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

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportBeach Profile PlotsThe 2007 beach profile data were used in conjunction with data from the historical surveys to createprofile plots and compute changes in beach width and sediment volume. The beach profile plotsdeveloped from the survey data are provided in Appendix 2-B. The range on each profile plotrepresents the distance in meters seaward of the survey origin measured along the transect alignment.The elevation is given in meters relative to NAVD88.Two sets of beach profile plots were generated for each transect. The first set of plots shows all of thebeach profile data available for each transect, while the second set of plots shows only those profilesobtained in during the two-year period encompassing the construction Bolsa Chica LowlandsRestoration Project (October 2005 to August 2007). The plots focusing on the recent two-year periodalso show the envelope of all available profile data that preceded the opening of the Bolsa Chicaentrance channel (May 1963 to March 2006). These plots also include two panels for each transect -one isolating the nearshore region of the profile and another displaying the entire profile length.Mean Sea Level Beach WidthsMean Sea Level (MSL) beach widths are provided in Appendix 2-C. The beach width was computedas the horizontal distance, in meters, between the landward edge of the beach sand and the point atwhich the beach profile intersected the plane of MSL Datum. In the Bolsa Chica area, MSL lies 0.79m above NAVD88. Notwithstanding the use of NAVD88 as the elevation reference for the profiledata, MSL was adopted as the shoreline reference in the belief that it provides a more accurateindicator of changes in beach configuration.Sediment VolumesSediment volume changes are provided in Appendix 2-D. The volume changes were computed alongeach transect for the entire width of the shorezone, and for that portion of the profile located aboveMSL (subaerial volume).The offshore boundary of the control volume for the beach above MSL was placed at the intersectionof the profile and a horizontal line corresponding to the elevation of MSL. The offshore boundary forthe shorezone was placed at the “statistical range of closure”. This parameter represents the distanceseaward of the transect origin, beyond which profile variations are smaller than the accuracy of thesurvey technique. As implied by its definition, the statistical range of closure was adopted as theoffshore boundary to separate the signal of true profile change from the noise of survey inaccuracy.The sea bottom elevation at the range of closure corresponds to the “depth of closure” or the depth atwhich sediment transport is not substantially affected by littoral processes.The statistical range of closure was determined for the five historical transects as part of the CCSTWS-OC. These boundaries, however, were no longer appropriate due to the profile changes that resulted atseveral locations from the placement of the ebb bar offshore of the Bolsa Chica entrance. As a result,the statistical range of closure was re-computed for each historical transect and for the two newtransects based on all available survey data collected since May 1963 (Table 2-5). The procedure usedto calculate the statistical range of closure for each transect was identical to that employed for theCCSTWS-OC (USACE, 2002).Merkel & Associates, Inc. 79