Feedbacks between bivalve density, flow, and suspended sediment concentration on patch stable states

We explore the role of biophysical feedbacks occurring at the patch scale (spatial scale of tens of meters) that influence bivalve physiological condition and affect patch stability by developing a numerical model for the pinnid bivalve, Atrina zelandica, in cohesive sediments. Simulated feedbacks involve bivalve density, flow conditions (assumed to be primarily influenced by local water depth and peak current speed), suspended sediment concentration (evaluated through a balance between background concentration, deposition, and erosion), and changes in the physiology of Atrina derived from empirical study. The model demonstrates that high bivalve density can lead to skimming flow and to a concomitant decrease in resuspension that will affect suspended sediment concentration over the patch directly feeding back on bivalve physiology. Consequently, for a given flow and background suspended sediment load, the stability of a patch directly depends on the size and density of bivalves in the patch. Although under a range of conditions patch stability is ensured independently of bivalve density, simulations clearly indicate that sudden changes in bivalve density or suspended sediment concentration can substantially affect patch structure and lead to different stable states. The model highlights the role of interactions between organisms, flow, and broader scale environmental conditions in providing a mechanistic explanation for the patchy occurrence of benthic suspension feeders.     

Characteristics of flow structure of free-surface flow in a partly obstructed open channel with vegetation patch

Free-surface flows over patchy vegetation are common in aquatic environments. In this study, the hydrodynamics of free-surface flow in a rectangular channel with a bed of rigid vegetation-like cylinders occupying half of the channel bed was investigated and interpreted by means of laboratory experiments and numerical simulations. The channel configurations have low width-to-depth aspect ratio (1.235 and 2.153). Experimental results show that the adjustment length for the flow to be fully developed through the vegetation patch in the present study is shorter than observed for large-aspect-ratio channels in other studies. Outside the lateral edge of the vegetation patch, negative velocity gradient (\(\partial \overline{u}/\partial z < 0\)) and a local velocity maximum are observed in the vertical profile of the longitudinal velocity in the near-bed region, corresponding to the negative Reynolds stress (\(- \overline{{u^{\prime}w^{\prime}}} < 0\)) at the same location. Assuming coherent vortices to be the dominant factor influencing the mean flow field, an improved Spalart–Allmaras turbulence model is developed. The model improvement is based on an enhanced turbulence length scale accounting for coherent vortices due to the effect of the porous vegetation canopy and channel bed. This particular flow characteristic is more profound in the case of high vegetation density due to the stronger momentum exchange of horizontal coherent vortices. Numerical simulations confirmed the local maximum velocity and negative gradient in the velocity profile due to the presence of vegetation and bed friction. This in turn supports the physical interpretation of the flow processes in the partly obstructed channel with vegetation patch. In addition, the vertical profile of the longitudinal velocity can also be explained by the vertical behavior of the horizontal coherent vortices based on a theoretical argument.     

Spatial evolution of coherent motions in finite-length vegetation patch flow

A number of experimental studies on submerged canopy flows have focused on fully-developed flow and turbulent characteristics. In many natural rivers, however, aquatic vegetation occurs in patches of finite length. In such vegetated flows, the shear layer is not formed at the upstream edge of the vegetation patch and coherent motions develop downstream. Therefore, more work is neededz to reveal the development process for large-scale coherent structures within vegetation patches. For this work, we considered the effect of a limited length vegetation patch. Turbulence measurements were intensively conducted in open-channel flows with submerged vegetation using Particle Image Velocimetry (PIV). To examine the transition from boundary-layer flow upstream of the vegetation patch to a mixing-layer-type flow within the patch, velocity profiles were measured at 33 positions in a longitudinal direction. A phenomenological model for the development process in the vegetation flow was developed. The model decomposed the entire flow region into four zones. The four zones are the following: (i) the smooth bed zone, (ii) the diverging flow zone, (iii) the developing zone and (iv) the fully-developed zone. The PIV data also confirmed the efficiency of the mixing-layer analogy and provided insight into the spatial evolution of coherent motions.     

Investigation of the flow structures through heterogeneous vegetation of varying patch configurations in an open channel

Environmental Fluid Mechanics - The hydrodynamics of flow passing through heterogeneous vegetation configurations is always complex and it becomes difficult to capture the flow structures through...     

Restoration of hyper-eutrophic water with a modularized and air adjustable constructed submerged plant bed

A modularized and air adjustable constructed submerged plant bed (CSPB) which can be used to restore the eutrophic water is introduced in this paper. This plant bed helps hydrophyte grow under poor conditions such as frequently changed water depth, impaired water transparency, algae bloom and substantial duckweed in summer, which are not naturally suitable for growing hydrophyte. This pilot study in Waihuan River of Tianjin, China, revealed that reduction of Chemical Oxygen Demand (COD), Total Nitrogen (TN) and Total Phosphorus (TP) by the use of CSPB could be reached 30%–35%, 35%–40%, 30%–40% respectively in the growing season (from March to October) and 5%–10%, 5%–15%, 7%–20% respectively in the winter (from November to February) when the detention time was 6 d. The relationships between the concentration of COD, TN, TP and the detention time fit the first-order kinetic equation well and the coefficients of determination (R²) were all above 0.9. The attenuation coefficients k of the kinetic equation were a function of the water temperature. When the water temperature was quite low or quite high, k was not significantly changed with increasing or decreasing water temperature. While when the temperature was in a moderate range, an increase or decrease of water temperature would lead to a rapid increase or decrease in k.     

Secondary flow within a river bed and contaminant transport

We have developed a numerical method to simulate the transport of non-sorbing contaminants within the sediment layer of a stream and the leaching of these contaminants in the steam. Typical stream bottom surfaces are uneven with triangularly shaped undulation forms. The flow of the water above such triangular surfaces causes external pressure changes that result in a “pumping effect” and a secondary flow within the sediment. The latter causes a significant contaminant advection within the sediment layer. The flow field in the porous sediment layer is obtained by solving numerically Darcy’s equations. The unsteady mass transfer equation is solved by using a finite-difference method with an up-wind scheme. The effects of parameters, such as channel slope, hydraulic head and dispersion, are studied by quantitatively comparing the numerical results of the total mass flow rate from the contaminant source, the concentration front propagation, and the contaminant mass flow rate into the water column. The “pumping effect,” increases the flow in the vertical direction and, thus, enhances the vertical advective mass transport of the contaminant. This bedform-shape induced flow is largely responsible for the mass transfer of contaminants into the water column. The numerical results also show that the mechanical dispersion inside the sediment bed will significantly increase the contaminant mass flow rate from the source.     

Modeling the buoyant flow of heated water discharged from surface and submerged side outfalls in shallow and deep water with a cross flow

In this study, a three-dimensional model was used to numerically study the buoyant flow, along with its mixing characteristics, of heated water discharged from the surface and submerged side outfalls in shallow and deep water with a cross flow. Hydraulic experimental data were used to evaluate the applicability of the model. The simulation results agree well with the experimental results, particularly, the jet trajectories, the dimensions of the recirculating zone, and the distribution of the dimensionless excess temperature. The level of accuracy of the simulation results of the present study is nearly identical to that of the results conducted by McGuirk and Rodi (1978). If the heated water is discharged into shallow water where the momentum flux ratio and the discharge densimetric Froude number are high, the submerged discharge method is better than the surface discharge method in terms of the scale of the recirculating zone and the minimum dilution. In deep water, where the momentum flux ratio and discharge densimetric Froude number are low, however, the submerged discharge method had few advantages. In shallow water, the discharge jet is deflected by the ambient cross flow, while forcing the ambient flow to bend towards the far bank for the full depth. For a submerged discharge in shallow water, the recirculating zone is the largest in the lowest layer but becomes smaller in the upper layer. As the water depth increases, the ambient flow goes over the jet and diminishes the blocking effect, thereby decreasing the bending of the jet.     

Effects of emergent and submerged rigid vegetation configurations on gravity current dynamics

Environmental Fluid Mechanics - The propagation and hydrodynamic processes of lock-exchange gravity current through vegetation regions were investigated experimentally. Experimental results show...     

Hydrodynamics and bed stability of open channel flows with submerged foliaged plants

In this work we investigate experimentally and numerically the flow structure around foliaged plants deployed in a channel with gravels on the bed under submerged and barely submerged conditions. Velocity and Reynolds stress were measured by using a NORTEK Vectrino profiler. Visual observation shows that the initial motion of gravels is easier to be triggered under the condition of flow with barely submerged vegetation. This is confirmed by the measured velocity, Reynolds stress and total kinetic energy (TKE) profiles. The velocity exhibits a speed up in the near-bed region, and the associated Reynolds stress and TKE increase there. A 3D numerical model is then verified against the experiments and used to investigate systematically the effect of degree of submergence of foliaged plants on the channel bed shear stress. The results show that the maximum bed shear stress occurs when the foliage is situated slightly below the water surface, which can enhance channel bed instability.     

沉水植物对沉积物及土壤垂向各形态无机磷的影响

该研究利用相同区域湖泊沉积物及土壤在室内模拟条件下培养黑藻,运用化学连续浸提法分离不同层次底质中各形态无机磷,并通过对水体中磷质量浓度和Eh值的测定,分析不同层次底质中各形态无机磷质量分数和间隙水中磷质量浓度的变化,揭示在沉水植物作用下淹水土壤和沉积物中不同层次各形态无机磷的迁移转化规律.结果表明,本试验条件下,黑藻主要影响上层和下层(根系所在层)底质中各形态无机磷的迁移转化;黑藻主要吸收利用弱吸附态磷和可还原态磷,并通过改变底质氧化还原环境影响可还原态磷和铁铝氧化态磷的迁移转化;黑藻的生长促进钙结合态磷的释放,但是黑藻的腐烂促进钙结合态磷的沉积;土壤中各形态无机磷比营养水平相当的沉积物中的磷容易迁移转化,黑藻对沉积物中各形态无机磷迁移转化的影响大于土壤.     

Assessment of spatial distribution of submerged vegetation in the Orbetello lagoon by means of a mathematical model

Coastal lagoons are characterized by a constant threat of eutrophication and a critical coexistence of differing submerged vegetation forms. This paper investigates the competitive equilibrium of macroalgae and phanerogams in the Orbetello lagoon in relation to physico-chemical and environmental factors, including wind, nutrients in the water column, and sediment characteristics. A mathematical model describing the evolution of the submerged vegetation as a function of the abiotic parameters is used here in conjunction with specific experimental studies to explain the relationship between phanerogams (seagrasses) prairie expansion, water movements, and sediment characteristics. The combination of specific sediment sampling and mathematical modelling shows that water circulation and the state of the upper sediment are both dominant factors in determining the phanerogams distribution in the lagoon and the mutually exclusive growth of these groups in differing parts of the lagoon. Water currents control the distribution of floating macroalgae, resulting in an uneven accumulation of decomposing biomass and phanerogams seed dispersal. The oxygenation provided by the rooted phanerogams affects the sediment characteristics, making them suitable for further prairie expansion. In addition to sediment analysis the use of a mathematical model combining the hydrodynamics and the water quality of the lagoon provides a thorough explanation of the expansion of the rooted vegetation in critical areas. A further result of this research is the validation of the model, originally calibrated with the lagoon central stations’ data, with the newly acquired data from several other parts of the ecosystem. The model predictions are in good agreement with the field observations under a number of environmental conditions and explain the observed expansion trend of phanerogams, which are beneficial for the lagoon ecology, more thoroughly than by relying on the sediment observations alone.     

Model of turbulence penetration into a suspension layer on a sediment bed and effect on vertical solute transfer

The vertical diffusional mass (solute) transfer through a suspended sediment layer, e.g. at the bottom of a lake, reservoir or estuary, by the propagation of velocity fluctuations from above was investigated. The attenuation of the velocity fluctuations in the suspension layer and the associated effect on solute transfer through the suspension layer was simulated. To represent large eddies traveling downstream in water over a high-concentration suspended sediment layer, a streamwise velocity fluctuation moving in downstream direction was imposed along the upper boundary of the suspension layer. Velocity fluctuations and downstream velocity were normalized by the shearvelocity (U_*) at the top of the suspension layer. Streamwise and vertical velocity components inside the suspension layer, were obtained from the 2-D continuity and the Navier–Stokes equations. The persistence of turbulence with depth—as it penetrates from the overlying water into the suspension layer—was found to depend on its amplitude, its period, and on the apparent viscosity of the suspension. The turbulence was found to propagate efficiently into the suspension layer when its frequency is low, and the apparent viscosity of the suspension is high. Effects on vertical mass transfer were parameterized by penetration depth and effective diffusion coefficient, and related to apparent viscosity of the suspension, Schmidt number and shear velocity on top of the suspension layer. The enhancement of turbulence penetration by viscosity is similar to the flow near an oscillating flat plate (Stokes’ second problem), but is opposite to turbulence penetration into a stationary porous and permeable sediment bed. The information is applicable to water quality modeling mear the sediment/water interface of lakes, river impoundments and estuaries.     

Modeling the longitudinal profiles of streamwise velocity in an open channel with a model patch of vegetation

This paper proposes a model for predicting the longitudinal profiles of streamwise velocities in an open channel with a model patch of vegetation. The governing equation was derived from the momentum equation and flow continuity equation. The model can estimate the longitudinal profiles of velocities both inside and outside a vegetation patch. Laboratory experiments indicate that the longitudinal profiles of velocities inside a patch and in the adjacent bare channel have the same adjustment distance in the longitudinal direction, but the profiles have different trends because the vegetation drag drives the flow from the patch to the adjacent bare channel. The model considers different dimensionless parameters in two flow adjustment regions upstream of and inside the patch. Sixteen sets of experimental data from different sources are used to verify the model. The model is capable of modeling the longitudinal profiles of velocities inside and outside patches of cylinders or cylinder-like plants. Compared to a previous model, the current model improves the modeling accuracy of longitudinal profiles of velocities.

     

底质中氨氮对沉水植物生长的影响

沉水植物的生长不但与水体营养盐浓度具有十分密切的关系,也受底泥中营养物质的影响,尤其是底泥中氨氮的含量对于植物生长具有重要的作用,因此研究底质中氨氮对高等沉水植物生长的影响,能够为沉水植被的重建及选种提供科学参考。作者在人工配制的7种含不同浓度铵态氮的底质条件下,通过测定苦草VallisnerianatansL.和伊乐藻Elodeanuttallii两种沉水植物的植株鲜物质量的变化、干物质量、根系的活力、过氧化物酶(POD)和过氧化氢酶(CAT)活性,以期明确这两种沉水植物与水体底质浓度之间的关系。试验结果表明,氨氮起始质量分数<50mg·kg-1和<500mg·kg-1时可以分别促进苦草和伊乐藻鲜物质量、干物质量和根系活力的增加,大于此范围则会抑制苦草VallisnerianatansL和伊乐藻Elodeanuttallii的生长。     

Double-average mean flow and local turbulence intensity profiles from PIV measurements for an open channel flow with rigid vegetation

This contribution presents particle image velocimetry measurements for an open channel stationary uniform and fully developed flow of water over a horizontal flat bed of uniform glass beads in presence of a staggered array of vertical cylindrical stems. The main objective was to explore and quantify the influence of the stems-to-flow relative submergence, h _v /h, over the mean flow and local turbulence intensities. A comparison with measurements for the non-vegetated flow over the same granular bed is presented. Results indicate a remarkable influence of h _v /h over the whole flow field. The time-average mean flow presents a strong spatial variation in the layer of the flow occupied by the stems. The local velocity fluctuations are strongly affected by the presence of the stems, with regions in between the stems where they reach peaks that are several times larger than those encountered in the flow in absence of vegetation. The turbulence intensity profiles are noticeably different when compared to those measured in the non-vegetated flow conditions. From previous works it was possible to derive an equation for the mean velocity, U _v , of the flow through the vegetated layer of height h _v . The prediction of this equation is in good agreement with the uniform value for the double-average longitudinal velocity profile in this layer. A final brief discussion about the possible impact of these vegetated-flow features on the sediment transport is presented.     

Quantifying a significance of sediment particle size to hyporheic sedimentary oxygen demand with a permeable stream bed

A mechanistic model of sedimentary oxygen demand (SOD) for hyporheic flow is presented. The permeable sediment bed, e.g. sand or fine gravel, is considered with hydraulic conductivity in the range $0.1 < K < 20$  cm/s. Hyporheic pore water flow is induced by pressure fluctuations at the sediment/water interface due to near-bed turbulent coherent motions. A 2-D advection–diffusion equation is linked to the pore water flow model to simulate the effect of advection–dispersion driven by interstitial flow on oxygen transfer through the permeable sediment. Microbial oxygen uptake in the sediment is expressed as a function of the microbial growth rate, and is related to the sediment properties, i.e. the grain diameter $(d_{s})$ and porosity $(\phi )$ . The model describes the significance of sediment particle size to oxygen transfer through the sediment and microbial oxygen uptake: With increasing grain diameter $(d_{s})$ , the hydraulic conductivity $(K)$ increases so does the oxygen transfer rate, while particle surface area per volume (the available surface area for colonization by biofilms) decreases reducing the microbial oxygen uptake rate. Simulation results show that SOD increases as the hydraulic conductivity $(K)$ increases before a threshold has been reached. After that, SOD diminishes with the increment of the hydraulic conductivity $(K)$ .     

外加碳源及沉水植物对沉积物各形态磷的影响

以原沉积物(处理1)和添加0.4%葡萄糖(碳源,处理2)的沉积物作为底质培养狐尾藻,采用室内模拟实验,研究了外加碳源及沉水植物对沉积物有机和无机磷形态的影响。结果表明:随着培养时间的延续,狐尾藻的生长促进了沉积物中磷的释放,其衰退增加了沉积物中磷的沉积;沉水植物对磷的不同赋存形态的影响并不相同,其中对无机磷形态中的Fe/Al-P和有机形态磷中的活性有机磷有显著的影响(p<0.05),而对其它形态磷的影响没有表现出统计差异;无论有无种植沉水植物,外加碳源可促进了沉积物Fe/Al-P和有机磷释放,增加了Ca-P固定;外加碳源显著提高了有机磷中活性有机磷的质量分数(p<0.05),降低了稳定性有机磷的质量分数,而种植沉水植物可显著降低活性有机磷质量分数。说明碳源和沉水植物对沉积物中磷形态分布有重要影响。