Sediment characterisation and analysis - WP 5 & WP 6 report SedInOut Project

Contemporary atmospheric temperature rise imposes profound physical changes to high- elevation mountain environments. These changes, by directly affecting the mountain cryosphere through glacier retreat and permafrost degradation, can alter the hydrologic regime of mountain drainage basins (Huggel et al., 2015), as well as the amount and caliber of sediment readily available for mass movements and fluvial transport. Although sediment supply exerts a primary control on channel stability and relevant geo-hazard potential, there is a general lack of standard procedures for characterizing sediment sources, sediment typology and for evaluating sediment availability. This gap, which is mainly associated with the variety of existing national and regional protocols of data collection, currently prevents pursuing an unbiased, transnational risk management strategy considering current climatic challenges. SedInOut, through a joint international effort, aims to develop methodologies for the quantification and characterization of sediment across representative pilot catchments, towards a sustainable land management that values geo-risk mitigation and sediment recycling.

In this report, we present a methodological approach that relies on existing geological mapping (CARG project), high-resolution digital topography, and historical aerial photos and orthophoto mosaics, while integrating field-based and proximal sensing data in conjunction with multi- temporal, remotely based mapping (Figure 1). Remotely sensed procedures include multi- temporal mapping of glacier extent (Section 3.1.1), Quaternary materials (e.g., bedrock, glacial till, talus debris, colluvium, and alluvium) (Section 3.1.2), the drainage network (Section 3.1.3) and rapid shallow failures (i.e., sediment sources including debris slides, debris flows and bank collapses) (Section 3.1.4). Field-based and proximal sensing data include measurements on shallow landslide geometry (Section 3.2.1), and characterization of surface (i.e., manual Wolman pebble count and photo sieving) and subsurface (i.e., bulk sampling, on-site preliminary sieving, and laboratory sieving) grain size distribution (GSD) conducted at six representative sites (i.e., M1 through M6; Figure 2a) along the mountain channel network that drains the glaciated landscape of Upper Mazia Valley (Sections 3.2.2, 3.2.3, 3.3, and 3.4). Field measurements on landslide geometry are critical for constraining an empirical landslide area-volume relation, which in turn is used for translating landslide areas, as mapped on sequential photo sets, into first-order volumetric estimates of mobilized debris. GSD data allows characterizing the spatial variability of characteristic sediment calibers (i.e., D50, D84, and D90) as well as the armoring ratio (an index of channel stability), starting at glacier and rock glacier fronts and moving downstream.

The multi-temporal mapping approach is structured as follows. Through visual inspection of sequential orthophoto sets, we first track changes in glacier extent. Subsequently, as glaciers retreat, we map and quantify the extent of newly exposed Quaternary materials, the occurrence of shallow rapid failures, and the relevant changes in the structure of the main drainage network.

In this document, we illustrate SedInOut methodological approach applied to Mazia Valley, here regarded as representative of conditions that characterize the Austroalpine geologic domain. In particular, we integrate two nested spatial scales: (i) Upper Mazia Valley (18.8 km2) over which we conducted extensive fieldwork to constrain the geometry of rapid shallow failures on the hillslopes and low-order streams, and to characterize alluvial sediment along the channel network (Figure 2a); (ii) Proglacial Mazia Area (8.4 km2), where we document decadal geomorphological changes following the retreat of the Mazia glacier (Figure 2b). Time and temporal scales of investigation are summarized in Table 1.