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

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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
Bolsa Chica Lowlands Restoration Monitoring 2008 Annual Report While tidal muting from oceanic conditions provides a good means by which drainage loss may be measured, it is more difficult and less direct than establishing a maintenance trigger that can be readily measured from recording instrumentation. Further, during periods of sea surface rise, such as during an El Nino Southern Oscillation (ENSO) event, it may be expected that there will be less difference between the oceanic low tides and those within the FTB, yet MTB and Freeman Creek drainage conditions will be even more severely affected than under normal shoal-driven muting. For this reason, it is worth considering the use of the lowest achieved tides within the FTB as a metric for maintenance dredging action. During periods 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. In addition, once the Freeman Creek gate is open to the FTB it will be important to assess the influence of FTB elevations on operational drainage from this basin as well. It is important to note that both the central and east MTBs have a lower designed operational elevation than does the west MTB. As a result, it is anticipated that maintaining limited muting within the FTB will be even more important to the intended operation of these basins than to the higher elevation west MTB. 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 Mean Low Tide muting of 0.15 m. • Add an interim trigger of the rolling average of four consecutive lowest tides achieved during spring tide series exceeding 0.12 m NAVD, as described above. • 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. • Remove the flood shoal in winter 2008/2009 as scheduled to reduce the tidal muting effects. Recommendations relating to this dredging trigger are currently considered preliminary in nature as the muting of the system affects the functioning of the MTBs, which have yet to be fully opened. As a result, final dredging triggers relating to tidal muting should only be set after opportunities exist to monitor the full performance of all of the MTBs under normal and muted conditions. Analysis of Beach Width Trigger The Bolsa Chica Monitoring Plan defined beach nourishment triggers based on the monthly beach width observations at the USACE measurement sites within the study area. The minimum permittable Merkel & Associates, Inc. 138
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2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...

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