2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...
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<strong>Bolsa</strong> <strong>Chica</strong> Lowlands Restoration <strong>Monitoring</strong><br />
<strong>2008</strong> <strong>Annual</strong> <strong>Report</strong><br />
measured second year shoaling was thus determined to be 79% of that predicted by the preliminary<br />
engineering model calculations.<br />
The variable seasonal influx of sand, and added complication of provision of local source sand in the<br />
pre-filled ebb bar and beach around the mouth likely have played a role in the high early infill rates.<br />
Further, early infill would have also added sand to the oversized entrance channel, thus decreasing the<br />
observed rate of shoaling from the true rate since the flood shoal survey assessment area does not<br />
extend to the full extent of the entrance channel. Subsequent reduced rates of infill may illustrate more<br />
rapid achievement of relative stability following the initial system loading. The lack of temporal<br />
precision and high variance associated with early system dynamics is to be expected with limited<br />
predictive methods.<br />
Through <strong>2008</strong>, the average monthly shoal accumulation rate remained remarkably stable with the<br />
average accumulation rate from January-June <strong>2008</strong> being 3,775 m 3 /month and July-December <strong>2008</strong><br />
being an average of 4,247 m 3 /month. The six-month accumulation rates, however, mask what are<br />
likely to be much more variable instantaneous rates that are dependent upon tide state, surf conditions,<br />
littoral transport volumes, and patterns of flow across at the flood shoal. Tidal monitoring in the FTB<br />
suggests that at least one punctuated change in the flood shoal may have occurred during <strong>2008</strong> (see<br />
Section 2.2). This may reflect the formation of a minor sill across the primary tidal channel, followed<br />
by a breach of the sill, or the cut of a new primary channel across the flood shoal.<br />
The flood shoal volume, area of shoaling, and shoaling rate have occurred similarly to processes<br />
predicted during the project design. The notable difference between predicted shoaling and that<br />
actually observed has been the bypass of much of the maintenance basin by the shoal formation and<br />
thus a greater penetration into the FTB than would be expected given the accretion volume manifested<br />
at this early period in shoal formation. Although ultimate shoal formation is expected to extend much<br />
further into the FTB (Figure 2-1), the early bypass of portions of the shoal maintenance basin allowed<br />
a more rapid progression of the shoal along the eastern edge of the basin than the western edge. In<br />
retrospect, this bypass should have been predictable given past observations of shoal development in<br />
systems such as Batiquitos Lagoon, Agua Hedionda Lagoon, and San Elijo Lagoon and the anticipated<br />
patterns of effective tidal flows. As the sand accumulation along the eastern edge of the basin<br />
develops a greater resistance to flows, the effective flow pattern is expected to shift into those portions<br />
of the maintenance basin that have not received sand accumulation, thus beginning to infill the full<br />
extent of the basin with shoal sands.<br />
Another principal difference in shoal development from that anticipated was the transverse bar<br />
development at the inside curve and the subsequent deep scouring on the opposite bank to the south.<br />
Given basin morphology, the observed development patterns of the <strong>Bolsa</strong> <strong>Chica</strong> flood shoal are<br />
anticipated to continue in the future. It is less clear how the future accretion rates will vary as the inlet<br />
conditions continue to evolve and respond to maintenance dredging, changes in littoral cell sand<br />
availability, coastal storm climates, and the addition of future prism with the opening of the remaining<br />
central and eastern MTBs.<br />
The manner in which the basin performs as expected or different from expected is a factor in<br />
determining the necessity for shoal dredging and the establishment of triggers. This is addressed in<br />
Section 3.<br />
Merkel & Associates, Inc. 106