Francesco Macchione

The reliability of flood event simulations is a crucial aspect in terms of saving lives. The worst flood scenarios derive from possible dam failures. For earthen dams, the breaching mechanism, triggered by overtopping or piping, is affected by several modelling uncertainties, which propagate downstream and may influence flood hazard mapping. In this work, the hypothetical failure of a real earthen dam was simulated. Two hypothetical dam‐breach hydrographs were computed using a simplified physically based model and propagated downstream, showing that the peak discharge undergoes an attenuation process which causes the differences between the two scenarios to progressively decrease with increasing distance from the dam. Beyond a certain distance, the attenuation of these differences is such that a large portion of the flooded area falls into the same hazard classes, even though the hydraulic parameters of the two different scenarios do not completely coincide. Therefore, this result is important for its practical implications for hazard assessment. For the analysed case study, uncertainties in the dam‐breach hydrograph had little influence in terms of downstream hazard mapping, because most of the exposed area was located beyond this distance. In similar cases, uncertainties in the hydrograph peak have a limited effect on hazard assessment.

A significant portion of Europe is prone to flooding, including severe events occurring over very small areas. Recent flood hazard mapping methods can cover large regions, but often fail to capture processes driven by small streams or direct rainfall. This study presents the authors’ experience in the application of a fully hydrodynamic model over an entire territory, with direct rainfall input (rain-on-grid approach at the basin scale). The case study is the Neto River basin in Calabria (Italy), covering approximately 1000 km², a region that represents an ideal natural laboratory for investigating flash flood processes in Europe. Simulations were carried out using the TUFLOW 2D commercial modelling tool. A key objective is to demonstrate that the Chicago hyetograph enables a constant return period across the entire domain. Additionally, specific procedures are proposed to represent numerous minor crossings (e.g., small bridges, culverts, and road and railway underpasses) and dam outlets without refining the computational grid or abandoning the Shallow Water Equations (SWE). This approach allows identification of major river floods, flash floods, runoff-related hydraulic effects, and pluvial flooding. Results show that the fully hydrodynamic rain-on-grid model is highly effective for flood hazard mapping, with strong agreement between simulations and observed events, confirming its predictive reliability and enabling high-resolution, comprehensive territorial analysis.

The failure of a dam is a catastrophic event, which is expected to induce drastic changes to the downstream bed morphology. However, knowledge about channel response due to dam-breach flood wave and its impact on flood hydrodynamics and hazard assessment is still poor. Therefore, we studied the river morphodynamics associated with dam-breach floods, in order to understand the possible extent and distribution of the erosion depths and deposition heights, as well as the consequences of these changes on the flood hydrodynamics and, above all, on the hazard assessment. In this work, these impacts were investigated by simulating the hypothetical failure of a real embankment dam, using a simplified physically based model for the computation of the dam-breach hydrograph, and a two-dimensional morphodynamic model for the downstream wave propagation. To account for some of the uncertainties related to both models, two breach shape scenarios and three mobile bed configurations were considered. It was found that bed erodibility affected the flood hazard much more than the breach shape. Furthermore, the mobile bed configuration significantly affected river morphology in the narrow and steep part of the river, where both erosion depths and deposition heights were very high, while bed elevation changes in the downstream plain part of the river were significantly less. Finally, consideration of river morphodynamics in dam-breach flood wave propagation has been shown to shorten the arrival times of the flood wave and lead to more intense flood hazard due to a remarkable increase in the unit discharge.

Obstructions at bridge can exacerbate flood hazard: when woody debris occlude a bridge, backwater increases, and sometimes a temporary reservoir can form. If the condition for the collapse of the jam exists, the phenomenon can evolve into a dam break flow resulting in an increase in downstream discharge or a steepening in the flow hydrograph. A rigorous description of backwater formed by debris at the bridge, and more generally of the interaction between flood and structure, is still a challenge and only few studies focus on the breakage phase and its consequence on flood dynamic. Unfortunately, analyzing the hydraulics of so many localized situations within a large area in detail would involve considerable modeling and calculation costs. Therefore, treatment methods are desirable that provide correct results in terms of the reference hydraulic variables for assessing the hazard without the need to model extremely local phenomena. The purpose of this study is to evaluate the possibility to reproduce an observed or estimated backwater by the use of an equivalent roughness at the place of the bridge and simulate the breakage phase by a sudden variation of its value. The method, applied to the numerical reconstruction of the experimental tests of dam break performed by the Waterways Experiment Station in 1961, has provided results that overlap well with the experimental values. Furthermore, it was applied to the reconstruction of the historical flood occurred in the Soverato river (Calabria, Italy). It did not require local grid refinement and provided results faithful to flood marks.

The Hydrologic Engineering Centre-River Analysis System (HEC-RAS), developed by the US Army Corps of Engineers, is one of the most known, analyzed and used model for flood mapping both in the scientific literature and in practice. In the recently released version (release 5.0.7), the HEC-RAS model has been enriched with novel modules, performing fully 2-D computations based on the 2-D fully dynamic equations as well as the 2-D diffusion wave equations; moreover the application of rainfall to each cell of the two-dimensional domain is now possible. Contrarily to the common applications for flood propagation in river reach, this specific module has never been analyzed in the literature. Therefore, the main purpose of this work is to assess the potential and the capabilities of the 2-D HEC-RAS model in rainfall-runoff simulations at the basin scale, comparing the results obtained using both the options (fully dynamic equations and diffusion wave equations) to the simulations obtained by using a 2-D fully dynamic model developed by the authors for research purposes. Both models have been tested in a small basin in Northern Italy to analyze the differences in terms of discharge hydrographs and flooded areas. The application of a criterion for hazard class mapping has shown significant variations between the two models. These results provide practical indications for the water engineering community in the innovative research field related to the use of 2-D SWEs at the basin scale.

This paper is focused on the contribution that the two‐dimensional Shallow Water Equations could provide in respect to the fields of research devoted to the river networks analysis. The novelty introduced in this work is represented, in particular, by the hydraulic characterization of the river drainage networks, starting from flow patterns simulated by the two‐dimensional Shallow Water Equations on high‐resolution digital elevation model (DEM) through the analysis of water depths flowing down the hillslopes, channelized in both the main river and in all the tributaries or stored in small depressions. The first finding of this research is the determination of scaling laws that describe the relations between the water depth threshold, used to identify the network cells, and a dimensionless area, related to the total area of the network cells themselves. The observed bimodal scaling behavior has been considered as representative of the flow patterns belonging to the channel networks (CN) and hillslope plus channel networks (HCN), the physical and geomorphological interpretation of which has been provided from a multifractal point of view. In particular, the analysis of the multifractal spectra highlights significant variations in the multifractal signatures, between the CN and the HCN structures, leading to the proposal of a novel criterion for channels heads detection that has provided encouraging predictions of field observations.