Residual Sediment Research Articles

Local sea level change scenarios for the end of the 21st century and potential physical impacts in the lower Ria de Aveiro (Portugal)

Sea level change is an important consequence of climate change due to its impact on society and ecosystems. Analyses of tide-gauge data have indicated that the global sea level has risen during the 20th century and several studies predict that the mean sea level will continue to rise during the 21st century, intensifying coastal hazards worldwide. In Portugal, the Ria de Aveiro is expected to be one of the regions most affected by sea level change. The main aim of this study is to evaluate the potential impacts of the mean sea level change on the hydrodynamics and morphodynamics of the Ria de Aveiro. With this purpose, local mean sea level change was projected for the period 2091–2100 relative to 1980–1999, for different Special Report on Emission Scenarios (SRES) scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). These projections revealed an increase in the mean sea level between 0.28 m under scenario B1 and 0.42 m under scenario A2. The results obtained for sea level rise scenario A2 projection were used to force the morphodynamic model MORSYS2D, previously implemented for the Ria de Aveiro. The modelling results were compared with model forecasts for the present sea level. The residual sediment transport and its balance at the lagoon inlet were computed and analysed for both situations. While the residual sediment transport is generally seaward, sediments tend to deposit inside the inlet due to the weak sediment transport at its mouth. The direction of the residual flux will not change with the sea level rise, but sediment fluxes will intensify, and accretion inside the inlet will increase. The rise in mean sea level will also affect the lagoon hydrodynamics. The tidal prism at the lagoon mouth will increase by about 28% in spring tide. In the lower lagoon only a slight increase of the tidal asymmetry is predicted.

Transverse structure of tidal and residual flow and sediment concentration in estuaries

An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped estuarine cross-sections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model. The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter space considered (average tidal velocity amplitude, 0.1–1 m s − 1 and maximum water depth, 10–60 m). Its skills are lower for along-channel residual flows if nonlinearities are moderate to high (strong tides in deep estuaries) and for transverse flows and residual sediment concentrations if the Ekman number is small (weak tides in deep estuaries). An important new aspect of the analytical model is the incorporation of tidal variations in the across-channel density gradient, causing a double circulation pattern in the transverse flow during slack tides. The gradient also leads to a new tidally rectified residual flow component via net advection of along-channel tidal momentum by the density-induced transverse tidal flow. The component features landward currents in the channel and seaward currents over the slopes and is particularly effective in deeper water. It acts jointly with components induced by horizontal density differences, Coriolis-induced tidal rectification and Stokes discharge, resulting in different along-channel residual flow regimes. The residual across-channel density gradient is crucial for the residual transverse circulation and for the residual sediment concentration. The clockwise density-induced circulation traps sediment in the fresher water over the left slope (looking up-estuary in the northern hemisphere). Model results are largely consistent with available field data of well-mixed estuaries.

A PROCESS-BASED APPROACH TO SEDIMENT TRANSPORT

A process-based model for the Yangtze Estuary is constructed to study the sediment transport in the estuary. The proposed model covers the entire tidal region of the estuary, the Hangzhou Bay and a large part of the adjacent sea. The dominant processes, fluvial and tidal, are included in the model. The calibration of the model against extensive flow, water level, salinity and suspended sediment data shows a good representation of observed phenomena. With the present calibrated and validated model, the residual flow field and the residual sediment transport field are obtained. The residual sediment transport pattern gives insight into the morphological behaviour of the mouth bars.

Mechanisms of sediment flux and turbidity maintenance in the Delaware Estuary

[1] An observational study was conducted to identify mechanisms of suspended sediment flux and turbidity maintenance in the Delaware River estuary. From March through October 2005, instrumented moorings were deployed to obtain continuous measurements of currents and suspended sediment concentration at sites along the estuarine channel and on flanking subtidal flats. Data time series were analyzed to determine the relative influence of nontidal advection and tidal pumping on residual fluxes of sediment. Results indicate that the estuarine channel is a strongly advective transport environment with residual sediment fluxes driven mostly by gravitational circulation. Tidal pumping is a contributing process of residual sediment flux in the channel near the estuarine null point and turbidity maximum, though the magnitude and direction of pumping vary with river flow and resident sediment inventory in the upper estuary. Sediment pumping in the channel is driven by tidal asymmetries in velocity and particle settling and perhaps by tidal variations in internal mixing in the stratified lower estuary. In contrast to the estuarine channel, residual sediment fluxes over the subtidal flats are weak and dominated by tidal pumping. Landward advective fluxes of sediment in bottom waters of the lower estuarine channel are strongest during neap tides; during large spring tides sediment is mixed high in the water column and the advective flux reverses to seaward under the residual surface outflow. Despite these transient seaward fluxes, the estuary has an enormous capacity to buffer extreme freshwater discharges and suppress export of suspended sediment to Delaware Bay.

Development of a mud transport model for the Scheldt estuary

A mud transport model for the Scheldt estuary is being developed. Its purpose is to support managers of the Scheldt estuary with the solution of a number of managerial issues. The model domain ranges from the tidal boundary at Gent down to the Belgian coastal cities of Nieuwpoort and Zeebrugge. The hydrodynamic simulations on which the mud transport model is based show realistic values for water levels, salinities and residual currents in the major part of the model domain. However, the propagation of the tidal wave is modelled less accurately upstream of Antwerp. Regarding the mud transport simulations, which are based on simple process formulations, it is shown that a satisfactory agreement is obtained between computed and observed concentration levels and their spatial and temporal variations. SPM levels in the Scheldt appear to be rather sensitive to the volume of harbour siltation and dumping, notably near Antwerp. The sediment budget computed by the model includes an unrealistically high residual sediment flux towards the North Sea. By enhancing the estuarine mud circulation, the modelled residual flux becomes smaller and agrees better with observation-based estimates. A longer hydrodynamic simulation period (presently up to 3 months) would help to analyse the sediment budget and seasonal dynamics in more detail.

Bioleaching of heavy metals contaminated sediment by pure and mixed cultures of A cidithiobacillus spp.

The aim of this study was to investigate the effect of bacterial strains on the bioleaching of Cr, Cu, Pb, and Zn from contaminated sediment samples taken from a stationary point in Izmir Inner Bay. Single and mixed cultures of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans were used separately to achieve metal solubilization in suspension. Of the three trials tested, single culture of A. thiooxidans decreased the pH to 0.7, resulting in the highest metal solubilization ratios. Solubilization efficiencies of Cr, Cu, and Zn were high (> 80%) with A. thiooxidans but Pb could be solubilized to a ratio of 63%. The efficiency of metal solubilization from contaminated sediment in decreasing order is: Zn > Cu > Cr > Pb. In order to check for the mass balance, metals remaining in the residual sediment were also determined. The effect of bioleaching on sequentially extracted (chemically distributed) metals was investigated in addition to the metal solubilization efficiencies. For most of the trials, it is observed that, the stable forms of heavy metals were transferred and accumulated in more soluble forms during bioleaching and they directly solubilized in the water when they were finally transferred to the exchangeable and/or reducible form.

Sediment transport and deposition on a river‐dominated tidal flat: An idealized model study

A 3‐D hydrodynamic model is used to investigate how different size classes of river‐derived sediment are transported, exported and trapped on an idealized, river‐dominated tidal flat. The model is composed of a river channel flanked by sloping tidal flats, a configuration motivated by the intertidal region of the Skagit River mouth in Washington State, United States. It is forced by mixed tides and a pulse of freshwater and sediment with various settling velocities. In this system, the river not only influences stratification but also contributes a significant cross‐shore transport. As a result, the bottom stress is strongly ebb‐dominated in the channel because of the seaward advance of strong river flow as the tidal flats drain during ebbs. Sediment deposition patterns and mass budgets are sensitive to settling velocity. The lateral sediment spreading scales with an advective distance (settling time multiplied by lateral flow speed), thereby confining the fast settling sediment classes in the channel. Residual sediment transport is landward on the flats, because of settling lag, but is strongly seaward in the channel. The seaward transport mainly occurs during big ebbs and is controlled by a length scale ratio Ld/XWL, where Ld is a cross‐shore advective distance (settling time multiplied by river outlet velocity), and XWL is the immersed cross‐shore length of the intertidal zone. Sediment trapping requires Ld/XWL < 1, leading to more trapping for the faster settling classes. Sensitivity studies show that including stratification and reducing tidal range both favor sediment trapping, whereas varying channel geometries and asymmetry of tides has relatively small impacts. Implications of the modeling results on the south Skagit intertidal region are discussed.

Idealized study on cohesive sediment flux by tidal asymmetry

The flux of cohesive sediment in an estuary is determined by many factors, including tidal asymmetry, wave effect, fluvial influence, phase difference between tidal velocity and tidal level fluctuations, sediment properties, flocculation, bed erodibility, bathymetry effect and other nonlocal effects. Our capability in predicting sediment fluxes in tide-dominant environments is critical to the morphodynamics and water quality of estuaries. Due to the difficulties in carrying out detailed measurement of sediment flux with high spatial and temporal resolutions, an one-dimensional-vertical (1DV) numerical model for cohesive sediment transport, previously verified and calibrated with field measured cohesive sediment concentration data, is utilized here to study some of the aforementioned factors in affecting tidal-driven sediment fluxes in idealized condition. Tidal-averaged sediment flux is shown to be correlated with tidal velocity skewness with a linear relationship. This linear relationship is different from that of non-cohesive sediment and it is demonstrated here to be mainly due to variable critical shear stress implemented for the mud bed in order to parameterize consolidation. The reason that tidal velocity skewness causes tidal-averaged residual sediment transport is shown to be due to nonlinear intra-tidal interactions between flow velocity and sediment concentration. Moreover, the effects of nonlinear intra-tidal interaction between tidal velocity and tidal level fluctuations is shown to mainly cause seaward transport, which is the most significant under progressive wave system (phase difference 90°) and almost negligible for standing wave system (phase difference 0°).

The effect of tidal asymmetry and temporal settling lag on sediment trapping in tidal estuaries

Over decades and centuries, the mean depth of estuaries changes due to sea-level rise, land subsi- dence, infilling, and dredging projects. These processes produce changes in relative roughness (friction) and mixing, resulting in fundamental changes in the char- acteristics of the horizontal (velocity) and vertical tides (sea surface elevation) and the dynamics of sediment trapping. To investigate such changes, a 2DV model is developed. The model equations consist of the width- averaged shallow water equations and a sediment balance equation. Together with the condition of mor- phodynamic equilibrium, these equations are solved analytically by making a regular expansion of the vari- ous physical variables in a small parameter. Using these analytic solutions, we are able to gain insight into the fundamental physical processes resulting in sediment trapping in an estuary by studying various forcings separately. As a case study, we consider the Ems es- tuary. Between 1980 and 2005, successive deepening of the Ems estuary has significantly altered the tidal and sediment dynamics. The tidal range and the surface sediment concentration has increased and the position of the turbidity zone has shifted into the freshwater zone. The model is used to determine the causes of these historical changes. It is found that the increase of the tidal amplitude toward the end of the embayment is the combined effect of the deepening of the estuary and a 37% and 50% reduction in the vertical eddy viscosity and stress parameter, respectively. The phys- ical mechanism resulting in the trapping of sediment, the number of trapping regions, and their sensitivity to grain size are explained by careful analysis of the var- ious contributions of the residual sediment transport. It is found that sediment is trapped in the estuary by a delicate balance between the M2 transport and the residual transport for fine sediment (ws = 0.2 mm s −1 ) and the residual, M2 and M4 transports for coarser sediment (ws = 2 mm s −1 ). The upstream movement of the estuarine turbidity maximum into the freshwa- ter zone in 2005 is mainly the result of changes in tidal asymmetry. Moreover, the difference between the sediment distribution for different grain sizes in the same year can be attributed to changes in the temporal settling lag.

Quantifying restoration success and recovery in a metal‐polluted stream: a 17‐year assessment of physicochemical and biological responses

Summary 1. Evaluating the effectiveness of stream restoration is often challenging because of the lack of pre‐treatment data, narrow focus on physicochemical measures and insufficient post‐restoration monitoring. Even when these fundamental elements are present, quantifying restoration success is difficult because of the challenges associated with distinguishing treatment effects from seasonal variation, episodic events and long‐term climatic changes. 2. We report results of one of the most comprehensive and continuous records of physical, chemical and biological data available to assess restoration success for a stream ecosystem in North America. Over a 17 year period we measured seasonal and annual changes in metal concentrations, physicochemical characteristics, macroinvertebrate communities, and brown trout Salmo trutta populations in the Arkansas River, a metal‐contaminated stream in Colorado, USA. 3. Although we observed significant improvements in water quality after treatment, the effectiveness of restoration varied temporally, spatially and among biological response variables. The fastest recovery was observed at stations where restoration eliminated point sources of metal contamination. Recovery of macroinvertebrates was significantly delayed at some stations because of residual sediment contamination and because extreme seasonal and episodic variation in metal concentrations prevented recolonization by sensitive species. 4. Synthesis and applications. Because recovery trajectories after the removal of a stressor are often complex or nonlinear, long‐term studies are necessary to assess restoration success within the context of episodic events and changes in regional climate. The observed variation in recovery among chemical and biological endpoints highlights the importance of developing objective criteria to assess restoration success. Although the rapid response of macroinvertebrates to reduced metal concentrations is encouraging, we have previously demonstrated that benthic communities from the Arkansas River remained susceptible to other novel anthropogenic stressors. We suggest that the resistance or resilience of benthic macroinvertebrate communities to novel stressors may be effective indicators of restoration success that can account for the non‐additive (e.g. synergistic) nature of compound perturbations.

Morphodynamic modeling of tidal channel evolution in comparison to empirical PA relationship

Tidal channels and inlets in alluvial environments are interconnected dynamic systems that react to changing physical conditions (such as sea level rise) as well as to anthropogenic impact (such as dredging and bank protection works). Past research resulted in an empirical equilibrium relationship for inlets between the tidal prism ( P) and the cross-sectional area in a tidal inlet ( A). Constant PA relationships were found along several tidal basins. Physical explanations of the PA relationship are based on a balance between littoral drift and tidal transport capacity (for inlets), on the concept of critical shear stress for sediment transport or on the appearance of constant spatial sediment concentrations, which is related to spatially uniform shear stresses. The current research aims to clarify the PA relationship by addressing the morphodynamic evolution of a tidal embayment over millennia. Use is made of a 2D process-based, numerical model that is capable of describing long-term morphodynamic basin evolution and pattern development. Wave impact (i.e. littoral drift) is neglected so that focus is on tidal motion only. Model results closely obey the empirical PA relationship by Jarrett (1976) even in the absence of waves. This suggests that tidal forcing alone plays a major role in the development of the PA relationship. Results also show that cross sections increase for similar tidal prisms on the long term. Explanation is found by small gradients in tide residual sediment transports that allow for (very slow) development over time. The scatter found around the empirical PA relationship by Jarrett (1976) may thus be partly explained by the age of the tidal basin. A shallow basin mouth attached to a deep ocean and a deep basin continuously deepens. The basin mouth evolution becomes strongly damped when it is attached to a shallow basin that exports sediment. The PA relationship is constant along the shallow basin which is attributed to constant tidal conditions along the basin. Further analysis shows that confined inlets require smaller cross-sectional areas than free inlets to convey the same tidal prism. This is attributed to their relatively deep cross-sectional area so that friction plays a smaller role. This is in line with different coefficients found for jettied inlets and free inlets in the empirical PA relationship of Jarrett (1976).

The influence of changes in tidal asymmetry on residual sediment transport in the Western Scheldt

This study fits into a wider research program from RWS RIKZ concerning the exchange of sediment between the coast and the tidal basins, Western Scheldt and Wadden Sea, which are the largest basins along the Dutch Coast, over different time-scales. For both basins, questions about the evolution of the import/export at the mouth recently arose. In case of the Western Scheldt, which is the subject of this study, mainly the uncertainty about the future developments after the change from import to export at the mouth was noticed in the 1990s, which necessitated a more detailed study of this area. To understand the sediment exchange between the Dutch coast and the tidal basin Western Scheldt, the mechanism responsible for this transport must be identified. This study focuses on only one possibly important mechanism: the interaction between the tides and bathymetry. An existing numerical, depth-averaged Delft3D model of the Western Scheldt has been applied. The bathymetry of the years 1970, 1983 and 2002 was selected based on previous sand balance studies. Thus three different situations at the mouth are represented: strong import, import and export of sediments. Each situation was forced by the same morphological tide, where a tide is selected in such a way that during ebb and during flood the computed sediment transport through a cross-section near Vlissingen is similar to the average ebb and flood transport for a complete spring–neap tidal cycle. A comparison was made of the vertical tidal asymmetry in the model with field data for calibration purposes. Then, both the vertical and horizontal tides were analysed. Time series of water levels, velocities and discharges were harmonically analysed to derive the M 2, M 4 and M 6 tidal constituents. The tidal asymmetry is expressed by the amplitude ratios and the phase differences. Furthermore the tide-driven sediment transport ascribed to a given bathymetry was determined. The residual transport patterns were analysed and a sand balance was derived from the modelled transports. This led to the evolution in time of the sediment exchange between, and the erosion/sedimentation within, different macro cells ( Winterwerp et al., 2000) of the Western Scheldt. The human interventions, the changes in tidal asymmetry and the evolution in the sediment transport interact. The increased landward transport inside the river is reproduced in the model; however, the change from sediment import to sediment export at the mouth could not be explained by tidal asymmetries only. Other processes such as winds and waves should also be studied to determine the mechanisms, which influence most of the observed sediment exchanges.

Effects of pH and total organic carbon (TOC) on the distribution of trace metals in Kubanni dam sediments, Zaria, Nigeria

The determination of Cd, Cr, Cu, Pb and Zn in the exchangeable, carbonate, Fe-Mn-oxides, organic matter and residual sediment chemical fractions of Kubanni Dam during dry and raining seasons was carried out using the spectrophotometric method after Tessiers’ sequential scheme for speciation had been carried out. The pH and Total Organic Carbon (TOC) of the sediment samples were also determined to study their effects on the distribution of metals. The results indicate that pH influenced the release or adsorption of each metal in the different sediment fractions. At low average pH of 5.21 recorded during dry season, desorption of metals from the reducible sediment fraction into the water column occurred, which may have effect on aquatic organisms. Due to increased metal complexation at high pH, total extractable fractions of metals generally increased. Higher TOC levels during the rainy season indicated larger adsorption surfaces for Cr (19.92%) Pb (19.97%) and Zn (21.14%) on the organic matter fraction, which, under oxidizing condition, these metals may be remobilized into the aquatic environment. Cu correlated positively with the organic carbon content of the sediment samples. The positive correlation of Pb, Cd and Cr with sediment pH may reflect the tendency of cation adsorption to increase at high pH. Key words: pH, TOC, Trace metals, Distribution, Sediments

Residual Urinary Volume and Urinary Tract Infection, A Life Threatening Illness: A Case Report

Multiple Sclerosis (MS) is an inflammatory disease that demyelinates the central nervous system causing progressive disability. The urodynamics of MS patients is an important consideration because these patients are susceptible to acute urinary bladder retention associated with a high fever. Treatment should include irrigation using a Foley catheter and an irrigation syringe to remove residual urine and sediment from the neurogenic bladder, intravenous fluid resuscitation, bacteriological analysis of the residual urine, and admission to an Intensive Care Unit for further evaluation by a urologist. After discharge, suprapubic cystostomy should be a consideration for patients who have chronic urinary tract infections associated with acute urinary bladder retention, as well as limited mobility to perform self catheterization, or limited personal care assistants trained to perform a urethral clean intermittent catheterization.

Transoceanic ships as vectors for nonindigenous freshwater bryozoans

AbstractAim The transport of organisms in ships’ ballast tanks is a dominant vector for aquatic invasions worldwide. Until recently, efforts to manage this vector have overlooked the potential transport of invertebrate resting stages in the residual waters and sediments within emptied ballast tanks, i.e. NOBOB (‘No Ballast On Board’) tanks. The resting stages (statoblasts) of freshwater bryozoans are often buoyant and locally abundant and thus can be taken up easily during ballasting operations. They are also resistant to extreme environmental conditions and can generate new colonies after being dormant for decades; as such, they would likely remain viable propagules after lengthy transport in ship ballast tanks. This study quantified the occurrence of freshwater bryozoan statoblasts in ballast tank sediments of transoceanic ships.Location North American Great Lakes.Methods We quantified the frequency of occurrence, abundance and diversity of bryozoans (as statoblasts) in residual sediment samples taken from 51 NOBOB tanks of 33 transoceanic ships visiting the Great Lakes from 2000 to 2002.Results Our study identified 11 species, comprising nearly 12% of the total number of freshwater bryozoans known worldwide. These include two exotic species unrecorded in the Great Lakes (Fredericella sultana and Lophopus crystallinus), an exotic species already established in the region (Lophopodella carteri) and three cosmopolitan species (Plumatella casmiana, P. fungosa and P. repens). Our estimates suggest that a ship with NOBOB tanks may carry up to 106 statoblasts.Main conclusions The discovery of species unrecorded in the Great Lakes and the potentially large numbers of statoblasts being transported in ship ballast tanks indicate a significant risk of new species introductions. Furthermore, the presence of cosmopolitan species and an exotic species already established in the Great Lakes suggests the strong possibility of cryptic invasions via the introduction of exotic genotypes.

Effect of bottom stress formulation on modelled flow and turbidity maxima in cross-sections of tide-dominated estuaries

A three-dimensional numerical model with a prognostic salinity field is used to investigate the effect of a partial slip bottom boundary condition on lateral flow and sediment distribution in a transect of a tidally dominated channel. The transect has a symmetrical Gaussian cross-channel bottom profile. For a deep, well-mixed, tidally dominated channel, partial slip decreases the relative importance of Coriolis deflection on the generation of cross-channel flow patterns. This has profound implications for the lateral distribution of residual salinity that drives the cross-channel residual circulation pattern. Transverse sediment transport, however, is always found to be governed by a balance between advection of residual sediment concentration by residual lateral flow on the one hand and cross-channel diffusion on the other hand. Hence, the changes in the cross-channel distribution of residual salinity modify the lateral sediment distribution. For no slip, a single turbidity maximum occurs. In contrast, partial slip gives a gradual transition to a symmetrical density distribution with a turbidity maximum near each bank. For a more shallow, partially mixed tidal channel that represents the James River, a single turbidity maximum at the left bank is found irrespective of the near-bed slip condition. In this case, semi-diurnal contributions to sediment distribution and lateral flow play an important role in cross-channel sediment transport. As vertical viscosity and diffusivity are increased, a second maximum at the right bank again exists for partial slip.

Marine dinoflagellate cysts in the ballast tank sediments of ships entering the Laurentian Great Lakes

(2009). Marine dinoflagellate cysts in the ballast tank sediments of ships entering the Laurentian Great Lakes. SIL Proceedings, 1922-2010: Vol. 30, No. 7, pp. 1035-1038.

Morphological evolution of the Dee Estuary, Eastern Irish Sea, UK: A tidal asymmetry approach

Asymmetry in the tide (unequal ebb and flood duration) is a dominant factor in causing residual sediment transport and morphological changes in estuaries. The evolution of estuarine morphology is a process of dynamic equilibrium in the short-term, while these features are ephemeral in the long-term. In this study we investigate the spatial distribution of tidal distortion and asymmetry in the Dee estuary, UK, by 3-dimensional numerical modelling methods. High resolution LIDAR surveys are used to underpin and explain our numerical modelling results in terms of basin hypsometry and areas of recent erosion and deposition. Harmonic analysis of the numerical modelling results showed that the shallower intertidal areas (sand and mud banks) were the most tidally asymmetric, showing flood dominance. The main navigation channels showed some ebb dominance but the tides here were relatively undistorted. This overall flood dominance is likely to induce net sediment import to the Dee, which explains known historical morphological changes (large scale accretion over the last two centuries) and also recent morphological changes as seen from the LIDAR surveys (which show predominantly net accretion between 2003 and 2006). Hypsometrical analysis suggests the Dee may be approaching equilibrium, and that the flood dominance and sedimentation rate may therefore decrease in the future. In an infilling estuary, an increase in the area and elevation of tidal flats can eventually shift an estuary towards ebb dominance, as shown by previous research and by ‘idealised estuary’ modelling results presented in this study. The large tidal amplitude to hydraulic depth ratio of the Dee, however, suggests that the tidal flats would have to be very extensive indeed for this to occur.

Modelling impact of dredging and dumping in ebb-flood channel systems

For a channel-shoal system in a funnel-shaped basin the impact of dredging and dumping is investigated using a complex process-based model. First, the residual flow and sediment transport circulations are analysed for the channel-shoal pattern, which has emerged after a long-term model simulation. Results are compared to the Western Scheldt estuary, which forms the inspiration for this study. Subsequently, different dredge and dump scenarios are modelled, according to a conceptual model, in which ebb-and flood-channels and enclosed shoals form morphodynamic units (cells) with their own sediment circulation. Model results show that dumping sediment in a channel further reduces the channel depth and induces erosion in the opposite channel, which enhances tilting of the cross-section of the cell and eventually can lead to the degeneration of a multiple channel system into a single channel. The impact of different dredging and dumping cases agrees with results from a stability analysis. This means that this type of model applied to a realistic geometry can potentially be used for better prediction of the impact of human interventions.

Aerobic metabolism of vitamin E by marine bacteria: Interaction with free radical oxidation (autoxidation) processes

The degradation of vitamin E by aerobic bacterial communities, isolated from marine sediment and microbial mat samples, was investigated. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) profiles and cloning/sequencing experiments revealed that biodegradation of vitamin E in sediments is mainly carried out by strains belonging to the genera Idiomarina and Bacillus, for which the DGGE pattern matched the pattern obtained from the second sediment subculture. Biodegradation appeared to involve an initial ω-oxidation of the isoprenoid side chain and subsequent β-oxidation, affording 2,5,7,8-tetramethyl-2(2′-carboxyethyl)-6-hydroxychroman (α-CEHC). This compound was not accumulated at the end of growth, showing that the bacterial degradation of vitamin E is not limited to the isoprenoid side chain. In cultures still containing residual sediment, the presence of metabolites with a shortened side chain and opened chroman ring (e.g., α-tocopheronolactone and α-tocopherylhydroquinonolactone) attested to the simultaneous involvement of biodegradation and autoxidation. The induction of autoxidation during these incubations was attributed to some of the sediment components, which could act as catalysts for free radical reactions. In oxic environments, the combination of free radical oxidation and aerobic biodegradation should result in very rapid degradation of vitamin E. Different pathways are proposed to explain the formation of the different compounds resulting from these interactions.