Consideration of the Mechanisms for Tidal Bore Formation in an Idealized Planform Geometry

Abstract

A tidal bore is a positive wave traveling upstream along the estuary of a river, generated by a relatively rapid rise of the tide, often enhanced by the funneling shape of the estuary. The swell produced by the tide grows and its front steepens as the flooding tide advances inland, promoting the formation of a sharp front wave, that is, the tidal bore. Because of the many mechanisms and conditions involved in the process, it is difficult to formulate an effective criterion to predict the bore formation. In this preliminary analysis, aimed at bringing out the main processes and parameters that control tidal bore formation, the degrees of freedom of the problem are largely reduced by considering a rectangular channel of constant width with uniform flow, forced downstream by rising the water level at a constant rate. The framework used in this study is extremely simple, yet the problem is still complex, and the solution is far from being trivial. From the results of numerical simulations, three distinctive behaviors emerged related to conditions in which a tidal bore forms, a tidal bore does not form, and a weak bore forms; the latter has a weakly steep front and after the bore formed it rapidly vanishes. Based on these behaviors, some criteria to predict the bore formation are proposed and discussed. The more effective criterion, suitably rearranged, is checked against data from real estuaries, and the predictions are found to compare favorably with the available data.

Tidal bore impact pressures on a trestle pier in the Qiantang River Estuary, China

“…41,42 Therefore, the occurrence of tidal bores is predictable to a certain extent. 43 The interaction between the tidal bore and the structures can be observed by installing well-spaced sensing equipment. To date, following a review of the relevant literature, there is still an incomplete understanding of the vertical distribution of tidal bore-induced pressure on elevated structures built across rivers with tidal bores.…”

Hydrodynamics of tidal bores: turbulent propagation and sediment transport

“…The inland flow during high flood tide is amplified by the gradually-narrowing funnelled estuary, forcing the leading edge to grow steeper until forming an upstream propagating discontinuous wave that is the tidal bore (Lighthill, 1978;Chanson, 2011a). The generation of tidal bores is extremely sensitive to the hydrodynamic balance between the tidal range, the seaward river flow conditions and the estuarine topography (Chanson, 2011a;Bonneton et al, 2015Bonneton et al, , 2016Viero & Defina, 2018;Filippini et al, 2019), the mechanism of which is analogous to the development of shock waves in gas dynamics (Stoker, 1957;Lighthill, 1978) and can also be analysed based on the theory of long wave deformation (Peregrine,1966;Lin, 2008). There are more than 400 major rivers and estuaries affected by tidal bores, worldwide on all continents only excluding Antarctica (Chanson, 2011a).…”