Debris flows on vegetated screes in the black mountain,Carmarthenshire

The Black Mountain Scarp in Carmarthenshire is currently being modified by debris-flow activity along gullies cut into the vegetated scree, which is a persistent feature of the scarp-front. Low angled, concave debris flow cones are accumulating at the mouths of the gullies. Mean rate of sediment yield from the sides of one gully was estimated to be 8·4 m³ in one year, and 9·8 m³ was shifted out of the gully by a single debris-flow in the same year. Movement by other transport processes, such as streamflow, was negligible in the gullies. No new gullies are being initiated at the present and all of them appear to be in a similar state of development. Consequently, it appears that some marked environmental change was responsible for initiating gully erosion. If present rates of gully-erosion are representative of the past, the gullies are only about 540–700 years old and may possibly have been initiated by sheep-grazing in the area.