Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California
Recent Erosional Changes at Point Año Nuevo, Implications for Beach Stability and Coastal Erosion in Northern Monterey Bay, California
Gerald E. Weber, Sr.
Earth Sciences Department, Univ of California, Santa Cruz, Santa Cruz, CA 95064, [email protected]
Historic maps and aerial photographs indicate extensive erosional changes at Point Año Nuevo during the past 400 years. These include; 1) formation of the channel between the point and island since 1603, 2) erosional destruction of the broad beach along the north shore that provided a sand source for the dune field, 3) and extensive erosion along the south shore of the point in the past 30 years, in an area where there had been no erosion for 120 years. Conversely, in northern Monterey Bay the sea cliffs south of New Brighton Beach have not experienced significant erosion over the past 150 years. Why?
Creation of the channel between Año Nuevo Point and the island temporarily changed the long term equilibrium in littoral drift along the Santa Cruz County coastline. The channel allowed a large volume of sand (13 to 20 million cubic yards) trapped behind the point to slowly move through the channel; adding yearly to the volume of sand from "normal" (pre channel) littoral drift moving along the Santa Cruz County coastline. Consequently, beaches down drift from the point widened, providing sea cliffs protection from wave attack for the past 250-350 years. Along beaches protected from the predominant northwest and westerly winter swells in southern Santa Cruz County, the additional sand widened beaches sufficiently to protect the late Holocene sea cliffs from wave attack except during the most severe winter storms, resulting in a 250-350 year period in which Northern Monterey Bay beaches have been abnormally wide.
The point source of sand at Point Año Nuevo is now exhausted. Beaches down drift from the point are thinning and sea cliff erosion is accelerating. The disequilibrium period of high volumes of littoral drift and reduced sea cliff erosion in Northern Monterey Bay is ending, and returning to the normal equilibrium that existed prior to the formation of the channel between Point Año Nuevo and Año Nuevo Island. This suggests that coastal erosion rates determined from short term (40-60 year) studies will underestimate long term erosion rates. Consequently a period of thin beaches and increased coastal erosion is near. It also indicates how difficult it is to determine long term rates of geomorphic processes from short term observations.
Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85224.htm). © Copyright 2005 The Geological Society of America (GSA).