References of "Pirotton, Michel"
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See detailFlow field in shallow reservoir with varying inlet and outlet position
Ferrara, Velia; Erpicum, Sébastien ULiege; Archambeau, Pierre ULiege et al

in Journal of Hydraulic Research (in press)

Shallow reservoirs are used for multiple purposes, such as storm water retention and trapping of sediments. Reliable predictions of the flow fields are necessary to inform the design and operation of ... [more ▼]

Shallow reservoirs are used for multiple purposes, such as storm water retention and trapping of sediments. Reliable predictions of the flow fields are necessary to inform the design and operation of these structures. Using numerical simulations, we performed a systematic analysis of the influence of the location of the inlet and outlet on the flow fields developing in rectangular shallow reservoirs of various sizes. Depending on the relative location of the inlet and outlet with respect to the reservoir centreline, contrasting flow patterns are obtained, involving either no flow reattachment, or a jet reattached on either of the reservoir sidewalls. The results reveal also the occurrence of bi-stable flow configurations, i.e. different steady state flow fields are reached depending on the flow history. This is of high relevance for the design of shallow reservoirs as such configurations should certainly be avoided to achieve a robust hydraulic sizing of the reservoir. [less ▲]

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See detailHydraulic determination of dam releases to generate warning waves in a mountain stream: performance of an analytical kinematic wave model
Stilmant, Frédéric; Pirotton, Michel ULiege; Archambeau, Pierre ULiege et al

in Journal of Hydraulic Engineering (in press)

In this case study, we study the generation of warning waves with prescribed characteristics in a mountain stream. We determine which dam release will generate the desired warning wave. We solve this ... [more ▼]

In this case study, we study the generation of warning waves with prescribed characteristics in a mountain stream. We determine which dam release will generate the desired warning wave. We solve this inverse problem following a two-model approach. An analytical kinematic model is used for a preliminary design of the dam release and a detailed two-dimensional (2D) fully dynamic model is used to converge to the final solution. Although the presented case study is far from an idealized academic case, the analytical model performs well and, beyond its role for preliminary design, turns out to be of prime interest for both understanding and discussing the results of the detailed 2D model. The complex interactions between the release hydrograph, the geometry of the river and the friction formula are brought to light by the analytical model, which highlights the complementarity of both models and the usefulness of such a two-model approach. [less ▲]

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See detailMaximum energy dissipation to explain velocity fields in shallow reservoirs
Westhoff, Martijn; Erpicum, Sébastien ULiege; Archambeau, Pierre ULiege et al

in Journal of Hydraulic Research (in press)

Shallow reservoirs are often used as sediment traps or storage basins, in which sedimentation depends on the flow pattern: Short rectangular reservoirs reveal a straight jet from inlet to outlet with on ... [more ▼]

Shallow reservoirs are often used as sediment traps or storage basins, in which sedimentation depends on the flow pattern: Short rectangular reservoirs reveal a straight jet from inlet to outlet with on both sides identical recirculation zones. In longer reservoirs, the main jet reattaches to the side of the reservoir leading to a small and a large recirculation zone. Previous studies found an empirical geometric relation describing the switch between these two flow patterns. In this study, we demonstrate, with a simple analytical model, that this switch coincides with a maximization of energy dissipation in the shear layer between the main jet and recirculation zones: Short reservoirs dissipate more energy when the flow pattern is symmetric, while longer reservoirs dissipate more energy with an asymmetric pattern. This approach enables to predict the flow patterns without detailed knowledge of small scale processes, potentially useful in the early phase of reservoir design. [less ▲]

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See detailPressure and velocity on an ogee spillway crest operating at high head ratio: experimental measurements and validation
Peltier, Yann; Dewals, Benjamin ULiege; Archambeau, Pierre ULiege et al

in Journal of Hydro-Environment Research (in press)

This paper aims at validating pressure and velocity measurements conducted in two physical scale models of an ogee spillway crest operating at heads largely greater than the design head. The design head ... [more ▼]

This paper aims at validating pressure and velocity measurements conducted in two physical scale models of an ogee spillway crest operating at heads largely greater than the design head. The design head of the second model is 50% smaller than the one of the first model. No pier effect or air venting is considered in the study. The velocity field is measured by Bubbles Image Velocimetry. The relative pressure along the spillway crest is measured using pressure sensors. Comparison of measured velocities between both spillways indicates low scale effects, the scaled-profiles collapsing in most parts of the flow. By contrast, measurements of relative pressure along the spillway crest differ for large heads. A theoretical velocity profile based on potential flow theory and expressed in a curvilinear reference frame is fitted to the velocity measurements, considered as reference, for extrapolating the velocity at the spillway crest. Comparing the extrapolated velocity at the spillway crest and the velocity calculated from the relative pressure considering a potential flow finally emphasizes that bottom pressure amplitudes seem overestimated for the larger spillway, while an averaging effect might operate for the pressure measurements on the smaller spillway. [less ▲]

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See detailProtection contre l’affouillement du barrage de Poses-Amfreville : études sur modèles réduit et numérique
Sixdenier, Jean Philippe; Shaiek, Salah; Erpicum, Sébastien ULiege et al

in Loudière, Daniel (Ed.) Hydraulique des barrages et des digues (2017, December)

The worrying development of a scouring downstream of the Poses dam decided Voies Navigables de France to carry on tests on experimental and numerical hydraulic models in order to design a suitable ... [more ▼]

The worrying development of a scouring downstream of the Poses dam decided Voies Navigables de France to carry on tests on experimental and numerical hydraulic models in order to design a suitable protection solution. In a first step, the study enabled to identify the operation situations for which scouring risk was the most important. Second, a rip rap protection made of 3-6 t blocks on a length of 30 m downstream of the dam has proved to be able to resist the whole realistic flow solicitations. [less ▲]

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See detailExperimental investigation of fluvial dike breaching due to flow overtopping
El Kadi Abderrezzak, Kamal; Rifai, Ismail; Erpicum, Sébastien ULiege et al

Conference (2017, December)

The failure of fluvial dikes (levees) often leads to devastating floods that cause loss of life and damages to public infrastructure. Overtopping flows have been recognized as one of the most frequent ... [more ▼]

The failure of fluvial dikes (levees) often leads to devastating floods that cause loss of life and damages to public infrastructure. Overtopping flows have been recognized as one of the most frequent cause of dike erosion and breaching. Fluvial dike breaching is different from frontal dike (embankments) breaching, because of specific geometry and boundary conditions. The current knowledge on the physical processes underpinning fluvial dike failure due to overtopping remains limited. In addition, there is a lack of a continuous monitoring of the 3D breach formation, limiting the analysis of the key mechanisms governing the breach development and the validation of conceptual or physically-based models. Laboratory tests on breach growth in homogeneous, non-cohesive sandy fluvial dikes due to flow overtopping have been performed. Two experimental setups have been constructed, permitting the investigation of various hydraulic and geometric parameters. Each experimental setup includes a main channel, separated from a floodplain by a dike. A rectangular initial notch is cut in the crest to initiate dike breaching. The breach development is monitored continuously using a specific developed laser profilometry technique. The observations have shown that the breach develops in two stages: first the breach deepens and widens with the breach centerline being gradually shifted toward the downstream side of the main channel. This behavior underlines the influence of the flow momentum component parallel to the dike crest. Second, the dike geometry upstream of the breach stops evolving and the breach widening continues only toward the downstream side of the main channel. The breach evolution has been found strongly affected by the flow conditions (i.e. inflow discharge in the main channel, downstream boundary condition) and floodplain confinement. The findings of this work shed light on key mechanisms of fluvial dike breaching, which differ substantially from those of dam breaching. These specific features need to be incorporated in flood risk analyses involving fluvial dike breach and failure. In addition, a well-documented, reliable data set, with a continuous high resolution monitoring of the 3D breach evolution under various flow conditions, has been gathered, which can be used for validating numerical models. [less ▲]

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See detailAvailable tools for the quantification of the evolution of future flood risk in Wallonia
Bruwier, Martin ULiege; Archambeau, Pierre ULiege; Pirotton, Michel ULiege et al

Conference given outside the academic context (2017)

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See detailInfluence de la géométrie du quadrant amont et comportement hydraulique sous forte charge des seuils profilés standard
Erpicum, Sébastien ULiege; Pirotton, Michel ULiege; Blancher, Benoit et al

in Loudière, Daniel (Ed.) Hydraulique des barrages et des digues (2017, November)

In order to study the variation of the hydraulic characteristics of ogee-crested weirs for upstream heads higher than the design head, several profiles have been tested on an experimental facility ... [more ▼]

In order to study the variation of the hydraulic characteristics of ogee-crested weirs for upstream heads higher than the design head, several profiles have been tested on an experimental facility specifically designed and instrumented. These profiles have been defined regarding the real profiles of spillways of dams operated by EDF as well as the most used geometry of the literature. The experimental tests have been supplemented by numerical modeling, whose results were found to be very similar. The analysis shows that the discharge coefficient increases with the upstream head until head ratios around 5.5. This evolution follows very well the power equations from the literature. For higher head ratios, the discharge coefficient abruptly decreases from a value around 0.6 to a value around 0.55. This decrease corresponds to the apparition between the main flow and the weir of a low velocity area (the lower nappe does no more correspond to the weir profile). The analysis also shows a significant reduction of the discharge coefficient (around 10%) if the upstream quadrant design head is higher than 2 times the design head of the downstream quadrant. On the contrary, no modification of the discharge coefficient is measured if the upstream quadrant design head is smaller than the downstream quadrant one. [less ▲]

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See detailPhysical modeling of overtopping induced fluvial dike failure: effects of the channel flow and floodplain innundation
Rifai, Ismail; Erpicum, Sébastien ULiege; Archambeau, Pierre ULiege et al

Conference (2017, August)

Fluvial dikes have been constructed as flood defense structures, but their failure may lead to casualties and major damages in the protected areas. Flow overtopping is listed as the main cause of dike ... [more ▼]

Fluvial dikes have been constructed as flood defense structures, but their failure may lead to casualties and major damages in the protected areas. Flow overtopping is listed as the main cause of dike failure. An accurate assessment of the breach evolution is a prerequisite to a sound flood risk assessment and management. in contrast, the current knowledge of the physical processes involved in fluvial dike failure by overtopping remains highly fragmented. This paper presents experimental tests on the breaching of homogenous sand-dikes in a fluvial configuration, i.e. the flow is parallel to the longitudinal dike axis. Overtopping is initiated over a pilot notch at the dike crest. Experiments are performed for different inflow discharges in the main channel and under different levels of floodplain confinement. The transient evolution of the dike geometry is monitored using a laser profilometry technique. The detailed reconstruction of the breach formation and expansion provides key insights into the mechanisms involved in the failure of fluvial dikes. Results show that the breach development differs highly from experiments disregarding the flow parallel to the dike axis (i.e. frontal configuration) as in the fluvial configuration the breach develops mainly in the flow direction and the floodplain water level strongly conditions the breach horizontal and vertical extent. [less ▲]

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See detailInfluence of urban patterns on flooding
Bruwier, Martin ULiege; Mustafa, Ahmed; Aliaga, Daniel et al

in E-proceedings of the 37th IAHR World Congress (2017, August)

The goal of this paper is to identify the respective influence of different characteristics of urban patterns on urban flooding. A set of 2,290 alternate urban patterns was generated randomly using an ... [more ▼]

The goal of this paper is to identify the respective influence of different characteristics of urban patterns on urban flooding. A set of 2,290 alternate urban patterns was generated randomly using an urban generator tool providing the geometry of buildings and their relative location to the ground, over a square area of 1 km². Steady 2-D hydraulic computations were performed for these 2,290 different urban patterns with identical hydraulic boundary conditions. The computational time was reduced by using an anisotropic porosity model. This model uses relatively coarse computational cells; but preserves information from the detailed topographic data through the use of porosity parameters. Based on the computed maps of waterdepths for the 2,290 urban patterns, a sensitivity analysis based on a multiple linear regression was performed to outline the most influential urban characteristics. [less ▲]

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See detailModèles de porosité pour les inondations urbaines
Dewals, Benjamin ULiege; Bruwier, Martin ULiege; El Saeid Mustafa, Ahmed Mohamed ULiege et al

Scientific conference (2017, July 25)

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See detailOn the transition between symmetric and asymmetric flow in rectangular shallow reservoirs - a case of maximum energy dissipation?
Westhoff, Martijn; Erpicum, Sébastien ULiege; Archambeau, Pierre ULiege et al

Conference (2017, June 28)

Shallow reservoirs are often used as sediment traps or storage basins, in which sedimentation depends on the flow pattern. Short rectangular reservoirs reveal a straight jet from inlet to outlet with ... [more ▼]

Shallow reservoirs are often used as sediment traps or storage basins, in which sedimentation depends on the flow pattern. Short rectangular reservoirs reveal a straight jet from inlet to outlet with identical recirculation zones on both sides. In longer reservoirs, the main jet reattaches to the side of the reservoir leading to small and large recirculation zones. Previous studies have found an empirical geometric relation describing the switch between these two flow patterns. In this study, we demonstrate, with a simple analytical model, that this switch coincides with a maximization of energy dissipation in the shear layer between the main jet and recirculation zones: Short reservoirs dissipate more energy when the flow pattern is symmetric, while longer reservoirs dissipate more energy with an asymmetric pattern. This approach enables the prediction of the flow patterns without detailed knowledge of small scale processes, potentially useful in the early phase of reservoir design. [less ▲]

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See detailOn the transition between symmetric ans asymmetric flow in rectangular shallow reservoirs - a case of maximum energy dissipation?
Westhoff, M.C.; Erpicum, Sébastien ULiege; Archambeau, Pierre ULiege et al

Conference (2017, June)

Shallow reservoirs are often used in hydraulic engineering as sediment traps or storage basins. For short rectangular reservoirs, the main jet flows straight from the inlet to the outlet with on both sides ... [more ▼]

Shallow reservoirs are often used in hydraulic engineering as sediment traps or storage basins. For short rectangular reservoirs, the main jet flows straight from the inlet to the outlet with on both sides symmetric recirculation zones. In longer reservoirs, however, the main jet reattaches to the side of the reservoir leading to a small and a large recirculation zone. Previous studies have found an empirical geometric relation describing the switch between these two flow pattems. In this study, we demonstrate with a simple analytical model, that this switch in flow pattem coincides with a maximization of energy dissipation in the shear layer between the main jet and the recirculation zones. We show that for short reservoir lengths, energy dissipation is higher in the case of a symmetric fiow pattern, while for longer reservoir the energy dissipation is higher for asymmetric flow pattems. The simulated switch between the two flow patterns appears to be very close to the empirical relation. The strength of this approach lies in the fact that no detailed knowledge of small scale processes is needed, which may prove useful at an early stage of reservoir design. [less ▲]

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See detailShallow water model with anisotropic porosity for flood modelling on Cartesian grids
Bruwier, Martin ULiege; Archambeau, Pierre ULiege; Erpicum, Sébastien ULiege et al

Conference (2017, June)

Detailed topographic data have become widely available for inundation mapping. While the use of such high-resolution data enables the computation of flow variables at a fine scale, the computation time ... [more ▼]

Detailed topographic data have become widely available for inundation mapping. While the use of such high-resolution data enables the computation of flow variables at a fine scale, the computation time remains too high for many practical applications. In contrast, models solving the shallow-water equations with porosity provide a useful tool to improve the computational efficiency, while preserving to some extent the detailed topographic information through porosity parameters. In this study, we present a new model solving the fully dynamic shallow water equations with anisotropic porosity based on Cartesian grids. Using a Cartesian grid leads to specific challenges, particularly as regards the definition of the conveyance porosities at the cell edges. Moreover, the presented model is further improved by a merging method so as to increase the computational efficiency without affecting the overall accuracy. The performance of the model has been evaluated based on a wide range of test cases, which confirm the validity of the model and the benefits of such a modelling framework. [less ▲]

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See detailA laser profilometry technique for monitoring fluvial dike breaching in laboratory experiments
Dewals, Benjamin ULiege; Rifai, Ismail; Erpicum, Sébastien ULiege et al

Conference (2017, April)

A challenging aspect for experimental modelling of fluvial dike breaching is the continuous monitoring of the transient breach geometry. In dam breaching cases induced by flow overtopping over the whole ... [more ▼]

A challenging aspect for experimental modelling of fluvial dike breaching is the continuous monitoring of the transient breach geometry. In dam breaching cases induced by flow overtopping over the whole breach crest (plane erosion), a side view through a glass wall is sufficient to monitor the breach formation. This approach can be ex- tended for 3D dam breach tests (spatial erosion) ifthe glass wall is located along the breach centreline. In contrast using a side view does not apply for monitoring fluvial dike breaching, because the breach is not symmetric in this case. We present a non-intrusive, high resolution technique to record the breach development in experimental models of fluvial dikes by means of a laser profilometry (Rifai et al. 2016). Most methods used for monitoring dam and dike breaching involve the projection of a pattern (fringes, grid) on the dam or dike body and the analysis of its deformation on images recorded during the breaching (e.g., Pickett et al. 201 1, Frank and Hager 2014). A major limitation of these methods stems from reflection on the water surface, particularly in the vicinity of the breach where the free surface is irregular and rippled. This issue was addressed by Spinewine et al. (2004), who used a single laser sheet so that reflections on the water surface were strongly limited and did not hamper the accurate processing of each image. We have developed a similar laser profilometry technique tailored for laboratory experiments on fluvial dike breaching. The setup is simple and relatively low cost. It consists of a digital video camera (resolution of 1920 >< 1080 pixels at 60 frames per second) and a swiping red diode 30 mW laser that enables the projection of a laser sheet over the dike body. The 2D image coordinates of each defomied laser profile incident on the dike are transformed into 3D object coordinates using the Direct Linear Transformation (DLT) algorithm. All 3D object coordinates computed over a swiping cycle of the laser are merged to generate a cloud of points. The DLT-based image processing method 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. Refraction of the light and laser rays across the water surface needs to be taken into account, because the dike is partially submerged during the experiments. An ad hoc correction is therefore applied using the Snell-Descartes law. For dtis purpose, planar approximations are used to describe the shape of the water surface. In the presen- tation, we will discuss the resulting uncertainty and will detail the validation of the developed method based on configurations of known geometry with various complexity. The presented laser profilometry technique allows for a rapid non-intrusive measurement of the dike geometry evolution. It is readily available for laboratory experiments and has proven its performance (Rifai et al. 2017). Further adjustments are needed for its application to cohesive dike material due to the reduced visibility resulting from the higher turbidity of water. [less ▲]

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See detailOvertopping induced failure of non-cohesive, homogenous fluvial dikes
Rifai, Ismail ULiege; Erpicum, Sébastien ULiege; Archambeau, Pierre ULiege et al

in Water Resources Research (2017), 53(4), 33733386

Accurate predictions of breach characteristics are necessary to reliably estimate the outflow hydrograph and the resulting inundation close to fluvial dikes. Laboratory experiments on the breaching of ... [more ▼]

Accurate predictions of breach characteristics are necessary to reliably estimate the outflow hydrograph and the resulting inundation close to fluvial dikes. Laboratory experiments on the breaching of sand-filled fluvial dikes were performed, considering a flow parallel to the dike axis. The breach was triggered by overtopping of the dike crest. A detailed monitoring of the transient evolution of the breach geometry was conducted, providing key insights into the gradual and complex processes involved in fluvial dike failure. The breach develops in two phases: (1) the breach becomes gradually wider and deeper eroding on the downstream side along the main channel, and (2) breach widening controlled by side slope failures, continuing in the downstream direction only. Increasing the inflow discharge in the main channel, the breach formation time decreases significantly and the erosion occurs preferentially on the downstream side. The downstream boundary condition has a strong influence on the breach geometry and the resulting outflow hydrograph. [less ▲]

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See detailTransition between symmetric and asymmetric flow in rectangular shallow reservoirs — a case of maximum dissipation?
westhoff, Martijn; Erpicum, Sébastien ULiege; Archambeau, Pierre ULiege et al

Conference (2017, April)

Shallow reservoirs are often used in hydraulic engineering as sediment traps or storage basins, Sedimentation within these reservoirs depends on the flow pattern, which in turn depends on the shape of the ... [more ▼]

Shallow reservoirs are often used in hydraulic engineering as sediment traps or storage basins, Sedimentation within these reservoirs depends on the flow pattern, which in turn depends on the shape of the reservoir. For short rectangular reservoirs, the main jet flows straight from the inlet to the outlet with on both sides identical recirculation zones. In longer reservoirs, however, the main jet reattaches to the side of the reservoir leading to a small and a large recirculation zone. Previous studies have found an empirical geometric relation describing the switch between these two different flow patterns. In this study, we demonstrate that this switch in flow pattern coincides with a maximization of energy dissipation in the shear layer between the main jet and the recirculation zones. To show this we described the power received from the jet by the recirculation zone as the product of a fluid-fluid friction coefficient and the square of the velocity difference times the shear velocity of the recirculation zone. This power is balanced by the bottom friction of the recirculation zone. Energy dissipation in the shear layer is then determined as the difference between the power performed by the jet and the power received by the recirculation zone. In this setup, energy dissipation is maximized by optimizing the friction coefficient. We show that for short reservoir lengths, energy dissipation is higher in the case of a symmetric flow pattern, while for longer reservoir the energy dissipation is higher for asymmetric flow patterns. The simulated switch between the two flow patterns appears to be very close to the empirical relation. This suggests that the flow pattem adapts in order to maximize energy dissipation between the jet and recirculation zones. The strength of this approach lies in the fact that no detailed knowledge of small scale processes is needed, while large scale structure formation can still be predicted. [less ▲]

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See detailLessons learned from combined experimental and numerical modelling of urban floods
Archambeau, Pierre ULiege; Bruwier, Martin ULiege; Finaud-Guyot, Pascal et al

Conference (2017, April)

Field data for validating hydraulic models remain scarce, They are often limited to inundation extents and water marks, which provide little insights into the dynamic features of the flow in urbanized ... [more ▼]

Field data for validating hydraulic models remain scarce, They are often limited to inundation extents and water marks, which provide little insights into the dynamic features of the flow in urbanized floodplains, such as the discharge partition in-between the streets and the velocity fields. To address this issue, a unique experimental setup representing a whole urban district was built in the laboratory ICube in Strasbourg and the state-of-the-art shallow- water model Wolf 2D was tested against the experimental measurements (An'ault et al, 2016), The numerical model was also used to extend and refine the analysis of the laboratory observations. The experimental model (5 m >< 5 m) represents a square urban district with a total of l4 streets of different Widths and 49 intersections (crossroads). The inflow discharge can be controlled in each street individually and the outflow discharges were measured downstream of each street. The numerical model Wolf was developed at the University of Liege and has been extensively used in flood risk research (Beckers et al, 2013, Bruwier et al. 2015, Detrembleur et al. 2015). Several lessons could be learned from this combined experimental and numerical analysis. First, we found that the discharge partition in—between the streets is primarily controlled by the street widths. Second, although the standard shallow-water equations reproduce satisfactorily tnost of the flow characteristics, adding a turbulence model improves the prediction of the shape and length of the flow recirculations in the streets. Yet, this has little influence on the discharge partition because the computed recirculation widths are hardly affected by the turbulence model. The experiments and the numerical model also show that the water depths in the streets remain fairly constant in~between two intersections, while they drop suddenly downstream of each intersection as a result of complex flow interactions at the intersections. This hints that friction has little influence on the water depths obtained in the experiments. However, tailored numerical tests demonstrate that this is a direct consequence of the distorted nature of the experimental setup. Indeed, the ratio between the water depth and the street width is close to 1 in the experiments, while it would be at least one order of magnitude lower in real-world conditions, even for extreme floods. Finally, remote sensing data, such as digital elevation models, are generally available on a regular grid, which makes it convenient to use also a Cartesian grid for hydraulic modelling, We show here that the discretization of the geometry of the buildings on such a Cartesian grid has a major influence on the modelling accuracy (overestimation of the overall flow resistance). An extended shallow-water model based on non-isotropic porosity parameters is shown to improve substantially the prediction of the discharge partition in-between the streets. It is therefore considered as a valuable tool to advance urban flood modelling in practice. From the lessons learned here, we recommend that future research focuses on the design and exploitation of a less distorted experimental model, as well as on the analysis of extra flow processes such as transient conditions and interactions between overland flow and pressurized flow in underground passages. [less ▲]

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