Erosion and Accretion on a Mudflat: The Importance of Very Shallow‐Water Effects

Abstract

Understanding erosion and accretion dynamics during an entire tidal cycle is important for assessing their impacts on the habitats of biological communities and the long‐term morphological evolution of intertidal mudflats. However, previous studies often omitted erosion and accretion during very shallow‐water stages (VSWS, water depths < 0.20 m). It is during these VSWS that bottom friction becomes relatively strong and thus erosion and accretion dynamics are likely to differ from those during deeper flows. In this study, we examine the contribution of very shallow‐water effects to erosion and accretion of the entire tidal cycle, based on measured and modeled time‐series of bed‐level changes. Our field experiments revealed that the VSWS accounted for only 11% of the duration of the entire tidal cycle, but erosion and accretion during these stages accounted for 35% of the bed‐level changes of the entire tidal cycle. Predicted cumulative bed‐level changes agree much better with measured results when the entire tidal cycle is modeled than when only the conditions at water depths of >0.2 m (i.e., probe submerged) are considered. These findings suggest that the magnitude of bed‐level changes during VSWS should not be neglected when modeling morphodynamic processes. Our results are useful in understanding the mechanisms of micro‐topography formation and destruction that often occur at VSWS, and also improve our understanding and modeling ability of coastal morphological changes.

On the Wave Bottom Shear Stress in Shallow Depths: The Role of Wave Period and Bed Roughness

“…Several and accurate analyses, carried out with the aim of reproducing the morphological evolution of the Lagoon of Marano and Grado over an annual average period [44], have shown that only wind speeds exceeding 6 m/s involve the main morphodynamic changes, being quite frequent and strong enough to resuspend sediments. Moreover, these values agree with those assumed by different studies performed on the distribution of wave bottom shear stress inside Venice Lagoon [13,22,27,45] and those reproduced in many numerical applications to investigate tidal flats response to waves and tides [12,19,46,47].…”

“…If the wind is strong enough, which means speed of at least 12 m/s, fw is The friction factor entering in Equation (13) has been computed by adopting the Soulsby formulation as given by Equation (4). This is an extensive choice taken by many authors [13,22,27,[45][46][47] and it presupposes that a rough turbulent motion is realized near the bed. Only for a few cases f w is assumed constant [21] and equal to a calibrated parameter.…”

External conditions drive optimal planting configurations for salt marsh restoration

“…This is new; previous studies have focused on currents as the main cause for tussock formation (Bouma et al, , ; van Wesenbeeck et al, ). Waves are shallow in marsh areas, typically <0.5 m as in this study; yet they create erosional shear stresses on the seabed that match or exceed those of currents (Shi et al, , ). For currents, dense vegetation diverts forcing around patches, causing acceleration of hydrological energy at the patch perimeter, which increases shear stress to form erosion gullies (Bouma et al, , ; van Wesenbeeck et al, ).…”

“…Here, we had a natural situation with both waves and currents, where only wave forcing differed between the three exposure sites, suggesting that wave–current interactions generated the observed differences in tussocks and gully formation between sites. The physics behind wave–current interactions on erosion processes are complex and not well understood (Maza et al, ; Shi et al, , ; Yang & Irish, ). We propose a few simple principles that might explain the observed wave‐current induced sediment patterns around the vegetation patches (Figure ).…”

“…Presently, gully formation is known to be current generated (Wesenbeeck et al, ). Yet, waves also create erosional shear stresses on the seabed, which can match or exceed those of currents (Shi et al, ; Shi, Yang, Wang, Bouma, & Zhu, ). We argue that waves have the potential to generate gullies and restrict the lateral expansion of marsh patches, contributing to the effect of currents.…”