SIMoN
  Sanctuary Integrated Monitoring Network
Monitoring Project

Estimated Sediment Yield from Coastal Landslides and Active Slope Distribution Along the Big Sur Coast, Monterey and San Luis Obispo Counties, California

Principal Investigator(s)

  • Cheryl Hapke
    United States Geological Survey (USGS)
Start Date: June 01, 2000
End Date: May 31, 2003

The Coast Highway Management Plan (CHMP) was established with the intention of developing highway management approaches and solutions collaboratively with the MBNMS. This study was undertaken as a direct result of the need identified by CalTrans and MBNMS staff for fundamental data on background sediment volumes entering the MBNMS from coastal landslides. The primary goals of this research were: to quantify the volume of sediment that enters the MBNMS through coastal landslide processes using historical and recent aerial stereo photographs; to map the temporal and spatial variations in landslide distribution along the coast, and; to relate the volume losses to the complex geology of the region to document the geologic controls on sediment yield from coastal landslides.

Using digital stereo photogrammetry, terrain models were created for two dates spanning a 52-year time period. Volume changes were then calculated by differencing the terrain models. The average sediment yield (volume loss per linear extent of coast per year) was 21,000 ±3,200 m3/km/yr, with a range of 1,000 ±240 m3/km/yr to 46,700 ±7,300 m3/km/yr. The largest sediment yields were within the weakest geologic units, concentrated in the southern part of the study area, while the lowest sediment yields were within the stronger rocks, located primarily in the northern part of the Big Sur coast.

In areas where the sediment yield could not be directly calculated, the yields were interpolated by correlating with the geology in areas where sediment yields were determined. Active slopes were associated with areas of higher sediment yield and Franciscan Complex bedrock geology. This technique of estimating the sediment yield from coastal landslides provides the necessary background data to determine the average volumetric sediment input along the coast.

Summary to Date

This study provides background information on the volumes of sediment and other material that historically enter the MBNMS along the Big Sur coast directly from coastal landslides, and maps both the spatial and temporal distribution of areas of active input. Volumetric change analysis from this investigation estimate the average sediment yield for the Coast Highway 1 corridor is approximately 21,000 ±3,200 m3/km/yr based on analyses of the nine study sections (see Figure 1 for location of sections).

Sections 1 and 2 have very low input rates (e.g. 1,000 ±240 m3/km/yr for section 1) compared to the other sections (see Figure 2); these sections are within the stronger, granitic material. Sections 6 and 7 have moderately high input rates for the coastline compared to other sections. Section 4 lies within the stronger, granitic material, but has anomalously high input rates compared to the surrounding areas (study sections 3 and 5). The high rate is attributed to a large landslide that occurred in 1983, within the 52-year time period of this analysis. Stabilization of this landslide was the largest earth-moving operation in the Big Sur region; a total of nearly 20 million cubic meters of material was removed by both natural processes and slope stabilization.

Sections 8 and 9 have the highest input rates for the coastline (e.g. 46,700 ±7,300 m3/km/yr for section 8) compared to other sections. The bedrock geology of this section, which has a history of large, well-known landslides, is comprised of faulted and sheared rocks of the Franciscan Complex. Results of the sediment yield analysis from digital terrain model (DTM) differencing show a strong correlation between the local geology and the sediment delivery rates.

Monitoring Trends

  • The average sediment yield (volume loss per linear extent of coast per year) was 21,000 ±3,200 m3/km/yr, with a range of 1,000 ±240 m3/km/yr to 46,700 ±7,300 m3/km/yr.
  • Results of the sediment yield analysis from digital terrain model (DTM) differencing show a strong correlation between the local geology and the sediment delivery rates (see Figure 3).

Study Parameters

  • Maps
  • Geological characterization

Study Methods

The primary tools used in this study were digital photogrammetry and geographic information system (GIS). Digital photogrammetry involves the processing of historical and recent vertical aerial photographs to produce Digital Terrain Models (DTMs) from 3-D stereo models. These time-sequence DTMs are brought into a GIS where volume changes are calculated, and then the spatial distribution of terrain changes can be analyzed and compared to local geology. The historical aerial photographs chosen for this study are from 1942 (1:30,000), and the recent photographs are from 1994 (1:24,000). These photographs provide the basis for determining a 52-year, end-point volumetric change for nine study sections. These particular groups of aerial photographs were chosen because their scales provided regional coverage of the coastline, the length of time between them provided the longest possible time period for long-term rate calculation, and their availability as stereo film positives (diapositives) minimized nonsystematic errors.

References:

Hall, C.A., Jr., 1991, Geology of the Point Sur-Lopez Point region, Coast Ranges, California—a part of the Southern California allochthon: Geological Society of America Special Paper 266, 40 p.


Wills, C.J., Mason, M.W., Brown, K.D, Davenport, C.W., and Domrose, C.J., 2001, Landslides in the Highway 1 Corridor: Geology and Slope Stability Along the Big Sur Coast, Report to the Coast Highway Management Plan Caltrans District 5, 29 p.

Figures and Images

Figure 1: Simplified geologic map of the Big Sur area showing general lithologies exposed along the coast. Numbers 1-9 represent the locations of the specific study sections. Modified from Wills and others (2001).

Reference: Wills, C.J., Mason, M.W., Brown, K.D, Davenport, C.W., and Domrose, C.J., 2001, Landslides in the Highway 1 Corridor: Geology and Slope Stability Along the Big Sur Coast, Report to the Coast Highway Management Plan Caltrans District 5, 29 p.

Figure 2: Example map of the Big Sur coast region showing sediment yield from coastal landslides and active slope distribution. Go to http://pubs.usgs.gov/sim/2004/2852/ for downloadable versions of all maps.

Figure 3: Relationship between lithology and sediment yield for the nine study areas near Big Sur, California. Sediment yield within the weak Franciscan Complex mélange is consistently greater than sediment yield in stronger, granitic rocks of the Sur Complex of Hall (1991) and the sedimentary rocks of the Franciscan Complex.

Reference: Hall, C.A., Jr., 1991, Geology of the Point Sur-Lopez Point region, Coast Ranges, California—a part of the Southern California allochthon: Geological Society of America Special Paper 266, 40 p.