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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
recorded tides were very close to the predicted values of the post-construction condition. By January
2008, the FTB experienced a tidal range of 2.19 m and a low tide muting of 0.51 m.
The winter of 2007-2008 marked a considerable change in the tidal range and low tide muting within
the FTB (Figure 2-6). 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.
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 (M&N 1999).
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, with the most distinct changes occurring during the 2007-08
winter and spring months. Additionally, the low tide lag increased by only 36 minutes throughout
2007. Over 2008, the increase in lag was 174 minutes, a nearly five-fold increase.
During preliminary engineering, tidal predictions were based on a theoretical average spring tidal
condition, not the maximum spring tide condition noted in the muting analysis (Table 2-2). 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. The MTB tidal conditions
are strongly influenced by the amount of stored water in them as a result of prior tidal history. Waters
step up in elevation during moderate neap tidal series in the FTB and drain down during the more
extreme low water levels of spring tide series. This un-natural tidal fill and drain pattern is required to
provide gravity driven muted tides to marshlands that are located at elevations well below mean sea
level.
The west MTB was opened to the FTB in March 2008. During the first few months post-opening,
adjustments were made to maximize tidal range in the basin. In October 2008, the gates were
readjusted to restrict tidal flows into the west MTB due to increasingly inadequate drain-out during
low tides. As a result of the loss of low tide range in the FTB, low water elevations slowly built to
higher base elevations in the west MTB, causing the high water elevations to exceed desired maximum
operating elevations. This caused intermittent overtopping the precautionary oil containment weir at
the water control structure, and spilling over the lowered spillway that connects the west MTB to the
central 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.
Merkel & Associates, Inc. 113