2007 Annual Monitoring Report (pdf 16MB) - Bolsa Chica Lowlands ...

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportModify the expectations of tidal range from 2.75 meters (9.0 feet) to 2.29 meters (7.5 feet),with revised expected tidal elevations of 2.02 meters (6.62 feet) to -0.27 meters (-0.88 feet)relative to NAVD.Remove the dredge trigger of the Mean Low Tide muting of 0.15 meters (0.5 feet). Monitortides in the MTBs to clarify maintenance dredging triggers related to tides.Continue the tidal monitoring program with frequent reporting to show effects of the firstmaintenance dredging event and to assess the relationship between flood bar shoaling and tidalmuting.Remove the flood shoal as scheduled to reduce the tidal muting effects.Analysis of Beach Width TriggerThe Bolsa Chica Monitoring Plan defined beach nourishment triggers based on the monthly beachwidth observations at the USACE measurement sites within the study area. The minimum permittablebeach width based on two consecutive monthly measurements was stipulated to be 15.2 m (50 ft). Asecond condition indicated that the 12-month rolling average beach width could not deviate from thelong-term mean beach width (based on the period January 1980 to January 2000) by more than twostandard deviations.Figures 2-17, 2-18, and 2-19 show the long-term rolling average berm width from October 2006 (preproject)to December 2007 at each of the three USACE measurement sites within the study area. Thetime 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 deviationsabove 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 rollingaverage berm width exceeded two standard deviations above the long-term mean. The 12-monthrolling average berm width was never less than two standard deviations below the long-term meanduring 2007. Given the beach width criteria it is not expected that beach erosion will trigger the needfor maintenance dredging and replenishment as long as the Surfside-Sunset nourishment programcontinues.Analysis of Subtidal Habitat TriggerThe flood shoal volume, the area of shoaling, and shoaling rate all have occurred similar to processespredicted during the project design. Maintenance dredging should occur as recommended in the designas well. A recommended maintenance dredge trigger is the reduction of intertidal habitat area. TheBasis of Design Report (M&N 2003) indicates that dredging should occur when habitat reductionreaches 10%. However, this criterion is probably too restrictive and should be reconsidered. Previousanalyses 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.A two-year maintenance dredge frequency would be appropriate considering measured versuspredicted shoaling volumes, tidal muting, and habitat loss. A revised dredge trigger based on habitatloss should be when 24% of low intertidal habitat converts to subtidal habitat or when 24% of thesubtidal basin is lost to intertidal flood shoaling near the ocean inlet. In either condition, the habitatfunctionality is considered to be impaired from the initial design objectives.Merkel & Associates, Inc. 96