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

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportInfluence of Entrance ChannelShoreline Changes Adjacent to Entrance Channel: Between October 2005 and October 2007, thebeaches upcoast (north) of the new entrance channel accreted, while those downcoast (south) of thechannel tended to erode (Figure 2-13). Figure 2-20 compares the shoreline changes immediatelynorth of the channel (Transect 318+30) to the changes for the remaining upcoast monitoring sites(Transects 249+30 and 311+12). The shoreline at the three upcoast transects responded similarlybetween October 2005 and January 2007. Following the opening of the new channel (January2007 to October 2007), the shoreline at Transect 318+30 retreated slightly, while the beaches at theother upcoast sites advanced.A time series of the shoreline changes at the four transects located downcoast (south) of theentrance channel is shown in Figure 2-21. The shoreline changes at Transect 333+30 (immediatelydowndrift of the channel) were nearly identical to those at Transects 350+71 and 423+89. Theshoreline changes at Transect 378+29 differed only modestly and indicate progressive shorelineadvance between March 2006 and October 2007.Volume Changes Adjacent to Entrance Channel: The subaerial volume changes upcoast of theentrance channel (Figure 2-22) responded similarly to the shoreline changes. Volume gainsoccurred throughout the period at Transects 249+30 and 311+22. The sediment volume initiallyincreased at Transect 318+30 (adjacent to the entrance channel), but losses prevailed after May2007.Downcoast of the entrance channel, modest subaerial volume gains occurred at three of the fourtransects (Figure 2-23). As discussed previously, the volume gains at Transect 333+30 appear tobe attributable to the preservation of the nourishment material placed landward of the berm despitethe loss of beach width. The most significant volume losses in the downcoast region occurred atTransect 333+30 (adjacent to the entrance channel) between January 2007 and October 2007.Because this period followed the opening of the entrance channel, particular vigilance is warrantedat this site during future monitoring activities. (Note: It is not possible to assess the shorezonevolume changes for the Bolsa Chica Monitoring period because the October 2005 profile does notextend below the waterline.)Sediment Trapping in the Full Tidal Basin: As indicated in Section 2.1, approximately 198,000 m 3of sediment was deposited in the lagoon during the 17-month period between August 2006 andJanuary 2008 (equivalent to approximately 140,000 m 3 /y). While a small fraction of this materialmay have resulted from redistribution of basin sediments or aeolian processes, nearly all of thesediment has entered the basin from the ocean. It is probable that the high shoaling rate is atransient effect attributable to inlet stabilization; drawing locally from the pre-filled ebb bar andwidened beaches adjacent to the inlet. Nevertheless, the shoaling rate is on the same order ofmagnitude as the alongshore sediment transport rates developed as part of the CCSTWS-OCsediment budgets (estimated to range from 108,000 m 3 /y to 125,000 m 3 /y). As a result, particularattention is warranted in monitoring the flood shoal accumulation rates and beach profiles overtime.In the event that trapping rates detected during the initial post-opening are not transitory, theserates are of a significant magnitude to be of major concern to longshore transport in the littoral cell.Merkel & Associates, Inc. 90