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
upstream of Cross Section A as shown on Essex County FIS Flood Profile Sheet 81P.) The
geometric configuration of the existing 5-span bridge was then ―superimposed‖ between them.
The geometric configuration of the existing bridge was derived directly from the structure‘s 1951
construction plan set 5 .
Note: The MassDOT Hydraulic Section is indebted to Applied Coastal Research and Engineering
(ACRE) personnel (the project tidal hydrodynamic analysis consultant) for providing upstream and
downstream bridge cross sections ―cut‖ from the detailed bathymetric bottom TIN (Triangular
Irregular Network) they developed in support of the project RMA2 hydrodynamic model.
4. A ―Post-Project (Revised) Conditions‖ HEC-RAS model was in turn developed by modifying the
―Pre-Project (Existing) Conditions‖ model to reflect construction of a new Whittier Bridge and its
associated approach roadways. This step involved substituting the geometric configuration of the
proposed bridge type for that of the existing Whittier Bridge. Pertinent existing and proposed
bridge geometric configuration parameters incorporated in the ―Pre-Project (Existing) Conditions‖
and Post-Project (Revised) Conditions‖ HEC-RAS models are presented in Table 1-6 and Table
1-7.
5. Base flood simulations were run for all three hydraulic models described above. The results of
these simulations were then tabulated and compared to quantify the subject project‘s potential
impact on the Merrimack River‘s currently effective BFE profile and thereby establish the degree
to which the project will meet applicable NFIP Base floodplain development performance
standards- and comply with EO 11988. Pertinent computational results are presented in Table 4
in Appendix B.
Table 1-5:
Pertinent “Current Effective” HEC-2 Model Data
Parameter
Attributes
Model Reach Length
28,200 Feet
Ave Channel Length Between Cross Sections
4,700 Feet
No of Model Cross Sections 7
Ave. Cross Section Length
1,820 Feet
Manning’s N Channel (All Cross Sections) 0.04
Manning’s N Overbanks (All Cross Sections 0.075
Coefficient Of Contraction (All Cross Sections 0.3
Coefficient Of Expansion (All Cross Sections) 0.4
Upstream Water Surface Boundary Condition
13.6 Feet, NGVD
Downstream Water Surface Boundary Condition 8.7 Feet, NGVD (10-Year Tidal Flood Elevation, Newburyport, MA)
Upstream Peak Discharge Flow Input
115,000 Cubic Feet Per Second
Conclusions
As shown in Table 1-7, insertion of the two existing Whittier Bridge cross sections into the ―Duplicate
Effective‖ HEC-RAS model resulted in 0.1 to 0.2 foot increase in the river‘s BFE profile upstream of
the I-95 crossing location. This result is reasonable and indicates that, most likely, the total energy
head loss a 100-year flood event will sustain as it passes through the existing bridge waterway
opening is relatively minor.
5
MassDOT Construction Plans, Br. No. A-07-016=N-11-017, January 1951.
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