A Bidimensional Model of the Tagliamento River

The propagation of a flood wave is a very challenging topic, crucial in managing the flood risk. In the literature, several numerical models have been proposed to deal with this issue; most of them need the roughness coefficients to be assigned by the operator. The bottom roughness calibration of floodplains and channels represents a key point for flood studies, because it can heavily influence the results of any kind of numerical simulation. In this study, a numerical model is applied to the Tagliamento River, in North-East Italy. One of the main characteristics of this river is its natural environment, which changes from a very wide braided channel in the middle course to a narrow meandering river moving towards the sea. This makes the bed roughness extremely variable along the river, with different kind of vegetation, braiding, different grain size, meandering, etc. In this regard, particular care should be devoted to the roughness coefficient attribution and calibration. In the present paper, we present the detailed step of calibration and validation of a bidimensional numerical model on the Tagliamento River. A novel method to assign and calibrate roughness coefficient is introduced. Finally, the model is validated against two main flood events occurred in 1966 and 1996.

Formation and infill of the Late Glacial incised valley of Concordia Sagittaria (Tagliamento River, NE Italy)

<p>At the end of LGM the alluvial plains extending along the southern side of the Alps experienced a strong phase of fluvial entrenchment because of the impressive decrease of sedimentary input, related to the withdrawn of the Alpine glaciers within their valleys. Since 19 ka cal BP and up to Early Holocene, few incised valleys formed from the apex of the alluvial megafans to their distal sector but, along the northern Adriatic, the mid and late Holocene fluvial and coastal depositions have largely buried these landforms. During the Late Glacial the incised valleys were the only fluvial corridors where transport and deposition of sediments could occur in the whole plain.</p><p>We investigated the distal sector of the alluvial megafan of Tagliamento River through the analysis of a dataset consisting of ca. 2300 mechanical and hand-made cores. These data, compared with LIDAR-derived DEM, radiocarbon and paleoenvironmental analyses, allowed a detailed reconstruction of the formation and evolution of the buried incised valley characterizing the area of Portogruaro and Concordia Sagittaria. The valley has been traced for over 25 km, is up to 1.2 km wide and with a depth of 20 m below the top of LGM surface.</p><p>The erosive valley has been mainly formed between 19 and 14 ka cal BP, leading also to its partial infill with about 10 m of gravels, that arrived up to the present coast. The fluvial activity has been after limited to the deposition of fine sediment almost until the end of Late Glacial and, according to paleobotanical information, for the first time in the Venetian–Friulian Plain, these deposits recorded the vegetation of the Younger Dryas period.</p><p>After the disconnection from active Tagliamento, swampy environments occupied the valley bottom and the Holocene marine transgression started to indirectly affect the valley around 9.5 ka cal BP, contrasting the drainage and favouring the formation of widespread lacustrine environments. Since 8 ka cal BP lagoon entered in the valley and, following the sea-level rise, led to the deposition of a ca. 15 m thick unit of lagoon muds up to historical time. The infill of the valley documents the evidence of anthropogenic activity since 6-5 ka cal BP, probably in relation to wood clearance and soil degradation. Anyhow, significant human impact occurred during Iron and Roman Age, when Concordia became an important city. In 6<sup>th</sup> century AD high-magnitude floods deposited up to 5 m of sediments and largely obliterated the valley.</p><p>The detailed 3D reconstruction of the valley of Concordia allowed also to highlight the importance of the groundwater-fed streams in affecting the formation of this and other large incised valleys of Tagliamento. In particular, we produced evidence that river piracy by minor rivers triggered the creation of other incised valleys in the distal sector of Tagliamento megafan.</p><p>The buried incised valley of Concordia can represent a reference model also for describing the fluvial evolution of the other main Alpine rivers in the coastal sector of the whole Venetian-Friulian Plain during Late Glacial and Early Holocene.</p>

Mind the gap: extending the conceptual model of river island development to different environmental conditions and tree species

<p>We present recent results of field observations on an island braided reach of the middle Tagliamento River, Italy, where riparian vegetation survival and establishment depends on an unstable balance between vegetation growth and flood disturbance. We combined field observations and information extracted from aerial images, airborne lidar data, and river flow time series for the period 1986-2017 to investigate the changing spatial distribution of woody vegetation and the associated changes in river topography. We also explored the role of Alnus incana (a member of the Betulaceae family), in an environment dominated by the Salicaceae family (e.g. Populus nigra).</p><p>We observed that gaps between established islands and/or floodplain offer shelter to vegetation, supporting higher colonisation success and different vegetation-landform evolution pathways.</p><p>In particular, A. incana predominantly grows in lines along channel, island and floodplain edges, bordering wooded areas dominated by P. nigra. Given their association with floodplain and island edges and the relationship of taller (older) trees with more elevated surfaces, A. incana in the study reach appears to complement the physical engineering of the dominant species, P. nigra. This suggests that P. nigra may facilitate colonisation by alder but then both species trap sediments to aggrade channel edges and bar surfaces and build island and floodplain landforms.</p><p>Time sequences of aerial images in combination with the flood disturbance time series allowed us to interpret vegetation dynamics and to identify the fate of sexual and asexual reproduction strategies by observing vegetation expansion from lines of young plants and shrubs and from uprooted deposited trees and pioneer islands, respectively. Field observations are then generalized to extend a conceptual model of island development.</p><p>Growing conditions, disturbance energy, and time (window of opportunity) between major floods are the main controls on vegetation colonization. These vary among rivers, among reaches along the same river and locally, as in the investigated gaps, allowing different tree species with different life history traits (e.g. Populus nigra, Alnus incana) to engineer local river landforms in different and complementary ways.</p><p>Although the conceptual model is inspired by observations on the Tagliamento River, consideration of species life history traits and the joint influences of growing conditions, disturbance energy and windows of opportunity provide a framework that may be applied to other temperate rivers where trees drive landform development.</p>

Linking the 2030 Agenda for Sustainable Development to Research, Newspapers, and Governance: The Case of the Last Free-Flowing Alpine River

Are academic, newspaper and regulatory documents aligned with the United Nations Sustainable Development Goals and the Sendai Framework for Disaster Risk Reduction (SENDAI)? To answer this question, we develop a framework to compare the most commonly occurring keywords across these document types, as well as their use of Sustainable Development Goals and SENDAI keywords. The approach is tested in a case study on the Tagliamento River in the Italian Alps to explore the degree of communication among academia, newspapers and governance. Across the analyzed documents, we found disconnection between academic sources and regulatory documents. Occurrences of SDG-related keywords are positively correlated in regulatory documents and newspapers (r = 0.6), and in academic literature and newspapers (r = 0.38), indicating some degree of agreement. However, no correlation emerges between academic and regulatory documents, indicating a critical gap for communication and understanding between academic research and governance.

Numerical Modelling of Cohesive Bank Migration

River morphological evolution is a challenging topic, involving hydrodynamic flow, sediment transport and bank stability. Lowland rivers are often characterized by the coexistence of granular and cohesive material, with significantly different behaviours. This paper presents a bidimensional morphological model to describe the evolution of the lower course of rivers, where there are both granular and cohesive sediments. The hydrodynamic equations are coupled with two advection–diffusion equations, which consider the transport of granular and cohesive suspended sediment concentration separately. The change of bed height is evaluated as the sum of the contributions of granular and sediment material. A bank failure criterion is developed and incorporated into the numerical simulation of the hydrodynamic flood wave and channel evolution, to describe both bed deformation and bank recession. To this aim, two particular mechanisms are considered: the former being a lateral erosion due to the current flow and consequent cantilever collapse and the latter a geostatic failure due to the submergence. The equation system is integrated by means of a finite volume scheme. The resulting model is applied to the Tagliamento River, in northern Italy, where the meander migration is documented through a sequence of aerial images. The channel evolution is simulated, imposing an equivalent hydrograph consisting of a sequence of flood waves, which represents a medium year, with reference to their effect on sediment transport. The results show that the model adequately describes the general morphological evolution of the meander.

Influence of the Geometry Alteration of the Landslide Slope on its Stability: A Case Study in the Carnian Alps (Italy)

The paper presents the principles of the slope reprofiling and proves the effectiveness of this stabilization measure. The case study of two adjacent landslides in the National Road 52 “Carnica” in the Tagliamento River valley, the Carnian Alps (46°23′49″N, 12°42′51″E) are the example allowing for illustration of this approach. The phenomena have been studied for more than a decade, making it possible to carry out a detailed geological and geomorphological reconstruction. That was done on the basis of a large amount of monitoring data collected during that period. Since the landslides are threatening an important road, countermeasure works to ameliorate the stability conditions of the slides need to be designed. The paper focuses on the creation of a numerical model consistent with monitoring data and capable of reconstructing the dynamics of both landslides. Two cross-sections, one for each landslide, were selected for the analysis. The geometry of the slip surface was determined on the basis of control points, such as slip surface readings from inclinometers, and geomorphological evidence for the contour. The FLAC2D code was used to evaluate the current stability of these landslides and to determine the effectiveness of changing the slope geometry by removing material from the upper part of the slope and putting it to the lowest part as reinforcement.