Tidal wetlands are increasingly valued for their role in coastal defense. Nevertheless, in situ observations of storm surge attenuation within wetlands are still scarce. We present water level measurements along a 4 km intertidal channel and on the surrounding marsh platform for regular spring to neap tides and two major storm surge tides, showing the effects of flood wave height and marsh geomorphology on the amount of flood wave attenuation. Undermarsh tides with peak water levels below marsh platform elevation are mostly amplified (up to 4 cm/km) within the channels. Overmarsh tides with peak water levels above the marsh platform are generally attenuated along the channels, with maximum attenuation rates of 5 cm/km for tides that inundate the marsh platform by 0.5–1.0 m. For lower or higher flood waves, including storm surges, attenuation rates decrease. Furthermore, the observations show that the maximum attenuation occurs along narrow channel transects where the width of the platform is larger, whereas attenuation rates are lower along wider channels with smaller adjacent marsh platforms. These observations are confirmed by an analytical approximation of tidal wave propagation through convergent channels. The analytical model indicates that differences in attenuation rates are induced by variations in the cross‐channel averaged friction between channel sections and between tides with varying peak water levels. Finally, the highest attenuation rates of up to 70 cm/km are observed over short distances on the vegetated marsh platform. We conclude that this study provides an empirical basis for the wider implementation of nature‐based flood defense strategies.

A model is developed to investigate the process which leads to the formation of sand waves in shallow tidal seas characterized by a heterogeneous sea bed composition. The main goal of the analysis is the evaluation of the effects that a graded sediment has on the formation of the bottom forms and the investigation of the sorting process induced by the growth of the bottom forms. The analysis is based on the study of the stability of the flat bed configuration, i.e. small amplitude perturbations are added to the flat bottom and a linear analysis of their time development is made. For an oscillatory tidal current dominated by one tidal constituent, the results show that the graded sediment can stabilize or destabilize the flat bottom configuration with respect to the uniform sediment case, depending on the standard deviation sigma* of the grain size distribution and on the ratio (r) over cap between the horizontal tidal excursion and the water depth. For moderate values of (r) over cap, i.e. values just larger than the critical value for which the sediment is moved and sand waves appear, the presence of a sand mixture stabilizes the flat bed. On the other hand, for large values or (r) over cap, the mixture has a destabilizing effect. In both cases the effect that a sand Mixture has on the stability of the flat bed configuration is relatively small. Moreover, for moderate values of (r) over cap, the fine fraction of the mixture tends to pile up at the crests of the bottom forms while the coarse fraction moves towards the troughs. For large values or (r) over cap, the grain size distribution depends on the value of sigma*. The results are physically interpreted and provide a possible explanation of the apparently conflicting field observations of the grain size distribution along the sand wave profile, carried Out in the North Sea

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[1] 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 seabed in a shallow tidal sea dominated by one tidal constituent. The heterogeneous bottom material is modeled by a bimodal sediment mixture and the transport of sediment close to the bottom, as well as in suspension, is considered. The active layer concept is employed and hiding/exposure effects are taken into account. The mechanisms giving rise to the sediment-sorting process along the bed forms are found to be unaffected by the fractional transport of sediment in suspension. On the other hand, 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 affects the contribution to the amplification rate related to the sorting process. This leads to the growth of bed forms 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. On the other hand, longer bed forms appear when the sediment mixture is characterized by a relatively fine mean grain size. Moreover, a destabilizing effect is identified in the former case while the flat bed configuration is stabilized in the latter case. A comparison of the theoretical predictions with field observations seems to support the model, even though it cannot be considered conclusive.Citation: Van Oyen, T., and P. Blondeaux (2009), Tidal sand wave formation: Influence of graded suspended sediment transport,