SLOPE FAILURE TYPES:

Debris flow

debris

SLOPE FAILURE NAME:

Debris flow

DEFINITION:

Mixture of fine and coarse material with a variable quantity of water, that forms a muddy slurry which moves downslope, usually in surges (COROMINAS, 1996).

MAIN CHARACTERISTICS:

Debris flows can result on the movement of coarse debris considerable distances from the source area. This movement is enhanced by high moisture contents that lubricate the flow.

Three elements can be clearly distinguished in a debris flow:

  • The source area, where the scar is present, can have a linear or horseshoe shape, but is always shallow.
  • The main track, always longer than wider.
  • The depositional toe, where the previous structure of the debris is not recognisable.

Although usually debris flows present a V shape on their track, leaving the coarser material on two lateral levées, this characteristic is not found at the Giant’s Causeway. Here debris tend to move by sliding over the slope surface, almost without dragging further material. As with ‘mudflows’, as the flows move downslope the material drains and the flow can develop into slide in which further movement is by plastic deformation.

On the attached maps, rockfalls that move down along a gully and travel far away from their original scar, have also been reported as debris flow.

CAUSES:

Usually high rainfall terms are the most common agent of these landslides. Also previous landslides can overload an specific part of the slope of debris and water, and cause a secondary debris flow.

AREAS PRONE TO FAILURE:

Debris flow usually take place on slopes covered by thin, unconsolidated rock ands soil debris. Rocky percolation lines, where block releases can channel, are also prone areas.

Shepherd’s Path roundabouts, where there is a thick debris mantle over the slope, is the most prone area for these failure events.

OCCURRENCE:

During or after high and continuous rainfall events.

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