Marloth Park Management Plan. - Nkomazi Local Municipality

However, reasonable management can limit soil erosion to an acceptable level, where
utilisation of the resources is not influenced negatively. This acceptable level is usually
determined as the level, where no perceptible loss of soil is observed.
The main causes of soil erosion are wind and water. Where the herbaceous cover of the soil is
thin, the top layer of the soil is compacted by raindrops. Therefore, water infiltration
decreases and water run-off increases, resulting in the displacement of the topsoil. In South
Africa water erosion is significant in areas receiving more than 100 mm of rain per year,
whereas wind erosion is restricted to areas receiving less than 400 mm of rain per year. Vertic
soils are more prone to erosion than sandy soils.
Prevention of soil erosion starts with the prevention of the deterioration of the veld and the
development of bare patches. Prevention of the formation of new erosion ditches and the
expansion of existing dongas is of importance. This management aim is reached by reducing
water run-off in catchments areas by establishing a good grass layer and diverting the run-off
water from the erosion ditch. Ditches that have only begun to form should be stabilised first,
before extensive dongas are reclaimed. Complete repair of short ditches is preferable to only
partial repair of extensive dongas. Existing erosion ditches need to be prevented from
becoming deeper, wider and longer. Erosion of the donga floor is controlled by the
construction of walls made of stone, concrete, wire baskets filled with stones (gabions) or a
combination of these materials. These walls are anchored deep in the floor and the sides of the
donga. Large stones are placed at the bottom and along the gully wall. Then smaller stones are
stacked on top to build the wall. The upstream side of the wall is constructed with a slope,
while the downstream side is vertical. Water cannot flow underneath or around these
structures. Silt accumulates on the upstream side and vegetation establishes itself on the
structures as well as the donga floor, where moisture is retained. To accelerate to process of
rehabilitation, creeping grass species such as couch grass Cynodon dactylon can be planted on
the slope of the artificial walls. The initial wall need not be high. As silt builds up, the wall
can be raised. However, the minimum height of a wall should be constructed in such a way
that water flowing over the dam does not exceed 0.6 m. Building walls at the head of a donga
or ditch prevents them from growing longer and wider. The wall needs to be constructed in
such a way, that water cannot flow into the ditch from the side and cause the walls to cave in.
Therefore, walls need to be higher than the ditch itself and additional low walls, parallel to the
edge of the ditch need to be constructed. The water is then directed into the gully by
predetermined channels.
© Ecological Associates/ Marloth Park 109