There are several ways of classifying erosion. One general way is to distinguish between accelerated and geologic erosion. Geological erosion occurs where soil is in its natural environment surrounded by its natural vegetation without human disturbance. Geologic erosion has been taking place naturally for millions of years and it helps to create balance in uncultivated soil that enables plant growth. It’s a relatively a slow continuous process that often goes on unnoticed. Accelerated erosion is a concept referring to an essentially natural process occurring at an increased rate under conditions of ecological disequilibrium. Accelerated erosion is the most dangerous type and it needs concerted efforts through careful planning and implementation of appropriate control measures. Another distinction is between rill, sheet and gully.
Splash erosion is the first stage of the erosion process. It occurs when raindrops hit bare soil. The explosive impact breaks up soil aggregates so that individual soil particles are ‘splashed’ onto the soil surface. The splashed particles can rise as high 60cm above the ground and move up to 1.5 metres from the point of impact. The particles block the spaces between soil aggregates, so that the soil forms a crust that reduces infiltration and increases runoff.
Sheet erosion refers to the uniform movement of a thin layer of soil across an expanse of land devoid of vegetative cover. Raindrops detach soil particles, which go into solution as runoff occurs and are transported downstream to a point of deposition. Deposition occurs when runoff slows to the point where soil particles can no longer remain in suspension. Tilled agricultural fields and construction sites are subject to sheet erosion.
When sheet flows begin to concentrate on the land surface, rill erosion occurs. While sheet erosion is generally invisible, rill erosion leaves visible scouring on the landscape. This type of erosion occurs when the duration or intensity of rain increases and runoff volumes accelerate. Rills may become stable through soil consolidation; however, they are still the major sediment transport route for soil detached on the interrill areas. Improved understanding of the ability of rain-impacted flows in rills to transport sediment is needed to improve our estimates of sediment transport and delivery.
Rill erosion evolves into gully erosion as duration or intensity of rain continues to increase and runoff volumes continue to accelerate. A gully is generally defined as a scoured out area that is not crossable with tillage or grading equipment.
Stream channel erosion
Stream channel erosion consists of both stream bed and stream bank erosion. Stream bed erosion occurs as flows cut into the bottom of the channel, making it deeper. This erosion process will continue until the channel reaches a stable slope. The resulting slope is dependant on the channel materials, and flow properties. As the stream bed erodes, and the channel deepens, the sides of the channel become unstable and slough off; resulting in stream bank erosion. Stream bank erosion can also occur as soft materials are eroded from the stream bank or at bends in the channel. This type of stream bank erosion results in meandering waterways. One significant cause of both steam bed and stream bank erosion is due to the increased frequency and duration of runoff events that are a result of urban development.
Tunnel erosion occurs when surface water moves into and through dispersive subsoils. Dispersive soils are poorly structured so they erode easily when wet.The tunnel starts when surface water moves into the soil along cracks or channels or through rabbit burrows and old tree root cavities. Dispersive claysare the first to be removed by the water flow. As the space enlarges, more water can pour in and further erode the soil. As the tunnel expands, partsof the tunnel roof collapse leading to potholes and gullies.Indications of tunnel erosion include water seepage at the foot of a slope and fine sediment fans downhill of a tunnel outlet.Remediation actions include breaking open existing tunnels, revegetation, and increasing soil organic matter. Extensive earthworks may be required.
Tillage erosion moves soil from the top of the field downward, exposing subsoil at the crest while burying soil at the bottom. After many years of tillage, topsoil accumulates at the bottom of the slope. No soil leaves the field due to tillage erosion, but the effects for productivity and increased yield variability can be huge.
Severe sheet and rill erosion on highly erodible soils in northwest Iowa (USA) after heavy rains. These soils had no protection against soil erosion.
Source: Lynn Betts, NRCS
Ephemeral gully erosion and severe rill erosion washes young corn plants from the ground as well as topsoil and nutrients from loess soils.
Source: Lynn Betts, NRCS
Gully erosion is evident in this unprotected field following a storm.
Soure: Tim McCabe, NRCS
Stream erosion and deposition
Source: Albert Copley, Oklahoma University