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

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Bolsa Chica Lowlands Restoration Monitoring 2008 Annual Report beach width based on two consecutive monthly measurements was stipulated to be 15.2 m. A second condition indicated that the 12-month rolling average beach width could not deviate from the longterm mean beach width (based on the period January 1980 to January 2000) by more than two standard deviations. The presumption is that the deviation from beach width must also be towards a declining width from the benchmark period. Figures 2-17, 2-18, and 2-19 show the long-term rolling average berm width from October 2006 (preproject) to December 2008 at each of the three USACE measurement sites within the study area. The time series plots of the monitoring data also show the minimum stipulated berm width (15.2 m) (red), the long-term mean berm width (green), and a red shaded area encompassing two standard deviations above and below the long-term mean berm width. The 12-month rolling average berm width remained well above the minimum stipulated berm width (15.2 m) throughout the period at each of the sites. At 307+88 and 424+44, the 12-month rolling average berm width exceeded two standard deviations above the long-term mean. The 12-month rolling average berm width was never less than two standard deviations below the long-term mean during 2007. Given the beach width criteria, it is not expected that beach erosion will trigger the need for maintenance dredging and replenishment as long as the Surfside-Sunset nourishment program continues, as this upcoast feed of littoral sand has been building beach width over time. At the present time, the beach response triggers established for the project maintenance dredging requirements are not particularly responsive to the relatively low volume of sand lost to entrainment and flood shoal capture. However, the project does not appear to be resulting in substantive broad scale changes in beach conditions beyond a localized influence of the jetties on beach form in the immediate vicinity of the inlet. As this maintenance trigger was developed to ensure protection of littoral beach conditions and it appears that these are being protected, no recommendation for a trigger change is made at this time. It would, however, be appropriate to consider reduction or even future elimination of the beach monitoring program given the highly unlikely condition that any of these maintenance triggers will be tripped prior to the requirements to perform maintenance dredging for tidal muting corrections. Analysis of Subtidal Habitat Trigger The flood shoal volume, the area of shoaling, and shoaling rate all have occurred similarly to processes predicted during the project design. Maintenance dredging should occur as recommended in the design as well. A recommended maintenance dredge trigger is the reduction of intertidal habitat area. The Basis of Design Report (M&N 2003) indicates that dredging should occur when habitat reduction reaches 10%. However, this criterion is probably too restrictive and should be reconsidered. Previous analyses as part of preliminary engineering studies show a rapid loss of 10% habitat within 1.3 years, and a subsequent habitat loss reaching 24% after 2 years (M&N 1999) at the predicted shoaling rate. The actual application of this trigger is confounded by the low frequency (not more than once a year and planned for lower frequency in the future) habitat mapping and bathymetric assessments, which are needed to assess intertidal habitat losses. As a result of these complications the maintenance dredging program should consider eliminating this trigger as the system matures and the intensity of biological and physical monitoring are diminished over time. In the interim period, while adequate data are being collected to complete these assessments (over the next 3-7 years) a two-year maintenance dredging frequency would be appropriate considering measured versus predicted shoaling volumes, tidal muting, and habitat loss. A revised dredge trigger based on habitat loss should be: when Merkel & Associates, Inc. 139
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 Merkel & Associates, Inc. 140
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2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...

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