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

The Commonwealth Of Massachusetts
Massachusetts Department of Transportation
Highway Division
Interoffice Memorandum
To:
Through:
From:
Thomas Donald, P.E., Director of Project Development
Alex Bardow, P.E., State Bridge Engineer
Richard Murphy, P.E., Hydraulic Engineer
Date: February 8, 2012
Subject: Amesbury/Newburyport: Br. No. A-07-016= N-11-017, BIN 2V1,
Route I-95/Merrimack River, Whittier Bridge Replacement Project,
Merrimack River Base Flood Elevation Profile and Regulatory Floodway Impact
Evaluation, Project File No. 601096
At the request of the MassDOT manager of the subject project, I have performed a
technical evaluation of the project’s probable impact on the Merrimack River’s base (100-Year)
flood elevation profile and regulatory floodway delineation as determined within the 2012 Essex
County Flood Insurance Study (FIS) (Reference 1) . An Executive Summary and brief narrative
sections discussing general project base floodplain regulatory compliance requirements,
engineering methods employed in this evaluation, and pertinent analytical results of the same
follow directly.
Executive Summary
The purpose of this evaluation is to quantitatively determine the probable impact of
project implementation on the Merrimack River’s existing National Flood Insurance Program
(NFIP) base flood elevation (BFE) profile and regulatory floodway delineation in proximity to
the Whittier Bridge crossing location. This evaluation was performed in a manner consistent
with current Federal Highway Administration (FHWA) and Federal Emergency Management
Agency (FEMA) base floodplain coordination guidelines (see References 9 and 10). The results
of this evaluation are critical to establishing whether or not project implementation will
necessitate revision of existing Merrimack River NFIP base floodplain mapping products, and
the degree to which this project complies with Executive Order 11988 (EO 11988), Floodplain
Management (see Reference 8).
Project Base (100-year) Floodplain Regulatory Compliance Requirements
The existing Whittier Bridge (Br. No. A-07-016=N-11-007) crosses a reach of the
Merrimack River with a NFIP Zone AE Special Flood Hazard Area (SFHA) coding and a
regulatory floodway delineated within the mapped base (100-Year) floodplain limits. This NFIP
SFHA designation means that the river’s BFE profile- and the horizontal limits of it’s regulatory
floodway – have been determined by application of detailed hydrologic/hydraulic engineering
methods- and are presented within the affected Communities Flood Insurance Study (FIS) and
Flood Insurance Rate (FIRM) mapping. The Merrimack River’s NFIP BFE profile at the Whittier
Bridge crossing is shown on Flood Profile Sheet 81P (see Attachment 1) of the 2012 Essex
County FIS. The limits of the river’s local regulatory floodway delineation are listed in Table 12,
Floodway Data of that FIS, and illustrated on Essex County Digital Flood Insurance Rate Map
(DFIRM) Number 25009C0108F.
The proposed Whittier Bridge Replacement Project is a federally funded action, with
the FHWA acting as the lead federal agency. As such, the project must comply with the terms
of the National Environmental Policy Act (NEPA). Because of the existing bridge’s location