Spatial, frequency and magnitude distribution variability of debris flow processes in the Alps:relationships with various boundary conditions

ESPP Doctoral School – 1st General Meeting

Sara Savi (TOPO-EUROPE, SedyMONT), Bern, Switzerland

Spatial, frequency and magnitude distribution variability of debris flow processes in the Alps:

relationships with various boundary conditions.

Introduction

Debris flows are rapid, gravity-induced mass movements consisting mostly of granular solids, soil debris and water that involve large volumes of material. Accordingly, debris-flow processes are among the most hazardous phenomena in mountain catchments. An improved understanding about the mechanisms of sediment production, sediment transfer and surface processes dynamics within a catchment is the prerequisite if the aim is to develop scenarios about future possible events.

The aim of this project is to analyze how the combination of environmental effects (e.g., precipitation rates) and intrinsic landscape properties have affect the magnitude-frequency distribution of debris-flow processes in a catchment located at the foothills of the Alps. More specifically it aims to understand the relationships between climate, sediment production, geological setting and the dynamics of sediment transfer and surface processes. Special attention will be focused on elaborating the timescales of sediment transfer in mixed debris-flow/fluvial catchments.

Studied Sites and Methodology

Research is carried out in two different catchments in the Alps: the Schimbrig catchment located at the northern foothills of the Alps near Entlebuch, central Switzerland, and the Zielbach catchment situated in the Southern Alps near Merano, North-East Italy. The two sites differ in the geological framework and the catchment’s size and thus in the timescales of sediment transfer. Both catchments, however, share the observation that sediment has been transferred by both floods and debris- flows. According to the differences in the catchment’s sizes (and thus in the contrasts of timescales of sediment transfer), we decided to apply two different quantitative approaches including dendrogeomorphology (at Schimbrig) and 10Be of river-born sand (Zielbach).

The Schimbrig catchment, located in the central Swiss Alps, is 2.5 km2 large and underlain by the sandstone-mudstone alternations of the Eocene Subalpine Flysch, and partially by Oligocene conglomerates of the Subalpine Molasse and Cretacous limestone-mudstone alternations exposed in the Helvetic thrust nappes. The largest volumes of sediment, however, are sourced in the Flysch. The elevations of the catchment range from 975 m a.s.l. at the outlet to 1780 m a.s.l at the drainage divide. The most important geomorphologic feature is the Schimbrig landslide, located in the eastern tributary segment, that is a >300’000 m2-large earth slide > 5-10 m thick. This landslide experienced a phase of high slip rates >2m day-1 between September 1994 and May 1995, transferring a total of 350’000 m3 of material. In contrast, the western side is characterized by a network of >50 m-deeply incised channels bordered by hillslopes that host shallow- and deep-seated landslides that measure <15’000 m2.

We asses timescales of sediment transfer using the technique of dendrogeomorphology that allow us to precisely restore the chronology of debris-flow processes during the past 200 years and to map the travel path of these flows. This methodology is based on the concept of “process – event – response” for a certain number of trees. When a tree is affected by an event (rock fall, landslide, debris flow etc.) it reacts in distinct ways by adjusting the growth rate. This reaction depends on the nature of the damage and the tree species. For instance, debris flows result in partial embedding of the lowermost stem segment by sediment, which results in an instantaneous reduction of the growth rates. In this way it’s possible to restore the time when sediment transfer altered the tree growth, and the nature of the transfer of sediment (e.g., debris flows, rock falls, floods).

The Zielbach catchment, located in the Südtirol in northern Italy, measures ca. 40 km2. The catchment hosts 1,3 km2 large glaciers in the uppermost reach. The highest point of the drainage basin reaches an elevation of 3337 m a.s.l (Mt. Rosso) and descends to 505 m a.s.l., where the trunk stream debouches into the Adige River. The bedrock comprises schists and high-grade gneisses of crystalline basement of Austroalpine metamorphic unit. The last recorded debris flow occurred in August 2008 and moved a mass of 70.000 m3 that reached the Adige River destroying 5 bridges along.

We use concentrations of 10Be of river-born sand to measure spatial patterns of denudation rates within the basin. In addition, concentrations of 10Be in soils permit us to calculate erosion rates on hillslopes that have been decoupled from the channel network. Accordingly, 10Be concentrations measured for hillslope samples, and for the bedload in the channel network, allows us to quantify the time scales of sediment transfer from the hillslopes into the channel network. In addition, the combination of data established from the 10Be technique and information about the chronology and magnitude of debris flow processes (measured by our Italian colleagues) will allow us to evaluate the importance of debris-flow versus fluvial processes on the sediment budget of the Zielbach catchment. Moreover, using the results of geomorphic and geologic mapping will allow us to precisely allocate the sediment sources for debris flows.

Preliminary results

In the past year the research was focused on mapping the geologic architecture and the nature of sediment transfer (and the resulting landforms) in both catchments, and on collecting 10Be and dendrochronologic samples.

Preliminary analysis of the dendrochonologic samples indicate that 64% of the trees grew up between 1900 and 2009; 34% of the tree samples showed germination dates between 1800 and 1900, and the

remaining 2% of the sampled specimens germinated before 1800. Dendrogeomorphic analyses depict that nearly 50% of the sampled trees were affected by debris-flow activity in the 1990s. This period of high activity might be related to enhanced sediment transfer to the river system in response to the high slip rates of the Schimbrig earth slide between 1994 and 1995, as documented by historical records in the region (e.g., Schwab et al., 2008, Geomorphology). Other periods of enhanced debris-flows activity seem to have occurred around 1960, 1950 and at the beginning of 20th century. The 10Be preparation is time consuming and will be part of this year’s activity.