Gully erosion occurs when concentrated surface water scours out the regolith and underlying rock, with the debris either deposited downslope or transported into river systems, causing significant downstream problems. The severity and shape of the gully are highly dependent on the rock type. The very severe to extreme gully erosion is limited to argillites (crushed), mudstone, and fine siltstones, each with its own distinct gully shape. In this article, we will discuss Gully Erosion which will be helpful for UPSC exam preparation.

Gully Erosion

What is Gully?

• A gully is a landform formed by running water, mass movement, or a combination of the two eroding sharply into soil or other relatively erodible material, most commonly on a hillside or in river floodplains or terraces.
• Gullies look like large ditches or small valleys, but they are metres to tens of metres deep and wide, with a distinct 'headscarp' or 'headwall' and progress by headward (i.e. upstream) erosion.
• Gullies are frequently associated with intermittent or ephemeral water flow, which is typically associated with localised intense or prolonged rainfall events or snowmelt.
• Gullies can be formed and accelerated by cultivation practices on farmland hillslopes, and they can form quickly in rangelands from existing natural erosion forms subject to vegetative cover removal and livestock activity.

What is Gully Erosion?

• Gully erosion is a visible type of soil erosion that reduces soil productivity and restricts land access and use.
• Gully soil can also cause infrastructure damage by burying fence lines, silting up waterways, clogging road culverts, and filling dams and reservoirs.
• Suspended sediments from an eroding gully can degrade water quality. Nutrients and pesticides are attached to these fine colloidal clay particles, which can remain suspended for long periods of time, clogging groundwater aquifers, polluting watercourses, and endangering aquatic flora and fauna.
• Controlling gully erosion can be challenging and expensive. It may be justified on higher-quality soils where there is a reasonable chance of success, or in strategic locations such as where a road or building is under threat by an advancing gully.
• Gully erosion is a more advanced stage of rill erosion in which surface channels have eroded to the point where tillage operations cannot remove them.
• A gully head forms when rill erosion deepens and widens, resulting in a distinctive nick point or headwall. As a result of headwall migration, most gullies extend up slope.
• However, the collapse and slumping of sidewalls is usually responsible for the greatest proportion of soil loss.
• Human land use, particularly changes in land use, may hasten gully expansion through head cutting, sidewall collapse, piping, floor erosion, and other processes, resulting in widespread land degradation and potential damage to human structures and activities.
• A gully is traditionally distinguished from a rill by having a critical cross-sectional area of at least one square foot, i.e., the size of a channel that can no longer be erased by normal tillage operations.
• Gullies are frequently associated with a wide range of on-site and off-site consequences.
  • On-site consequences include direct loss of land, trees, and crops, as well as decreased trafficability. These restrict agricultural and other land-use options.
    • Gullies can also severely damage roads, buildings, and other infrastructure. In severe cases, such destructions may result in a large number of casualties.
  • Off-site consequences of gully erosion include changes in catchment hydrology, such as lower river baseflows and higher peak runoff discharge.
    • Gullies can indirectly contribute to sediment loads by increasing runoff and sediment connectivity between upland areas, valley bottoms, and river networks or lakes.
• As a result, gully erosion is a major concern in many areas around the world. It is a key process in land degradation and desertification, posing a significant threat to various ecosystems and ecosystem services.

Causes of Gully Erosion

• Cultivation or grazing on gully-prone soils.
• Increased runoff caused by land use changes such as tree removal in a catchment or the construction of new residential areas.
• Furrows, contour banks, waterways, dams by washes, stock pads, fences, tracks, or roads all contribute to runoff concentration.
• Waterways in cropping areas that are improperly designed, built, or maintained.
• A lack of vegetation cover caused by overgrazing, fires, or salinity issues.
• Long periods of low flows or seepage flows.
• 'Down cutting' in a creek causes gullies to advance up the drainage lines that flow into it.
• Diversion of a drainage line to an area of high erosion risk, such as a steep creek bank or erodible soil.
• Gully erosion occurs when water is channelled across unprotected land and washes away the soil along drainage lines.
• Under natural conditions, runoff is moderated by vegetation, which generally holds the soil together, protecting it from excessive runoff and direct rainfall.
• Excessive clearing, inappropriate land use, and soil compaction caused by grazing often leave the soil exposed and unable to absorb excess water.
• Surface runoff then increases and concentrates in drainage lines, allowing gully erosion to develop in vulnerable areas.

Problems Caused by Gully Erosion

• Property dissection causing access and management issues.
• Loss of productive land (gullies often occur in the most productive area of the catchment).
• Reduced amenity and property values, including the destruction of farm improvements such as fences or tracks.
• Discoloration of water supply and sedimentation of waterways, dams, and lower paddocks.
• Provides a haven for vermin.

Prevention of Gully Erosion

• The primary, long-term control method for gully erosion is vegetation.
• Stock must be kept out of all gullies, and the gully floor, sidewalls, and surrounding areas must be revegetated. Because of moisture stress, it is often difficult to establish vegetation on gully sidewalls.
• Replant the gully floor with fast-growing grasses.
• Sidewalls and surrounding areas have been replanted with trees and grasses.
• It is possible that irrigation, hydroseeding, and mulching will be required.
• Replant the catchment above the gully (increase water usage).
• If runoff can be diverted and safely disposed of, gully erosion may be controlled.
• Contour drains and diversion banks are used to divert water away from the gully head.
• In many gully systems, properly designed and installed engineering structures may be the only way to stop headward progress.
• Preserving remnant vegetation along drainage lines and removing grazing from these areas.
• Increasing water usage by planting deep-rooted perennial pastures, trees, or an appropriate mixture of both, thereby maintaining healthy, vigorous levels of vegetation.
• Identifying drainage lines as a distinct land class in which vegetation must be protected.
• Immediate sheet or rill erosion stabilization.
• Ensuring that track runoff is evenly distributed across paddocks to dissipate its energy.
• Maintaining high levels of organic matter in the soil.
• Avoiding excessive cultivation (fence and manage the land according to its capability).

Conclusion

Gully erosion is an advanced stage of water damage to land when the surface channels have been eroded to the point where tillage operations would be useless. Apart from causing massive soil losses and the destruction of farmland, it also reduces water quality by increasing the sediment load in streams. Gully erosion, once established, can be difficult to control. In most cases, a combination of approaches is required, such as the use of vegetation, fencing, diversion banks, and engineering structures. Gullies are widely regarded by the scientific community as an important landform for understanding environmental change and soil erosion risks, and they can play an important role in raising public awareness about these issues.