2007 Annual Monitoring Report (pdf 16MB) - Bolsa Chica Lowlands ...

Bolsa Chica Lowlands Restoration Monitoring2007 Annual Reportexceeded the predicted muting following shoal development. Interestingly, the phase lags are alsogreater than anticipated in design. There has been a gradually increasing degree of muting over time,with variable rates of muting – likely coincident with significant drainage restriction by the floodshoal.No changes are recommended to the tidal monitoring program.BEACH MONITORINGHistorically, the beaches along the Bolsa Chica study area have benefited as the downdrift recipient ofthe Surfside-Sunset nourishment material. During the 34-year period between 1963 and 1997, thebeaches advanced at four of the five historical transects included in the Bolsa Chica monitoringprogram. Mean sea level (MSL) shoreline advance ranged from 14 m to 71 m within the present studyarea. The only occurrence of shoreline retreat during the 34-yr period was a loss of 18 m at a transectlocated at Huntington Cliffs. The volume of sand above MSL increased in parallel to the beach widthchanges during the period. The shorezone volumes in the study area, which incorporate the sedimentchanges further offshore, increased at all of the sites. The greatest gains typically occurred prior to1978.During the two-year period encompassing the construction of the Bolsa Chica Lowlands RestorationProject (October 2005 to August 2007), the shoreline advanced at the three transects located north ofthe entrance channel, with the greatest gain being 22 m. Shoreline retreat predominated at the surveysites located south of the entrance channel, with losses of approximately 7 m of beach width. Thesubaerial volume (sand above MSL) changes north of the channel were very similar to the beach widthchanges. In contrast to the predominance of shoreline retreat south of the entrance channel, subaerialvolume loss occurred at only one transect in this region. This apparent discrepancy immediately southof the entrance channel can be explained by the conservation of the project beach nourishment materialplaced landward of the berm despite the loss of beach width. While it is not possible to quantitativelyassess shorezone volume changes during the recent two-year period (the October 2005 profile does notextend below the waterline), the beach profiles at three sites near new inlet show volume gainsattributable to ebb bar pre-filling during project construction. Despite the overall volume gains locally,particular attention is warranted in the region south of the entrance during future monitoring activitiesbecause minor shoreline retreat did occur in this area during the period.Approximately 198,000 m 3 of sediment was deposited in the lagoon during the 17-month periodbetween August 2006 and January 2008 (equivalent to approximately 140,000 m 3 /y). While a smallfraction of this material may have resulted from redistribution of basin sediments or aeolian processes,nearly all of the sediment has entered the basin from the ocean. It is probable that the high shoalingrate is a transient effect attributable to inlet stabilization; drawing locally from the pre-filled ebb barand widened beaches adjacent to the inlet. Nevertheless, the shoaling rate is on the same order ofmagnitude as the alongshore sediment transport rates previously developed for Orange County(estimated to range from 108,000 m 3 /y to 125,000 m 3 /y). As a result, particular attention is warrantedin monitoring the flood shoal accumulation rates and beach profile changes over time.In the event that trapping rates detected during the initial post-opening are not transitory, these ratesare of a significant magnitude to be of major concern to alongshore transport in the littoral cell. If leftunchecked and unmanaged, the primary implication of a substantial reduction of the alongshoresediment supply is shoreline erosion downdrift of the entrance channel. The Bolsa Chica project,Merkel & Associates, Inc. 6