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David Lockington - One of the best experts on this subject based on the ideXlab platform.
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Tidally driven pore water exchange in offshore intertidal Sandbanks: Part I. Field measurements
Estuarine Coastal and Shelf Science, 2008Co-Authors: Badin Gibbes, Clare Robinson, H. Carey, David LockingtonAbstract:in recent years blooms of the toxic marine cyanobacteria Lyngbya majuscula have been frequently observed in a system of offshore intertidal Sandbanks in Moreton Bay, Australia. Past research suggests that these blooms are linked to the presence of bio-available forms of iron. Using hydraulic and pore water chemistry data collected from a shore normal transect at an offshore bloom site, the role of tidally driven exchange as a potential mechanism for delivery of bio-available iron across the sediment-water interface was examined. Field data revealed a residual pore water flow system in the Sandbank, with seawater entering the upper Sandbank platform and discharging through the bank edge. Upward flow and elevated near-surface dissolved Fe(II) concentrations (>20 mu M Fe(II) at -0.05 m depth) were measured simultaneously in the discharge zones at the Sandbank edge. The measured concentrations were more than four times greater than concentrations previously shown to stimulate L majuscula growth. These results suggest that the tidally driven exchange mechanism might be capable of delivering dissolved Fe(II) to sites within offshore interticlal Sandbanks where blooms of L majuscula have been observed. While the source of the iron was not identified, potential candidates are discussed. These findings have implications for the current conceptual model for L. majuscula blooms in offshore interticlal Sandbanks within Moreton Bay. Further investigations are required to fully understand the role of tidally driven exchange in controlling the export of bio-available iron to coastal waters at the field site. In particular there is a need to better assess the link between the pore water flows and the geochemical reactions that might occur along the flow path. (C) 2008 Elsevier Ltd. All rights reserved.
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Tidally driven pore water exchange within offshore intertidal Sandbanks: Part II numerical simulations
Estuarine Coastal and Shelf Science, 2008Co-Authors: Badin Gibbes, Clare Robinson, David LockingtonAbstract:Field measurements presented by [Gibbes, B., Robinson, C., Li, L, Lockington, D.A., Carey, H., 2008. Tidally driven pore water exchange within offshore intertidal Sandbanks: Part I Field measurements. Estuarine, Coastal and Shelf Science 79, pp. 121-132.] revealed a tidally driven pore water flow system within an offshore intertidal Sandbank in Moreton Bay, Australia. The field data suggested that this flow system might be capable of delivering nutrients, and in particular bio-available iron, across the sediment-water interface. Bio-available iron has been implicated as a key nutrient in the growth of the toxic marine cyanobacteria Lyngbya majuscula and therefore this pore water exchange process is of interest at sites where L majuscula blooms have been observed. In this study two-dimensional numerical simulations were used in conjunction with hydraulic data from field measurements to further investigate the tidally induced pore water flow patterns. Simulation results generally showed good agreement with the field data and revealed a more complex residual pore water flow system in the Sandbank than shown by the field data. The flow system, strongly influenced by the geometry of the Sandbank, was characterized by two circulation cells which resulted in pore water discharge at the bank edge and also to a permanently ponded area within the Sandbank interior. Simulated discharge volumes in these two zones were in the order of 0.813 m(3) and 0.143 m(3) per meter width (along shore) of Sandbank per tidal cycle at the bank edge and Sandbank interior respectively. Transit times of pore water circulating through these cells were found to range from approximate to 17 days to > 60 years with an average time of 780 days. The results suggest that the tidally driven flow systems might provide a mechanism for transport of bio-available iron across the sediment-water interface. This flow could constitute a previously unrecognized source of bio-available iron for L. majuscula blooms in the Bay. (C) 2008 Elsevier Ltd. All rights reserved.
Eric Chaumillon - One of the best experts on this subject based on the ideXlab platform.
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Wave control on the rhythmic development of a wide estuary mouth Sandbank: A process-based modelling study
Marine Geology, 2016Co-Authors: Thomas Guérin, Xavier Bertin, Eric ChaumillonAbstract:This study presents the numerical hindcast of the morphological changes and stratigraphy of a wide estuary mouth Sandbank located along the French Atlantic coast (Marennes-Oléron Bay), and primarily investigates the respective contribution of tide and waves to its pluri-decadal evolution. Firstly, the morphodynamic hindcast appears strongly improved when tide and waves are considered in the forcing rather than only tide. Secondly, the modelling results suggest that the strong seasonal variation of the wave climate in this area explains the rhythmic lateral accretion of the bank (i.e. normal to the currents main direction) observed in seismic reflection profiles. The grain size variation in the area of sediment accretion is also characterized by a seasonal cycle. Finally, repetitive bathymetric surveys and seismic profiles revealed that the lateral accretion of the bank is associated with about one seismic reflector per year, which is in agreement with our modelling results when suggesting that these seismic reflectors are due to the seasonal variation of the grain size. These new results may be considered for the morphological evolution and the stratigraphy of other tidal Sandbanks potentially affected by waves.
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Wave control on the rhythmic development of a wide estuary mouth Sandbank: A process-based modelling study
Marine Geology, 2016Co-Authors: Thomas Guérin, Xavier Bertin, Eric ChaumillonAbstract:Abstract This study presents the numerical hindcast of the morphological and stratigraphic evolution of a wide estuary mouth Sandbank located along the French Atlantic coast (Marennes-Oleron Bay), and primarily investigates the respective contribution of tide and waves to its pluri-decadal evolution. Firstly, the morphodynamic hindcast appears strongly improved when tide and waves are considered in the forcing rather than only tide. Secondly, the modelling results suggest that the strong seasonal variation of the wave climate in this area explains the rhythmic lateral accretion of the bank (i.e. normal to the main currents direction) observed in seismic reflection profiles. The grain size variation in the area of sediment accretion is also characterized by a seasonal cycle. Finally, repetitive bathymetric surveys and seismic profiles revealed that the lateral accretion of the bank is associated with about one seismic reflector per year, which is in agreement with our modelling results when assuming that these seismic reflectors are due to the seasonal variation of the grain size. These new results may be considered when studying the morphological evolution and the stratigraphy of other tidal Sandbanks potentially affected by waves.
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Multi time-scale evolution of a wide estuary linear Sandbank, the Longe de Boyard, on the French Atlantic coast
Marine Geology, 2008Co-Authors: Eric Chaumillon, Xavier Bertin, Hélène Falchetto, Jonathan Allard, Nicolas Weber, Patrice Walker, Nicolas Pouvreau, Guy WöppelmannAbstract:Abstract This trans-disciplinary work combines modeling and observational approaches and offers a unique dataset to study the behaviour of a wide estuary linear Sandbank over different time-scales. The Sandbank studied, the Longe de Boyard, lies in a macrotidal estuary environment off the French Atlantic coast. Side scan sonar data combined with shipek grab samples and numerical modeling of waves and tides revealed its short-term dynamics. Historical (1824) and present-day (2000 and 2003) bathymetric data combined with numerical simulations of waves and tides and tide-related sand transport in 1824, and seismic profiling, were then used to demonstrate the long-term evolution of the Sandbank and how this correlates with the short-term dynamics. The geological evolution (centuries to millennia) was finally deduced from seismic stratigraphy combined with an analysis of vibrocore samples. Most of the long-term morphological changes in the ‘Longe de Boyard’ can be explained by the short-term dynamics involving sand transport convergence driven by both tides and waves. Seaward, the changes in the axial part of the bank correspond mainly to erosion and can be explained by wave and tide ravinment. Shoreward, sediment accretion is related to the convergence of tide-related sand transport during ebb and flood due to the dam-effect of the crest of the bank. The changes in the Sandbank since 1824 can also be related to a decrease in the tidal channel cross section. The latter was the result of a 10% decrease in tidal currents and tidal prisms subsequent to the rapid sediment infill, and a related 20% reduction in the water volume of the estuary system where the Sandbank lies. Seismic stratigraphy and cores showed that the modern Sandbank consists of upper clinoforms made of fine sand built up over a core made of coarse sand and gravel related to high-energy environments. Hence, the Longe de Boyard is not only the result of sand convergence driven by both tides and waves but also integrates decreases in the tidal prism subsequent to sediment infilling of the surrounding estuaries on a century and millenia time-scale.
Badin Gibbes - One of the best experts on this subject based on the ideXlab platform.
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Tidally driven pore water exchange in offshore intertidal Sandbanks: Part I. Field measurements
Estuarine Coastal and Shelf Science, 2008Co-Authors: Badin Gibbes, Clare Robinson, H. Carey, David LockingtonAbstract:in recent years blooms of the toxic marine cyanobacteria Lyngbya majuscula have been frequently observed in a system of offshore intertidal Sandbanks in Moreton Bay, Australia. Past research suggests that these blooms are linked to the presence of bio-available forms of iron. Using hydraulic and pore water chemistry data collected from a shore normal transect at an offshore bloom site, the role of tidally driven exchange as a potential mechanism for delivery of bio-available iron across the sediment-water interface was examined. Field data revealed a residual pore water flow system in the Sandbank, with seawater entering the upper Sandbank platform and discharging through the bank edge. Upward flow and elevated near-surface dissolved Fe(II) concentrations (>20 mu M Fe(II) at -0.05 m depth) were measured simultaneously in the discharge zones at the Sandbank edge. The measured concentrations were more than four times greater than concentrations previously shown to stimulate L majuscula growth. These results suggest that the tidally driven exchange mechanism might be capable of delivering dissolved Fe(II) to sites within offshore interticlal Sandbanks where blooms of L majuscula have been observed. While the source of the iron was not identified, potential candidates are discussed. These findings have implications for the current conceptual model for L. majuscula blooms in offshore interticlal Sandbanks within Moreton Bay. Further investigations are required to fully understand the role of tidally driven exchange in controlling the export of bio-available iron to coastal waters at the field site. In particular there is a need to better assess the link between the pore water flows and the geochemical reactions that might occur along the flow path. (C) 2008 Elsevier Ltd. All rights reserved.
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Tidally driven pore water exchange within offshore intertidal Sandbanks: Part II numerical simulations
Estuarine Coastal and Shelf Science, 2008Co-Authors: Badin Gibbes, Clare Robinson, David LockingtonAbstract:Field measurements presented by [Gibbes, B., Robinson, C., Li, L, Lockington, D.A., Carey, H., 2008. Tidally driven pore water exchange within offshore intertidal Sandbanks: Part I Field measurements. Estuarine, Coastal and Shelf Science 79, pp. 121-132.] revealed a tidally driven pore water flow system within an offshore intertidal Sandbank in Moreton Bay, Australia. The field data suggested that this flow system might be capable of delivering nutrients, and in particular bio-available iron, across the sediment-water interface. Bio-available iron has been implicated as a key nutrient in the growth of the toxic marine cyanobacteria Lyngbya majuscula and therefore this pore water exchange process is of interest at sites where L majuscula blooms have been observed. In this study two-dimensional numerical simulations were used in conjunction with hydraulic data from field measurements to further investigate the tidally induced pore water flow patterns. Simulation results generally showed good agreement with the field data and revealed a more complex residual pore water flow system in the Sandbank than shown by the field data. The flow system, strongly influenced by the geometry of the Sandbank, was characterized by two circulation cells which resulted in pore water discharge at the bank edge and also to a permanently ponded area within the Sandbank interior. Simulated discharge volumes in these two zones were in the order of 0.813 m(3) and 0.143 m(3) per meter width (along shore) of Sandbank per tidal cycle at the bank edge and Sandbank interior respectively. Transit times of pore water circulating through these cells were found to range from approximate to 17 days to > 60 years with an average time of 780 days. The results suggest that the tidally driven flow systems might provide a mechanism for transport of bio-available iron across the sediment-water interface. This flow could constitute a previously unrecognized source of bio-available iron for L. majuscula blooms in the Bay. (C) 2008 Elsevier Ltd. All rights reserved.
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Pore Water Exchange Processes in Offshore Intertidal Sandbanks
2007Co-Authors: Badin GibbesAbstract:In recent years blooms of the toxic marine cyanobacteria Lyngbya majuscula have been frequently observed in a system of offshore intertidal Sandbanks in Moreton Bay, Australia. Past research suggests that these blooms are linked to the presence of bioavailable forms of iron. Submarine groundwater discharge (SGD) is thought to provide an important nutrient (particularly iron) transport mechanism at locations where L. majuscula blooms are observed. In this study the feasibility of the current conceptual model for SGD to offshore L. majuscula bloom sites was examined using a combination of field measurements and numerical models. Numerical simulations indicated that SGD could occur several kilometres offshore within the Sandbank system if a continuous, low hydraulic conductivity aquitard extends from the onshore aquifer to the offshore Sandbank. Models that incorporated significant discontinuities or gaps in the aquitard unit resulted in SGD at the location of these discontinuities before reaching the offshore Sandbank system. Improved hydrogeological data, particularly in relation to the presence and continuity of any aquitard units, is required to enable a better assessment of SGD mechanisms in this area. Simulated maximum instantaneous vertical flow velocities in the near-surface zone of the sediments around the offshore exit point were an order of magnitude lower the velocities associated with local-scale pore water flow processes that are driven by tidal and subtidal pumping. As such these local-scale flow processes (instead of SGD) are likely to provide the dominant pore water exchange mechanisms in the offshore Sandbank systems. Field data was collected from a L. majuscula bloom site at an offshore intertidal Sandbank within the Eastern Banks of the Bay to examine these local-scale flow processes. The measurements identified a tidally-induced residual pore water flow system in the Sandbank, with seawater entering the upper Sandbank platform and discharging through the bank edge. Furthermore the field data revealed concentrations of bio-available iron in pore waters that were more than four times greater than concentrations previously shown to stimulate L. majuscula growth. Upward flow and elevated near-surface dissolved Fe(II) concentrations (> 20 µM Fe(II) at -0.05 m depth) were measured simultaneously in the discharge zones at the Sandbank edge. Analysis of dissolved Fe(II) profiles within pore waters at the site showed that measurable concentrations of dissolved Fe(II) were absent at the base (-2 m depth) of all monitoring stations. This result seems to indicate that terrestrially derived SGD is not an iron source at the site. Numerical simulations of the tidally-induced pore water flow at the field site generally showed good agreement with the field data. Simulation results revealed a more complex residual pore water flow system in the Sandbank than shown by the field data. Both numerical simulations and field measurements suggest that the residual flow might provide a mechanism for transport of bio-available iron across the sediment-water interface. During periods when these Sandbanks are submerged a number of pore water exchange processes, collectively termed subtidal pumping, are likely to be operating at the site. Numerical models were developed to examine one particular form of subtidal pumping, i.e., current-bedform interactions. These models were based on a one-way sequentially coupled model of surface water and porous medium flow. Interactions between unidirectional and bi-directional currents with simple two-dimensional fixed bedforms were considered. Results suggested that while pore water exchange was likely in distinct zones around the Sandbank edges, a whole-of-bank scale pore water circulation was unlikely to occur. Smaller scale (< 1.0 m) variations in the Sandbank topography might exert a stronger influence on the pore water flow and associated pore water exchange. Simulations showed more complex pore water flow paths and longer pore water residence times in systems under bi-directional flow conditions compared to systems subject to unidirectional flows. The consideration of bi-directional flows is important in modelling current-bedform interactions in tidally influenced environments. The findings of this study have implications for the current conceptual model for L. majuscula blooms in offshore intertidal Sandbanks within Moreton Bay. This conceptual model should be updated to include tidal and subtidal pumping as potential delivery mechanisms for bio-available iron. Further investigations are required to fully understand the role of tidal and subtidal pumping in controlling the export of bio-available iron to coastal waters at the field site. In particular there is a need to better assess the link between the pore water flows and the geochemical reactions that might occur along the flow path. Improved data on pore water chemistry as well as the solid phase mineralogy is required to develop reactive transport models for these flow systems. Such models would allow further investigation of the importance of tidal and subtidal pumping as a mechanism for exporting dissolved chemicals across the sediment-water interface.
Laura L Govers - One of the best experts on this subject based on the ideXlab platform.
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linking the morphology and ecology of subtidal soft bottom marine benthic habitats a novel multiscale approach
Estuarine Coastal and Shelf Science, 2020Co-Authors: Sebastiaan Mestdagh, A R Amirisimkooei, Karin J Van Der Reijden, L Koop, Sarah Oflynn, Mirjam Snellen, Christiaan Van Sluis, Laura L GoversAbstract:High-resolution surveying techniques of subtidal soft-bottom seafloor habitats show higher small-scale variation in topography and sediment type than previously thought, but the ecological relevance of this variation remains unclear. In addition, high-resolution surveys of benthic fauna show a large spatial variability in community composition, but this has yet poorly been linked to seafloor morphology and sediment composition. For instance, on soft-bottom coastal shelves, hydrodynamic forces from winds and tidal currents can cause nested multiscale morphological features ranging from metre-scale (mega)ripples, to sand waves and kilometre-scale linear Sandbanks. This multiscale habitat heterogeneity is generally disregarded in the ecological assessments of benthic habitats. We therefore developed and tested a novel multiscale assessment toolbox that combines standard bathymetry, multibeam backscatter classification, video surveying of epibenthos and box core samples of sediment and macrobenthos. In a study on the Brown Bank, a Sandbank in the southern North Sea, we found that these methods are greatly complementary and allow for more detail in the interpretation of benthic surveys. Acoustic and video data characterised the seafloor surface and subsurface, and macrobenthos communities were found to be structured by both Sandbank and sand wave topography. We found indications that acoustic techniques can be used to determine the location of epibenthic reefs. The multiscale assessment toolbox furthermore allows formulating recommendations for conservation management related to the impact of sea floor disturbances through dredging and trawling.
Dominic E. Reeve - One of the best experts on this subject based on the ideXlab platform.
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Investigation of deep sea shelf Sandbank dynamics driven by highly energetic tidal flows
Marine Geology, 2016Co-Authors: Antonia Chatzirodou, Harshinie Karunarathna, Dominic E. ReeveAbstract:Abstract In this paper we describe a numerical modelling study carried out to investigate the prevailing sediment dynamics of two large Sandbanks located at a site designated for future development of tidal stream energy extraction, in the Inner Sound Channel of Pentland Firth, Scotland, UK. A calibrated and validated 3D Delft3D hydrodynamic model covering Pentland Firth channel was combined with a morphodynamic model. The sea bed changes occurring around the Sandbanks during a period of two spring-neap tidal cycles are described and discussed in detail. It was found that both Sandbanks, which are located in a deep shelf region (depths > 18 m), are morphodynamically active and their existence and integrity are strongly linked with the existing hydrodynamic regime.
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Morphodynamic behaviour of a nearshore Sandbank system: The Great Yarmouth Sandbanks, U.K.
Marine Geology, 2008Co-Authors: Jose M. Horrillo-caraballo, Dominic E. ReeveAbstract:Offshore Sandbanks provide a major source of sediment and may have a significant impact on the evolution of the nearby shoreline. The prediction of Sandbank evolution is extremely important for coastal engineers and managers. However, it is evident from the existing literature that additional investigations into the evolution of offshore Sandbanks are needed. The present paper describes qualitative and quantitative investigations of the evolution of the Great Yarmouth Sandbanks. A depth-integrated tidal model has been used to calculate the residual currents arising from the sequence of Sandbank configurations observed from historic charts over a 150 year period. Tidal simulations were performed with a nested model covering the Southern North Sea and Great Yarmouth zone (east coast of the UK). Simulations were carried out for a period corresponding to several spring-neap cycles to calculate residual currents and were compared directly with field measurements. Model calibration and validation was performed against published tidal elevation charts based on many observations, tidal predictions and field measurements. These detailed comparisons have been used to determine the errors in the model predictions. Overall, the performance of the model is considered good. The model output has been analysed to investigate qualitative links between the Sandbank configuration and the residual flow statistics. The results demonstrate that the long-term changes in Sandbank morphology can be explained from the pattern of tidal residual currents.
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Statistical analysis and forecasts of long-term Sandbank evolution at Great Yarmouth, UK
Estuarine Coastal and Shelf Science, 2008Co-Authors: Dominic E. Reeve, Jose M. Horrillo-caraballo, Vanesa MagarAbstract:Abstract A data-driven model has been developed to analyse the long-term evolution of a Sandbank system and to make ensemble predictions in a period of 8 years. The method uses a combination of empirical orthogonal function (EOF) analysis, (to define spatial and temporal patterns of variability), jack-knife resampling, (to generate an ensemble of EOFs), a causal auto-regression technique, (to extrapolate the temporal eigenfunctions), and straightforward statistical analysis of the resulting ensemble of predictions to determine a ‘forecast’ and associated uncertainty. The methodology has been applied to a very demanding site which includes a curved shoreline and a group of mobile nearshore Sandbanks. The site is on the eastern coast of the UK and includes the Great Yarmouth Sandbanks and neighbouring shoreline. A sequence of 33 high quality historical survey charts reaching back to 1848 have been used to analyse the patterns and to predict morphological evolution of the Sandbank system. The forecasts demonstrate an improved skill relative to an assumption of persistence, but suffer in locations where there are propagating features in the morphology that are not well-described by EOFs.
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Eigenfunction Analysis of Decadal Fluctuations in Sandbank Morphology at Gt Yarmouth
Journal of Coastal Research, 2001Co-Authors: Dominic E. Reeve, Neil ThurstonAbstract:Empirical orthogonal function (EOF) analysis (or Principal component analysis) has become an established technique for investigating interannual beach level fluctuations. In this paper, EOF techniques are applied to a series of historical bathymetric surveys of a nearshore Sandbank system in order to investigate their long term morphological behaviour. The Sandbanks cover an area approximately 30km alongshore and 10km cross-shore and lie ~2 kilometres offshore from Gt. Yarmouth which is situated on the East Coast of the UK. Geographical Information Systems (GIS) techniques were used to create digital models of the seabed from historic survey data. The models were analysed using EOF and GIS methods to identify any trends or cyclic behaviour in the morphology of the Sandbanks. Evidence of changes in gross Sandbank configuration, long term trends and potentially recurrent behavior have been identified, and their significance for current coastal management practice are discussed.
