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

3.10.2 Existing Conditions at Wetland H Mitigation site
Whittier Bridge/I-95 Improvement Project FEIR
Chapter 3.0: Mitigation and Commitments
The existing upland in the proposed mitigation area (Wetland H, Site 1) consists of a relatively
disturbed steep sandy slope leading down to the wetland. The canopy is open and patchy, and
contains mostly small trees and saplings with grasses and early successional shrubs in the
understory. The bank ranges from 20-25 feet above the surface of the wetland. Considerable trash
was observed in the upland near the mitigation site. The existing conditions on the site are shown on
Sheet 1 (map pocket at end of chapter). Section 1.4.5 includes information on the existing
conditions at the mitigation site.
3.10.3 Proposed Mitigation at Wetland H
The proposed mitigation at Wetland H will include excavating the existing upland slope to
approximately the 28-foot contour. The goal in the proposed wetland replication will be to establish
forested wetland adjacent to the existing marsh by planting young saplings and shrubs, and seeding
to establish a dense herbaceous layer.
This wetland replication will replace the lost acreage at Wetlands I and H at an approximately 1:1
ratio and will replace the lost principal functions of Floodflow Alteration, Sediment/Toxicant Retention
and Nutrient Removal. Functions gained by the wetland replication will include flood storage,
sediment/toxicant retention, nutrient transformation/retention, and wildlife habitat. The functional
quality for wildlife habitat of the replication wetland will be limited by the disturbed nature of the
existing wetland caused by the dominance by invasive species (common reed and purple
loosestrife) and the developed upland surrounding most of the area. Detailed information on the
mitigation design is included in section 1.4.5. Full size copies of the mitigation site plans are
included in map pockets at the end of this chapter.
3.11 FLOODPLAINS
The project will not result in a measureable impact to the 100-year floodplain or the Regulatory
Floodway in the Merrimack River; therefore, no mitigation measures are proposed.
Hydraulic modeling of the proposed Whittier Bridge cross sections into the ―Duplicate Effective‖
HEC-RAS model resulted in 0.1 to 0.2 foot increase in the Merrimack River Base Flood Elevation
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.
Base Flood Elevation profiles computed with the ―Pre-Project (Existing) Conditions‖ and ―Post-
Project (Revised) Conditions‖ HEC-RAS models are virtually identical. This result directly infers that
the construction of the preferred replacement bridge type will meet applicable NFIP base floodplain
development performance standards and thereby assure overall project compliance with the terms
of EO 11988. This result also directly infers that implementation of this alternative will not
necessitate further project coordination with FEMA pursuant to revising existing NFIP base
floodplain mapping products for the Merrimack River.
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