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

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Bolsa Chica Lowlands Restoration Monitoring 2008 Annual Report downcoast. To compensate for anticipated short-term sediment losses from the littoral budget due to the natural formation of an ebb bar, initial lagoon shoaling, and fillet formation along the jetties, the ebb bar located offshore of the entrance channel was pre-filled, and supplemental sand was placed as beach nourishment adjacent to the channel at the time of construction. These pre-fills were intended to minimize littoral sand loss to ebb bar formation and provide supplemental sand for early inlet stabilization. In addition, the long-term project sediment management plan provides for periodic down-coast beach nourishment using sediment derived from the FTB during maintenance dredging operations, restoring the sediment lost from the littoral budget to the downdrift beaches. MAINTENANCE DREDGING AND DREDGING TRIGGERS Parameters of tidal muting, beach width, loss of subtidal habitat, closure risk, muted tidal basin function, and water quality were analyzed to evaluate the functioning of the system and determine when dredging should be performed. Some of these parameters have pre-established triggers including tidal muting, beach width, and loss of subtidal habitat. Other parameters do not presently have established criteria for triggering a dredging event. 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 the program’s 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. It is more likely that maintenance dredging will be required to address an intrinsic system need related to the functionality of the MTB tidal control structures and Freeman Creek. Final triggers to address this issue will need to be set once all of the MTBs are open to the FTB and have operated under both normal and muted FTB conditions. During periods in 2008 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. 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 the Mean Low Tide muting of 0.152 m. • Add an interim trigger of the rolling average of four consecutive lowest tides achieved during spring tide series exceeding 0.12 m NAVD, described in detail in Section 3. • 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. Merkel & Associates, Inc. 9
Bolsa Chica Lowlands Restoration Monitoring 2008 Annual Report • Continue the tidal monitoring program to show effects of the first maintenance dredging event and to assess the relationship between flood shoaling and tidal muting. • Dredge accumulated sands from within the flood shoal in winter 2008/2009 as scheduled since effects of the flood shoal impede the tidal ebbing from the entire site and adversely effect function of the MTBs and Freeman Creek Water Control structure. • The beach width dredging trigger should be modified to reflect a more current set of beach width data that includes the effects of the 2002 Surfside-Sunset nourishment and the scheduled 2009 Surfside-Sunset nourishment. In addition, the trigger should indicate that dredging should be performed when the beach width is less than two standard deviations from the mean beach width, since being greater than two standard deviations does not indicate a need for dredging. • Consider phasing out the beach width triggers, as these are not likely to ever be tripped prior to maintenance dredging triggers that address muting and impairment of the MTBs. • Continue bathymetric monitoring, and anticipate another maintenance dredging event in two years. Additional adjustments to dredging triggers are anticipated in response to future performance analysis of the MTBs and additional analysis of shoaling after the first maintenance dredging cycle is completed. The first maintenance-dredging event is scheduled to occur in early 2009. For future dredging events, consideration should be given to dredging to the permitted depth of the final engineering design depths to extend the period between maintenance events. Dredging at the time of initial construction was not completed to full design depths within the maintenance basin. If deepening of the maintenance basin were completed, this would garner additional time between dredging events and would improve dredging efficiency by capturing a greater volume of sediment in a more localized and recoverable area nearer the inlet. Additionally, the pre-dredging contracting process can consume a considerable period of time and thus work should be completed to streamline and pre-prepare to the maximum extent practical prior to maintenance triggers being tripped. This would allow for a reduced period over which the system functions in an impaired condition prior to completing maintenance dredging. To accomplish this would require: preparation of the majority of the plans and specifications, completion of permitting based on a maintenance basin plan and dredge volume range, preparation of bid and contract documents, and obtaining maximum flexibility for the dredging window of work. Long-term Corps permits for maintenance dredging are possible, including ten-year permits to include dredging triggers and pre-dredging notification and approvals that are considerably shorter than applying for a new permit each time dredging is to be performed. Merkel & Associates, Inc. 10
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