Gully formation may be severe in areas with deep soils and steep slopes. On steep slopes, the velocity of the water is very high and the scouring effect will be great. A deep soil profile with only little cohesion is susceptible to rapid and deep gully formation during heavy rainfall.
Purpose of gully control
The purpose of gully control is not so much erosion control as an attempt to limit the effects of erosion which is taking place upstream from the gully. Of course, existing gullies should be prevented from developing further. What measures are taken to prevent or control the process of gully formation depends on the size of the gully and the area to be drained (the amount of water to be diverged). First of all we try to check the amount of water coming into the gully by protecting the soil upstream or even by diverging the water. The velocity of the water in the gully also has to be checked so that it doesn’t scour out further.
Smaller gullies can be kept in check by the farmers themselves as follows: As far as possible the water is kept in the middle of the gully so that the walls cannot be undermined. In shallow gullies small dams with an overflow can be laid out with rubble, twigs, stones and wire bolsters.
The water can then ooze through these fairly open constructions whereas any transported silt is held back upstream. In this way the longitudinal slope is reduced and with it the flow velocity too. If available, wire netting supported by wooden posts, can be used for smaller gullies.
Protection of the head of the gully can be done by protecting the soil with broken stone, rubble, twigs or similar material. It is also advisable to keep the area around the head of the gully planted up with trees or a tuft forming crop, for example vetiver grass. To prevent trampling by cattle (sometimes even the direct cause of gully formation) preventive measures should be taken by making a good fence (thorny hedge for example). By leading the water along the lower parts of the land you must be sure that water actually comes into the gully.
Points of attention
When laying out these check dams, the following points should be taken into account: – The principle is to shorten the length of the slope in the gully over which the water flows, so that the flow velocity (and with it the chance of further erosion) decreases. -The gully walls at the position of the dam and also a part upstream are graded to a slope of 1:2 (going up 1 meter over a distance of 2 meters) or less, so that the chance of breaking is minimized. – There should be good contact between the dam and the gully wall (well anchored); otherwise the temporary structure will wash away.
Fencing poles are always driven deep into the soil so this applies to these structures too. The dam should be lowest in the middle where the flow has to concentrate at the overflow. The gully floor should be strengthened down stream against the scouring force of the water. This can be done by making a type of mattress or cover which is well sealed. (For example broken stone, discarded car tires filled up, concrete rubble etc.). After overflowing, the water is very turbulent. Even though the stream is concentrated in the middle of the gully, the walls will have to be extra strengthened.