Erosion of California Cliffs

 

from Swirad and Young

The majority of California's coastline is lined with cliffs with spectacular views of the Pacific Ocean. However,  the erosion of the cliffs threatens highways, railways, wastewater facilities, commercial structures, pubic  structures, and residential structures. Large cliff failures occur episodically and have historically have seemed random, making cliff-retreat forecasts a major challenge. Cliff failures in recent decades have caused fatalities and significant infrastructure damage. I recall watching a house I regularly passed on Route 1 up north slide into the Ocean on the evening news.

There are some areas of the California coast that experience high magnitude cliff failures, repeatedly.  Daly City, Portuguese Bend, and San Onofre are such examples. (Hapke et al., 2009; Young et al., 2009; Young, 2015, Young, 2018). Large-scale quantitative studies of California cliff erosion and failure have been done with Adam Young and Zuzanna Swirad building on their and others previous work. The goal is to someday be able to accurately predict which areas are likely to collapse in the future. 

Despite the general understanding of what drives coastal change (waves, rainfall), scientists are unable to predict cliff erosion at specific locations and times. Because of the wide range of causes and types of erosion, and highly variable geologic, oceanographic, and climatic settings; land planners and local officials could not know which areas were unsafe. Young found no significant correlation between cliff erosion rates and environmental factors such as rainfall, groundwater, waves, and relative sea level change. However, the highest cliff face retreat rates occurred at locations with what he described as weak rocks.

The current study was proceeded by several earlier studies. Hapke et al. (2009) measured cliff top retreat by comparing 1920s–1930s topographic maps and 1998/2002 LiDAR datasets for 353 km of cliffs spread throughout the state (20% of the California coast). This study found generally higher retreat rates in northern and central California (on average) compared to southern California.

Young (2018) used 1998 and 2009–2011 LiDAR datasets to measure cliff erosion and cliff face and top retreat rates for 595 km of the southern and central California coastline (35% of the California coast), and found higher retreat rates for cliffs fronted by sandy beaches than those without a beach.

Swirad and Young  used the  2009–2011 and 2016 LiDAR datasets to measure cliff erosion for a 595 km portion of the coast followed by a study of a different 866 km (53%) of the California coastline. They identified landslide volume frequency relationships, and quantified state- and county-averaged cliff face retreat rates. The new results were compared to the previous study  and yielded similar statistical results of cliff face retreat. These statistics help account for episodic erosional events, and help improve model predictions.

For the current study using LiDAR, Swirad and Young were able to measure both the cliff top change and changes within the cliff face. The goal is to be able to predict collapse. The research has found it difficult to isolate events that happen seasonally or annually and have tended in their model building towards a statistical approach.   Swirad and Young created a one-meter digital elevation models and evaluated the cliff erosion and retreat between 2009-2011 and 2016 in five-meter segments along 866 kilometers of California’s coast. “Because we found statistical agreement with the previous time period, 1998 and 2009-2010, we may be more confident that the statistical approach is the way to do it,” Swirad said. Erosion was detected along more than half of the cliffs.

The limited time frame for the data sets is a weakness of the studies. It is too narrow a time period for developing a predictive model for geologic events. A new statewide data collection began last year. This will provide a third time span to see if the statistical results remain consistent with those from the earlier periods and could provide additional support for the statistical modeling approach. 

This study was funded by the California Ocean Protection Council and California Department of Parks and Recreation and using data collected for previous research funded by California Sea Grant. The data e is available on the website California Coastal Cliff Erosion Viewer. Users Though the website is designed for coastal planning and development decision-makers, anyone can browse any cliff in the state to see its past rate of erosion and related retreat statistics. You can read the current study here.

Zuzanna M. Swirad, Adam P. Young, Spatial and temporal trends in California coastal cliff retreat, Geomorphology, Volume 412, 2022, 108318, ISSN 0169-555X, https://doi.org/10.1016/j.geomorph.2022.108318.