EP034-02
Long-term late Holocene cliff retreat rate in Del Mar, California revealed from shore platform 10Be concentrations and numerical modeling
Thursday, 10 December 2020: 17:33
Virtual
Travis Wellington Clow1, Jane K. Willenbring1, Adam Young2, Hironori Matsumoto3 and Alan Hidy4, (1)Stanford Earth Sciences, Stanford, CA, United States, (2)Scripps Institution of Oceanography, La Jolla, CA, United States, (3)University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, (4)Lawrence Livermore National Laboratory, Livermore, CA, United States
Abstract:
Slowly eroding coasts make up the majority of the coastlines on Earth, including the western coast of the United States, and host critical infrastructure like roads, railways, and residential structures. Coastal cliff retreat presents a danger to these communities, potentially amplified under rising sea level conditions, yet constraints on retreat rates used for mitigating future hazards are often limited to those derived from historical imagery and maps dating back 10-100 years. A recently developed combination of in situ-produced cosmogenic 10Be surface exposure dating in conjunction with a new numerical model of shore platform profile development that takes into account sea level rise, intertidal weathering and wave erosion on cliff retreat provides quantification of cliff retreat histories over hundreds to thousands of years via cliff-normal 10Be sample transects.
Here, we use nine cosmogenic 10Be samples from a sandy claystone shore platform preserved along a narrow and sandy beach backed by a near vertical ~20-meter-tall cliff in Del Mar, California to present a long-term cliff retreat rate of 5.5 +/- 0.5 cm a-1 over the last two millennia for this site. This is the first long term cliff retreat rate for any coast in North America determined by this new methodology. Existing decadal retreat rates at and proximal to this site range from 5-20 cm a-1, suggesting that cliff retreat here may be accelerating. Preliminary modeling results suggest that uplift-corrected sea level rise in Southern California, which remained constant during the late Holocene (0.8 mm a-1) but doubled in the last century, cannot alone explain this potential increase, as modeled platform geometries and associated development rates show a stronger dependence on the imposed weathering rate than wave erosion efficacy. We further explore this and other drivers (e.g. increase in storm frequency, land use change) for this potential increase.
