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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
surveys, the method provides a cost-effective means to supplement the more accurate MSL beach
widths derived from the semi-annual beach profile survey data.
Results
The beach profile plots are provided in Appendix 2-B. MSL beach widths and sediment volume data
are presented in Appendices 2-C and 2-D, respectively. Appendix 2-E contains the beach width
measurements obtained for the Bolsa Chica monitoring program, while Appendix 2-F contains those
collected by the U.S. Army Corps of Engineers.
Beach Profile Plots
The 2008 beach profile data were used in conjunction with data from the historical surveys to create
profile plots and compute changes in beach width and sediment volume. The beach profile plots
developed from the survey data are provided in Appendix 2-B. The range on each profile plot
represents the distance in meters seaward of the survey origin measured along the transect alignment.
The elevation is given in meters relative to NAVD88.
Two sets of beach profile plots were generated for each transect. The first set of plots shows all of the
beach profile data available for each transect, while the second set of plots shows only those profiles
obtained in during the three-year period encompassing the end of construction of the Bolsa Chica
Lowlands Restoration Project and the first two years post-construction (October 2005 to October
2008). The plots focusing on the recent three-year period also show the envelope of all available
profile data that preceded the opening of the Bolsa Chica entrance channel in August 2006 (May 1963
to March 2006). These plots also include two panels for each transect - one isolating the nearshore
region of the profile and another displaying the entire profile length.
Mean Sea Level Beach Widths
Mean Sea Level (MSL) beach widths are provided in Appendix 2-C. The beach width was computed
as the horizontal distance, in meters, between the landward edge of the beach sand and the point at
which the beach profile intersected the plane of MSL Datum. In the Bolsa Chica area, MSL lies 0.79
m above NAVD88. Notwithstanding the use of NAVD88 as the elevation reference for the profile
data, MSL was adopted as the shoreline reference in the belief that it provides a more accurate
indicator of changes in beach configuration.
Sediment Volumes
Sediment volume changes are provided in Appendix 2-D. The volume changes were computed along
each transect for the entire width of the shorezone, and for that portion of the profile located above
MSL (subaerial volume).
The offshore boundary of the control volume for the beach above MSL was placed at the intersection
of the profile and a horizontal line corresponding to the elevation of MSL. The offshore boundary for
the shorezone was placed at the “statistical range of closure”. This parameter represents the distance
seaward of the transect origin, beyond which profile variations are smaller than the accuracy of the
survey technique. As implied by its definition, the statistical range of closure was adopted as the
offshore boundary to separate the signal of true profile change from the noise of survey inaccuracy.
The sea bottom elevation at the range of closure corresponds to the “depth of closure” or the depth at
which sediment transport is not substantially affected by littoral processes.
The statistical range of closure was determined for the five historical transects as part of the CCSTWS-
OC. However, these boundaries were no longer appropriate due to the profile changes that resulted at
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