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<div> <div> <p><span data-contrast="auto">By creating pools and retaining sediment and organic matter, instream wood provides habitats for a vast variety of different species. It creates a complex river bed and is essential for a healthy ecosystem (Wohl et al., 2019). However, during extreme weather conditions, floods can mobilize the wood and transport it, causing a hazard to downstream infrastructure. Therefore it is important better understand river wood dynamics, such as storage and transport regimes. These regimes are influences by individual log characteristics (e.g. shape, density and orientation), but also individual river weather, climate and geographical factors. In the last decade, an increasing amount of case studies have been performed, although still limited in amount of logs tracked in European rivers (Wyzga et al., 2017). In our current contribution, we deploy a tracking and monitoring system in an Alpine river in the canton of Vaud, Switzerland. The Avancon the Nant is located in the Vallon de Nant, a valley that has been protected since 1969 (Vittoz and Gmür, 2009), and can therefore be argued to have a close to natural wood regime.</span></p> <p><img src="" alt="" /></p> <div> <div> <p><span data-contrast="auto">Figure: Locations of instream wood in 2022 as compared to 2021. In grey, 3 special sections (wider sections and sections with multiple streams) of river are represented.</span></p> </div> </div> <div> <div> <p><span data-contrast="auto">In the summer of 2021, 948 (0001 to 0948) pieces of instream wood were tagged with a unique number and 2 unique RFID tags. One year later, in another field campaign, the movement of the pieces was assessed (see figure). From the pieces that have been recovered (7% were lost), a total of 20 pieces were found to have moved with an average of 260 meters. These movements took place in specific sections, primarily in single-threaded narrow sections. The two lower special river sections (w1 and w2) were found to contain pieces with a larger diameters as compared to the other sections. As the tree density decreases when moving up the river, also the total volume of wood storage and the amount of pieces decreased. Furthermore, more pieces with a high degree of decat were found as compared to fresher pieces. This indicated that in recent years, less wood recruitment has taken place.</span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">REFERENCES </span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">Vittoz, P., & Gmür, P. 2009: Introduction aux Journées de la biodiversité dans le Vallon de Nant (Bex, Alpes vaudoises), </span><span data-contrast="auto">Mémoire de la Société vaudoise des Sciences naturelles, 23, 3-20.</span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">Wohl, E., Kramer, N., Ruiz-Villanueva, V., Scott, D. N., Comiti, F., Gurnell, A. M., Piegay, H., Lininger, K. B., Jaeger, K. L., Walters, D. M., & Fausch, K. D. 2019: The natural wood regime in rivers, BioScience, 69, 259–273.</span><span data-ccp-props="{"> </span></p> </div> <div> <p><span data-contrast="auto">Wyzga, B., Mikus, P., Zawiejska, J., Ruiz-Villanueva, V., Kaczka, R. J. & Czech, W. 2017: Log transport and deposition in incised, channelized, and multithread reaches of a wide mountain river: Tracking experiment during a 20-year flood, Geomorphology, 279, 98-111.</span><span data-ccp-props="{"> </span></p> </div> </div> </div> </div>

<p>Large wood (LW) has earned increased attention as a component of fluvial systems as its ecological and physical benefits, as well as its contributions to damages during flood events, have been realized. As LW found in river networks had originated from outside of the channel corridor, significant efforts have been made to identify recruitment processes that supply LW to channels. Evidence has proved treefall, landslides, bank erosion, debris flows, and fluvial entrainment contribute to LW recruitment. Prediction and identification of the areas prone to these processes are very challenging but could serve to better understand wood dynamics. Therefore, identifying areas prone to recruitment processes, estimating available LW, and determining LW connections in a watershed will help design management strategies aimed at mitigating LW’s impacts as well as provide insight on the movement and recruitment of LW in fluvial systems. Analogous challenges exist when dealing with sediment dynamics.</p><p>We applied the graph theory (GT) to instream LW supply and transfer. A GT is a set of nodes representing different entities (i.e., wood sources) with edges connecting nodes based on determined relationships (i.e., wood recruitment processes). The GT proves useful in exploring landscape connectivity with the capability of identifying critical nodes or regions, measuring properties of connectivity, identifying process coupling based on spatial patterns, and defining related geomorphological processes such as that of sediment cascades in which landscape components are coupled based on properties effecting sediment transfer.</p><p>GT proves capable of defining connections between LW recruitment from hillslopes to the channel and from channel segment to channel segment. Currently, the fuzzy logic toolbox presented by Ruiz-Villanueva and Stoffel (2018) has been utilized to delineate the connected, recruitment process prone areas for landslides, debris flows, and bank erosion in the study area of Vallon de Nant, Canton of Vaud, Switzerland. The delineated areas have been used in ArcPro in coalition with vegetation data to extract hillslope-to-channel connections and channel-to-channel connections. The channel or fluvial network has been segmented based on the presence of features which reduce downstream transfer of LW such as channel widening and presence of obstructions. The determined connections will be applied in the R package, igraph, to extract network properties of the constructed, instream LW GT model.</p><p>GT aids complex network analyses by providing a technique which retains only the critical information. Therefore, following the rigorous work of determining the system components, connections, and constructing the graph model, additional analysis can be performed with streamlined performance. Through our graph representation of instream LW supply and transfer, we plan to use the mathematical framework and algorithms from graph theory to further our understanding of instream LW such as the likely origins based on cost analysis.</p><p>“This work is supported by the SNSF Eccellenza project PCEFP2-186963 and the University of Lausanne.”</p><p>Ruiz-Villanueva, V., Stoffel, M. (2018). Application of fuzzy logic to large organic matter recruitment in forested river basins. Proceedings of the 5th IAHR Europe Congress New Challenges in Hydraulic Research and Engineering, 467-468. doi:10.3850/978-981-11-2731-1_047-cd</p>

<p>Instream large wood (LW), downed trees, trunks, root wads, branches, and fragments of wood at least one meter in length and ten centimeters in diameter, contribute to maintaining rivers´ physical and ecological integrity. Additionally, LW can be hazardous to infrastructure, which makes management of LW a priority for local services. Post-analysis of flood events has demonstrated recruitment of LW through several mechanisms such as bank erosion, landslides, and debris flows. Such recruitment and transfer of LW to fluvial networks vary spatially and temporally, which makes predictions of these processes challenging.</p><p>The current studies lack a method to express the structure of supply and transfer of LW throughout a catchment. An improvement would be to provide a means to interpret the structural connectivity of LW or the potential impact one recruitment area may have on another. Therefore, we propose to apply the graph theory to construct the network of LW recruitment from its source location to the fluvial network and to a designated outlet. In so doing, we aim to identify critical spatial connections that have previously not been realized, provide a means to view LW recruitment and transfer for an entire catchment wholistically, and explore the application of the graph theory in the realm of LW, which has not previously been done.</p><p>The graph theory requires a simple sequence for input data. The sequence is constructed based on physical connections that exist in nature. For instance, a forested region which has been determined to be intersected by a landslide prone area would represent a source of LW and the process which would supply the LW to the channel would be the landslide. Graph theory interprets the source area of LW as well as the location along the river to which the landslide connects as two unique nodes. The mechanism supplying the LW, in this case landslides, would represent an edge that would connect the two nodes. This same principal is then applied to the entire network for all pertinent recruitment mechanisms. The network can then be analyzed for a variety of different purposes including the identification of critical hot spots where multiple processes may be coinciding or locations along the channel where LW recruitment is relatively absent. Each node can additionally be given attributes which can later be extracted, such as the amount of LW available.</p><p>Determination of recruitment process prone areas has been based on previously developed methods for landslides, bank erosion, and debris flows. Swiss inventories of the forested area have been applied to delineate the forested area.</p><p>This contribution will show preliminary results done in the Vallon de Nant, Canton of Vaud, Switzerland. The catchment is relatively small (approx. 13km<sup>2</sup>) with active landslide, debris flow, bank erosion, and avalanche processes which supply LW. Current findings indicate landslide and bank erosion processes to be more widespread throughout the catchment with debris flows occurring more intermittently. The constructed LW network will provide a means to relate different recruitment mechanisms to one another and to LW found in rivers.</p>

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