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

• Published in: Journal of geophysical research. Earth surface 2023-04, Vol.128 (4)
• Main Authors: Clow, T., Willenbring, J. K., Young, A. P., Matsumoto, H., Hidy, A. J., Shadrick, J. R.
• Format: Article
• Language: English
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2022JF006855

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 10 Be 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. Erosion in rocky coastal environments is hazardous to coastal communities and infrastructure, but our ability to constrain how fast erosion and cliff retreat is occurring is typically limited to the last century. This limitation is problematic for understanding the full picture of coastal cliff retreat to properly mitigate risk. We use a newly developed method to estimate cliff retreat over thousands of years in Del Mar, California, where extensive infrastructure exists just tens of meters from an eroding cliff. A new data set that estimates the length of time a shore platform is exposed is combined with a model that simulates cliff retreat and shore platform development to quantify cliff retreat rates that steadily range from 5.0 to 12.5 cm yr −1 over the last 2,000 years. This long‐term rate is consistent with mod