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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
24% of low intertidal habitat converts to subtidal habitat or when 24% of the subtidal basin is lost to
intertidal flood shoaling near the ocean inlet, dredging is warranted. In either condition, the habitat
functionality would be considered to be impaired from the initial design objectives.
Analysis of Closure Risk Trigger
The closure risk trigger is not specifically defined or quantified, but is presented as an opinion of risk
due to the shoaling pattern of the inlet. A review of the bathymetric changes at the inlet shows that the
major cross-sectional flow area varies by season and is presently located within a very narrow thalweg
on the southern side of the inlet (Figure 2-2). Based on the quantity of material passing through the
inlet and the significant narrowing of the thalweg, it is prudent to recommend dredging as needed to
prevent a catastrophic closure of the inlet in a large storm event.
Of greatest concern relative to an episodic closure event are large accumulations of sand within the
inlet channel, high and over-steepened channel banks, and the sinuous course of flow between high
sand bars and adjacent to the armored shoreline. In such cases as these, a major storm event, or series
of events, combined with a weak neap tide series could lead to either full or partial closure of the
mouth. This could then result in loss of drainage and rising water levels within the MTBs and
Freeman Creek.
As conditions necessary to cause a catastrophic closure event have not yet materialized, however,
response to such an occurrence should be included in emergency planning contingencies. In the event
such a closure occurred, it is likely that tidal flows could be restored to pre-event conditions by
excavation of the channel during a strong ebbing tide.
Analysis of Water Quality Trigger
This trigger for maintenance dredging would be met if the water quality within the FTB were to begin
to exhibit conditions of impaired circulation resulting in degradation of field measurable parameters of
dissolved oxygen, considerably rising temperatures during summer months, or high resident plankton
blooms that were not reflected in the open coastal waters. At the present time, the FTB waters exhibit
conditions comparable to those of the open coast with oceanic variability strongly driving conditions in
the areas of the FTB located closest to the inlet and only moderate variance from ocean conditions
further into the basin. The FTB closely matched the ocean temperature in the winter, with slightly
higher temperatures than the ocean in the summer months, a condition typically seen in other coastal
embayments in the region. Dissolved oxygen levels measured at Bolsa Chica were within the expected
range and reflected the strong influence of diurnal tidal flow, and were generally above healthy levels
of 5.5 mg/L, with daily tidal peaks in the 7.5 to 8.5 mg/L range. There were no significant local
plankton blooms during either 2007 or 2008. Based on these conditions, water quality triggers for
maintenance dredging were not tripped during the 2008 monitoring period.
3.3 DREDGE TRIGGERS - CONCLUSIONS AND RECOMMENDATIONS
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 its 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. This is not to say that beaches would not benefit from replenishment with flood shoal sand
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