Gully Erosion

Gully Erosion is an advanced stage of rill erosion, which, in turn, is a more advanced form of sheet erosion. It occurs when water flow concentrates in channels, cutting deep channels into the soil, leading to significant soil loss. These channels, often referred to as gullies, are too large to be repaired through normal tillage. The Soil Conservation Society of America defines a gully as a “channel or miniature valley cut by concentrated runoff, through which water typically flows only during and immediately after heavy rains.”

In India, areas like ravine regions suffer from an estimated erosion rate of 33 tons of soil per hectare per year due to gullies (Shekinah and Saraswathy, 2005). The impact of gully erosion is significant, as it can lead to the loss of productive soil, affecting agricultural productivity.

Development of Gullies

Gully formation involves two main processes: waterfall erosion and channel erosion.

• Waterfall Erosion: This process mainly happens at the head of the gully and causes the gully to extend. It occurs when water flows over a steep slope and erodes the surface in a waterfall-like manner.
• Channel Erosion: This process is more common in the body of the gully. It scours the sides and bottom of the gully, deepening and widening the channel.

Stages of Gully Development

Gullies develop in four stages:

1. Formation Stage: The gully starts as a small depression where runoff water begins to concentrate, leading to soil erosion. The rate of erosion is slower in this stage if the topsoil is resistant to erosion.
2. Development Stage: The gully head moves upstream, and the gully widens and deepens. In this stage, the gully may cut into the subsoil (C-horizon) as water flows with increased intensity.
3. Healing Stage: Vegetation starts to grow within the gully, helping stabilize the soil and prevent further erosion.
4. Stabilization Stage: The gully reaches a stable gradient, and vegetation cover effectively anchors the soil, allowing the development of new topsoil.

Classification of Gullies

Gullies are classified based on their size, shape, and formation.

Based on Size (Depth and Drainage Area)

• Small Gullies: Depth < 1 meter, drainage area < 2 hectares.
• Medium Gullies: Depth 1–5 meters, drainage area 2–20 hectares.
• Large Gullies: Depth > 5 meters, drainage area > 20 hectares.

Based on Shape (Cross-section)

• U-Shaped Gullies: These are formed where both the topsoil and subsoil are equally susceptible to erosion. The result is a nearly vertical gully wall.
• V-Shaped Gullies: These are the most common and occur when the subsoil is more resistant to erosion than the topsoil, causing the gully to form a “V” shape.
• Trapezoidal Gullies: These form when the bottom of the gully is made of more resistant material than the topsoil. The sides are steeper, and further erosion is limited.

Based on the Formation of Branches

• Continuous Gullies: These gullies have a main channel with several branch channels. They form networks and may eventually merge with other gullies or water systems.
• Discontinuous Gullies: These develop independently, typically after a landslide, and do not have a clear junction with the main channel at first.

Principles of Gully Control

The primary principle of gully control is to reduce the surface runoff that leads to gully formation. This can be achieved through the following methods:

1. Improvement of Gully Catchments: Measures such as improving vegetation and reducing runoff in the surrounding area can help control gully formation.
2. Diversion of Surface Water: This involves diverting runoff away from the gully area to reduce the flow that causes erosion.
3. Stabilization of Gullies: This includes structural measures and re-vegetation to stabilize the gully and prevent further erosion.

Gully Control Measures

Gully control involves both preventive and corrective methods. The primary goal is to reduce or redirect runoff and stabilize the gully.

Prevention

• Retention of Runoff on the Drainage Area: This can be achieved through practices like contour farming, strip cropping, terracing, and bunding. These methods reduce the volume and velocity of runoff that reaches the gully.
• Diversion of Runoff: In some cases, the most effective way to control gullies is to divert runoff away from them. This can be done through terraces or diversion ditches.

Conveyance of Runoff through the Gully: When it is not possible to divert runoff, the runoff must be conveyed through the gully itself. This can be done by establishing vegetation or constructing soil conservation structures that help control erosion.

Classification of Gully Control Measures

Biological or Vegetative Measures

These methods focus on using plants and natural materials to stabilize gullies.

1. Anti-Erosion Crops: These crops can help stabilize gully areas while also providing supplementary income.
2. Grassed Waterways: Small to medium-sized gullies can be converted into grassed waterways, which involve planting grass species that help reduce water velocity and soil erosion.
3. Sod Flumes: Used to control overfalls in gullies, sod flumes prevent waterfall erosion by providing a protected surface for water to flow over.
4. Sod Strip Checks: These are used in small gullies to slow down runoff and promote vegetation growth.
5. Low Sodded Earthfills: These are temporary structures made from earth and sod to control gully erosion until vegetation is established.
6. Trees and Shrubs: These are planted to stabilize severely eroded gully areas.

Engineering Measures

These measures include both temporary and permanent structures to control gully erosion.

• Temporary Gully Control Structures (TGCS): These include woven wire check dams, brush dams, loose rock dams, and log check dams. They are designed to last for 3 to 8 years and help manage smaller amounts of runoff.
• Permanent Gully Control Structures (PGCS): For larger, more persistent gullies, permanent structures such as drop spillways, chute spillways, and earthen check dams are used.

Design Criteria of TGCS

• Height of Temporary Checks: Typically, the overall height of temporary checks should not exceed 75 cm, with an effective height of about 30 cm.
• Spillway Capacity: The spillway should be designed to handle peak runoff expected once every 5 to 10 years.
• Spacing of Check Dams: The spacing should be such that the crest elevation of one check dam is the same as the bottom of the next one upstream, helping to reduce erosion effectively.