Physical modeling investigation of rainfall infiltration in steep layered volcanoclastic slopes

Infiltration processes in layered slopes can be strongly affected by the different hydraulic properties of the soils constituting the layers, with potential downslope flow diversion that can have effects on slope stability as well as on runoff generation and groundwater recharge. In this respect, volcanoclastic soil covers represent a typical example, as layers with strongly contrasting textures are deposited during the various eruptive phases. In this paper, the results of a transient infiltration test, carried out in a densely instrumented physical model of a layered volcanoclastic sloping cover, are presented. The soil cover was constituted by a layer of gravelly pumices interbedded between two layers of finer ashes (sandy Items). Even with such an extreme contrast in texture, capable of significantly delaying the advancement of infiltration through the layer of pumices, flow diversion occurred only temporarily at the interface between the upper layer of ashes and the pumices. In fact, although a long-lasting intense rainfall was applied into an initially dry soil profile, the downslope diverted water volume in form of a subsurface runoff was just a small fraction of the total applied rainfall. In fact, the accumulation of water above the upper edge of the pumices, responsible of the subsurface runoff, soon led to the establishment of a large water potential gradient, which redirected the infiltrating flow through the pumices and stopped the downslope flow diversion.