QZ
Q. Zhu
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1
We apply a 2-D horizontal process-based model (Delft3D) to study the feedback mechanisms that control the long-term evolution of a fringing intertidal flat in the Western Scheldt Estuary. The hydrodynamic model is validated using a comparison with measurements on the intertidal flat and the sediment transport module is calibrated against long-term morphology data. First, the processes that lead to net sediment exchange between channel and flat are studied. Then, long-term simulations are performed and the dependency of sediment fluxes on the tidal flat bathymetry, and the corresponding morphodynamic feedback mechanisms are explained. In the long run, relatively stable states can be approached, which are shown to be typical for wave-dominated fringing mudflats. The system behavior can be explained by the typical feedback mechanisms between the intertidal bathymetry and the hydrodynamic forces on the flat. In the subtidal domain, the impact of small (5–10 cm) wind waves increases with a rising elevation due to decreasing water depths. In the intertidal domain, the wave impact
increases with increasing cross-sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide-induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing
feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves. ...
increases with increasing cross-sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide-induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing
feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves. ...
We apply a 2-D horizontal process-based model (Delft3D) to study the feedback mechanisms that control the long-term evolution of a fringing intertidal flat in the Western Scheldt Estuary. The hydrodynamic model is validated using a comparison with measurements on the intertidal flat and the sediment transport module is calibrated against long-term morphology data. First, the processes that lead to net sediment exchange between channel and flat are studied. Then, long-term simulations are performed and the dependency of sediment fluxes on the tidal flat bathymetry, and the corresponding morphodynamic feedback mechanisms are explained. In the long run, relatively stable states can be approached, which are shown to be typical for wave-dominated fringing mudflats. The system behavior can be explained by the typical feedback mechanisms between the intertidal bathymetry and the hydrodynamic forces on the flat. In the subtidal domain, the impact of small (5–10 cm) wind waves increases with a rising elevation due to decreasing water depths. In the intertidal domain, the wave impact
increases with increasing cross-sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide-induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing
feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves.
increases with increasing cross-sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide-induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing
feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves.
Short-term bed-level variability in tidal wetlands has important implication both for ecology and engineering. In this study, we combined in situ measurements with model simulations to quantify short-term bed-level changes on a meso-macrotidal wetland in the Yangtze River Delta. On the middle flat, we observed erosion during neap-to-mean tides under onshore moderate-to-strong winds, and bed recovery during subsequent spring tides, when winds were both offshore and weaker, suggesting that winds can overturn the neap–spring cyclicity of bed-level changes even on meso–macrotidal mudflats. The magnitude of bed-level changes was smaller on both sides of the middle flat, while the smallest changes occurred on the salt marsh. Observed bed-level changes were reconstructed using a single-point bed-level change model, which incorporates in situ measured parameters of hydrodynamics (waves and currents), suspended sediment concentrations, and bed sediment properties. We conclude that the relative importance of waves and tides in intertidal wetland erosion and accretion can vary temporally (due to changes in balance between wave and tidal energies) and spatially (because of changes in elevation and vegetation in the cross-shore profile). Our study also reflects the advantage of combination of in situ measurement with simulation in detecting short-term variability of tidal flats.
...
Short-term bed-level variability in tidal wetlands has important implication both for ecology and engineering. In this study, we combined in situ measurements with model simulations to quantify short-term bed-level changes on a meso-macrotidal wetland in the Yangtze River Delta. On the middle flat, we observed erosion during neap-to-mean tides under onshore moderate-to-strong winds, and bed recovery during subsequent spring tides, when winds were both offshore and weaker, suggesting that winds can overturn the neap–spring cyclicity of bed-level changes even on meso–macrotidal mudflats. The magnitude of bed-level changes was smaller on both sides of the middle flat, while the smallest changes occurred on the salt marsh. Observed bed-level changes were reconstructed using a single-point bed-level change model, which incorporates in situ measured parameters of hydrodynamics (waves and currents), suspended sediment concentrations, and bed sediment properties. We conclude that the relative importance of waves and tides in intertidal wetland erosion and accretion can vary temporally (due to changes in balance between wave and tidal energies) and spatially (because of changes in elevation and vegetation in the cross-shore profile). Our study also reflects the advantage of combination of in situ measurement with simulation in detecting short-term variability of tidal flats.
Sediment dynamics on intertidal mudflats
A study based on in situ measurements and numerical modelling
Tidal flats provide essential ecosystem services (e.g. coastal protection and function as habitats sustaining coastal food webs). They are also under pressure due to climate change and human interventions. To investigate the sediment dynamic processes on tidal flats for a better understanding of the morphological development, in situ measurements were carried out on three tidal flats in the Yangtze Estuary (China) and in the Westerschelde Estuary (the Netherlands). The results show that erosion and deposition stages alternate due to the competition between hydrodynamic forces and bed strength, and due to variability of suspended sediment concentration. A bed-level change (BLC) model has been developed based on this understanding. Sediment dynamic processes on tidal flats are governed by tidal forcing exhibiting neap-spring variation, as well as by random wind events. The impacts from wind events were investigated by an integrated approach based on in situ measurements and the BLC model. This integrated approach also helped to reveal the spatial variability of bed erosional characteristics of tidal flats under influence of diatoms. This work not only improves the prediction of morphological changes of tidal flats, but also has implications for coastal engineering and for studies of coastal ecology and the coastal environment.
...
Tidal flats provide essential ecosystem services (e.g. coastal protection and function as habitats sustaining coastal food webs). They are also under pressure due to climate change and human interventions. To investigate the sediment dynamic processes on tidal flats for a better understanding of the morphological development, in situ measurements were carried out on three tidal flats in the Yangtze Estuary (China) and in the Westerschelde Estuary (the Netherlands). The results show that erosion and deposition stages alternate due to the competition between hydrodynamic forces and bed strength, and due to variability of suspended sediment concentration. A bed-level change (BLC) model has been developed based on this understanding. Sediment dynamic processes on tidal flats are governed by tidal forcing exhibiting neap-spring variation, as well as by random wind events. The impacts from wind events were investigated by an integrated approach based on in situ measurements and the BLC model. This integrated approach also helped to reveal the spatial variability of bed erosional characteristics of tidal flats under influence of diatoms. This work not only improves the prediction of morphological changes of tidal flats, but also has implications for coastal engineering and for studies of coastal ecology and the coastal environment.
Journal article
(2017)
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H. F. Yang, S. L. Yang, K. H. Xu, H. Wu, B. W. Shi, Q. Zhu, W. X. Zhang, Z. Yang
Deltas are widely threatened by sediment starvation and climate change. Erosion potential is an important indicator of delta vulnerability. Here, we investigate the erosion potential of the Yangtze Delta. We found that over the past half century the Yangtze's sediment discharge has decreased by 80% due to the construction of >50,000 dams and soil conservation, whereas the wind speed and wave height in the delta region have increased by 5-7%, and the sea level has risen at a rate of 3 mm/yr. According to hydrodynamic measurements and analyses of seabed sediments, the period when bed shear stress due to combined current-wave action under normal weather conditions exceeds the critical bed shear stress for erosion (τcr) accounts for 63% of the total observed period on average and can reach 100% during peak storms. This explains why net erosion has occurred in some areas of the subaqueous delta. We also found that the increase with depth of τcr is very gradual in the uppermost several metres of the depositional sequence. We therefore expect that the Yangtze subaqueous delta will experience continuous erosion under sediment starvation and climate change in the next decades of this century or even a few centuries.
...
Deltas are widely threatened by sediment starvation and climate change. Erosion potential is an important indicator of delta vulnerability. Here, we investigate the erosion potential of the Yangtze Delta. We found that over the past half century the Yangtze's sediment discharge has decreased by 80% due to the construction of >50,000 dams and soil conservation, whereas the wind speed and wave height in the delta region have increased by 5-7%, and the sea level has risen at a rate of 3 mm/yr. According to hydrodynamic measurements and analyses of seabed sediments, the period when bed shear stress due to combined current-wave action under normal weather conditions exceeds the critical bed shear stress for erosion (τcr) accounts for 63% of the total observed period on average and can reach 100% during peak storms. This explains why net erosion has occurred in some areas of the subaqueous delta. We also found that the increase with depth of τcr is very gradual in the uppermost several metres of the depositional sequence. We therefore expect that the Yangtze subaqueous delta will experience continuous erosion under sediment starvation and climate change in the next decades of this century or even a few centuries.
Accurate estimations for the bed shear stress are essential to predict the erosion and deposition processes in estuaries and coasts. This study used high-frequency in situ measurements of water depths and near-bed velocities to estimate bed shear stress on an open intertidal flat in the Yangtze Delta, China. To determine the current-induced bed shear stress (tc) the in situ near-bed velocities were first decomposed from the turbulent velocity into separate wave orbital velocities using two approaches: a moving average (MA) and energy spectrum analysis (ESA). tc was then calculated and evaluated using the log-profile (LP), turbulent kinetic energy (TKE), modified TKE (TKEw), Reynolds stress (RS), and inertial dissipation (ID) methods. Wave-induced bed shear stress (tw) was estimated using classic linear wave theory. The total bed shear stress (tcw) was determined based on the GranteMadsen waveecurrent interaction model (WCI). The results demonstrate that when the ratio of significant wave height to water depth (Hs/h) is greater than 0.25, tcw is significantly overestimated because the vertical velocity fluctuations are contaminated by the surface waves generated by high winds. In addition, wind enhances the total bed shear stress as a result of the increases in both tw and tc generated by the greater wave height and reinforcing of vertical turbulence, respectively. From a comparison of these various methods, the TKEw method associated with ESA decomposition was found to be the best approach because: (1) this method generates the highest mean index of agreement; (2) it uses vertical velocities that are less affected by Doppler noise; and (3) it is less sensitive to the near-bed stratification structure and uncertainty in bed location and roughness.
...
Accurate estimations for the bed shear stress are essential to predict the erosion and deposition processes in estuaries and coasts. This study used high-frequency in situ measurements of water depths and near-bed velocities to estimate bed shear stress on an open intertidal flat in the Yangtze Delta, China. To determine the current-induced bed shear stress (tc) the in situ near-bed velocities were first decomposed from the turbulent velocity into separate wave orbital velocities using two approaches: a moving average (MA) and energy spectrum analysis (ESA). tc was then calculated and evaluated using the log-profile (LP), turbulent kinetic energy (TKE), modified TKE (TKEw), Reynolds stress (RS), and inertial dissipation (ID) methods. Wave-induced bed shear stress (tw) was estimated using classic linear wave theory. The total bed shear stress (tcw) was determined based on the GranteMadsen waveecurrent interaction model (WCI). The results demonstrate that when the ratio of significant wave height to water depth (Hs/h) is greater than 0.25, tcw is significantly overestimated because the vertical velocity fluctuations are contaminated by the surface waves generated by high winds. In addition, wind enhances the total bed shear stress as a result of the increases in both tw and tc generated by the greater wave height and reinforcing of vertical turbulence, respectively. From a comparison of these various methods, the TKEw method associated with ESA decomposition was found to be the best approach because: (1) this method generates the highest mean index of agreement; (2) it uses vertical velocities that are less affected by Doppler noise; and (3) it is less sensitive to the near-bed stratification structure and uncertainty in bed location and roughness.