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

elative to the Merrimack River’s NFIP base floodplain- NEPA regulations require that MassDOT
demonstrate the project’s compliance with EO 11988 as part of the project NEPA review
process.
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 a regulatory floodway delineation, there are two critical NFIP development
performance standards that project activities must meet- construction of the new Whittier
Bridge cannot neither result in any increases to the Merrimack River’s existing BFE profile, nor
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, 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. See Reference 1 for
computational procedures.
Base flood Hydraulic Analyses
The following five step procedure as 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 USACOE 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 complied in 1977 by the USACOE
NED 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 ft 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 USACOE 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, it’s 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 Appendix 1, Table 1.
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 -
USACOE 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