Velocity and sediment surge: What do we see at times of very shallow water on intertidal mudflats?

Zhang, Qian; Gong, Zheng; Zhang, Changkuan; Townend, Ian; Jin, Chuang; Li, Huan

doi:10.1016/j.csr.2015.12.003

Cited by 22 publications

(25 citation statements)

References 29 publications

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“…These hydrodynamic and sediment processes are likely to govern morphodynamics in such very shallow‐water environments, and could cause the large and rapid bed‐level changes as compared with relatively deep‐water conditions. This inference is also supported by previous studies (e.g., Bassoullet et al, ; Bayliss‐Smith et al, ; Gao, ; Zhang et al, ). For example, Gao () showed that a large amount of sediment is deposited on/eroded from on the bottom bed during the very shallow‐water period, causing large bed‐level changes.…”

Section: Discussionsupporting

confidence: 87%

“…Although there was a small variability in SSCs (Figure b) within each submergence period during Tides 1–3 and 5–9, the peak value generally appeared at the beginning of the flood tide, as observed on other intertidal flats (Black, ; Li et al, ; Shi et al, ; Wang et al, ; Zhang et al, ). Wave periods derived from the wave–tide recorder ranged from 2 to 5 s during field measurements, suggesting that local winds lead to wind waves and all the dynamics are local because the waves are generated by local wind.…”

Section: Resultsmentioning

confidence: 56%

“…Firstly, a tidal front typically occurs, characterized by a significantly variation in bottom velocity from zero up to tens of centimeters per second within a few seconds (i.e., abrupt changes in velocity) (e.g., Gao, ; Nowacki & Ogston, ). Secondly, VSWS are generally accompanied by a peak value of SSCs, which tend to be the maximum value within the entire tidal cycle (e.g., Zhang et al, ), thereby leading to a marked increase in depositional flux based on equation . These hydrodynamic and sediment processes are likely to govern morphodynamics in such very shallow‐water environments, and could cause the large and rapid bed‐level changes as compared with relatively deep‐water conditions.…”

Section: Discussionmentioning

confidence: 99%

“…The coastal hydrodynamics are influenced by a large radial sand‐ridge system in the southern Yellow Sea off the Jiangsu coast, and characterized by a radial current field and high turbidity due to abundant sediment supply from the Yangtze River estuary and the abandoned Yellow River mouth (e.g., Shi et al, ; Wang, ; Zhang et al, ).…”

Section: Study Areamentioning

confidence: 99%

“…As a result, there is a lack of knowledge on the errors in predictive models of bed‐level change when water is very shallow. In addition, the maximum changes in bed‐level within an entire tidal cycle are likely to occur when the water depth is less than 0.2 m (Fagherazzi & Mariotti, ), because such stages are generally characterized by pulses of current velocity, potential breaking wave impacts, and high suspended sediment concentrations (SSCs) (Zhang et al, ). These hydrodynamics are likely to result in strong sediment re‐suspension, as well as high erosional and depositional fluxes associated with horizontal advection, even though the VSWS commonly last for only a few minutes or even a few seconds (Gao, ).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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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.

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Section: Discussionsupporting

confidence: 87%

Section: Resultsmentioning

confidence: 56%

Section: Discussionmentioning

confidence: 99%

Section: Study Areamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Hindered erosion: The biological mediation of noncohesive sediment behavior

Chen

Zhang

Paterson

et al. 2017

Water Resources Research

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Extracellular polymeric substances (EPS) are ubiquitous on tidal flats but their impact on sediment erosion has not been fully understood. Laboratory-controlled sediment beds were incubated with Bacillus subtilis for 5, 10, 16, and 22 days before the erosion experiments, to study the temporal and spatial variations in sediment stability caused by the bacterial secreted EPS. We found the biosedimentary systems showed different erosional behavior related to biofilm maturity and EPS distribution. In the first stage (5 days), the biosedimentary bed was more easily eroded than the clean sediment. With increasing growth period, bound EPS became more widely distributed over the vertical profile resulting in bed stabilization. After 22 days, the bound EPS was highly concentrated within a surface biofilm, but a relatively high content also extended to a depth of 5 mm and then decayed sharply with depth. The biofilm increased the critical shear stress of the bed and furthermore, it enabled the bed to withstand threshold conditions for an increased period of time as the biofilm degraded before eroding. After the loss of biofilm protection, the high EPS content in the sublayers continued to stabilize the sediment (hindered erosion) by binding individual grains, as visualized by electron microscopy. Consequently, the bed strength did not immediately revert to the abiotic condition but progressively adjusted, reflecting the depth profile of the EPS. Our experiments highlight the need to treat the EPS-sediment conditioning as a bed-age associated and depth-dependent variable that should be included in the next generation of sediment transport models. Plain Language Summary Sedimentology and geomorphology have traditionally been seen as fields in which physical and chemical processes dominate. However, microbial communities should never be bystanders, because they suffuse all sedimentary environments on earth. Under hydrodynamic forces, they take part in an impressive range of sediment processes and thus exercising a formative influence on coastal evolutions. Bio-sediments exhibit more complex characteristics than abiotic systems, and lead to different modelling methods compared to those in traditional settings. For instance, the thresholds for sediment initiation and subsequent erosion rates are no longer solely related to particle properties (e.g., particle size, the most widely used), but mediated by glue-like extracellular polymeric substances (EPS) secreted by microbes. From this point of view, it is easy to understand why sediments in field observations behave differently from predictions, usually appearing considerably strengthened. Our results indicate that the EPS mediation in sediment stability may vary with the rhythms of microbial growth, and re-profile the sediment stability during different stages of cementing processes. A conceptual framework for sediment erosion is hence put forward to transform traditional sediment system to EPS-sediment system.

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Assessing the relative contributions of the flood tide and the ebb tide to tidal channel network dynamics

Geng

Gong

Zhou

et al. 2019

Earth Surf Processes Landf

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Flood and ebb currents provide different contributions to the initiation and evolution of tidal channel networks, generating diverse network structures and channel cross-sections. In order to separate the effects of these contributions, a physical model of a sloping tidal-flat basin was set up in the laboratory. Depending on the degree of tidal asymmetry imposed offshore, either flood or ebb currents can be enhanced. The experimental results show that the ebb current has a higher capability to initiate and shape tidal networks than the flood current. Headward erosion is mainly induced by the ebb flow. The slightly inclined flat surface tends to reduce the energy of the flood current and to enhance the ebb current, thus prolonging the duration of morphodynamic activity as well as sediment motion. Overall, flood-dominated tides favour the formation of small-scale channel branches in the upper basin zone, while long lasting ebb-dominated tides result in more complex, wider and deeper tidal networks.

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Velocity and sediment surge: What do we see at times of very shallow water on intertidal mudflats?

Cited by 22 publications

References 29 publications

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

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

Hindered erosion: The biological mediation of noncohesive sediment behavior

Assessing the relative contributions of the flood tide and the ebb tide to tidal channel network dynamics

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