Soil Erosion and Flood Risk Management

Gullies are also located and mapped on the pediment and in the glacis, the latter marked as an area with a
high accumulation of sediments (alluvial fans, Fig. 7 and Map 6).
Classification of gully systems
The upper areas belong to the Inselbergs and are characterized by several rills. Rills are small drainage
channels which have a very parallel course in the steeper parts of the mountain. This kind of erosion is
similar to a shallow gully system. There are rills in the pediment and glacis areas too. That means this type of
erosion is not limited by strict slope and specific soil structures, vegetation and land use types (Fig. 8A).
The most gully systems in the investigated area correspond to complex gully systems in the pediment (Fig.
8B). These gully types are characterized by a lot of smaller gullies and rills flowing into the main gully.
Wide spread catchments also characterise complex gully systems (Fig 7 and 8). The area of the glacis
represent often gully systems which are characterized as simple, isolated, linear structures. In general these
types of gullies are very wide and deep (Fig. 8C)
Fig. 8: A = rills on eastside of Mwakingali Mountain, B = complex gully system in the pediment, C = simple isolate
gully in the glacies
4.3.5 Discussion
The combination of field mapping and interpretation of topographic maps and satellite images produce good
results during the research in the northeast subarea. The accuracy of gully location depends on the skills and
experiences of the mapper.
The GPS measurements support the exact localization of the gullies with an accuracy of 3 m which is
sufficient for integrating this information into the Digital Elevation Model. For example the elevation of
Mwakingali Mountain from the topographic map approximates 818 metres. The GPS measures the same
elevation of 818 metres on the top of the mountain.
The majority of the gullies are situated in the pediment where the slope is less and correspond to complex
gully systems (Fig. 8B). These gully types are characterized by a lot of smaller gullies and rills flowing into
the main gully. Streets represent often the starting point of a gully system. The dominating process in the
gully heads is sheet and rill erosion resulting from the high level of slope and the position of bedrock close to
the surface. The area of the glacis represent often gully systems which are characterized as simple, isolated,
linear structures. In general these types of gullies are very wide and deep.
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