Limitations of empirical sediment transport formulas for shallow water and their consequences for swash zone modelling

TL;DR

This paper reveals that standard sediment transport formulas in shallow water models often produce physically impossible sediment concentrations, leading to unreliable swash zone predictions, and proposes an ad hoc limit to address this issue.

Contribution

It identifies the limitations of existing empirical sediment transport formulas in shallow water and demonstrates their impact on swash zone modeling accuracy.

Findings

Standard formulas can produce sediment concentrations exceeding physical limits by up to 10^5 times.

Implementing a concentration limit significantly alters model predictions.

Current models are unreliable due to neglecting shallow water sediment storage constraints.

Abstract

Volumetric sediment concentrations computed by phase-resolving swash morphodynamic models are shown to exceed unity minus porosity (i.e. the maximal physically possible concentration value) by up to factor of $10^5$ when using standard expressions to compute the sediment transport rate. An ad hoc limit of sediment concentration is introduced as a means to evaluate consequences of exceeding physically realistic concentration by standard expressions. We find that implementation of this ad hoc limit strongly changes the quantitative and qualitative predictions of phase-resolving swash morphodynamic models, suggesting that existing swash predictions are unreliable. This is because standard expressions inappropriately consider or ignore the fact that the shallow swash water depth limits the storage capacity of transported sediment.