Chapters 1 - U.S. Fish and Wildlife Service

Chapter 4
lead to a breach in the dike that separates the Bolsa Cell from the mitigation channel. The
condition of the culverts at the center of the Cell is currently unknown.
To provide adequate tidal exchange between the existing main wetlands complex and the two
eastern restoration sites, 7 th Street Pond and Perimeter Pond, the southernmost tidal channel in
the main wetlands complex was extensively modified. The main length of this east/west aligned
channel from just northeast of Hog Island to a point to the southeast of the terminus of 7 th Street
was widened, and a new connector channel was dredged from the main channel southeast to the
proposed site of Perimeter Pond (refer to Figure 4-11). In addition, a box culvert was installed at
the south end of the 7 th Street Pond to convey tidal flows from the modified channel under an
existing unpaved access road and into the 7 th Street Pond.
Shortly after construction, concerns were also raised regarding the high velocities observed during
incoming and outgoing tides in the vicinity of the box culvert installed at the south end of the 7 th
Street Pond. An extreme hydrodynamic condition known as a vortex was observed in the area
during times when high speed water currents were being drawn through the submerged culvert.
This condition, which continues today, was evaluated during the five-year monitoring period for the
Port of Long Beach mitigation project. The extreme vortex formation occurring adjacent to the 7 th
Street Pond culvert, as well as erosion occurring in some of the created tidal basins and around
associated culverts was evaluated during this monitoring period (Sea Dyn, Inc. 1993). The vortex
formation was a concern because of the adverse effect it was having on diving birds. To eliminate
or reduce the strength of the vortices forming at the 7 th Street Pond culvert, the study
recommended installing flow vanes within the existing culvert wing walls. The flows vanes would
consist of steel plates bolted onto the culvert (Sea Dyn, Inc. 1993). The results of this analysis
indicated that altering the culvert in this manner would not be expected to impact the current tidal
flow into and out of the 7 th Street basin; however, the channel slope located opposite of the culvert
would likely have to be armored. No actions have been implemented to date to address this issue,
and prior to considering this or any other alterations, additional modeling and analysis would be
conducted to more fully understand the potential effects to existing hydraulic conditions and the
surrounding habitat, as well as to large fish, marine mammals, sea turtles, and other marine
organisms traveling through the existing culvert.
Tidal current, water levels, and habitat areas were monitored by MEC Analytical Systems, Inc.
between March 16, 1990 to December 18, 1990 and March 27, 1992 to October 7, 1992 (MEC 1995).
Monitoring of tidal currents revealed that the currents were high enough to cause sediment
transport throughout the channel system in the restored areas, as well as local scour in the vicinity
of the culverts. The subsequent 1993 erosion study (Sea Dyn, Inc. 1993) concluded that scour in
the 7 th Street basin and the associated supply channel appeared to have reached equilibrium and
that scour opposite the culvert exit will probably continue, but at a much slower and diminishing
rate. The 1993 report also concluded that “given the present equilibrium state, no slope protection
measures are appropriate at the present time. However, if any changes are made to the tidal
response of the basin or the flow field around the culvert, slope protection measures may be
required” (Sea Dyn, Inc. 1993). Periodic visual inspection of the slopes was recommended.
Erosion and scour processes within the restoration area were once again analyzed in late 2006 and
early 2007 by Everest International Consultants, Inc. (2007) due to continued concerns about the
stability of the slopes and channels in the restored area. Everest concluded that erosion continues
to occur within the restoration area and that erosion appears to have expanded to areas that were
not identified as problems in prior reports. A previously undocumented flow vortex was also
observed in the vicinity of the large culvert that conveys tidal waters from the main marsh complex
under Bolsa Avenue and through the Bolsa mitigation channel to the Forrestal Pond and Case
4-20 Seal Beach National Wildlife Refuge