Tidal sand wave formation: Influence of graded suspended sediment transport

The influence of graded suspended sediment transport on the formation of tidal sand waves is investigated. Hereto, in the framework of a linear stability analysis, we examine the time development of small amplitude perturbations superimposed to a flat heterogeneous sea bed in a shallow tidal sea dominated by the semidiurnal tidal constituent. The heterogeneous bottom material is modeled by a bimodal sediment mixture and the active layer concept is employed. The transport of sediment close to the bottom and in suspension is considered accounting for hiding/exposure effects. It is found that the mechanisms giving rise to the sediment sort- ing process along the bedforms are quantitatively, only slightly affected and qualitatively, not influenced by the fractional transport of sediment in sus- pension. On the contrary, the formation of sand waves is quantitatively, substantially affected by the graded suspended sediment transport and the wavelength of the most amplified mode is found to depend strongly on the characteristics of the sediment mixture. In particular, the graded suspended sediment transport largely effects the contribution to the amplification rate related to the sorting process. This leads to the growth of bedforms characterized by a smaller wavelength compared with that predicted for a uniform sediment, when the mean grain size of the sediment mixture is relatively coarse, and results in the appearance of longer bedforms when the sediment mixture is characterized by a relatively fine mean grain size. Moreover, a stabilizing effect is identified in the latter case while the flat bed configuration is destabilized in the former. A comparison of the theoretical predictions with field observations seems to support the model, however, cannot be considered conclusive.