California's Retreating Coastline: What Next?

摘要

The coast of California is retreating in response to a continuous sea level rise over the past 18,000 years. Of more immediate concern to California, however, are the impacts of severe El Nino events, specifically, large storm waves and elevated sea levels as we experienced in 1982-83 and 1997-98. The state's coastal property values are at all time highs, with houses literally on the sand for sale in the $5-$10,000,000 range. While the entire state's shoreline has migrated eastward 5-15 miles over the past 15,000 years, because of the investment and high property values, significant public and private funds have been expended in efforts to slow or halt any additional retreat. Our future options are limited, however, and they need to be both sustainable and cost-effective over the long term. Over the past 50-75 years, the typical response to coastal retreat has been the construction of seawalls or revetments. Seawalls, however, which have a number of impacts, are designed to protect bluffs and cliffs, not to preserve beaches. Thus a conflict has developed between cliff-top homeowners and the public who use the fronting beaches. While some states have banned all new hard protective structures, proposals for new seawalls in California are still frequent but face increasing public opposition. Simultaneously, local governments have organized with the help of lobbyists to push for funding for beach replenishment and nourishment. While there have been millions of cubic yards added to the beaches of southern California from large coastal construction projects, there have been very few sites to date where sand was imported solely for nourishment. Due to the high littoral drift rates that characterize California's coast, it can be expected that the life span of nourished beaches in most locations will be relatively short. While artificial beach nourishment is very expensive and short lived, trapping the sand so that more of it stays on the beach could provide greater long-term benefits. Many of California's beaches exist because of littoral barriers such as headlands that serve as groins. Trapping this sand on the beaches would provide significant and sustained benefits as would the release or removal of the large volumes of sand trapped behind the many stream impoundments.