ABSTRACT
The major geomorphological hazards in Los Angeles County (and indeed throughout southern California) are associated with valley-side slopes and channels in the mountains, and with the drainage channels and alluvial plains beyond the mountains. The principal components of these natural geomorphological systems are shown schematically in Figure 1.1. The valley-side slope activity includes three main processes: rock decomposition, and the movement of material downslope by slope erosion or slope failure. Slope erosion comprises surface movement of debris either under the predominant influence of gravity (dry erosion) or in conjunction with flowing Principal features of the fluvial geomorphological systems in Los Angeles County https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003168645/7476f66d-c70d-415d-9213-8053dc2b4539/content/fig1_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> 2water (wet erosion). Slope failure may take many forms, but the most significant are surface-soil slipping and landslides. Both slope erosion and slope failure pose problems for people and property in hilly and mountainous areas. Both are also the main contributors of sediment to the channel network - the second major locus of geomorphological hazards. Hazards associated with the mountain channels and the channelled alluvial plains beyond the mountains arise from the way in which water and sediment are moved through drainage systems in semi-arid areas. The flows are ephemeral, range from river flows low in debris to relatively viscous debris flows, and, unless controlled, often follow relatively unpredictable courses across alluvial slopes. When and where flows leave their channels, the resulting floods of sediment and water have posed problems for plains dwellers in much of Los Angeles County.
