Tidal Bore Flow Reversal Classification Must Account for Reference Frame, Simulations Show

The study deploys laboratory PIV alongside weakly compressible SPH numerics to track velocity fields across a propagating nonlinear surge on a counter-current. Global reversal occurs only when bore velocity exceeds background discharge; otherwise local pockets reverse while the net river flow persists. This resolves contradictory field reports from estuaries such as the Severn and Qiantang by showing that earlier classifications omitted the reference-frame shift imposed by prior tidal and fluvial forcing. Spatial inhomogeneity in tidal range further produces mid-passage transitions between bore regimes, altering sediment resuspension and navigation windows. Prior observational studies treated reversal as an intrinsic bore attribute, missing how the same bore height yields opposite outcomes under differing background currents. The frame-aware taxonomy directly informs flood and sediment models used by coastal authorities. Three-dimensional field campaigns with mobile PIV arrays in macrotidal estuaries are now required to test the 2D predictions under realistic bathymetry and stratification. Such data would also quantify how accelerating sea-level rise modifies the velocity ratio threshold that separates reversal regimes.