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Coastal reservoirs can alleviate freshwater shortages occurring in the nearshore zone. Deep pools patched at the bottom of reservoirs at greater depths than their adjacent beds maintain a high salinity and possibly provide preferential paths for saltwater backfill. However, such processes are not well understood. Laboratory experiments and numerical simulations were conducted to investigate the effect of the deep pool on subsurface flow and the salinization of coastal reservoir and adjacent aquifers. When seawater intruded into the initially fresh aquifer, the deep pool accelerated the uptake of saltwater to the reservoir from the salt wedge, forming a temporary salt plume around the pool. The saltwater plume also accelerated salt ingress and the evolution of the total salt mass in the coastal reservoir, resulting in the earlier attainment of a quasi-steady state (a classical saltwater wedge) than that when the deep pool was absent. Under steady-state conditions, the deep pool greatly enhanced the exchange of water across the reservoir-aquifer interface by 107% and raised the salinization level of the coastal reservoir by at least 10.3%. Sensitivity analysis suggested that a deeper pool and/or a pool located closer to the reservoir dam may enhance the water exchange and salinity level in the reservoir because it accelerates the water/salt inflow to the freshwater body to a greater extent. These findings may further contribute to improving the predictability and management of water quality in these coastal facilities.

The method of filtering and dewatering waste slurry using geotextiles has been widely used in recent years. It is important to adopt suitable geotextiles to get a high dewatering rate and a high retention performance. In view of the unclear influence of the geotextile properties on the cake and the unclear soil particle retention mechanism in geotextiles, this paper uses the Lattice Boltzmann method-Discrete Element method-DLVO (LBM-DEM-DLVO) coupling method to simulate the mesoscopic process of slurry filtration process using geotextiles. The numerical results show that the nature of the geotextile does not affect the hydraulic conductivity of the cake. And the retention mechanism for soil particles being trapped inside geotextile was proposed. The mechanism is related to the relative relationship between the interaction force between the geotextile fiber and the soil particle Finter and the drag force of the soil particle F d y in the geotextile. The dimensionless ratio αsl = F i n t e r / ( F d y + G ) was proposed to quantify this retention mechanism; the smaller αsl is, the greater the probability that the geotextile will lose particles. When αsl ≥ 1.04, the geotextile does not lose particles. According to these results, some suggestions of geotextiles for dewatering slurry were proposed.

The Lattice Boltzmann method has been widely adopted to simulate flow in porous media. The choice of appropriate boundary schemes is essential to achieve simulation accuracy; however, the criteria for the most suitable boundary treatment in the simulation of flow in porous media flow remain unresolved. Here, three types of the most commonly used boundary conditions are tested: interpolation bounce back (IBB), partial saturated method (PSM), and immersed boundary method (IBM). The dimensionless drag of face-centered cubic (FCC) sphere array and the dimensionless permeability of a random closely packed (RCP) sphere array are calculated and compared at different viscosities and resolutions. In the FCC sphere array case where spheres are not contacted, the IBB and PSM exhibit the same accuracy and both are of the second-order convergence rate. The IBM is less accurate and is of the first-order convergence rate. In the RCP sphere array case where the spheres are contacted, the IBB shows finer results and a second-order convergence rate. PSM underestimates the dimensionless permeability and increases resolution only slightly improved the results of PSM. The IBM overestimates the dimensionless permeability. These results indicate that among the three methods, the IBB is the most accurate. The PSM has the same accuracy as the IBB when sediments are not contacted; however, it loses its accuracy in the simulation of flow in closely packed porous media. This work could serve as a benchmark for further research in choosing the most appropriate method in the simulation of flow in porous media.

A better understanding of phosphorus-transfer process and influence factors at Sediment-Water Interface (SWI) is essential to develop effective and efficient river managements strategies. In this study, overlying water, pore water and riverbed sediment samples were collected in the Huaihe River (HR), a highly polluted river in Eastern China, in May 2013, July 2013 and June 2016, respectively. Models were developed to analyze influence factors on phosphorus by Bayesian Networks (BNs), which could describe complex interdependencies between dependent/independent variables conveniently compared to traditional statistical models. The transfer process and effects of land use on phosphorus at the SWI were evaluated. Results indicated that phosphorus concentration decreased in overlying water and pore water similarly but increased in riverbed sediment from 2013 to 2016. The concentrations in overlying/pore water reached a maximum at the middle of the research area, while those in riverbed sediment increased from upstream to downstream. Phosphorus was transferred from overlying water to riverbed sediment through pore water mainly in a dissolved phase, indicating that riverbed sediment could absorb contaminants and act as “pollution sink” in the HR. Upstream water and sediment were important sources of downstream phosphorus with the influence coefficients ranging from 0.294 to 0.491. Land use within 1 km river buffer had most significant influence on total phosphorus in overlying water and 2 km river buffer to riverbed sediment while land use had the most noteworthy effects on particulate/dissolved phosphorus in 2 km and 8 km river buffer respectively. Farmland, urban and rural residential land were important sources of phosphorus at the SWI, and farmland contributed most (>30%) in the HR. The study not only provides insights into phosphorus-transfer process and influence factors at the SWI in the HR, but the proposed model also could be applicable in other polluted rivers.

Contaminants that entered the streambed during previous surface water pollution events can be released to the stream, causing secondary pollution of the stream and impacting its eco-environmental condition. By means of laboratory experiments and numerical simulations, we investigated density effects on the release of solute from periodic bedforms. The results show that solute release from the upper streambed is driven by bedform-induced convection, and that density effects generally inhibit the solute release from the lower streambed. Density gradients modify the pore water flow patterns and form circulating flows in the area of lower streambed. The formation of circulating flows is affected by density gradients associated with the solute concentration and horizontal pressure gradients induced by stream slope. The circulating flows near the bottom of the streambed enhance mixing of the hyporheic zone and the ambient flow zone.

Attenuation of groundwater ammonium (NH

Sediment microbial communities play critical roles in the health of fish and the biogeochemical cycling of elements in aquaculture ecosystems. However, the response of microbial communities to temporal and spatial variations in interconnected aquaculture pond and ditch systems remains unclear. In this study, 61 sediment bacterial samples were collected over one year from 11 sites (including five ponds and six ditches) in a 30-year-old fish aquaculture farm. The 16S rRNA approach was used to determine the relative abundances of microbial communities in the sediment samples. The relationships among nutrients, heavy metals, and abundant microorganisms were analyzed. Our results showed that Proteobacteria, Bacteroides and Chloroflexi were the predominant phyla in the sediments of aquaculture pond, with average abundances of 36.33%, 18.60%, and 14.58%, respectively. The microbial diversity in aquaculture sediments was negatively correlated (P 0.05) with the concentrations of total nitrogen and total phosphorus in sediments, indicating that the microbial diversity is highly associated with the remediation of nutrients in sediments. The sediment samples with high similarities were discovered by the t-distributed stochastic neighbor embedding (t-SNE) method. The site-specific correlations between specific microorganisms and heavy metals were explored. The network analysis revealed that the microbial diversities in aquaculture ponds were more stable than that in aquaculture ditches. The network analysis also illustrated that the microbial genera with low relative abundances may become key groups of microbial communities in sediment ecosystems. Our work deepens the understanding of the relationships between microbial communities and the spatiotemporal characteristics of surface water and sediments in aquaculture farms.

Tides and seasonally varying inland freshwater input, with different fluctuation periods, are important factors affecting flow and salt transport in coastal unconfined aquifers. These processes affect submarine groundwater discharge (SGD) and associated chemical transport to the sea. While the individual effects of these forcings have previously been studied, here we conducted physical experiments and numerical simulations to evaluate the interactions between varying inland freshwater input and tidal oscillations. Varying inland freshwater input was shown to induce significant water exchange across the aquifer-sea interface as the saltwater wedge shifted landward and seaward over the fluctuation cycle. Tidal oscillations led to seawater circulations through the intertidal zone that also enhanced the density-driven circulation, resulting in a significant increase in the total SGD. The combination of the tide and varying inland freshwater input, however, decreased the SGD components driven by the separate forcings (e.g., tides and density). Tides restricted the landward and seaward movement of the saltwater wedge in response to the varying inland freshwater input in addition to reducing the time delay between the varying freshwater input signal and landward-seaward movement in the saltwater wedge interface. This study revealed the nonlinear interaction between tidal fluctuations and varying inland freshwater input will help to improve our understanding of SGD, seawater intrusion, and chemical transport in coastal unconfined aquifers.

Hyporheic flow transports fine particles into the riverbed, which can lead to clogging of the bed and in turn affect hyporheic flow and exchange processes. Field measurements and numerical simulations show the formation of a low-permeability layer (LPL) near the bed surface due to fine particle clogging, and consequently reduction of exchange fluxes between the bed and river water. A characteristic porosity (e*) and time scale were derived to quantify the clogging process and effects on transport. Both the exchange flux and mean solute residence time were found to follow a power law relationship with e*. For the normalized particle exchange flux, the exponent is close to unity, i.e., a linear relationship with e*. The results also showed significant effects of the fine particle concentration, pressure difference, sediment collision efficiency and fine particle diameter on the bed clogging. Large values of these parameters led to intensified clogging, with the formation of different types of LPL.

Previous studies of bedform-induced hyporheic flow focused on processes driven by longitudinal hydraulic head variations at the sediment-water interface (SWI) due to river flow and bedform interactions. By way of numerical simulations, we examined the transverse hyporheic flow in a river system with compound sections, comprising a main river channel and a floodplain connected by a bank. The results show that head fluctuations across the compound beds associated with the transverse secondary flow can cause significant lateral hyporheic flow through two pore water circulating cells. Such a 2-D compound bedfrom have a higher flux across the SWI than the 2-D longitudinal dunes since the 2-D compound cross-section form induces more drag. The circulating flow cells originate from the bank surface and terminate at the main channel and floodplain bed surfaces, respectively. The hyporheic exchange concentrates on the bank region below the SWI where an intensive solute transfer concurrently occurred. The head gradient along the SWI is sensitive to the bank slope angle, which consequently determines transverse hyporheic flow dynamics and solute transport processes. Larger bank slope angles induce higher hyporheic fluxes and smaller hyporheic areas and residence time. The effects of increasing flux on solute transport in the compound section are partly influenced by a sharply decreasing hyporheic area. Thus the solute consequently transfers downward within the hyporheic zone with a lower rate at larger bank slope angle. Larger river discharges may promote both the transverse hyporheic flow and solute transformation across the bed surface. These physical processes may have implications for the biogeochemical processing within the hyporheic zone across a compound river system.

A2D density-dependent model was proposed to describe the salt pollution and N2O production-consumption process in the HZ Themodel used in this study is based on the OpenFOAM(https://openfoam.org/) and COMSOL (https://cn.comsol.com/)

Colloids are often in forms of inorganic and organic particles whose particle sizes (d) are ranging from 1.0 nm to 10.0 μm. On the one hand, their transport processes in the hyporheic zone such as deposition, resuspension, clogging and release are substantially affected by hyporheic exchange. On the other hand, the existence of colloids can modify the hydraulic structure of the hyporheic zone due to clogging. Despite the general acknowledgement that particle size plays an important role in contaminant adsorption and clogging, it remains unclear how the particle size of colloids changes during their transport processes in the hyporheic zone. This study aims to investigate the variation of the particle size of colloids in the overlying water and the effects of settlement and convection-diffusion via laboratory experiment and numerical simulation. The results show that both settlement and convection-diffusion play roles in the exchange of colloids between the stream and the streambed. More specifically, settlement is the dominant factor affecting the exchange process of large-sized particles (d > 3.06 μm) in the hyporheic zone as the high settling velocity dominates the outflux of colloids in the overlying water; the exchange process of small-sized particles (d < 1.10 μm) is more affected by convection-diffusion and some of them can be released from the streambed to the overlying water; while the exchange process of middle-sized particles (1.10 μm < d < 3.06 μm) is affected by both convection-diffusion and settlement. Thus, this study may provide important insights into the variation of the particle size of colloids in the overlying water and the effects of settlement and convection-diffusion.

Interactions among multiple nutrients uptake certainly have a great effect on their retention in headwater streams, yet little research has been made to explore the quantitative characteristics of their interactions, especially in mesotrophic streams. In response, we conducted an identical series of instantaneous nutrient addition experiments, using ammonium nitrogen (NH4-N) and phosphate phosphorus (PO4-P) alone or together, in two mesotrophic agricultural headwater streams in Chaohu Lake Basin, China, to quantify the relationships between nutrient concentrations and uptake rates, and examine how NH4-N and PO4-P interact to affect their individual uptake. Both the Michaelis-Menten (M-M) equation and response surface model were utilized to analyze coupled NH4-N and PO4-P uptake patterns across a range of nutrient concentrations, by fitting the kinetic processes of NH4-N and PO4-P uptake in single- and dual-nutrient additions. The capacity of both NH4-N and PO4-P uptake was increased in different degrees in dual-nutrient additions. Response surface models could quantitatively characterize the three-dimensional dynamic evolution trend of NH4-N or PO4-P uptake rates at different concentrations. The influence of PO4-P additions on NH4-N uptake was generally greater than that of NH4-N on PO4-P uptake in the five tracer tests. In addition, results of correlation analysis indicated that water temperature might be the main factor affecting the coupling of N and P uptake in mesotrophic streams and followed by hydrological factors (e.g., discharge) and channel geomorphology.

In this study, both laboratory experiments and numerical simulations were conducted to investigate the effect of density-driven flow on the transport of high-concentration pollutants in the hyporheic zone. The results show that the density gradient can change the flow of pore water and the strong density-driven flow can lead to an unstable flow, which increases the effect of preferential flow and thus causes the appearance of solute fingers in the hyporheic zone. Notably, these solute fingers become more obvious with the increase of depth. The appearance of solute fingers depends on the relative strength of the pumping exchange and density gradient, which are represented by the dimensionless number M* and N* respectively. Finger flows appear near the interface when M* is less than 0.5 N*. This study may contribute to better understanding the transport and destination of solutes and thus may provide some insights into the assessment on pollution incidents.

Colloids exist widely in rivers which can act as contaminants or carries of contaminants. Hyporheic exchange drives colloids to transport into the hyporheic zone. However, the variation of the particle size of colloids has seldom been considered in previous transport theories of colloids. This study aims to investigate the variation of the particle size of colloids and functions for different sized particles via laboratory experiments and simulations. The results show that the settlement and convection-diffusion of colloids play a dominant role in the exchange of colloidal particles between the stream and the streambed. Large particles can settle into the streambed more rapidly as settlement domains the process, which however can hardly be detected in the overlying water during the later period of the experiment; the exchange process of small-sized particles is affected more by convection and diffusion, and a retarded trace release can be monitored from the streambed to the stream; while for middle-sized particles, mass transfer coefficient and settling velocity affect exchange process together. Because the variation of particle size (affecting specific surface area, surface electrochemical characteristics) can have a substantial effect on the contaminants carriers (colloids) in the hyporheic zone, the knowledge of it should be taken into account in future studies.

A carbon solution composed of nanoparticles (d50 = 85.7 nm) was used in experiments designed to explore nanoparticle transport characteristics within the hyporheic zone of a riverbed. Experiments and numerical simulations demonstrated that nanoparticle transport in the hyporheic zone is affected by hydraulic head gradients due to river flow-bedform interactions as well as density gradients associated with the nano-carbon solution. Differences with similar flow/transport situations were examined, and it was found that particulate-enhanced density can change hyporheic transport appreciably. In addition to density, particle settling enhances downward movement of the nano-carbon plume in the riverbed. While nanoparticle transport in the upper hyporheic zone is largely controlled by advection due to flow driven by head gradients at the bed surface, density gradients and particle settling influence the transport process significantly in the lower hyporheic zone. During the transport process, nanoparticles become deposited due to attachment to sand particles and filtration by small pores in the bed. Compared with transport where density variations are minimal, the particulate-induced density gradient induces downward transport of nanoparticles and entrained liquids, leading to deposition/accumulation at the base of the bed.

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.

Water management is an important practice with significant effect on greenhouse gases (GHG) emission from soils. Nitrous oxide (N2O) and carbon dioxide (CO2) emissions and their global warming potentials (GWPs) from subsurface watering soil (SUW) were investigated, with surface watering (SW) as a control. Results indicated that the N2O and CO2 emissions from SUW soils were somewhat different to those from SW soil, with the peak N2O and CO2 fluxes from SUW soil reduced by 28.9% and 19.4%, and appeared 72 h and 168 h later compared with SW. The fluxes of N2O and CO2 from SUW soils were lower than those from SW soil in both pulse and post-pulse periods, and the reduction was significantly (p 0.1) lower that from SW soil. Moreover, N2O and CO2 fluxes from both watering treatments increased exponentially with increase of soil water-filled pore space (WFPS) and temperature. Our results suggest that watering soil from subsurface could significantly reduce the integrative greenhouse effect caused by N2O and CO2 and is a promising strategy for soil greenhouse gases (GHGs) mitigation. And the pulse period, contributed most to the reduction in emissions of N2O and CO2 from soils between SW and SUW, should be a key period for mitigating GHGs emissions. Response of N2O and CO2 emissions to soil WFPS and temperature illustrated that moisture was the dominant parameters that triggering GHG pulse emissions (especially for N2O), and temperature had a greater effect on the soil microorganism activity than moisture in drier soil. Avoiding moisture and temperature are appropriate for GHG emission at the same time is essential for GHGs mitigation, because peak N2O and CO2 emission were observed only when moisture and temperature are both appropriate.

Coastal reservoirs can alleviate the shortage of freshwater in the nearshore zone. Inland groundwater and tides can cause hydraulic gradients between seawater and reservoir water and affect the salt concentration of coastal reservoirs; however, the mechanism is poorly understood. Laboratory experiments and numerical simulations were conducted to investigate the combined effect of inland groundwater inputs and tides on the surface/subsurface flow and salinization dynamics of the coastal reservoir and adjacent aquifer. The results indicated that inland groundwater followed two pathways before discharging into the sea, regardless of the tidal conditions. The predominant pathway led inland groundwater to be discharged to the coastal reservoir and then to sea. The other pathway encompassed the entire aquifer underlying the reservoir bed and terminated at the aquifer–sea interface. As the inland groundwater input increased, the groundwater entering the reservoir also increased, whereas the ratio of the fresh groundwater transported through the reservoir to the total groundwater input decreased. Tidal pumping allowed more seawater to intrude into the coastal reservoir and adjacent aquifer compared with nontidal conditions. This caused a larger proportion of groundwater to be discharged to the coastal reservoir, with the density-driven flow and water/salt exchange across the aquifer-reservoir interface also enhanced. As the inland groundwater input increased, salinization of the coastal reservoir decreased and the inhibition of the inland groundwater input on salinization also decreased. This study contributes to the improved management of such coastal infrastructures.

Utilizing detention basin to store flood water and reduce the risk of levee failure, as well as guarantee the security of important regions downstream, is an important flood control measure in some large river catchments. However, the populated and economic detention basins in China are facing the dual challenges of protecting the safety of lives and property and alleviating the pressure of flood control downstream. Maximizing the total benefits of a flood control system by balancing the flooding losses in the detention basin and the flood control benefit of the flood protected regions downstream has been the key factor in river basin flood management. In this paper, a framework is developed to optimize the flood diversion siting and operating policy of a detention basin, which consists of three modules: (1) the flood simulation module, which simulates the flood routing and flood diversion process via a one-dimensional and two-dimensional coupled hydrodynamic model; (2) the cost and benefit calculation module, which estimates the flood damage in the detention basin and the flood control benefit in flood protected regions based on the results of the flood simulation model, with the diversion structure cost considered; and (3) the optimization module, which optimizes the diversion siting and control strategy with the objective of the maximum system benefit by balancing the flood loss in the detention basin and flood control benefit of the protected area. The framework is applied to the Huayanghe detention basin, the most downstream detention basin along the Yangtze River basin, China. The results state that gate control upstream is the optimal strategy to decrease the flood losses in the detention basin, but the potential flood losses of the protected region increase significantly under this policy. Moreover, an operating policy that balances moderately the flooding losses in the detention basin and the flood control benefit of flood protected regions downstream is provided though the framework. Overall, this study highlights flood simulations and optimization approaches represent a promising solution for filling the gap of densely populated detention basin modeling and real-world management decision-making.

Benzene is a toxic contaminant and can harm many aquatic species and cause serious damages to the river eco-system, if released to rivers. In 2012, a major spill accident occurred on the Huaihe River in Eastern China with 3 tons of benzene released to the river section 70 km upstream of a natural reserve. Two emergency measures were taken to minimize the impact of the accident on the natural reserve: 1) flow control by adjusting upstream sluices to delay the arrival of the contaminant plume at the reserve and 2) in-situ treatment using activated carbons to reduce the contaminant concentration. Here we develop a process-based mathematical model to analyze the monitoring data collected shortly after the accident, and explore not only how effective the adopted measures were over the incident but more importantly the mechanisms and critical conditions underlying the effectiveness of these measures. The model can be used as a tool for designing optimal management responses to similar spill accidents in regulated river systems, combining flow control and in-situ treatment.

Large amounts of toxic metals are discharged into rivers and lakes, but little is known about the factors that drive the adsorption and transformation of these metals in the hyporheic zone and the exchange flux across the sediment-water interface. To better understand transport and transformation of metal ions in the hyporheic zone, flume experiments and numerical simulations were performed in a streambed with periodic bedforms using zinc ions. Compared to non-adsorbing contaminant, the results show that adsorption leads to a more rapid decrease in the concentration of Zn2+ in the overlying water, and a lower final concentration is reached. The mass of adsorbed ions is several times higher than that of free ions in the bedform's water phase. Indeed, metal adsorption is in the shallow layer of the streambed. Although this prevents heavy metal groundwater contamination, the same cannot be said of shallow layer of the hyporheic zone. Knowledge of the migration and transformation of metal ions in the hyporheic zone provides insights pertinent to the restoration of polluted rivers.

Hyporheic exchange is affected by bedform geometry, which induces complex flow paths within the bedform. Additional factors that influence flow and solute transport in the hyporheic zone are layered profile sediments and density-driven flow. This study explored the combined effects of these factors on hyporheic exchange through laboratory experiments and numerical simulations involving infiltrating solute displacing less-dense resident water in a layered bedform with a low permeability layer (LPL). The bedform consisted of three horizontal layers, in which the hydraulic conductivity of the middle layer (LPL) was less than that of the top (TL) and bottom layers (BL). The results demonstrated that a previously unexplored combination of mechanisms (density effects and layered bedform) produces irregular spatial patterns of solute transport in the hyporheic zone. For instance, the width of solute plume within the bottom layers becomes narrowed compared with tracer transport. With increasing density contrast between infiltrating solute and resident water, the solute plume becomes much narrower, forming fingers. Numerical modeling further shows that the hydraulic conductivity contrast (HCC) and relative thickness (RT) of the hyporheic zone layers also affect the spatial solute transport patterns. As the hydraulic conductivity contrast or relative thickness increases, the plume becomes much narrower. Horizontal ambient flow (HAF) dominated in the bottom layers, and lateral solute spreading and mixing intensified with a higher hydraulic conductivity contrast and relative thickness. Furthermore, the vertical solute plume was detached by the horizontal ambient flow in the bottom layers with a discontinuous low permeability layer, forming a discontinuous zone of vertical solute transport.

Discrete modeling of particle-fluid interaction was of great importance for its wide applications in chemical, petroleum and geotechnical engineering. Using Immersed Boundary Method (IBM) to couple Discrete Element Method (DEM) with Lattice Boltzmann Method (LBM) was adopted by many researchers to study particle-fluid systems. However, to accurately simulate the behavior of a large number of particles in the fluid, a huge computational power wasrequired to fully resolved the motion of each particle and an intensive search of particles’ contacts was needed to account for the collision between particles. In this paper, a periodic boundary was proposed for coupling DEM with LBM using IBM and an efficient particle contact detection algorithm was developed to further reduce the computation cost. The model was validated by several well-defined benchmark including: a single particle settling in a box, particle spinning in a channel and the well-known ‘Drafting, Kissing and Tumbling’ (DKT) effect of two settling disks. The model's applicability was exhibited in the simulation of sediments movement in a channel, which agreed well with previous experiments. These results showed the potential usage of present numerical model to investigate in particle-fluid interaction systems.

The rainfall patterns play an important role in determining the response of Suspended Sediment (SS) and Total Phosphorus (TP) to influence factors, which is complex and needs to be better understood. The Bayesian Network (BN), with each variable depending on only its immediate parent variables, can help to describe the complex processes involved. In this study, BNs were developed to assess the impacts of catchment characteristics and human activities on SS and TP loads under different rainfall patterns in the Huaihe River Basin (HRB). The results showed that SS made significant contributions to TP in rivers which increased with the rainfall intensity. Catchment characteristics (sub-catchment area, slope and soil erosion area) affected the SS and TP loads; however the influence of human activities (sewage outfalls and land use) was more significant. With the rainfall intensity rising, farmland switched from a “sink” to a “source” of nutrients. When the antecedent three-day-average rainfall intensity was high but the current one was low, the farmland became the most significant source of SS and TP. The urbanization had significant positive effects on SS (ranging from 10.9% to 15.1%) and TP (ranging from 8.3% to 10.5%), which was insensitive to rainfall patterns as a result of the high proportions of impervious surface coverage in the urban area. These results improve our understanding of influence factors on river water quality and will contribute to effective water environment management for rivers.

To better manage water environment in highly polluted rivers, the influence factors on water quality need to be investigated. With the effects of oxygen-demanding contaminants, it is difficult to resolve the complex interdependencies of the various factors using conventional methods. The Bayesian Networks (BNs), in which each variable only depends on its immediate parent variables, can solve this problem. In this study, the BNs were developed to assess the impacts of land use and sewage outfalls on Ammonia Nitrogen (AN) and Dissolved Oxygen (DO) concentrations in the Huaihe River Basin (HRB) for different seasons and spatial scales, where AN was a typical oxygen-demanding contaminant and the most serious contaminant in the area. The BNs gave the best explanations for variations in AN (NSE = 0.80) and DO (NSE = 0.72) concentrations by using land use and sewage outfalls data at the local scale (less than 20 km radii around monitor stations), suggesting that controlling water contaminant sources at local scales can improve water quality efficiently. AN negatively affected DO concentration, which was more significant in dry seasons. Wastewater from sewage outfalls was the largest contributor (26.2%) to AN pollution in dry seasons, which was weakened in wet seasons by an intensive dilution process. Farmland acted as a “sink” in dry seasons and as a “source” in wet seasons. The transition between two states was caused by large variations in surface runoff between dry and wet seasons. Urban land made a disproportionately large contribution to water pollution compared to other kinds of land use. These findings improve our understanding of influence factors on water quality and will contribute to effective river management.

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.

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.

The uncertainty and sensitivity of predicted positions and thicknesses of seawater-freshwater mixing zones with respect to uncertainties of saturated hydraulic conductivity, porosity, molecular diffusivity, longitudinal and transverse dispersivities were investigated in both head-control and flux-control inland boundary systems. It shows that uncertainties and sensitivities of predicted results vary in different boundary systems. With the same designed matrix of uncertain factors in simulation experiments, the variance of predicted positions and thickness in the flux-control system is much larger than that predicted in the head-control system. In a head-control system, the most sensitive factors for the predicted position of the mixing zone are inland freshwater head and transverse dispersivity. However, the predicted position of the mixing zone is more sensitive to saturated hydraulic conductivity in a flux-control system. In a head-control system, the most sensitive factors for the predicted thickness of the mixing zone include transverse dispersivity, molecular diffusivity, porosity, and longitudinal dispersivity, but the predicted thickness is more sensitive to the saturated hydraulic conductivity in a flux-control system. These findings improve our understandings for the development of seawater-freshwater mixing zone during seawater intrusion processes, and give technical support for groundwater resource management in coastal aquifers.

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.