Chengji Shen

A high-resolution method for the depth-integrated solute transport equation based on an unstructured mesh

This paper presents a high-resolution numerical method for solving mass transport problems involving advection and anisotropic diffusion in shallow water based on unstructured mesh. An alternating operator-splitting technique is adopted to advance the numerical solution with advection and diffusion terms solved separately in two steps. By introducing a new r-factor into the Total Variation Diminishing (TVD) limiter, an improved finite-volume method is developed to solve the advection term with significant reduction of numerical diffusion and oscillation errors. In addition, a coordinate transformation is introduced to simplify the diffusion term with the Green-Gauss theorem used to deal with the anisotropic effect based on unstructured mesh. The new scheme is validated against three benchmark cases with separated and combined advection and diffusion transport processes involved. Results show that the scheme performs better than existing methods in predicting the advective transport, particularly when a sharp concentration front is in presence. The model also provides a sound solution for the anisotropic diffusion phenomenon. Anisotropic diffusion has been largely neglected by existing flow models based on unstructured mesh, which usually treat the diffusion process as being isotropic for simplicity. Based on the flow field provided by the ELCIRC model, the developed transport model was successfully applied to simulate the transport of a hypothetical conservative tracer in a bay under the influence of tides. Highlights? New r-factor for TVD limiter is presented to reduce artificial diffusion. ? Green-Gauss theorem is used to approximate the anisotropic diffusion term. ? The method can be incorporated into the ocean models based on unstructured mesh.

Effects of salinity variations on pore water flow in salt marshes

Spatial and temporal salinity variations in surface water and pore water commonly exist in salt marshes under the combined influence of tidal inundation, precipitation, evapotranspiration, and inland freshwater input. Laboratory experiments and numerical simulations were conducted to investigate how density gradients associated with salinity variations affect pore water flow in the salt marsh system. The results showed that upward salinity (density) gradients could lead to flow instability and the formation of salt fingers. These fingers, varying in size with the distance from the creek, modified significantly the pore water flow field, especially in the marsh interior. While the flow instability enhanced local salt transport and mixing considerably, the net effect was small, causing only a slight increase in the overall mass exchange across the marsh surface. In contrast, downward salinity gradients exerted less influence on the pore water flow in the marsh soil and slightly weakened the surface water and groundwater exchange across the marsh surface. Numerical simulations revealed similar density effects on pore water flow at the field scale under realistic conditions. These findings have important implications for studies of marsh soil conditions concerning plant growth as well as nutrient exchange between the marsh and coastal marine system.

Improvement of the hillslope‐storage Boussinesq model by considering lateral flow in the unsaturated zone

Unsaturated flow is an important factor that affects groundwater motion. Among various drainage models, the nonlinear Hillslope-storage Boussinesq (HSB) model has been commonly used to predict water flux along a slope. In this study, we improved this model by considering lateral flow in the unsaturated zone. Using modified van Genuchten functions, we analytically expressed the concept of equivalent propagation thickness in the vadose zone. This analytical expression was then incorporated into the HSB model to reflect two different stages of the drainage process and to simulate the hillslope drainage process more accurately. The model results indicated that lateral flow has significant effects in the unsaturated zone during the hillslope drainage process. Even in sandy aquifers, the amount of water contributed by the unsaturated zone is a key factor that enables a decrease in the water table during the middle and late stages of the process. A comparison between the measured and simulated results based on both convergent-type and divergent-type hillslope drainage processes revealed that the thickness of the saturated zone decreases as the unsaturated flow increases. This study emphasizes the necessity of considering unsaturated flow in the HSB model to improve the accuracy of predicting groundwater outflow rates and develop more accurate hydrographs. The concept of equivalent propagation thickness also provides a criterion for assessing the importance of unsaturated lateral flow for future drainage research.

Effects of unstable flow on solute transport in the marsh soil and exchange with coastal water

Recent studies of marsh hydraulics have focused on tide-induced pore-water circulation as the main drive for solute transport in the marsh soil and exchange with coastal water. Our study revealed another important mechanism provided by unstable fingering flow, which largely modified solute transport paths. In the marsh interior, downward penetration of salt fingers forced ambient pore-water and solute plumes to move upward and exit the marsh soil through marsh platform at relatively high concentrations, up to two orders of magnitude higher than exit solute concentrations at the tidal creek bed. The mixing of solute with ambient pore water in the marsh interior was intensified greatly by fingering flow. A critical distance to the creek was determined based on a field-scale model simulation to distinguish tidal circulation-dominated and fingering flow-dominated solute transport zones. The new transport mechanism has implications for understanding the fate of solutes in particularly salt marshes of low creek densities.

Predictability and Quantification of Complex Groundwater Table Dynamics Driven by Irregular Surface Water Fluctuations

Shallow groundwater interacts strongly with surface water across a quarter of global land area, affecting significantly the terrestrial eco-hydrology and biogeochemistry. We examined groundwater behavior subjected to unimodal impulse and irregular surface water fluctuations, combining physical experiments, numerical simulations, and functional data analysis. Both the experiments and numerical simulations demonstrated a damped and delayed response of groundwater table to surface water fluctuations. To quantify this hysteretic shallow groundwater behavior, we developed a regression model with the Gamma distribution functions adopted to account for the dependence of groundwater behavior on antecedent surface water conditions. The regression model fits and predicts well the groundwater table oscillations resulting from propagation of irregular surface water fluctuations in both laboratory and large-scale aquifers. The coefficients of the Gamma distribution function vary spatially, reflecting the hysteresis effect associated with increased amplitude damping and delay as the fluctuation propagates. The regression model, in a relatively simple functional form, has demonstrated its capacity of reproducing high-order nonlinear effects that underpin the surface water and groundwater interactions. The finding has important implications for understanding and predicting shallow groundwater behavior and associated biogeochemical processes, and will contribute broadly to studies of groundwater-dependent ecology and biogeochemistry.

Salt Dynamics in Coastal Marshes: Formation of Hypersaline Zones

Salt is a key solute in salt marshes and under the influence of evapotranspiration can accumulate to a high concentration level in the marsh soil and precipitate in the solid form to become a significant stressor for plants, affecting marsh plant productivity and ecological zonation. Numerical simulations of coupled pore water flow and salt transport were conducted to examine how spring-neap tides and evaporation combine to influence salt dynamics and distribution patterns in marshes. The salt pan formation was simulated with a sandy loam marsh soil subjected to a medium rate of potential evaporation. The critical condition for the salt pan formation was underpinned by hydraulic connection between the marsh surface and water table to sustain evaporation in the supratidal zone. Both low soil permeability and overly high potential evaporation were found to break the hydraulic connection. In this case, the surface soil salinity increased gradually over the intertidal zone to a maximum around the spring high tide mark followed by a rapid decrease to a lower constant level across the supratidal zone. This salinity distribution pattern has also been observed in the field. In both salt marshes with and without salt pans, excessive salt accumulated on the marsh surface due to evaporation was removed by tidally induced circulating flow and/or flow driven by density gradients associated with the accumulated salt. The salt dynamics and distribution patterns revealed here, especially the salt pan formation simulated for the first time, have important implications for studies of marsh plant growth and overall eco-functions.

Effects of Unsaturated Flow on Hillslope Recession Characteristics

Recession flow analysis is usually conducted to infer hydraulic parameters of hillslope aquifers. Various Boussinesq equation-based models, both linear and nonlinear, have been used to analyze the recession curves for sloping aquifers, with a focus on the long-time recession behavior. Based on a modified Boussinesq equation with capillarity incorporated, we demonstrate the significant effect of unsaturated flow on the recession curve, which result in three (instead of two) power law regimes with two transition points (instead of one) corresponding to the formation of a fully unsaturated zone at the adjacent area of the upslope boundary and across the whole domain, respectively. The results show that the power of the second and third recession regime is variable, depending on the slope angles, soil types, and hillslope geometries. The unsaturated flow effects also lead to the absence of drastic drop of (Formula presented.) at the transition between the first and second regime, which was predicted by previous numerical models but has not been observed in the field or laboratory experiments. These findings have important implications for recession flow analysis in studies of hillslope aquifers.

Analysis of the Morphological Changes and Related Sediment Transport Mechanisms of the Baisha Shoal in the Qiongzhou Strait, China

ABSTRACT Kong, J.; Pan, M.; Shen, C.; Hua, G., and Zhao, H., 2016. Analysis of the morphological changes and related sediment transport mechanisms of the Baisha Shoal in the Qiongzhou Strait, China. The morphological changes and evolutionary mechanism of the Baisha Shoal in the Qiongzhou Strait were investigated using various methods, including bathymetric chart comparisons, geomodeling, hydrodynamic modeling, and hydrologic statistical analysis. Water depths were extracted from digitized charts to explore the evolution of the Baisha Shoal and to quantitatively estimate the changes in the shoal area within the 5-m isobath. The grain-size trend analysis (GSTA) model was introduced to explain the features of bed sediment distribution and to analyze the sources of the sediment. Tidal flow models and wave models were adopted to simulate the tidal flows and wave fields around the Baisha Shoal and to reveal the sediment transport dynamics. Further comparisons based on a Mann-Kendall trend analysis and remote-sensing image data from different years were used to discuss the reason for the recent evolution of the Baisha Shoal. The results show that the formation of the Baisha Shoal was caused by the combined effects of reciprocating flow in the eastern and western directions and waves from the NE direction. However, in recent years, the Baisha Shoal has demonstrated an unsteady geomorphological state and is transitioning from a deposition-dominated state to an erosion-dominated state. Erosion has become most significant since 2004, as reflected in the shoal area within the 5-m isobath, which is decreasing by 0.732 km2/y. The analysis demonstrated that a reduction in sediment load and increased sand mining are the two most significant factors that are progressively altering the shoal morphology. If these issues cannot be resolved, the strong dynamic surroundings will aggravate the erosion of the Baisha Shoal. This study on the Baisha Shoal highlights the necessity of combining different approaches to better understand coastal evolutionary features, dynamic mechanisms, and evolutionary causes, as well as to guide future coastal protection engineering design.

Analytical research on the tidally-induced groundwater wave propagation mechanism in coastal aquifers

ABSTRACT Kong, J., Shen, C.J., Ye, R.H., 2013. Analytical research on the tidally-induced groundwater wave propagation mechanism in coastal aquifers. With increasingly more attention paid to coastal environment, research on the interaction between subsurface and surface water has become a hot subject. Being different from surface water, tidally-induced groundwater moves slower with special over-height far away from the coastal line, which is higher than the mean sea level. Such a special hydrodynamic characteristic hints the special propagation mechanism is different from traditional surface wave. In order to reveal the tidally-induced groundwater wave propagation mechanism and analyze the influence of latent coastal beach profile variation because of mean sea level rise, theoretical research is focused on a newly derived high-order analytical solution corresponding to non-linear Boussinesq equation applied on sloping beach. Research indicates that: Firstly, the tidal induced groundwater wave energy mainly concentrates on the former three order constituent waves. The variation of beach profile because of sea level rise will not change the proportion relationship among the constituent waves; Secondly, the over-heights increase is influenced by the conductivity decrease, porosity enlargement or tidal amplitude widening; Thirdly, mild beach slope and large conductivity are favorable for wave propagating with longer wavelength. In the end, the rise of mean sea level increases the aquifer thickness and then leads to the faster propagation speed of groundwater wave, lower over-height and makes the interaction between surface and subsurface water more intensively reflected in farther tidal wave signal landwards.

Study on the Role of Sediment Dike in the Port Construction near Silt Coast

With the rapid development of social economy, silt coast exploitation becomes the main approach to make up the imbalance of harbor resources in China, although the inherent problem of sedimentation has not been fully addressed. Due to the special geomorphology and complex sediment transport near silt coastal zone, the sediment dike, as a traditional construction, plays an important role in maintaining port operation and decreasing the siltation. In present paper, using the Guang-Ao harbor district planning project in the south coast of China, different dike arrangement schemes have been compared based on numerical simulations. The sediment transport characteristics and the latent erosion and siltation trend under these schemes have been discussed. Results show the importance and necessity of a proper design of dike near silt coastal zone, which can guide the sand transport path and reduce the possibility of sand intrusion in harbor zone. Present research provides a reference for other engineers to design and optimize the harbor design, especially in zones with high concentration of suspended sediment.