Abstract. This paper presents numerical simulations of free surface flows induced by a dam break comparing the shallow water approach to fully three-dimensional simulations. The latter are based on the solution of the complete set of Reynolds-Averaged Navier-Stokes (RANS) equations coupled to the Volume of Fluid (VOF) method.The methods assessment and comparison are carried out on a dam break over a flat bed without friction, a dam break over a triangular bottom sill and a dam break flow over a 90 • bend. Experimental and numerical literature data are compared to present results.The results demonstrate that the shallow water approach, even if able to sufficiently reproduce the main aspects of the fluid flows, loses some three-dimensional phenomena, due to the incorrect shallow water idealization that neglects the three-dimensional aspects related to the gravity force.

Unmanned Aerial Vehicles (UAVs) are now filling in the gaps between spaceborne and ground-based observations and enhancing the spatial resolution and temporal coverage of data acquisition. In the realm of hydrological observations, UAVs play a key role in quantitatively characterizing the surface flow, allowing for remotely accessing the water body of interest. In this paper, we propose a technology that uses a sensing platform encompassing a drone and a camera to determine the water level. The images acquired by means of the sensing platform are then analyzed using the Canny method to detect the edges of water level and of Ground Control Points (GCPs) used as reference points. The water level is then retrieved from images and compared to a benchmark value obtained by a traditional device. The method is tested at four locations in an artificial lake in central Italy. Results are encouraging, as the overall mean error between estimated and true water level values is around 0.05 m. This technology is well suited to improve hydraulic modeling and thus provides reliable support to flood mitigation strategies.

The inspection of strategic works such as dams is of vital importance both for their maintenance and for the safety of downstream populations. The reduced accessibility, both for uptake needs and for their strategic nature, and the large time needed for an inspection by traditional method do not facilitate the investigation of this type of structures. The new unmanned aerial vehicle (UAV) technology, equipped with high-performance cameras, allows for rapid photographic coverage of the whole dam system. Apart from the placement on the structure of a high number of markers, the correct geo-referencing and validation of the model also requires an important terrestrial topographic campaign by total station, Global Positioning System and laser scanner. Punctual, linear and surface analysis shows the high accuracy of the drone acquiring technique. The product is suitable for a detailed survey of the conservation status of the materials and the complete metric reconstruction of the dam system and the adjacent land. The present work explains firstly a UAV acquisition and then the first dense point cloud validation procedure of a concrete arch gravity dam. The Ridracoli dam is the object of the survey, located in the village of Santa Sofia in central Italy.

A large-scale floodplain delineation algorithm is applied to identify potentially inundated areas at the basin scale. The model, which mainly uses a digital elevation model (DEM) and design flood peak discharge at the outlet as input data, is implemented within a geographic information system (GIS). It implements a preliminary GIS-based terrain analysis framework for estimating the stream network, surface flow direction and drainage grids, while the core algorithm implements an automated fluvial cross-section extraction for discharge and flow height estimation. The delineation is then implemented by filtering the floodplain cells as those cells whose elevation is lower than the corresponding channel flow height. The proposed 'hydrogeomorphic floodplain', obtained on the Tiber River basin (approx. 17 000 km ) avec la reconnaissance topographique à 90 m de résolution mondiale par le radar de la navette spatiale de la NASA (SRTM) est comparée aux cartes officielles des inondations de l'Autorité du bassin du fleuve Tibre. L'étude de cas présentée montre la possibilité d'utiliser un algorithme de SIG automatisé et les données de télédétection largement disponibles pour l'identification préalable de l'empreinte des plaines inondables à l'échelle mondiale.

Abstract:In this paper, we investigate the performance of three-dimensional (3D) hydraulic modeling when dealing with river sinuosity and meander bends. In river bends, the flow is dominated by a secondary current, which has a key role on the flow redistribution. The secondary flow induces transverse components of the bed shear stress and increases the velocity in outward direction, thus generating local erosion and riverbed modifications. When in river bends, the 3D processes prevail, and a 3D computational fluid dynamics (CFD) model is required to correctly predict the flow structure. An accurate description of the different hydrodynamic processes in mildly and sharply curved bends find a relevant application in meanders migration modeling. The mechanisms that drive the velocity redistribution in meandering channels depend on the river's roughness, the flow depth (H), the radius curvature (R), the width (B) and the bathymetric variations. Here, the hydro-geomorphic characterization of sharp and mild meanders is performed by means of the ratios R/B, B/H, and R/H, and of the sinuosity index. As a case study, we selected the Malpasset dam break on the Reyran River Valley (FR), as it is perfectly suited for investigating performances and issues of a 3D model in simulating the inundation dynamics in a river channel with a varying curvature radius.

This paper investigates the accuracy of models obtained by drone surveys. To this end, this work analyzes how the placement of ground control points (GCPs) used to georeference the dense point cloud of a dam affects the resulting three-dimensional (3D) model. Images of a double arch masonry dam upstream face are acquired from drone survey and used to build the 3D model of the dam for vulnerability analysis purposes. However, there still remained the issue of understanding the real impact of a correct GCPs location choice to properly georeference the images and thus, the model. To this end, a high number of GCPs configurations were investigated, building a series of dense point clouds. The accuracy of these resulting dense clouds was estimated comparing the coordinates of check points extracted from the model and their true coordinates measured via traditional topography. The paper aims at providing information about the optimal choice of GCPs placement not only for dams but also for all surveys of high-rise structures. The knowledge a priori of the effect of the GCPs number and location on the model accuracy can increase survey reliability and accuracy and speed up the survey set-up operations.

This paper presents the hydrological analysis and grain size characteristics of fluvial sediments in a river basin and their combination to characterize a flood event. The overall objective of the research is the development of a practical methodology based on experimental surveys to reconstruct the hydraulic history of ungauged river reaches on the basis of the modifications detected on the riverbed during the dry season. The grain size analysis of fluvial deposits usually requires great technical and economical efforts and traditional sieving based on physical sampling is not appropriate to adequately represent the spatial distribution of sediments in a wide area of a riverbed with a reasonable number of samples. The use of photographic sampling techniques, on the other hand, allows for the quick and effective determination of the grain size distribution, through the use of a digital camera and specific graphical algorithms in large river stretches. A photographic sampling is employed to characterize the riverbed in a 3 km ungauged reach of the Tescio River, a tributary of the Chiascio River, located in central Italy, representative of many rivers in the same geographical area. To this end, the particle size distribution is reconstructed through the analysis of digital pictures of the sediments taken on the riverbed in dry conditions. The sampling has been performed after a flood event of known duration, which allows for the identification of the removal of the armor in one section along the river reach under investigation. The volume and composition of the eroded sediments made it possible to calculate the average flow rate associated with the flood event which caused the erosion, by means of the sediment transport laws and the hydrological analysis of the river basin. A hydraulic analysis of the river stretch under investigation was employed to verify the validity of the proposed procedure.

This study examined the potential of using high-resolution image data acquired by micro-UAV (unmanned aerial vehicle) for analysing large-scale environmental and hydraulic parameters. A procedure based on image data acquisition and processing is discussed and applied to a test case, Trasimeno Lake. This lake was selected for this study because of its importance in cultural, environmental and social context. This study focused on the temporal and spatial scale extension of reed beds (Common Reed). The Common Reed can strongly influence the ecological balance of the lake and particularly its evapotranspiration levels, which significantly influence the lake's water balance. Moreover, the lake provides valuable ecosystem services, including phytoremediation water treatment and the absorption of carbon (carbon sink). A presentation of the study area from a geographical and morphological perspective is provided; then, the platform (UAV and sensors) and the processing system of the images are discussed. The results concerning the accuracy of the method for the construction of georeferenced orthophotographs, and the preliminary results obtained from flights performed at different times and growing seasons of reed beds of Trasimeno Lake, are presented. This study also provides convincing evidence that the orthophotographs obtained with a low-cost UAV system can be used to analyse changes in the extension of the Phragmites australis (Common Reed) of Trasimeno Lake, and the alteration of some environmental parameters of the vegetation, in a temporal and spatial scale. It also highlights that this system can be used to facilitate analyses performed by traditional remote sensing method.