Sediment Yield from Big Sur Coastal Landsides

Along the Big Sur coastline in central California, the rugged Santa Lucia Mountains descend abruptly into the Pacific Ocean, creating one of the steepest coastal slopes in the contiguous United States. Weak rocks and steep topography provide ideal conditions for frequent large landslides. In addition, this region experiences both high amounts of precipitation and high wave energy in the winter months. All these factors combine to produce an area of chronic landslides that may block, undermine, or damage Highway 1, at the edge of the coastal slope.

Figure 1: Location of the study sections along the Big Sur coast in central California. The numbers 1-9 shown on the map correspond to the specific study sections.

The California Department of Transportation (Caltrans) is responsible for maintaining the Highway 1 corridor and for providing safe access for both local residents and tourists. Prior to the establishment of the Monterey Bay National Marine Sanctuary, road-opening measures sometimes involved disposal of some landslide material and excess material generated from slope stabilization onto the seaward side of the highway. It is assumed that this disposed material, either directly, or indirectly through subsequent erosion, was transported down slope into the adjacent ocean. In addition to the landslides that initiate above the road, natural slope failures also occur on the steep slopes below the road, delivering material to the base of the coastal mountains where it is eroded and dispersed by waves and nearshore currents. As a result, any coastal slope landslide, whether through natural or anthropogenic processes, can result in sediment entering sanctuary waters. The disposal practices had the potential to disrupt biological communities by converting marine habitats from rocky substrate to soft bottom and increasing nearshore zone suspended sediment concentrations. However, natural landslide processes provide material for protection from waves at the base of the slope, and sediment entering the water provides nutrients and material for various nearshore habitats. Restricting any disposal may starve a system of necessary nutrients and sediments and actually increase the rate of cliff erosion.

Since 2000 the United States Geological Survey (USGS) has been conducting a study to provide an estimate of the historical volume of sediment (sediment yield) that enters the coastal system directly from coastal slope failures along nine sections of the Big Sur coastline (Figure 1). Its purposes are to provide background data for the sanctuary and the Caltrans Coast Highway Manage- ment Plan as well as to advance the fundamental understanding of coastal landslide input rates and processes along this stretch of coastline.

The primary tools used in this study are digital photogrammetry and GIS (geographic information systems). Digital photogrammetry involves a technique of processing aerial photographs with computer software to produce 3-dimensional topographic models of the terrain – digital terrain models (DTMs). DTMs from two dates are brought into a GIS where the DTMs are subtracted, and the volume changes are calculated. The spatial distribution of the terrain changes is also compared to the local geology. The historical aerial photographs chosen for the study are from 1942 and the recent photographs are from 1994, thus providing the base for determining a fifty-two-year sediment yield (the volume loss per linear extent of coast per year).

The results of the volumetric change analysis are shown in Figure 2 (p. 7). The average sediment yield for the Big Sur Highway 1 corridor is approximately 21,000 + 3,200 m3 per kilometer per year (43,200 + 6,500 yd3 per mile per year) based on the analysis for the completed nine sections. The rocks along the Big Sur coastline are a complex mixture of sheared rocks of the Franciscan Complex and granitic rocks of the Sur complex. The rocks of the Franciscan Complex tend to be weaker than those of the Sur complex. However, the lithology within the Franciscan Complex varies dramatically, and softer, highly sheared mélange is more prone to landsliding than the various sedimentary strata and volcanic rocks.

Figure 2: Relationship between sediment yield and lithology (physical characteristics) for the nine study sections of coastline. The sediment yield within the weak mélange is consistently greater than the yield in the stronger sedimentary units and the local granitic rocks. (FC = rocks of the Franciscan Complex)

Sections 1 and 2 have very low input rates for the coastline compared to other sections and are within the stronger granitic material (see Figure 1). Sections 6 through 9 have high input rates for the coastline compared to the other sections. Section 4, which lies within the stronger granitic material, demonstrates anomalously high input rates compared to the surrounding areas (sections 3 and 5). This high rate is attributed to the particularly large JP Burns landslide that occurred in 1983 and is therefore within the fifty-two-year time period of this analysis. A total of nearly 20 million cubic meters of material was removed by a combination of natural processes and slope stabilization. Sections 6 and 8 have the highest input rates for the coastline compared to other sections. The rocks along these sections of coastline are faulted and sheared rocks of the Franciscan Complex with a history of large historic landslides.

In 2000 the sanctuary and Caltrans determined that in order to deal with the issue of landslide material disposal along the Big Sur coast, it was necessary to understand the historical inputs to the coast from the chronic landslides. The results of this sediment yield study have been incorporated into the Caltrans Coastal Highway Management Plan and are being used by the sanctuary to investigate the possibility of controlled disposal of landslide material along selected coastline sites where valuable marine resources would be least affected. Future collaborative work between the sanctuary, Caltrans, and the USGS will involve high-resolution analyses of three specific landslide sites to understand not only how much material is entering the sanctuary waters but what the rates of input are and by what processes material is delivered to the nearshore.

Cheryl Hapke
United States Geological Survey