Modeling the effect of progressive grain-size sorting on the scale dependence of back-barrier tidal basin morphology

• Tidal basin morphodynamics coupled with multiple grain sizes.
• Constant feedback of gain-size sorting process on morphological evolution.
• Explanation of erosion processes balance.
• Explanation of increasing grain-size difference between upper and lower tidal flats.

Two categories of empirical relationships between morphological parameters and spatial scale factors have been studied by field observations of tidal basins along the Dutch and German North Sea coast, one being about channel-flat system parameters, i.e., the relative channel area and the ratio of channel volume to tidal prism, the other on tidal flat morphology, i.e., the mean tidal flat elevation and the convexity of tidal flat hypsometry. In the present study, the morphodynamic evolution of tidal basins has been coupled with grain size such that there was a constant feedback of the progressive gain-size sorting process on the morphological evolution itself. In order to investigate sorting effects on morphological scaling relationships, a series of numerical modeling exercises were carried out with multiple grain-size fractions in schematized rectangular back-barrier tidal basins similar to those found in the German Wadden Sea. To identify the effect of progressive grain-size sorting on the morphological evolution, the results of the multiple size-fraction model was compared with those of single fraction models. The comparison suggests that, although the same scaling relationships apply for the channel-flat system as a whole, the scaling of tidal flats proceeds at reduced rates, i.e., the tidal flat hypsometry curves modeled with multiple fractions are almost identical to those with a single fraction for small basins but more convex-up for large basins. The former can be explained by the balance of erosion processes between mobile fine sediments and stable coarse grains, the latter by the increasing difference in grain size between upper and lower tidal flats.