Abstract:The cataclysmic 1980 eruption of Mount St Helens radically reduced the infiltration characteristics of ¾60 000 ha of rugged terrain and dramatically altered landscape hydrology. Two decades of erosional, biogenic, cryogenic, and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption. We assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra, but now revegetated primarily with grasses and other plants, to evaluate hydrological modifications due to erosion and natural turbation. Eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50-fold. Between 1980 and 2000, apparent infiltration capacities of plots on the hillslope increased as much as ten fold, but remain approximately three to five times less than the probable pre-eruption capacities. Common regional rainfall intensities and snowmelt rates presently produce little surface runoff; however, high-magnitude, lowfrequency storms and unusually rapid snowmelt can still induce broad infiltration-excess overland flow. After 20 years, erosion and natural mechanical turbation have modulated, but not effaced, the hydrological perturbation caused by the cataclysmic eruption.

It is widely recognized that the effects of a phase shift of fine sediment in large-scale debris flows are likely to be large. Therefore, in numerical simulations, it is essential to describe fine sediments in the fluid phase, and not in the solid phase. Recently, the "Kanako" numerical simulator has been widely used for a variety of objectives, particularly because it has a graphical user interface. However, to date, there is no widely available numerical simulation model for large-scale debris flows that includes the effects of phase shifts. Here, we present a modified version of Kanako to describe this phase shift for fine sediment. In the new numerical simulator, which we refer to as Kanako-LS, we assume that sediments can be classified into two groups in terms of sediment diameter (fine and coarse), and define the critical diameter of the sediment (Dc) as the smallest diameter at which sediments behave as a solid phase. Then, we test the applicability of Kanako-LS using an example of debris flows triggered by a deep-seated rapid (catastrophic) landslide in Japan. Our results suggest that Kanako-LS may be useful for a variety of types of large-scale debris flow, particularly if the amount of fine sediment and the magnitude of the interstitial fluid turbulence are sufficient.

In order to clarify the erosion processes on a marly bare slope in the Southern Alps, the erosion processes in a steep and erodible slope composed of the Black Marls Formation were observed by a time-lapse video camera. The observations revealed that miniature debris flows (MDFs) occurred at the time of the rainfall-runoff event in which the most severe erosion took place in the whole observation period of 3 months. Analysis of the camera images showed some characteristics of the MDFs, and these are discussed in the context of real rainfall-runoff phenomena observed at the outlet of a small experimental basin including the visually observed slope. The following results were obtained. (1) A rough estimation of the total amount of sediment discharge by the MDFs showed that it was not negligible quantitatively. (2) The MDFs occurred only during the rising limb of the hydrograph during 6 minutes. (3) Based on this observation and a review of the literature, in a very steep and highly erodible slope, MDFs or similar phenomena might play an important role in the erosion and transport processes.

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