Changkuan Zhang

Suspended sediment fluxes in the radial sand ridge field of South Yellow Sea

ABSTRACT Zhang C. K., Yang Y. Z., Tao, J. F., Chen, Y. P., Yao, P., Su. M., 2013. Suspended sediment fluxes in the radial sand ridge field of South Yellow Sea. The radial sand ridge at the South Yellow Sea is one of unique geomorphic units in the world. It is famous of its radial current pattern and high suspended sediment concentration, forming abundant tidal flat resources which could be reclaimed for the land use. In order to make a scientific plan for the land reclamation, it is necessary to quantitatively understand the distribution of suspended sediment fluxes in this field. In this study, a 2D tidal current and suspended sediment transport model is developed and validated by the hydrologic survey data measured in 2006. The suspended sediment fluxes are calculated by the period-averaged multification of the simulated tidal current and sediment concentration in two consecutive tidal periods. The numerical results show that, in either winter or summer season, there has a positive sediment flux comes from the north and the south boundaries, indicating a net gain of sediment for the sand ridge field from both boundaries. The numerical results also show that a clockwise sediment flux system exists in the middle of the sand ridge field, but a small portion of sediment leaves the sand ridge area at the north wing. The overall gain of the sediment in this field is positive, implying that the radial sand ridge is still in the growing process.

Hindered erosion: The biological mediation of noncohesive sediment behavior

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 bio-sedimentary systems showed different erosional behaviour related to biofilm maturity and EPS distribution. In the first stage (5 days), the bio-sedimentary 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 stabilisation. 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 sub-layers continued to stabilise 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.

Estimation of long-term wave statistics in the East China Sea

ABSTRACT Chen, Y. P., Xie, D. M., Zhang, C. K, Qian, X. S., 2013. Estimation of long-term wave statistics in the East China Sea. This study used a state-of-the-art approach to estimate the long-term wave statistics in the East China Sea. The wave climate in the past 60 years (1950–2009) was simulated by a third generation wave model driven by the NCEP/NCAR reanalysis wind data. Based on the statistical analysis of the numerical results, the mean wave fields and the extreme wave fields with 100-year and 50-year return period on the whole computational domain were obtained. The spatial variations of the wave fields show that the statistical waves decrease from south to north and from sea to coast in general, which can be associated with the local topography of the East China Sea. The seasonal variations show that the mean waves in the winter and autumn are slightly larger than those in the spring and summer, while the extreme waves in the summer are much larger than those in the other seasons, which is mainly due to the seasonal variation of monsoonal winds and the occurrence of typhoon events in the summer. The outputs of this paper are helpful to understand the distribution of mean and extreme waves in the East China Sea, and also useful to calculate the design waves in the coastal area if nearshore wave model like the SWAN model is introduced.

Stabilizing effects of bacterial biofilms : EPS penetration and re-distribution of bed stability down the sediment profile

Biofilms, consisting of microorganisms and their secreted extracellular polymeric substances (EPS), serve as “ecosystem engineers” stabilizing sedimentary environments. Natural sediment bed provides an excellent substratum for biofilm growth. The porous structure and rich nutrients allow the EPS matrix to spread deeper into the bed. A series of laboratory-controlled experiments were conducted to investigate sediment colonization of Bacillus subtilis and the penetration of EPS into the sediment bed with incubation time. In addition to EPS accumulation on the bed surface, EPS also penetrated downwards. However, EPS distribution developed strong vertical heterogeneity with a much higher content in the surface layer than in the bottom layer. Scanning Electron Microscope (SEM) images of vertical layers also displayed different micro-morphological properties of sediment-EPS matrix. In addition, colloidal and bound EPS exhibited distinctive distribution patterns. After the full incubation, the bio-sedimentary beds were eroded to test the variation of bed stability induced by biological effects. This research provides an important reference for the prediction of sediment transport, and hence deepens the understanding of the biologically mediated sediment system and broadens the scope of the burgeoning research field of “biomorphodynamics”.

Modelling of a non-buoyant vertical jet in waves and currents

A generic numerical model using the large eddy simulation (LES) technique is developed to simulate a non-buoyant vertical jet in wave and/or current environments. The experimental data obtained in five different cases, i.e., one case of the jet in a wave only environment, two cases of the jet in a cross-flow only environment and two cases of the jet in a wave and cross-flow coexisting environment, are used to validate the model. The grid sensitivity tests are conducted based on four different grid systems and the results illustrate that the non-uniform grid system C (205×99×126 nodes with the minimum size of 1/10 jet diameter) is sufficiently fine for the modelling. The comparative study shows that the wave-current non-linear interaction should be taken into account at the inflow boundary while modelling the jet in wave and cross-flow coexisting environments. All numerical results agree well with the experimental data, showing that: (1) the jet under the influence of the wave action has a faster centerline velocity decay and a higher turbulence level than that in the stagnant ambience, meanwhile the “twin peaks” phenomenon exists on the cross-sectional velocity profiles, (2) the jet under a cross-flow scenario is deflected along the cross-flow with the node in the downstream, (3) the jet in wave and cross-flow coexisting environments has a flow structure of “effluent clouds”, which enhances the mixing of the jet with surrounding waters.

On the stability relationships between tidal asymmetry and morphologies of tidal basins and estuaries: STABILITY RELATIONS BETWEEN TIDAL ASYMMETRY AND ESTUARINE MORPHOLOGY

Simple stability relationships are practically useful to provide a rapid assessment of coastal and estuarine landforms in response to human interventions and long‐term climate change. In this contribution, we review a variety of simple stability relationships which are based on the analysis of tidal asymmetry (shortened to “TA”). Most of the existing TA‐based stability relationships are derived using the one‐dimensional tidal flow equations assuming a certain regular shape of the tidal channel cross‐sections. To facilitate analytical solutions, specific assumptions inevitably need to be made e.g. by linearising the friction term and dropping some negligible terms in the tidal flow equations. We find that three major types of TA‐based stability relationships have been proposed between three non‐dimensional channel geometric ratios (represented by the ratio of channel widths, ratio of wet surface areas and ratio of storage volumes) and the tide‐related parameter a/h (i.e. the ratio between tidal amplitude and mean water depth). Based on established geometric relations, we use these non‐dimensional ratios to re‐state the existing relationships so that they are directly comparable. Available datasets are further extended to examine theutility of these TA‐based relationships. Although a certain agreement is shown for these relationships, we also observe a large scatter of data points which are collected in different types of landscape, hydrodynamic and sedimentologic settings over the world. We discuss in detail the potential reasons for this large scatter and subsequently elaborate on the limited applicability of the various TA‐based stability relationships for practical use. We highlight the need to delve further into what constitutes equilibrium and what is needed to develop more robust measures to determine the morphological state of these systems.

A numerical study of equilibrium states in tidal network morphodynamics

The long-term morphodynamic evolution of tidal networks on tidal flats is investigated using a two-dimensional numerical model. We explore the physical processes related to the development of the morphology and the presence of equilibrium configurations. Tidal networks are simulated over a rectangular domain representing a tidal platform, a different setting compared to estuaries (subject to riverine influence) and lagoons (offshore bars constricting the flow). In the early and middle phases of the tidal network evolution, large sediment patches with rhombus-like shape form and gradually migrate in the flood direction, even though the overall sediment flux is ebb-directed. A cross-section-averaged “equilibrium” state is asymptotically approached after about 500 years. The area and peak discharge of the lower flat cross-sections at year 500 approximately show a 1:1 relationship, which is in agreement with field observations. We also show that model results are consistent with the Q-A relationship (peak discharge Q versus cross-sectional area A), which is obtained under the assumption of a constant Chézy friction.

Predicting water content using linear spectral mixture model on soil spectra

Abstract Remote sensing has been widely applied for soil moisture estimation. However, such estimates become difficult to obtain and can be inaccurate when applied to complex earth surfaces with more than one soil type because of the interference of spectral signals from different soil components. This study aims to develop a moisture prediction method that is insensitive to soil types; this is based on in situ samples collected from an intertidal zone in Jiangsu Province in China and on laboratory measurements of soil spectra. The results demonstrate that for a reflectance-based method, moisture content is closely related to reflectance on the three wavebands centered at 2143, 1760, and 742 nm for four types of soil (sand, silty sand, sandy silt, and silt) considered separately; the relationship is not close if all soil types are mixed together ( R 2 = 0.77 ). To develop the desired model, a linear spectral mixture model (LSMM) was employed to extract parameter water abundance (Wa: information on soil water content) in advance, while eliminating redundant information from other soil components. Wa has a relatively higher correlation ( R 2 = 0.82 ) than reflectance with moisture content for a mixed soil type. Thus, employing the LSMM helps realize a practical water content estimation model for predicting moisture over complex earth surfaces, because it has the potential of eliminating spectral effects from soil components.

Radial tidal current field in a semi-enclosed rectangular basin： formation and evolution

The radial tidal current field accounts for the formation of the radial sand ridges in the South Yellow Sea. Understanding the formation and evolution of this radial tidal current field is vital to assessing the morphodynamic features in the area. A semi-enclosed rectangular basin with and without a coastal barrier was schematized from the topography of the Bohai Sea and Yellow Sea. The 2D tidal current field in this basin was simulated using the DELFT3D-FLOW model. The concept of tidal wave refraction, which highlights the effect of the sloped or stepped submarine topography on the propagation of the tidal waves, was introduced to explain the formation of the radial tidal current field. Under the effect of tidal wave refraction, co-phase lines of the counterclockwise rotating tidal wave and incident tidal wave are transformed into clockwise and counterclockwise deflections, respectively, leading to the convergence and divergence of the flow field. Regardless of whether a coastal barrier exists or not, the outer radial tidal current field might emerge over certain topography. The responses of the radial tidal current field in this basin to the environmental variations such as coastline changes and bottom erosions were discussed. Results show that local protrusion near the focal point of the radial tidal current field will have limited effects on the location of the tidal system. However, a remarkable shift of the amphidromic point toward the entrance and central axis of this basin and a movement of the focal point of the radial tidal current field toward the entrance could be caused by the significant seaward coastline advance and submarine slope erosion.