Land characteristics and management factors involved in gully & tunnel erosion

Processes involved:

Gully and tunnel erosion occur when the forces due to rainfall, flowing water and gravity overcome the cohesion and weight of the soil particles/aggregates.

Processes involved are:

detachment of exposed surface soil by
- raindrop impact
- channelised overland flow
- cracking
detachment of subsoil by
- subsurface flow in permeable strata and along cracks and tunnels
-cracking
transport of particles/aggregates by
- channelised overland flow
- subsurface flow
- gravity collapse
deposition

Gully erosion is regarded as having occurred when the channel is too deep to be crossed or cannot be obliterated by tillage.

Off-site effects include increased sedimentation and run-on in streams and on lower lands.

	Land characteristics affecting processes	Factors affected by land characteristics	Management factors that modify land characteristics
Vegetation	- structure, percent surface cover (including litter) - leaf area, rooting depth and perenniality	Exposure of surface soil Intensity of raindrop impact Velocity of channelised flow and hence particle detachment and transport Transpiration, and hence infiltration rate and volume of surface and subsurface flow	All aspects of the vegetation are affected by selection of species and control of biomass by practices such as cultivating clearing trafficking fertilising grazing trampling harvesting burning
Climate	- rainfall intensity/duration - seasonal rainfall/evapotranspiration regime	Intensity of raindrop impact Volume of surface and sub-surface flow Volume of surface and sub-surface flows via regulation of soil water content
Geology	- perviousness of rock or unconsolidated sediments	Soil water content and hence infiltration rate and volume of surface and subsurface flow Lateral or vertical movement of water
Topography	- microrelief (both of channel and catchment to a site) - channel slope degree and length - position in landscape and catchment area - catchment slope degree and length - slope and land-form shape	Infiltration/run-off ratio Velocity of surface flow Infiltration/run-off ratio Velocity of surface flow Volume of surface and sub-surface flows reaching site Infiltration/run-off ratio Velocity of surface flow Tendency to concentrate surface flow	Contour and diversion banking, strip cropping and contour cultivating reduce catchment slope length and catchment area; they also affect microrelief Contour and diversion banking, strip cropping and contour cultivating reduce catchment slope length and catchment area; they also affect microrelief
Soil	- profile permeability - depth and water-holding capacity - size/weight of soil particles/aggregates - cohesion of particles/aggregates, including tendency to crack, slake and disperse - differential permeability within a horizon due to the presence of cracks and channels - percent stone cover	Infiltration rate and hence volume of surface and sub-surface flow Lateral or vertical movement of soil water Volume of surface and sub-surface flow Detachment and transport Detachment Movement of water along preferred channels Volume surface flow	Type and amount of biomass production will affect soil organic matter content, which will in turn affect most listed soil characteristics Soil disruption and compaction by trampling, burrowing, cultivating and trafficking will affect profile permeability, water-holding capacity and size/weight and cohesion of soil particles/aggregates

Aldrick, J.M. et. al. (1988). A study of the Land in the Catchment of the Gippsland Lakes. Department of Conservation Forests and Lands, Victoria

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