Topographic and road control of mega-gullies in Kinshasa (DR Congo)

• Gully erosion in Kinshasa is due to urban expansion.
• The first study of the topographical control of gullying in an urbanized area
• Urban development/road network intensification and the risk of gullying
• A new concept of ‘cumulative road length control’ to gullying, the S–Lc relation.
• First tool of urban as well as rural mega-gully prevention in a sandy soil area

Diachronic mapping (1957, 1967, 2007 and 2010) shows an exponentially growing mega-gully network since roads were constructed through in the forests and plantations which occupied the sandy soils of the high town of Kinshasa. We found that the spatial occurrence of the mega-gullies (width ≥ 5 m) in this newly urbanized environment is controlled by two factors. First, there is a topographic control, given by the relation S = 0.00008A− 1.459, with S being the slope gradient (m m− 1) of the soil surface at the gully head and A the drainage area (ha) above the head. There is also a ‘road’ control, expressed by S = 22.991Lc− 1.999, with Lc being the cumulated length of roads in the basin above the gully head. The co-existence of both controls reflects the fact that the local sands are highly permeable and hence roads are more important generators of continuous runoff. The S–A relation noted above should not be applied outside the town where the road network is less dense. In contrast, the S–Lc relation may be used in both the town and rural areas underlain by porous soils where roads are the only generators of continuous runoff. We further conclude that the high town of Kinshasa is one of the most vulnerable places for gullying, and gullying can potentially transform the town into a badland. ‘Artisanal’ gully treatment is more successful than generally believed and the S–Lc relation can be a tool for mega-gully prevention.