feet below the 10-Year Return Period
Flood (7.4 feet NAVD88). This will allow high water events and return period flood events to
pass with minimal flow obstructions during construction.
5. The foundation designs for the new piers along with the footprints for the cofferdams have
been optimized to lessen the hydraulics impacts to the river during construction. The
cofferdams will be tightly positioned around the pier‘s footings to minimize flow obstruction
and backwater effects. The optimization of the pier design resulted in no effect on the
existing National Flood Insurance Program (NFIP) base flood elevation (BFE) profile and
regulatory floodway delineations at the bridge.
6. The new bridge abutments have also been designed and repositioned to lessen their
encroachment along the river banks. The abutments will be positioned 50 feet further
landward along the north and south sides of the Merrimack River.
1.4.9 Cofferdam Dewatering Protocol
The DEIR Certificate and the MassDEP comment letter requested that a protocol for daily
dewatering discharges to the Merrimack River from the cofferdams be developed in order to limit
turbidity impacts to the Merrimack River fishery and to avoid discharge of lead or asbestos in the
dewatering effluent.
MassDOT will require that the design/build contractor develop a cofferdam dewatering protocol to
control the discharge of water from the cofferdams. The protocol will address the clarity and turbidity
of water being discharged into the river. The protocol will ensure that all discharge water passes
through a sedimentation tank and that the tank effluent will be monitored to ensure discharges into
the river remain clear will not create a visible plume. In general, the cofferdam water should be able
to pass directly through the properly sized sedimentation tank into the river until the water level is
lowered to within three feet of the bottom of the cofferdam. At that time, additional measures such as
use of lower volume pumps or intermittent pumping would be used so that previously settled solids
would not be placed into suspension and discharged into the river.
During the final drawdown to three feet from the bottom of the cofferdam, the discharge effluent
would be directed to a sedimentation tank with appropriate baffles and/or increased settling time to
capture sediment prior to discharge to the river. The final six inches of discharge water would be
directed to a localized sump using a low volume pump within the cofferdams to collect sediment prior
to discharge into the sedimentation tank and subsequently to the river.
1.4.10 Chapter 91 Compliance
The DEIR Certificate and the MassDEP comment letter requested that the information submitted in
the Chapter 91 License application be included in the FEIR. This section includes information on the
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