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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
BEACH MONITORING
Beach profile data were obtained in May and October 2008 and compared to historic data and data
collected in 2007. Historically, the beaches along the Bolsa Chica study area have benefited as the
downdrift recipient of the Surfside-Sunset nourishment material. During the 34-year period between
1963 and 1997, the beaches advanced at four of the five historical transects included in the Bolsa Chica
beach monitoring program. Mean sea level (MSL) shoreline advance ranged from 14 m to 71 m within
the present study area. The only occurrence of shoreline retreat during the 34-yr period was a loss of
18 m at a transect located at Huntington Cliffs. The volume of sand above MSL increased in parallel
to the beach width changes during the period. The shorezone volumes in the study area, which
incorporate the sediment changes further offshore, increased at all of the sites. The greatest gains
typically occurred prior to 1978.
During the three-year period encompassing the last year of the construction of the Bolsa Chica
Lowlands Restoration Project and the first two years post-restoration (October 2005 to October 2008),
the shoreline advanced at the three transects located north of the entrance channel, with the greatest
gain being 24 m. Shoreline retreat predominated at the survey sites located south of the entrance
channel, with beach width changes ranging from a gain of 1 m to a loss of 14 m. The subaerial volume
changes were very similar to the beach width changes, with gains occurring north of the entrance
channel and losses predominating south of the entrance channel. While it is not possible to
quantitatively assess shorezone volume changes during the recent three-year period (the October 2005
profile does not extend below the waterline), a trend of shorezone volume loss has prevailed at each
transect between January 2007 and October 2008. This may be attributable dispersal of the pre-filled
ebb bar and natural erosion between Surfside-Sunset nourishment intervals.
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 high initial shoaling rates and the stabilization of the aforementioned local
sediment sources (nourished beaches and ebb bar). Nevertheless, the shoaling rate measured during
the initial 17-month period is on the same order of magnitude as the alongshore sediment transport
rates previously developed for Orange County (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 recent 2008 dredging activities to understand if the initial sedimentation was transitory or
should be expected following future dredging episodes.
In the event that the high sediment trapping rates detected following the initial inlet opening persist
following the upcoming dredging activities forecast for 2009 (i.e., the sedimentation rates are not
transitory), these rates are of a significant magnitude to be of major concern to alongshore transport in
the littoral cell. If left unchecked and unmanaged, the primary implication of a substantial reduction of
the alongshore 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
Merkel & Associates, Inc. 8