For millennia, fluvial dikes (also called levees) have been built along long stretches of the world's rivers to protect population and property from flooding (Ward et al., 2017). However, failure of fluvial dikes leads to devastating human, economic, and environmental consequences worldwide (e.g.,

A nonintrusive, high-resolution laser profilometry technique (LPT) has been developed for continuous monitoring of the three-dimensional (3-D) evolving breach in laboratory models of noncohesive fluvial dikes. This simple and low-cost setup consists of a commercial digital video camera and a sweeping red diode 30 mW laser projecting a sheet over the dike. The 2-D image coordinates of each deformed laser profile incident on the dike are transformed into 3-D object coordinates using the direct linear transformation (DLT) algorithm. All 3-D object coordinates computed over a laser sweeping cycle are merged to generate a cloud of points describing the instantaneous surface. The DLT-based image processing algorithm uses control points and reference axes, so that no prior knowledge is needed on the position, orientation, and intrinsic characteristics of the camera, nor on the laser position. Because the dike is partially submerged, ad hoc refraction correction has been developed. Algorithms and instructions for the implementation of the LPT are provided. Reconstructions of a dike geometry with the LPT and with a commercial laser scanner are compared in dry conditions. Using rigid dike geometries, the repeatability of the measurements, the refraction correction, and the dike reconstruction have been evaluated for submerged conditions. Two laboratory studies of evolving fluvial dike breaching due to flow overtopping have been conducted to demonstrate the LPT capabilities and accuracy. The LPT has advantages in terms of flexibility and spatiotemporal resolution, but high turbidity and water surface waves may lead to inaccurate geometry reconstructions.

With the surge of extreme meteorological events and intensification of urbanization downstream of hydraulic structures, the need for predicting failure of dams and dikes has become of paramount importance for establishing emergency response procedures (Zhong et al., 2021). To this end, numerical models are instrumental tools for simulating the embankment breaching process. Existing models can be classified into three categories. First, statistical (or parametric) models are based solely on regression analysis of data from past events or laboratory campaigns. They describe some breaching parameters (e.g., final breach width, failure duration or maximum breach discharge) as a function of dam or reservoir properties. These simple, computational-efficient models may lack generality because they entirely rely on data from specific cases without considering underlying physics (Chen et al., 2019;De Lorenzo & Macchione, 2014;Lee, 2019). Conversely, distributed physically based models can describe the phenomenon in greater detail, as they solve the flow and sediment governing equations using a computational mesh of the domain. Their results may be accurate but only if reliable data is available and if physical processes are numerically well represented, for example, erosion of non-homogeneous dam material, slope failure or 3D-flow patterns (Cantero-Chinchilla et al., 2019;Onda et al., 2019;Pheulpin et al., 2020;Shimizu et al., 2020). Additionally, the time required to run distributed physically based models can be substantial (ASCE, 2011). Simplified physics-based models offer a good trade-off (Wu, 2013). Without spatially distributing the flow description nor the embankment morphology, they enable simulating hydraulic and dam breach variables (e.g., time-evolution of breach discharge and dimensions) by describing selected physical processes (

<p>Failure of fluvial dykes often leads to devastating consequences in the protected areas. Overtopping flow is, by far, the most frequent cause of failure of fluvial dykes. Numerical modeling of the breaching mechanisms and induced flow is crucial to assess the risk and guide emergency plans.</p><p>Various types of numerical models have been developed for dam and dyke breach simulations, including 2D and 3D morphodynamic models (e.g., <em>Voltz et al.</em>, 2017 ; <em>Dazzi et al.</em>, 2019 ; <em>Onda et al.</em>, 2019). Nevertheless, simpler models are a valuable complement to the detailed models, since they enable fast multiple model runs to test, e.g. a broad range of possible breach locations or to perform uncertainty analysis. Moreover, unlike statistical formulae, physically-based lumped models are reasonably accurate and remain interesting in terms of process-understanding (<em>Wu</em>, 2013 ; <em>Zhong et al.</em>, 2017 ; <em>Yanlong</em>, 2020).</p><p>Nonetheless, existing lumped physically-based models were developed and tested mostly in frontal configurations, i.e. for the case of breaching of an embankment dam and not a fluvial dyke. Despite similarities in the processes, the breaching mechanisms involved in the case of fluvial dykes differ due to several factors such as a loss of symmetry and flow momentum parallel to the breach (<em>Rifai et al.</em>, 2017). Therefore, there is a need to assess the transfer of existing lumped physically-based models to configurations involving fluvial dyke breaching.</p><p>Here, we have developed a modular computational modeling framework, in which we are able to implement various physically-based lumped models of dyke breaching. In this framework, we started with our own implementation of the model presented by <em>Wu</em> (2013) and we incorporated a number of changes to the model. Next, we evaluated the model performance for a number of laboratory and field tests covering both frontal (<em>Frank</em>, 2016; <em>Hassan and Morris</em>, 2008) and fluvial (<em>Rifai et al.</em>, 2017; 2018; <em>Kakinuma and Shimizu</em>, 2014) configurations. The modular framework we have developed proves also particularly suitable for testing the sensitivity and uncertainties arising from assumptions in the model structure and parameters.</p>

<p>Overtopping of fluvial dikes occurs frequently during major floods and may lead to dike failure, with severe consequences in the protected areas. Mechanisms of fluvial dike breaching remain incompletely understood, while predicting the breach hydrograph is of paramount importance for the flood risk management.</p><p>Here, we present a new series of laboratory experiments, in which the evolving 3D fluvial dike geometry was monitored in detail using the laser profilometry technique. The experimental setup extends over about 20 m by 7 m and accommodates a 15 m long main channel and a 7 m-long dike section. The facility is located at LNHE of EDF-R&D (France). The present study extends former experiments by Rifai et al. (2017, 2018), which were conducted with uniform coarse sand (d<sub>50</sub> = 1.03 mm). In the new tests, various mixtures of coarse (d<sub>50</sub> = 1.03 mm) and fine (d<sub>50</sub> = 0.24 mm) sands were used as dike material (Rifai et al., 2020). The fraction of fine sand was varied systematically to assess its influence on the breaching process, specifically as regards the apparent cohesion.</p><p>The experimental observations reveal that the frequency of breach slope collapse tends to decrease as the fraction of fine sand is increased; but the collapsing volumes become larger. Consequently, in the tested configurations, the addition of fine sand to the dike material has virtually no effect on the overall breaching dynamics, due to compensation between less frequent but larger collapsing material volumes. In the presentation, the relative importance of the effects will be discussed in comparison with other influencing parameters such as the main channel discharge, floodplain backwater effects and the dike geometry.</p><p>All experimental data, including high resolution 3D dynamic models of the breach geometry, are publicly available online (Rifai et al., 2019).</p><p><strong>References</strong></p><p>Rifai, I., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., El kadi Abderrezzak, K., & Dewals, B. (2017). Overtopping induced failure of noncohesive, homogeneous fluvial dikes. Water Resources Research, 53(4), 3373-3386.</p><p>Rifai, I., El kadi Abderrezzak, K., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., & Dewals, B. (2018). Floodplain backwater effect on overtopping induced fluvial dike failure. Water Resources Research, 54(11), 9060-9073.</p><p>Rifai, I., El kadi Abderrezzak, K., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., & Dewals, B. (2019). Flow and detailed 3D morphodynamic data from laboratory experiments of fluvial dike breaching. Scientific data, 6(1), 53.</p><p>Rifai, I., El kadi Abderrezzak, K., Hager, W.H., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., & Dewals, B. (2020). Apparent cohesion effects on overtopping-induced fluvial dike breaching. Journal of Hydraulic Research. In press. https://doi.org/10.1080/00221686.2020.1714760.</p>