Cliff erosion and bluff retreat along the California coast

While the cliffs and bluffs making up the California coastline may appear at first glance to provide a resistant barrier against the Pacific Ocean, nothing could be further from the truth. Eighteen thousand years ago the coastline of the state was as much as 45 km offshore to the west. With the melting of the icecaps and glaciers at the end of the last Ice Age, and the warming and thermal expansion of the world oceans, /spl sim/43,000,000 km/sup 3/ of seawater was released which raised sea level about 120 meters globally. As a result, the coastline of California migrated east and continues to do so but at a reduced rate as sea level rise has slowed over the past /spl sim/3000 years. The coastline has been advancing and retreating for as long as a landmass recognizable as California has existed and sea level has fluctuated. The shoreline of today is, therefore, only a single point on a constantly changing curve. Seventy-two percent of California's 1760 km coastline consists either of high steep cliffs or lower bluffs eroded into nearly horizontal marine terraces. Four processes and factors interact to produce sea cliff retreat. Two of these are very large in scale and provide the overall regional framework for coastal evolution: sea level change and tectonic activity. The other two, the inherent properties of the cliffs or bluffs (rock strength, structural weaknesses, presence of groundwater, for example) and the external forces (wave energy, tidal range, degree of protection offered by a beach, climate, and frequency and magnitude of severe storm events) are the most influential local factors. While the entire coastline of the state is eroding, the weaker sedimentary rocks typically erode at average rates of 10 to 30 cm/year, whereas the harder igneous and metamorphic rocks, which often make up the resistant headlands and steep mountainous areas, erode at slower rates. The wide, flat, uplifted marine terraces, which are characteristic of much of California's coast and have usually been eroded into sedimentary rock, are easily developed and have been the sites where many of California's coastal communities have evolved. The low cliffs and bluffs fronting these terraces, however, are often easily eroded and coastal land loss in these areas has become a statewide concern. Cliff erosion hot spots are well recognized and public and private property damage during the last twenty-five years dominated by El Nino conditions has been high. Historical approaches to dealing with the ongoing erosion of the state's cliffs and bluffs have focused historically on seawalls and revetments, but concerns with the long-term effectiveness and impacts of these structures have begun to restrict additional armor emplacement.