AS: Flows

Flow

-       Fast, need well lubricated material

-       The material behaves like a viscous fluid

-       Material size – large boulders – small grains

-       The debris avalanche (large boulders) is the fastest of the flows

-       Some other types of flows include earthflows and debris flows (small grain sized slows).

-       Occurs because there is a decrease in internal/ shear strength. Heavy rain infiltrates the regolith – lubricates the material by filling the pores thus increasing pore pressure. Shear strength < external stress i.e. gravity

-       Flows can be triggered by earthquakes would increase shear stress.

-       The most important point about flows is that there is a decrease in movement with depth. The top middle moves the fastest and the front extends the furthest (an area known as the ‘toe/lobe’). The internal deformation of material down the slope and as the material goes down the slope there is a decrease in velocity.  A scar is left at the top of the slope where the flow began. This is a steeper section of the slope.

-       The overall impact of the slope: Scar, gentler gradient at the base of the slope and material may spread widening slope foot.

-       Mudflows: Rapid movements, occurring on steeper slopes, exceeding 1km/hr. They are most likely to occur following periods of intensive rainfall, where both volume and weight are added to the soil giving it a higher water content than an earthflow

-       Earthflows: When the regolith slopes 5-15⁰ becomes saturated with water, it begins to flow downhill at a rate varying between 1 and 15 km per year. The movement of material may produce short flow tracks and small bulging lobes or tongues, yet may not be fast enough to break the vegetation.

Solifluction

-       This process meaning ‘soil flow’ is a slightly faster movement averaging between 5cm to 1m per year.

-       Often takes place under periglacial conditions where vegetation cover is limited.

-       During winter both the bedrock and regolith are frozen. In summer, the surface layer thaws but the underlying layer remains frozen and acts like impermeable rock. Because surface water cannot infiltrate downwards and temperatures are too low for effective evaporation, any topsoil will become saturated and will flow as an active layer over the frozen layers.

-       This process produces solifluction sheets and lobes (rounded tongue like features), and heads (a mixture of sand and clay formed in valleys and at the foot of sea cliffs).