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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
measured second year shoaling was thus determined to be 79% of that predicted by the preliminary
engineering model calculations.
The variable seasonal influx of sand, and added complication of provision of local source sand in the
pre-filled ebb bar and beach around the mouth likely have played a role in the high early infill rates.
Further, early infill would have also added sand to the oversized entrance channel, thus decreasing the
observed rate of shoaling from the true rate since the flood shoal survey assessment area does not
extend to the full extent of the entrance channel. Subsequent reduced rates of infill may illustrate more
rapid achievement of relative stability following the initial system loading. The lack of temporal
precision and high variance associated with early system dynamics is to be expected with limited
predictive methods.
Through 2008, the average monthly shoal accumulation rate remained remarkably stable with the
average accumulation rate from January-June 2008 being 3,775 m 3 /month and July-December 2008
being an average of 4,247 m 3 /month. The six-month accumulation rates, however, mask what are
likely to be much more variable instantaneous rates that are dependent upon tide state, surf conditions,
littoral transport volumes, and patterns of flow across at the flood shoal. Tidal monitoring in the FTB
suggests that at least one punctuated change in the flood shoal may have occurred during 2008 (see
Section 2.2). This may reflect the formation of a minor sill across the primary tidal channel, followed
by a breach of the sill, or the cut of a new primary channel across the flood shoal.
The flood shoal volume, area of shoaling, and shoaling rate have occurred similarly to processes
predicted during the project design. The notable difference between predicted shoaling and that
actually observed has been the bypass of much of the maintenance basin by the shoal formation and
thus a greater penetration into the FTB than would be expected given the accretion volume manifested
at this early period in shoal formation. Although ultimate shoal formation is expected to extend much
further into the FTB (Figure 2-1), the early bypass of portions of the shoal maintenance basin allowed
a more rapid progression of the shoal along the eastern edge of the basin than the western edge. In
retrospect, this bypass should have been predictable given past observations of shoal development in
systems such as Batiquitos Lagoon, Agua Hedionda Lagoon, and San Elijo Lagoon and the anticipated
patterns of effective tidal flows. As the sand accumulation along the eastern edge of the basin
develops a greater resistance to flows, the effective flow pattern is expected to shift into those portions
of the maintenance basin that have not received sand accumulation, thus beginning to infill the full
extent of the basin with shoal sands.
Another principal difference in shoal development from that anticipated was the transverse bar
development at the inside curve and the subsequent deep scouring on the opposite bank to the south.
Given basin morphology, the observed development patterns of the Bolsa Chica flood shoal are
anticipated to continue in the future. It is less clear how the future accretion rates will vary as the inlet
conditions continue to evolve and respond to maintenance dredging, changes in littoral cell sand
availability, coastal storm climates, and the addition of future prism with the opening of the remaining
central and eastern MTBs.
The manner in which the basin performs as expected or different from expected is a factor in
determining the necessity for shoal dredging and the establishment of triggers. This is addressed in
Section 3.
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