Difference between revisions of "Testpage1"
Dronkers J (talk  contribs) 
Dronkers J (talk  contribs) 

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==Introduction==  ==Introduction==  
+  [[Image:DordogneTidalBore.jpgcenter700pxthumbFigure 1. Left image : Partially breaking tidal bore on the Dordogne River (Courtesy of Antony “YEP” Colas). Right image : Undular tidal bore on the Garonne River (Bonneton et al. 2011 <ref name=B11a>Bonneton, P, Parisot, JP., Bonneton, N., Sottolichio, A., Castelle, B., Marieu, V., Pochon, N. and Van de Loock, J. 2011. Large amplitude undular tidal bore propagation in the Garonne River, France, Proceedings of the 21st ISOPE Conference: 870874, ISBN 9781880653968</ref>).]]  
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+ Table 1. Estuaries and rivers with substantial tidal bores (bore height of half a meter up to several meters). The maximum tidal range is the maximum range recorded at a tide gauge situated close to the location in the estuary where the tidal range is highest.  + Table 1. Estuaries and rivers with substantial tidal bores (bore height of half a meter up to several meters). The maximum tidal range is the maximum range recorded at a tide gauge situated close to the location in the estuary where the tidal range is highest.  
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The estuarine shape (depth and width profiles) plays an important role in tidal bore formation. This is illustrated by the observation that engineering works (dredging, weirs) during the past century have weakened or even completely suppressed tidal bores in many rivers, for example in the rivers Seine, Loire, Charente and Petitcodiac. We will discuss in the following general conditions for bore formation in terms of estuary geometry and flow conditions.  The estuarine shape (depth and width profiles) plays an important role in tidal bore formation. This is illustrated by the observation that engineering works (dredging, weirs) during the past century have weakened or even completely suppressed tidal bores in many rivers, for example in the rivers Seine, Loire, Charente and Petitcodiac. We will discuss in the following general conditions for bore formation in terms of estuary geometry and flow conditions.  
−  Illustrations of tidal bores are shown in  +  Illustrations of tidal bores are shown in Fig. 1. 
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==Tidal bore formation in funnelshaped estuaries==  ==Tidal bore formation in funnelshaped estuaries==  
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+  [[Image:BoreFormationCondition.jpgleft400pxthumbFigure 4. Estuarine classification in parameter space (<math>\epsilon, \phi</math>). Maximum surface elevation slope <math>A_{max}</math>, the white dashed line represents the <math>\epsilon_c</math> curve, namely the limit for tidal bore appearance following the criterion <math>A_{max} \ge 10^{3}</math>. Each point in the plane (<math>\epsilon, \phi</math>) represents a numerical tidal wave solution for an idealized convergent estuary; this figure relies on 225 numerical simulations (Filippini et al., 2018)<ref></ref>.]]  
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The analysis of nonlinear tidal wave transformation in estuaries, in terms of tidal forcing at the estuary mouth and largescale geometrical properties of the channel, has received considerable attention (see [[Tidal asymmetry and tidal basin morphodynamics]]). By contrast, the extreme nonlinear tidalwave case where tidal bores form is much less studied.  The analysis of nonlinear tidal wave transformation in estuaries, in terms of tidal forcing at the estuary mouth and largescale geometrical properties of the channel, has received considerable attention (see [[Tidal asymmetry and tidal basin morphodynamics]]). By contrast, the extreme nonlinear tidalwave case where tidal bores form is much less studied.  
−  [[Image:GaronneObservationOndularBoreDevelopment.jpg center  +  [[Image:GaronneObservationOndularBoreDevelopment.jpg center800pxthumbFigure 2. Tidal wave distortion and bore formation observed in the GirondeGaronne estuary (Bonneton et al. 2015<ref name=B15></ref>) at spring tide. Left panel: Tide gauge locations on the estuary. Right top panel: Time series of tide elevation at 4 different locations, from the estuary mouth (Le Verdon) to the upper estuary (Podensac); Right bottom panel: Tidal bore illustrations at Podensac field site, located 126 km upstream the river mouth.]] 
−  [[Image:TidalBoreSimulation.jpg center600pxthumb Figure  +  [[Image:TidalBoreSimulation.jpg center600pxthumb Figure 3. Tidal wave distortion and bore formation in a convergent estuary. Numerical simulations by Filippini et al. 2019 <ref name=F> Filippini, A. G., Arpaia, L., Bonneton, P., & Ricchiuto, M. 2019. Modeling analysis of tidal bore formation in convergent estuaries. European Journal of MechanicsB/Fluids 73: 5568</ref> Left panel: Rising tide elevation at different increasing times. Right panel: Zoom of left panel.]] 
−  The most intense tidal bores occur in long and shallow tidedominated funnelshaped estuaries (e.g. Qiantang, Severn, Kampar, Gironde/Garonne/Dordogne, …) . The formation of these tidal bores is governed by the progressive distortion of the tidal wave as it propagates up the estuary. When the ebbflood asymmetry (ebb duration longer than the flood and larger flood than ebb currents) is strong enough generally in the upper estuary (see tidal wave elevation in Fig.  +  The most intense tidal bores occur in long and shallow tidedominated funnelshaped estuaries (e.g. Qiantang, Severn, Kampar, Gironde/Garonne/Dordogne, …) . The formation of these tidal bores is governed by the progressive distortion of the tidal wave as it propagates up the estuary. When the ebbflood asymmetry (ebb duration longer than the flood and larger flood than ebb currents) is strong enough generally in the upper estuary (see tidal wave elevation in Fig. 2 at Bordeaux and Podensac), tidal bore can form (see Figs. 2 and 3). 
This extreme nonlinear deformation of a tidal wave occurs in a limited number of estuaries. To determine the conditions favourable to tidal bore occurrence, a scaling analysis can eb carried out for a schematic channel geometry. A usual schematization is based on the approximations <ref>Lanzoni, S. and Seminara, G. 1998. On tide propagation in convergent estuaries, J. Geophys. Res. 103: 30793–30812</ref><ref>Savenije, H.H.G. 2012. Salinity and Tides in Alluvial Estuaries, second ed., Salinity and Tides in Alluvial Estuaries, second ed., www.salinityandtides.com</ref><ref>Dronkers, J. 2017. Convergence of estuarine channels. Continental Shelf Res. 144: 120–133</ref>:  This extreme nonlinear deformation of a tidal wave occurs in a limited number of estuaries. To determine the conditions favourable to tidal bore occurrence, a scaling analysis can eb carried out for a schematic channel geometry. A usual schematization is based on the approximations <ref>Lanzoni, S. and Seminara, G. 1998. On tide propagation in convergent estuaries, J. Geophys. Res. 103: 30793–30812</ref><ref>Savenije, H.H.G. 2012. Salinity and Tides in Alluvial Estuaries, second ed., Salinity and Tides in Alluvial Estuaries, second ed., www.salinityandtides.com</ref><ref>Dronkers, J. 2017. Convergence of estuarine channels. Continental Shelf Res. 144: 120–133</ref>:  
*Uniform channel depth <math>D</math>,  *Uniform channel depth <math>D</math>,  
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River flow and riverbed slope also influence tidal bore formation. River flow contributes to tidal wave deformation by enhancing the longitudinal velocity gradient and riverbed slope by upstream reduction of the water depth. However, river flow and riverbed slope also contribute to tidal wave damping and thus oppose tidal bore formation <ref> Horrevoets, A. C., Savenije, H. H. G., Schuurman, J. N. and Graas, S. 2004. The influence of river discharge on tidal damping in alluvial estuaries. Journal of Hydrology 294: 213228</ref>. The latter effect usually dominates. Observations show that the tidal bore in the Garonne and Dordogne (France) is suppressed at high river runoff<ref name=B16>Bonneton, P., Filippini, A.G., Arpaia, L., Bonneton, N. and Ricchiuto, M. 2016. Conditions for tidal bore formation in convergent alluvial estuaries. Estuarine, Coastal and Shelf Science. 172: 121127</ref>. A similar effect is observed in the Daly estuary by Wolanski et al. (2006), who relate the occurrence of the tidal bore at low river discharge to the small water depth during such conditions. The opposite effect is reported for the GuamáCapim river system near the mouth of the Amazon River, where tidal bores are observed only at high river discharges, in conjunction with high equinoctial tides<ref>Freitas, P.T.A., Silveira, O.F.M. and Asp, N.E. 2012. Tide distortion and attenuation in an Amazonian tidal river. Brazilian journal of oceanography, 60: 429446</ref>.  River flow and riverbed slope also influence tidal bore formation. River flow contributes to tidal wave deformation by enhancing the longitudinal velocity gradient and riverbed slope by upstream reduction of the water depth. However, river flow and riverbed slope also contribute to tidal wave damping and thus oppose tidal bore formation <ref> Horrevoets, A. C., Savenije, H. H. G., Schuurman, J. N. and Graas, S. 2004. The influence of river discharge on tidal damping in alluvial estuaries. Journal of Hydrology 294: 213228</ref>. The latter effect usually dominates. Observations show that the tidal bore in the Garonne and Dordogne (France) is suppressed at high river runoff<ref name=B16>Bonneton, P., Filippini, A.G., Arpaia, L., Bonneton, N. and Ricchiuto, M. 2016. Conditions for tidal bore formation in convergent alluvial estuaries. Estuarine, Coastal and Shelf Science. 172: 121127</ref>. A similar effect is observed in the Daly estuary by Wolanski et al. (2006), who relate the occurrence of the tidal bore at low river discharge to the small water depth during such conditions. The opposite effect is reported for the GuamáCapim river system near the mouth of the Amazon River, where tidal bores are observed only at high river discharges, in conjunction with high equinoctial tides<ref>Freitas, P.T.A., Silveira, O.F.M. and Asp, N.E. 2012. Tide distortion and attenuation in an Amazonian tidal river. Brazilian journal of oceanography, 60: 429446</ref>.  
−  Available field data suggest that tidal bores form in convergent estuaries characterized by large values of the convergence ratio <math\delta</math>. Bonneton et al. (2016)<ref name=B16> Bonneton, P., Filippini, A.G., Arpaia, L., Bonneton, N. and Ricchiuto, M. 2016. Conditions for tidal bore formation in convergent alluvial estuaries. Estuarine, Coastal and Shelf Science. 172: 121127</ref> and Filippini et al. (2018)<ref name=F></ref> have analysed tidal bore occurrence as a function of the dimensionless parameters <math>\epsilon</math> and <math>\phi</math> for a fixed value of the convergence ratio <math>\delta=2.4</math>. Fig.  +  Available field data suggest that tidal bores form in convergent estuaries characterized by large values of the convergence ratio <math\delta</math>. Bonneton et al. (2016)<ref name=B16> Bonneton, P., Filippini, A.G., Arpaia, L., Bonneton, N. and Ricchiuto, M. 2016. Conditions for tidal bore formation in convergent alluvial estuaries. Estuarine, Coastal and Shelf Science. 172: 121127</ref> and Filippini et al. (2018)<ref name=F></ref> have analysed tidal bore occurrence as a function of the dimensionless parameters <math>\epsilon</math> and <math>\phi</math> for a fixed value of the convergence ratio <math>\delta=2.4</math>. Fig. 4 presents the position of convergent estuaries in the parameter space (<math>\epsilon, \phi</math>). We can observe a clear separation between tidalbore estuaries and non tidalbore estuaries. Tidal bores occur when the nonlinearity parameter <math>\epsilon</math> is greater than a critical value, <math>\epsilon_c</math>, which is an increasing function of the friction parameter<math>\phi</math>. For small <math>/phi</math>values (friction parameter <math>\phi \approx 15</math>), tidal bores can form for <math>\epsilon</math> greater than 0.2. By contrast, for large <math>\phi</math> values the tidal bore formation requires much larger nonlinearity parameters. These results show that bore formation is mainly controlled by the competition between two physical processes: (a) the nonlinear distortion of the tidal wave, which is favourable to bore inception; (b) the friction dissipation of the tidal wave, which is unfavourable to bore formation. 
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Secondly, large river discharges induce strong tidal damping (Horrevoets et al. 2004)<ref> Horrevoets, A. C., Savenije, H. H. G., Schuurman, J. N. and Graas, S. 2004. The influence of river discharge on tidal damping in alluvial estuaries. Journal of Hydrology 294: 213228</ref>, especially in the upper estuary, where tidal bores generally occur. Wolanski et al. 2006 showed that in the Daly estuary tidal bore occur at spring tides only for low river discharge. In the Garonne River, tidal bore formation is strongly controlled by river discharge (Bonneton et al. 2015, 2016). For large discharge (order of 1000 m3/s), tidal bores do not form. By contrast, for low discharge (order of 150 m3/s) tidal bore almost always occurs in the Garonne River and can even forms at neap tide. These findings was confirmed by Filippini et al. (2019)<ref name=F></ref> from numerical simulations. The opposite effect is reported for the GuamáCapim river system near the mouth of the Amazon River, where tidal bores are observed only at high river discharges, in conjunction with high equinoctial tides<ref>Freitas, P.T.A., Silveira, O.F.M. and Asp, N.E. 2012. Tide distortion and attenuation in an Amazonian tidal river. Brazilian journal of oceanography, 60: 429446</ref>.  Secondly, large river discharges induce strong tidal damping (Horrevoets et al. 2004)<ref> Horrevoets, A. C., Savenije, H. H. G., Schuurman, J. N. and Graas, S. 2004. The influence of river discharge on tidal damping in alluvial estuaries. Journal of Hydrology 294: 213228</ref>, especially in the upper estuary, where tidal bores generally occur. Wolanski et al. 2006 showed that in the Daly estuary tidal bore occur at spring tides only for low river discharge. In the Garonne River, tidal bore formation is strongly controlled by river discharge (Bonneton et al. 2015, 2016). For large discharge (order of 1000 m3/s), tidal bores do not form. By contrast, for low discharge (order of 150 m3/s) tidal bore almost always occurs in the Garonne River and can even forms at neap tide. These findings was confirmed by Filippini et al. (2019)<ref name=F></ref> from numerical simulations. The opposite effect is reported for the GuamáCapim river system near the mouth of the Amazon River, where tidal bores are observed only at high river discharges, in conjunction with high equinoctial tides<ref>Freitas, P.T.A., Silveira, O.F.M. and Asp, N.E. 2012. Tide distortion and attenuation in an Amazonian tidal river. Brazilian journal of oceanography, 60: 429446</ref>.  
−  Variations of local river geometry and water depth can also significantly affect tidal bore dynamics (Bonneton et al. (2011)<ref nameB11a></ref> and Keevil et al. (2015)<ref>Keevil, C. E., Chanson, H. and Reungoat, D. 2015. Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision. Earth Surface Processes and Landforms 40: 15741586</ref>, Fig. 10 ??. Observations show that tidal bores often develop in shallow rivers that discharge from the higher upstream zone into a broad estuary. If the tidal wave has already acquired sufficient asymmetry when travelling through the estuary, a tidal bore develops when the tidal wave surges into the shallow river. The presence of a mouth bars (large <math>\epsilon</math>) is favourable to the development of a strong asymmetry of the tidal wave and tidal bore formation. On the contrary, the presence of local channel constrictions (large <math>\phi</math>) is unfavourable to tidal bore formation.  +  Variations of local river geometry and water depth can also significantly affect tidal bore dynamics (Bonneton et al. (2011)<ref nameB11a></ref> and Keevil et al. (2015)<ref>Keevil, C. E., Chanson, H. and Reungoat, D. 2015. Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision. Earth Surface Processes and Landforms 40: 15741586</ref>, Fig. 10 ??. Observations show that tidal bores often develop in shallow rivers that discharge from the higher upstream zone into a broad estuary. If the tidal wave has already acquired sufficient asymmetry when travelling through the estuary, a tidal bore develops when the tidal wave surges into the shallow river. The presence of a mouth bars (large <math>\epsilon</math>) is favourable to the development of a strong asymmetry of the tidal wave and tidal bore formation, see Fig. 5. On the contrary, the presence of local channel constrictions (large <math>\phi</math>) is unfavourable to tidal bore formation. 
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−  +  [[Image:SeineQiangtang.jpgcenter700pxthumbFigure 5. Many natural estuaries have large shoals in the mouth zone. These socalled mouth bars strongly enhance distortion of the tidal wave entering the estuary. Left image: Tidal bore in the Seine River around 1960, before the mouth bar was dredged. Right image: Tidal bore in the Qiangtang estuary, where a large mouth bar is still present (https://en.wikipedia.org/wiki/Qiantang_River ).]]  
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==Tidal bore characteristics==  ==Tidal bore characteristics==  
−  A tidal bore can be schematically represented by a propagating transition between two streams of water depth <math>D_1</math> and <math>D_2</math>, with <math>D_1 < D_2</math> (see Fig.  +  A tidal bore can be schematically represented by a propagating transition between two streams of water depth <math>D_1</math> and <math>D_2</math>, with <math>D_1 < D_2</math> (see Fig. 6). Once such a hydraulic jump has developed, its flow velocities ahead and behind the jump can be derived from the mass and momentum balance equations (Fig. 6): 
<math>v_1=u_1 – c =  \sqrt{\Large \frac{g D_2 (D_2 + D_1)}{2 D_1}} \normalsize , \quad \quad (1) </math>  <math>v_1=u_1 – c =  \sqrt{\Large \frac{g D_2 (D_2 + D_1)}{2 D_1}} \normalsize , \quad \quad (1) </math>  
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Knowing that <math>D_2 > D_1</math>, it is straightforward to see that <math>F_1 > 1</math> and <math>F_2 < 1</math>. The jump correspond to the passage from a supercritical flow (<math>F_1 > 1</math>) to a subcritical flow (<math>F_2 < 1</math>). The bore intensity is characterized equally by <math>\Delta D / D_1</math> or by the Froude number <math>F_1</math>.  Knowing that <math>D_2 > D_1</math>, it is straightforward to see that <math>F_1 > 1</math> and <math>F_2 < 1</math>. The jump correspond to the passage from a supercritical flow (<math>F_1 > 1</math>) to a subcritical flow (<math>F_2 < 1</math>). The bore intensity is characterized equally by <math>\Delta D / D_1</math> or by the Froude number <math>F_1</math>.  
−  [[Image:TidalBorePropagation.jpgright400pxthumbFigure  +  [[Image:TidalBorePropagation.jpgright400pxthumbFigure 6. Schematic representation of a hydraulic jump propagating with velocity <math>c</math>. In the frame moving with velocity <math>c</math> the hydraulic jump appears stationary; time derivatives are zero. The corresponding mass and momentum balance equations from which the propagation characteristics can be derived are indicated in the figure. The symbols used stand for: <math>u</math>= flow velocity in a fixed frame, <math>v</math>= flow velocity in the moving frame, <math>D</math>= water depth, <math>g</math>=gravitational acceleration; subscripts 1 and 2 indicate upstream and downstream conditions, respectively.]] 
If <math>F_2 = 1</math> (the bore height <math> \Delta D</math> is zero) the bore velocity <math> c </math> is equal to  If <math>F_2 = 1</math> (the bore height <math> \Delta D</math> is zero) the bore velocity <math> c </math> is equal to  
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===Undular bores===  ===Undular bores===  
−  For <math>F_1</math> smaller than approximately 1.3, the bore transition is smooth and followed by a wave train (Fig.  +  For <math>F_1</math> smaller than approximately 1.3, the bore transition is smooth and followed by a wave train (Fig. 1 right). The bore then consists of a mean jump between two water depths (see Fig. 6) on which secondary waves are superimposed. This type of bore is usually called an undular bore<ref name=C>Chanson, H. 2009. Current knowledge in hydraulic jumps and related phenomena. A survey of experimental results. European Journal of MechanicsB/Fluids 28: 191210</ref> and Favre (1935) <ref>Favre, H. 1935. Etude théorique et expérimentale des ondes de translation dans les canaux découverts (Theoretical and experimental study of travelling surges in open channels), Dunod, Paris</ref> was the first to describe this phenomenon from laboratory experiments. That is why undular bores are sometimes referred to as Favre waves. This phenomenon is a dispersive wave process (http://www.coastalwiki.org/wiki/Dispersion_(waves)), which is also named “dispersive shock” in the mathematical community. 
A simple model describing the secondary waves attached to the bore was proposed by Lemoine (1948) <ref name=L></ref>. He considers that secondary waves can be approximated by the linear theory. The propagation speed in the fixed frame is, according to linear [[shallowwater wave theory]],  A simple model describing the secondary waves attached to the bore was proposed by Lemoine (1948) <ref name=L></ref>. He considers that secondary waves can be approximated by the linear theory. The propagation speed in the fixed frame is, according to linear [[shallowwater wave theory]],  
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As this approach ignores waveinduced mass transport it is valid only for very small bores (<math>F_1</math> close to 1)<ref>Wilkinson, M.L. and Banner, M.L. 1977. 6th Australasian Hydraulics and Fluid Mechanics Conf. Adelaide, Australia, 59 December 1977</ref>. Nevertheless, equations (7) and (8) appear to give a correct estimate of the observed wavelength and amplitude of undular bore propagating in rectangular channels (<ref>Binnie, A. M., & Orkney, J. C. (1955). Experiments on the flow of water from a reservoir through an open horizontal channel II. The formation of hydraulic jumps. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 230(1181), 237246</ref><ref>Chanson, H. (2010). Undular tidal bores: basic theory and freesurface characteristics. Journal of Hydraulic Engineering, 136(11), 940944</ref>).  As this approach ignores waveinduced mass transport it is valid only for very small bores (<math>F_1</math> close to 1)<ref>Wilkinson, M.L. and Banner, M.L. 1977. 6th Australasian Hydraulics and Fluid Mechanics Conf. Adelaide, Australia, 59 December 1977</ref>. Nevertheless, equations (7) and (8) appear to give a correct estimate of the observed wavelength and amplitude of undular bore propagating in rectangular channels (<ref>Binnie, A. M., & Orkney, J. C. (1955). Experiments on the flow of water from a reservoir through an open horizontal channel II. The formation of hydraulic jumps. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 230(1181), 237246</ref><ref>Chanson, H. (2010). Undular tidal bores: basic theory and freesurface characteristics. Journal of Hydraulic Engineering, 136(11), 940944</ref>).  
−  However, natural estuary and river channels are nonrectangular and present most of the time a variable crosssection with a nearly trapezoidal shape and gently sloping banks. The propagation of undular bores over channels with variable crosssections was studied by Treske (1994)<ref name=Tr>Treske, A. 1994. Undular bore (Favrewaves) in open channels  Experimental studies, J. Hydraulic Res. 32: 355370</ref> in the laboratory and by Bonneton et al. (2015)<ref name=B15></ref> in the field. Both studies identified a transition around <math> F_t=1.15</math>. For <math>F_1 > F_t</math> the secondary wave field in the mid channel is very similar to the dispersive waves (Favre waves) described above, and follow the relations (7) and (8)<ref name=B15></ref>. For <math>F_1 < F_t</math>, the secondary wave wavelength in the whole channel is at least two to three times larger than in a rectangular channel for the same Froude numbers. It was shown that this new undular bore regime (Fig. A c,d and B b) differs significantly from classical dispersive undular bores in rectangular channels (i.e. Favre waves). Chassagne et al. (2019) <ref name=  +  However, natural estuary and river channels are nonrectangular and present most of the time a variable crosssection with a nearly trapezoidal shape and gently sloping banks. The propagation of undular bores over channels with variable crosssections was studied by Treske (1994)<ref name=Tr>Treske, A. 1994. Undular bore (Favrewaves) in open channels  Experimental studies, J. Hydraulic Res. 32: 355370</ref> in the laboratory and by Bonneton et al. (2015)<ref name=B15></ref> in the field. Both studies identified a transition around <math> F_t=1.15</math>. For <math>F_1 > F_t</math> the secondary wave field in the mid channel is very similar to the dispersive waves (Favre waves) described above, and follow the relations (7) and (8)<ref name=B15></ref>. For <math>F_1 < F_t</math>, the secondary wave wavelength in the whole channel is at least two to three times larger than in a rectangular channel for the same Froude numbers. It was shown that this new undular bore regime (Fig. A c,d and B b) differs significantly from classical dispersive undular bores in rectangular channels (i.e. Favre waves). Chassagne et al. (2019) <ref name=CF>Chassagne, R., Filippini, A., Ricchiuto, M. and Bonneton, P. 2019. Dispersive and dispersivelike bores in channels with sloping banks. Journal of Fluid Mechanics 870: 595616. doi:10.1017/jfm.2019.287</ref> recently showed that this undular bore regime (named dispersivelike bore) is controlled by hydrostatic nondispersive wave properties, with a dynamics similar to [[Infragravity wavesedgewaves]] in the nearshore. The transition between dispersive and dispersivelike bores is illustrated on Fig. 7 and 8. The dispersivelike bores are characterized by low wave steepness, which make them difficult to visually observe in the field. It is why such tidal bores are generally ignored and why tidal bore occurrence in the field is strongly underestimated <ref> Bonneton, P., Van de Loock, J., Parisot, JP., Bonneton, N., Sottolichio, A., Detandt, G., Castelle, B., Marieu, V. and Pochon, N. 2011. On the occurrence of tidal bores – The Garonne River case. Journal of Coastal Research, SI 64: 114621466</ref> and 2012 ??). 
−  [[Image:DispersiveBore.jpgright500pxthumbFigure  +  [[Image:DispersiveBore.jpgright500pxthumbFigure 7. Illustration of the two undular tidal bore regimes (Garonne River, Bonneton et al. 2015). Panels a, b: Dispersive regime (“mascaret” in French), F1=1.27. Panels c, d: Dispersivelike regime, Fr=1.08. Black line: elevation in the midchannel; magenta line: elevation close to the river bank. Panel a:Twodimensional (2D) phase structure. Panel c: Quasi1D phase structure.]] 
−  [[Image:  +  [[Image:DispersiveUndularBores.jpgleft300pxthumbFigure 8. Numerical simulations of the two undular tidal bore regimes. Panel a: Dispersive regime, F1=1.20. Panel b: Dispersivelike regime, F1=1.05. From Chassagne et al. 2019<ref name=CF></ref>.]] 
===Breaking bores===  ===Breaking bores===  
−  For large Froude numbers, bores correspond to turbulent breaking fronts (  +  For large Froude numbers, bores correspond to turbulent breaking fronts (Fig. 5), where the energy is dissipated by turbulent eddies<ref>Tu, J. and Fan, D. 2017. Flow and turbulence structure in a hypertidal estuary with the world's biggest tidal bore. Journal of Geophysical Research: Oceans, 122: 34173433</ref>. The transition between undular and turbulent bores is shown in Fig. 11. 
+  Before arrival of the tidal bore the current velocity is generally low and in the ebb direction. At the arrival of the bore front the current velocity is almost instantaneously reversed into the flood direction and reaches a high value within a minute. Fig. 9 shows the sharp increase in the current velocity and suspended sediment concentration recorded in the megatidal Baie du Mont Saint Michel at spring tide when the tidal flood wave enters the tidal flat area and the small rivers at the head of the Baie <ref name=F> Furgerot, L., Mouazé, D., Tessier, B., Perez, L., Haquin, S., Weill, P., & Crave, A. (2016). Sediment transport induced by tidal bores. An estimation from suspended matter measurements in the Sée River (MontSaintMichel Bay, northwestern France). Comptes Rendus Géoscience, 348(6), 432441</ref>. The sudden strong change of the current velocity has an important stirring effect on the bed sediments, causing a sharp increase of the suspended sediment concentration. Turbulent bores contribute significantly to upstream sediment transport<ref>Reungoat, D., Lubin, P., Leng, X. and Chanson, H. 2018. Coastal Engineering Journal 60: 484498</ref>.  
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+  [[Image:TidalBoreSedimentSeeRiver_Furgerot.jpgcenter900pxthumbFigure 9. Measurement of sediment transport by a tidal bore in the Sée River on 8 May 2012 (Furgerot et al, 2016 <ref name=F></ref>). Panel 1: Tidal bore entering the Sée channel at the head of the Baie du Mont Saint Michel. The monastery is visible at the background. Panel 2: Map of the inner part of the Baie du Mont Saint Michel. The measuring location in the Sée River is indicated by the red star. Panel 3: Temporal variation of the vertical distribution of suspended sediment concentration (top), current velocity (middle) and sediment flux just (bottom) before and after the arrival of the tidal bore. The current is ebbdirected with very low velocity just before arrival of the tidal bore and reverses to a flooddirected velocity simultaneously with the arrival of the tidal bore (middle panel). A very high short peak in the suspended sediment concentration near the bed occurs at the passage of the tidal bore front. High suspended sediment concentrations over the entire vertical (mainly fine silty sand) are recorded about 5 minutes later representing the sediment advected by the tidal bore (top panel). After a very high peak at the passage of the bore front the sediment flux is again high and in upriver direction some minutes later. The sediment flux is much lower during the remaining flood period (bottom panel) and similar to the upriver sediment flux when a tidal bore is absent.  
+  A video of the arrival of the tidal bore can be viewed by clicking [https://www.youtube.com/watch?v=waeS1MvhT9M here]. The photo at the right is a still from the video.]]  
+  
In many tidal rivers the bore propagation near the shallow channel banks differs from the bore propagation in the deeper middle part of the channel. The local Froude number is relatively lower at the deeper parts of the channel, where the bore has often an undulating character, while a higher breaking bore occurs in the shallower parts. This phenomenon is illustrated in Fig. 10 for the tidal bore in the Petitcodiac River.  In many tidal rivers the bore propagation near the shallow channel banks differs from the bore propagation in the deeper middle part of the channel. The local Froude number is relatively lower at the deeper parts of the channel, where the bore has often an undulating character, while a higher breaking bore occurs in the shallower parts. This phenomenon is illustrated in Fig. 10 for the tidal bore in the Petitcodiac River.  
Line 306:  Line 311:  
Recent approaches, based on the resolution of the general Navier Stokes equations in their multiphase form (water phase and air phase), allow a detailed description of bore structure, turbulence and air entrainment in a roller at the bore front<ref name=L>Lubin, P., Chanson, H. and Glockner, S. 2010. Large eddy simulation of turbulence generated by a weak breaking tidal bore. Environmental Fluid Mechanics 10: 587602</ref><ref>Berchet, A., Simon, B., Beaudoin, A., Lubin, P., Rousseaux, G. and Huberson, S. 2018. Flow fields and particle trajectories beneath a tidal bore: A numerical study. International Journal of Sediment Research 33: 351370</ref>. Figure 13 presents the simulation of recirculating structures under a breaking bore. NavierStokes approaches are dedicated to small scale bore processes that cannot be applied to tidal bores at the estuarine scales because of limited computation power. Such applications would require longwave models, where small scale vorticity motions are parametrized and not directly resolved.  Recent approaches, based on the resolution of the general Navier Stokes equations in their multiphase form (water phase and air phase), allow a detailed description of bore structure, turbulence and air entrainment in a roller at the bore front<ref name=L>Lubin, P., Chanson, H. and Glockner, S. 2010. Large eddy simulation of turbulence generated by a weak breaking tidal bore. Environmental Fluid Mechanics 10: 587602</ref><ref>Berchet, A., Simon, B., Beaudoin, A., Lubin, P., Rousseaux, G. and Huberson, S. 2018. Flow fields and particle trajectories beneath a tidal bore: A numerical study. International Journal of Sediment Research 33: 351370</ref>. Figure 13 presents the simulation of recirculating structures under a breaking bore. NavierStokes approaches are dedicated to small scale bore processes that cannot be applied to tidal bores at the estuarine scales because of limited computation power. Such applications would require longwave models, where small scale vorticity motions are parametrized and not directly resolved.  
−  
The most common longwave model is the depthaveraged Saint Venant or Non Linear Shallow Water (NSW) model, which assumes hydrostatic pressure. This model gives a good description of the largescale tidal wave transformation<ref name=S></ref>. The properties of breaking turbulent bores, are also quite well described by the nondispersive NSW equations with jump conditions <ref>Chanson, H. 2012. Tidal bores, aegir, eagre, mascaret, pororoca: Theory and observations. World Scientific</ref>.  The most common longwave model is the depthaveraged Saint Venant or Non Linear Shallow Water (NSW) model, which assumes hydrostatic pressure. This model gives a good description of the largescale tidal wave transformation<ref name=S></ref>. The properties of breaking turbulent bores, are also quite well described by the nondispersive NSW equations with jump conditions <ref>Chanson, H. 2012. Tidal bores, aegir, eagre, mascaret, pororoca: Theory and observations. World Scientific</ref>. 
Revision as of 16:21, 16 August 2019
Tidal bore dynamics
A tidal bore is a sudden elevation of the water surface that travels upstream an estuary with the incoming flood tide. This article describes the processes involved in this ultimate stage of tidal wave deformation and the modelling of these processes. For an introduction to the topic of tidal wave deformation the reader is referred to the article Tidal asymmetry and tidal basin morphodynamics.
Contents
Introduction
Estuary/river name  Maximum tidal range [m]  Tide gauge location  Country 

Shannon River  5.6  Shannon  Ireland 
Humber,Trent  8.3  Hull  UK 
Great Ouse  7.4  King's Lynn  
Severn  12  Portishead  
Dee  9.8  Flint  
Mersey  10  Liverpool  
Ribble  10.2  Lytham  
Morecambe bay, Nith River, River Kent  10.9  Morecambe  
River Eden  10.3  Silloth  
Seine  8.5  Honfleur  France 
Canal de Carentan  7  Carentan  
Baie du Mont Saint Michel – Sélune River  14  Granville  
Garonne, Dordogne  6.3  Podensac  
Pungue  7  Beira  Mozambique 
Qiantang  5  Ganpu  China 
Indus  4  Port Muhammad Bin Qasim  Pakistan 
Hooghly  5.7  Sagar Island  India 
Brahmaputra  5.7  Chittagong  Bangladesh 
Sittaung  6.3  Moulmein  Myanmar 
Batang Lupar River  5.6  Kuching  Sarawak 
Kampar River  5.3  Pulo Muda  Sumatra 
Hooghly  5.7  Sagar Island  India 
Fly , Bamu, Turamu Rivers  4.2  estuary  West Papua 
Styx  6.4  Mackay, Queensland  Australia 
Daly River  7.9  estuary  
Turnagain Arm, Knick Arm  7.9  Anchorage  Alaska 
Bay of Fundy, Petitcodiac and Salmon rivers  16  Truro  Canada 
Colorado River  7.7  San Filipe  Mexico 
Amazon, Araguira, Guama, Capim and Mearim Rivers  3.4  Macapa  Brazil 
When a tidal wave propagates upstream into an estuary, its shape is progressively distorted. In many estuaries the highwater (HW) tidal wave crest propagates faster upestuary than the lowwater (LW) tidal wave trough. HW propagates considerably faster than LW if the mean channel depth [math]D_0[/math] is not much greater than the spring tidal range [math]2a[/math] (same order of magnitude or a few times larger) and if the intertidal area is smaller than the tidal channel surface area. The tidal rise period is shortened and the tidal wave becomes steeper during propagation. If the tidal wave can propagate sufficiently far upstream the river without strong damping, the tidal wave front may become so steep that a water level jump – a socalled tidal bore, can form at the beginning of the rising tide.
Table 1 presents estuaries and tidal rivers in which significant tidal bores have been observed (see also BartschWinkler and Lynch (1988) ^{[2]} and Colas (2017)^{[3]}). It appears that tidal bores generally occur for large tidal ranges (Tr0) at the estuary mouth. However, such a simple criterion is not sufficient to classify estuaries in terms of tidal bore occurrence. For instance, Furgerot (2014) ^{[4]} showed that in the Sée/Mont Saint Michel estuary, Tr0 must be larger than 10 m for tidal bore formation in the Sée River and, on the other hand, Bonneton et al. 2015 observed tidal bores in the Gironde/Garonne estuary even for Tr0 smaller than 2 m.
The estuarine shape (depth and width profiles) plays an important role in tidal bore formation. This is illustrated by the observation that engineering works (dredging, weirs) during the past century have weakened or even completely suppressed tidal bores in many rivers, for example in the rivers Seine, Loire, Charente and Petitcodiac. We will discuss in the following general conditions for bore formation in terms of estuary geometry and flow conditions.
Illustrations of tidal bores are shown in Fig. 1.
Tidal bore formation in funnelshaped estuaries
The analysis of nonlinear tidal wave transformation in estuaries, in terms of tidal forcing at the estuary mouth and largescale geometrical properties of the channel, has received considerable attention (see Tidal asymmetry and tidal basin morphodynamics). By contrast, the extreme nonlinear tidalwave case where tidal bores form is much less studied.
The most intense tidal bores occur in long and shallow tidedominated funnelshaped estuaries (e.g. Qiantang, Severn, Kampar, Gironde/Garonne/Dordogne, …) . The formation of these tidal bores is governed by the progressive distortion of the tidal wave as it propagates up the estuary. When the ebbflood asymmetry (ebb duration longer than the flood and larger flood than ebb currents) is strong enough generally in the upper estuary (see tidal wave elevation in Fig. 2 at Bordeaux and Podensac), tidal bore can form (see Figs. 2 and 3). This extreme nonlinear deformation of a tidal wave occurs in a limited number of estuaries. To determine the conditions favourable to tidal bore occurrence, a scaling analysis can eb carried out for a schematic channel geometry. A usual schematization is based on the approximations ^{[7]}^{[8]}^{[9]}:
 Uniform channel depth [math]D[/math],
 Exponential convergence of the width [math]B(x)[/math].
The convergence length [math]L_b [/math], defined as [math]L_b=B(x)/dB/dx[/math], is assumed independent of the alongchannel coordinate [math]x[/math]. The forcing tidal wave at the estuary mouth can be characterized by its tidal frequency [math]\omega=2 \pi / T[/math] and its amplitude [math]A_0=Tr_0/2[/math], where [math]Tr_0[/math] is the estuary mouth tidal range. Another important parameter which controls tide propagation in estuary is the dimensionless mean friction coefficient [math]c_D[/math]. The tidal wave dynamics is then controlled, if we neglect fresh water discharge effects, by three dimensionless parameters:
 the nonlinearity parameter, [math]\epsilon=A_0/D[/math],
 the friction parameter, [math]\phi=c_D \sqrt{gD}/(\omega D)[/math],
 the convergence ratio, [math]\delta=\sqrt{gD}/(\omega L_b)[/math].
The formation of tidal bores is mainly governed by the progressive distortion of the tidal wave as it propagates up the estuary. This extreme nonlinear deformation of the tidal wave occurs under special conditions, in particular^{[5]} ^{[10]}:
 a large tidal amplitude [math]a[/math],
 a long, shallow and convergent channel.
River flow and riverbed slope also influence tidal bore formation. River flow contributes to tidal wave deformation by enhancing the longitudinal velocity gradient and riverbed slope by upstream reduction of the water depth. However, river flow and riverbed slope also contribute to tidal wave damping and thus oppose tidal bore formation ^{[11]}. The latter effect usually dominates. Observations show that the tidal bore in the Garonne and Dordogne (France) is suppressed at high river runoff^{[12]}. A similar effect is observed in the Daly estuary by Wolanski et al. (2006), who relate the occurrence of the tidal bore at low river discharge to the small water depth during such conditions. The opposite effect is reported for the GuamáCapim river system near the mouth of the Amazon River, where tidal bores are observed only at high river discharges, in conjunction with high equinoctial tides^{[13]}.
Available field data suggest that tidal bores form in convergent estuaries characterized by large values of the convergence ratio <math\delta</math>. Bonneton et al. (2016)^{[12]} and Filippini et al. (2018)^{[6]} have analysed tidal bore occurrence as a function of the dimensionless parameters [math]\epsilon[/math] and [math]\phi[/math] for a fixed value of the convergence ratio [math]\delta=2.4[/math]. Fig. 4 presents the position of convergent estuaries in the parameter space ([math]\epsilon, \phi[/math]). We can observe a clear separation between tidalbore estuaries and non tidalbore estuaries. Tidal bores occur when the nonlinearity parameter [math]\epsilon[/math] is greater than a critical value, [math]\epsilon_c[/math], which is an increasing function of the friction parameter[math]\phi[/math]. For small [math]/phi[/math]values (friction parameter [math]\phi \approx 15[/math]), tidal bores can form for [math]\epsilon[/math] greater than 0.2. By contrast, for large [math]\phi[/math] values the tidal bore formation requires much larger nonlinearity parameters. These results show that bore formation is mainly controlled by the competition between two physical processes: (a) the nonlinear distortion of the tidal wave, which is favourable to bore inception; (b) the friction dissipation of the tidal wave, which is unfavourable to bore formation.
Conditions for tidal bore formation
More generally, the analysis of tidal bores observed in natural estuaries suggests the following conditions for tidal bore formation.
The first requirement for a tidal bore is sufficient length of the estuarinetidal river system, i.e. the tide can penetrate far enough into the system. This does not depend so much on the convergence length [math]L_b[/math], but mainly on the average slope of the estuarytidal river system and river flow. A steep slope and a large flow limit the penetration of the tide.
The tidal bore develops in a section of the estuarytidal river system where the nonlinearity parameter [math]\epsilon[/math] is large (shallow depth, large tidal range) and where the tidal wave is already rather asymmetrical at the estuarine mouth. Sometimes the tidal bore is already forming close to the mouth of the estuary. This is the case of estuaries that have a large mouth bar, such as the Qiangtang and the Charente (before the largescale interventions). But this is rather the exception. Most often the tidal bore is formed in a section of the estuarytidal river system further upstream, in the tidal river or on the tidal flats at the head of the estuary (as for the Garonne, Seine, Sée, etc.). A low river flow (or low ebb tide) seems favorable to trigger the tidal bore. The convergence of the width plays a smaller role. If the longitudinal channel slope is weak, the tidal bore can travel over a long distance without much dissipation. A small channel slope is generally indicative of lowtomoderate current velocities and friction (steep slopes are generally related to strong friction generated by high current velocities and coarse bed sediments).
The condition of a large tidal range and a strong tidal asymmetry depends on friction; one should therefore distinguish between muddy and nonsilty estuaries. For estuaries where the solid fluvial flow is important and containing a large fraction of cohesive mud (mainly estuaries in the (sub) tropic regions, but also the Garonne), the estuarine channel bed is generally smooth so the friction is weak (small [math]\phi[/math]); the tidal wave is not or hardly damped. In this case a small relative channel depth (large [math]\epsilon[/math]) is the main condition to obtain a strong asymmetry of the tidal wave. Estuaries with low solid discharge or supplying sandy/coarse sediments have sandy or gravelly bottom channels in the mouth region that produces strong friction (large [math]\phi[/math]) and strong tidal energy dissipation. In this case, the main condition for obtaining a large tidal range is a strong convergence of the width of the estuary (large [math]/delta[/math]). Secondly, large river discharges induce strong tidal damping (Horrevoets et al. 2004)^{[14]}, especially in the upper estuary, where tidal bores generally occur. Wolanski et al. 2006 showed that in the Daly estuary tidal bore occur at spring tides only for low river discharge. In the Garonne River, tidal bore formation is strongly controlled by river discharge (Bonneton et al. 2015, 2016). For large discharge (order of 1000 m3/s), tidal bores do not form. By contrast, for low discharge (order of 150 m3/s) tidal bore almost always occurs in the Garonne River and can even forms at neap tide. These findings was confirmed by Filippini et al. (2019)^{[6]} from numerical simulations. The opposite effect is reported for the GuamáCapim river system near the mouth of the Amazon River, where tidal bores are observed only at high river discharges, in conjunction with high equinoctial tides^{[15]}.
Variations of local river geometry and water depth can also significantly affect tidal bore dynamics (Bonneton et al. (2011)Cite error: Invalid <ref>
tag;
refs with no name must have content and Keevil et al. (2015)^{[16]}, Fig. 10 ??. Observations show that tidal bores often develop in shallow rivers that discharge from the higher upstream zone into a broad estuary. If the tidal wave has already acquired sufficient asymmetry when travelling through the estuary, a tidal bore develops when the tidal wave surges into the shallow river. The presence of a mouth bars (large [math]\epsilon[/math]) is favourable to the development of a strong asymmetry of the tidal wave and tidal bore formation, see Fig. 5. On the contrary, the presence of local channel constrictions (large [math]\phi[/math]) is unfavourable to tidal bore formation.
Tidal bore characteristics
A tidal bore can be schematically represented by a propagating transition between two streams of water depth [math]D_1[/math] and [math]D_2[/math], with [math]D_1 \lt D_2[/math] (see Fig. 6). Once such a hydraulic jump has developed, its flow velocities ahead and behind the jump can be derived from the mass and momentum balance equations (Fig. 6):
[math]v_1=u_1 – c =  \sqrt{\Large \frac{g D_2 (D_2 + D_1)}{2 D_1}} \normalsize , \quad \quad (1) [/math]
[math]v_2=u_2 – c =  \sqrt{\Large \frac{g D_1 (D_2 + D_1)}{2 D_2}} \normalsize , \quad \quad (2) [/math]
where [math]c[/math] is the bore celerity, [math]v[/math] the velocity in the moving frame and [math]u[/math] in the fixed frame. These equations show that the problem is entirely determined by the depth ratio [math]D_2 / D_1[/math], or equivalently by [math]\Delta D / D1[/math], where [math]\Delta D = D_2 – D_1[/math] is the bore height. The Froude numbers around the jump, in the moving frame can be obtained from equations (1) and (2):
[math]F_1^2 = \Large \frac{v_1^2}{g D_1}\normalsize = \large \frac{D_2 (D_1+D_2)}{2 D_1^2} \normalsize , \quad \quad (3) [/math]
[math]F_2^2 = \Large \frac{v_2^2}{g D_2}\normalsize = \large \frac{D_1 (D_2+D_1)}{2 D_2^2} \normalsize . \quad \quad (4)[/math]
Knowing that [math]D_2 \gt D_1[/math], it is straightforward to see that [math]F_1 \gt 1[/math] and [math]F_2 \lt 1[/math]. The jump correspond to the passage from a supercritical flow ([math]F_1 \gt 1[/math]) to a subcritical flow ([math]F_2 \lt 1[/math]). The bore intensity is characterized equally by [math]\Delta D / D_1[/math] or by the Froude number [math]F_1[/math].
If [math]F_2 = 1[/math] (the bore height [math] \Delta D[/math] is zero) the bore velocity [math] c [/math] is equal to
[math]c_2 = u_2 + \sqrt{g D_2}[/math].
Another requirement for a tidal bore is therefore [math]c \lt c_2[/math] or [math]F_2 \lt 1[/math] and [math]F_1 \gt 1[/math]. Because [math]c_2[/math] is the propagation speed of longwave disturbances upstream of the bore, these disturbances (undulations) will catch up to the bore. A slowly propagating bore ([math]F_2 \lt \lt 1[/math]) will thus grow faster and become higher than a fast propagating bore ([math]F_1, F_2 [/math] close to 1) ^{[17]}.
It can be shown that shown that conservation of mass and momentum at the transition does not imply conservation of energy. From mass and momentum equations he obtained the following expression for the energy dissipation ^{[18]} ^{[19]}:
[math]\Delta E = \rho g \Delta H = \rho g \Delta D + 0.5 \rho (v_2^2 v_1^2) = \Large \frac{\rho g}{4} \sqrt{\frac{g (D_1+D_2)}{2 D_1 D_2}}\normalsize (D_2 – D_1)^3 , \quad \quad (5)[/math]
Two forms of energy dissipation can occur at the transition, leading to two different types of bores: undular and turbulent bores.
Undular bores
For [math]F_1[/math] smaller than approximately 1.3, the bore transition is smooth and followed by a wave train (Fig. 1 right). The bore then consists of a mean jump between two water depths (see Fig. 6) on which secondary waves are superimposed. This type of bore is usually called an undular bore^{[20]} and Favre (1935) ^{[21]} was the first to describe this phenomenon from laboratory experiments. That is why undular bores are sometimes referred to as Favre waves. This phenomenon is a dispersive wave process (http://www.coastalwiki.org/wiki/Dispersion_(waves)), which is also named “dispersive shock” in the mathematical community.
A simple model describing the secondary waves attached to the bore was proposed by Lemoine (1948) ^{[22]}. He considers that secondary waves can be approximated by the linear theory. The propagation speed in the fixed frame is, according to linear shallowwater wave theory,
[math] c_w = u_2 + \sqrt{gD_2 (1 \frac{k^2 D_2^2}{3})} , \quad \quad (6)[/math]
where [math]k=2 \pi / \lambda[/math] is the wave number and [math]\lambda[/math] the wavelength. The secondary waves are stationary in a frame moving with the bore front, i.e. [math]c_w = c[/math]. This equality provides an estimate of the wavelength:
[math]\lambda=\sqrt{2/3} \pi D_2 (F_1 – 1)^{1/2} . \quad \quad (7)[/math]
The secondary waves are stationary in the frame moving with the bore front, but in this frame wave energy is radiated behind the front. Equating the energy flux with the mean bore dissipation (Eq. 6) gives the secondary wave amplitude as a function of [math]F_1[/math]:
[math]a_w=\frac{4}{3\sqrt{3}} D_2 (F_1 – 1) . \quad \quad (8)[/math]
As this approach ignores waveinduced mass transport it is valid only for very small bores ([math]F_1[/math] close to 1)^{[23]}. Nevertheless, equations (7) and (8) appear to give a correct estimate of the observed wavelength and amplitude of undular bore propagating in rectangular channels (^{[24]}^{[25]}). However, natural estuary and river channels are nonrectangular and present most of the time a variable crosssection with a nearly trapezoidal shape and gently sloping banks. The propagation of undular bores over channels with variable crosssections was studied by Treske (1994)^{[26]} in the laboratory and by Bonneton et al. (2015)^{[5]} in the field. Both studies identified a transition around [math] F_t=1.15[/math]. For [math]F_1 \gt F_t[/math] the secondary wave field in the mid channel is very similar to the dispersive waves (Favre waves) described above, and follow the relations (7) and (8)^{[5]}. For [math]F_1 \lt F_t[/math], the secondary wave wavelength in the whole channel is at least two to three times larger than in a rectangular channel for the same Froude numbers. It was shown that this new undular bore regime (Fig. A c,d and B b) differs significantly from classical dispersive undular bores in rectangular channels (i.e. Favre waves). Chassagne et al. (2019) ^{[27]} recently showed that this undular bore regime (named dispersivelike bore) is controlled by hydrostatic nondispersive wave properties, with a dynamics similar to edgewaves in the nearshore. The transition between dispersive and dispersivelike bores is illustrated on Fig. 7 and 8. The dispersivelike bores are characterized by low wave steepness, which make them difficult to visually observe in the field. It is why such tidal bores are generally ignored and why tidal bore occurrence in the field is strongly underestimated ^{[28]} and 2012 ??).
Breaking bores
For large Froude numbers, bores correspond to turbulent breaking fronts (Fig. 5), where the energy is dissipated by turbulent eddies^{[29]}. The transition between undular and turbulent bores is shown in Fig. 11. Before arrival of the tidal bore the current velocity is generally low and in the ebb direction. At the arrival of the bore front the current velocity is almost instantaneously reversed into the flood direction and reaches a high value within a minute. Fig. 9 shows the sharp increase in the current velocity and suspended sediment concentration recorded in the megatidal Baie du Mont Saint Michel at spring tide when the tidal flood wave enters the tidal flat area and the small rivers at the head of the Baie ^{[6]}. The sudden strong change of the current velocity has an important stirring effect on the bed sediments, causing a sharp increase of the suspended sediment concentration. Turbulent bores contribute significantly to upstream sediment transport^{[30]}.
In many tidal rivers the bore propagation near the shallow channel banks differs from the bore propagation in the deeper middle part of the channel. The local Froude number is relatively lower at the deeper parts of the channel, where the bore has often an undulating character, while a higher breaking bore occurs in the shallower parts. This phenomenon is illustrated in Fig. 10 for the tidal bore in the Petitcodiac River.
Modelling
Tidal bore formation involves a large range of temporal and spatial scales, from the estuary to the turbulence scale. For this reason it is difficult to model these processes as a whole, both from physical models (laboratory experiments) and numerical approaches.
Physical models
Due to the large range of scales, it is impossible to design a laboratory experiment in close similitude with natural tidal bores. However, leaving aside the tidal wave transformation and bore formation, the bore in itself (i.e. hydraulic jump in translation) can be studied in detail from flume experiments. The bore is commonly generated in a rectangular flume by using a fastclosing gate at the upstream end of the flume^{[26]}^{[20]}. This method allows the study of the different bore regimes (see Fig. 11) and provides valuable insights in secondary wave structure and vortical motions.
To avoid the abrupt bore generation of the above method, Rousseaux et al. (2016)^{[10]} proposed a novel approach. This method mimics the tidal asymmetry met in nature between the ebb and the flood. Fig. 12 shows an example of a tidallike bore generated with this method.
Numerical models
Recent approaches, based on the resolution of the general Navier Stokes equations in their multiphase form (water phase and air phase), allow a detailed description of bore structure, turbulence and air entrainment in a roller at the bore front^{[22]}^{[31]}. Figure 13 presents the simulation of recirculating structures under a breaking bore. NavierStokes approaches are dedicated to small scale bore processes that cannot be applied to tidal bores at the estuarine scales because of limited computation power. Such applications would require longwave models, where small scale vorticity motions are parametrized and not directly resolved.
The most common longwave model is the depthaveraged Saint Venant or Non Linear Shallow Water (NSW) model, which assumes hydrostatic pressure. This model gives a good description of the largescale tidal wave transformation^{[32]}. The properties of breaking turbulent bores, are also quite well described by the nondispersive NSW equations with jump conditions ^{[33]}. However, the onset of a tidal bore and its evolution upstream is controlled by nonhydrostatic dispersive mechanisms^{[34]}. If the onset of the tidal bore can be well described by classical weakly dispersive weakly nonlinear Boussinesqtype equations (^{[34]}, see Boussinesq equations), the subsequent nonlinear evolution, for highintensity tidal bores, requires the use of the basic fully nonlinear Boussinesq equations, named SerreGreen Naghdi (SGN) equations^{[35]}^{[36]}^{[6]}^{[20]}. This modelling approach allows an accurate description of both, the tidal bore formation at the estuarine scale (Fig. 4) and the bore structure at the local scale (Fig. 14).
See also
 Tidal asymmetry and tidal basin morphodynamics
 Ocean and shelf tides
 Morphology of estuaries
 Estuaries
Further reading
Dronkers, J.J. 1964. Tidal computations in rivers and coastal waters. NorthHolland Publ. Co., 518 pp.
References
 ↑ Bonneton, P, Parisot, JP., Bonneton, N., Sottolichio, A., Castelle, B., Marieu, V., Pochon, N. and Van de Loock, J. 2011. Large amplitude undular tidal bore propagation in the Garonne River, France, Proceedings of the 21st ISOPE Conference: 870874, ISBN 9781880653968
 ↑ BartschWinkler, S., and Lynch, D. K. 1988. Catalog of worldwide tidal bore occurrences and characteristics. US Government Printing Office
 ↑ Colas, A. 2017. Mascaret, prodige de la marée. Atlantica Editions
 ↑ Furgerot, L. (2014). Propriétés hydrodynamiques du mascaret et de son influence sur la dynamique sédimentaire: Une approche couplée en canal et in situ (estuaire de la Sée, Baie du Mont Saint Michel) (Doctoral dissertation, Caen)
 ↑ ^{5.0} ^{5.1} ^{5.2} ^{5.3} Bonneton, P., Bonneton, N., Parisot, JP. and Castelle, B. 2015. Tidal bore dynamics in funnelshaped estuaries. J. Geophys. Res.: Ocean 120: 923941. DOI: 10.1002/2014JC010267
 ↑ ^{6.0} ^{6.1} ^{6.2} ^{6.3} ^{6.4} ^{6.5} Filippini, A. G., Arpaia, L., Bonneton, P., & Ricchiuto, M. 2019. Modeling analysis of tidal bore formation in convergent estuaries. European Journal of MechanicsB/Fluids 73: 5568 Cite error: Invalid
<ref>
tag; name "F" defined multiple times with different content  ↑ Lanzoni, S. and Seminara, G. 1998. On tide propagation in convergent estuaries, J. Geophys. Res. 103: 30793–30812
 ↑ Savenije, H.H.G. 2012. Salinity and Tides in Alluvial Estuaries, second ed., Salinity and Tides in Alluvial Estuaries, second ed., www.salinityandtides.com
 ↑ Dronkers, J. 2017. Convergence of estuarine channels. Continental Shelf Res. 144: 120–133
 ↑ ^{10.0} ^{10.1} ^{10.2} Rousseaux, G., Mougenot, J. M., Chatellier, L., David, L. and Calluaud, D. 2016. A novel method to generate tidallike bores in the laboratory. European Journal of MechanicsB/Fluids 55:3138
 ↑ Horrevoets, A. C., Savenije, H. H. G., Schuurman, J. N. and Graas, S. 2004. The influence of river discharge on tidal damping in alluvial estuaries. Journal of Hydrology 294: 213228
 ↑ ^{12.0} ^{12.1} Bonneton, P., Filippini, A.G., Arpaia, L., Bonneton, N. and Ricchiuto, M. 2016. Conditions for tidal bore formation in convergent alluvial estuaries. Estuarine, Coastal and Shelf Science. 172: 121127 Cite error: Invalid
<ref>
tag; name "B16" defined multiple times with different content  ↑ Freitas, P.T.A., Silveira, O.F.M. and Asp, N.E. 2012. Tide distortion and attenuation in an Amazonian tidal river. Brazilian journal of oceanography, 60: 429446
 ↑ Horrevoets, A. C., Savenije, H. H. G., Schuurman, J. N. and Graas, S. 2004. The influence of river discharge on tidal damping in alluvial estuaries. Journal of Hydrology 294: 213228
 ↑ Freitas, P.T.A., Silveira, O.F.M. and Asp, N.E. 2012. Tide distortion and attenuation in an Amazonian tidal river. Brazilian journal of oceanography, 60: 429446
 ↑ Keevil, C. E., Chanson, H. and Reungoat, D. 2015. Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision. Earth Surface Processes and Landforms 40: 15741586
 ↑ Dronkers, J.J. 1964. Tidal computations in rivers and coastal waters. NorthHolland Publ. Co., 518 pp.
 ↑ Lemoine, R. 1948. Sur les ondes positives de translation dans les canaux et sur le ressaut ondulé de faible amplitude. Houille Blanche: 183185
 ↑ Stoker, J.J. 1957. Water Waves. Interscience, New York
 ↑ ^{20.0} ^{20.1} ^{20.2} Chanson, H. 2009. Current knowledge in hydraulic jumps and related phenomena. A survey of experimental results. European Journal of MechanicsB/Fluids 28: 191210
 ↑ Favre, H. 1935. Etude théorique et expérimentale des ondes de translation dans les canaux découverts (Theoretical and experimental study of travelling surges in open channels), Dunod, Paris
 ↑ ^{22.0} ^{22.1} ^{22.2} Lubin, P., Chanson, H. and Glockner, S. 2010. Large eddy simulation of turbulence generated by a weak breaking tidal bore. Environmental Fluid Mechanics 10: 587602
 ↑ Wilkinson, M.L. and Banner, M.L. 1977. 6th Australasian Hydraulics and Fluid Mechanics Conf. Adelaide, Australia, 59 December 1977
 ↑ Binnie, A. M., & Orkney, J. C. (1955). Experiments on the flow of water from a reservoir through an open horizontal channel II. The formation of hydraulic jumps. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 230(1181), 237246
 ↑ Chanson, H. (2010). Undular tidal bores: basic theory and freesurface characteristics. Journal of Hydraulic Engineering, 136(11), 940944
 ↑ ^{26.0} ^{26.1} Treske, A. 1994. Undular bore (Favrewaves) in open channels  Experimental studies, J. Hydraulic Res. 32: 355370
 ↑ ^{27.0} ^{27.1} Chassagne, R., Filippini, A., Ricchiuto, M. and Bonneton, P. 2019. Dispersive and dispersivelike bores in channels with sloping banks. Journal of Fluid Mechanics 870: 595616. doi:10.1017/jfm.2019.287
 ↑ Bonneton, P., Van de Loock, J., Parisot, JP., Bonneton, N., Sottolichio, A., Detandt, G., Castelle, B., Marieu, V. and Pochon, N. 2011. On the occurrence of tidal bores – The Garonne River case. Journal of Coastal Research, SI 64: 114621466
 ↑ Tu, J. and Fan, D. 2017. Flow and turbulence structure in a hypertidal estuary with the world's biggest tidal bore. Journal of Geophysical Research: Oceans, 122: 34173433
 ↑ Reungoat, D., Lubin, P., Leng, X. and Chanson, H. 2018. Coastal Engineering Journal 60: 484498
 ↑ Berchet, A., Simon, B., Beaudoin, A., Lubin, P., Rousseaux, G. and Huberson, S. 2018. Flow fields and particle trajectories beneath a tidal bore: A numerical study. International Journal of Sediment Research 33: 351370
 ↑ Cite error: Invalid
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tag; no text was provided for refs namedS
 ↑ Chanson, H. 2012. Tidal bores, aegir, eagre, mascaret, pororoca: Theory and observations. World Scientific
 ↑ ^{34.0} ^{34.1} Peregrine, D. H. 1966. Calculations of the development of an undular bore. Journal of Fluid Mechanics 25: 321330
 ↑ Bonneton, P., Barthélemy, E., Chazel, F., Cienfuegos, R., Lannes, D., Marche, F. and Tissier, M. 2011. Recent advances in Serre–Green Naghdi modelling for wave transformation, breaking and runup processes. European Journal of MechanicsB/Fluids 30: 589597
 ↑ ^{36.0} ^{36.1} Tissier, M., Bonneton, P., Marche, F., Chazel, F., & Lannes, D. 2011. Nearshore dynamics of tsunamilike undular bores using a fully nonlinear Boussinesq model. Journal of Coastal Research SI 84: 603607
 ↑ SoaresFrazão S. and Zech Y. 2002. Undular bores and secondary waves  Experiments and hybrid finitevolume modeling. Journal of Hydraulic Research, International Association of Hydraulic Engineering and Research (IAHR) 40: 3343
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