Susceptibility to Mass Movement

Susceptibility to Mass Movement

Mass movement encompasses erosion processes in which gravity is the primary force acting to dislodge and transport land surface materials. It is a function of the gravitational stress acting on the land surface and the resistance of the surface soils and/or rock materials to dislodgement. When the gravitational stress exceeds this resistance, mass movement occurs. The occurrence of mass movement depends on the interaction of various factors including landform, lithology, soil type, rainfall intensity and duration, drainage characteristics and vegetation cover.

Landslip is seldom the result of a single factor as failure is the end result of activities and processes that have taken place over many years prior to the actual movement. In general, failures occur when the weight of the slope exceeds its restraining capacity. The most common triggering agent is the infiltration of water into the sloping land surface, which has the effect of both reducing the shear strength of the soil material and increasing the mass loading on the slope.

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Within the study area there are a number of steeply sloping areas which may be susceptible to landslip if not managed correctly. The land characteristics and management factors involved in mass movement or landslip are described in Table 7.

Table 7 Land characteristics and management factors involved in mass movement

Processes	Land features affecting processes	Factors affected by land characteristics	Management factors that modify land characteristics
Landsliding occurs when the shear forces exceed soil/regolith strength; this generally occurs when soil/regolith strength is reduced by an increase in water	Vegetation leaf area, rooting depth perenniality total leaf area and canopy type root depth and mass	Transpiration and hence soil water content Volume of water held by canopy and hence volume available for infiltration Anchorage of soil by roots harvesting	All aspects of the vegetation are affected by selection of species and control of biomass by practices such as cultivating clearing trafficking fertilizing grazing trampling burning
Processes involved are:	Climate seasonal rainfall/ evapotranspiration regime	Soil water content
infiltration ofwater wetting of basal plane saturation of soil (mudflow)	Geology perviousness of rock or unconsolidated sediments wet strength of rock / regolith angle of dip	Soil water content Shearing tendency Shearing tendency
shearing and movement of soil mass by gravity	Topography slope degree microrelief and position in landscape	Lateral gravitational component Run-on, site drainage and hence soil water content
Other processes that may be involved include: loading of soil mass resulting in an increase in shear strength removal of material from slope toe resulting in reduced slope support Types of landslides covered by this table are: rock and earth slides earth flow (downslope movement of unsaturated soil and weathered rock on a lubricated basal shear plane) mudflow (movement of saturated soil and rock) combination slide/flows	Soil topsoil permeability presence of slowly permeable layer cohesion of particles/aggregates including tendency to slake and disperse depth clay mineralogy	Soil topsoil permeability presence of slowly permeable layer cohesion of particles/aggregates including tendency to slake and disperse depth clay mineralogy	Compaction and soil disruption by stock and vehicles, and by cultivating, will affect profile permeability