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Tidal bore dynamics
A tidal bore is the ultimate stage of distortion of a tidal wave that propagates upstream a river. 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  Bordeaux  
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. 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 highwater (HW) wave crest will propagate faster upestuary than the lowwater (LW) wave trough. The tidal rise period is shortened and the tidal wave is becoming steeper. If the tidal wave can propagate sufficiently far upstream the river without strong damping, the tidal wave front may become so steep that tidal rise corresponds to a water level jump – a socalled tidal bore.
Table 1 gives an overview of estuaries and tidal rivers in which significant tidal bores have been observed. It can be concluded from the table that tidal bores often occur in megatidal zones  areas where the maximum tidal range is greater than 6 m. However, it appears that substantial tidal bores can also occur in areas where the tidal range is smaller, although this is the exception rather than the rule. This exception seems to be limited to tropical and subtropical countries. Rivers are generally shallower in these regions due to abundant supply of fine sediments. Another possible reason is the relative importance of K1 and O2 diurnal tidal components, which can combine with the semidiurnal M2 tide to yield persistent tidal asymmetry ^{[1]} (see also the article Ocean and shelf tides). However, for the rivers listed in Table 1 this does not seem to play an important role ^{[2]}.
Engineering works (weirs, dredging) 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.
Illustrations of tidal bores are shown in Fig. 1.
Tidal bore formation
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 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^{[3]} ^{[4]}:
 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 ^{[5]}. The latter effect usually dominates. Observations show that the tidal bore in the Garonne and Dordogne (France) is suppressed at high river runoff^{[6]}. 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^{[7]}.
Although most studies of tidal bore formation have focused on estuaryriver systems with gradual bed slopes, there is strong evidence that steep gradients in the mean depth may induce tidal bore formation. This phenomenon is well known for tsunami waves propagating onto the shoreface or for swell waves collapsing on the beach. 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 different stages of tidal bore formation are sketched in Fig. 2 ^{[8]}. In this case river discharge is favorable to tidal bore development. Fig. 3 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 of the landward inner basin.
Propagation of tidal bores
Nonlinear processes become increasingly important in the final stages of bore development. These processes can no longer be analyzed with analytical methods, but only with numerical simulation models. The hydrostatic longwave equations cannot be applied to describe the emergence of a breaking tidal wave. Nonhydrostatic effects must be taken into consideration. This will be discussed further in the next sections.
A bore is a propagating transition between two streams of water depths [math]D_1[/math] and [math]D_2[/math] with speed [math]c=(D_2u_2D_1u_1)/ (D_2D_1)[/math], where [math]D_1 \lt D_2[/math]. Once such a hydraulic jump has developed, its propagation characteristics can be derived from the mass and momentum balance equations, see Fig. 4. By eliminating [math]v_1=cu_1[/math] or [math]v_2=cu_2[/math] from these equations two equivalent expressions can be derived for the height of the tidal bore:
[math]\Delta D= \Large \frac{D_1}{2} [\normalsize 3 + \sqrt{1 + 8 F_1^2} \Large ] \normalsize = \Large \frac{D_2}{2} [\normalsize 3  \sqrt{1 + 8 F_2^2} \Large ] \normalsize, \quad \quad (1)[/math]
where [math]\Delta D=D_1 D_2[/math] is the bore height and the Froude numbers are given by
[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 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 (2)[/math]
For [math]\Delta D\lt 0.4 h_2[/math] the tidal bore propagation speed [math]c[/math] can be fairly well represented by
[math]c \approx \Large \frac{1}{2} \normalsize (c_1 + c_2) , \quad \quad (3)[/math]
where [math] c_1 = u_1 + \sqrt{g D_1} , \quad c_2 = u_2 + \sqrt{g D_2}[/math].
If the bore velocity [math] c [/math] is equal to [math]c_2[/math] (i.e. [math]F_2 = 1[/math]) the bore height [math] \Delta D[/math] is zero. 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) ^{[10]}.
Tidal bore characteristics
In the previous section the tidal bore is represented by a hydraulic jump separating upstream and downstream regions of constant water depth. This simplification ignores the fact that energy loss occurs in the transition zone. Conservation of mass and momentum at the transition does not imply conservation of energy; there is a positive energy head loss
[math]\Delta E = \rho g \Delta H = \rho g \Delta D + 0.5 \rho (v_2^2 v_1^2) , \quad \quad (6)[/math]
where [math]\rho[/math] is the density of water.
Two forms of energy dissipation can occur at the transition, leading to two different types of 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. 5). The bore then consists of a mean jump between two water depths on which secondary waves are superimposed. This type of bore is usually called an undular bore^{[11]}. Favre (1935) ^{[12]} was the first to describe this phenomenon from laboratory experiments. That is why undular bores are sometimes referred to as Favre waves.
The propagation speed of short waves upstream of the bore is, according to linear shallowwater wave theory,
[math] c_w = u_2 + \sqrt{\Large \frac{g}{k}\normalsize \tanh(kD_2)} \approx u_2 + \sqrt{gD_2 (1 \frac{k^2 D_2^2}{3})} , \quad \quad (5)[/math]
where [math]k=2 \pi / \lambda[/math] is the wave number and [math]\lambda[/math] the wavelength. The latter approximation holds only if [math]kD_2 \le 1[/math]. Such short waves are stationary in a frame moving with the bore front if [math]c_w – u_2 = v_2[/math]. This equality provides an estimate of the wavelength of these waves that follow the bore front,
[math]\lambda \approx \Large \frac{2 \pi }{\sqrt{3}}\normalsize D_2 (1F_2^2)^{1/2} . \quad \quad (6)[/math]
In undular bores, energy is dissipated by the wave train following the bore front. The energy flux per unit width radiated by the wave field can be approximated by [math]P=0.5 \rho g c_g a^2[/math], where [math]a[/math] is the wave amplitude and [math]c_g[/math] the wave group velocity in the moving frame following the bore front (see shallowwater wave theory). The wave group velocity [math]c_g[/math] is given by
[math]c_g = u_2 + n (c  u_2)  c = v_2 \; (1n), \quad n=\large (\frac{1}{2}+ \frac{kD_2}{\sinh(2kD_2)}) \normalsize . \quad \quad (7)[/math]
Lemoine (1948) ^{[15]} equated this energy flux with the flux of energy corresponding to the energy head loss (Eq. 4), [math]P= D_2 v_2 \Delta E[/math], to obtain an estimate for the wave amplitude [math]a[/math] as a function of [math]D_1[/math] and [math]D_2[/math]. As this approach ignores waveinduced mass transport it is valid only for very small bores ([math]F_1[/math] close to 1)^{[16]}.
Breaking bores
For large [math]F_1[/math], bores correspond to turbulent breaking fronts (figures 1 and 7), where the energy head loss is dissipated by turbulent eddies^{[17]}. The transition between undular and turbulent bores is shown in figure 9. The turbulent eddies exert strong shear stresses on the channel bed, causing high concentrations of suspended sediment, see Fig. 3. Turbulent bores contribute significantly to upstream sediment transport^{[18]}.
In many estuaries and tidal rivers the bore propagation near the shallow channel banks differs from the bore propagation in the deeper middle part of the channel. The Froude number [math]F_1[/math] 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. 7 for the tidal bore in the Petitcodiac River. In the next section the interaction between different propagation characteristics at the channel centre and the channel banks will be discussed more in detail.
Dispersive tidal bores
Even in the absence of strong bathymetric gradients crosssectional depth variations can significantly influence tidal bore properties ^{[14]}^{[19]}. Most laboratory, theoretical and numerical bore studies did not consider transverse variations of the channel bed. 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)^{[21]} in the laboratory and by Bonneton et al. (2015)^{[3]} 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 Favre waves. 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. 8 a,b) differs significantly from classical dispersive undular bores in rectangular channels (i.e. Favre waves). Chassagne et al. (2019) ^{[20]} 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. 8.
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^{[21]}^{[11]}. This method allows the study of the different bore regimes (see Fig. 9) 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)^{[4]} proposed a novel approach. This method mimics the tidal asymmetry met in nature between the ebb and the flood. Fig. 10 shows an example of a tidallike bore generated with this method.
Numerical models
The secondary waves associated with undular bores have dispersive properties and thus cannot be described by the nonlinear shallowwater (NSW) equations, which hold for longwave phenomena (such as tidal motion) and assume hydrostatic pressure. The basic theoretical approach for undular tidal bores is based on weakly dispersive Boussinesqtype equations ^{[22]}. Weakly dispersive Boussinesqtype equations are an extension of the NSW equations by including terms that take into account vertical fluid accelerations, see Boussinesq equations.
The properties of breaking turbulent bores, on the contrary, are quite well described by the nondispersive NSW equations with jump conditions ^{[23]}.
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^{[8]}^{[24]}. Figure 11 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 description is the Saint Venant or Non Linear Shallow Water (NLSW) model. This model gives a good description of the largescale tidal wave transformation^{[25]}. However, the onset of a tidal bore and its evolution upstream is controlled by nonhydrostatic dispersive mechanisms^{[22]}. If the onset of the tidal bore can be well described by classical weakly dispersive weakly nonlinear Boussinesqtype equations, the subsequent nonlinear evolution, for highintensity tidal bores, requires the use of the basic fully nonlinear Boussinesq equations, named SerreGreen Naghdi (SGN) equations^{[26]}^{[27]}^{[28]}^{[11]}. This modelling approach allows an accurate description of both, the tidal bore formation at the estuarine scale and the bore structure at the local scale (Fig. 12).
See also
 Tidal asymmetry and tidal basin morphodynamics
 Ocean and shelf tides
 Morphology of estuaries
 Estuaries
Further reading
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.
Dronkers, J.J. 1964. Tidal computations in rivers and coastal waters. NorthHolland Publ. Co., 518 pp.
References
 ↑ Hoitink, A.F.J., Hoekstra, P. and van Mare, D.S. 2003. Flow asymmetry associated with astronomical tides: Implications for residual transport of sediment. J.Geophys.Res. 108: 131  138
 ↑ Song, D., X. H. Wang, A. E. Kiss, and Bao, X. 2011. The contribution to tidal asymmetry by different combinations of tidal constituents. J. Geophys. Res., 116, C12007
 ↑ ^{3.0} ^{3.1} ^{3.2} 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
 ↑ ^{4.0} ^{4.1} ^{4.2} 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
 ↑ 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
 ↑ 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
 ↑ ^{8.0} ^{8.1} ^{8.2} ^{8.3} Lynch, D.K. 1982. Tidal bores. Scientific American 247: 146157 Cite error: Invalid
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tag; name "L" defined multiple times with different content  ↑ Migniot, C. 1998. Mission Mont Saint Michel  Synthèse des connaissances hydrosédimentaires. Direction Départementale de l'Equipement de la Manche, 111pp.
 ↑ Dronkers, J.J. 1964. Tidal computations in rivers and coastal waters. NorthHolland Publ. Co., 518 pp.
 ↑ ^{11.0} ^{11.1} ^{11.2} 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
 ↑ 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
 ↑ ^{14.0} ^{14.1} 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
 ↑ Lemoine, R. 1948. Sur les ondes positives de translation dans les canaux et sur le ressaut ondulé de faible amplitude. Houille Blanche: 183185
 ↑ Wilkinson, M.L. and Banner, M.L. 1977. 6th Australasian Hydraulics and Fluid Mechanics Conf. Adelaide, Australia, 59 December 1977
 ↑ 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
 ↑ 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
 ↑ ^{20.0} ^{20.1} 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
 ↑ ^{21.0} ^{21.1} Treske, A. 1994. Undular bore (Favrewaves) in open channels  Experimental studies, J. Hydraulic Res. 32: 355370
 ↑ ^{22.0} ^{22.1} Peregrine, D. H. 1966. Calculations of the development of an undular bore. Journal of Fluid Mechanics 25: 321330
 ↑ Chanson, H. 2012. Tidal bores, aegir, eagre, mascaret, pororoca: Theory and observations. World Scientific
 ↑ 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
 ↑ Savenije, H. H. 2006. Salinity and tides in alluvial estuaries. Elsevier
 ↑ 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
 ↑ ^{27.0} ^{27.1} 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
 ↑ ^{28.0} ^{28.1} 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
 ↑ 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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