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During the last decades, several Italian gravel-bed rivers suffered from different types and levels of human\ud pressures that changed their morphological and vegetation patterns. This study aims to analyze the medium-\ud and short-term evolution of vegetation cover, fluvial islands and main channel characteristics along a gravel-\ud bed reach of the Piave River (North-eastern Italy), with special emphasis on the changes in the vegetation\ud cover type and structure. The Piave River experienced different types of human disturbance such as gravel min-\ud ing, hydropower schemes, and land use changes. A sequence of aerial photographs (1960, 1970, 1982, 1991,\ud 1999, 2006, 2010 and 2012) have been analyzed to detect the medium-term (about 50 yr) evolution of riparian\ud vegetation along a reach about 30 km-long in the middle course of the Piave River. In addition, LiDAR data (2003)\ud and seven repeated topographic surveys (done between 2007 and 2011) have been used to quantify changes in\ud three cross sections on a sub-reach about 2 km-long, in order to analyze the channel evolution over a short period\ud (1–8 yr). The medium-term analysis revealed that changes in the river evolutionary trend depended on the\ud variations in human activities both in the main channel and at basin scale. In fact, during the last five decades\ud there has been a consistent and continuous increase of riparian vegetation within the river corridor, from around\ud 50% up to 68%, which corresponds to a continuous decrease from around 46% to 29% in the area of exposed gravel\ud and low flow channels. Considering the different vegetation cover types, there was a predominant increase of sta-\ud ble and tall vegetation from around 34% up to a maximum of around 67% (1999), then a slight decrease to around\ud 62% (2010 and 2012). After a slight recovery phase subsequent to the cessation of gravel mining in the late 1990s\ud and associated to flood events in the early 2000s, the Piave River appears to have been in an equilibrium phase in\ud terms of bed elevation, planform morphology and areas of vegetation, which is becoming taller and more mature,\ud as a result of the combination of flow regulation, reduced bedload input from upstream and lack of relevant flood\ud events. These results suggest that in highly regulated — in terms of flow and sediment fluxes — rivers the cessation\ud of gravel mining alone is not sufficient to revert a degradation trend and thus restore prior morphological patterns

The possibility of analysing the characteristics and volume of inchannel large wood (LW) is of importance for river management but the traditional manual field activities are usually time-consuming and not easy to apply at a larger spatial scale. This paper presents an alternative and faster method to detect the characteristics and measurements of large wood in rivers by using the terrestrial laser scanner (TLS) technology. Field-measurements data and TLS scans were collected in August 2013 along 14 ha of the Piave River (Italy) analysing 230 and 208 woody elements for the manual method and the TLS one, respectively. TLS data were processed using the Cyclone 7 software and the LW measurements were extracted adopting two specific tools. The resulting low margin of error in the comparison between field data and those derived from TLS surveys confirmed the ability of TLS in the detection of large wood and wood jams characteristics. The greatest deviations were found for wood jams height which the TLS showed a tendency to overestimate (+24.37%) and LW length with a slight underestimation (19.76%). Considering the wood volume, the relative difference between the TLS and manual method was within a negligible margin of error of ±7%. Characteristics and measurements of LW in rivers can be obtained from TLS surveys, but some progress in this technique is still needed to allow a better management of the 3D point cloud and a faster extraction of the wood measurements. The proposed method represents an alternative tool for faster and repeated surveys of wood characteristics in a complex river environment, ensuring a reliable quantification of spatial and temporal variation of wood volume

No abstract

River islands are defined as discrete areas of woodland vegetation located in the riverbed and surrounded by either water-filled channels or exposed gravel. To be defined as such, they also have to exhibit stability over time and remain exposed during bank-full flows. River islands are very important from both a morphological and an ecological point of view, as they represent the most natural state of a fluvial system and are to a large degree influenced by human pressure. This study aims to analyze morphological and vegetation characteristics of three types of river islands, pioneer, young and stable, and their dynamics after different flood events in the Tagliamento River, a natural gravel bed river with highly natural conditions. The Tagliamento River, with a length of 178 km, is the main river of the Friuli-Venezia Giulia region, and it represents a link between Alpine and Adriatic zones. Because of the limited human pressure, the main course of Tagliamento is characterized by highly complex fluvial dynamics. The analyses were carried out in two sub-reaches characterized by different dominant morphology but equally low human pressure. Island dynamics, in relation with flood events, have been analyzed using aerial photos taken over two consecutive years, 2010 and 2011. Mean elevation of surface and maximum height of vegetation were estimated for each river island using digital models obtained from two LiDAR datasets. The results suggest that river islands lie at different elevations, and this implies a different resistance capacity during flood events. Pioneer river islands and young river islands lie at lower elevations than stable islands causing a total or partial incapacity to survive during considerable flood events. In most cases, the islands’ typology (pioneer, young, or stable) is cancelled by ordinary floods. Stable islands lie at higher elevations, and only intense and infrequent flood events, i.e. recurrence interval (RI) >10–15 years, are able to determine substantial erosions. The presence of high vegetation, in some of our study cases higher than 30 m, contributes to increase the resistance of stable islands. River islands are very dynamic elements, strictly associated with the occurrence of major floods and to the morphological characteristics of the fluvial system. The main morphological changes have been documented in the braided sub-reach; the highly dynamic response to changes of the fluvial reach also affects the islands stability. However, the different resistance capacity during flood events of pioneer, young, and stable islands as well as the different bank erosion capacity are important aspects to better understand dynamics of fluvial islands. In this sense, the study could be helpful to predict the hydraulic hazard linked to the morphological changes and to formulate more effective flood management programs