National Assessment of Shoreline Change Part 4: Historical Coastal Cliff Retreat along the California Coast

Coastal cliff retreat, the landward migration of the cliff face,
is a chronic problem along many rocky coastlines in the United
States. As coastal populations continue to grow and community
infrastructures are threatened by erosion, there is increased demand
for accurate information regarding trends and rates of coastal cliff
retreat. There is also a need for a comprehensive analysis of cliff
retreat that is consistent from one coastal region to another. To meet
these national needs, the U.S. Geological Survey is conducting an
analysis of historical coastal cliff retreat along open-ocean rocky
coastlines of the conterminous United States and parts of Hawaii,
Alaska, and the Great Lakes. One purpose of this work is to develop
standard repeatable methods for mapping and analyzing coastal cliff
retreat so that periodic updates of coastal erosion can be made
nationally that are systematic and internally consistent. This report
on the California Coast is an accompaniment to a report on long-term
sandy shoreline change for California. This report summarizes the
methods of analysis, interprets the results, and provides explanations
regarding long-term rates of cliff retreat. Neither detailed
background information on the National Assessment of Shoreline Change
Project nor detailed descriptions of the geology and geomorphology of
the California coastline are presented in this report. The reader is
referred to the shoreline change report (Hapke et al., 2006) for this
type of background information. Cliff retreat evaluations are based on
comparing one historical cliff edge digitized from maps, with a recent
cliff edge interpreted from lidar (Light Detection and Ranging)
topographic surveys. The historical cliff edges are from a period
ranging from 1920-1930, whereas the lidar cliff edges are from either
1998 or 2002. Long-term (~70-year) rates of retreat are calculated
using the two cliff edges. The rates of retreat presented in this
report represent conditions from the 1930s to 1998, and are not
intended for predicting future cliff edge positions or rates of
retreat. Due to the geomorphology of much of California's rocky coast
(high-relief, steep slopes with no defined cliff edge) as well as to
gaps in both the historical maps and lidar data, we were able to
derive two cliff edges and therefore calculate cliff retreat rates for
a total of 353 km. The average rate of coastal cliff retreat for the
State of California was -0.3±0.2 m/yr, based on rates averaged from
17,653 individual transects measured throughout all areas of
California's rocky coastline. The average amount of cliff retreat was
17.7 m over the 70-year time period of our analysis. Retreat rates
were generally lowest in Southern California where coastal engineering
projects have greatly altered the natural coastal system. California
permits shoreline stabilization structures where homes, buildings or
other community infrastructure are imminently threatened by
erosion. While seawalls and/or riprap revetments have been constructed
in all three sections of California, a larger proportion of the
Southern California coast has been protected by engineering works,
due, in part, to the larger population pressures in this area.
[Summary provided by the USGS.]