The methods available for gully control are –
(1) Diversion of runoff,
(2) Vegetative methods,
(3) Construction of temporary structures, and
(4) Construction of permanent structures.
(1) Diversion of Runoff:
Diversion of runoff is achieved by constructing diversion drains. The diversion drain is a shallow channel put across the slope above the gully. The design principles of diversion drains are exactly the same as that of grassed waterways.
In case of diversion drains the slope is kept generally less than 0.5 per cent and usually between 0.1 to 0.25 per cent. The diversion drains intercept the runoff coming from the area above the gully. The intercepted runoff is let off at a point in the gully well protected so that no further erosion at that point occurs.
(2) Vegetative Methods:
In these methods, the objective is to control gully erosion using vegetation.
i. Natural Vegetation:
If the runoff that is causing the gully is diverted and grazing is controlled from the eroded area, natural vegetation begins to get established. The growth of natural vegetation will eventually protect the gully area with grasses, shrubs and trees native to the area in question. In some areas, the development of natural vegetation may be stimulated by application of fertilizers or by spreading some mulches to conserve moisture.
ii. Artificial Vegetation:
Artificial vegetation of the gully beds and banks accelerates the process of establishing the vegetation and will help in stabilizing the gully. Where the gully banks are not deep and contain soil, they are sloped and then vegetation is established on the slopes.
Selection of grasses, shrubs or trees should be carefully done and should suit the local conditions as well and they should be of some economic importance. Grass is ordinarily a primary vegetative cover used in gully control. It is established both on the bed and the sides either by seeding or by sodding.
Sod flumes are successfully used to control overfalls at the head of the small gullies. The head of the gully is shaped to a gentle slope not exceeding 6 : 1 and grass is established by sodding.
Enough width is provided so that the flow velocities do not damage the grassed surfaces. In order to stabilize the toe of the flume, a wall of bricks or stones could be constructed.
(3) Temporary Structures:
Temporary structures for gully control are designed to retard the flow of water and reduce the channel erosion. In addition, they retain some quantities of sediment and moisture which helps in establishment of vegetation.
Construction of temporary structures for gully control purposes are advantageous as –
(1) They are cheap as compared to permanent structure,
(2) Locally available material can be used, and
(3) No technical skill is needed for their construction.
There are a wide variety of designs developed for temporary structures.
These may be classified as –
(1) Brushwood dams,
(2) Loose rock dams,
(3) Rock filled dams, and
(4) Woven wire dams.
Brushwood check dams use locally available vegetation cuttings in their construction. Two types of constructions are generally followed, viz. single post row brushwood check-dams and double post row brushwood check dams.
The double post row dam is used when the expected runoff is in larger quantities. In the woven wire dam, a wire mesh is used to hold the stones in place. All the check dams using involving stones are to be adopted in areas where stones are available easily.
The rock fill dams and the woven wire dams are more lasting than the loose rock dams. There are no standard principles of the design of these temporary structures. They are to be designed keeping in view of the needs and availability materials in a given situation.
(4) Permanent Structures for Gully Control:
Permanent structures are constructed when the benefits from such structures are justifiable compared to the cost of construction. The permanent structures protect the gullies from further development and at the same time, help in storage of water.
General requirements of the permanent structures for gully control are:
(1) They should be constructed with permanent materials,
(2) They should have adequate capacity to handle the runoff, and
(3) They should help in stabilizing the gully and store water wherever necessary.
For the purpose of gully control, three types of structures are adopted.
The three types of structures are:
(1) Chute spillway,
(2) Drop spillway, and
(3) Drop-inlet or pipe spillway.
Chute spillways are used at the head to convey the water safely to the gully bed. The drop spillways are used along the gully bed to act as control points so that the gully bed is not eroded below the crest level of the structure. The drop inlet spillways are used at appropriate locations in the gully for storage of water.
The permanent structures for gully control consist of three main components are:
i. The inlet,
ii. Conduit and
iii. The outlet.
Water enters the structure through the inlet and is conveyed through the conduit. The water leaves the structure through the outlet. The outlet is mainly responsible for dissipating the energy of the water so that the water flowing through the structure does not cause any erosion downstream of the structure.
Even though these structures are mentioned as gully control structures, these are also used for other purpose as well. Similar structures are used as outlets in farm ponds, as outlets in bunding and bench terracing projects and in irrigation water management.
The design principles essentially remain the same even though some minor modifications in the structures are made to suit particular situations. Design of similar structures for irrigation practice is given in Garg (1987).