Gully Erosion

 Gully erosion is an advanced stage of rill erosion. It is a process where surface (or subsurface) water concentrates in narrow flow paths and removes the soil resulting in incised channels that are too large to be destroyed by normal tillage operations. Although gully erosion is a natural process, it is most often triggered or accelerated by human activities such as clearing vegetation and overstocking.

The gully development is recognized in four stages:

 Formation Stage: Scouring of topsoil in the direction of general slope occurs as the runoff water concentrates. It normally proceeds slowly where the topsoil is fairly resistant to erosion.

 Development Stage: Causes upstream movement of the gully head and enlargement of the gully in width and depth. The gully cuts to the C-horizon of soil, and the parent materials are removed rapidly as water flows.

 Healing Stage: Vegetation starts growing in the gully.

 Stabilization Stage: Gully reaches a stable gradient, gully walls attain a stable slope and sufficient vegetation cover develops over the gully surface to anchor the soil and permit the development of new topsoil.

Classification of Gullies

Gullies can be classified based on three factors viz. their size, shape (cross-section), and formation of branches or continuation. The detailed classification is discussed below.

Based on Size (depth and drainage area)

Based on Shape

The classifications of gullies based on shape are as follow

 U-Shaped: These are formed where both the topsoil and subsoil have the same resistance against erosion. Because the subsoil is eroded as easily as the topsoil, nearly vertical walls are developed on each side of the gully.

 V-Shaped: These gullies develop where the subsoil has more resistance than topsoil against erosion. This is the most common form of the gully.

 Trapezoidal: These gullies are formed where the gully bottom is made of more resistant material than the topsoil. Below the bottom of the gully, the subsoil layer has much more resistance to get eroded, and thus the development of a further depth of the gully is restricted.

Based on the Formation of Branches or Continuation

 Continuous Gullies: More prominent on gentle slopes. These gullies consist of many branches. A continuous gully has the main gully channel and many mature or immature branch gullies. A gully network is made up of many continuous gullies.

 Discontinuous Gullies: More prominent on steep slopes. These may develop on hillsides after landslides. They are also called independent gullies. At the beginning of its development, a discontinuous gully does not have a distinct junction with the main gully or stream channel. Principles of Gully Control

 Generally, gullies are formed by an increase in surface runoff. Therefore, minimizing surface runoff is essential in gully control.

 The rate of gully erosion depends primarily on the runoff-producing characteristics of the watershed.

 Watersheds deteriorate because of misuse of the land (man-made changes), short intensive rainstorms, prolonged rains of moderate intensity, and rapid snow melts. The precipitation factors turn into a high runoff, develop flooding, and form gullies. In gully control, the following three methods should be applied according to the order given:

i. Improvement of gully catchments to reduce and regulate the runoff rates (peak flows).

iii. Stabilization of gullies by structural measures and accompanying re-vegetation.

Temporary Gully Control Structures

 They have a life span of 3 to 8 years and they are pretty effective where the amount of runoff is not too large.

 These are made of locally available materials.

 Basic purposes they serve are to retain more water as well as soil for proper plant growth and prevent channel erosion until sufficient vegetation is established on the upstream side of the gully

Types of temporary Gully Control Structures

 Woven wire check dams

Used in gullies of moderate slopes (not more than 10 percent) and small drainage areas that do not have flood flows that carry rocks and boulders.

Help in the establishment of vegetation for permanent control of erosion.

The dam is built in a half-moon shape with an open end upstream.

The amount of curvature is arbitrary: but an offset equal to 1/6th of the width of the gully at the dam site is optimum.

 Brush dams

Cheap and easy to build, but least stable of all types of check dams.

Best suited for gullies with small drainage area.

The Center of the dam is kept lower than the ends to allow water to flow over the dam rather than around it

 Loose rock dams

Suitable for gullies with small to medium size drainage area.

Used in areas where stones or rocks of appreciable size and suitable quality are available.

Flat stones are the best choice for dam making.

Stones can be laid in such a way that the entire structure is keyed together.

If round or irregular-shaped stones are used, the structure is generally encased in woven wire to prevent outside stones from being washed away.

If the rocks are small, they should be enclosed in a cage of woven wire.

 Plan or slab dams

These dams are suitable in areas where timber is plentiful, and dams can be constructed with much less labor as compared to other types of temporary structures.

These dams can generally be used in gullies with larger drainage areas.

 Log check dams

They are similar to plank or slab dams. Logs and posts used for the construction are placed across the gully. They can also be built of planks, heavy boards, slabs, poles, or old railroad ties. The main objectives of log check dams are to hold fine and coarse material carried by flowing water in the gully and to stabilize gully heads.

They are used to stabilize incipient, small and branch gullies generally not longer than 100 m and with catchment areas of less than two hectares. The maximum height of the dam is 1.5 m from the ground level

 Boulder check dams

Boulder check dams placed across the gully are used mainly to control channel erosion and to stabilize gully heads. A gully system or multiple-gully system having a catchment area of 20 ha or less with a length of about 900 m can be stabilized by boulder check dams.

These dams can be used in all regions. The maximum total height of the dam is 2 m and the thickness of the dam at spillway level is 0.7 to 1.0 m

Permanent Gully Control Structures

 These are built of masonry, reinforced concrete or earth are efficient supplemental control measures in soil and water conservation. They are helpful in situations where vegetative measures or temporary structures fail to serve the purpose of controlling the concentration of runoff or reclaim a gully.

 They are generally used in medium to large gullies with medium to large drainage areas designed to handle runoff from the heaviest rains that may be expected once in 25 to 50 years or more depending upon the estimated life of the structure.

Three basic permanent structures, generally employed in stabilizing gullies are:

 Drop spillway

 Drop inlet spillway

 Chute spillway