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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
settlement of the East MTB water control structure (WCS) on which the FTB tide gauge is mounted.
As this structure settled downward, it suggested a rising water elevation in the FTB and thus the
subtracted difference from LAOH tide station data would indicate negative values. This artifact in the
data is an order of magnitude less important than the measured muting rates within the FTB, but it is
important to understanding sources of tidal measurement error at the site.
The overall differences in the high tide elevations are relatively small as shown in Table 2-2, with a
2007-2008 average difference of less than 0.007m. Given the anticipated settlement of WCSs, and thus
the attached FTB gauge and the relatively minor variation from LAOH it is clear that no consequential
muting of the high tides within the FTB has occurred.
When examining the maximum spring tide muting that occurs at low tide on a spring-neap tidal cycle
basis, the severity is much more pronounced than is evidenced during mean low tides or higher low
tides. Table 2-2 also presents the low spring tide muting in the FTB in comparison to oceanic
conditions at LAOH. This was calculated by subtracting the spring low tide in the FTB from the
equivalent oceanic low tide at the LAOH gauge. When the low tide muting is graphed as a function of
time (Figure2-6) several aspects of the FTB muting are revealed. First, the data indicate the presence
of seasonal variation in muting, with an increase in the winter and mid-summer months when larger
than average tidal ranges occur, and decreased muting during the spring and fall months when smaller
than average tidal cycles occur. Figure 2-6 also shows fairly substantial changes in muting rates
between months, including a relatively precipitous acceleration in the extent of muting in the system
overall beginning in January 2008. Prior to that month, little change in the extent of muting was seen.
After January 2008, there was a sharp increase in muting through approximately April 2008 after
which time the spring tide low tide muting remained fairly constant through the remainder of the year.
1
0.9
0.8
Spring Tide Low Tide Muting (m)
0.7
0.6
0.5
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0.3
0.2
0.1
0
Nov-06
Dec-06
Jan-07
Feb-07
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Apr-07
May-07
Jun-07
Jul-07
Aug-07
Sep-07
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Jan-08
Feb-08
Mar-08
Apr-08
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Feb-09
Figure 2-6. Maximum spring low tide muting (Muting reflects the maximum difference between the FTB and
corresponding LAOH low tides)
Merkel & Associates, Inc. 111