Dynamics and functions of large wood have become integral considerations in the science and management of river systems. Study of large wood in rivers took place as monitoring of fish response to wooden structures placed in rivers in the central United States in the early 20th century, but did not begin in earnest until the 1970s. Research has increased in intensity and thematic scope ever since. Awide range of factors has prompted these research efforts, including basic understanding of stream systems, protection and restoration of aquatic ecosystems, and environmental hazards in mountain environments. Research and management have adopted perspectives from ecology, geomorphology, and engineering, using observational, experimental, and modelling approaches. Important advances have been made where practical information needs converge with institutional and science leadership capacities to undertake multi-pronged research programmes. Case studies include ecosystem research to inform regulations for forest management; storage and transport of large wood as a component in global carbon dynamics; and the role of wood transport in environmental hazards in mountain regions, including areas affected by severe landscape disturbances, such as volcanic eruptions. As the field of research has advanced, influences of large wood on river structures and processes have been merged with understanding of streamflow and sediment regimes, so river form and function are now viewed as involving the tripartite system of water, sediment, and wood. A growing community of researchers and river managers is extending understanding of large wood in rivers to climatic, forest, landform, and social contexts not previously investigated. © 2020 John Wiley & Sons, Ltd.

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Reflections on the history of research on large wood in rivers

Quantification of fluvial wood using UAVs and structure from motion

Connectivity has become an increasingly used concept in hydrological and sediment research. In order to quantify it, various indices have been proposed since the start of the 21st century including the index of connectivity. This index is based on a limited number of factors, the most important one being topography. Sediment connectivity indices values probably depend on the digital elevation model (DEM) resolution. The aim of this study was, first, to compare the effect of DEM pixel size (between 0.25 and 10m, using an UAV) in the Belgian loess belt, a lowland area. We show that the index values were lower when the pixel size decreased (a difference of about 20% in value between 0.25 and 10m). In addition, the impact of linear features in the watershed (e.g. grass strip, bank and road) was lower with the largest pixel sizes, and the connectivity pattern was affected with a pixel size of 5m or more. At lower pixel sizes (1m or below), some more disconnected regions appeared. These corresponded with zones where there had been water stagnation during and after rainfalls, and was corroborated by field observations. This confirmed the need for a proper resolution according to the objectives of the study. The second aim of this study was to deduce a minimum pixel size for connectivity study, helping local erosion or sedimentation location and consequent land management decisions. In our context, 1m stands as the optimum DEM resolution. This pixel size permitted location of all ‘key areas’ in terms of erosion. Very high resolutions (<0.5m) did not generate much more information, and their calculation time was far greater. Copyright © 2017 John Wiley & Sons, Ltd.

How pixel size affects a sediment connectivity index in central Belgium : Pixel size and weighting factor in a sediment connectivity index

Deglaciation controls on sediment yield: towards capturing spatio-temporal variability

The morphological response of the Tegnas alpine catchment (Northeast Italy) to a Large Infrequent Disturbance.

Sediment connectivity changes in an Andean catchment affected by volcanic eruption

https://iris.unito.it/bitstream/2318/1765590/3/Martinietal_20_Open.pdf

Assessing the effect of fire severity on sediment connectivity in central Chile.

Sediment connectivity underlies the relationships between sediment source areas and stream network and the use of topography‐based indices, such as the index of connectivity (IC), has become a common approach to investigate these relationships. However, few studies have tested the potential of the IC as an independent variable for predicting observable processes. In this study, the aim is the validation of the IC as a semi‐quantitative tool for depicting structural connectivity and predicting sediment dynamics linking sediment sources to the channel network after a large infrequent disturbance in the Rio Cordon catchment (Italy). First, the assessment of lateral connectivity was carried out using a combination of remote sensing data and field observations. In this way, it was possible to classify the sediment sources into disconnected, connected or functionally connected. Second, logistic regression analyses were performed using different IC variants, with different weighting factors and spatial resolutions as independent variables and connectivity status as dependent variable. Hence, the predictive capacity of the IC was tested and a threshold was derived to distinguish between low and high sediment connectivity. The results showed that only 15% of sediment sources are connected to the stream network. Moreover, the IC proved to be a reliable tool for depicting structural connectivity when using surface roughness as weighting factor and high‐resolution digital terrain models. On the contrary, it failed at predicting sediment dynamics as it did not classify correctly the functionally connected areas, so it cannot infer functional sediment connectivity in this case study. As a consequence, it was possible to obtain an IC threshold of −2.32, capable of differentiating between low and high structural sediment connectivity and useful for future management decisions. The threshold, in fact, represents an objective boundary between disconnected and connected sources in the Rio Cordon catchment.

Predicting sediment connectivity in a mountain basin: A quantitative analysis of the index of connectivity

To achieve a reliable analysis of the impacts induced in mountain basins by large and infrequent floods, all their main components, from the spatial-temporal distribution of meteorological agents to the hydrological, geomorphic and sedimentological response should be considered. Comprehensive study of the hydro-geomorphic responses is extremely valuable to increase the awareness of large floods, especially, in highly populated mountain areas. Such type of investigation requires a solid and wide dataset, which is why only few studies had the chance to describe the response in such a holistic way. This work comprehensively analyzed the high magnitude/low frequency Vaia event, a severe storm that affected northeastern Italy in October 2018 and thus the Rio Cordon study basin. The 80 h precipitation registered in the basin showed a total rainfall equal to 29.8% of the mean annual precipitation. The temporal distribution of rainfall presented two phases, i.e., a first characterized by moderate but persistent precipitation and a second more intense, exhibiting recurrence intervals over 50 years. A combination of indirect methods permitted the reconstruction of the hydraulic forcing acted in the Rio Cordon. Despite the implicit uncertainty, these methods clearly highlighted the high magnitude expressed by October 2018 flood, which generated a unit peak discharge equal to 3.3 m3 s−1 km−2 and a peak of unit stream power of 3865 W m−2, i.e., the highest hydraulic forcing conditions ever observed in 34 years. In terms of geomorphic changes, the use of pre- and post-event LiDAR data stressed out a moderate response of the hillslopes, where the (re)activation of the sediment sources was limited. Only few of these acted as sediment supplier to the main channel. The channel network instead, exhibited an evident response, with the Rio Cordon severely altered by wide lateral widening, deep streambed incision and armouring removal. The hydraulic and geomorphic forcing generated by October 2018 flood caused extensive streambed remobilization and boulder mobility that, in turn, induced the transport of a massive sediment volume. In this sense, the match between bedload observed and bedload predicted suggested the outstanding hydraulic forcing occurred and the quasi-unlimited supply conditions acted during the flood. The long-lasting monitoring program maintained in the Rio Cordon basin provided the rare opportunity to compare two high magnitude/low frequency floods and their induced effects. The September 1994 and October 2018 events were caused by different rainfall conditions, which resulted in different hydrological- and, especially, geomorphic- and sedimentological-responses. Particularly, the October 2018 flood induced an unprecedented alteration on the fluvial system, the effects of which could persist over the long-term.

Lorenzo Martini

Papers

Sediment connectivity changes in an Andean catchment affected by volcanic eruption

The morphological response of the Tegnas alpine catchment (Northeast Italy) to a Large Infrequent Disturbance.

Assessing the effect of fire severity on sediment connectivity in central Chile.

Predicting sediment connectivity in a mountain basin: A quantitative analysis of the index of connectivity

Hydrological, geomorphic and sedimentological responses of an alpine basin to a severe weather event (Vaia storm)