Hazard index maps for woody material recruitment and transport in alpine catchments

Abstract

Abstract.A robust and reliable risk assessment procedure for hydrologic hazards deserves particular attention to the role of transported woody material during flash floods or debris flows. At present, woody material transport phenomena are not systematically considered within the procedures for the elaboration of hazard maps. The consequence is a risk of losing prediction accuracy and of underestimating hazard impacts. Transported woody material frequently interferes with the sediment regulation capacity of open check dams and moreover, when obstruction phenomena at critical crosssections of the stream occur, inundations can be triggered. The paper presents a procedure for the determination of the relative propensity of mountain streams to the entrainment and delivery of recruited woody material on the basis of empirical indicators. The procedure provided the basis for the elaboration of a hazard index map for all torrent catchments of the Autonomous Province of Bolzano/Bozen. The plausibility of the results has been thoroughly checked by a backward oriented analysis on natural hazard events, documented since 1998 at the Department of Hydraulic Engineering of the aforementioned Alpine Province. The procedure provides hints for the consideration of the effects, induced by woody material transport, during the elaboration of hazard zone maps.

Tracking log displacement during floods in the Tagliamento River using RFID and GPS tracker devices

“…Nevertheless, wood in-channel could be a threat to some human activities on rivers, including disruption of the navigation on large rivers and commercial marine operations (Piégay, 2003). Moreover, if wood is transported massively during highmagnitude flood events, it could represent a risk for sensitive structures such as bridge piers (Mazzorana et al, 2009;Rigon et al, 2012). Given the important positive and potentially very negative effects of wood in rivers, increasing attention has been given to the assessment of wood budget at the catchment and reach scales (Sedell and Froggatt, 1984;Palik et al, 1998;Piégay et al, 1999;Marcus et al, 2011;Schenk et al, 2013).…”

Physical and numerical modelling of a bedload deposition area for an Alpine torrent

“…Studies like Mazzorana et al (2009), emphasize the importance of a qualitative and quantitative assessment of woody debris recruitment and transport during flood events. However, for this study no data on woody debris yield to the fan is available, so test cases including woody debris are based on rough estimates.…”

Modelling woody material transport and deposition in alpine rivers

“…An analysis of recent flood events documented in the Province of Bolzano showed that wood vegetation in the river bed has a higher relative tendency to produce woody material (depending on wood stand volume) compared to wood vegetation on stream banks. In absolute terms, the amount of woody material recruited from the stream banks exceeded the volume recruited from the riverbank (Mazzorana et al 2009). This fact can be attributed to the wood stand volume being on average larger on the bank slopes than in the river bed.…”

“…These wood-covered areas within a buffer width are categorised as active wood buffer (AWB) areas. This buffer width corresponds either to the potential tree height derived from the forest typology map (Klosterhuber et al 2007) or to the real tree height retrieved from the Lidar-based digital surface model and the digital terrain model (Mazzorana et al 2009). The difference between the surface model and the terrain model can give hints about the reference tree height and therefore about the width of the active wood buffer.…”

“…By a qualitative analysis of documented flood events (Mazzorana et al 2009), the following assessment procedure for woody material recruitment could be established (see Fig. 4): (1) Identification and spatial delimitation of possible woody material recruitment areas, (2) assignment of these recruitment areas to the appropriate river morphology categories (e.g., stream bed, river banks, flood plain), (3) determination of the wood structure characteristics and definition of the respective structure typologies (see Table 1), (4) calculation of an indicator describing the hydraulic impact (Eq.…”