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 />
The tide logger was held in an aluminum bracket mounted to the FTB bulkhead at the east water<br />
control structure. The initial bracket configuration resulted in a sensor elevation of –0.05 m NAVD.<br />
This elevation was not sufficient to capture the lower low tides during spring tidal cycles. On January<br />
19, 2007 at 13:00, the bracket was extended, resulting in a sensor elevation of –0.68 m NAVD. A<br />
subsequent elevational measurement of the FTB bulkhead by Coastal Frontiers resulted in adjustment<br />
of the sensor elevation to –0.72 m NAVD. This sensor elevation was used for data reported in the<br />
2007 annual report. In <strong>2008</strong>, it was learned that nearby Orange County elevational benchmarks had<br />
been adjusted 7 cm to account for local subsidence. Thus, the tide stations sensor elevation was<br />
corrected to –0.79 m NAVD. This change, along with a minor calculation error discovered during<br />
analysis of the <strong>2008</strong> data, resulted in minor changes to previously reported values. When appropriate,<br />
these corrections are provided in the below text.<br />
Tidal elevations were calculated by adjusting the 6-minute water depth data with the sensor elevation.<br />
The tidal muting analyses are based on data collected from January 20, 2007 through December 31,<br />
<strong>2008</strong>.<br />
Recorded tides were compared with tides measured at the nearest tidal station, located 22.5 km (14<br />
miles) north in Los Angeles Outer Harbor (LAOH) (NOAA Station 9410660). The NOAA gauge is<br />
located immediately adjacent to the open ocean, and the recorded tides represent the ocean tidal<br />
conditions. The data were obtained from the NOAA Tides and Currents website<br />
(http://tidesandcurrents.noaa.gov). The obtained data were not temporally corrected based on distance<br />
to the study site because the correction is less than the logging period.<br />
Results<br />
Comparison of the lower low tide data for each sampling date since January 20, 2007 against the<br />
NOAA tide data for LAOH shows that the FTB does not completely drain to local oceanic sea levels<br />
during lower low spring tides (Figure 2-5a). Moreover, the data indicate that tidal muting trends<br />
observed in 2007 were continued and became more pronounced during <strong>2008</strong>. In <strong>2008</strong>, lower low tides<br />
in the FTB only went as low as LAOH during very mild neap tides.<br />
Plotting the differences between the minimum observed tidal elevations for all daily lower low tides at<br />
LAOH versus the FTB numerically illustrates increased tidal muting (Figure 2-5b). From January to<br />
December 2007, tidal muting within the FTB had increased by an average of 0.07 m (previously<br />
reported as 0.10 m). Muting accelerated during <strong>2008</strong>. By the end of <strong>2008</strong>, tidal muting averaged<br />
approximately 0.40 m. The maximum low tide differential between LAOH and the FTB was 0.86 m<br />
on July 2, <strong>2008</strong>. Notably, on infrequent occasions the tidal elevation of the ocean tides marginally<br />
exceeded the elevation within the FTB. This occurred principally at periods of neap tide and may be<br />
the result of a variety of factors including inlet morphology, tidal lag influences, or instrument variance<br />
at particular times.<br />
Table 2-2 summarizes the bi-weekly spring highest and lowest tidal elevations observed in the FTB<br />
and LAOH. Because the FTB gauge was lowered on January 19, 2007 only a partial spring-neap tidal<br />
cycle was analyzed for the first cycle in January 2007. The column identified as “Difference” under<br />
“Spring High Tide” show the spring high tide differences as calculated by subtracting the highest tide<br />
in the FTB by that at LAOH. A negative number indicates that the ocean high tide is higher than that<br />
in the FTB, and a positive number indicates the FTB has a higher high tide than that in the ocean. The<br />
general change from slightly positive values in 2007 to negative values in <strong>2008</strong> may be associated with<br />
Merkel & Associates, Inc. 108