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
The basic EO 11988 compliance requirement for this project is that the new Whittier Bridge, when
constructed, will conform to applicable NFIP base floodplain development performance standards.
Because the existing bridge crosses a waterway with both a computed BFE profile and regulatory
floodway delineation, there are two critical NFIP development performance standards that project
activities must meet. Construction of the new Whittier Bridge cannot either result in any increases to
the Merrimack River‘s existing BFE profile, or necessitate alteration to the river‘s existing regulatory
floodway delineation, at any point within the affected communities. If implementation of the new
bridge will not meet both of the above requirements, NFIP regulations (and, by direct extension, the
terms of EO 11988) require that MassDOT file a pre-construction Conditional Letter of Map Revision
(CLOMR) with FEMA pursuant to revision of the river‘s currently effective base floodplain mapping
products.
Engineering Methods
Base Flood Hydrologic Analysis
Peak base (100-year) flood discharges for the Merrimack River used in this evaluation were adopted
from the Essex County FIS and are presented in Table 1-5.
The following five step procedure was followed to determine the probable impact of the Whittier
Bridge Project on the Merrimack River‘s existing NFIP BFE profile and regulatory floodway
delineation. This procedure is consistent with FHWA and FEMA base floodplain encroachment
review guidelines set forth in References 9 (44 CFR Part 65) and 10 (23 CFR Part 650.115).
1. A ―hard‖ copy of the input file for the USACE HEC-2 Water Surface Profile Model developed to
determine the Merrimack River currently effective BFE profile and regulatory floodway delineation
in Newburyport was secured from the FEMA NFIP Engineering Library in Alexandra, VA. This
―Currently Effective‖ HEC-2 model was completed in 1977 by the USACE New England District
during the Newburyport FIS. The river reach considered in the model was about 28,200 feet long,
extending from a point located about 13,100 feet downstream of the I-95 crossing to the
Newburyport/West Newbury corporate limit. The model itself consists of seven river cross
sections linked along a linear river channel alignment. The USACE apparently intended that this
model serve as a means of simulating the occurrence of the flood of March 1936 over the study
river reach. So, its river cross section grid was deliberately assembled so as to reflect built and
natural environmental conditions along the banks of river just prior to that flood event.
Consequently the model did not incorporate any cross sections surveyed at the location of
bridges and other shoreline structures constructed after 1936 (including the existing Whittier
Bridge). Pertinent ―Current Effective‖ HEC-2 model configuration data are presented in Table 1-5.
2. The entire content of the effective Merrimack River HEC-2 model input file was manually
programmed into a contemporary, Windows-based water surface profile modeling application, the
USACE HEC-RAS (River Analysis System) Version 4.1. The resulting HEC-RAS model was then
calibrated so as to reproduce the river‘s BFE profile as shown on Flood Profile Sheet 81P of the
Essex County FIS within 0.1 feet, and thus constitute a ―Duplicate Effective‖ Merrimack River
hydraulic model.
3. The ―Duplicate Effective‖ HEC-RAS model was then modified to reflect current local topographic
and built environment conditions with the existing Whittier Bridge in place and thereby yield a
―Pre-Project (Existing) Conditions‖ HEC-RAS model. This process involved first inserting two river
cross sections (one acquired just upstream and another acquired just downstream of the existing
bridge waterway opening) at present bridge crossing location into the duplicate effective model‘s
cross section grid. (The insertion point of these cross sections was located about 590 feet
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