Location and probability of shoal margin collapses in a sandy estuary

Channel bank failure, and collapses of shoal margins and beaches due to flow slides, have been recorded in Dutch estuaries for the past 200 years but have hardly been recognized elsewhere. Current predictions lack forecasting capabilities, because they were validated and calibrated for historic data of cross‐sections in specific systems, allowing local hindcast rather than location and probability forecasting. The objectives of this study were to investigate where on shoal margins collapses typically occur and what shoal margin collapse geometries and volumes are, such that we can predict their occurrence. We identified shoal margin collapses, generally completely submerged, from bathymetry data by analyzing digital elevation models of difference of the Western Scheldt for the period 1959–2015. We used the bathymetry data to determine the conditions for occurrence, specifically to obtain slope height and angle, and applied these variables in a shoal margin collapse predictor. We found 299 collapses along 300 km of shoal margin boundaries over 56 years, meaning that more than five collapses occur on average per year. The average shoal margin collapse body is well approximated by a 1/3 ellipsoid shape, covers on average an area of 34 000 m² and has an average volume of 100 000 m³. Shoal margin collapses occur mainly at locations where shoals take up a proportionally larger area than average in the cross‐section of the entire estuary, and occur most frequently where lateral shoal margin displacement is low. A receiver operating characteristic curve shows that the forecasting method predicts the shoal margin collapse location well. We conclude that the locations of the shoal margin collapses are well predicted by the variation in conditions of the relative slope height and angle within the Western Scheldt, and likely locations are at laterally relatively stable shoal margins. This provides hypotheses aiding the recognition of these features in sandy estuaries worldwide. Copyright © 2018 John Wiley & Sons, Ltd.     

Numerical computation of the tidally induced Lagrangian residual current in a model bay

With a depth-averaged numerical model, the tidally induced Lagrangian residual current in a model bay was studied. To correctly reflect the long-term mass transport, it is appropriate to use the Lagrangian residual velocity (LRV) rather than the Eulerian residual velocity (ERV) or the Eulerian residual transport velocity (ETV) to describe the residual current. The parameter κ, which is defined as the ratio of the typical tidal amplitude at the open boundary to the mean water depth, is considered to be the indicator of the nonlinear effect in the system. It is found that the feasibility of making the mass transport velocity (MTV) approximate the LRV is strongly dependent on κ. The error between the MTV and the LRV tends to increase with a growing κ. An additional error will come from the various initial tidal phases due to the Lagrangian drift velocity (LDV) when κ is no longer small. According to the residual vorticity equation based on the MTV, the Coriolis effect is found to influence the residual vorticity mainly through the curl of the tidal stress. A significant difference in the flow pattern indicates that the LRV is sensitive to the bottom friction in different forms.     

Longitudinal variation in lateral trapping of fine sediment in tidal estuaries: observations and a 3D exploratory model

This study investigates the longitudinal variation of lateral entrapment of suspended sediment, as is observed in some tidal estuaries. In particular, field data from the Yangtze Estuary are analysed, which reveal that in one cross-section, two maxima of suspended sediment concentration (SSC) occur close to the south and north sides, while in a cross-section 2 km down-estuary, only one SSC maximum on the south side is present. This pattern is found during both spring tide and neap tide, which are characterised by different intensities of turbulence. To understand longitudinal variation in lateral trapping of sediment, results of a new three-dimensional exploratory model are analysed. The hydrodynamic part contains residual flow due to fresh water input, density gradients and Coriolis force and due to channel curvature-induced leakage. Moreover, the model includes a spatially varying eddy viscosity that accounts for variation of intensity of turbulence over the spring-neap cycle. By imposing morphodynamic equilibrium, the two-dimensional distribution of sediment in the domain is obtained analytically by a novel procedure. Results reveal that the occurrence of the SSC maxima near the south side of both cross-sections is due to sediment entrapment by lateral density gradients, while the second SSC maximum near the north side of the first cross-section is by sediment transport due to curvature-induced leakage. Coriolis deflection of longitudinal flow also contributes the trapping of sediment near the north side. This mechanism is important in the upper estuary, where the flow due to lateral density gradients is weak.     

PCB and HCB in the sediments and waters of the Tiber estuary

The contamination by polychlorinated biphenyls (PCB) and hexachlorobenzene (HCB) in sea waters and sediments of Tiber River estuary has been studied in 1976 and 1977.PCBs have been detected in all the samples analysed at mean levels of 0.297 ppb (1976) and 0.135 ppb (1977); these levels are much higher than those proposed as acceptable for surface waters. Contamination increases with increasing distance from the shore.HCB has been identified in only 16% of the water samples analysed and it was found absent in sediment samples.The results are discussed relative to both the levels of PCB in other sea waters and the toxicity of these products to the aquatic animals.     

Chlorophyll a and its derivatives in sediments of the Odra estuary as a measure of its eutrophication

Chlorophyll a and its selected derivatives, as well as chlorophylls b and c have been determined in samples of recent (0-10 cm) sediments, collected at 30 stations in the Szczecin Lagoon, Pomeranian Bay and the Bornholm Deep. Samples were retrieved in different seasons between 1992 and 2001, including the time before and after the great flood of July 1997. Regarding the content of chlorophyll a and its derivatives in sediments, the Szczecin Lagoon may be classified as permanently eutrophic and the Pomeranian Bay and Bornholm Deep as mesotrophic. A lack of correlation between chlorophylls b, c and chlorophyll a indicated that intensive blooms of cyanobacteria occur in the Szczecin Lagoon, which is a characteristic of eutrophic zones. The chlorin a results are related to selected parameters characterizing the aquatic environment. The values for nutrients, salinity and temperature in near-bottom waters, as well as granulometry and organic carbon in the sediments verify the above conclusions.     

Plutonium,radiocesium and radiocobalt in sediments of the Hudson River estuary

Anthropogenic radionuclides have reached the Hudson estuary as global fallout from nuclear weapons testing and through local releases from commercial nuclear reactors. Significant activities of²³⁸Pu and^239,240Pu (fallout-derived),¹³⁴Cs and⁶⁰Co (reactor-released), and¹³⁷Cs (derived from both sources), have accumulated in the sediments throughout the estuary, with the primary zone of accumulation near the downstream end of the system in New York harbor. The estuary appears to have trapped nearly all of the^239,240Pu delivered as fallout, and consequently, ocean dumping of dredged harbor sediment is currently the primary means for the net transport of these nuclides to coastal waters. In contrast, only 10–30% of the¹³⁷Cs,¹³⁴Cs and⁶⁰Co delivered to the estuary have been retained on the fine particles which accumulate at a rapid rate in the harbor.The primary factors which have governed the distribution of anthropogenic radionuclides in Hudson sediments are: (1) spread of fine particles labeled with both fallout and reactor nuclides throughout the axis of the estuary, (2) differences in timing of the peak fallout years (1962–1964) and years of maximum reactor releases (1971–1972), (3) large variations in sediment accumulation rates, ranging from a few millimeters per year or less to many tens of centimeters per year, (4) appreciable desorption of¹³⁷Cs and¹³⁴Cs from particles at higher salinities, and (5) possible enhanced desorption of⁶⁰Co at higher salinities (relative to¹³⁴Cs and¹³⁷Cs) which may be associated with the release of reduced manganese from the harbor sediments.     

Primary production in a turbid estuary, the Ems-Dollard: field and model studies

The Ems-Dollard Estuary has a steep axial gradient in turbidity which strongly regulates primary production of phytoplankton and microphytobenthos. A combination of the turbidity and nutrient gradients causes a high pelagic primary production in the outer, seaward part of the estuary, and a high benthic production on high-level mudflats (with long effective photoperiods) in the inner part. Model simulations were used to study phytoplankton succession and the factors influencing this succession. Light limitation was evaluated in a sensitivity run which used a lower compensation light intensity.     

Sediment transport in distributary channels and its export to the pro-deltaic environment in a tidally dominated delta: Fly River, Papua New Guinea

Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb- and flood-dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10 g l⁻¹ from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100 cm above the bed) drop off from around 100 cm s⁻¹ within the distributary channels to <50 cm s⁻¹ on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35 h to traverse the 20-km-wide, low-gradient (2×10⁻³ degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates.     

Phosphorus and organic carbon in the sediments of a polluted subtropical estuary,and the influence of coastal reclamation

In July 1978, total phosphorus and organic carbon were determined in the sediments of Tolo Harbour, a sewage-polluted estuary in north-east Hong Kong. Concentrations were correlated with % silt-clay in each of three areas. Phosphorus concentrations were highest in central Tolo Harbour, lower by about 1.5μg at. P g^?1 in the outer estuary, Tolo Channel, and lowest in the polluted inner reaches near large coastal reclamations. The latter values, about 3.0 μg at. P g^?1 lower than in central Tolo Harbour, might reflect a selective adsorption of phosphate by reclamation sediments. Organic carbon concentrations were high in the inner reaches and decreased towards the outer channel. Correlations between phosphorus and organic carbon were compared with a published correlation for the east coast of England: in Hong Kong, phosphorus concentrations showed a smaller increase as organic carbon increased, and reached only one-third of the English values as organic carbon approached 2.0%.     

Historical overview of 2D and 3D hydrodynamic modelling of shallow water flows in the Netherlands

Ocean Dynamics - This paper contains a historical overview of 2D and 3D hydrodynamic modelling in the Netherlands from the 1960s till now. It started in the 1960s with two-dimensional...     

Plutonium occurrence and phase distribution in sediments of the Wyre estuary,northwest England

The occurrence of plutonium isotopes derived from the Windscale reprocessing facility discharges and weapons test fallout in intertidal sediments of the Wyre estuary, northwest England, is reported. Windscale plutonium appears to be slowly transported to the estuary in association with sedimentary material, and its distribution in surface and buried sediments is controlled by the variations in organic matter content of the sediments and/or their grain size characteristics. Results from chemical leaching experiments suggest that very little plutonium is ion exchangeable or in humic substances, but it is in other organic and Fe/Mn hydrous oxide fractions of the sediments. Plutonium shows a similar depositional behaviour to mercury which enters the Wyre directly in local industrial discharges.     

The role of sulphide in the formation of dimethyl mercury in river and estuary sediments

The role of sulphide-containing species in sediments in the mobilization of mercury in the environment is demonstrated. The mobile species is the volatile hydrophobic dimethyl mercury. Transport to atmosphere in this form may account for a loss of about 12% annually of methyl mercury from sediments and may therefore be a substantive part of the biogeochemical cycling of mercury in the environment.     

Morphological controls in sandy estuaries: the influence of tidal flats and bathymetry on sediment transport

The morphodynamics of shallow, vertically well-mixed estuaries, characterised by tidal flats and deeper channels, have been investigated. This paper examines what contributes to flood/ebb-dominant sediment transport in localised regions through a 2D model study (using the TELEMAC modelling system). The Dyfi Estuary in Wales, UK has been used as a case study and, together with idealised estuary shapes, shows that shallow water depths lead to flood dominance in the inner estuary whilst tidal flats and deep channels cause ebb dominance in the outer estuary. For medium sands and with an artificially ‘flattened’ bathymetry (i.e. no tidal flats), the net sediment transport switches from ebb-dominant to flood-dominant where the parameter a/h (local tidal amplitude ÷ local tidally averaged water depth) exceeds 1.2. Sea level rise will reduce this critical value of a/h and also reduce the ebb-directed sediment transport significantly, leading to a flood-dominated estuarine system. A similar pattern, albeit with greater transport, was simulated with tidal flats included and also with a reduced grain size. This suggests that analogous classifications for flood/ebb asymmetry of the tide in estuaries as a whole may not represent the local sediment transport in sufficient detail. Through the Dyfi simulations, the above criterion involving a/h is shown to be complicated further by augmented flow past a spit at the estuary mouth which gives rise to a self-maintaining scour hole. Simulations of one year of bed evolution in an idealised flat-bottomed estuary, including tidal flow past a spit, recreate the flood/ebb dominance on either side of the spit and the formation of a scour hole in between. The erosion rate at the centre of the hole is reduced as the hole deepens, suggesting the establishment of a self-maintaining equilibrium state.     

A two-dimensional model for suspended sediment transport in the southern branch of the Rhine–Meuse estuary,The Netherlands

For the southern branch of the Rhine–Meuse estuary, The Netherlands, a two-dimensional horizontal suspended sediment transport model was constructed in order to evaluate the complicated water quality management of the area. The data needed to calibrate the model were collected during a special field survey at high river runoff utilizing a number of techniques: (1) turbidity probes were used to obtain suspended sediment concentration profiles; (2) air-borne remote sensing video recordings were applied in order to obtain information concerning the spatial distribution of the suspended sediment concentration; (3) an acoustic probe (ISAC) was used to measure cohesive bed density profiles and (4) an in situ underwater video camera (VIS) was deployed to collect video recordings of the suspended sediment. These VIS data were finally processed to fall velocity and diameter distributions and were mainly used to improve insight into the relevant transport processes, indicating significant erosion of sand from the upstream Rhine branch. For quantitative calibration of the model, the data from the turbidity profiles were used. Sedimentation and erosion were modelled according to Krone and Partheniades. The model results showed a good overall fit to the measurements, with a mean absolute error of 18 per cent (standard fault = 1 per cent), corresponding to concentrations of about 0·020 (upstream) to 0·005 kg m⁻³ (downstream). The overall correlation between observed and simulated suspended sediment concentrations was 0·85. The remote sensing video recordings were used for a qualitative calibration of the model. The distribution pattern of the suspended sediment on these photos was reproduced quite well by the model. However, a more accurate calibration technique is needed to enable the use of aerial remote sensing as a quantitative calibration method. Copyright © 1999 John Wiley & Sons, Ltd.     

Critical erosion profiles in macro-tidal estuary sediments: Implications for the stability of intertidal mud and the slope of mud banks

Vertical profiles of the critical erosion threshold (τ_crit) in sediment have been measured at 11 stations along the axis of the Tamar Estuary and at a single station in a tributary of the Tamar at St. John's Ford. The τ_crit of surface sediment increased from 0.04 Pa in the upper, brackish estuary to 0.09 Pa in the lower estuary. In the upper estuary τ_crit only increased slightly with depth whereas in the marine estuary τ_crit increased rapidly from 0.09 Pa at the surface to 0.25 Pa at 15 cm below the sediment surface. The results showed that the relationship between τ_crit and bulk density (ρ_b) obtained previously for surface sediment was also applicable to sediments from depths of 10–15 cm and probably deeper. Profiles of ρ_b were measured to depths of 70 cm using a corer. In the lower (marine) estuary ρ_b increased with depth in the sediment from 1580 kg m⁻³ at the surface to 1720 kg m⁻³ at 70 cm. In the upper estuary ρ_b values were lower at 1170–1200 kg m⁻³ and profiles were almost homogeneous indicating that consolidation was not occurring. The mid-estuary was transitional between these two situations. These results are consistent with the seasonal accumulation and loss of ‘mobile’ sediment observed previously in the upper estuary with changes in river flow, and with the apparent stability of intertidal mud in the lower marine estuary deduced from historical bathymetric survey records. The slopes of the intertidal mud banks ranged from 1–2% in the lower estuary to 20–25% in mid-estuary but, instead of continuing to increase in steepness towards the head as the estuary became narrower, the measured slopes actually decreased. It is speculated that the lack of consolidation through continual mobilisation and settlement cycles combined with an increase in silt content in the upper estuary resulted in sediment that lacked the mechanical strength to maintain steep slopes.