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

Bolsa Chica Lowlands Restoration Monitoring2007 Annual Reporthowever, incorporates two sand management measures to actively address the potential for downdrifterosion by eliminating or substantially reducing the net long-term loss of sand downcoast. Tocompensate for anticipated short-term sediment losses from the littoral budget due to the naturalformation of an ebb bar, initial lagoon shoaling, and fillet formation along the jetties, the ebb barlocated offshore of the entrance channel was pre-filled, and supplemental sand was placed as beachnourishment adjacent to the channel at the time of construction. These pre-fills were intended tominimize littoral sand loss to ebb bar formation and provide supplemental sand for early inletstabilization. In addition, the long-term project sediment management plan provides for periodicdown-coast beach nourishment using sediment derived from the FTB during maintenance dredgingoperations, restoring the sediment lost from the littoral budget to the downdrift beaches.MAINTENANCE DREDGING AND DREDGING TRIGGERSParameters of tidal muting, beach width, loss of subtidal habitat, closure risk, muted tidal basinfunction, and water quality were analyzed to evaluate the functioning of the system and determinewhen dredging should be performed. Some of these parameters have pre-established triggers includingtidal muting, beach width, and loss of subtidal habitat. Other parameters do not presently haveestablished criteria for triggering a dredging event.In reviewing the established dredging triggers, it is clear that some of the triggers may never be metexcept under extreme circumstances, while more significant triggers may exist that have not as yetbeen quantified. Chronic beach erosion triggers are not likely to be met because of the ongoingreplenishment 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 attrigger point transects. This is not to say that beaches would not benefit from replenishment with floodshoal sand bypass. Rather, it acknowledges that beach erosion is not likely to occur to the extent thatwould trigger an obligatory maintenance-dredging event for replenishment purposes. It is more likelythat maintenance dredging will be required to address an intrinsic system need related to thefunctionality of the MTB tidal control structures. These triggers will need to be set once the MTBs areopen to the FTB.At the present time, the following preliminary recommendations are made. Additional adjustments todredging triggers are anticipated in response to future performance analysis of the MTBs andadditional analysis of shoaling after the first maintenance dredging cycle is completed.Modify the expectations of tidal range from 2.75 meters (9.0 feet) to 2.29 meters (7.5 feet), withrevised expected tidal elevations of 2.02 meters (6.62 feet) to -0.27 meters (-0.88 feet) relative toNAVD.Remove the dredge trigger of the Mean Low Tide muting of 0.152 meters (0.5 feet). Monitor tidesin the MTBs to clarify maintenance dredging triggers related to tides in those areas.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.Merkel & Associates, Inc. 7