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

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