2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
downcoast. To compensate for anticipated short-term sediment losses from the littoral budget due to
the natural formation of an ebb bar, initial lagoon shoaling, and fillet formation along the jetties, the
ebb bar located offshore of the entrance channel was pre-filled, and supplemental sand was placed as
beach nourishment adjacent to the channel at the time of construction. These pre-fills were intended to
minimize littoral sand loss to ebb bar formation and provide supplemental sand for early inlet
stabilization. In addition, the long-term project sediment management plan provides for periodic
down-coast beach nourishment using sediment derived from the FTB during maintenance dredging
operations, restoring the sediment lost from the littoral budget to the downdrift beaches.
MAINTENANCE DREDGING AND DREDGING TRIGGERS
Parameters of tidal muting, beach width, loss of subtidal habitat, closure risk, muted tidal basin
function, and water quality were analyzed to evaluate the functioning of the system and determine
when dredging should be performed. Some of these parameters have pre-established triggers including
tidal muting, beach width, and loss of subtidal habitat. Other parameters do not presently have
established criteria for triggering a dredging event.
In reviewing the established dredging triggers, it is clear that some of the triggers may never be met
except under extreme circumstances, while more significant triggers may exist that have not as yet
been quantified. Chronic beach erosion triggers are not likely to be met because of the ongoing
replenishment at Surfside-Sunset and the program’s effect on long-term beach growth trends.
Similarly, acute erosion triggers are not likely to be met due to the generally broad beach profiles at
trigger point transects. It is more likely that maintenance dredging will be required to address an
intrinsic system need related to the functionality of the MTB tidal control structures and Freeman
Creek. Final triggers to address this issue will need to be set once all of the MTBs are open to the FTB
and have operated under both normal and muted FTB conditions.
During periods in 2008 when the average of the lowest spring tides in each tide series achieved
elevations at or below –0.05 m NAVD, the west MTB functioned well. When the average of the
lowest spring tides in each tide series achieved elevations at or above 0.28 m NAVD, the function of
the west MTB was impaired and operational ranges were necessarily curtailed to avoid flooding above
designed operational levels. As an interim-operating trigger for maintenance dredging, it is
recommended that the occurrence of four or more consecutive low spring tides in the FTB that fail to
achieve low elevations of 0.12 m NAVD or lower, on a running average basis, should suggest dredging
is likely necessary. It is anticipated that maintenance triggers will need to be further modified in the
future as the central and east MTBs are opened to tidal flows.
Recommendations
• Modify the expectations of tidal range in the FTB from 2.75 m to 2.29 m, with tidal elevations
ranging from 2.02 to -0.27 m NAVD.
• Remove the dredge trigger of the Mean Low Tide muting of 0.152 m.
• Add an interim trigger of the rolling average of four consecutive lowest tides achieved during
spring tide series exceeding 0.12 m NAVD, described in detail in Section 3.
• Continue the tidal monitoring program with frequent reporting to show effects of the first
maintenance dredging event occurring in 2009 and to assess the relationship between flood
shoaling and tidal muting.
Merkel & Associates, Inc. 9