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

Whittier Bridge/I-95 Improvement Project FEIR
Chapter 2.0: Response to Comments on the Environmental Assessment/Draft Environmental Impact Report
Response: This comment has been investigated. The stormwater improvements proposed within
the SHLO, west of the Martignetti parcel will include a sediment forebay, detention basin (3B), and
infiltration basin (3A). Access for construction of the basins will be from the SHLO to the west.
Access for the operation and maintenance of the basins will be served through the Shared-
Use_Path. The existing topography of the SHLO for this portion of the project varies in elevation
between EL=57 to EL=40. As further described below, a majority of the relocated NB roadway and
adjacent stormwater facilities will require a significant earth excavation and will primarily be
constructed at an elevation that is much lower than the subject property.
The sediment forebay proposed at station 221+75 will require an earth excavation of approximately
15 feet (bottom of forebay EL=42 vs existing grade EL=57). Removal of large boulders and bedrock
is anticipated for the construction of the forebay and adjacent NB roadway. The elevation of the
Matignetti property is approximately EL=50 at this location resulting in a proposed forebay that is
approximately 8 feet lower in elevation than the subject property.
The detention basin proposed between ST 222 and ST 224+50 will require an earth excavation of
approximately 10 feet for a majority of the basin, and an excavation of 15 feet at its southern limits
adjacent to the interface with the forebay. The bottom of the proposed basin is EL=39 whereas the
adjacent subject property ranges in elevation between EL=46 to EL=48, resulting in a basin bottom
that will be atleast 7 feet below the subject. The primary purpose of the detention basin is provide for
the temporary storage of stormwater during large storm events. It is intended to drain completely
through an outlet control structure within 24 hours of any major storm event. It is anticipated that the
installation of the detention basin will require the removal of rock and or shallow bedrock. Under the
scenario of existing shallow bedrock at this location, consequently a strong potential for a perched
water table; removal of the bedrock under this condition to the elevations previously described would
likely result in the lowering of the groundwater table in the immediate vicinity.
The infiltration basin that is proposed between ST 225 and ST 226 will require an earth excavation of
10 feet at its southern limits to an earth excavation of 2 feet at its northern limits. The bottom of the
proposed infiltration basin is EL=37.5, the adjacent subject property ranges in elevation between
EL=45 to EL=47. The basin bottom is proposed at least 7.5 feet below the subject. The primary
purpose of the infiltration basin is to provide water quality treatment and groundwater recharge
consistent with the stormwater management performance standards. The basin is designed to stage
up with approximately 2.2 feet of stormwater for precipitation events that contain at least ½ inch of
rainfall. Based on the subsurface explorations that have been completed to date, we understand that
the seasonal high groundwater at this location is approximately 2 feet below the basin.
Conservatively speaking, assuming that the groundwater mound as a result of the proposed
infiltration basin reaches an elevation EL=39.7 in a uniform direction around the basin, the mound
would be 5 feet lower in elevation than the surface elevations of the adjacent subject. The public
records available through the municipal website indicted that the building does not have a full cellar
foundation which leads us to conclude that even under a worst case mound scenario the impacts to
the groundwater condition should not have a negative impact to the subject.
The stormwater improvements proposed as part of the project should be constructed in a manner as
to not impact adjacent properties. The conclusions drawn in this response are based on a
combination of limited subsurface explorations, existing survey, and assumptions previously stated.
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