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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
Discussion
The shoreline change assessment is based on the 45-year period between 1963 and 2008. Particular
emphasis is placed on the three-year period encompassing the end of the construction of the Bolsa
Chica Lowlands Restoration Project and the first two years post-restoration (October 2005 to October
2008). This three-year period will be referred to as the “Bolsa Chica Monitoring Period”. The project
components that influence coastal changes include placing approximately 929,326 m 3 (1,214,579 y 3 )
of sand in an ebb bar located offshore of the entrance channel during Winter 2005/2006, providing
approximately 102,500 m 3 (133,962 y 3 ) of beach nourishment (50:50 north and south) to the shoreline
adjacent to the channel in Summer 2006 and establishing tidal exchange at the entrance channel in
August 2006.
As indicated previously, the beaches along the Bolsa Chica study area has regularly benefited from the
downdrift dispersal of the Surfside-Sunset nourishment material. A comprehensive account of the
coastal changes in the area during the 34-year period between 1963 and 1997 can be found in the
CCSTWS-OC (USACE, 2002).
Profile Changes
Long-Term Profile Changes (1963 to 2008): The above-water beach profiles obtained in 2008 are
each near or seaward of the upper bound of the historical profile envelope at six of the seven Bolsa
Chica area transects. The exception was Transect 378+29, located at Huntington Cliffs. These
findings are consistent with the long-term trend of beach width and sediment volume gains identified
in the CCSTWS-OC for the Bolsa Chica study area. Of the five historical transects, the accretion trend
was absent only at Transect 378+29.
Bolsa Chica Monitoring Period Changes (2005 to 2008): During the three-year Bolsa Chica
Monitoring Period, above-water profile accretion occurred at the three transects located north of the
entrance channel (249+30, 311+22, and 318+30). Modest gains also occurred at the southernmost
transect (423+89). The greatest above-water volume gains occurred at Transect 311+22 and 318+30,
located immediately north of the entrance channel. These gains may be explained by a combination of
the beach nourishment placed in Summer 2006, onshore migration of the material placed in the ebb
bar, and upcoast sediment trapping at the entrance jetties. Immediately south of the entrance channel
at Transect 333+30, substantial above-water profile erosion occurred seaward of the berm. Volume
gains are evident at this location landward of the berm, however, and are likely an artifact of the 2006
beach nourishment. Further downcoast, the above-water beach at Transects 350+71 and 378+29 was
characterized by modest erosion or stability.
Offshore Ebb Bar: Approximately 1.5 million m 3 of sand was placed in an ebb bar located offshore of
the Bolsa Chica entrance channel between November 2005 and May 2006. This bar is evident in the
2007 and 2008 profiles at Transects 311+22, 318+30 and 333+30. Comparison of the 2007 and 2008
profiles indicates the onshore migration of the ebb bar during the one-year period between the surveys.
The most significant changes were isolated to depths above 8 m.
Beach Width Changes
2008 Beach Widths: Figure 2-11 shows the beach widths in the Bolsa Chica study area at the time of
the May 2008 and October 2008 surveys. Each figure also includes the range of Fall and Spring beach
widths for all available data between 1963 and 2002. At the time of the May 2008 survey, beach
widths ranged from 21 m at Transect 378+29 to 109 m at Transect 423+89. The greatest beach width
Merkel & Associates, Inc. 123