Susceptibility to Gully and Tunnel Erosion

Susceptibility to Gully & Tunnel Erosion

The susceptibility of land to tunnelling and gully erosion depends on a number of interrelated factors. These are principally rainfall intensity, vegetation cover, rooting depth, microrelief, slope, position in landscape, contributing upslope area, soil permeability, soil depth, soil cohesion and dispersibility. As the volume of overland flow increases and becomes channelised, the erosive power increases and resistance of the soil aggregates and particles to detachment becomes critical. The size and weight of the soil particles and their cohesion, or the tendency to slake or disperse will determine the resistance. Gully erosion occurs particularly in areas on Ordovician sediments, but is also common on granitic and glacial parent material. When gradational soils and stony loams on crests and upper slopes are cleared of the native deep-rooted vegetation, some rain percolates through the soil profile to the watertable, but some becomes overland flow with the potential to sheet erode the sloping land and scour out drainage depressions.

The presence of gullies and tunnels adversely affects productivity in a number of ways. As well as the land directly lost from production, the soil adjacent to the gully or tunnel is excessively drained thus reducing the vigour and number of plant species able to survive.

Broad overview susceptibility mapping across the North East region. To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

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The land characteristics and management factors involved in gully and tunnel erosion are described in Table 6.

Table 6 Land characteristics and management factors involved in gully and tunnel erosion

Processes	Land features affecting processes	Factors affected by land characteristics	Management factors that modify land characteristics
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	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
Processes involved are: detachment of exposed surface soil by - raindrop impact - channelised overland flow - cracking	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
detachment of subsoil by - subsurface flow in permeable strata and along cracks and tunnels cracking	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
transport of particles/ aggregates by - channelised overland flow - subsurface flow - gravity collapse deposition	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 concentratesurface 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
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 runon in streams and on lower lands	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

Source: Aldrick, et. al. (1988)