EROSIONAL PROCESSES OF MEDITERRANEAN BADLANDS - A NEW EROSIVITY INDEX FOR PREDICTING SEDIMENT YIELD FROM GULLY EROSION

The experimental catchment basin of St. Genis (Hautes-Alpes, France) was used to study pluvial erosion of the Terres Noires (Jurassic marls) in badlands during the years 1986 to 1989. The regional climate is of Mediterranean type. Three simple gullies (262 m2, 1110 m2, 1620 m2) were equipped for collecting pluviometric and hydrosedimentologic data. The annual average rate of erosion, calculated during the study period, is of the order of 190 t ha-1 yr-1 for an average rainfall of 840 mm/yr. The hydrodynamic behavior of marly slopes regulates the evolution of overland flow, and therefore erosional processes. Rainfall simulations in the field emphasized the low permeability of these marls, which is 5 mm/h for an initially dry soil. On the other hand, minimum rainfall required (P(t) in mm) to produce overland flow is a function of the duration of the initial period of dryness (T(d) in days) according to the equation: P(t) = 3T(d)0.4 The combination of the hydrologic characteristics of marls and of the kinetic energy of rainfall defined an erosivity index EKE (effective kinetic energy) which correlates well with measurements of sediment yield obtained at the mouth of simple gullies. This index is used to model the relationships between rainfall events and specific sediment yield (SSY) by the equation: SSY (t ha-1) = 3.38EKE X 10(-2) whose linear correlation coefficient is R = 0.91. This equation provides a good estimate of ssy at the scale of an individual episode of rainfall as well as at the scale of an annual pluviometric cycle.