Assessing natural and human alterations to coastal sediment supply in California and the impacts on regional beach sustainability

The long-term sustainability of California's beaches depends on periodic deliveries of sand and gravel from coastal rivers and streams. To assess the long-term health of California's beaches, this study characterized the current state of fluvial sediment delivery and quantified, on a littoral cell basis, the cumulative impacts of dams in decreasing annual discharge. Presently, more than 500 dams impound over 42,000 km2 (or 38%) of California's coastal watershed area. Flow modeling suggests that these dams have reduced the average annual sand and gravel flux to 20 major littoral cells by 2.8 million m3/yr (or 25%) by diminishing flood hydrographs. Approximately 23% (or 274 km) of the 1,193 km of beaches in California are downcoast from rivers that have had sediment supplies diminished by one-third or more. Moreover, 192 km or 70% of these threatened beaches are located in southern California, where most of the state's beach recreation and tourism activities are concentrated.; With the passage of Assembly Bill 64, the California Public Beach Restoration Act, in 1999, the State of California has made its first enduring commitment to large-scale public beach nourishment. However, there have been very few studies to date of regional beach changes documenting long-term trends of erosion that would justify artificially adding sand to California's beaches. This study completed the first high-precision investigation of changes in sub-aerial beaches in the Santa Cruz littoral cell over most of the twentieth century. In the Santa Cruz cell, no natural, persistent long-term trends of erosion or accretion were found at any of the beach study sites. Instead, 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. Although a persistent century-scale trend was not apparent, decadal variability was present at each site. The variation in fluvial sediment inputs may be the dominant factor behind decadal variability in beach width. El Niño events are responsible for most of the major fluvial sediment discharge events along the central California coast and therefore may play a key role in long-term beach sustainability.