Guangqiu Jin

Tidal influence on seawater intrusion in unconfined coastal aquifers

Studies of seawater intrusion in unconfined coastal aquifers typically neglect oceanic oscillations such as tides and assume a static seaward boundary condition defined by the mean sea level. Laboratory experiments and numerical simulations were conducted to investigate the influence of tidal oscillations on the behavior of the saltwater wedge. For the conditions examined, the experiments showed that an upper saline plume formed in the intertidal zone due to tide-induced seawater circulation. The presence of the upper saline plume shifted the fresh groundwater discharge zone seaward to the low-tide mark and restricted the intrusion of the saltwater wedge. The overall seawater intrusion extent, as indicated by the wedge toe location, was reduced significantly compared with the nontidal (static) case. Results from the numerical model matched these experimental observations and further demonstrated the similar type of tidal influence on the saltwater wedge in a field-scale aquifer system. The Glover (1959) solution for predicting the saltwater wedge was modified to account for the tidal effect by including the tide-induced circulation as a “recharge” to the aquifer. The findings highlight the significant impact of the tide in modulating the groundwater behavior and salt-freshwater dynamics, not only within but also landward of the intertidal zone.

Hyporheic flow under periodic bed forms influenced by low-density gradients

Small density variations due to low solute concentrations in the stream water commonly exist in streams and rivers. Using laboratory experiments and numerical modeling, we demonstrated that even such small density variations could influence the hyporheic flow in the streambed with the presence of periodic bedforms. The circulating pore water flow patterns in the bed were modified constantly as the solute front moved downward. Density-induced head gradients eventually overwhelmed the regional hydraulic gradient and drove the circulating flow below a hydraulic divide that would have existed without the density influence. The density-modified hyporheic flow provided a relatively fast solute transport mechanism and enhanced the overall mass exchange between the stream and bed. These results highlight the important role of weak, upward density gradients in modulating hyporheic flow.

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.

Lattice Boltzmann simulations of settling behaviors of irregularly shaped particles.

We investigated the settling dynamics of irregularly shaped particles in a still fluid under a wide range of conditions with Reynolds numbers Re varying between 1 and 2000, sphericity ϕ and circularity c both greater than 0.5, and Corey shape factor (CSF) less than 1. To simulate the particle settling process, a modified lattice Boltzmann model combined with a turbulence module was adopted. This model was first validated using experimental data for particles of spherical and cubic shapes. For irregularly shaped particles, two different types of settling behaviors were observed prior to particles reaching a steady state: accelerating and accelerating-decelerating, which could be distinguished by a critical CSF value of approximately 0.7. The settling dynamics were analyzed with a focus on the projected areas and angular velocities of particles. It was found that a minor change in the starting projected area, an indicator of the initial particle orientation, would not strongly affect the settling velocity for low Re. Periodic oscillations developed for all simulated particles when Re>100. The amplitude of these oscillations increased with Re. However, the periods were not sensitive to Re. The critical Re that defined the transition between the steady and periodically oscillating behaviors depended on the inertia tensor. In particular, the maximum eigenvalue of the inertia tensor played a major role in signaling this transition in comparison to the intermediate and minimum eigenvalues.

A Hybrid Sampling Method for the Fuzzy Stochastic Uncertainty Analysis of Seawater Intrusion Simulations

ABSTRACT Zhao, Z.; Zhao, J.; Xin, P.; Jin, G.; Hua, G., and Li, L., 2016. A hybrid sampling method for the fuzzy stochastic uncertainty analysis of seawater intrusion simulations. The traditional fuzzy stochastic hybrid method requires hundreds or even thousands of simulations to obtain a statistically stable result, which is a significant challenge for some nonlinear problems, such as simulating seawater intrusion. A hybrid sampling (HS) method was developed based on the Monte Carlo (MC) uncertainty analysis whose input parameters are characterized by both stochastic variables and fuzzy numbers. The HS method is a restricted sampling method that fully captures statistical information on the stochastic variables and the fuzzy memberships provided by fuzzy numbers. In HS, samples of stochastic variables and fuzzy numbers are generated using Latin hypercube sampling and restricted stratified sampling, respectively. After they have been generated, samples of different variables are paired to form inputs in a restricted manner for the simulations. This ensures that the samples are distributed across each variables range of uncertainty. The correlations between different variables are also controlled during the restricted sampling process. The simulations of seawater intrusions show that the means and variances of the samples generated using the HS method converge more quickly compared with those generated using a random sampling method. The number of MC simulations required was significantly reduced by using the HS method, which improves the effectiveness of predicting seawater intrusion for the management of coastal aquifers.

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.

Prolonged river water pollution due to variable-density flow and solute transport in the riverbed

A laboratory experiment and numerical modeling were used to examine effects of density gradients on hyporheic flow and solute transport under the condition of a solute pulse input to a river with regular bed forms. Relatively low-density gradients due to an initial salt pulse concentration of 1.55 kg m23 applied in the experiment were found to modulate significantly the pore-water flow and solute transport in the riverbed. Such density gradients increased downward flow and solute transport in the riverbed by factors up to 1.6. This resulted in a 12.2% increase in the total salt transfer from the water column to the riverbed over the salt pulse period. As the solute pulse passed, the effect of the density gradients reversed, slowing down the release of the solute back to the river water by a factor of 3.7. Numerical modeling indicated that these density effects intensified as salt concentrations in the water column increased. Simulations further showed that the density gradients might even lead to unstable flow and result in solute fingers in the bed of large bed forms. The slow release of solute from the bed back to the river led to a long tail of solute concentration in the river water. These findings have implications for assessment of impact of pollution events on river systems, in particular, long-term effects on both the river water and riverbed due to the hyporheic exchange.

Assessing temporal variations of Ammonia Nitrogen concentrations and loads in the Huaihe River Basin in relation to policies on pollution source control

To assess the quality of a water environment, an in-depth analysis of temporal patterns of contaminant concentrations in water body should be carried out based on unbiased water quality datasets. In this study, we developed a modified log-linear model to account for non-stationary seasonal variations of contaminant concentrations over multiple periods. The model was applied to analyze temporal changes of the Ammonia Nitrogen (AN) concentration at Middle Reaches of Huaihe River (MRHR) and two major tributaries, Shaying River (SR) and Guo River (GR). The modified model outperformed the original models and fitted the data well with Pearson correlation coefficients ranging from 0.67 to 0.86. Temporal patterns of AN concentrations, loads and sources were identified from 1998 to 2015 in connection to the implementation of Five-Year Plans (FYPs, policies for controlling water pollution) in the Huaihe River Basin (HRB). The results show that the AN concentration experienced a significant decrease. Since FYPs focused on controlling AN point sources, the proportion of AN loads derived from point sources decreased from 48-86% to 1-17% in the MRHR and from 66-92% to 2-56% in the SR and GR. However, rebounds of AN concentration occurred in the first year of each FYP period possibly due to discontinuity of the policy implementation over the transition between two consecutive FYPs. High AN concentration anomalies were found in flood seasons, related to pollution discharge beyond limits and/or irrational regulation of sluices. These results have implications for future pollution control policies in the HRB, particularly, the need to reduce the upper limits of contaminant loads for flood seasons, continuity of the policies implementation, reduction of non-point source pollution, rational sluice regulation and integrated pollution prevention programs. The developed model and approach are applicable to other polluted river basins to facilitate water quality assessment and evaluation of pollution control policies.

A non-negative and high-resolution finite volume method for the depth-integrated solute transport equation using an unstructured triangular mesh

This paper proposes a new high-resolution finite volume method for solving the two-dimensional (2D) solute transport equation using an unstructured mesh. A new simple r-factor algorithm is introduced into the Total Variation Diminishing flux limiter to achieve a more efficient yet accurate high-resolution scheme for solving the advection term. To avoid the physically-meaningless negative solutions resulted from using the Green–Gauss theorem, a nonlinear two-point flux approximation scheme is adopted to deal with the anisotropic diffusion term. The developed method can be readily coupled with a two-dimensional finite-volume-based flow models under unstructured triangular mesh. By integrating with the ELCIRC flow model, the proposed method was verified using three idealized benchmark cases (i.e., advection of a circle-shaped solute field, advection in a cyclogenesis flow field and transport of a initially square-shaped solute plume), and further applied to simulate the non-reactive solute transport process driven by irregular tides in the Deep Bay, eastern Pearl River Estuary of China. These cases are also simulated by models using other existing methods, including different r-factor for advection term and the Green–Gauss theorem for diffusion term. The comparison between the results from the new method and those from other existing methods demonstrated the new method could describe advection induced concentration shock and discontinuities, and anisotropic diffusion at high resolution without providing spurious oscillations and negative values.

Modelling study on subsurface flows affected by macro-pores in marsh sediments

Recent hydrological research on salt marshes based on mathematical models has suggested links of tidally driven pore water flows with both soil aeration rates and plant vegetation patterns, demonstrating the importance of subsurface flows in the salt marsh ecology. Macro-pores, such as crab burrows are typically distributed in marsh sediments, but little attention has been paid to their potential effects on pore water flows in the marsh soil. In this paper, we present the first 3-D model for simulating pore water flows affected by crab burrows. High hydraulic conductivity zones were used to simulate the burrows. We will also discuss the effects of soil compressibility and varying total stress induced by tidal fluctuations. The simulation results show that crab burrows distributed in an upper soil layer can act as water collectors as long as the lower soils is more hydraulically conductive than the overlying soil layer. During the ebb tide, crab burrows collect pore water in the upper soil layer and discharge it into the lower layer. Such a mechanism may enhance pore water exchange between the mash soil and tidal creek, improve soil aeration conditions and increase material exchange between the marsh and coastal water.