Coastal Cliff Retreat Rates Along the Big Sur Coast, Monterey and San Luis Obispo Counties, California
- Cheryl Hapke
United States Geological Survey (USGS)
End Date: May 31, 2003
Coast Highway 1 along the Big Sur coast is regularly subjected to damage from coastal cliff retreat, because the steep slopes experience both high amounts of precipitation and high wave energy during winter months. This study provides coastal cliff retreat rates along nine discontinuous study sections of the Big Sur coast. Three-dimensional stereo models from a previous study were used to digitize cliff edges, and the retreat rates were calculated for a 52-year period from 1942 to 1994.
The average cliff retreat rate was 18 ±6 cm/yr along the total 45 km of coast measured in this study. The lowest retreat rates (12 ±7 cm/yr) are associated with the stronger granitic rocks, located primarily in the northern part of the Big Sur coast. The highest rate (25 ±5 cm/yr) is along the coast just south of the town of Big Sur where it appears a large promontory on the slope completely eroded away or collapsed during the time of the analysis (1942 to 1994).
Summary to DateThe average cliff retreat rate for the total of 45 km of coastline studied along Coast Highway 1 is 18 ±6 cm/yr based on the analysis for the nine study sections (see Figure 1 for location of sections). The error represents 2 standard deviations to provide a 95% confidence interval. This assumes the error is random and follows a normal distribution. The cliff retreat rates vary substantially and range from 12 ±7 cm/yr in study section 1 to a high of 25 ±5 cm/yr in study section 3, near the town of Big Sur. In section 3, the average rate is higher because of an episodic failure along a very small (<50 m) part of the cliff.
Cliff retreat rates are presented as yearly averages. While average values are often very useful for long-term management planning, the actual processes of cliff retreat are highly episodic; a large retreat event during a series of storms may account for most of the retreat in any given area. With the exception of study section 8, in which the retreat is relatively evenly distributed, most areas have one or two erosion hotspots that account for much of the average cliff retreat rate.
The cliff retreat rates are correlated to the variable geology of the area to assess the relation between lithology and cliff retreat (see Figure 2). There is a general trend that suggests that stronger granitic rocks have lower cliff retreat rates, and the weaker materials (mélange of the Franciscan Complex) have higher rates. However, study sections 3, 4 and 5 have the highest retreat rates, respectively, and are associated with cliffs in the stronger granitic rocks (study section 4) and sedimentary units of the Franciscan Complex (study sections 3 and 5).
These results imply that there are parameters in addition to material strength that influence how rapidly cliffs are retreating along the Big Sur coast. While it is beyond the scope of this study to explore these factors, such parameters may include prior history of slope disturbance (natural and anthropogenic), distribution and density of geologic structures, hydrologic conditions, and protection (or lack thereof) at the base of the slope such as beaches or debris fans.
- The average cliff retreat rate for the total of 45 km of coastline studied along Coast Highway 1 is 18 ±6 cm/yr based on the analysis for the nine study sections.
- The cliff retreat rates vary substantially and range from 12 ±7 cm/yr in study section 1 to a high of 25 ±5 cm/yr in study section 3, near the town of Big Sur.
- There is a general trend that suggests that stronger granitic rocks have lower cliff retreat rates, and the weaker materials (mélange of the Franciscan Complex) have higher rates.
- Geological characterization
Study MethodsThe primary tools used for this analysis were digital photogrammetry and geographic information systems (GIS). Digital photogrammetry was used to process the historical (1942) and recent (1994) vertical aerial photographs to generate three-dimensional stereo models. The cliff edges were digitized directly from the models while viewing in stereo
to ensure that the actual break in slope was digitized as the erosion reference feature (cliff edge). The cliff edges were then brought into a GIS where a shore-parallel baseline was generated. The baseline can follow the shape of the coast, or it may be a straight line with the same azimuth as the average orientation of the coastline. Orthogonal transects extending from the baseline to the coast were generated using the Digital Shoreline Assessment System, or DSAS (Thieler and others, 2003). The spacing of transects for the average erosion rate calculations was 15 m. DSAS was then used to calculate the positional difference between the two cliff edges along each transect to establish cliff retreat rates.
Reference: Thieler, E.R., D. Martin, and A. Ergul. (2003) Digital Shoreline Analysis System, version 2.0: Shoreline change measurement software extension for ArcView; USGS Open-File Report 03-076.
Figures and Images
Figure 1: Map showing locations of the nine study sections of coast near Big Sur, California, and the average cliff retreat rate associated with each study section.
Figure 2: Relationship between lithology and cliff retreat rates for the nine study sections of coast near Big Sur California.
Figure 3: Oblique aerial photograph of Big Sur coast. Red line shows feature digitized as cliff edge. Letters A, B, and C represent different morphologies of the cliff edge: A, sharp cliff; B, active slope extending to road grade; C, active slope not visibly active to road grade. White arrow shows an engineered slope above road.