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Debris flows are one of the most relevant natural hazards in mountain regions. A combination of structural (e.g. check-dams) and non structural (e.g. warning systems) measures is needed in most cases to reduce debris flow risks, and both benefit from experimental data on debris-flow characteristics. A new experimental site is being equipped in the Autonomous Province of Bolzano (Italy) for both monitoring purposes and testing warning systems. The study site (Gadria basin) is a small channel subjected to frequent debris flows. The overall system includes the following components: 4 rain gauges located at different elevations, 5 radar sensors and 5 geophones, and 3 video cameras and flash lights to record the propagation and the deposition of the debris flow. Transmission of data and alerts from the instruments will use radio technology because GSM coverage is not available in the basin.

Debris flows are efficient processes of sediment transfer from slope base to alluvial fans in mountain drainage basins. To advance current understanding of debris-flow sediment dynamics at the regional scale we analyze a historical (1998-2009) database of debris flows in 82 selected basins of the Alto Adige Province, north-eastern Italy. By combining historical information on event-based debris-flow volumetric deposition, high-resolution (LiDAR-derived) digital topography, and mapping of Quaternary sediment stores we are able to (i) characterize the principal topographic conditions at which debris-flow initiation, transportation, and deposition operate; (ii) detail debris-flow sedimentary signatures across spatial scales; and (iii) link debris-flow sediment flux to morphometry, lithologic variability, and sediment availability. We show that basin-wide specific yield obeys a negative scaling relation with basin area, which in turn is strongly controlled by dominant bedrock geology and abundance of Quaternary deposits. When the event-based sediment flux is subsumed across spatial scales, a more complex sedimentary signature is apparent, regardless of sediment availability. We hypothesize that this signature is characteristic of most debris-flow dominated settings.

This report provides the results of Work Package 7 (WP7). WP7 focuses on the integration of methodological approaches across scales and provides guidelines for policy development in the context of sediment and large wood management. Two guidelines with methodological explanations and good practice examples are provided as expected output of this WP. The contents of the first guideline are the estimation of sediment budget and scenarios determination to be used for flood mitigation in Alpine basins. The identification of morphological impacts related to hydropower plants and gravel extraction is dealt with in the second guideline. Sediment budget analysis consists in the evaluation of fluxes, sources and sinks from different processes. Different factors influence sediment and wood transport and need to be carefully evaluated within specific spatial and temporal scales. Correct approaches and tools to estimate sediment and wood budget that consider the river reach of interest, with its specific basin and channel processes in order to evaluate the availability of transportable material are focused on in the first guideline. Sediment mobility ranges from bedload (mountain and foothill reaches) to hyperconcentrated and debris flows (mountain reaches). Wood, is instead mainly delivered by floating, bank erosion processes and landslides. Different scenarios concerning the interaction with structures, sediment continuity, torrent-control strategies for sediment erosion and debris flows in relation to an efficient structure design are dealt with in the second part of the first guideline. In addition, sediment retention management and policy are discussed to highlight recent trends in the regulatory instruments. Dams represent one of the most significant human interventions in the hydrological cycle. Dams and reservoirs are built and operated for several purposes such as power generation, flood protection, drinking water or agricultural water supply, recreation, fishing, and others. The construction of a dam results in discontinuities to the river continuum; their presence can strongly interact with the sediment continuity from the upper parts of the basin to the area downstream of dams. While the 20th century focused on dam construction, the 21st will focus on sustaining the function of existing infrastructure as it becomes increasingly affected by sedimentation. The objective of sediment management is to manipulate the river-reservoir system to achieve sediment balance while retaining as much beneficial storage as possible, and minimizing environmental impacts and socioeconomic costs. Procedures for the identification of morphological impacts related to hydropower plants and gravel extraction are provided in the second guideline. The first part of the guideline is focused on reservoir sediment management plans and proposals for management improvement. Indicators for the evaluation of fluviomorphological impacts due to sediment removal or sediment supply reduction are presented in the second part of this guideline. Finally, recommendations and implications for policy, practitioners and for research are provided for both guidelines.

A very detailed geodatabase of floods, landslides and debris flows is available and kept up to date for the territory of the Autonomous Province of Bolzano, in northeastern Italy (7400 km2). The database is fed by field surveys carried out after the occurrence of instability phenomena and reports locations and attributes of the events for the last 15 years. The on-site analysis grants the gathering of important information and documentation resulting from field measurements and interviews to witnesses of the events. Particular attention is paid to the recognition of location, magnitude and path of landslides and debris flows. Data on shallow landslide locations and debris-flow initiation sites have provided the input to a regional-scale debris flow model aimed at the assessment of debris flow paths and inundation areas. A simple mass propagation model based on topographic attributes (Huggel et al., 2003) has been applied to simulate mass movement phenomena for selected basins using initiation sites listed in the regional database. In the model, debris-flow propagation is given in probability-related values representing the hazard potential for the selected locations. All the simulations have been carried out using a high-resolution (2.5m) LiDAR-derived Digital Terrain Model (DTM). The combination of a georeferenced database of landslides and debris flows and a simple topography-based model constitutes therefore a reliable tool for a fast and preliminary debris-flow hazard estimate, assessment and mapping. References: Huggel C, Kääb A, Haeberli W, Krummenacher B. 2003. Regional-scale GIS-models for assessment of hazards from glacier lake outbursts: Evaluation and application in the Swiss Alps. Natural Hazards and Earth System Sciences 3(6):647-662.

In mountainous catchments, the quantification of sediment yield is of paramount relevance for land-use planning and design of sediment control structures. However, deciphering the contribution of the different sediment transport processes (debris flows, debris floods and bedload transport) is often challenging as they are strongly controlled by basin morphometry, hydrological regime, and sediment supply. Therefore, long-term instrumental monitoring through catchment-scale sensor networks can provide precious information, especially if coupled with high-resolution topographical surveys. The Gadria catchment, located in the eastern Italian Alps, offers the possibility to perform a systematic monitoring of sediment transport processes. This catchment typically features several low-magnitude flood episodes and a few debris-flow events per year, from late spring to early fall. Starting from 2011, various instruments mainly devoted to debris-flow detection (geophones, video cameras, flow stage sensors) have been installed along the main channel, just upstream of a retention basin. High-resolution topographical surveys of the retention basin are carried out each year, at the beginning and at the end of the summer season and after debris-flow events. Rainfall is measured in the intermediate part of the catchment and in the headwaters, while PIT-tracing of bedload was performed in the main channel. In this work, we present the reconstruction of the sediment dynamics at the catchment scale during the 2014 and 2015 monitoring seasons. Instrumental monitoring was used to estimate the contribution of the different flow processes, and data from topographical surveys to quantify the transported volumes. Results show that (i) coarse sediment yield is driven by sporadic debris flows while flood events allow the continuous fine-sediment migration along the channel network; (ii) volume estimations may be significantly different - up to 30% lower - if performed through a DEM of Difference (DoD) analysis of the retention basin or by analyzing monitoring data; (iii) a multi-parametric monitoring is needed to decipher sediment dynamics at catchment scale.

Debris flow is an efficient process of sediment transfer from slope base to piedmont depositional fans in mountain drainage basins. To advance understanding of debris-flow sediment dynamics at the regional scale, we analyze a historical (1998–2009) database of debris flows from 77 basins of Alto Adige Province, northeastern Italy. By combining information on event volumetric deposition, high-resolution digital topography, and Quaternary sediment mapping we are able to link debris-flow sediment flux to morphometry, lithologic variability, and sediment availability. We show that basin-wide specific sediment yield (SSY) scales as an inverse power function of basin area. This function is strongly controlled by the way rock type and abundance of Quaternary deposits affect the rate of downstream sediment recruitment. When sediment flux associated with each debris-flow event is subsumed across discrete spatial increments of the entire region, a complex sedimentary signature in the area-SSY space is apparent. That is, SSY increases downstream up to areas as large as 1 km2, and starts to decline beyond this scale, regardless of sediment availability. We propose that this area-SSY relation is characteristic of debris flow–dominated settings.