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

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