Morfologia, suoli ed erosione in Valle Marchiazza (Biellese)

Morphology, erosion and soil types in Marchiazza Valley (District of Biella, Piedmont, Italy)

Abstract

The hydrographic network of this basin is given by water courses showing in their talwegs two steps, close each other, yielded by two different stages of regressive erosion. In both hillsides, 4 or 5 terrace levels may easily be recognized: the upper one is likely due to Riss Glaciation, the following to Würm, whereas the two lowest, related to river-bed’s steps, have neotectonic origin. In his lower part, the basin shows a typical « Richter’s hillside shape», given by a slightly concave base (equilibrium slope of transport and light ablation). Soil types may be grouped in a few associations, in which are prevailing lithic Udorthents, typic Dystrochrepts and slightly leached Dystyc Eutrochrepts. Regression forms due to rejuvenation, or directly rock outcroppings, occur when erosion is pretty active. Slope deposits give rise to typic Udifluvents or colluvial soils. Ablation occurs in hillsides because of a widespread surface water-creeping, whereas solifluction phenomena are pretty lacking. Max-erosion moment clashes with a period of short and heavy rainfalls, so that the sheer power of waterstreams, at top of basins, may erode thoroughly detritic materials coming from hillsides. By the other hand, alluvium occurs when talweg slope decreases. Far downstream a charge decrement fosters un increase of net power which causes erosion in river-beds and outcropping of bedrocks, aided in this by regressive erosion in progress. Soil chains occurring in slopes confirm the suggested erosion model.