Structures of Lateral flow and turbulence in a breaking tidal bore rushing through a curved channel of the Qiantang Estuary

Abstract

Tidal bores form in funnel-shaped estuaries and bays initially and cause unique flow and turbulence dynamics when passing through meandering channels, such as in the Qiantang Estuary, Amazon River Estuary, and Seine River Estuary. To understand the lateral currents and turbulence processes of tidal bores in curved channels, velocity profiles and free-surface elevations are sampled for four semidiurnal tidal cycles in a curved channel located upstream of the Qiantang Estuary. During flood tides, the evolution of lateral currents experiences three distinct phases: first, there are inner-bank-toward lateral currents at the front of tidal bores, followed by two-layer helical currents in the middle of the flood tides, and finally, outer-bank- pointing lateral currents at the end of flood tides. Tidal bore breaking creates outbursts of turbulent kinetic energy. The enhanced turbulence emerges above the middle layers and persists for more than ten minutes after the breaking front. The lateral momentum balance indicates that the decreasing lateral barotropic pressure gradient (LBTPG) and the increasing summation of centrifugal and Coriolis acceleration give rise to the variation in lateral currents. The phase lead of bores near the outer bank induced by shoal-channel topography generates inner-bank pointing LBTPG at the bore front and then gradually weakens it. Significant turbulence following bore breaking may be induced via the wave-induced turbulence mechanism by violent secondary waves. This research shows that complicated lateral currents are an important component of tidal bores flowing through meandering channels and that secondary waves after bore breaking can continually feed turbulence.