Splash and discontinuous runoff as creators of fine sandy lag deposits with kalahari sands

Abstract

Earlier field experiments have shown that the sandy dome-shaped hills south of Kinshasa are denuded by the combined action of splash and discontinous runoff. The sandy cover of a typical hill becomes progressively finer downhill where the slope is steeper. Laboratory experiments on similar sands demonstrate that splash has a preference for the coarse modal fraction. Simulated disturbed rough laminar runoff exports slightly coarser sands than does splash, even over gentle slopes where the regime is subcritical. But splash leaves the grains over 0.6 mm while discontinuous run-off selects even these for rapid transportation. Study of the transportation rates of both mechanisms conclusively demonstrates that splash creates a coarse lag on the summital sections of the Amba hill but that discontinuous runoff leaves a progressively finer residue, which is being transported, as the slope becomes steeper. Silt is also protected from erosion but it is not known to what extent the creation of a filtration pavement is responsible for this. The critical grain-size limit, below which the relative concentration increased, is 0.215 mm, both in the field and in the laboratory. This critical limit is some-what finer than the median grain-size of the Kalahari sands covering the hilltops and also of the fabricated bimodal sands.