Extended Abstract (2.3M)4.6
California's retreating coastline-where do we go from here?
Gary B. Griggs, University of California, Santa Cruz, Santa Cruz, CA
California’s coastline is approaching a crisis point. This has resulted from a combination of natural processes and cycles, combined with human intervention and population growth. The coast of California is retreating in response to a continuous sea level rise over the past 18,000 years. While sea level rise rates have slowed over the last 3000-5000 years, there is no indication that this will subside in the near future; in fact, most scientists predict an increase in this rate. 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 beginning of a two-decade long period of more frequent ENSO events in 1978 altered our perception of coastal hazards in California. Most oceanfront residents and government agencies that have some shoreline responsibility have responded to the damage and losses with proposals for reconstruction or repair, seawalls or revetments, or requests for beach nourishment projects.
California’s population reached 35 million in 2002, a doubling since 1965. The state’s coastal property values are at all time highs, with houses literally on the sand for sale in the $5-$10,00,000 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. The lack of any certainty in the maximum elevation that sea level will ultimately rise to, or knowledge of when this will occur, make if very difficult to develop sensible and balanced long-term strategies for responding to coastal erosion.
Over the past 50-75 years, the typical response to coastal retreat has been the construction of seawalls or revetments. Twenty-seven miles of the California coast was protected in 1971, and by 1998 this has increased to 110 miles or 10% of the entire state’s coastline, a four-fold increase in 27 years. Armoring is far more extensive in Southern California with 33% of the combined coastlines of Ventura, Los Angeles, Orange and San Diego counties having now been protected. Seawalls, however, are designed to protect bluffs and cliffs, not to preserve beaches. Thus a conflict has developed between cliff-top homeowners trying to protect their property, and the public who use the fronting beaches. Many of the concerns that have been raised about seawalls, including visual impacts, restrictions on beach access, reduction of sand supply, and the loss of beach beneath or in front of seawalls, revolve around the issue of to what degree should private property owners be allowed to impact public beaches as they attempt to protect their own cliff or bluff top property. Or, in the case of government-funded projects, how much taxpayer’s money should be spent on attempts to stabilize the position of an otherwise eroding coastline if it means beach loss?
While several states have banned all new hard protective structures, proposals for new seawalls in California are still frequent but face increasing public and increased Coastal Commission scrutiny. Simultaneously, local governments have organized to lobby for beach replenishment and nourishment. Their objectives are to widen or rebuild beaches that could provide increased recreation area for both tourists and residents, and also help buffer the shoreline from wave attack. Arguments have been made that the beaches of California are eroding due to sand supply reduction. While significant impoundment of upstream sand supplies from the streams draining into the littoral cells of southern California has been well documented, it is not clear that this reduction has been directly reflected in sand supply at the coastline. To date, there have been no long-term assessments of systematic regional change in beach width or volume. The artificial widening of many beaches between the 1940’s and the 1960’s due to sand nourishment from the many large coastal construction projects, however, complicates any evaluation of long-term beach change in southern California. The beaches of the Santa Monica cell, for example, have been gradually returning to their natural widths at a time when sand reduction from dams and other diversions, and a return to more frequent and severe ENSO events, have significantly impacted the beaches of portions of southern California.
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 where sand was imported solely for nourishment. One of the first relatively large-scale beach nourishment projects dredged a total of 2 million yds3 of sand from six offshore sites and placed it on 12 San Diego County beaches at a cost of $17.5 million, or $8.75/yd3. Beach surveys, however, indicate by early 2002 that much of the sand had already been transported offshore or downcoast. 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 or rocky stream deltas that serve as groins. There are many locations along the state’s coastline where groins or other barriers could be built and initially charged with sand that could provide significant recreational area and shoreline protection. Hundreds of thousands of cubic yards of sand permanently leave California’s beaches each year through submarine canyons. 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.
Session 4, MEASAURING THE CHALLENGES OF LIVING IN THE COASTAL ZONE
Wednesday, 12 January 2005, 8:30 AM-2:30 PM
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