ALLUVIUM-BUTTRESSED LANDSLIDES: CONCEPTUAL MODEL AND EXAMPLES FROM CALIFORNIA

Large, deep-seated landslides typically occur in hillside settings without any natural buttressing, and many of these landslides have relatively low factors of safety (FS, the ratio of driving to resisting forces). However, where deep-seated landslides failed into valleys that subsequently experienced alluvial aggradation, a natural buttress of alluvium may be deposited over the landslide toe, leading to an increase in FS.

The eustatic model for alluvial buttressing of Quaternary landslides involves failure of slopes near the time of sea level low stand at the last glacial maximum (LGM, approximately 21 to 29 ka). Following LGM, mean sea level rose by approximately 134 meters from low stand to its present elevation. This rise in base level resulted in deposition of alluvial sediment in coastal valleys that had been v-shaped and downcutting prior to LGM. These valleys now have broad, low gradient floors formed by alluvium, and the thick alluvial strata that fill these valleys locally cover the toes of late Pleistocene landslides.

This poster briefly summarizes three examples of large, deep-seated Pleistocene landslides that are buttressed by alluvium. The McCracken Hill Landslide in southern California and the Potrero Canyon Landslide Complex in central California are located approximately 1.5 and 6 km, respectively, from the modern Pacific shore and closely fit the eustatic model of alluvial buttressing. At Knights Valley, in the upper Russian River watershed of northern California, a deep-seated alluvium-buttressed landslide is located approximately 81 km from the Pacific shore (measured along the Russian River and relevant tributaries). The alluvium in Knights Valley may have ponded due to Quaternary tectonic uplift of hills downstream from Knights Valley that are bounded by the Maacama and Knights Valley faults. The streams that drain Knights Valley appear to be incised into bedrock where they cross these hills. Also, limited radiocarbon dating of alluvium in Knights Valley indicates that most of the alluvium was deposited before the early stages of post-LGM sea level rise. Thus, the deposition of alluvium in Knights Valley does not appear to have been controlled by late Quaternary sea level; this demonstrates a rare exception to the broadly applicable eustatic model.