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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
As shown in Figure 2-23, subaerial volume losses prevailed after January 2007 at the two downcoast
transects nearest the entrance channel (333+30 and 350+71). Because this period followed the opening
of the entrance channel, particular vigilance is warranted at this site during future monitoring activities.
The remaining downcoast sites were relatively stable during this period. (Note: It is not possible to
assess the shorezone volume changes for the Bolsa Chica Monitoring period because the October 2005
profile does not extend below the waterline).
Sediment Trapping in the Full Tidal Basin: As indicated in Section 2.1, approximately 158,000 m 3 of
sediment was deposited in the lagoon during the 17-month period between August 2006 and January
2008. Sedimentation was reduced substantially during the second year (11-month period between
January 2008 to December 2008) to approximately 46,000 m 3 . While a small fraction of this material
may have resulted from redistribution of basin sediments or aeolian processes, nearly all of the
sediment has entered the basin from the ocean. It is possible that the high shoaling rate during the first
year was a transient effect attributable to inlet stabilization, and increased propensity for sedimentation
due to the proximately of the pre-filled ebb bar and widened beaches adjacent to the inlet. The reduced
shoaling rate during the second year is likely attributable to a reduced tidal prism due to infilling of the
FTB and the stabilization of the aforementioned local sediment sources (nourished beaches and ebb
bar).
The shoaling rate measured during the initial 17-month period was on the same order of magnitude as
the alongshore sediment transport rates developed as part of the CCSTWS-OC sediment budgets
(estimated to range from 108,000 m 3 /y to 125,000 m 3 /y). As a result, particular attention is warranted
in monitoring the flood shoal accumulation rates following the maintenance dredging in early 2009 to
understand if the initial sedimentation was transitory or should be expected following future dredging
episodes.
In the event that trapping rates detected during the initial post-opening are not transitory, these rates
are of a significant magnitude to be of major concern to longshore transport in the littoral cell. If left
unchecked and unmanaged, the primary implication of a substantial reduction of the longshore
sediment supply is shoreline erosion downdrift of the entrance channel. The Bolsa Chica project,
however, incorporates two sand management measures to actively address the potential for downdrift
erosion by eliminating or substantially reducing the net long-term loss of sand 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. This bypassing operation essentially restores the sediment lost from the littoral budget to
the downdrift beaches over the long-term. Taken together, these measures are anticipated to maintain
the historical supply of sediment to the beaches located south of the entrance channel. The first such
maintenance dredging will be conducted in early 2009.
Merkel & Associates, Inc. 135