Large‐Scale Scour in Response to Tidal Dominance in Estuaries

• Published in: Journal of geophysical research. Earth surface Vol. 126; no. 5
• Main Authors: Leuven, J. R. F. W., van Keulen, D., Nienhuis, J. H., Canestrelli, A., Hoitink, A. J. F.
• Format: Journal Article
• Language: English
• Published: 01.05.2021
• Subjects: channels; delta; estuary; modeling; scour; tides
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2020JF006048

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 SB, and on the ratio between the specific peak tidal discharge at the mouth and the specific river discharge at the landward boundary (qtide/qriver). Scours develop if (qtide/qriver)/SB exceeds 0.3. Scour conditions were found to occur globally across a range of scales, which allows its prediction in estuaries under future changes. Plain Language Summary Estuaries form the transitions from rivers to the ocean. The maximum depth of an estuary often occurs at the mouth. Occasionally, estuaries have their maximum depth further inland of the mouth. Such a depression in the bed‐level profile often occurs over a distance much larger than the channel width. We call this “large‐scale scour.” So far, two explanations for large‐scale scour exist, and in this article, we investigated a third mechanism that we term tide‐induced scour. We show with simplified numerical models that there are two main factors that control whether tide‐induced scours can form: (a) the planform shape of the channel and (b) the extent of tidal influence with respect to the river discharge. When the planform geometry is more strongly convergent in landward direction and/or when the tidal influence is relatively large compared to the fluvial influence, the potential for tide‐induced scour is large. In this study, we present a predictor that allows to establish the tide‐induced scour potential in estuaries worldwide. Understanding of tide‐induced scour and the prediction tool help anticipating how estuaries will evolve in the future under changing climate, land use, and sea‐level rise. Key Points Tidal funneling creates large‐scale scours when channel convergence exceeds friction effects such that peak velocities are amplified A simple metric shows that large‐scale scours appear in channels with large tidal amplitudes and short convergence lengths Estuary shape, fluvial, and tidal discharge predict scouring behavior now and under future changes, such as sea‐level rise