Variations In Hydrodynamic Conditions Research Articles

Alluvial-lacustrine record of Mid- to Late-Holocene moisture variations trend verified by multiple proxies in the middle and lower reaches of the Hutubi River, northwest China

Alluvial-lacustrine deposits are important carriers for studying paleo-hydrological information and the evolution of paleo-environments. In this study, a total of 6 optically stimulated luminescence (OSL) samples from 2 profiles in the lower and middle reaches of the Hutubi River in the North Tianshan Mountains were dated, and a comprehensive analysis was conducted by integrating sedimentary structures, grain sizes, and magnetic susceptibility characteristics to explore the sedimentary environment patterns and regional evolution processes during the Mid to late Holocene. The results revealed the following findings: (1) The OSL signal of quartz samples indicates the predominance of fast components, allowing reliable dating of regional fluvial sediments using the appropriate condition-tested coarse-grained quartz single-aliquot regenerative dose (SAR) protocol within the 90–125 μm range. (2) Analysis of sedimentary structures, grain sizes, and magnetic susceptibility characteristics reveals lithofacies of heterotopic synchronous or homotopic asynchronous in exposed profiles, with predominantly poorly sorted fine and very fine sand. Magnetic susceptibility is primarily controlled by the content of magnetic minerals in the coarse particles, while also being influenced by the combined effects of provenance input and depositional environment, indicating a complex regional sedimentary environment influenced by variable hydrodynamic conditions driven by fluvial processes. (3) Based on OSL dating results and the analysis of various environmental proxy indicators, it is determined that the climatic environment in the Middle and Late-Holocene in the downstream areas of the northern piedmonts of the Tianshan Mountains demonstrates a fluctuating pattern of wet and dry alternations. The sedimentary environment shifted from a stable state in the Mid-Holocene to an unstable state in the Late-Holocene. The intermittent appearance of wet records may be associated with increased river activity caused presumably by the release of high-altitude ice due to rising temperatures and warming conditions.

Effects of typhoon events on coastal hydrology, nutrients, and algal bloom dynamics: Insights from continuous observation and machine learning in semi-enclosed Zhanjiang Bay, China

Typhoons can induce variations in hydrodynamic conditions and biogeochemical processes, potentially escalating the risk of algal bloom occurrences impacting coastal ecosystems. However, the impacts of typhoons on instantaneous changes and the mechanisms behind typhoon-induced algal blooms remain poorly understood. This study utilized high-frequency in situ observation and machine learning model to track the dynamic variations in meteorological, hydrological, physicochemical, and Chlorophyll-a (Chl-a) levels through the complete Typhoon Talim landing in Zhanjiang Bay (ZJB) in July 2023. The results showed that a delayed onset of algal bloom occurring 10 days after typhoon's arrival. Subsequently, as temperatures reached a suitable range, with an ample supply of nutrients and water stability, Chl-a peaked at 121.49 μg L−1 in algal bloom period. Additionally, water temperature and air temperature decreased by 1.61 °C and 2.8 °C during the typhoon, respectively. In addition, wind speed and flow speed increased by 1.34 and 0.015 m s−1 h−1 to peak values, respectively. Moreover, the slow decline of 8.2 % in salinity suggested a substantial freshwater input, leading to an increase in nutrients. For instance, the mean DIN and DIP were 2.2 and 8.5 times higher than those of the pre-typhoon period, resulting in a decrease in DIN/DIP (closer to16) and the alleviation of P limitation. Furthermore, pH and dissolved oxygen (DO) were both low during the typhoon period and then peaked at 8.93 and 19.05 mg L−1 during the algal bloom period, respectively, but subsequently decreased, remaining lower than those of the pre-typhoon period. A preliminary learning machine model was established to predict Chl-a and exhibited good accuracy, with R2 of 0.73. This study revealed the mechanisms of eutrophication status formation and algal blooms occurrence in the coastal waters, providing insights into the effects of typhoon events on tropical coastal biogeochemistry and ecology.

Influence of seasonal variability on the trophic structure of pelagic communities

Seasonal variations in hydrodynamic conditions play a critical role in prey availability at the base of the food web and thus have implications for trophic interactions at higher trophic levels. Here we use the combination of stomach content analysis (SCA) and stable isotope analysis (SIA) to investigate predator-prey interactions and trophic structure of the pelagic fish community of the Bay of Biscay (BoB) in spring and late summer of 2020 and 2021. We found that trophic guilds were more influenced by intra-specific affinities rather than seasonal variability. Main prey for the pelagic fish comprised a set of species belonging to different trophic positions (TPs), from low TP class Thaliacea (salps) to mid TP Engraulis encrasicolus. Vertically migrating meso- and macrozooplankton (mainly copepods and krill) also played an important role as food source for the pelagic fish community showing high trophic overlap among most of species. No differences were detected between the trophic structure in spring and late summer with fish, squids and chaetognaths at the top TPs, whereas krill, meso-zooplankton and salps had the lowest values. However, the TPs estimated using both SCA and SIA approaches were consistently lower in spring, likely due to higher densities and, thus, low TP prey availability in spring. When analyzing ontogenic changes in TPs with predator’s size, significant and positive relationships were found when TP was estimated using SIA but not when using SCA. Overall, our results reveal new insights into the seasonal trophic dynamics of the pelagic fish community of the BoB.

The coastal front modulates the timing and magnitude of spring phytoplankton bloom in the Yellow Sea

Major seasonal quasi-stationary fronts on shelves play an important role in regulating the spatiotemporal variations in the phytoplankton community. However, knowledge of their effects on the timing and magnitude of spring phytoplankton bloom (SPB) remains limited. Here, based on decadal satellite data (2003–2020), we examine the climatological relationship between the Shandong coastal front (SCF) and SPB in the Yellow Sea. The results show that the onset of SPB occurs either in March (∼56% of the seasons examined) or in April (44%). The peak of SPB most often occurs in April (∼56% of the seasons examined) or is advanced to March (16%) or delayed to May (28%), and that the peak ranges from 1.04 to 2.54 mg Chl-a m−3. The onset of SPB matches with lower turbulence, particularly when the rate of generation of turbulent kinetic energy (TKERT) reaches zero. A higher magnitude of bloom is associated with a greater change in front and a lower TKERT. The in situ observations along the SCF transects in the Yellow Sea indicate that weakened SCF in spring associated with a shallower mixing layer enhances the transport of nutrients from the coastal to the shelf waters. Weakened frontal structure and atmospheric forcing in spring can further increase the water stability and decrease turbulence in the upper waters. The variation in hydrodynamic conditions allows shelf phytoplankton to stay longer in the upper waters with sufficient light and nutrients and consequently generate a Chl-a peak. The results suggest that the seasonal changes in front intensity and structure and turbulence are important prerequisites for initiating SPB on the shelf, and that further determines the magnitude of SPB.

Friction factor evaluation in tidal rivers and estuaries

The Saint-Venant equations are usually used for simulation of unsteady flow in various hydraulic environments including tidal estuaries and adjacent river sections called tidal rivers. These equations form the basis of most contemporary one-dimensional hydrodynamic models. Unlike lowland rivers, tidal rivers are characterized by the high variability of hydraulic resistance during a tidal cycle. The rapid variation in hydrodynamic conditions reflects changes in the ratio of forces responsible for the water movement (gravity, inertia, friction). The expression and the parametrization of the flow resistance for the unsteady currents usually are the same as for the uniform flows. As a result, the friction slope can be calculated as the hydraulic gradient from Chezy, Chezy-Manning or Darcy–Weisbach equations. The proposed method was developed to measure, calculate and analyze the variation of the friction factor in tidal rivers and estuaries characterized by reverse flow.•The method allows to identify the range of every term in the Saint-Venant momentum equation and to evaluate the contribution of different forces causing water movement during a tidal cycle.•The experimental evaluation of the friction factor and the range of its variation is a key point for accurate hydrodynamic modeling of tidal rivers.

Pollution characteristics and source analysis of heavy metals in surface sediments of Luoyuan Bay, Fujian

The concentrations of eight heavy metals, V, Cr, Co, Ni, Cu, Zn, Cd, and Pb, were analyzed to explore the environmental pollution characteristics, ecological risk levels, and sources of heavy metals in the surface sediments of Luoyuan Bay in Fujian Province. The results indicated that the mean concentrations of V, Cr, Co, Ni, Cu, Zn, Cd, and Pb were 110.44, 97.76, 17.35, 41.99, 36.78, 137.26, 0.09, and 35.81 mg⋅kg−1, respectively. The mean concentrations of Cr, Cu, and Zn exceeded the first category of the marine sediment quality standards, indicating a moderate regional comprehensive potential ecological hazard level. Due to variable hydrodynamic conditions, high concentrations of heavy metals were observed in the north and low concentrations were observed in the south, with high values generally recorded near steel plants and shipyard docks. Correlation and principal component analyses revealed that the V, Al, Co, Ni, and Pb originated from the weathering and erosion of rocks, and industrial wastewater discharge. Contrastingly, Cu, Zn, Cr, and Cd were mainly associated with the transportation and repair of ships, and marine aquaculture activities.

Tidal and subtidal transport in short, tidally-driven estuaries with low rates of freshwater input

Tidal and subtidal current velocity structure and forcing mechanisms are important to forecasting estuary conditions linked to water quality problems in coastal areas. This study combined current velocity, salinity, temperature, and water level data with analytical and numerical models to understand dynamics in a small estuary in Downeast Maine, USA. Both the tidal and subtidal flow, as well as the drivers of the subtidal flow, were found to have substantial spatial variation along the estuary. An analysis of the along-channel momentum balance indicates that advection and friction alternate in balancing Stokes drift and the barotropic pressure gradient along the estuary, switching at locations of estuary width constriction or expansion. These dynamics influenced along-channel subtidal flows producing highest velocity at the shoals and lowest velocity in the channel in the mid-reaches of the estuary when advection was more influential than friction. This pattern reversed when friction dominated over advection, producing the highest velocity in the center of the channel, which is corroborated by field measurements. The idealized model produced a spatial structure of along-channel residual flows that do not align with field measurements and numerical simulation results due to the simplicity of the estuary geometry used in the model. The outcome of the analyses suggests that slight variations in along-channel geometry and bathymetry can alter tidal and subtidal flows and transport, even in small (<10 km in length) tidally dominated estuaries with relatively little spatial variation in hydrodynamic conditions within a tidal cycle.

Interannual variations in the hydrodynamics of a tropical river-wetland system: a case study for the Abras de Mantequilla wetland, Ecuador

Wetlands are sensitive to hydrological conditions and human influence. The Abras de Mantequilla wetland (AdM) located at the centre of the Guayas River Basin (Western Ecuador), was declared a Ramsar site (2000) due to its important role in the conservation of bird fauna diversity. The wetland also supports significant populations of fish, including endemic species. The area exhibits two seasons: (i) a wet season (January–May); and (ii) a dry season (June–December). For the area of interest data is scarce, and a hydrodynamic model did not exist. In order to evaluate the interannual hydrodynamics and inundation patterns variability of this wetland system, a 2D hydrodynamic model was developed using the Delft3D software suite. Local knowledge was combined with field observations and expert elicitation to construct an appropriate model for the area. Results show that the Abras de Mantequilla is a river-wetland system that experiences extreme variations in hydrodynamic conditions from low water depths and almost stagnant conditions during the dry season, to a very dynamic behaviour during the rainy season. The inundation area ranges between 5 to 27 km2, water depths vary from 0.4 to 9 m and flow velocities from 0.0 up to 0.9 m/s. The “Nuevo River” is the main inflow to the wetland, with an overall contribution of about 85–90%, with peak discharges during February and March. A numerical tracer analysis indicated that the wetland can be divided into three main areas (upper, middle and low), with higher residence times found in the middle area. Overall, based on numerical hydrodynamic simulations it was possible to obtain adequate indications of the relative importance of the different river inflow conditions, as well as spatial and temporal inundation patterns, velocities and residence times under hydrological conditions varying from dry to extremely wet years. Furthermore, the hydrodynamic model provides a basis for other studies such as water quality assessment and primary production dynamics, as well as for determining the habitat suitability for fish.

Lithofacies characteristics and sedimentary model of a gravelly braided river-dominated fan: a case study of modern Poplar River alluvial fan (northwest Junggar Basin, China)

Conglomerate reservoirs in alluvial fans commonly contain economic crude oil, but it is challenging to depict the distribution of conglomerate reservoirs or to summarise the sedimentary model in alluvial fans deposits owing to their complex lithofacies changes and variable hydrodynamic conditions. This paper focuses on the Poplar River alluvial fan in the northwest Junggar Basin, China, as a special case to analyse its sedimentary processes and to build a sedimentary model through detailed outcrop mapping and grainsize analysis. The Poplar River alluvial fan is a braided river-dominated fan with the characteristics of a large fan size (approximate 327.6 km2), gentle slope (∼<0.04–0.07°), coarse sediments (mainly cobbles or finer pebbles) and rare fine-mud sediments deposited from floods. In this alluvial fan, 15 lithofacies are identified that mainly formed in five hydrodynamic environments: (i) debris-flow deposits; (ii) high-flow traction current deposits; (iii) low-flow traction current deposits; (iv) still-water deposits; and (v) eolian deposits. Moreover, the sedimentary processes on the Poplar River alluvial fan can be divided into flood and interflood periods. During flood periods, sediment deposition plays a major role on the alluvial fan. From the proximal to distal parts, six lithofacies associations can be identified: feeder-channel and sheet-flow deposits in the proximal part, sheet flood and braided-stream deposits in the intermediate part, and channel deposits and wetland deposits in the distal fan areas. During the interflood periods, sediment reworking is the most important function, and three lithofacies associations are identified: main-channel deposits in the proximal fan, braided-trench deposits in the intermediate fan and wetland deposits in the distal area. The braided-stream deposits, braided-trench deposits and sheet-flood deposits have the best potential for high-quality aquifers or oil reservoirs based on their moderately well-sorted grains and high primary permeable intervals. KEY POINTS Fifteen lithofacies and eight lithofacies associations are identified in the Poplar River braided river-dominated alluvial fan. The aggradation process of the studied alluvial fan can be divided into flood and interflood periods. The flood-period braided-fluvial deposits, interflood main-channel and braided-trench channel deposits are the facies with best reservoir potential.

Preparation of delayed-release multiparticulate formulations of diclofenac sodium and evaluation of their dissolution characteristics using biorelevant dissolution methods

Diclofenac sodium was used as a model drug for preparation of delayed-release (DR) multiparticulates, which were further processed into solid oral dosage forms such as capsules and tablets. Multiple unit pellets systems (MUPS) were prepared from different types of starter pellets (inert cores) including microcrystalline cellulose pellets, sugar spheres, isomalt pellets and novel calcium phosphate-based pellets. The study results showed that the material of the inert cores affected both mechanical properties of the drug-loaded pellets and the dissolution characteristic of the model drug. Biorelevant dissolution method carried out with the help of a pHysio-grad device allowed thorough examination of the developed formulations in the environment mimicking pH conditions along gastrointestinal tract. This method revealed significant differences between the formulations and their sensitivity to variable hydrodynamic conditions.

Reconstruction of monsoon evolution in southernmost Sumatra over the past 35 kyr and its response to northern hemisphere climate changes

Reconstruction of monsoon evolution in the tropical Indian Ocean and evaluation of its influence on large-scale ocean circulation and sea-air interaction processes can help us understand climate driving mechanisms. Herein, we used a gravity core SO184-10043 to present the marine sedimentary record from the southernmost Sumatra, spanning the past 35 kyr. The age model is well constrained by 12 accelerator mass spectrometry (AMS) radiocarbon dates determined in mixed planktic foraminifera. We also measured sediment grain size and major and trace elements to reconstruct the variations in hydrodynamic conditions and chemical weathering intensity. These data were then used to reveal the evolution of the northwest (Indonesian) monsoon system over the past 35 kyr. The results showed that both monsoon current and chemical weathering coincided with summer insolation in the Northern Hemisphere and responded to glacial-interglacial climate changes in the northern Atlantic. Higher grain-size values of sensitive components, increased chemical index of alteration (CIA), and Ba/Sr ratios, together with lower CaO/MgO ratios, reflect a stronger monsoon current and chemical weathering during the late marine isotope stage 3 and Holocene. Higher intensity of chemical weathering might be attributed to a stronger northwest monsoon carrying warm and moist air from the Asian continent to Indonesian waters. Lower values of sensitive components, CIA, and Ba/Sr, along with higher CaO/MgO values, reveal a weaker monsoon current and lower intensity of chemical weathering during the last glaciation-deglaciation phase, which was controlled by a weaker monsoon system. At a millennial timescale, cold events occurred during the following intervals: 30–26 ka BP, Heinrich Stadial 1 (HS1; 18–15 ka BP), Last Glacial Maximum (LGM; 23–18 ka BP), Younger Dryas (YD; 11–10 ka BP), and 8.2 ka BP. The same cold events were identified from core SO184-10043, indicating a perfect match with climatic cooling in the Northern Hemisphere. We also observed periodicities of ~ 7.9 ka, 3.0–3.1 ka, 2.2–2.3 ka, and ~ 1.0 ka in our core records. These periodicities are coincident with solar-induced climate changes and support the hypothesis that monsoon evolution in the tropical Indian Ocean during the late Pleistocene coincided with the climate change in the Northern Hemisphere.

The Anatomy of a Mississippian (Viséan) carbonate platform interior, UK: Depositional cycles, glacioeustasy and facies mosaics

Late Viséan platform interior carbonate strata of northern England and Wales have traditionally been interpreted to comprise shallowing-up cyclothems, the upper boundaries of which are marked by unconformities that record subaerial exposure events. These strata were deposited during what is understood to be a greenhouse-to-icehouse transition, and the cyclothems are assumed to reflect glacioeustatic sea-level oscillations associated with this climatic change. Importantly, these interpretations reflect a qualitative model which assumes that facies respond in a simple way to water depth variations and so migrate and stack in a predictable pattern as eustatic sea-level fluctuates. Exposure surfaces and most facies are, however, laterally discontinuous across the platform tops. Quantitative statistical analysis and simple 1D stratigraphic forward modelling suggest that the observed vertical facies stacking patterns on the Derbyshire and North Wales Platforms are not cyclical. The most ordered arrangement of facies that could be interpreted as “ideal cycles” in each section are variable within and across the platforms. This observational and analytical evidence suggests that instead of a simple, dominant external glacioeustatic forcing on sedimentation, a model commonly proposed for the Late Viséan carbonate strata of northern England and Wales, strata were deposited as a facies mosaic, under variable hydrodynamic conditions influenced by autogenic processes as well as local environmental fluctuations in salinity and temperature, and perhaps in conjunction with glacioeustasy and syn-rift tectonic processes which occurred from metre-scale to kilometre-scale, as suggested by the ideal cycles.

The effect of diatoms on the grain size of lake sediments: a case study of the sediments of Lake Kanas

The grain size of lake sediments has been widely used to reconstruct environmental processes. However, the various components of lake sediments are influenced by different factors, and therefore the environmental significance of grain-size variations may be complex. For example, in situ authigenic biological siliceous components cannot be used to indicate variations in hydrodynamic conditions. Here we present a case study of the effects of diatoms on the grain size of the sediments of Lake Kanas in NW China. The analyses consist of measurements of bulk sediment grain size and biogenic silica, together with microscope observations. Based on both light and scanning electronic microscope observations, two large diatom species, Campylodiscus hibernicus Ehrenberg and Ellerbeckia arenaria (Moore) Crawford, were found to be abundant in sediment samples with a large bulk-sediment grain size, and they contributed substantially to the biogenic silica content and the measured bulk sediment grain size of the core. A quantitative assessment of the effects on grain size was made, and the study has a broad significance for evaluating the effects of diatoms on sediment grain-size distributions in other lakes. Because diatoms may deposit in situ or could be transported differently from other clastic material, the explanation of hydrodynamics could be misled if the influence of diatom has not been excluded. Therefore, we suggest BSi content should be tested to assess the diatom effect prior to grain-size analysis of lake sediments, together with microscope check of diatoms if BSi content is higher than 10%.

Distance from rocky shores affects infaunal recolonization in a subtropical tidal flat

Hard bottoms can negatively affect the surrounding infauna by hosting predatory fauna and modifying local hydrodynamics and sedimentation rates. Considering that these effects depend on the distance to the hard bottoms, we expected that the infaunal structure and recolonization would change accordingly. We assessed whether the distance from a rocky shore alters infaunal recolonization in a subtropical tidal flat. Sediment was defaunated on sites near and far from a rocky shore and the subsequent faunal recolonization was followed for 40 days. The sites near and far from the rocky shore displayed different assemblage structures, which we attributed to local variation in hydrodynamic conditions. Both the total infaunal abundance and the abundance of the dominant species recovered faster at the near site. We indicate that changes in infaunal recolonization at varying distances from natural rocky shores are primarily driven by the availability of adults. Infaunal recovery took less than 2 weeks, but recolonization rates increased near the rocky shore, as a function of its naturally variable assemblage structure and relatively lower species richness and abundances. We suggest that models of infaunal distribution and recolonization should incorporate landscape features such as the presence and distance from rocky bottoms.

Long term (1997-2014) spatial and temporal variations in nitrogen in Dongting Lake, China.

In order to protect the water quality of Dongting Lake, it is significant to find out its nitrogen pollution characteristics. Using long-term monthly to seasonally data (1997–2014), we investigated the spatial and temporal variations in nitrogen in Dongting Lake, the second largest freshwater lake in China. The average concentrations of total nitrogen (TN) in the eastern, southern, and western parts of the lake were 1.77, 1.56, and 1.35 mg/L, respectively, in 2014. TN pollution was generally worse in the southern area than in the western area. Concentrations showed temporal variation, and were significantly higher during the dry season than during the wet season. Based on the concentration and growth rate of TN, three different stages were identified in the long term lake data, from 1997 to 2002, from 2003 to 2008, and from 2009 to 2014, during which the concentrations and the growth rate ranged from 1.09–1.51 mg/L and 22.09%-40.03%, 1.05–1.57 mg/L and -9.05%-7.74%, and 1.68–2.02 mg/L and 57.99%-60.41%, respectively. The main controls on the lake water TN concentrations were the quality and quantity of the lake inflows, spatial and temporal variations in hydrodynamic conditions within the lake (flow velocity, flow direction), and point and nonpoint inputs from human activities. Diffuse nutrient losses from agricultural land are a significant contributor. As a priority, the local government should aim to control the pollutant inputs from upstream and non-point nutrient losses from land.

Klinkenberg effect in hydrodynamics of gas flow through anisotropic porous materials

This study discusses results of experiments on hydrodynamic assessment of gas flow through backbone (skeletal) porous materials with an anisotropic structure. The research was conducted upon materials of diversified petrographic characteristics, both natural origin (rocky, pumice) and process materials (char and coke). The study was conducted for a variety of hydrodynamic conditions, using air, as well as for nitrogen and carbon dioxide. The basis for assessing hydrodynamics of gas flow through porous material was a gas stream that results from the pressure forcing such flow. The results of measurements indicate a clear impact of the type of material on the gas permeability, and additionally – as a result of their anisotropic internal structure – to a significant effect of the flow direction on the value of gas stream.

Modelling trace metal transfer in large rivers under dynamic hydrology: A coupled hydrodynamic and chemical equilibrium model

Trace metals (TMs) in rivers experience a complex chemistry that affects their partitioning between the dissolved and particulate fractions. Predictive modelling of this partitioning is challenging, particularly under variable hydrodynamic conditions in rivers. This paper presents a coupled hydrodynamic–geochemical model for trace metal dynamics in rivers, to highlight key parameters and set a basis for developing simplified models. A coupled model based on the hydrodynamic model MOHID and the chemical speciation model WHAM was applied to the middle part of the Garonne River (SW-France). The predictions of the coupled model are compared to those of a fixed Kd partitioning model.The model accurately simulates trace metal behaviour under varying hydrological conditions. The importance of the riverbed geomorphology and the diurnal variations in pH on TM concentrations in the water column are underlined. The sensitivity analysis ranked all physico-chemical parameters according to their influence on TM behaviour.

Determination of Wave Reflection Formulae for Vertical and Sloped Seawalls Via Experimental Modelling

This paper presents and discusses the results of experimental investigations using both vertical and sloping seawalls configurations to determine the optimal wave reflection characteristics of seawalls for protecting beaches against erosion under a variety of hydrodynamic conditions. The different experiments include both of rectangular or triangular serrated blocks and slotted seawalls with or without triangular serrations as energy dissipaters. The linear wave theory is utilized. Moreover, the Dalrymple method is considered to predict the ordinates of the resultant standing wave due to the partial wave reflection. Using the dimensional analysis, lab measurements, and SPSS, predictive formulae are proposed to predict the reflection coefficient for the five tested models by using SPSS software. The findings of the present investigation could be applied to optimize the design criteria of shore protection structures.

Characteristics of nitrogen release from Lake Dongting sediments under variable water level and velocity in the two-way annular flume

Sediment-water interface is a crucial place for internal nitrogen release in the shallow lake ecosystem and the hydrodynamic factors have significant influence on sediment nitrogen release. After the construction of the Three Gorges Dam,hydrodynamic conditions for shallow lakes in the middle and lower reaches of the Yangtze River have changed apparently. Water and sediments samples collected from Lake Dongting region were used to simulate the release characteristics of nitrogen from sediments under experience conditions of variable water level and velocity. The results showed that as the water level / velocity increased,concentrations of the suspended solids( SS) and total nitrogen( TN) in the overlying water increased and the nitrogen release intensity from sediments to overlying water enhanced. This internal nitrogen release caused by variable hydrodynamic conditions should not be ignored. Under the simulation conditions there was an optimal disturbance water level of 20 cm,where concentrations of the SS and TN were lowest in the water level nearby. The influence of variable water level and velocity on ammonium nitrogen and nitrate nitrogen in the overlying water and sediment-water interface were unapparent,while there was a form transformed between ammonium nitrogen and nitrate nitrogen in the pore water.

Tethered cells in fluid flows—beyond the Stokes’ drag force approach

Simulations of tethered cells in viscous sub-layers are frequently performed using the Stokes’ drag force, but without taking into account contributions from surface corrections, lift forces, buoyancy, the Basset force, the cells’ finite inertia, or added mass. In this work, we investigate to what extent such contributions, under a variety of hydrodynamic conditions, influence the force at the anchor point of a tethered cell and the survival probability of a bacterium that is attached to a host by either a slip or a catch bond via a tether with a few different biomechanical properties. We show that a consequence of not including some of these contributions is that the force to which a bond is exposed can be significantly underestimated; in general by ∼32–46%, where the influence of the surface corrections dominate (the parallel and normal correction coefficients contribute ∼5–8 or ∼23–26%, respectively). The Basset force is a major contributor, up to 20%, for larger cells and shear rates. The lift force and inertia contribute when cells with radii >3 μm have shear rates >2000 s−1. Buoyancy contributes significantly for cells with radii >3 μm for shear rates <10 s−1. Since the lifetime of a bond depends strongly on the force, both the level of approximation and the biomechanical model of the tether significantly affect the survival probability of tethered bacteria. For a cell attached by a FimH–mannose bond and an extendable tether with a shear rate of 3000 s−1, neglecting the surface correction coefficients or the Basset force can imply that the survival probability is overestimated by more than an order of magnitude. This work thus shows that in order to quantitatively assess bacterial attachment forces and survival probabilities, both the fluid forces and the tether properties need to be modeled accurately.