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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
III. MAINTENANCE DREDGING PROGRAM
The maintenance dredging program is planned as a sand management action to maintain “no net loss”
of sand to the downcoast beaches as required in the EIR/EIS and project permits, as well as to ensure
the vitality of the tidal system. During regular maintenance dredging operations, sand will be removed
from the flood shoal of the FTB within the original dredging footprint, in a region that can extend from
the tidal inlet north to approximately the position of the Freeman Creek culvert. The final area may be
slightly larger or smaller depending on shoaling patterns determined from pre-dredge surveys. Sand
dredged from the basin will be placed at the beach or nearshore areas based on the results of beach
monitoring as well as 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) estimated
the quantity of sand that would accrete in the lagoon would be on the order of 126,000 m 3 (165,000
yd 3 ) during the first year, 102,000 m 3 (134,000 yd 3 ) during the second year, 49,000 m 3 (64,000 yd 3 )
during the third year, and 7,600 m 3 (10,000 yd 3 ) during the fourth year post opening. The reduced
sand influx rate in later years was predicted as a result of anticipated system muting. Therefore, the
need for maintenance dredging would arise before later low influx rates would be realized.
Maintenance dredging plans included provisions for dredging deeper than the original dredge depth but
within originally permitted dredge depths. This could add as much as 400,000 m 3 (550,000 yd 3 ) of
dredging to maintenance sand removals from the FTB. This additional advance maintenance quantity
would provide a longer interval between dredging cycles if it were implemented.
3.1 DREDGING TRIGGERS
The following parameters were monitored and analyzed to evaluate the functioning of the system and
determine when dredging should be performed. Some of these parameters have previously established
dredging triggers associated with them, as indicated, while others have thresholds that were established
by the monitoring team, based on the need to sustain the biological and hydrological functioning of the
system.
Tidal Muting
Muting of the average low tide elevations (Mean Low Water) on the order of 0.5 feet would
indicate that the flood shoal maintenance dredging was warranted (Biological Monitoring and
Follow-up Plan [USFWS 2001]).
Muted Tidal Basin Function
The flood shoal should be dredged if the tidal drainage in the MTBs is impeded and the MTB
function is degraded as a result of inefficient drainage. Tidal monitoring in the FTB will help
determine the dredging trigger related to tidal drainage in the MTBs (Monitoring Team
determination).
Beach Width
Flood shoal dredging should occur if any beach is found to be narrower than 50 feet, based on two
consecutive monthly beach width measurements, and/or if any 12-month rolling average of beach
widths which deviate more than 2 standard deviations from the mean beach width, using the 20-
year historic record to establish these means and standard deviations (Beach Monitoring Plan
[USFWS, 2001]).
Merkel & Associates, Inc. 136