Large-Scale Scour in Response to Tidal Dominance in Estuaries

Channel beds in estuaries and deltas often exhibit a local depth maximum close to the river mouth. There are two known mechanisms of large-scale (i.e., >10 river widths along-channel) channel bed scours: width constriction and draw-down during river discharge extremes, both creating flow acceleration. Here, we study a potential third mechanism: tidal scour. We use a 1D-morphodynamic model to reproduce tidal dynamics and scours in estuaries that are in morphologic equilibrium. A morphologic equilibrium is reached when the net (seaward) sediment transport matches the upstream supply along the entire reach. The residual (river) current and river-tide interactions create seaward transport. Herein, river-tide interactions represent the seaward advection of tide-induced suspended sediment by the river flow. Tidal asymmetry typically creates landward transport. Scours form when tidal flow is amplified through funneling of tidal energy. Scours simultaneously reduce the residual (river) current and the river-tide interaction contribution to sediment transport, thereby maintaining morphologic equilibrium. When tidal influence is relatively large, and when channel convergence is strong, an equilibrium is only obtained with a scouring profile. We propose a predictor dependent on the width convergence, quantified as S_B, and on the ratio between the specific peak tidal discharge at the mouth and the specific river discharge at the landward boundary (q_tide/q_river). Scours develop if (q_tide/q_river)/S_B exceeds 0.3. Scour conditions were found to occur globally across a range of scales, which allows its prediction in estuaries under future changes.