2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
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<strong>Bolsa</strong> <strong>Chica</strong> Lowlands Restoration <strong>Monitoring</strong><br />
<strong>2008</strong> <strong>Annual</strong> <strong>Report</strong><br />
24% of low intertidal habitat converts to subtidal habitat or when 24% of the subtidal basin is lost to<br />
intertidal flood shoaling near the ocean inlet, dredging is warranted. In either condition, the habitat<br />
functionality would be considered to be impaired from the initial design objectives.<br />
Analysis of Closure Risk Trigger<br />
The closure risk trigger is not specifically defined or quantified, but is presented as an opinion of risk<br />
due to the shoaling pattern of the inlet. A review of the bathymetric changes at the inlet shows that the<br />
major cross-sectional flow area varies by season and is presently located within a very narrow thalweg<br />
on the southern side of the inlet (Figure 2-2). Based on the quantity of material passing through the<br />
inlet and the significant narrowing of the thalweg, it is prudent to recommend dredging as needed to<br />
prevent a catastrophic closure of the inlet in a large storm event.<br />
Of greatest concern relative to an episodic closure event are large accumulations of sand within the<br />
inlet channel, high and over-steepened channel banks, and the sinuous course of flow between high<br />
sand bars and adjacent to the armored shoreline. In such cases as these, a major storm event, or series<br />
of events, combined with a weak neap tide series could lead to either full or partial closure of the<br />
mouth. This could then result in loss of drainage and rising water levels within the MTBs and<br />
Freeman Creek.<br />
As conditions necessary to cause a catastrophic closure event have not yet materialized, however,<br />
response to such an occurrence should be included in emergency planning contingencies. In the event<br />
such a closure occurred, it is likely that tidal flows could be restored to pre-event conditions by<br />
excavation of the channel during a strong ebbing tide.<br />
Analysis of Water Quality Trigger<br />
This trigger for maintenance dredging would be met if the water quality within the FTB were to begin<br />
to exhibit conditions of impaired circulation resulting in degradation of field measurable parameters of<br />
dissolved oxygen, considerably rising temperatures during summer months, or high resident plankton<br />
blooms that were not reflected in the open coastal waters. At the present time, the FTB waters exhibit<br />
conditions comparable to those of the open coast with oceanic variability strongly driving conditions in<br />
the areas of the FTB located closest to the inlet and only moderate variance from ocean conditions<br />
further into the basin. The FTB closely matched the ocean temperature in the winter, with slightly<br />
higher temperatures than the ocean in the summer months, a condition typically seen in other coastal<br />
embayments in the region. Dissolved oxygen levels measured at <strong>Bolsa</strong> <strong>Chica</strong> were within the expected<br />
range and reflected the strong influence of diurnal tidal flow, and were generally above healthy levels<br />
of 5.5 mg/L, with daily tidal peaks in the 7.5 to 8.5 mg/L range. There were no significant local<br />
plankton blooms during either 2007 or <strong>2008</strong>. Based on these conditions, water quality triggers for<br />
maintenance dredging were not tripped during the <strong>2008</strong> monitoring period.<br />
3.3 DREDGE TRIGGERS - CONCLUSIONS AND RECOMMENDATIONS<br />
In reviewing the established dredging triggers, it is clear that some of the triggers may never be met<br />
except under extreme circumstances, while more significant triggers may exist that have not as yet<br />
been quantified. Chronic beach erosion triggers are not likely to be met because of the ongoing<br />
replenishment at Surfside-Sunset and its effect on long-term beach growth trends. Similarly, acute<br />
erosion triggers are not likely to be met due to the generally broad beach profiles at trigger point<br />
transects. This is not to say that beaches would not benefit from replenishment with flood shoal sand<br />
Merkel & Associates, Inc. 140