Landslides on coastal sage-scrub and grassland hillslopes in a severe El Nino winter; the effects of vegetation conversion on sediment delivery

During the 1997-1998 El Nino, record rainfall triggered >150 shallow landslides within a 9.5 km (super 2) area near Santa Barbara, California. They were studied to analyze the sediment delivery to valley floors from landslides in coastal sage scrub and converted grasslands. The conversion of coastal sage to grasslands, primarily to provide pasturage for cattle, is common in the region, and the landscape's response may affect water quality, reservoir infilling, and debris flow hazards. We explore the relationship between lateral-root reinforcement and landslide volume by developing a slope-stability analysis that incorporates root cohesion along the sides of the failure. The stability analysis correctly predicts an inverse relationship between landslide volume and hillslope angle in the sage. The volumes of failures in the grasslands do not vary systematically with slope and are generally smaller than those in the sage. From aerial-photograph analysis and field mapping, we find that there are 22.9 failures per square kilometer in the grasslands compared to 13.2 failures per square kilometer in the sage. Despite the lower failure density in the coastal sage, greater failure volumes and longer transport distances delivered more sediment to valley floors, with a specific volumetric flux of 2.8 X 10 (super -2) m (super 3) .m (super -1) for this El Nino compared to 1.7 X 10 (super -2) m (super 3) .m (super -1) in the grasslands. We conclude that the conversion from vegetation with stronger and deeper roots (coastal sage) to vegetation with weaker and shallower roots (grass) has caused a pulse of increased landsliding in the grasslands because the soils are currently too thick for the prevailing root reinforcement. We suggest that, over time, soils in the grassland hollows will become thinner as the evacuation by landslides is repeated until the landsliding rate declines to balance the soil supplied from local colluvium production and diffusive processes upslope.