Bed texture changes caused by check dams on ephemeral channels in Mediterranean semiarid environments

摘要

The hydrological correction of ephemeral channels by means of check dams has recently become a common defense practice against erosion and desertification in semi-arid Mediterranean environments. Among its effects are important bed texture changes which bring general stability to the affected bed reaches. In this article surface bed texture changes both upstream and downstream from check dams are shown for two ephemeral channels of the Segura basin, south-eastern Spain (the Torrecilla and Carcavo catchments), in relation to their degree of in-fill and slope variations. The armoring processes caused downstream from the check dams are studied with regard to the size of the structures, the hydro-dynamic characteristics of the flow and the sediment Supply rates. From a comparative analysis Of the surface and sub-surface particle sizes the longitudinal armouring trend of the bed along the corrected channels is described. In the Carcavo, armouring at the dam foot tends to increase progressively going downstream as bed slope decreases. Conversely, all down the Torrecilla rambla there is no linear trend agreeing with decrease in slope, armouring being more influenced by other environmental factors (geological constrictions, dam type and dimensions, etc.). Finally the surface size variation and the changes in roughness are analysed in accordance with the distance from the check dams and the bed slope.