The arid region piedmonts: glacis and alluvial fans
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Figure 15.27. Segmented fan surface. The upper segments are highly eroded by gulling, but also by the
dissection of the Jachal River. The picture was taken from a place located between Rodeos and San Jos6 de
Jachal, San Juan Province, Argentina.
movement by plastic flow, in which the solid and liquid phases move in a cohesive way.
The secondary processes, or reworking processes, lead to fan degradation and erosion
through the remobilisation of the sediments previously deposited by the primary
processes. Many of these processes work during prolonged spans of time on to the fan
surfaces, whereas the sediment supply events are generally triggered by low- frequency
and high- intensity storms.
2.4.1. Sediment supply processes
These may be divided into two classes: fluid-gravity and sediment-gravity flows (Blair and
McPherson, 1994a,b). The fluid-gravity flows can be described as typical fluvial flows,
whereas the sediment-gravity flows are mass movements on the colluvial slopes or on
fragmented bedrock within the drainage basins. These mass movements can be rockfalls