Author(s): D. P. Viero; A. Defina

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Keywords: Tidal bores; Predictive criterion; Numerical simulations; Estuarine hydrodynamics

Abstract: Tidal bores are positive waves travelling upstream along the estuary of many rivers worldwide, with important implications on ecology, morphology, and social activities. A predictive criterion for tidal bore formation is proposed and applied to real estuaries. Tidal bores, one of the most fascinating and widely known phenomena observed in tidal rivers, are positive waves travelling upstream along the estuary of a river. Their occurrence is related to a relatively rapid rise of the tide, and often enhanced by the funnelling shape of the estuary. As the flooding tide advances upstream along an estuary, the swell due to the tide grows and its front steepens, thus promoting the formation of a sharp front wave, i.e., the tidal bore. Tidal bores play a significant role on the ecology and morphodynamics of an estuary, as well as on the social activities that take place in this environment. Turbulent mixing and dispersion are enhanced at the passage of a tidal bore, and significant bed erosion and sediment resuspension take place; the bed material is suspended, aerated, advected upstream with the bore, and redeposited on the retreat of the tide. This process has a positive and significant influence on the breeding of many small, estuarine invertebrates such as shrimps, molluscs and worms, which in turn feed several species of fish and provide important feeding grounds for wading birds and estuarine wildlife. Tidal bores provide opportunity also for recreational activities such as surfing, thus acting as a major tourist attraction. Given its importance and appeal, tidal bore has long been studied theoretically, numerically, experimentally, and with field investigations. However, possibly because of the many mechanisms and conditions that determine whether a tidal bore forms or not (e.g., freshwater river flow velocity and depth, bed slope and friction, the funnelling shape of the estuary, etc.), the prediction of bore occurrence through effective criteria, that are not just qualitative, remains a challenge. In this study, a phenomenological analysis of numerical results is carried out to shed light on the main processes and parameters controlling the formation of tidal bore. The problem is largely simplified by performing a wide series of numerical simulations in a rectangular channel of constant width, with a uniform subcritical flow forced downstream by raising the water level at a constant rate. In the numerical simulations, the rate of downstream level rise resembles the maximum rising rates typical of semi-diurnal tides of real estuaries where tidal bores form. Although the framework assumed in the present study to assess the formation and development of a tidal bore is extremely simple, yet the problem is still complex, and solutions are far from being trivial. From the results of numerical simulations, we identify three distinctive behaviours, 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, its fate is to rapidly vanish. With reference to the schematic, we denote with “F” the foot of the front, and with “U” the cross-section where the flow reverses. When the section F travels upstream faster than the section U, no bore can form; otherwise, bores were found to form when U meets F. When U meets F, either the two sections align their velocities to move closely with a common speed (well-formed bore) or, alternatively, F speeds up and U slows down, so that the bore just formed progressively reduces its height until it vanishes (weak bore). This first basic criterion allows to univocally determine if a bore forms.

DOI: https://doi.org/10.3850/978-981-11-2731-1_081-cd

Year: 2018