FLOOD01621 - Laserfiche WebLink

Home Browse Search

3.2 Description of Alluvial Fan Flows Flows on alluvial fans are characterized by transient phenomena such as debris laden frontal waves, surging and channel avulsion. This section contains a discussion of the properties and behavior of various types of flood flows on alluvial fans. The four categories of floodin'" and mass wasting processes on alluvial fans promulgated by the Na1:ional Research Council (NRC) are landslides, mudflows, mud floods ,. and clear-water floods (NRC, 1982). Conventional water floods and mud floods are classified as floods under Flood Insurance Program (NFIP). Both types of flooding behave like water floods and are referred to as hydraulic flows. Mud floods were defined by the NRC as transporting high sediment concentrations (as much as 50 percent by volume). Mudflows were categorized as a specific subdivision of landslides where the "transporting mechanism is that of a flow having sufficient viscosity to support large boulders within a matrix of smaller-sized particles." The term "landslide" refers to the down- slope earth movement by such mechanisms as falling, toppling, sliding and spreading, and may be e,i ther wet or dry. In laboratory experiments, the categories of mud flood and mud flow were found to encompass a range of sediment concentrations from 20 to 50 percent by volume (O'Brien and Julien, 1985). Noncohesive sediment flows (granular flows) on Southwest fans have less sediment concentration than mudflows and should be classified as mud floods or water floods. The, classification of all these flows are shown in Table 2 as a function of sediment concentration. This classification system was based 011 terminology defi.ned by the NRC in 1982 and research conducted at: Colorado State University (O'Brien, 1986). Turbulence and dispersive stresses are dominant in the exchange of flow momentum in water floods and noncohesive sediment flows. Dispersive stresses arise from the collision of sediment partiCles and help to transfer the momentum from the core of the flow to the bed. Viscosity and yi,ald stress are assumed to be negligible in these flows. Sediment deposition occurs as transport capacity decreases with slope. Very viscous, hyperconcentra1::e,d sediment flows are often referred to as either mud or de,bris flows. Mudflows are nonhomogeneous, nonNewtonian, transient flood events whose fluid properties change dramatically as .they flow down steep watershed channels or across alluvial fans. ~[udflow behavior is a function of fluid matrix properties, channel geometry, slope and roughness. The fluid matrix consists of wat:E!r and fine sediments. At sufficiently high concentrations, t:hE' fine sediments alter the properties of the fluid, including density, viscosity, and yield stress. The colloidal forces betwe"H1 fine sediment partiCles give rise to the fluid's nonNewtonian be,havior. 5