Debris flows in a weathered granitic massif: Sierra de Gredos, Spain

Sierra de Gredos is composed of Hercynian granitic rock, the parent material for a weathering mantle, which extends throughout the range except in the areas that were eroded by glaciers during the late Pleistocene. Recent debris flows have formed on the steep slopes covered by the weathering mantle, causing considerable destruction. This study analyzes the impact of the mantleʹs structure and chemical composition on the formation and evolution of debris flows. The results show that the weathering mantle, despite its incipient state, is very thick in certain areas and that debris flows always originate here forming head scarps and channels that funnel the slope wash. This action leads to gullying, which quickly destroys the debris flow deposits and channels. Debris flows are influential in developing the drainage system in Sierra de Gredos, and the research suggests that debris flows occurring in other weathered granitic mountain ranges may have a similar impact.