Physical and numerical modeling of overbank flow with a groyne on the floodplain

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1 Physical and numerical modeling of overbank flow with a groyne on the floodplain (1) Y. Peltier, (1) S. Proust, (2) A. Bourdat, (1) F. Thollet, (3) N. Rivière, (1) A. Paquier (1) Hydrology-Hydraulics Research Unit, CEMAGREF, Lyon, France (2) Hydraulic laboratory, Compagnie Nationale du Rhône (CNR), Lyon, France (3) LMFA,CNRS-Université de Lyon, INSA de Lyon, Villeurbanne, France ABSTRACT: This paper focuses on both physical and numerical modeling of flow in the vicinity of a groyne, blocking off the floodplain of an asymmetric compound channel, and perpendicular to the main flow direction. Experiments were conducted in a laboratory flume and in a small-scale model. Various lengths of groyne d were used in both the laboratory flume and the small-scale model. Water levels, velocity field and spreading of the recirculation zone behind the obstacle were measured for five flow configurations. The influence on the flow parameters of aspect ratio d/b (where B is the total width of the channel), of total discharge Q and of relative depth ratio h r were examined. Particularly, a hydraulic disjunction may occur between the subcritical flow and the supercritical flow respectively located upstream and immediately downstream the obstacle, depending on the values of discharge Q and aspect ratio d/b. Flows were simulated with the 2D-H program RUBAR 20 (Cemagref). Two simple turbulence closures that are often used by engineers were tested: a constant eddy viscosity and the Elder s model u *, h. Simulations are compared to experimental measurements, focusing on the streamwise profiles of water level and discharge distribution across the channel, and on the spreading of the recirculation zone. The influence of the turbulence model on flow parameters is then investigated. Keywords: Compound Channel, Groyne, Recirculation zone, Physical modeling, 2D-H modeling 1 INTRODUCTION Overbank flows in rivers are complex 3D flows characterized by interactions between bottom frictions, interfacial shearing and mass transfers between the flows in the Main Channel (MC) and in the Floodplain (FP). To understand these specific processes, numerous experimental studies were conducted under uniform flow conditions in straight geometries (from Sellin, 1964, to Shiono and Knight, 1991). As uniform flows are quite uncommon in the field, non-uniform flows were also investigated. Three types of non-uniform flows can be distinguished: flows in compound channel with skewed floodplain boundaries (Sellin, 1993); flows in a 2-stage meandering compound channel (Shiono and Muto, 1998); and finally flows in converging and diverging compound channels (Bousmar, 2002, Proust, 2005, Proust et al., 2006, Bousmar et al., 2006). Due to the continuous variation in the geometry, these three types of flow can be considered as gradually varied. To go further in the understanding of overbank flows, investigation of strongly varied flows is necessary. These flows occur in the presence of strong variations in the floodplain width (e.g. due to groynes or embankments). In this case, flows are mainly characterized by large streamwise gradients of flow depth and by the development of a recirculation zone behind the obstacle. The flow in the vicinity of a groyne set up on the floodplain may interact with the flow in the main channel, depending on the ratio between the groyne length d and the overall width B, and on the relative depth h r. Consequently, a recirculation zone may change mass exchanges, discharge distribution and water levels across the compound channel. To our knowledge, compound channel flows

2 with recirculation zones have not been widely studied. A theory was built to predict the length of the recirculation zone (L r ) in the case of a sudden enlargement in a single channel (Babarutsi et al., 1996). In this context, L r is a function of the enlargement length (d), of the mean water depth (h) and of the Darcy-Weisbach coefficient (f) in the enlargement cross-section. This theory was tested and adapted for flows in the vicinity of a groyne for both single and compound geometries (Rivière et al., 2004, Martinez, 2005, Proust, 2005, Bourdat, 2007). Numerical simulations were conducted with depth-averaged 2D or 3D codes for flows around a groyne in a straight channel with a single crosssection only (Ouillon and Dartus, 1997, Tingsanchali and Maheswaran, 1990, Paquier et al., 2001, Paquier et al., 2003). First, this paper presents flow experiments with a groyne on the floodplain in a laboratory flume and in a small-scale model. Second, numerical simulations carried out with the 2D-H code RUBAR 20 (Cemagref) are compared to the experimental measurements. The aim is to evaluate the ability of a 2D-H code like RUBAR 20, using a 0 th order turbulence closure, to predict the flow pattern near a groyne. Finally, the influence of the turbulence model on flow parameters is addressed. 2 METHODS 2.1 Experimental set-up Experiments were conducted in two compound channel flumes, located at the Laboratoire de Mécanique des Fluides et d Acoustique (LMFA), Lyon, France, and at the Compagnie Nationale du Rhône (CNR), Lyon, France. The crosssection is asymmetrical in both flumes. The CNR flume is slightly curved and the main channel cross-section is trapezoidal (see Proust et al., 2006). The LMFA flume is straight and the main channel cross-section is rectangular. Relative roughness (Ks r =Ks fp / Ks mc, with Ks = Strickler coefficient) is equal to 1 in the LMFA flume (PVC made) and 0.9 in the CNR flume (concrete-covered). In the CNR flume, the main channel and the floodplain are supplied by the same inlet (single reservoir). In the LMFA flume, the upstream stilling basin is separated in two chambers, which enables the subsection discharges in the main channel (Q mc ) and in the floodplain (Q fp ) to be adjusted independently. This inlet accelerates the development of uniform flow conditions along the channel, by deleting the excess in floodplain discharge obtained when using a single reservoir (see Bousmar et al., 2005, Proust, 2005). The geometrical parameters of the flumes are reported in Table 1 and cross-sectional and plan views of the two flumes are presented in Fig.1. Figure 1. Cross-sectional and plan views of the two asymmetric compound channel flumes. Table 1. Geometrical characteristics of the CNR and LMFA flumes L [m] B [m] B/B fp [-] h bf [cm] S 0 [-] LMFA CNR Before evaluating the influence of a groyne on a compound channel flow, three flows were carried out without obstacle. They are considered in the sequel as reference flows. Hydraulic parameters of these flows (relative depth and floodplain discharge) are summarized in Table 2. Five flow configurations with a groyne on the floodplain were then investigated, keeping the same boundary conditions as reference flows and varying the length of the obstacle (see Tab. 2). It should be recalled that groynes are perpendicular to the main flow direction (x-axis). Their positions from the upstream boundary are: Xg = 4.05 m at CNR; Xg(d = 30) = 3 m and Xg(d = 50) = 2.5 m at LMFA as indicated in Fig. 2(a-c). For each flow with a groyne, water levels were measured with an ultrasonic probe at LMFA (accuracy = 0.1 mm) and with a point gauge at CNR (accuracy = 0.3 mm). Intensity of velocities were measured with a micro-propeller (with a threshold value of 5 cm/s) connected to a vane to obtain the direction of local flow. Six

3 measurements were carried out on a vertical profile (Z-axis) in the main channel, and between three and one in the floodplain with respect to the flow depth. At LMFA, water depths and velocity were measured each y = 5 cm across the channel and each x = 50 cm in the streamwise direction. At CNR, y =10 cm and x was variable between 0.5 and 2 m. Table 2. Geometrical and hydraulic parameters of the reference flows and of the flows with a groyne Reference flows Flows with a groyne Q [l/s] h r ref. [-] d [m] d/b [-] h r [-] LMFA CNR (a) 2.2 2D-H modeling The numerical program used in this study is a 2D-H code, RUBAR 20, developed by CEMA- GREF. This code solves the depth-averaged 2D shallow water equations, using a second order in space and time finite volumes method. Two models of eddy viscosity were tested: a constant viscosity ν t ; and a viscosity related to shear velocity u * and water depth h (Elder s model). A constant eddy viscosity ν t is generally used when the momentum transfer due to turbulent exchanges in a horizontal plan is insignificant. In this case, ν t accounts rigorously for turbulent diffusion, numerical diffusion, as well as dispersion of velocities on a vertical profile. Elder s eddy viscosity is used when turbulence is controlled by bed frictions, and writes as: = 2 2 ( q q ) λ u h = g + C υ t λ * x y (1) where C is the Chézy coefficient and λ is an empirical dimensionless parameter, ranging from 0.13 to 0.16 for large experimental flumes (Rodi, 1980) and from 0.6 to 2 for natural rivers (Wark et al., 1990). Meshes are regular and were interpolated from the actual channel topography ( x = 10 cm and y = 6 cm). At the upstream boundary condition, the unit discharges are fixed and equal to experimental values. At the downstream boundary condition, the actual water depths are taken into account for a subcritical regime, otherwise (supercritical regime) the outflow is free. 2.3 Recirculation zone (b) (c) Figure 2. Depth-averaged velocities for three flow configurations: (a) LMFA: h r = [ ] and d/b = 0.25; (b) LMFA: h r = [ ] and d/b = 0.41; (c) CNR: h r = [ ] and d/b = The boundaries of the recirculation zones are indicated by full lines. The formalism developed by McGill University for flows in a single channel with a sudden enlargement (Babarutsi et al., 1989; Babarutsi et al.,1996 ; Chu et al., 2004) was adapted to flows in a compound channel for either sudden enlargements or groynes on the floodplain (Rivière et al., 2004, Martinez, 2005, Proust, 2005). In the presence of a groyne, the physics within the recirculation zone is related to a dimensionless friction number S, written as: S = f.d/(8h) (2) Where f =Darcy-Weisbach coefficient; h = water depth in the cross-section of the groyne. Depending on the S values, two asymptotic regimes are distinguished. When S < 0.02, the regime is called Deep Water Flow (DWF), the flow is controlled by the groyne and the length of

4 the recirculation zone (L r ) is merely proportional to the groyne length d, as: L r = 12.09d (3) When S > 0.07, the regime is called Shallow Water Flow (SWF), the physics is controlled by bed frictions and the recirculation length (L r ) writes: L r = 5.44h/f (4) In this study, the ability of a 2D-H model to predict both the width and the length of a recirculation zone is analyzed. The length L r corresponds to the intersection between the line of vorticity centres downstream the groyne and the lateral floodplain wall (Fig. 3). The lateral boundary of the recirculation zone was defined as including the area where depth-averaged velocities U d are less than 5 cm/s (threshold value of the micro-propeller). Figure 3. Schematic plan view of a recirculation zone. 3 EXPERIMENTAL RESULTS 3.1 Relative depth, local Froude numbers, discharge ratio in the floodplain Flows with a groyne on the floodplain are compared to the reference flows (Fig. 4) for both LMFA and CNR flumes. In the presence of a groyne, the flow is disturbed on the whole length of the flume, as shown by relative depths in Fig. 4a. and by floodplain discharge ratios in Fig. 4b. Upstream the groyne, an increase of +34% (resp. +33%) in the relative depth was observed in the CNR flume (resp. LMFA flume) for the d/b ratio of 0.47 (resp. 0.41). In the cross-section of the groyne and downstream, the decrease in the relative depth can reach -100% in comparison with the reference flows. The shallow flow depths and the high values of velocities U d immediately downstream the obstacle result in the development of a supercritical flow zone. The extension of this zone rises when increasing the d/b aspect ratio or the total discharge Q (see Table3). Table 3. Froude numbers in the main channel and the floodplain immediately downstream the groyne. d/b Q [l/s] h r ref. Fr mc Fr fp < 1 > > 1 > < 1 > > 1 > 1 Given a constant discharge Q = 24,7 l/s at LMFA, with d/b = 0.41 both the floodplain flow and the main channel flow are supercritical downstream the groyne (Fig. 4d), while with d/b = 0.25 a hydraulic disjunction occurs in the floodplain only. (Fig. 4c). Given a constant d/b ratio = 0.47 at CNR, the flow in the main channel is subcritical with Q = 150 l/s and is supercritical with Q = 260 l/s downstream the groyne, while floodplain flow remains supercritical for the two discharges. As expected, the strength of mass transfers between subsections varies with respect to the d/b aspect ratio. For a given discharge Q, an increase in the d/b ratio accentuates the contraction and the acceleration of the flow in the crosssection of the groyne, as shown in Fig. 2a and Fig.2b. Downstream the obstacle, the influence of the left-hand lateral bank of the main channel is highlighted for ratio d/b = 0.41 only. In this case, a change in the velocities direction downstream the groyne occurs, especially on the floodplain (Fig. 2b). The streamwise profiles of discharge ratio in the floodplain are presented in Fig4b. They enable the mass transfer to be quantified. At LMFA, a significant decrease in the floodplain discharge is observed downstream the groyne: by -40% and -70% for d/b = 0.25 and 0.41 respectively (in comparison with the reference flow). More far downstream, the direction of mass transfers changes (from the floodplain to the main channel). At the downstream boundary, no flow reaches the reference flow conditions. A longer distance appears to be required. However, floodplain discharge ratios are very close to their reference values upstream the groyne at station X/L = 0.2. At the CNR (white labels), the decrease in the floodplain discharge reaches -86% for Q = 150 l/s and d/b = At the last station of measurements, the discharge Q fp is still far from the value of the reference flow. The CNR flow appears to be more disturbed than the LMFA flows. As the reference flow at LMFA is uniform and the reference flow at CNR is far from

5 (a) (b) Water level Z [cm] Q = 150 l/s ; d = 143 cm (c) Lateral axis Y [cm] X = 2.5 m X = 4.05 m (groyne) X = 4.5 m X = 8.25 m (e) (f) Figure 4. (a) Relative depths and (b) discharge ratios in the floodplain for the reference flows and the flows with a groyne (LMFA: Q = 24.7 l/s, d = 30 and 50 cm; CNR: Q = 150 l/s, d = 143 cm). Local Froude number for (c) Q = 24.7 l/s and d = 30 cm ; (d) Q = 24.7 l/s and d = 50 cm. (e-f) Lateral profiles of water level Z in the small-scale model for d/b =0.47: (e) Q = 150 l/s and (f) 260 l/s. Water level Z [cm] (d) Q = 260 l/s ; d = 143 cm X = 2.5 m X = 4.05 m (Groyne) X = 4.5 m X = 8.25 m Lateral axis Y [cm] equilibrium, it is difficult to conclude. The two other CNR flows (Q = 260 l/s, d = 77 cm and 143 cm) are not reported in Fig. 4(a-b), because their recirculation zone extends until the downstream boundary, which means beyond. This prevents from giving any conclusion on the influence of the groyne in the downstream part of the flow. Finally, it should be mentioned that transverse gradients of water surface were observed in and downstream the groyne section, as shown in Fig. 4(e-f). The lateral slope of water surface increases with d/b ratio (for a given Q) and with Q (for a given d/b). This cannot be identified in Fig.4a as relative depths are mean values on the total width B. 3.2 Recirculation zone

6 The lengths of the experimental recirculation zones L r are reported in Table 4. They are compared to the theoretical values computed with Eq. 3 and Eq. 4. It should be recalled that the recirculation zone spreads beyond the downstream boundary for two flows (Q = 260 l/s, d/b = 0.25 and 0.47). Table 4. Length of the recirculation zone L r : experimental measurements versus theoretical values (Eq. 3 and 4). Exp. Theory Regime L Q L r L r d/b r S SWF DWF [l/s] [m] [m] [m] DWF near DWF SWF > > Comparing experimental values of L r with the theoretical values, each flow can be linked to the 2 asymptotical regimes previously defined. The CNR flow with d/b = 0.47 and Q = 150 l/s clearly belongs to the Shallow Water Flow regime, which means that physics is governed by bed friction. The LMFA flow with d/b = 0.25 and Q = 24.7 l/s clearly belongs to the Deep Flow regime, which means that physics is mainly controlled by the groyne. The LMFA flow with d/b = 0.41 is located in the transition zone near DWF regime. 4 NUMERICAL MODELLING 4.1 Calibration of eddy viscosity Flows were simulated with the 2D-H code RUBAR 20, using two types of turbulence closure: a constant eddy viscosity ν t ; and the Elder s eddy viscosity ν t = λ.u * h. The two turbulence models were first calibrated under uniform flow conditions. Figure 5a and 5b present lateral profiles of depth-averaged velocity U d using different values of λ parameter in the Elder s model (a) and different values of constant ν t (b). Using Elder s model, the minimal discrepancies between simulated and experimental velocities U d are obtained for ν t = 0.1.u * h in Fig.5a, in agreement with the magnitude order of λ in Rodi (1980) for large open channels. In this case, the mean relative error between numerical and experimental velocities is 8.9 %. With the constant eddy viscosity model, ν t = m²/s is the more appropriate value with a 10.9 % mean relative error. Hence, simulations of a flow with a groyne on the floodplain were carried out with both ν t = 0.1.u * h and ν t = m²/s. Depth-averaged velocity Ud (m/s) Depth-averaged velocity Ud (m/s) 0,8 0,7 0,6 0,5 0,4 0,3 Meas. Nut = 0.001u*h Nut = 0.01u*h Nut = 0.1u*h Nut = 1u*h 0,2 0 0,2 0,4 0,6 0,8 1 1,2 0,8 0,7 0,6 0,5 0,4 0,3 Meas. Nut = 0 Nut = Nut = Nut = 0.01 Lateral axis Y (m) (a) 0,2 0 0,2 0,4 0,6 0,8 1 1,2 Lateral axis Y (m) (b) Figure 5. Lateral profiles of depth-averaged velocities U d under uniform flow conditions (LMFA flume, Q = 24.7 l/s): 2D-H simulations with (a) ν t = λ.u*h and (b) constant ν t versus experimental measurements. 4.2 Water level and floodplain discharge The 2D-H simulations of water level Z, of floodplain discharge Q fp and extension of the recirculation zone are compared to experimental data in Figure 6, computing relative errors between numerical and experimental results. Water levels Z are mean value on the total width B. The boundaries of the recirculation zone are defined as enclosing an area with velocities U d less than a threshold value of 5 cm/s. Simulations are carried out with ν t = 0.1.u*h, ν t = m²/s, and also with ν t = 0 m²/s to estimate the influence of turbulence diffusion in the momentum exchange. The two flows with a recirculation zone extending until the downstream boundary were not simulated. The two flows in the LMFA flume are sensitive to turbulent diffusion, which is not the case for the CNR flow. When accounting for turbu

7 (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 6. 2D-H simulations versus experimental measurements of flows with a groyne on the floodplain: (a-d-g) Relative errors on the water levels Z; (b-e-h) Relative errors on floodplain discharge Q fp ; (c-f-i) Extension of the recirculation zone (bounded by a threshold value of 5 cm/s). lence, the Elder s model appears to be the more appropriate. With ν t = 0.1.u * h, maximum relative errors are [+12%;-8%] on Z values, and [+37%;-17%] on Q fp values for the three flows. When the d/b ratio is increased, relative errors on both Z and Q fp rise (compare Fig6a with 6d and Fig6b with 6e). For a given Q = 24,7l/s, the floodplain discharge is over-estimated downstream the obstacle by +16% (resp. +37%) for ratio d/b = 0.25 (resp ) when using ν t = 0.1.u * h. This is not surprising since the flow becomes increasingly 3D when the d/b ratio is rised, highlighting the limits of shallow water equations in this context. For the three flows, maximum relative errors on Z and Q fp occur between the groyne crosssection and a station located 2m downstream the groyne. Upstream the groyne, both Z and Q fp are accurately modeled. As expected, it is easier to simulate a converging flow than a diverging flow. Simulations with ν t = 0 m 2 /s confirm that turbulent diffusion has no influence on the CNR flow (d/b = 0.47, Q = 150 l/s), both upstream and downstream the obstacle. For the two LMFA flows, results get worse when using ν t = 0 m 2 /s, but the results are different between configurations d/b = 0.25 and d/b = Upstream the groyne (zone of flow convergence), turbulence appears to have more effect on flow parameters for the small d/b ratio (Fig.6b). This is coherent with what was observed using a quasi-1d model in converging compound channels (Proust et al., 2008a, 2008b): the role of turbulent diffusion decreases with an increase in the converging angle.

8 Downstream the groyne (zone of flow divergence), the opposite phenomena is observed. When using ν t = 0 m 2 /s, the results are more erroneous with the largest groyne (compare Fig. 6b and 6e and Fig 2a and 2b) with an underestimation of 80% in the floodplain discharge Q fp. This means that momentum exchange due to turbulent diffusion increases with the flow divergence behind the groyne, i.e. with increasing d/b ratios. 4.3 Recirculation zone The simulated and experimental recirculation zones are presented in Fig.6c-f-i. Relative errors on the calculation of recirculation length L r are reported in Table 5. The Elder s model ν t = 0.1.u * h provides the best results in agreement with the simulations of water level and floodplain discharge. The recirculation length L r is accurately modelled for the two LMFA flows, while L r is over-estimated by -30% for the CNR flow. The 2D-H simulations are less accurate in terms of lateral expansion of the recirculation zone, notably for the LMFA flows. The width of the zone is underestimated e.g. by -57% at station X = 3.5 m for Q = 24.7 l/s and d/b = 0.25 (Fig.6c). When using ν t = 0 m 2 /s, the 2D-H code cannot reproduce the shape of the recirculation zone for the LMFA flow with the large groyne (Fig. 6f). In particular, length L r cannot be determined as the recirculation zone extends until the downstream boundary. As previously mentioned, turbulence plays a specific role in the region of flow divergence for this flow configuration. For the CNR flow, using ν t = 0 m 2 /s instead of ν t = 0.1.u * h has no effect, which confirms the negligible influence of turbulence on this flow. Table 5. Relative error on calculation of recirculation length L r Relative error on calculation of L r [%] d/b Regime νt = 0 νt = νt = 0.1u*.h [m 2 /s] [m 2 /s] [m 2 /s] 0.25 DWF Near DWF > SWF DISCUSSION This study showed that two types of overbank flow should be distinguished. The first type of flow is represented by the shallow CNR flow with a groyne length d = 143 cm and with floodplain flow depth behind the obstacle in the range 1cm to 2cm (relative depth h r [ ]. ). This flow appears to be mainly controlled by bed friction and mass transfers between the floodplain and the main channel. Vertical Reynolds stresses τ xx and τ xy induced by the groyne are negligible relative to the other terms of the 2D-H momentum equations. The turbulent model has no effect on the simulated flow parameters. The shape of the recirculation zone is well modeled, notably in the lateral direction. The recirculation length L r is merely related to the mean water depth h and the Darcy- Weisbach coefficient f in the groyne crosssection. The second type of flow is represented by the LMFA flow with a groyne length d = 50 cm and floodplain flow depth in the range 1cm to 2cm (h r [ ]). Using the 2D-H equations, the lateral extension of the recirculation zone is poorly modelled (the width is underestimated by 55 to 70%) and the bulging form near the groyne corner is not reproduced, while the recirculation length L r is accurately simulated. Moreover, simulations of water level Z and floodplain discharge Q fp and of the spreading of the recirculation zone are very sensitive to the turbulent diffusion. The vertical Reynolds stresses τ xx and τ xy induced by the groyne play a major role in the 2D-H equations for this flow. As the modelling consistency appears to be better in the longitudinal direction than in the lateral direction, the actual turbulent diffusion appears to be anisotropic. A 2D approach with isotropic eddy viscosity is less relevant in this context. As shown by the LFMA flow with d = 30 cm, a transition zone exists between the two types of flow previously described, where both bed frictions and vertical Reynolds stresses influence the flow. These preliminary results showed that it would be more relevant to consider an eddy viscosity with two components ν t and ν t to ac- x y count for the asymmetry in the momentum exchange due to turbulent diffusion. Using the Elder s model, this means to calibrate the 2 components λ x and λ y. 6 CONCLUSION

9 Five flows with a groyne on the floodplain were performed in a small-scale model and in a laboratory flume. These flows were compared to reference flows (same boundary conditions without obstacle). The presence of the groyne induces an increase in the relative depth of +33% upstream the groyne, and a decrease of 100% downstream the groyne. Significant decrease in the floodplain discharge are observed behind the groyne (up to 70%). Due to the flow acceleration and the shallow flow depths downstream the groyne, the flow becomes supercritical either in the floodplain only or in the two subsections, depending on aspect ratio d/b (groyne length/total width) and total discharge Q. Vortices are generated by the groyne corner and are advected along the recirculation zone boundary. These structures may be 3 dimensional and anisotropic, depending on the aspect ratio h/d (mean flow depth/groyne length). When the total cross-section is compound, two issues are superimposed: (1) the interaction between the obstacle and the flow in the vicinity; (2) the interaction between the main channel flow and the floodplain. From an engineering point of view, the question is to know to what extent can 2D-H shallow water equations model mean flow parameters, when using a 0 th order turbulent closure with an isotropic eddy viscosity (Boussinesq assumption)? Flows were simulated with the 2D-H programme RUBAR 20 (Cemagref). Simulations were carried out considering either a constant eddy viscosity, or a Elder s eddy viscosity, or ignoring turbulent diffusion. The turbulent models were calibrated under uniform flow conditions. Results were compared to experimental data, focusing on the evaluation of water level Z, of floodplain discharge Q fp and of the extension of the recirculation zone behind the groyne. When the turbulence model influence the flow parameters, Elder s model appears to be the more appropriate. Comparing numerical and experimental measurements, maximum relative errors are [+12%;-8%] on Z values, and [+37%;-17%] on Q fp values. These errors are located between the groyne cross-section and 2 metres downstream. Errors on both Z and Q fp rise when the d/b ratio is increased, for a given total discharge Q. For instance, with Q = 24.7 l/s, maximum error on floodplain discharge Q fp is +15% with d/b = 0.25, and +39% with d/b = In agreement with previous work of the literature, two types of flow regime were underlined. For the Shallow Water Flow regime (SWF), the flow is mainly controlled by bed friction and mass transfers between the main channel and the floodplain. The vertical Reynolds stresses τ xx and τ xy that are physically produced by the groyne play a minor role, and consequently, accounting for turbulence in the simulations has no influence on Z, Q fp and on the recirculation zone. In this case, there is no link between the recirculation length L r and the groyne length d, and the lateral extension of the recirculation zone is quite accurately modelled. For the Deep Water regime (DWF), the significant effect of vertical Reynolds stresses is underlined. The length L r is proportional to the groyne length d and is accurately modeled. However, the lateral expansion of the recirculation zone is always underestimated (up to 66%), which has a direct influence on the calculation of Q fp and Z. As the consistency of the 2D-H modelling is better in the longitudinal direction than in the lateral direction, it clearly demonstrates that the turbulent exchanges are anisotropic behind the groyne. To divide the eddy viscosity in x y two components ν t and ν t appears unavoidable in this context when using a 0 th order turbulent closure. REFERENCES Babarutsi, S., Ganoulis, J., & Chu, V.H Experimental investigation of shallow recirculating flows. Journal of Hydraulic Engineering, ASCE, Vol.115, No.7, Babarutsi, S., Nassiri, M., & Chu, V.H Computation of shallow recirculating flow dominated by friction. Journal of Hydraulic Engineering, ASCE, Vol.122, No.7, Bourdat, A Débordements des cours d eau en presence de remblais routiers dans les lits majeurs. Projet de fin d'études, ENSHMG de Grenoble, CEMA- GREF, 48p. Bousmar, D Flow modelling in compound channels / Momentum transfer between main channel and prismatic or non-prismatic floodplains. Ph-D thesis, Université catholique de Louvain, Faculté des Sciences Appliquées, Louvain, 306 p. Bousmar, D., Rivière, N., Proust, S., Paquier, A.., Morel, R. & Zech, Y Upstream discharge distribution in compound-channel flumes. Journal of Hydraulic Engineering, ASCE, Vol.131, No.5, Bousmar, D., Proust, S., and Zech, Y Experiments on the flow in a enlarging compound channel." Proc. of the int. conf. on fluvial hydraulics, River flow 2006, 6-8 september, Lisbon, Portugal, Chu, V.H., Fang, L., Altai, W Friction and confinement effects on a shallow recirculating flow, J. Environmental and Eng. Science, 3 (5),

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