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

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportIII. MAINTENANCE DREDGING PROGRAMThe maintenance dredging program is planned as a sand management action to maintain “no net” lossof sand to the downcoast beaches as required in the EIR/EIS and project permits, as well as to ensurethe vitality of the tidal system. Sand will be removed from the flood shoal of the FTB within theoriginal dredging footprint in a region that can extend from the tidal inlet through the basin toapproximately the position of the Freeman Creek culvert. The final area may be slightly larger orsmaller depending on shoaling patterns determined from pre-dredge surveys. Sand dredged from thelagoon will be placed at the beach or nearshore areas based on the results of beach monitoring as wellas a consideration of the volume of material to be dredged.Preliminary engineering studies (M&N 1999) and the Basis of Design Report (M&N 2003) estimatedthe quantity of sand that would accrete in the lagoon would be the order of 230,000 m 3 (300,000 yd 3 )every 2 years. Maintenance dredging includes provisions for dredging deeper than the original dredgedepth but within originally permitted dredge depths. This could add as much as 400,000 m 3 (550,000yd 3 ) of dredging. This additional advance maintenance quantity would provide a longer intervalbetween dredging cycles if it were implemented.3.1 DREDGING TRIGGERSThe following parameters were monitored and analyzed to evaluate the functioning of the system anddetermine when dredging should be performed. Some of these parameters have established dredgingtriggers associated with them, as indicated, while others have thresholds that were established by themonitoring team, based on the need to sustain the biological and hydrological functioning of thesystem.Tidal MutingMuting of the average low tide elevations (Mean Low Water) on the order of 0.5 feet wouldindicate that the flood shoal maintenance dredging was warranted based on the BiologicalMonitoring and Follow-up Plan (USFWS, 2001).Beach WidthFlood shoal dredging should occur if any beach is found to be narrower than 50 feet, based on twoconsecutive monthly beach width measurements, and/or if any 12-month rolling average of beachwidths which deviate more than 2 standard deviations from the mean beach width, using 20 yearhistoric record to establish these means and standard deviations based on the Beach MonitoringPlan (USFWS, 2001).Loss of Subtidal HabitatThe flood shoal should be dredged if a 10% decrease in habitat acreage occurs based on the Basisof Design Report (M&N, 2003).Closure RiskThe flood shoal should be dredged if it is determined that the inlet is at risk of closure in a singlestorm scenario due to the localized shoaling pattern (Monitoring Team determination).Merkel & Associates, Inc. 94