Residual Sediment Transport by Tidal Straining near Sloping Topography

Tidal straining is known to be a crucial factor for the generation of residual currents and sediment transport in estuaries and Regions of Freshwater Influence (ROFIs) of the coastal ocean. Essential for this process is the presence of a lateral density gradient, resulting either from freshwater runoff or from differential heating. Here, we show that near sloping topography, tidal straining triggered by the projection of a purely vertical density gradient onto the slope may be observed even if freshwater runoff and differential heating do not play a significant role. A one-dimensional (slope-normal) numerical model is used to illustrate the basic mechanisms and implications of this new process, focusing on residual currents and sediment transport. We identify the key non-dimensional parameters governing the problem, and use them to study the physically relevant parameter range. Our main finding is that for a wide range of conditions, tidal straining results in an effective upslope transport of sediment. This process may be relevant in a broad context because, in contrast to classical tidal straining, it does not require an externally imposed lateral density gradient.