  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.
