2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
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<strong>Bolsa</strong> <strong>Chica</strong> Lowlands Restoration <strong>Monitoring</strong><br />
<strong>2008</strong> <strong>Annual</strong> <strong>Report</strong><br />
BEACH MONITORING<br />
Beach profile data were obtained in May and October <strong>2008</strong> and compared to historic data and data<br />
collected in 2007. Historically, the beaches along the <strong>Bolsa</strong> <strong>Chica</strong> study area have benefited as the<br />
downdrift recipient of the Surfside-Sunset nourishment material. During the 34-year period between<br />
1963 and 1997, the beaches advanced at four of the five historical transects included in the <strong>Bolsa</strong> <strong>Chica</strong><br />
beach monitoring program. Mean sea level (MSL) shoreline advance ranged from 14 m to 71 m within<br />
the present study area. The only occurrence of shoreline retreat during the 34-yr period was a loss of<br />
18 m at a transect located at Huntington Cliffs. The volume of sand above MSL increased in parallel<br />
to the beach width changes during the period. The shorezone volumes in the study area, which<br />
incorporate the sediment changes further offshore, increased at all of the sites. The greatest gains<br />
typically occurred prior to 1978.<br />
During the three-year period encompassing the last year of the construction of the <strong>Bolsa</strong> <strong>Chica</strong><br />
Lowlands Restoration Project and the first two years post-restoration (October 2005 to October <strong>2008</strong>),<br />
the shoreline advanced at the three transects located north of the entrance channel, with the greatest<br />
gain being 24 m. Shoreline retreat predominated at the survey sites located south of the entrance<br />
channel, with beach width changes ranging from a gain of 1 m to a loss of 14 m. The subaerial volume<br />
changes were very similar to the beach width changes, with gains occurring north of the entrance<br />
channel and losses predominating south of the entrance channel. While it is not possible to<br />
quantitatively assess shorezone volume changes during the recent three-year period (the October 2005<br />
profile does not extend below the waterline), a trend of shorezone volume loss has prevailed at each<br />
transect between January 2007 and October <strong>2008</strong>. This may be attributable dispersal of the pre-filled<br />
ebb bar and natural erosion between Surfside-Sunset nourishment intervals.<br />
Approximately 158,000 m 3 of sediment was deposited in the lagoon during the 17-month period<br />
between August 2006 and January <strong>2008</strong>. Sedimentation was reduced substantially during the second<br />
year (11-month period between January <strong>2008</strong> to December <strong>2008</strong>) to approximately 46,000 m 3 . While<br />
a small fraction of this material may have resulted from redistribution of basin sediments or aeolian<br />
processes, nearly all of the sediment has entered the basin from the ocean. It is possible that the high<br />
shoaling rate during the first year was a transient effect attributable to inlet stabilization, and increased<br />
propensity for sedimentation due to the proximately of the pre-filled ebb bar and widened beaches<br />
adjacent to the inlet. The reduced shoaling rate during the second year is likely attributable to a<br />
reduced tidal prism due to high initial shoaling rates and the stabilization of the aforementioned local<br />
sediment sources (nourished beaches and ebb bar). Nevertheless, the shoaling rate measured during<br />
the initial 17-month period is on the same order of magnitude as the alongshore sediment transport<br />
rates previously developed for Orange County (estimated to range from 108,000 m 3 /y to 125,000<br />
m 3 /y). As a result, particular attention is warranted in monitoring the flood shoal accumulation rates<br />
following the recent <strong>2008</strong> dredging activities to understand if the initial sedimentation was transitory or<br />
should be expected following future dredging episodes.<br />
In the event that the high sediment trapping rates detected following the initial inlet opening persist<br />
following the upcoming dredging activities forecast for 2009 (i.e., the sedimentation rates are not<br />
transitory), these rates are of a significant magnitude to be of major concern to alongshore transport in<br />
the littoral cell. If left unchecked and unmanaged, the primary implication of a substantial reduction of<br />
the alongshore sediment supply is shoreline erosion downdrift of the entrance channel. The <strong>Bolsa</strong><br />
<strong>Chica</strong> project, however, incorporates two sand management measures to actively address the potential<br />
for downdrift erosion by eliminating or substantially reducing the net long-term loss of sand<br />
Merkel & Associates, Inc. 8