Late Holocene Cliff Retreat in Del Mar, CA, Revealed From Shore Platform ¹⁰Be Concentrations and Numerical Modeling

Abstract

Rocky coast cliff retreat presents a hazard to coastal communities and infrastructure that is potentially amplified under rising sea level conditions, among other factors. Unfortunately, constraints on retreat rates are typically limited to those derived from imagery and maps spanning the last ∼100 years. Here, we use a newly developed coupled model of shore platform profile evolution and cosmogenic radionuclide production that considers the influence of relative sea level (RSL) rise, weathering, material resistance, and wave height decay to model cliff retreat over millennial timescales and its potential drivers in Del Mar, California. We demonstrate the ability to use topographic and bathymetric measurements from a narrow shore platform along with a limited data set of nine cosmogenic 10Be concentrations extending ∼125 m from the cliff base to obtain modeled cliff retreat rates that steadily range from 5.0 to 12.5 cm yr−1 over the last two millennia until 100 years before present. These rates are consistent with modern retreat rates of about 2–19 cm yr−1 here. RSL rise in Southern California remained relatively constant during the late Holocene, potentially explaining the relatively stable modeled cliff retreat rate over this time. We also explore the relative influence of weathering, material resistance, and wave erosion efficacy and find that both weathering and wave‐driven erosion are necessary to replicate the measured data at this location, with the latter exerting a stronger control on model acceptance, suggesting that waves may provide a possible mechanism by which RSL rise may influence coastal cliff erosion in southern California.

Modeling future cliff-front waves during sea level rise and implications for coastal cliff retreat rates

“…The nature of this association is clarified through three years of weekly laser-scan observations from Del Mar, California, that reveal cliff-toe erosion is correlated with wave impact height and duration, whereas erosion higher on the profile is better correlated with rainfall 10 . At this site, Clow et al 11 measured and modeled cosmogenic 10 Be concentrations across the shore platform fronting the eroding cliff, and found that cliff retreat rates over the last 2000 years were similar to modern erosion rates, consistent with relatively stable SLR rates over the same period. These results suggest that in southern California, waves provide a mechanism by which SLR can contribute to cliff erosion rates.…”