Gully erosion is a more advanced form of rill erosion, which is a more advanced form of sheet erosion. It’s the most visually stunning type of erosion. Any amount of surface runoff has the potential to cause gully erosion. A gully is defined as “a channel or tiny valley created by concentrated runoff but through which water generally flows only during and soon after heavy rains,” according to the Soil Conservation Society of America. It might be branched or dendritic, or it can be straight, long, thin, and constant in breadth.” In ravine regions in India, the rate of soil erosion from gullies is 33 t/ha/yr (Shekinah and Saraswathy, 2005). The only difference between a ravine, a gully, and a rill is their size. Normal tillage operations can’t fill a gully since it’s too big. A ravine is a narrow, deep gully. It’s a little bigger than a gully, and it’s generally worn down by water. Gully erosion is projected to harm around 4 million hectares of land in India (Michael and Ojha, 2012).
Gullies’ Development
Waterfall erosion and channel erosion are the two basic mechanisms that cause gullies to form. Both of these erosions are widespread in the same gully. The gully head is frequently extended by waterfall erosion, whereas channel erosion enlarges the depth and width of gullies by scouring the bottom and sides. Channel erosion is the most common cause of gullies. When an overflow occurs at the gully’s head, the gully continues to erode due to waterfall erosion. Figure 1 depicts waterfall erosion at the gully head and the gully’s progress towards the watershed’s upper boundary.
Fig. 1. Waterfall erosion at gully head.
There are four stages to the gully development:
Formation Stage: As runoff water concentrates, it scours the top soil in the general slope direction. It usually takes a long time if the top soil is quite resistant to erosion.
Development Stage: Causes the gully head to travel upstream and the gully to expand in breadth and depth. The gully breaks through the soil’s C-horizon, and the parent materials are swiftly washed away as water runs.
Healing Stage: In the gully, vegetation begins to grow.
Stabilization Stage: Gully walls achieve a stable slope, and sufficient plant cover grows over the gully surface to anchor the soil and allow the growth of new topsoil.
Classification of Gullies
The size, shape (cross section), and creation of branches or continuation of gullies are the three variables that may be used to classify them. The categorization in detail is addressed further down.
Based on Size (depth and drainage area)
Gully classification based on the size is presented in Table 1.
Table 1. Gully classification based on size
|
Classification |
Depth (m) |
Drainage area (ha) |
| Small |
< 1 |
< 2 |
| Medium |
1 to 5 |
2 to 20 |
| Large |
> 5 |
> 20 |
Based on Shape
The classification of gullies based on shape is shown in Fig 2.
U-Shaped: These are generated when the erosion resistance of the topsoil and subsoil is equal. Because the subsoil erodes at the same rate as the topsoil, virtually vertical walls form on both sides of the gully.
V-Shaped: These gullies form where the subsoil is more resistant to erosion than the topsoil. The most prevalent type of gully is this one.
Trapezoidal: When the gully bottom is made of a more resistant material than the topsoil, these gullies form. The subsoil layer below the bottom of the gully is much more resistant to erosion, limiting the depth of the gully’s development.
Fig. 2. Gully classes based on the shape of gully cross-section.
Based on the Branches’ Formation or Continuation
Continuous Gullies: There are various branching in these gullies. There is a main gully channel and many mature or immature branch gullies in a continuous gully. A gully network is made up of a series of interconnected gullies. A multiple-gully system is made up of various gully networks.
Discontinuous Gullies: After landslides, they can form on slopes. They’re also known as self-contained gullies. A discontinuous gully does not have a definite confluence with the main gully or stream channel when it first forms. A fairly flat surface is covered by flowing water in a discontinuous gully. It eventually makes its way to the main gully channel or stream. Independent gullies can be found between the branches of a continuous gully or they can take up a whole region when no continuous gullies exist.
Gully Control Principles
Gullies are often generated by an increase in surface runoff. As a result, gully mitigation requires limiting surface runoff. The rate of gully erosion is essentially determined by the watershed’s runoff generating features, watershed area, soil characteristics, gully size-shape, and slope, among other factors. Watersheds degrade due to land abuse (man-made modifications), short, intense rainstorms, extended, moderate-intensity rainstorms, and quick snow melt. The precipitation elements that cause heavy runoff, floods, and the formation of gullies. The following three approaches for gully control should be used in the order listed:
- Gully catchment improvements to minimise and manage runoff rates (peak flows).
- Surface water is diverted above the gully area.
- Gully stabilization by structural interventions and subsequent re-vegetation.
Small or incipient gullies can be stabilised without the need of the third approach when the first and/or second procedures are used in several temperate climate nations. For successful gully reduction in tropical and subtropical nations with significant rainfall (monsoons, typhoons, tropical cyclones, and so on), all three strategies must be used.
Measures to Control the Gully
It is far simpler to prevent the creation of a gully than it is to regulate it after it has occurred. The management of runoff from the drainage area is one of the most important elements in a gully control scheme. The following is a list of the many runoff control measures that can be examined in order:
- Retention of Runoff on the Drainage Area: It’s achievable because to effective crop management and conservation measures including contouring, strip cropping, bunding, and terracing, among others. Runoff is considerably decreased when contour bunds are employed. Small and medium-sized gullies in cultivated areas can also be recovered by laying a sequence of earth fills across the gully.
- Diversion of Runoff Around the Gullied Area: The most efficient way to prevent gullies is to remove all runoff from the gullied region. This can be accomplished by redirecting runoff from the gully and directing it to a suitable exit at a non-erosive pace. Runoff is typically diverted from its natural exit using terraces and diversion ditches. Terraces are excellent for controlling minor gullies on cultivated fields, as well as medium-sized shallow gullies. If the slope above a gully is too steep for terracing or the drainage region is grassland or woods, diversion ditches can be utilized to divert runoff away from the gully.
- Conveyance of Runoff through the Gully: If neither retaining nor diverting drainage is practicable, runoff must be carried through the gully itself. Only if vegetation can be developed in the gullies, or if soil conservation structures can be constructed at important spots to provide primary control, is this viable.
