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