Sheet erosion is the most damaging type of erosion because it is not often recognised and seldom treated. When raindrops fall, particles of soil are knocked loose and then carried away by the runoff. This runoff further strips unprotected areas of their valuable topsoil and becomes muddy water that ends up in drains, streams and rivers. Sheet erosion leads to streams and gullies, the main focus of most conservation efforts these days. Sheet erosion does leave some visible marks as shown in the figure below.

Figure 1.

Soil collecting behind obstructions on aslope such as [A] stones left by runoff because they were too heavy to be carried away. [B] Moldy mounds of soil and other debris trapped under trapped under branches, twigs or even clumps of straw. [C]

Figure 2.

This shows the effects of sheet erosion where the loss of soil exposes the roots of the trees. Water then easily passes beneath the trunks of the tress and between their roots. Once all the soil that supported them and gave them life is washed away the trees will be washed out of the ground as well.

It has been noted that in schools where the broom is excessively used in the name of cleanliness, the soil in the yard is washed away or swept away, causing pools of water to form within the schoolyard during the rainy season.

• Farming steep slopes
• Bad farming methods
• Over grazing deforestation
• Veld fires
• Overpopulation

As soil erosion is mainly a man made problem - caused by human activities, it can, of course, be prevented. That’s why the knowledge of the factors that affect soil erosion is essential for any effort to design erosion control measures.

Figure 3 A,B,C The Planting Material

Any measure which helps keep the soil covered by vegetation will prevent or at least slow down erosion. The second factor that can be influenced is the speed of the running water. Good farming methods together with extra measures like ridging and planting hedges, trees or vetiver, will slow down the water and thus reduce or stop soil losses. It is also of great importance for people to obey laws made in an effort to prevent soil erosion and siltation.

Although many grasses and trees have been tried over the years as measures to prevent soil erosion, to date only vetiver grass has stood the test of the time. The following characteristics derived from observations of vetiver grass show that this plant is ideally suited for the vegetative system of soil and moisture conservation. No other grass is known to rival it in hardiness or diversity.

The plants must form a hedge, otherwise the system cannot act as a barrier against soil loss. Planting the slips too far apart would render the system almost useless, because the grass would take too long to form a hedge and would provide little protection.

• Vetiver Grass – A Hedge Against Erosion by Bill Fraser [1993]
• Kwekwe Environmental Education Programme [KEEP] Newsletter 6/96