Delineating Long-Term Trends in Beach Change, Central California: Fid-Bau Portal

Delineating Long-Term Trends in Beach Change, Central California

Willis, Cope M. / Griggs, Gary B.

We investigated the temporal oscillation of five representative beaches distributed along 150 km of the Santa Cruz littoral cell to assess their stability over decadal time scales. Using high resolution, soft-copy photogrammetry, we generated highly accurate orthophotographs of our study sites from aerial photographs taken at five to ten year intervals between 1931 and 2001. To reduce the influence of the significant seasonal variability in beach size, we focused our investigation on aerial photographs taken during the summer and early fall. Our results indicate that, while sub-aerial beach width varied by as much as ±35% from the long-term average beach width, no persistent erosional trend was present at any of the five study sites. We believe that a dynamic equilibrium has been maintained over the past 70 years because the primary sources of beach sediment—rivers and bluffs—have not been disrupted significantly by watershed or coastal development. Average annual sediment inputs have been diminished by less than 10% due to the construction of dams that reduce fluvial sediment delivery to the coast and shore protection structures that temporarily halt sediment inputs from bluff erosion. In addition, no evidence currently available suggests that losses of sediment from longshore or offshore transport out of the cell have increased. Thus, the Santa Cruz cell beaches appear to be in balance with the relatively unaltered sediment budget. This type of littoral cell scale assessment that combines an investigation of (1) historical beach changes and (2) reductions or augmentations to the primary sand sources and losses provides a reliable indication of regional beach sustainability over the typical 50-year planning horizon of coastal managers. The methodology employed in this study could serve as a model for similar studies in other parts of the state, particularly southern California where dams have significantly diminished fluvial sediment supplies, to provide the requisite scientific background to evaluate the need for and effectiveness of beach nourishment.