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

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Bolsa Chica Lowlands Restoration Monitoring 2008 Annual Report dynamic due to dynamic tidal and sedimentation processes. The January 2007 survey showed that the inlet thalweg was in the middle of the inlet channel and the shoal was on the southeast side of the inlet channel as shown in Figure 2-2. This reflects the infancy state of the wetland geomorphology at that time, with sediment not yet having been deposited along the inside bank of the tidal inlet channel. The June 2007 survey shows sedimentation along the inside bank of the tidal inlet channel, with the channel being forced toward the outside bank. That pattern continued to evolve over time and is also reflected in the January, July, and December 2008 surveys. From mouth opening through December 2008, each survey shows the flood shoal growing progressively larger, corresponding to a period of time when low tides in the tidal basin were becoming increasingly truncated and tides were becoming more muted. The effects of the shoaling on tidal muting are analyzed in Section 2.2. Also noted in the progression of surveys are a series of new channel cuts through the flood shoal as the resistance to tidal flow along the primary channel caused breakouts and new channel formation into the larger FTB basin. These breakout channels resulted in the expansion of the flood shoal to the west, away from the primary linear shoal accumulation and into the boat ramp area along the west shore to the north of the inlet. Another morphologic feature noted in the shoal formation was the generation of an elevated and shell fragment-armored nose on the inside radius of the inlet channel as it enters the FTB. This elevated shoal was the result of both current-transported and wave-built littoral sand accumulation. The subsequent winnowing of sand from this bar resulted in an armoring of the feature by shell fragments carried in the littoral sands. Because of the significant bar development, the channel thalweg was pushed off the tip of the bar towards the opposite bank and narrowed significantly. The narrowing of the flow resulted in significant bed scouring at the base of a riprap-armored nose on the opposite bank. The scour displaced sands below the riprap toe and resulted in some loss of bank protection at this location. As a result, rock was replaced at the toe of the riprap slope on the south side of the inlet to restore protection to this area. Based on the bathymetric assessments, it could be estimated that the flood shoal volume deposited during the first twelve months post-opening, August 2006 through July 2007, was approximately 127,524 m 3 (166,667 yds 3 ) (derived by taking the 122,105m 3 surveyed in June of 2007 and adding 5,419 m 3 (a single month times the average estimated deposition rate for the subsequent period, June 2007 through January 2008). This method of annualizing the shoal volume would be expected to result in an underestimate of shoaling due to the generally declining shoaling rate through time. Using this estimating approach, the actual shoaling rate was estimated to be only 1% higher than the 126,200 m 3 (165,000 yds 3 ) predicted as the first year shoal volume (M&N 1999). As expected, the subsequent sediment accretion rate shown in Figure 2-3 decreases gradually, as the flood shoal developed towards an equilibrium state. The second year shoaling rate from August 2007 through July 2008 is roughly estimated at 53,381 m 3 /year (69,395 yds 3 /year) (derived by subtracting the estimated August 2007 volume from the July 2008 surveyed volume. This rate is substantially lower than the 102,500 m 3 /year (134,000 yds 3 /year) shoaling volume predicted by preliminary engineering modeling for the second year after the inlet was to be connected to the ocean (M&N 1999). However, when taken as a whole, the model-predicted two-year accumulation volume of 230,000 m 3 (300,000 yds 3 ) compares very favorable to the early period measurements and rate-based escalation two-year volume of 180,905 m 3 (237,000 yds 3 ) from the post-construction monitoring. The Merkel & Associates, Inc. 105
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. Merkel & Associates, Inc. 106
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2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...

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