2007 Annual Monitoring Report (pdf 16MB) - Bolsa Chica Lowlands ...

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportThe tide logger was mounted in an aluminum bracket mounted to the FTB bulkhead at the east watercontrol structure. The initial bracket configuration resulted in a sensor elevation of –0.05 m NAVD (-0.15 ft). This elevation was not sufficient to capture the lower low tides during spring tidal cycles. OnJanuary 19, 2007 at 13:00, the bracket was extended, resulting in a sensor elevation of –0.68 m NAVD(–2.24 ft). Tidal elevations were calculated by adjusting the 6-minute water depth data with the sensorelevation. The tidal muting analyses are based on data collected over a one-year period from January20, 2007 through January 31, 2008.Recorded tides were compared with tides measured at the nearest tidal station, located 22.5 kilometers(14 miles) north in Los Angeles Outer Harbor (LAOH) (NOAA Station 9410660). The NOAA gaugeis located immediately adjacent to the open ocean, and the recorded tides represent the ocean tidalconditions. The data were obtained from the NOAA Tides and Currents website(http://tidesandcurrents.noaa.gov). The obtained data were not temporally corrected based on distanceto the study site because the correction is less than the logging period.ResultsComparison of the lower low tide data for each sampling date since January 20, 2007 against theNOAA tide data for LAOH shows that the FTB does not completely drain to local oceanic sea levelsduring lower low spring tides (Figure 2-5a). Moreover, simple inspection of the data indicates thattidal muting was becoming more pronounced through the 2007 monitoring period as the flood shoalbuilt.Plotting the difference between the minimum observed tidal elevations for all daily lower low tides atLos Angeles versus the FTB illustrates increased tidal muting during 2007 (Figure 2-5b). Mean tidalmuting within the FTB has increased by 0.10 m (0.34 ft) between January 20, 2007 and January 31,2008. This equates to approximately 8.5 mm (0.028 ft) per month during the monitoring period(Figure 2-4b). Notably, on infrequent occasions the tidal elevation of the ocean tides marginallyexceeded the elevation within the FTB. This occurred principally at periods of neap tide and may bethe result of a variety of factors ranging from inlet morphology, tidal lag influences, to instrumentvariance at particular times. While the mean low tide muting has increased by only a moderateamount, more significant muting occurs during spring tide series when the greatest tidal range isobserved (Figure 2-4a).Table 2-3 summarizes the monthly highest and lowest tidal elevations observed in the FTB and LAOH.Because the FTB gauge was lowered on January 19, 2007 only a partial month was analyzed inJanuary 2007. The monthly maximum tidal ranges shown in Table 2-3 column (4) for the FTB andcolumn (7) for the LAOH were calculated by subtracting the recorded highest tide by the lowest tidefor each month. Column (8) shows the monthly high tide differences by subtracting the highest tide inthe FTB by that at LAOH. A negative number indicates that the ocean high tide is higher than that inthe FTB, and a positive number indicates the FTB has a higher high tide than that in the ocean. Thelatter could be the result of disturbances in the measurements or other error. Without substantialepisodic event shoaling of the inlet, the high tide should theoretically rise to a higher level on the opencoast than within the Bolsa Chica basin. However, the differences in the high tide elevations are verysmall, which indicates the overall measurement is reliable and there is no consequential muting of thehigh tides. Column (9) shows the low tide muting in the FTB, which is calculated by subtracting themonthly low tide in the FTB by that in the ocean. The low tides are muted, and muting conditions arechanging over time as a result of flood shoal development in the basin.Merkel & Associates, Inc. 67