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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
edge of the maintenance basin, therefore an additional survey was conducted by CLE to fill this gap on
January 28, 2009. It was assumed that these data generally reflected the conditions in this area at the
time of the December 23, 2008 survey, as this area is not an area of high deposition, nor erosion.
Survey work reported in this report was conducted prior to the first maintenance dredging event, which
occurred in early 2009 and which will be reported on in the next annual report.
Data from the vessel-based bathymetric assessment were post-processed to correct for tidal elevations
at the time of the survey, and the boat and shore data was used to develop bathymetric contour plots for
the basin. These contour plots were aligned with a plot of the same area from the post-construction,
pre-opening, contour data collected on August 20, 2006 that was provided by Moffatt & Nichol.
To estimate the rate of sand influx in the FTB, a sediment assessment polygon encompassing the area
of shoal formation was established. The changes in volume within this polygon over time were
quantified, both between surveys and in comparison to the pre-opening conditions in August 2006.
Results
The contour plots for the pre-opening survey and the five post-opening surveys are presented in Figure
2-2. A small shoal had formed in the inlet by January 2007 and continued to expand through 2007 and
2008. The net volume of sediment (composed entirely of littoral sand) accreted within the assessment
polygon was compared to the pre-opening conditions and is presented in Table 2-1. The net accretion
takes into the account minor losses of sediment due to erosion.
Table 2-1. Net increase in inlet sediment volume in comparison to pre-opening conditions.
Survey date Net Sediment Accretion (m 3 )
August 2006 (pre-opening) 0
January 2007 + 59,481
June 2007 + 122,105
January 2008 + 158,403
July 2008 + 180,905
December 2008 + 204,623
The total rate of volume change from the basin opening in August 2006 to the December 2008 survey,
roughly 28 months later, was approximately 240 m 3 /day. It is important to note, however, that this
average rate does not represent the actual accretion per day, as deposition and erosion occurred
throughout the period at an uneven rate.
To examine this variable rate, the contour plots of each survey were compared to each other to quantify
areas of erosion and accretion between surveys. These comparisons are presented graphically in
Figure 2-3. Included in the figure is a table that tracks the accretion and erosion of sediment over time
within the sediment assessment polygon. As anticipated, there was a large input of sand between the
basin opening on August 24, 2006 and the first survey on January 19, 2007, with an average of 402
m 3 /day. The influx rate then decreased between subsequent surveys, to an average of 134 m 3 /day in
2008.
Merkel & Associates, Inc. 101