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 />
Loss of Subtidal Habitat<br />
The flood shoal should be dredged if a 10% decrease in habitat acreage occurs (Basis of Design<br />
<strong>Report</strong> [M&N, 2003]).<br />
Closure Risk<br />
The flood shoal should be dredged if it is determined that the inlet is at risk of closure in a single<br />
storm scenario due to the localized shoaling pattern (<strong>Monitoring</strong> Team determination).<br />
Water quality<br />
Tidal circulation in the FTB will be slightly less efficient under the muted tidal condition.<br />
However, the FTB should still have an excellent circulation condition with a residence time of a<br />
few days. The water quality will degrade if the inlet is closed. At present, the large size of the<br />
FTB and the significant wave fetch is believed to be adequate to sustain good water quality even in<br />
highly muted conditions. However, substantial deviations in water quality parameters that suggest<br />
isolation from strong oceanic influences should trigger dredging of the flood shoal (<strong>Monitoring</strong><br />
Team determination).<br />
3.2 TRIGGER ANALYSIS<br />
Analysis of Tidal Muting Trigger<br />
A review of the tidal ranges in the FTB and LAOH indicates that the differences in the high tide<br />
elevations between the two sites are very small. This implies that the overall measurement is reliable<br />
and there was no muting of the high tides in <strong>2008</strong>. The low tides were muted in <strong>2008</strong> and the degree<br />
of muting was a result of flood shoal accretion in the basin. While the original tidal muting trigger<br />
within the Biological <strong>Monitoring</strong> and Follow-up Plan (USFWS 2001) was based on muting of the<br />
average low tide elevations (Mean Low Water) on the order of 0.5 feet, this trigger is too high to avoid<br />
adverse effects on tidal circulation and drain-out conditions from the MTBs. As a result, a different<br />
trigger based on lower tide levels is appropriate.<br />
Several factors are important to consider in setting tidal muting or tidal drainage triggers for<br />
maintenance dredging events. Tidal muting is positively correlated with tidal range, with greater<br />
muting occurring during spring tides and less muting occurring during neap tides. The trend of tidal<br />
muting is for gradually increased muting over time, with greater punctuated increases and reductions in<br />
muting that are likely coincident with significant changes in littoral transport and flow patterns across<br />
the flood shoal (this occurred during the winter of 2007-08, Figures 2-5B and 2-6). The amount of<br />
muting shown in Table 2-2 increased over time as the flood shoal expanded in the FTB.<br />
Tidal plots in the Appendix 2-A and the low tide muting in Table 2-2 show an average spring low tide<br />
muting of 0.29 m from oceanic conditions over 2007 (January through December). For that same<br />
monitoring period in <strong>2008</strong>, the spring low tide muting averaged 0.54 m. However, high water level<br />
management issues in the west MTB, necessitating gate closures and self-regulating tide gate<br />
adjustments to reduce tidal range, extended from approximately May <strong>2008</strong> through the remainder of<br />
the year. During this period, the average low spring tide was muted 0.62 m from oceanic conditions.<br />
For the preceding few months during which the west MTB was open, the muting level was 0.39 m and<br />
constraints were minimal.<br />
Merkel & Associates, Inc. 137