A total of 1860 beach profiles were surveyed monthly from October 1995 to December 2003 at 29 control points along the coast of Rosarito (Baja California, Mexico). Sand volumes were estimated for the subaerial beach and related to deep-water wave records off California and nearshore waters at two localities off Rosarito. The spectral analysis of the sand volume time series revealed two main spectral peaks of 12- and 36-month periods. The latter cycle was identified for the first time along Rosarito beaches, showing a slightly larger amplitude on the beaches near the Federal Electricity Commission´ s breakwater and in the southern part of the study area. The contribution to sand movement produced by the annual cycle was 80 m3 m–1, while the transport induced by the 36-month cycle was 50 m3 m–1. The erosion and accretion cycle corresponded to winter and summer, respectively, but this pattern was inverted at two localities. The annual or seasonal cycle of sand volume change, defined in terms of the Dean number (Ω = Hs/wsT), is controlled by the seasonal wave pattern, while the inverted cycle is due to local changes in beach morphology and approach angle of breaking waves. The factor that accounted for this cycle is associated with interannual wave pattern variations and this, in turn, with ENSO events.
Punta Banda Estuary, located within Todos Santos Bay, on the northwest coast of the Baja California Peninsula, is separated from the bay by a single sandbar, approximately 8 km long. From 1972 to 2003 this sandbar showed a longitudinal net growth of 420 m, which decreased the width of the estuary mouth by more than 400 m. Despite the bar's rapid growth it has been suggested that the estuary mouth remains open because in spring, summer and autumn the longshore sand transport is northeastward, but during winter the longshore sand transport should be toward the southwest, away from the mouth, establishing a balance between the amount of sediment transported both ways. In this study we analyze wave data acquired from November 2004 to March 2005 on the bay side of the sandbar, and for the first time provide the magnitude and direction of the longshore sand transport during winter. The results show that during this season the dominant waves approach the bay from the west-northwest and the sandbar from the west, inducing a net longshore sand transport of 817 m 3 day-1 to the northeast. This result contradicts the hypothesis of a longshore sand transport balance, and shows that throughout a typical year the net longshore sand transport is to the northeast. The fact that the estuary mouth has not closed up should be attributed to other causes, such as extraordinary amounts of rain during the rainy season or inequalities between the ebb and flood tidal current velocities to the estuary.
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