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

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
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