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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
tides and that tidal muting was becoming more pronounced through the 2008 monitoring period as the
flood shoal built in the inlet. In 2008, lower low tides in the FTB only went as low as LAOH during
very mild neap tides.
From January to December 2007, tidal muting within the FTB had increased by an average of 0.07 m.
The winter of 2007-2008 marked a considerable change. From December 2007 through April 2008,
tidal range decreased by approximately 0.24 m and tidal muting increased by an equivalent amount.
From May 2008 through the remainder of the year, muting and tidal range remained fairly consistent
with little additional muting being evidenced. By the end of 2008, the tidal range within the FTB had
been reduced from that of the open coast by an average of approximately 0.6 m with the maximum
observed range loss reaching 0.86 m during July 2008. The collected data indicated 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. There were also 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 through approximately April 2008, after which time the spring tide low tide muting
remained fairly constant through the remainder of the year.
The lag of the low tide in the FTB compared to that in the ocean was approximately 78 minutes on
January 19, 2007, 114 minutes on January 21, 2008, and 288 minutes by December 13, 2008.
It was expected that the tidal range would gradually decrease and muting of the low tide would
increase over time. It was further expected that muting and phase lag would become more severe due
to effects of flood shoal development in the FTB until the implementation of the first dredging event,
occurring in 2009. Preliminary engineering predictions of the effect of shoaling on tidal muting were
that the tide range would reach 2.256 m and muting of the low tide to reach 0.244 m. Generally, the
muted tidal range under the post-construction condition met the target of the “full tidal range”
objective of the project planning documents during 2007, but with further shoal development in 2008
the range substantively diminished.
During preliminary engineering, tidal predictions were based on a theoretical average spring tidal
condition, not the maximum spring tide condition. Because of the high importance of the low tide
muting and lag to the drain-fill hydraulics of the MTBs, these maximum drain-out conditions are of
key interest as they pertain to proper functioning of the MTBs. Although the FTB would still be
considered fully tidal in 2008, the diminishing drainage from the basin reached such a point as to
restrict drainage from the open west MTB.
Although the Freeman Creek water control structure slide gates remained closed during 2008, the
muting of the FTB would have otherwise restricted the full drain-out potential if they had been open,
since the drainage of Freeman Creek is by gravity to the FTB. FTB water levels were higher than the
creek in 2008 and would have precluded proper drainage.
As a result of the shoal-associated muting and its controlling influence on the functioning of the MTBs
and Freeman Creek, along with other variables (shoal volume and area of low intertidal habitat lost),
maintenance dredging was warranted in 2008 and first scheduled to occur in the fall of 2008.
Continuous tidal monitoring will continue in 2009.
Merkel & Associates, Inc. 7