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Abstract: Landsliding not only changes the morphology of the terrain but is also an effective factor in sediment production. Extreme events such as earthquake-induced large rock falls or rainfall-induced landslides may, during the event itself or in a period after it, greatly increase the sediment delivery and river sediment transport. The effects of strong earthquakes and large landslides on sedimentation are evident in the Upper Soča River basin, NW Slovenia. The annual sediment production in the headwaters of this typical alpine valley in western Slovenia is estimated to average>1000 m3 km−2 a−1. In the last few years, two large landslides triggered there were of comparable volume but differed regarding sediment delivery to the fluvial network. The 2000 Stože landslide at 1.5 million m3 was a debris flow that delivered more than 1 million m3 to the Koritnica River. The Strug complex landslide was initiated in 2001 by a rock slide and soon amounted to an estimated volume of 310,000 m3. In 2002, it released over 20 rainfall-induced debris flows on the order of 100 to 1000 m3 to the fluvial system. In mountainous terrain in the headwaters of the Upper Soča River, slope failures may occur during strong earthquakes depending on the event magnitude. The earthquake of April 12, 1998 (M S 5.6) caused more than 100 failures, among them 50 rock falls. About 260,000 m3 of rock fall material will remain on hill slopes with no potential of reaching the river network, while an estimated volume of 480,000 m3 may be released to watercourses in a longer period during extreme events. About 200,000 m3 of rock fall debris was deposited in areas from where the material was released to watercourses during rainfall events. As a consequence of the 1998 earthquake, hyperconcentrated flows were observed during floods in some torrential tributary channels of the Upper Soča River. After the earthquake of July 12, 2004 (M S 4.9), 50 rather superficial slope failures including 38 rock falls were registered. From 2000 until 2004, more than 150,000 m3 sediments were dredged from the Upper Soča River channel. [Copyright &y& Elsevier]

Abstract: Large rock slides frequently cause secondary unstable phenomena. Many such instability processes happened after December 2001, when the Strug rockslide estimated at 95,000 m3 was triggered above the Koseč village near Kobarid in the Julian Alps, W Slovenia. It was initiated at the contact between high permeable calcareous rocks (Cretaceous scaglia) thrusted over nearly impermeable clastic rocks (Cretaceous flysch). Soon after the rockslide initiation, a rock fall with a volume of 45,000 m3 was initiated within the rockslide. The kinetic push of the rock fall caused the movement of a translational soil landslide with a volume of 180,000 m3 that partially slipped into the torrential ravine of the Brusnik Stream. After a sudden drop of 15 m in December 2001, the rockslide average velocity exponentially slowed down to less than 10 m/year till the end of 2002, and came to a practical stillstand in 2003. After the rainfall in spring 2002, small debris flows made of clayey gravels with a volume of up to 1000 m3 started to flow from the zone of accumulation of the rock fall over the soil landslide to and along the channel of the Brusnik Stream. In 2002, more than 20 debris flow events were registered. The statistical analysis of the measured local rainfall intensities showed that debris flows were initiated at daily rainfall reaching from 20 to 30 mm, depending on the antecedent precipitation. This value may be taken as a specific hydrologic threshold for this site. Because in 2003 no more debris flows were registered, a conclusion was drawn that debris flow events were rainfall-induced but governed on the same time by the availability of rock fall debris in its zone of accumulation. [Copyright &y& Elsevier]

In two events, on November 15 and 17, 2000, near the Mangart Mountain (2679 m a.s.l.), NW Slovenia, two translational landslides (debris flow slides) with a total volume of more than 1.5 million m3 occurred on the Stozˇe slope composed of morainic material filled with silt fraction. The first landslide was associated with a dry and the second landslide with a wet debris-flow, respectively. The rain gauging station in the village of Log pod Mangartom recorded 1638.4 mm of rainfall (more than 60% of the average annual precipitation) in the 48 days before the events (rainfall intensity of 1.42 mm/h in 1152 h). The recorded rainfall depth has a recurrence interval of more than 100 years. Other recorded rainfall depths of shorter duration (481.6 mm in 7 days, 174.0 mm in 24 h, 70 mm in 1 h) have recurrence intervals of much less than 100 years. A hydrological analysis of the event showed that the increase in runoff coefficients during the wet period in autumn 2000 before the landslide was as high as two- to threefold. An analysis using natural isotopes of δ18O and tritium of water samples from the Stozˇe landslide area has shown permanent but slow exfiltration of underground waters from a reservoir in the slope. In the case of low-intensity and long-duration rainfall in autumn 2000, relatively low permeable (10−7 m/s) morainic material was nearly saturated but remained stable (average porosity 21%, water content 20%, liquid limit 25%) until high artesian pressures up to 100 m developed in the slope by slow exfiltration from the relatively high permeable (10−5 m/s) massive dolomite. The Stozˇe landslide (two debris flow slides) was triggered by high artesian pressures built in the slope after long-duration rainfall. The devastating debris-flows formed from the landslide masses by infiltration of rainfall and surface runoff into the landslide masses and by their liquefaction. [Copyright &y& Elsevier]