678 results on '"SEDIMENT transport"'
Search Results
102. Effects of bottom trawling on the Ebro continental shelf sedimentary system (NW Mediterranean).
- Author
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Palanques, Albert, Puig, Pere, Guillén, Jorge, Demestre, Montserrat, and Martín, Jacobo
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DREDGING (Fisheries) , *SEDIMENTATION & deposition , *DISSOLVED organic matter , *HYDROGRAPHY , *SEDIMENT transport , *TURBIDITY - Abstract
Abstract: A monitoring effort to address the physical effects of bottom trawling was conducted on the Ebro prodeltaic mud belt during the RESPONSE project. The monitoring was carried out for 14 months covering periods of different trawling intensities and a close season for the trawling fleet. The seabed morphology was studied by side-scan sonar and sediment texture and organic carbon content were analysed. Suspended sediment vertical distribution was recorded by CTD+turbidity hydrographic profiles and sediment transport was computed using time series from moored turbidimeters and current meters. The results show that the seabed of the fishing ground is strongly affected by scraping and ploughing induced by bottom trawling. Part of the finer fraction of the prodeltaic mud resuspended by trawling is winnowed, increasing the silt content of the settling sediment and also near-bottom turbidity. Sediment resuspended by trawling is incorporated in the bottom nepheloid layer and transported across- and along-shelf, increasing sediment fluxes. Trawling also induces an increase in the organic carbon content in the bottom sediment. All these effects induced by trawling have occurred during the last few decades, changing natural conditions in the fishing ground. [Copyright &y& Elsevier]
- Published
- 2014
- Full Text
- View/download PDF
103. Impact of winter storms on sediment erosion in the Rhone River prodelta and fate of sediment in the Gulf of Lions (North Western Mediterranean Sea).
- Author
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Dufois, François, Verney, Romaric, Le Hir, Pierre, Dumas, Franck, and Charmasson, Sabine
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WINTER storms , *EROSION , *SEDIMENT transport , *WATER waves - Abstract
Abstract: In this study a three-dimensional sediment transport model was developed. The model accounts for both current and wave forcing on the sediment and was implemented over the Gulf of Lions. A two-way nesting technique was used to focus on the Rhone River prodelta which is considered as a sink for riverine sediment. In addition, to understand the resuspension of trapped sediment over the Rhone prodelta, an in situ experiment, called SCOPE, was conducted during the winter 2007–2008. The experiment consisted of measuring hydro-sedimentary parameters using a mooring station comprising a current profiler (ADCP) and an altimeter (acoustic transducer) located in the eastern part of the Rhone prodelta. The three-dimensional transport model was validated using these data, and used to investigate the effect of sediment dynamics at the prodelta and Gulf of Lions scale. Both modelling and data analysis highlighted the impact of the two strong storms from the south-east which characterised the experimental period. Erosion of bed material (about 2cm) and an increase in suspended material (up to about 50–100mg/l) in the water were the result of each storm as recorded at the mooring station. The erosion capacity due to waves, combined with a strong current, due to both wind and wave forcing, resulted in strong south-westward export over the whole prodelta. Each storm was responsible for an off-prodelta export estimated at around 2.1Mt. This study demonstrates that the Rhone River sediments trapped over the Rhone prodelta are subject to strong resuspension during episodic events. [Copyright &y& Elsevier]
- Published
- 2014
- Full Text
- View/download PDF
104. Process-based modelling of bank-breaking mechanisms of tidal sandbanks
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Suzanne J.M.H. Hulscher, T.J. van Veelen, Pieter C. Roos, and Marine and Fluvial Systems
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Nonlinear morphodynamics ,010504 meteorology & atmospheric sciences ,Flow (psychology) ,Tidal sandbanks ,Geology ,Mechanics ,Transient evolution ,Aquatic Science ,010502 geochemistry & geophysics ,Oceanography ,Rotation ,01 natural sciences ,Deposition (geology) ,Current (stream) ,Process-based modelling ,Orientation (geometry) ,Wind wave ,Bank-breaking mechanisms ,S-shape ,Sediment transport ,Physics::Atmospheric and Oceanic Physics ,0105 earth and related environmental sciences ,Bed load - Abstract
To improve our understanding of transient longterm plan view dynamics of tidal sandbanks, we have developed a nonlinear idealized morphodynamic model. Current mechanisms describe how an isolated bank breaks into multiple banks, but they lack process-based support.We aim to provide such support with the inclusion of nonlinear interactions between tidal flow, topography, Coriolis and bottom friction in the hydrodynamic formulation and depthdependent wind wave stirring and slope effects in the bedload sediment transport formulation. We distinguish three generic evolution paths. (I) Rotation of the bank ends towards the preferred angle of deposition and separate growth, causing bank-breaking. (II) Rotation of the bank ends towards the preferred angle of deposition and central growth, triggering an S-shape. (III) Straight outline and central growth, triggering an S-shape, but much faster than path II. In addition to the path-dependent dynamics, the banks also experience pattern expansion, elongation and amplitude growth. Bank-breaking (path I) requires two condition. The initial bank may not have an orientation in the angle of preferred deposition and must be sufficiently long to trigger separate growth. Alternatively, the nonlinear flow response to an elevated centre triggers evolution towards an S-shape. The proposed paths are consistent with mechanisms in Huthnance (1982b) and Smith (1988). The characteristic final Sshape shows resemblance to meandering banks in the North Sea.
- Published
- 2018
105. Morphodynamic responses of Caofeidian channel-shoal system to sequential large-scale land reclamation
- Author
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Zhichao Dong, Liming Zuo, Honglin Song, Xuejian Han, Yue Ma, Huidi Liang, Youcai Liu, Cuiping Kuang, and Maitane Olabarrieta
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Hydrology ,geography ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Flood myth ,Shoal ,Sediment ,Geology ,Aquatic Science ,010502 geochemistry & geophysics ,Oceanography ,Inlet ,01 natural sciences ,Siltation ,Land reclamation ,Environmental science ,Sediment transport ,Beach morphodynamics ,0105 earth and related environmental sciences - Abstract
Understanding the influence of reclamation projects on a channel-shoal system is of vital importance regarding the management and sustainable development of coastal regions. This study addresses the issue by applying a processes-based morphological model to investigate hydro-morphodynamics of Caofeidian channel-shoal in five phases of large-scale reclamation project from year 2003 to 2020. Forced by the spring-neap tidal cycles and representative waves, the model considers multi-fraction sediment dynamics regarding strong spatial variations of mud-sand mixtures in the study area. The numerical results demonstrate that sequential large-scale land reclamation induces continuous loss of sediment volume of tidal flat by reducing its sediment storage capability. The morphodynamics of channels is complex in space and perturbation of reclamation location increases this complexity. For the channels in the western uninterrupted coast, reclamation affects their morphological evolution by damping the long-shore sediment transport. The tidal asymmetry is a key factor in the geomorphology of channels in the eastern inlet-interrupted coast (Laolonggou lagoon-inlet system), which is impacted by the reclamation location. Particularly, reclamation constricting the inlet dampens the flood tidal asymmetry and triggers the deepening of inlet, while tidal flats reclamation in lagoon enhances flood tidal asymmetry which induces the siltation of inlet. The model also reveals different behaviors of multi-fraction sediments, of which the silt with larger settling velocity is subject to the hydrodynamic change induced by reclamation. Although this study is site-specific, the finding provides valuable information for sustainable management of channel-shoal systems in muddy-silt coast under intermittent or sequential land reclamation.
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- 2018
106. Morpho-stratigraphic features of the northern shelf of the Strait of Gibraltar: Tectonic and sedimentary processes acting at different temporal scales
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Miguel Bruno, Francisco J. Lobo, M. Luján, and S. de Castro
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geography ,Bedform ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Continental shelf ,Geology ,Contourite ,Aquatic Science ,010502 geochemistry & geophysics ,Oceanography ,01 natural sciences ,Sedimentary depositional environment ,Horst and graben ,Paleontology ,Basement (geology) ,Sedimentary rock ,Sediment transport ,0105 earth and related environmental sciences - Abstract
The northern shelf of the Strait of Gibraltar adjacent to Camarinal Sill, defined here as the Cape Paloma continental shelf, has been investigated by analyzing a set of geophysical data including multibeam bathymetric images, a side-scan sonar mosaic and high-resolution seismic profiles, and the simulation of water-mass circulation patterns along the northern coastal margin. The aim of the study was to establish the significance of factors determining the evolution of this shallow margin at different temporal scales and to assess the implications for bedform generation in strait settings, taking into account the complex tectonic evolution and the energetic hydrodynamic regime of the strait. Deformed basement rocks are part of the Betic-Rif thrust wedge, western Gibraltar Arc, mainly formed by the materials of the Flysch Complex units and covered by Pliocene to Quaternary post-orogenic deposits. A central high (Bajo de los Cabezos High) is delimited by lateral depressions, that nucleated two major depocentres with distinctive filling histories. The eastern depocentre is controlled by WNW-ESE faults cutting the Cretaceous-Miocene basement rocks; these faults generate horsts and grabens that could have contributed to the opening of the Strait of Gibraltar during the Pliocene. The largest and westernmost depocentre is related to the complete infilling of a shelf palaeovalley. The sediment cover is molded by different fields of submarine dunes and comet marks that indicate the influence of hydrodynamic processes on sediment transport at the coastal margin. The observations in the study area regarding bedform development must be placed into a wider context of strait sediment dynamics. The Cape Paloma continental shelf exhibits both erosional and depositional forms, due to its intermediate location between the strait, mostly dominated by erosional processes, and the Barbate Platform (northwest of the study area), mostly characterized by depositional forms. The long-term evolution of the sediment depocentres in the study area appears to be mainly influenced by the morpho-tectonic configuration of the margin, which in turn was established to a large extent by differential uplifting along the coast. In the shelf east of the central high, the basement horst and graben structure trapped sediments in the physiographic lows and fostered the formation of large-scale sediment banks. In the shelf west of the central high, the occurrence of a major infilled palaeovalley is in agreement with a gentle subsidence trend. The physiographic configuration is also thought to play a major role in defining short-term processes, particularly in confining a cyclonic eddy to the east of the Bajo de los Cabezos High during specific conditions of the tidal cycle. This eddy favors the recirculation of sediments in the coastal margin, as evidenced by small bedform fields that apparently show a wider distribution that the larger-scale, confined sediment banks, due to the instauration of the modern sediment dynamics after the complete shelf flooding. The sediment transport pattern established in the study area seems to be eventually captured by a submarine channel that provide an efficient mechanism for sediment export toward deep-water settings, where an extensive contourite depositional system has been documented.
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- 2018
107. Observations and modelling of shoreline and multiple sandbar behaviour on a high-energy meso-tidal beach
- Author
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Joan Oltman-Shay, Maria V.G. Gonzalez, Kristen D. Splinter, Jantien Rutten, and Robert A. Holman
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Shore ,geography ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Severe weather ,Winter storm ,Sediment ,Shoal ,Geology ,Forcing (mathematics) ,Aquatic Science ,010502 geochemistry & geophysics ,Oceanography ,01 natural sciences ,Submarine pipeline ,Sediment transport ,0105 earth and related environmental sciences - Abstract
This contribution describes 10 years of observed sandbar and shoreline cross-shore position variability at a meso-tidal, high energy, multiple sandbar beach. To examine relationships between the temporal variability in shoreline/sandbar position with offshore wave forcing, a simple equilibrium model is applied to these data. The analysis presented in this paper shows that the equilibrium model is skilled at predicting the alongshore-averaged, time-varying position of the shoreline (R = 0.82) and the outer sandbar position (R = 0.75), suggesting that these end members of the nearshore sediment system are most strongly influenced by offshore wave forcing in a predictable, equilibrium-forced manner. The middle and inner bars are hypothesized to act as sediment transport pathways between the shoreline and the outer bar. Prediction of these more transient features by an equilibrium model was less skilful. Model coefficients reveal that these two end members (outer bar and shoreline) in the sediment system act in opposite directions to changes in the annual offshore wave forcing. During high wave events, sediment is removed from the shoreline and deposited in the nearshore sediment system with simultaneous landward retreat of the shoreline and offshore migration of the outer sandbar. While both end member features have cycles at annual and inter-annual scales, their respective equilibrium response factor differs by almost a factor of 10, with the shoreline responding around an inter-annual mean (ϕ = 1000 days) and the outer bar responding around a seasonal mean (ϕ = 170 days). The model accurately predicts shoreline response to both mild (e.g. 2004/05, 2008/09) and extreme (e.g. 2005/06, 2009/10) winter storms, as well as their summer recovery. The more mobile and dynamic outer sandbar is well-modelled during typical winters. Summer onshore sandbar migration of the outer bar in 2005 and 2006 is under-predicted as the system transitioned between a triple (winter) and double (summer) sandbar system. The changing of the number of bars present in the system is something that this simple model cannot predict. Analysis of the data suggests that this multi-bar system adjusts its cross-shore seasonal movement when there is a significant change in the sediment supply to the system (e.g., nourishment projects, severe storms).
- Published
- 2018
108. The Açu Reef morphology, distribution, and inter reef sedimentation on the outer shelf of the NE Brazil equatorial margin
- Author
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Moab Praxedes Gomes, Helenice Vital, and Luzia Liniane do Nascimento Silva
- Subjects
geography ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Continental shelf ,Atoll ,Geology ,Aquatic Science ,010502 geochemistry & geophysics ,Oceanography ,01 natural sciences ,Paleontology ,Ridge ,Facies ,Sedimentary rock ,Siliciclastic ,Sediment transport ,Reef ,0105 earth and related environmental sciences - Abstract
Submerged reefs, referred to as the Acu Reefs, have been newly observed on both sides of the Acu Incised Valley on the northeastern equatorial Brazilian outer shelf. This study aims to understand the roles of shelf physiography, its antecedent morphologies, and its inter reef sedimentation on the different development stages of the biogenic reef during last deglacial sea-level rise. The data sets consist of side-scan sonar imagery, one sparker seismic profile, 76 sediment samples, and underwater photography. Seven backscatter patterns (P1 to P7) were identified and associated with eleven sedimentary carbonate and siliciclastic facies. The inherited relief, the mouth of the paleo incised valley, and the interreef sediment distribution play major controls on the deglacial reef evolution. The reefs occur in a depth-limited 25–55 m water depth range and in a 6 km wide narrow zone of the outer shelf. The reefs crop out in a surface area over 100 km2 and occur as a series of NW-SE preferentially orientated ridges composed of three parallel ridge sets at 45, 35, and 25 m of water depth. The reefs form a series of individual, roughly linear ridges, tens of km in length, acting as barriers in addition to scattered reef mounds or knolls, averaging 4 m in height and grouped in small patches and aggregates. The reefs, currently limited at the transition between the photic and mesophotic zones, are thinly covered by red algae and scattered coral heads and sponges. Taking into account the established sea-level curves from the equatorial Brazilian northeastern shelf / Rochas Atoll and Barbados, the shelf physiography, and the shallow bedrock, the optimal conditions for reef development had to occur during a time interval (11–9 kyr BP) characterized by a slowdown of the outer shelf flooding, immediately following Meltwater Pulse-1B. This 2 kyr short interval provided unique conditions for remarkable reef backstepping into distinct parallel ridge sets. Furthermore, the Acu Reefs have trapped relict siliciclastic sediments within the three sets of reefs, west of the Acu Incised Valley and adjacent coasts. Lines evidence of easterly nearshore currents carried sediments from the old Acu Incised Valley and adjacent coasts. These incipiently drowned reefs influence the water circulation patterns of the modern shelf system, its carbonate sedimentation, and sediment transport. This study provides a new example of reef occurrence which might be more commonly observed on similar equatorial continental shelves.
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- 2018
109. How do changes in suspended sediment concentration alone influence the size of mud flocs under steady turbulent shearing?
- Author
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Duc Tran, Kyle Strom, and Rachel Kuprenas
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Shearing (physics) ,Flocculation ,Turbidity current ,010504 meteorology & atmospheric sciences ,Turbulence ,Drop (liquid) ,0208 environmental biotechnology ,Geology ,Soil science ,02 engineering and technology ,STREAMS ,Aquatic Science ,Oceanography ,01 natural sciences ,020801 environmental engineering ,Settling ,Environmental science ,Sediment transport ,0105 earth and related environmental sciences - Abstract
Modeling the size and settling velocity of sediment under the influence of flocculation is crucial for the accurate prediction of mud movement and deposition in sediment transport modeling of environments such as agricultural streams, large coastal rivers, estuaries, river plumes, and turbidity currents. Yet, collecting accurate and high resolution data on mud flocs is difficult. As a result, models that account for the influence of flocculation on mud settling velocity are based on sparse data that often present non-congruent relationship in floc properties with basic influencers of flocculations such as suspended sediment concentration. This study examines the influence of suspended sediment concentration on floc size populations within a turbulent suspension. Specifically, the work investigates: (1) the relationship between the equilibrium floc size and suspended sediment concentration under conditions of steady concentration and turbulent shearing; and (2) the speed at which mature flocs adapt to an unsteady drop in the concentration when turbulent shear is constant. Two sets of experiments were used to investigate the target processes. All work was conducted in laboratory mixing tanks using a floc camera and a newly developed image acquisition method. The new method allows for direct imaging and sizing of flocs within turbulent suspensions of clay in concentrations ranging from 15 to 400 mg/L, so that no transfer of the sample to another settling column or imaging tank is needed. The primary conclusions from the two sets of experiments are: (1) that the equilibrium floc size in an energetic turbulent suspension is linearly and positively related to concentration over the range of C = 50 to 400 mg/L, yet with a smaller-than-expected slope based on previous data and models from low-energy environments; and (2) that floc sizes decrease quickly (with a time lag on the order of 1–15 min) to time-varying decreases in concentration at turbulent shearing of G = 50 s - 1 . Overall the data illustrate that equilibrium floc size is a positive function of concentration, but that the rate of increase is weaker than expected. The data also suggest that approximating the size or settling velocity of some muds with a simple equilibrium model might be appropriate if the time steps of interest are on the order of 10 min or larger. The data also shows the importance of calibrating historic mud settling velocity equations for accurate predictions.
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- 2018
110. Recent coarsening of sediments on the southern Yangtze subaqueous delta front: A response to river damming
- Author
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Benwei Shi, Xiangxin Luo, Y. Meng, Shilun Yang, C. S. Wu, Kehui Xu, and Haifei Yang
- Subjects
Hydrology ,Delta ,Tidal range ,010504 meteorology & atmospheric sciences ,Sorting (sediment) ,Sediment ,Geology ,Aquatic Science ,010502 geochemistry & geophysics ,Oceanography ,01 natural sciences ,Current (stream) ,Wave height ,Erosion ,Sediment transport ,0105 earth and related environmental sciences - Abstract
After more than 50,000 dams were built in the Yangtze basin, especially the Three Gorges Dam (TGD) in 2003, the sediment discharge to the East China Sea decreased from 470 Mt/yr before dams to the current level of ~140 Mt/yr. The delta sediment's response to this decline has interested many researchers. Based on a dataset of repeated samplings at 44 stations in this study, we compared the surficial sediment grain sizes in the southern Yangtze subaqueous delta front for two periods: pre-TGD (1982) and post-TGD (2012). External factors of the Yangtze River, including water discharge, sediment discharge and suspended sediment grain size, were analysed, as well as wind speed, tidal range and wave height of the coastal ocean. We found that the average median size of the sediments in the delta front coarsened from 8.0 µm in 1982 to 15.4 µm in 2012. This coarsening was accompanied by a decrease of clay components, better sorting and more positive skewness. Moreover, the delta morphology in the study area changed from an overall accretion of 1.0 cm/yr to an erosion of − 0.6 cm/yr. At the same time, the riverine sediment discharge decreased by 70%, and the riverine suspended sediment grain size increased from 8.4 µm to 10.5 µm. The annual wind speed and wave height slightly increased by 2% and 3%, respectively, and the tidal range showed no change trend. Considering the increased wind speed and wave height, there was no evidence that the capability of the China Coastal Current to transport sediment southward has declined in recent years. The sediment coarsening in the Yangtze delta front was thus mainly attributed to the delta's transition from accumulation to erosion which was originally generated by river damming. These findings have important implications for sediment change in many large deltaic systems due to worldwide human impacts.
- Published
- 2018
111. Near bed suspended sediment flux by single turbulent events
- Author
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Eva Kwoll, Seyed Mohammad Amirshahi, and Christian Winter
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Physics ,010504 meteorology & atmospheric sciences ,Meteorology ,Turbulence ,0208 environmental biotechnology ,Sediment ,Geology ,02 engineering and technology ,Reynolds stress ,Aquatic Science ,Oceanography ,Atmospheric sciences ,01 natural sciences ,020801 environmental engineering ,Physics::Fluid Dynamics ,Flux (metallurgy) ,Turbulence kinetic energy ,Acoustic Doppler velocimetry ,Sediment transport ,Physics::Atmospheric and Oceanic Physics ,Intensity (heat transfer) ,0105 earth and related environmental sciences - Abstract
The role of small scale single turbulent events in the vertical mixing of near bed suspended sediments was explored in a shallow shelf sea environment. High frequency velocity and suspended sediment concentration (SSC; calibrated from the backscatter intensity) were collected using an Acoustic Doppler Velocimeter (ADV). Using quadrant analysis, the despiked velocity time series was divided into turbulent events and small background fluctuations. Reynolds stress and Turbulent Kinetic Energy (TKE) calculated from all velocity samples, were compared to the same turbulent statistics calculated only from velocity samples classified as turbulent events ( Re events and TK E events ). The comparison showed that Re events and TKE events was increased 3 and 1.6 times, respectively, when small background fluctuations were removed and that the correlation with SSC for TKE could be improved through removal of the latter. The correlation between instantaneous vertical turbulent flux ( w ′ ) and SSC fluctuations ( SSC ′ ) exhibits a tidal pattern with the maximum correlation at peak ebb and flood currents, when strong turbulent events appear. Individual turbulent events were characterized by type, strength, duration and length. Cumulative vertical turbulent sediment fluxes and average SSC associated with individual turbulent events were calculated. Over the tidal cycle, ejections and sweeps were the most dominant events, transporting 50% and 36% of the cumulative vertical turbulent event sediment flux, respectively. Although the contribution of outward interactions to the vertical turbulent event sediment flux was low (11%), single outward interaction events were capable of inducing similar SSC ′ as sweep events. The results suggest that on time scales of tens of minutes to hours, TKE may be appropriate to quantify turbulence in sediment transport studies, but that event characteristics, particular the upward turbulent flux need to be accounted for when considering sediment transport on process time scales.
- Published
- 2018
112. Tectonic and eustatic control on channel formation, erosion, and deposition along a strike-slip margin, San Diego, California, USA
- Author
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Neal W. Driscoll, Jillian M. Maloney, and Lana G Graves
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geography ,geography.geographical_feature_category ,Sediment ,Geology ,Aquatic Science ,Fault (geology) ,Oceanography ,Strike-slip tectonics ,Deposition (geology) ,Paleontology ,Passive margin ,Subaerial ,Sediment transport ,Marine transgression - Abstract
Active margins are characterized by small mountainous streams, which contribute significantly to global sediment transport from land to sea. The response of active margin streams to base-level change due to eustatic sea-level cycles is less well understood than passive margin streams, which form the basis of most sequence stratigraphic models. The southern California margin offers an opportunity to investigate the evolution of small mountainous streams along a strike-slip margin over a sea-level cycle. High-resolution CHIRP data were examined from the Silver Strand littoral cell in San Diego, California, which is located in a region of localized transtension associated with a step in the Newport Inglewood-Rose Canyon fault zone. CHIRP data reveal active faulting on multiple fault strands, a subaerial surface likely formed from MIS 4 to MIS 2, fluvial incision associated with MIS 2, and the transgressive surface formed during sea-level rise from MIS 2 to MIS 1. Interpreted acoustic units observed in the data include the Cretaceous outcropping Rosario Group, Miocene to Pliocene prograding clinoforms, subaerial deposits associated with MIS 4 to MIS 3, channel fill and transgressive lag deposits formed during the most recent sea-level transgression, and Holocene marine sediment. The CHIRP data examined in this study build upon findings from other seismic studies, sediment cores, trenching, and outcrops that have been examined on- and offshore San Diego. Combined, these data suggest that the fault-controlled pull-apart basin formed in this region influences sediment transport and deposition throughout the last glacial cycle. During highstand, the fault-controlled wide shelf, subsiding basin, and circulation patterns result in trapping of sediment in San Diego Bay and nearshore, cutting off sediment delivery to the deep-sea. During lowstand, the fault controlled morphology creates a drainage divide that deflects streams to the south which lengthens and reduces slope of the drainage pathways. These findings are in contrast to observations in other fault controlled littoral cells along the southern California margin where sediment delivery to the deep-sea remains active during highstand, highlighting the variability of stream response to base-level change along active margins.
- Published
- 2021
113. Relevance of historical nautical charts in sediment dynamic assessments on a high energy temperate shelf
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Chloe Leach, Rafael Cabral Carvalho, David M. Kennedy, Daniel Ierodiaconou, and Mary Young
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Shore ,geography ,geography.geographical_feature_category ,Nautical chart ,Geology ,Aquatic Science ,Oceanography ,Deposition (geology) ,Dredging ,Aerial photography ,Bathymetry ,Coastal management ,Sediment transport - Abstract
Shipping has been of critical importance for European colonisation and development in many parts of the world. These required the charting of safe shipping approaches, which gradually evolved into our modern nautical charts. Colonial charts provide an unique historical perspective and are of global scientific relevance for understanding the sedimentary dynamics and quantifying human alterations in the coastal zone. In areas where significant changes occurred, volumetric differences can be calculated providing a relative degree of certainty, whereas in broader areas the uncertainties are greater than the volumetric changes. In this study we looked at bathymetric and shoreline change since the mid 19th century based on information available on historical charts. We compared lead line soundings to contemporary Multibeam Echosounding (MBES) bathymetry and Light Detection and Ranging (LiDAR) data at four locations along the swell-dominated west coast of Victoria, Australia. Shorelines extracted from these charts were also used in conjunction with aerial photography and satellite imagery to show how changes following the construction of coastal infrastructure has affected natural processes of sediment transport and had long-term impacts on the adjacent coastline. We demonstrate how the construction of the port of Portland breakwaters trapped approximately 1,450,000 ± 550,000 m3 of sand, an equivalent amount of material eroded from the downdrift coastline; the closing of the southwest passage and the construction of the training walls of Port Fairy reduced the availability of sand to East Beach; the viaduct and breakwater construction at Warrnambool created a sand trap that accumulated 1,028,181 ± 575,613 m3 of sand, which made the shoreline prograde fast and reduced the harbour depth; and the dredging required to maintain the port of Apollo Bay created new land with the deposition of 291,712 ± 265,203 m3 of sand adjacent to the disposal area but caused erosion further downdrift.
- Published
- 2021
114. Hydrodynamics, sediment transport, and water quality of two contrasting dredge pits on the Louisiana shelf
- Author
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Robert Bales, Kanchan Maiti, Kehui Xu, Guandong Li, and Haoran Liu
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Hydrology ,Water mass ,Water column ,Environmental science ,Sediment ,Geology ,Submarine pipeline ,Storm ,Water quality ,Aquatic Science ,Oceanography ,Sediment transport ,Seabed - Abstract
Sediment is needed for coastal restoration in Louisiana and is often excavated offshore from mud-capped and sandy dredge pits. To better understand the post-dredging effects on hydrodynamics, sediment transport, and water quality, two tripods were deployed at sandy Caminada pit and mud-capped Sandy Point pit in summer 2018 and 2019, respectively, and profiling data were collected during a total of 34 casts in 2018 and 2019. The results reveal similarities and differences between pit and adjacent waters and are used to compare two pits. Tropical storm events were observed during each deployment which provided insight concerning the effects these storms have on dredge pits. Water masses inside Caminada pit were relatively stable and sluggish which resulted from the pit's deep depth and location. Sandy Point dredge pit was highly impacted by the Mississippi River plume due to its proximity, which effectively stratified the water column in summer 2019, affecting ventilation near the seabed. Strong winds, taller waves, higher shear stresses, elevated turbidity and increased dissolved oxygen were generally found during the passages of tropical storms over both pits. Comparing with outside tripod station, the inside station of Caminada pit experienced a longer duration of low dissolved oxygen in bottom waters. Water mass movements and oxygen consumption from resuspension of pit bottom are likely key mechanisms driving oxygen dynamics. Our results provide observational data for future studies on post-dredging morphologic change, marine communities, and oxygen dynamics of dredge pits on the Louisiana shelf and other coastal dredge areas worldwide.
- Published
- 2021
115. Roles of advection and sediment resuspension-settling in the turbidity maximum zone of the Changjiang Estuary, China
- Author
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Yuan Li, Ya Ping Wang, Qingguang Zhu, Hao Wu, and Ajay B. Limaye
- Subjects
geography ,geography.geographical_feature_category ,Advection ,Drainage basin ,Sediment ,Geology ,Estuary ,Aquatic Science ,Oceanography ,Settling ,Erosion ,Environmental science ,Turbidity ,skin and connective tissue diseases ,Sediment transport - Abstract
Understanding mechanisms of suspended sediment transport in estuaries is critical for predicting estuarine geomorphic evolution, managing navigation and fisheries, and monitoring the health of estuarine ecosystems. Yet monitoring suspended sediment transport in estuaries is challenging due to the influences of both riverine and marine processes, and traditional in situ observations cannot directly differentiate contributions of multiple physical processes (e.g., advection and resuspension/settling). Here we investigated the effects of advection and sediment resuspension/settling on suspended sediment transport within the turbidity maximum zone at the South Passage of the Changjiang Estuary. The Changjiang catchment is notable for its long history of intensive human activities including dam construction, which has caused significant geomorphic changes along the river and its estuary. We conducted synchronized hydrodynamic measurements at three monitoring stations along the river channel and imported time series of current velocity and suspended sediment concentration (SSC) at these stations into a simplified, one-dimensional box model to separate in situ SSC into advection- and resuspension/settling-induced components. Our results show that high SSC consisted of a background SSC of 0.93 kg m−3 maintained by advection that contributes to 97.9% of the total average SSC, and a fluctuating SSC controlled by sediment resuspension and settling. The background SSC was higher than the fluctuating SSC at all times, indicating that sediment advection was stronger than local sediment resuspension in the river channel. We compared time series of SSC components, bed-level change and suspended sediment transport rates in the South Passage with previous observations in the nearby subaqueous delta, and found that the South Passage of the Changjiang Estuary experienced net accretion during summer because of the relatively high background SSC maintained by advection of fluvial sediment. In contrast, the subaqueous delta experienced erosion due to strong local sediment resuspension and weak sediment delivery by advection. Our results highlight the spatial variability in the magnitudes of sediment advection and resuspension/settling as key drivers of geomorphic changes of deltas and estuarine channels in response to sediment reduction caused by upstream dam construction.
- Published
- 2021
116. A physically-modified spectral model for prediction of sand wave migration
- Author
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Jiang Wenbin, Lin Mian, and Cao Gaohui
- Subjects
Bedform ,Scale (ratio) ,Geology ,Soil science ,Aquatic Science ,Oceanography ,Stability (probability) ,Sand wave ,Typhoon ,Long period ,Spectral analysis ,Sediment transport ,Physics::Atmospheric and Oceanic Physics - Abstract
Sand waves are rhythmic bedforms existing in shallow seas and typically induced by the interaction of currents and topography. In this study, a physically-modified spectral model for simulating small-scale sand wave migration is proposed by integrating non-linear spectral analysis, typhoon wind field and sediment transport theory. We introduce a power-average velocity to modify the spectral analysis, which gives explicit physical meanings to the morphological method and improves its stability and reliability. Then, the effects of typical and extreme weather are decoupled and the short-term migration caused by typhoons is removed from the total migration. Results show that a single typhoon will cause large sand wave migration, but the migration caused by multiple typhoons in a long period of time will cancel each other out in the measured data. Moreover, the bilateral reverse migration trend of one-group sand waves in a small scale (∼3 km) is captured, which is consistent with field data and previous researches. Compared with existing researches, the proposed model is able to identify the influences of extreme conditions on sand wave migration and provide a feasible and high efficiency way to predict sand wave migration when the available hydrodynamic data is limited.
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- 2021
117. Quantitative geochemistry as a provenance indicator for surface sediments in the north Jiangsu radial sand ridges (NJRSR) in the South Yellow Sea, East China
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Qing Liu, Wenting Shi, Jicheng Cao, Yifei Zhao, and Min Xu
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Current (stream) ,Provenance ,Continental margin ,Terrigenous sediment ,Geochemistry ,Sediment ,Geology ,Sedimentary rock ,Aquatic Science ,Silt ,Oceanography ,Sediment transport - Abstract
The North Jiangsu radial sand ridges (NJRSR) are typical sedimentary bodies distributed along the continental margin of China. Historically, the Yellow and Changjiang Rivers transported terrigenous sediments into the sea, depositing them in the study area. However, the provenance of the sediments in the NJRSR is not conclusively understood. In this study, the grain sizes and major elemental geochemistry of 33 surface sediment samples were investigated to discern the provenance of the sediments in the NJRSR. The results indicate that the sedimentary components are primarily silt (53.60%), sand (35.60%), and clay (10.80%). The grain size (Mz) of the NJRSR coastal surface sediments was 2.5–8.5 Φ, with an average of 5.1 Φ, and had high SiO2 and Al2O3 contents (on average, SiO2 and Al2O3 accounted for >68%). The statistical results of the geochemical analyses were reflected in the control on sediment elemental components by riverine transport in the study area, which were then used to divide the study area into three sedimentary provinces. Elemental ratios (Mg/Al, Fe/Al, K/Al) were used to discern the provenance of the NJRSR surface sediments, indicating that the inputs from the Yellow and Yangtze rivers are the dominant sediment sources in the study area. Using the inverse model, a quantitative estimation revealed that average sediment contributions of the Abandoned Yellow and Yangtze rivers were 67.4% and 32.6%, respectively. In addition, modern current systems play an important role in controlling sediment transport from the Abandoned Yellow and Yangtze rivers to the NJRSR. This study demonstrates the importance of geochemistry for quantitatively determining the provenance of the NJRSR, which is especially useful for quantitatively investigating Earth surface processes in global shelf-marginal seas.
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- 2021
118. Hydrodynamic and sediment transport modelling in the Pearl River Estuary and adjacent Chinese coastal zone during Typhoon Mangkhut.
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Yang, Yun, Guan, Weibing, Deleersnijder, Eric, and He, Zhiguo
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TYPHOONS , *SEDIMENT transport , *COASTS , *WAVE-current interaction , *ORBITAL velocity , *SUSPENDED sediments - Abstract
Typhoon Mangkhut, in 2018, was one of the worst typhoons in recent history that has made landfall in the Pearl River Estuary (PRE) China. It swept along the coast of Guangdong Province, causing severe damage to many areas and affecting more than one million people. To study the hydrodynamics and sediment dynamics in different sub-regions of the PRE during the typhoon, we implemented a 3D wave-current-sediment coupled ocean model based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM). A series of numerical experiments were conducted to study the variation in suspended sediment concentration (SSC) under different external forcing mechanisms, including tides, waves, river discharges and strong winds. Moderate Resolution Imaging Spectroradiometer (MODIS) images, collected after the typhoon landed, were used to assess the simulation results. We found that in the area where the water depth was shallower than 20 m, the increased total suspended sediment concentration was dominated by the regional erosion of fine sediment which was caused by near-bottom wave orbital velocity and bottom shear stress, especially at the four western outlets and along the western coast of the PRE. For the deeper coastal zone, at depths between 20 and 30 m offshore of the western coast, the high total SSC resulted from the contribution of fine sediment transported from the upper and middle estuaries by southwestward advection. A small amount of sand eroded and migrated locally, owing to the corresponding increase in the bottom shear stress. The variation in SSC during the typhoon had a time lag compared with other dynamic conditions, resulting in a maximum 1 h delayed response after the typhoon made landfall. Based on the results of the sensitivity experiments under the same typhoon conditions, we found that, as a result of wave-enhanced bottom shear stress, the SSC values associated with wave-current interaction were significantly higher, by as much as 1.5 and 1.3 times those of simulations without the waves at the surface and near-bottom layers, respectively. This indicated that the sediment resuspension induced by waves was important during typhoon landfall. However, the strong wind-induced current was weakened owing to the joint effect of wave radiation stress and enhanced bottom stress; thus, considering wave effects, the magnitude of the resulting sediment transport rate was smaller than that without considering wave effects, especially in shallow waters. During the approach, landfall and retreat of Typhoon Mangkhut, Lingding Bay experienced two transitions from a slight state of sediment loss to a relatively high state of sediment gain, followed by an even greater state of sediment loss. During an M 2 tidal cycle after Typhoon Mangkhut made landfall, the net seaward suspended load sediment transport in Lingding Bay reached 0.37 megatonnes. For the typhoon period covering three M 2 tidal cycles, the overall state of suspended load sediment transport in Lingding Bay was sediment loss, with the magnitude of sediment loss on the west side being larger than that of sediment gain on the east side. The magnitude of the overall sediment loss in Lingding Bay would be enlarged unrealistically by approximately 73% if the wave effects were excluded. In each period, the extent of sediment transport between the two sides of the bay was strengthened by the wave-current interaction under typhoon conditions. We determined that the net effect of wave-current interaction and strong wind-induced currents under typhoon conditions was a key factor for erosion, which resulted in SSC-influenced areas being approximately 8.7 and 4.4 times as large as those under normal weather conditions for the surface and near-bottom layers, respectively. The turbidity maximum moved from the West Shoal of Lingding Bay under normal weather to the Modaomen Estuary, which was the most affected area during the typhoon when wave effects were considered. We also showed that the relative location of the typhoon path with respect to the estuary had a more significant effect on erosion than the intensity of the typhoon. ● A 3D wave-current-sediment coupled ocean model was set up and validated to simulate sediment transport during Typhoon Mangkhut. ● Waves were primarily responsible for resuspension of both fine sediment and sand during the event. ● Strong wind-induced currents were weakened owing to the joint effect of wave radiation stress and enhanced bottom stress. ● Turbidity maximum moved westward along the path of the typhoon. [ABSTRACT FROM AUTHOR]
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- 2022
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119. Textures, provenances, and transport patterns of sediment on the inner shelf of the East China Sea.
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Cong, Jingyi, Zhang, Yong, Hu, Gang, Mi, Beibei, Kong, Xianghuai, Xue, Biying, Ning, Ze, and Yuan, Zhongpeng
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SEDIMENT transport , *SEDIMENTATION & deposition , *COASTAL sediments , *WATER depth , *MINERAL analysis , *CLAY minerals , *ANALYSIS of river sediments , *ESTUARIES - Abstract
Although many studies have been carried out on the mud wedge in the inner shelf of the East China Sea (ECS), the sediment sources and seismic textures of the mud wedge are still unclear and remain controversial. Therefore, this study utilizes grain size and clay mineral analyses of surficial sediment samples and high-resolution sub-bottom seismic profile interpretation to identify the provenance and texture of these mud wedge sediments. The results show that sediment sources on the inner shelf of the East China Sea are mixed by the Yangtze River, small mountainous rivers along the Zhejiang-Fujian, and western Taiwan coasts. The sediments derived from the Ou and Min rivers are mainly concentrated in the shallowest shelf area within a water depth of 20 m which is located north of the Min River Estuary. Their contribution to the estuaries and coastal areas reflects the different sediment transport patterns of the two small mountainous rivers. The thickness of the Holocene mud wedge between Hangzhou Bay and Taizhou Bay is greater than 20 m at a water depth of approximately 10 m. The cause of this phenomenon is possibly related to the presence of the Zhoushan Islands, which are distributed along the Zhejiang coast and appear to effectively trap sediments from the Yangtze River, facilitating sediment accumulation. The findings of this study will help improve our understanding of the sediment transport and deposition mechanism of deposition on the inner shelf of the East China Sea. • We produced grain size and clay mineral analyses and high-resolution sub-bottom seismic profile interpretation. • Sediments derived from small mountainous rivers are mainly concentrated in the shallowest shelf area. • The thickness of the Holocene mud wedge is locally greater than 20 m in the shallowest shelf area. [ABSTRACT FROM AUTHOR]
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- 2022
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120. Isolating the role of shore-parallel sediment transport in continental shelf build-up
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Uri Schattner
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0106 biological sciences ,Shore ,geography ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Continental shelf ,010604 marine biology & hydrobiology ,Sediment ,Geology ,Aquatic Science ,Oceanography ,01 natural sciences ,Paleontology ,Continental margin ,Sedimentary rock ,Sediment transport ,Sedimentary budget ,0105 earth and related environmental sciences ,Marine transgression - Abstract
While the role of shore-parallel transport is extensively documented, its contribution to source-to-sink and provenance studies is often overshadowed by tectonic variations, sediment influx, and sea-level change. To isolate the sedimentary contribution of shore-parallel transport, this study examines the central Levant continental margin (easternmost Mediterranean), where the other three factors are well constrained. New high-resolution multibeam and seismic reflection data collected in a dedicated cruise aboard the RV Bat Galim are used for analyzing current-indicating morphologies formed since the Last Glacial Maximum (LGM). Results show six different current indicators, which attest to the ongoing shore-parallel Levant Jet System (LJS) flow. Throughout the post-LGM transgression and coastal retreat, the LJS shifted landward to flow over the submerged shelf and upper slope. This shift, along with periodic/seasonal alterations in the flow intensity, regulated the sediment transport, distribution and erosion, along and across the margin, during the lowstand, transgression, and highstand conditions. This case study emphasizes the central role of shore-parallel currents in regulating the sedimentary budget. It provides a reference case for continental margins where the shore-parallel role is obscured by more dominant tectonic-eustatic-sedimentation factors.
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- 2021
121. The seabed boundary layer beneath waves opposing and following a current.
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Holmedal, Lars Erik, Johari, Jona, and Myrhaug, Dag
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OCEAN bottom , *OCEAN currents , *OCEAN waves , *TURBULENCE , *HYDRODYNAMICS , *WATER depth , *COMPUTER simulation - Abstract
Abstract: The effect of streaming on the sea bed boundary layer flow beneath combined waves and current has been investigated for waves following and opposing a current, taking both linear waves and second order Stokes waves into account. This flow results from an interaction between the classical wave–current seabed boundary layer mechanism and two different streaming mechanisms. The classical wave–current seabed boundary layer mechanism leads to a reduced mean velocity relative to the current alone; the two streaming mechanisms are streaming caused by turbulence asymmetry in successive wave half-cycles (beneath asymmetric forcing) and streaming caused by the presence of a vertical wave velocity within the seabed boundary layer as earlier explained by Longuet-Higgins. The effect of wave asymmetry, wave length to water depth ratio, magnitude of current and bottom roughness have been investigated by numerical simulations for realistic physical situations. Mean Eulerian quantities as well as the mass transport (wave-averaged Lagrangian velocity) are presented. The resulting sediment dynamics near the ocean bottom has been investigated; results for both suspended load and bedload are presented. For wave dominated situations, the velocity profiles beneath opposing waves and current are substantially different from those beneath following waves and current, both for linear and second order Stokes waves. As the flow becomes current dominated, the velocity profiles beneath opposing and following waves and current approach the velocity profile beneath horizontally uniform sinusoidal forcing. The net transport of suspended sediments and bedload is in the direction of the current both for waves following and opposing the current. For wave dominated situations the net sediment transport is strongly affected by the streaming mechanisms, both for linear and second order Stokes waves and current. [Copyright &y& Elsevier]
- Published
- 2013
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122. Influence of freshwater discharges and tides on the abundance and distribution of larval and juvenile Munida gregaria in the Baker river estuary, Chilean Patagonia.
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Meerhoff, Erika, Castro, Leonardo, and Tapia, Fabián
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ESTUARIES , *TIDES , *FRESHWATER ecology , *SEDIMENT transport , *PLANKTON - Abstract
Abstract: Zooplankton time series collected with different temporal resolution and coverage were examined to characterize seasonal and diel patterns in the abundance of Munida gregaria larvae and juveniles in the Baker river estuary. Zoeae were more abundant in late winter and spring, coinciding with the season of lower sediment transport and higher primary production in the region. The occurrence of juveniles was exclusively in summer. There was a significant correlation between the abundance of zoeae and high-frequency temperature variability near the pycnocline over periods of 7–20 and 26–30 days prior to each plankton sampling. These time scales of correlation suggest that internal motions may be a proximal environmental cue for lunar rhythms in larval hatching, rather than directly causing the aggregation of larvae at the sampling area. To characterize shorter-term patterns in larval abundance and vertical distribution, stratified samples were collected every 3h over a full late-spring day (November 2008) near one of the monitoring stations. Zoeae were significantly more abundant at 10–25m depth (p=0.039), and changes in depth-integrated abundance of both zoea and megalopae were strongly associated with the tidal cycle. Together, these results suggest that the spatial structure and population dynamics of M. gregaria in this region may respond to the combined forcing of seasonal changes in freshwater inputs, tidally-driven processes such as lateral transport of larvae and juveniles, and internal-wave mediated changes in local conditions. [Copyright &y& Elsevier]
- Published
- 2013
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123. Surficial sediment distribution and the associated net sediment transport pattern retain-->in the Pearl River Estuary, South China.
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Zhang, Wei, Zheng, Jinhai, Xiaomei, Ji, Hoitink, A.J.F., van der Vegt, M., and Zhu, Yuliang
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ESTUARIES , *SEDIMENTS , *PARTICLE size distribution , *SEDIMENT transport , *PHYSICAL geography - Abstract
Abstract: Spatial variations in grain-size parameters contain information on sediment transport patterns. Therefore, in this study, 106 surficial sediment samples taken from the Pearl River Estuary (PRE), South China, were analyzed, to better understand the net sediment transport pattern in this region. The PRE is an area of fine-grained sediment and shows muddy patches with relatively coarse sediment in the north. The sorting coefficient of surficial sediment over the whole estuary is higher than 2, indicating a very poor sorting in this region. The relationship between the sand, silt and clay contents of the sediments was analyzed by a log-ratio analysis, and it was indicated that the selective deposition is a non-linear function of the sediment mixture composition. The net sediment transport pathways indicate four distinct characteristics of sediment transport over the PRE. The sediment is transported obviously southeastward in the upper part of the estuary due to the relatively stronger fluvial dynamics and northwestward in the lower part, due to the stronger tidal dynamics. However, in the central part of the PRE, the sediment transport vectors display a clockwise rotation trend and net deposition is taking place, as indicated by the convergence of the grain size trend. The trend vectors show that some of the sedimentary material is supplied from upper and lower parts of the estuary to this depocenter. This result is consistent with the residual current circulation pattern near Neilingding Island. The patterns of sediment transport reveal directions with the main areas of deposition and possible dispersal patterns in the PRE. [Copyright &y& Elsevier]
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- 2013
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124. Interannual changes in seafloor surficial geology at an artificial reef site on the inner continental shelf.
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Raineault, Nicole A., Trembanis, Arthur C., Miller, Douglas C., and Capone, Vince
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CONTINENTAL shelf , *SUBMARINE topography , *REEFS , *COASTAL development , *SEDIMENTS , *STRATIGRAPHIC geology - Abstract
Abstract: The influence of reef structures on seafloor surface sediments has implications for marine spatial planning and coastal development, including use of the coastal zone for offshore wind energy. We present results of interannual changes in seafloor surficial geology at the Redbird artificial reef site, located on the continental shelf offshore of Delaware Bay. The Redbird reef is composed of NYC subway cars, barges, tugboats, and other sunken objects. Since objects were added sporadically between 1996 and 2009, the survey area acts as a natural laboratory to study the evolution of the surrounding seafloor at a structural reef habitat through time. Annual side-scan surveys from 2008 through 2011, and one bathymetric survey in 2010 provide information about surface geology and morphology. Local wave and current data for this time period were analyzed to determine the main morphological agents. Automated backscatter segmentation show that three bottom types dominate and that these large-scale (>10m) surface sediment patterns persist from year to year. Grab samples reveal that the bottom types are silty sand with clay and sandy gravel. Clear sediment and biological patterns emerged revealing the influence of the objects on the seafloor. Comet-shaped moats of sandy gravel surround single objects and grow to form large-scale coalesced patches around groups of objects. Alignment of sediment patches suggests the periodic hydrodynamic influence of seasonal storms. The abundance and diversity of organisms increases with decreasing clay/silt content. Evidence of scour includes the removal of fine sediments, the formation of moats 1–30m in diameter and 0.5–1m deep around the reef objects, and the >1m settling of objects into the seafloor. Data suggest subway cars reached equilibrium with the environment in 6–7 years, but that larger objects or clusters of objects take a longer time to equilibrate and have farther-reaching effects. Knowledge of local wave and current climate and stratigraphy could inform decisions of object clustering and orientation to decrease scour impacts. [Copyright &y& Elsevier]
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- 2013
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125. Distribution patterns of particle-reactive radionuclides in sediments off eastern Hainan Island, China: Implications for source and transport pathways
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Huang, Dekun, Du, Jinzhou, Deng, Bing, and Zhang, Jing
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MARINE sediments , *RADIOISOTOPES , *MARITIME shipping , *POLLUTANTS , *SPECTROMETERS , *OCEAN surface topography - Abstract
Abstract: The study of sediment sources and transport processes from land to ocean can help in predicting the fate of the pollutants released from land or the potential change in sediment delivery to coastal areas and/or open oceans. The activities of 7Be, excess 210Pb (210Pbxs), excess 234Th (234Thxs) and 137Cs in surface sediments collected offshore of eastern Hainan Island, China, in August of 2008 were measured by an HPGe γ-spectrometer to evaluate the sediment source and transport processes. The results showed that all the surface sediments were silt or sand, and the mean grain sizes of the northern locations were higher than those in the other regions. The ranges of activities of 7Be, 210Pbxs, 234Thxs and 137Cs in surface sediment were 0.14–12.7, 37.4–199, 2.24–176 and 0.02–1.06Bqkg−1, with averages of 3.78±4.77, 110±8.1, 66.7±8.9 and 0.52±0.22Bqkg−1, respectively. The activities of the radionuclides increased from coast to offshore in the northern section. The upwelling may cause high particle fluxes with high activities of 210Pbxs and 234Thxs. A comparison of the source and transport of the suspended sediments with river discharge along the coast shows that the coastal current and offshore upwelling are the dominant factors for the transport and sources of surface sediment in the study region. The sediment was transported from south to north by the coastal current, and sediments with a large grain size may be deposited via the north loop current. The ratios of the nuclide activities indicated that the suspended particles need approximately one year to be removed from the water column into the seabed and that the main source of the sediments off eastern Hainan Island in the study regions was terrigenous deposits. [Copyright &y& Elsevier]
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- 2013
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126. Numerical prediction of medium-term tidal flat evolution in the Yangtze Estuary: Impacts of the Three Gorges project
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Kuang, Cuiping, Liu, Xu, Gu, Jie, Guo, Yakun, Huang, Shichang, Liu, Shuguang, Yu, Weiwei, Huang, Jing, and Sun, Bo
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- *
NUMERICAL analysis , *TIDAL flats , *ESTUARIES , *ESTUARINE sediments , *SIMULATION methods & models , *STREAM measurements , *SEDIMENT transport - Abstract
Abstract: A two-dimensional (2-D) numerical model and a three-dimensional (3-D) numerical model of the Yangtze Estuary, including tidal flow, suspended sediment transport and bed evolution, were established based on the Delft3D-FLOW model respectively. The models were calibrated with the field data. The calibrated models were employed to predict the morphological evolution process of the Yangtze Estuary for the next 20 years, with the predicted area covering the Chongming Eastern Beach and the Nanhui Tidal Flat. According to the simulated results in the medium term, due to the progress of the Three Gorges, erosion will happen in the southeast and south region and deposition will happen in the northeast and north region of the Chongming Eastern Beach, while a deep channel will be formed gradually in the middle part. The Nanhui Tidal Flat will develop southeastwards. Five scenarios of different river discharges and sediment loads are obtained from an exponential regression equation between flow rate and suspended sediment concentration deduced from the monthly averaged flow rate and suspended sediment concentration during 2003–2009. These five scenarios were used for investigating the topographic responses of the Chongming Eastern Beach and the Nanhui Tidal Flat to different river discharge and sediment supply conditions. The predictions show that the erosion rate will decrease and the deposition rate will increase with the increase of sediment supply, i.e., less the sediment supply leads to more recession. [Copyright &y& Elsevier]
- Published
- 2013
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127. Climate controls on longshore sediment transport
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Splinter, Kristen D., Davidson, Mark A., Golshani, Aliasghar, and Tomlinson, Rodger
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SEDIMENT transport , *CLIMATE change , *OCEAN waves , *SOUTHERN oscillation , *REGRESSION analysis - Abstract
Abstract: Subtle changes in the directional wave climate driven by changes in large scale climate variability have the potential to result in spatial changes (erosion and accretion) of sandy coastlines due to deviations in longshore sediment transport supply. 50+ years of hindcast offshore directional wave conditions for southeast Queensland, Australia were used to derive yearly wave climates that were input into a spectral wave model to estimate nearshore breaking wave conditions and resulting longshore sediment transport along a 35km stretch of sandy coastline. Resulting temporal deviations in net annual longshore transport were smoothed using a 5-year Hamming filter and compared against two climate indices known to influence the Australian climate: the Southern Oscillation Index (SOI) and the Inter-decadal Pacific Oscillation (IPO). During the negative IPO phase (more El Niño like) deviations in estimated transport were significantly correlated to the SOI index at −1 year lag and correlated to the IPO index at +1 year lag. During positive phases of the IPO (more La Niña like) highest correlations were found at 6 year lag (SOI) and −10 year lag (IPO). A linear regression model combining the influence of both indices explained 65% of the predicted variability in longshore transport during negative phases of the IPO and 48% of the variance during positive phases of the IPO. During positive phases of the IPO, the SOI index was shown to dominate model response and may indicate an increase in the influence of the South Pacific Convergence Zone with respect to the wave climate off the east coast of Australia. A model spanning both phases and only considering positive lags (knowledge of past climate indices) explained approximately half of the predicted variability in longshore transport over the entire record length. The results presented here indicate strong links between these large scale climate indices and estimated longshore transport ), such that a simple model derived from knowledge of these indices could be used as a first pass estimate of spatial and temporal variability in longshore sediment transport and resulting large scale coastal evolution. [Copyright &y& Elsevier]
- Published
- 2012
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128. Cross-shore variation of intermittent sediment suspension and turbulence induced by depth-limited wave breaking
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Yoon, Hyun-Doug and Cox, Daniel T.
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SUSPENDED sediments , *SEASHORE , *TURBULENCE , *QUANTITATIVE research , *ADVECTION , *WAVE energy , *FLUMES , *ENERGY dissipation - Abstract
Abstract: Intermittent features of turbulence and sediment suspension were investigated in a large-scale laboratory flume, under erosive and accretive conditions with measured free surface elevation, fluid velocity, and sediment concentration across the surf zone. Intermittent events of turbulence and sediment suspension occurred for a small portion of the time series but contained a significant amount of motions in these events. Comparison of intermittency statistics with previous studies conducted under different experimental conditions showed similar results, indicating that intermittency is a general aspect of turbulence and sediment suspension in the surf zone, including the bar crest, the bar trough, and the inner surf zone. The relationship of these intermittent turbulence and suspension events was examined with conditional probabilities. Here, it was found that only 20–35% of the turbulent events were associated with sediment suspension events, implying that much of the intermittent turbulent motion may act to dissipate wave energy rather than suspend sediments. On the other hand, 50–65% of the sediment suspension events were associated with turbulent events, implying that intermittent turbulent motion is one of the fundamental mechanisms for the initiation of sediment suspension in the surf zone. The sediment suspension events that were uncorrelated with turbulent events were mostly induced by strong offshore low-frequency motion, suggesting that advection plays a key role. The vertical structures of intermittency showed that the fraction of intermittent events for both turbulence and sediment concentration were vertically uniform and quantitatively similar each other, although their thresholds varied vertically. The motion contained in the intermittent suspension events increased toward the bed, while it was vertically uniform in the intermittent turbulent events. Finally, it was found that the intermittent sediment suspension events play an important role in each bathymetric change. [Copyright &y& Elsevier]
- Published
- 2012
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129. The influence of limestone reefs on storm erosion and recovery of a perched beach
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Gallop, Shari L., Bosserelle, Cyprien, Eliot, Ian, and Pattiaratchi, Charitha B.
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LIMESTONE , *REEFS , *STORMS , *BEACH erosion , *PARAPERCIS colias , *LANDFORMS , *COASTAL sediments , *LAGOONS , *SEDIMENT transport - Abstract
Abstract: Mechanisms through which naturally-occurring hard landforms, such as rock and coral reefs, influence coastal sediment transport are still poorly understood. Therefore, field investigations were undertaken during storm conditions on the sandy beaches of Yanchep Lagoon in southwestern Australia, which are perched on Quaternary limestone reefs. During two consecutive winter storms, the response of three subaerial beach profiles were quantified at: (a) an Exposed Profile which was fronted to seaward by a predominantly sandy substrate; (b) a Reef Profile that was fronted directly seaward by limestone outcrops submerged below mean sea level; and (c) a Bluff Profile where the dry beach was perched on a limestone bluff that reached above mean sea level and that contained a shallow coastal lagoon. The subaerial beach response to the storms had considerable spatial variation alongshore and was strongly dependent on the local rock topography. The Exposed Profile eroded most with a 2m-high scarp cut into the dune while the dunes at the Reef and Bluff Profiles were stable. The Bluff Profile also eroded considerably and the coastal lagoon widened and deepened. The Reef Profile was the most stable overall because erosion was balanced by short periods of accretion during the storm period which was partly due to sediment supplied by longshore transport through the coastal lagoon from the Bluff Profile. During the month after the storms wave energy was relatively low and the beach at the Exposed Profile accreted almost to the pre-storm volume, although the scarp in the dune was still present. The Reef Profile accreted most in the month after the storms while recovery at the Bluff Profile was low. It appeared that the bluff inhibited onshore sediment transport during and after the storms and in addition, strong currents in the lagoon transported sediment alongshore to supply the other beach profiles. These observations indicated that rock topography, especially elevation relative to sea level determined if beach erosion was reduced during storms and whether accretion was dampened in the post-storm recovery phase. [Copyright &y& Elsevier]
- Published
- 2012
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130. On the application of horizontal ADCPs to suspended sediment transport surveys in rivers
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Moore, S.A., Le Coz, J., Hurther, D., and Paquier, A.
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SUSPENDED sediments , *RIVER sediments , *WATER currents , *SEDIMENT transport , *RIVER surveys , *STREAMFLOW , *TURBIDITY currents , *SAND - Abstract
Abstract: In the last five years, horizontal acoustic Doppler current profilers have come onto the market as an instrument designed to measure real-time horizontal current profiles from a permanent fixed mount. The focus of this paper is on the feasibility of using these commercial instruments to measure suspended sediment fluxes in rivers. Measurements are presented from a study site that was equipped with an optical turbidity meter and three horizontal ADCPs operating at 300kHz, 600kHz and 1200kHz between November 2009 and June 2010. Laser grain size measurements showed the primary particles of the suspended sediment to be predominantly silts with some clay and occasionally some fine sand. Measurements of sediment attenuation by the three horizontal ADCPs are presented for two high concentration events during which concentrations reached 2.5 and 8kg/m3, respectively. Very clear linear relationships are seen between sediment attenuation and particle concentration and the sediment attenuation is consistent with the theory for viscous absorption by fine sediments. Using the relationship between attenuation and concentration, we are able to reconstruct the concentration time series for events during which the turbidity data are unavailable. Our findings show that, when properly positioned, horizontal ADCPs can provide suitable measurements of sediment transport during extreme river flow conditions such as floods, at least for predominantly silt-sized particles. This ability is of primary interest for river survey applications since the long term sediment transfer is believed to be driven by these intense events. [Copyright &y& Elsevier]
- Published
- 2012
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131. Impact of oceanic floods on particulate metal inputs to coastal and deep-sea environments: A case study in the NW Mediterranean Sea
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Roussiez, Vincent, Heussner, Serge, Ludwig, Wolfgang, Radakovitch, Olivier, Durrieu de Madron, Xavier, Guieu, Cécile, Probst, Jean-Luc, Monaco, André, and Delsaut, Nicole
- Subjects
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OCEANOGRAPHY , *FLOODS , *PARTICULATE matter , *COASTAL ecology , *CASE studies , *CONTINENTAL shelf , *SEDIMENTS - Abstract
Abstract: An exceptional flood event, accompanying a marine storm, was investigated simultaneously at the entrance and the exit of the Gulf of Lion’s hydrosystem (NW Mediterranean) in December 2003. Cs, Cr, Co, Ni, Cu, Zn, Cd and Pb signatures of both riverine and shelf-exported particles indicate that continental inputs and resuspended prodeltaic sediments were intensively mixed with resuspended sediments from middle/outer shelf areas during advective transport. As a result, particles leaving the Gulf of Lion inherited the mean signature of shelf bottom sediments, exporting anthropogenic Pb and Zn out into the open sea. When assessing the particulate metal budget in relation with the event, it appears that the output fluxes accounted for between 15% and 60% of the input fluxes, depending on the element and the period of reference. This trend is also observed for annual budgets, which were drawn up by compiling the data from this study and the literature. Results evidenced that, except some element fluxes during extreme output scenario, outputs never counterbalance the inputs. In its current functioning, the Gulf of Lion’s shelf seems to act as a retention/sink zone for particulate metals. Regarding anthropogenic fluxes, the contribution of the oceanic flood of December 2003 to the mean annual scenario is considerable. Environmental impacts onto coastal and deep-sea ecosystems should therefore tightly depend on both the intensity and the frequency of event-dominated sediment transport. [Copyright &y& Elsevier]
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- 2012
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132. Storm-induced inner-continental shelf circulation and sediment transport: Long Bay, South Carolina
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Warner, John C., Armstrong, Brandy, Sylvester, Charlene S., Voulgaris, George, Nelson, Tim, Schwab, William C., and Denny, Jane F.
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SEDIMENT transport , *BEACH nourishment , *HABITATS , *STORMS , *FLOW meters , *OCEAN currents , *FRONTS (Meteorology) - Abstract
Abstract: Long Bay is a sediment-starved, arcuate embayment located along the US East Coast connecting both South and North Carolina. In this region the rates and pathways of sediment transport are important because they determine the availability of sediments for beach nourishment, seafloor habitat, and navigation. The impact of storms on sediment transport magnitude and direction were investigated during the period October 2003–April 2004 using bottom mounted flow meters, acoustic backscatter sensors and rotary sonars deployed at eight sites offshore of Myrtle Beach, SC, to measure currents, water levels, surface waves, salinity, temperature, suspended sediment concentrations, and bedform morphology. Measurements identify that sediment mobility is caused by waves and wind driven currents from three predominant types of storm patterns that pass through this region: (1) cold fronts, (2) warm fronts and (3) low-pressure storms. The passage of a cold front is accompanied by a rapid change in wind direction from primarily northeastward to southwestward. The passage of a warm front is accompanied by an opposite change in wind direction from mainly southwestward to northeastward. Low-pressure systems passing offshore are accompanied by a change in wind direction from southwestward to southeastward as the offshore storm moves from south to north. During the passage of cold fronts more sediment is transported when winds are northeastward and directed onshore than when the winds are directed offshore, creating a net sediment flux to the north–east. Likewise, even though the warm front has an opposite wind pattern, net sediment flux is typically to the north–east due to the larger fetch when the winds are northeastward and directed onshore. During the passage of low-pressure systems strong winds, waves, and currents to the south are sustained creating a net sediment flux southwestward. During the 3-month deployment a total of 8 cold fronts, 10 warm fronts, and 10 low-pressure systems drove a net sediment flux southwestward. Analysis of a 12-year data record from a local buoy shows an average of 41 cold fronts, 32 warm fronts, and 26 low-pressure systems per year. The culmination of these events would yield a cumulative net inner-continental shelf transport to the south–west, a trend that is further verified by sediment textural analysis and bedform morphology on the inner-continental shelf. [Copyright &y& Elsevier]
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- 2012
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133. A detailed, event-by-event analysis of suspended sediment concentration in the swash zone
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Cáceres, Iván and Alsina, José M.
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OCEAN waves , *FLUMES , *SUSPENDED sediments , *WAVE energy , *DATA analysis , *HYDRODYNAMICS , *TIME series analysis , *ENVIRONMENTAL impact analysis - Abstract
Abstract: This work presents a detailed analysis of suspended sediment concentration measured in the inner surf and swash zone under large-scale wave flume conditions. Controlled hydrodynamic conditions tested in the laboratory corresponded to highly energetic (erosive) conditions. Hydrodynamic, bottom evolution and suspended sediment concentration (SSC) data were acquired at several locations within the inner surf and the swash zone. A limited number of hydrodynamic type of events have been identified to induce important suspended sediment concentrations. These hydrodynamic conditions include incident broken wave, wave capture, weak and strong wave backwash interactions or pure backwash events. The acquired data showed the variability of the suspended sediment concentration pattern depending on the cross-shore location from which measurements were taken. This suspension mechanism also varied depending on the time series characteristic. There was a clear predominance of incident wave/bore as a forcing term in the surf zone, whilst the SSC measured in the swash zone showed that wave-swash interaction controls the most significant events. Finally, the importance of a wave-by-wave analysis is highlighted in this study. It was observed that despite the fact that most of the time series induce profile accretion in the mid and upper swash area, only one or two swash events are able to erode extensive parts of the beach and change the final output of the entire time record from accretion to erosion. [Copyright &y& Elsevier]
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- 2012
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134. Estimating suspended sediment loads in the Pearl River Delta region using sediment rating curves
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Zhang, Wei, Wei, Xiaoyan, Jinhai, Zheng, Yuliang, Zhu, and Zhang, Yanjing
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PARAMETER estimation , *SUSPENDED sediments , *DATA analysis , *RIVER sediments , *SEDIMENT transport - Abstract
Abstract: In this study, sediment rating curves are employed to study the variations in relationships between water discharge and suspended sediment concentration based on the recent 50 years of monthly data set in the three major rivers of the Pearl River Delta. Results show that sediment rating parameters vary with time. The lowest rating coefficient, , and the highest rating exponent, b, mostly occur in the 1980s, indicating that sediment transport reached its peak in this decade at the same level as water discharge. This upward shift of sediment load is probably caused by exacerbated karst rocky desertification in the upper reaches of the Pearl River. However, since the beginning of the 1990s sediment loads from the Pearl River to its estuary began to show a dramatically decreasing trend, which is attributed mainly to deposition in the reservoirs, leading to an increase of and a decrease of b. Furthermore, the sediment rating curve in 1957 to1970 is applied to estimate potential sediment load (1971 to 2006) in the absence of human influences. It is also estimated quantitatively by the sediment rating curves that in the 1980s, the annual sediment load decreased by 7.59×106 t/yr because of natural factors, while sediment increase induced by human activities was 20.07×106 t/yr, which resulted in an actual increased sediment load of 12.47×106 t/yr compared with the reference level in 1957 to 1970. In the last two decades, the difference between measured and estimated sediment loads became considerable, and the annual deficit sharply increased to 26.80×106 t/yr in the 1990s, and 50.46×106 t/yr in the 2000s, indicating that human activities, mainly referring to dam and reservoir construction, play a dominant role in the decrease of sediment load. The decrease in sediment supply from the Pearl River should be paid special attention because it may cause serious impacts on the river delta and the coastal ocean. [Copyright &y& Elsevier]
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- 2012
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135. Prediction of time-evolving sand ripples in shelf seas
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Soulsby, R.L., Whitehouse, R.J.S., and Marten, K.V.
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LOGICAL prediction , *OCEAN bottom , *SEDIMENT transport , *OCEAN waves , *OCEAN temperature , *DATA analysis , *TIDAL currents - Abstract
Abstract: The ability to predict the presence and physical properties of small-scale sand ripples on the sea bed is important for determining the bed roughness felt by currents and waves, for sediment transport applications, and for acoustic applications. A fully time-evolving model is presented for predicting the height, length and orientation of sand ripples generated by currents, waves or both, which includes processes dealing with threshold of motion, ripple wash-out and biological degradation. We believe that this is currently the only predictor to feature all these properties, and is thus more widely applicable to shelf seas than other more sophisticated, but less comprehensive, methods. Tests against a large data-set of observations using an Acoustic Ripple Profiler at a site on the east coast of England, where ripples are generated by both tidal currents and waves, showed that the predictor captures most of the main features observed. A modified criterion for the wave/current dominance in the prediction equations improved some aspects of the agreement. The properties of current-generated ripples are predicted accurately, but the wavelength, and to a lesser extent the height, of wave-generated ripples show some under-prediction. Ripple orientations are modelled in an approximate way, but the main features of the observed orientations are captured. The bio-degradation feature of the predictor could not be tested with this data-set. [Copyright &y& Elsevier]
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- 2012
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136. Modeling transport and deposition of the Mekong River sediment
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Xue, Zuo, He, Ruoying, Liu, J.Paul, and Warner, John C.
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SEDIMENTATION & deposition , *TRANSPORTATION , *SOUTHEAST Asians , *SEDIMENT transport , *EXPERIMENTS , *MONSOONS , *SEA level - Abstract
Abstract: A Coupled Wave–Ocean–Sediment Transport Model was used to hindcast coastal circulation and fine sediment transport on the Mekong shelf in southeastern Asian in 2005. Comparisons with limited observations showed that the model simulation captured the regional patterns and temporal variability of surface wave, sea level, and suspended sediment concentration reasonably well. Significant seasonality in sediment transport was revealed. In summer, a large amount of fluvial sediments was delivered and deposited near the Mekong River mouth. In the following winter, strong ocean mixing, and coastal current lead to resuspension and southwestward dispersal of a small fraction of previously deposited sediments. Model sensitivity experiments (with reduced physics) were performed to investigate the impact of tides, waves, and remotely forced ambient currents on the transport and dispersal of the fluvial sediment. Strong wave mixing and downwelling-favorable coastal current associated with the more energetic northeast monsoon in the winter season are the main factors controlling the southwestward along-shelf transport. [Copyright &y& Elsevier]
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- 2012
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137. The impact of typhoon Morakot on the modern sedimentary environment of the mud deposition center off the Zhejiang–Fujian coast, China
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Li, Yunhai, Wang, Aijun, Qiao, Lei, Fang, Jianyong, and Chen, Jian
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TYPHOONS , *SEDIMENTATION & deposition , *MUD , *HYDROGRAPHY , *WATER temperature , *SALINITY , *TURBIDITY - Abstract
Abstract: Typhoon Morakot, which first made landfall in central Taiwan on Aug. 7th and then in Fujian on Aug. 9th after crossing through the Taiwan Strait was the strongest typhoon to impact the East China Sea (ECS) in 2009. Two surveys were conducted, respectively on Aug. 1st and Aug. 12th, in the mud deposition center off the Zhejiang–Fujian coast to study the impacts of the typhoon on the marine environment and sediment transport. Continuous in-situ hydrographic data (water temperature, salinity, depth and turbidity) were recorded at 29 stations during the first survey and 23 stations during the second survey. The water and surface sediment samples were collected at the same time. A comparison of the measurements from the two surveys shows that significant variations in temperature, salinity and turbidity distributions occurred in the water column. The upper water layer was well-mixed after the passage of Morakot. The water temperature and salinity dropped approximately 0.2–1.4°C and 0.3–3.3psu, respectively, while the water turbidity increased from less than 1FTU to 1–30FTU. In the bottom layer, the temperature increased approximately 2.5–4.0°C, and the salinity dropped approximately 0.2–0.6psu due to the water mixing. The water turbidities generally increased to greater than 50FTU (greater than 150FTU in some locations), which were several times to more than tenfold greater than those in the first survey, which were generally between 10 and 60FTU. The thermocline, which existed at a depth of approximately 10m during the first survey, disappeared after the typhoon. A halocline appeared at approximately 10m depth in the near-shore area due to abundant fresh water supply, whereas the salinity was generally homogeneous during the first survey. Concurrently, the relatively warm and fresh water extended downward to the whole medial water layer due to the water mixing. The turbidity and volume of the bottom turbid layer was greatly increased at the second survey compared with the first survey. The intrusion of the low-temperature and high-salinity deep water (below 50m depth) was significantly reduced at the same time. [Copyright &y& Elsevier]
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- 2012
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138. Rapid sediment removal from the Columbia River plume near field
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Nowacki, Daniel J., Horner-Devine, Alexander R., Nash, Jonathan D., and Jay, David A.
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RIVER sediments , *PLUMES (Fluid dynamics) , *ESTUARIES , *EDDY flux , *PARTICLE size distribution , *SAND , *SEDIMENT transport - Abstract
Abstract: We compute rates of sediment removal from the Columbia River, USA plume near field using a control volume technique. By using data from repeated transect passes along the plume axis, we account for variations in plume width, depth, and velocity, and we construct a framework for determining the dominant terms in the complete sediment balance. The contribution due to turbulent sediment flux is estimated using collocated measurements of turbulent shear and suspended-sediment concentration. The maximum sediment clearance rate on the landward end of the transect, 4km from the river mouth, is approximately 0.1gm−2 s−1, which corresponds to an effective removal velocity of 10mms−1. This is significantly higher than the settling velocities expected for single-grain and aggregate particles observed in the estuary. The plume clearance rate decreases seaward, with values of 0.01gm−2 s−1 6.5km from the river mouth. We investigate several potential mechanisms to explain the along-axis variability in sediment removal, including gravitational settling of several particle-size distributions and removal by turbulent mixing. A simple model that accounts for the settling of the distribution of particles observed in the estuary with the addition of a very fine sand settling class best explains the observed magnitude and the seaward decrease in the sediment removal rate. Although it is unlikely that fine sands are retained in the plume from the estuary due to their high settling speeds, we hypothesize that they may be locally resuspended landward of the transect. The turbulent flux of sediment out of the plume is almost an order of magnitude smaller than the total removal rates observed on the landward end of the transect. However, the turbulent flux generally is greater than the removal rate predicted for unflocculated fine silt, indicating that it may be important for smaller particle size distributions or farther from shore. [Copyright &y& Elsevier]
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- 2012
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139. Tide-induced suspended sediment transport: Depth-averaged concentrations and horizontal residual fluxes
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Yu, Qian, Wang, Ya Ping, Flemming, Burg, and Gao, Shu
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- *
TIDES , *SEDIMENT transport , *WATER depth , *ESTUARIES , *SUSPENDED sediments , *OCEANOGRAPHY , *DETERMINANTS (Mathematics) - Abstract
Abstract: In coastal and estuarine waters, depth-averaged suspended sediment concentration (DASSC) at a fixed observation site has two sources: local resuspension and advection along a horizontal gradient. The empirical decomposition method can separate the horizontal residual suspended sediment flux (HRF) into several terms, e.g., Eulerian flux, Stokes'' drift and tidal pumping. A simple depth-averaged 1D model is solved analytically for an observational site over one semi-diurnal tidal cycle to explore the determinants of the forcing factors comprising residual, M2 and M4 current velocities, mean water depth, M2 tidal amplitude and the DASSC gradient. The analytical solutions provide clear physical explanations of tidal DASSC variations and HRF. The solutions are applied to fit and explain the observations at a fixed station in a macro-tidal channel located along the Jiangsu Coast, China. The results suggest a dependence of the M4 DASSC variation on resuspension induced by the M2 velocity, and the dependence of the M2 DASSC variation on the advection and resuspension induced by the interaction between the M2 and M4 velocities. The horizontal DASSC gradient results in large seaward tidal pumping, which is offset by the landward Lagrangian flux to produce a reduced landward HRF. [Copyright &y& Elsevier]
- Published
- 2012
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140. Tidally-induced sediment transport patterns in the upper Bay of Fundy: A numerical study
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Wu, Yongsheng, Chaffey, Jason, Greenberg, David A., Colbo, Keir, and Smith, Peter C.
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SEDIMENTS , *TIDAL currents , *NUMERICAL analysis , *HYDRODYNAMICS , *OCEAN bottom , *REMOTE sensing , *IMAGE processing - Abstract
Abstract: The Minas Basin, the eastern end of the Bay of Fundy, is well known for its high tide ranges and strong tidal currents, which can be exploited to extract electricity power. The properties of the tidally-induced sediment transport in the Minas Basin, where significant changes in tidal processes may occur due to a recently proposed tidal power project, have been studied with a three-dimensional hydrodynamic model, an empirical bed load sediment transport model and surface sediment concentrations derived from the remotely-sensed images. The hydrodynamic model was evaluated against independent observational data, which include tidal elevation, tidal current (in the full water column and bottom layer), residual current profile and tidal asymmetry indicators. The evaluation shows that the model is in good agreement with the observations. The sediment transport includes two components, bed load and suspended particulate load. The bed load is calculated using the modelled bottom shear stress and the observed grain size data. The estimated features of bed load transport roughly agree with the observed patterns of the erosion and deposition in the Minas Basin and Cobequid Bay. The transport of the suspended load is estimated using the modelled velocity fields and the surface sediment concentration derived from remote-sensing images. The comparisons between the modelled results and the limited observations illustrate that the observed directions of suspended sediment transport are basically reproduced by the model. The modelled net suspended sediment input into the Minas Basin through Minas Passage is 2.4×106 m3 yr−1, which is comparable to the observed value of 1.6×106 m3 yr−1. The variations of the bed load and the suspended load in space and time are also presented. The total net transport, defined as the mean value of the sum of bed and suspended load transports during the tidal cycle, shows strong spatial variability. The magnitude of the transport flux ranges from 0.1 to 0.2kgm−1 s−1 in Minas Channel and Minas Passage, 0.1kgm−1 s−1 in Cobequid Bay, to 0.01kgm−1 s−1 in the central Minas Basin and Southern Bight. In Minas Channel, the sediment transport follows the structure of the tidal residual circulation, which features a large anticlockwise gyre. The sediment in Minas Passage moves eastward and deposits into the central Minas Basin. However, the sediment from the eastern part of the Basin moves westward and deposits in the central Minas Basin as well. In the Cobequid Bay, sediment moves eastward and deposits in the upper bay. [Copyright &y& Elsevier]
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- 2011
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141. Modelling impact of northerly wind-generated waves on sediments resuspensions in the Dover Strait and adjacent waters
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Guillou, Nicolas and Chapalain, Georges
- Subjects
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SEDIMENTS , *NUMERICAL analysis , *SIMULATION methods & models , *HYDRODYNAMICS , *SEDIMENT transport , *PREDICTION models - Abstract
Abstract: A numerical approach based on the coupling of the wave propagation module SWAN (Simulating WAves Nearshore) with the three-dimensional circulation module COHERENS (COupled Hydrodynamical-Ecological model for RegioNal and Shelf seas) is developed to simulate the impact of a northerly wind-generated storm on the resuspensions of heterogeneous sediments in the Dover Strait and adjacent waters. Suspended sediment transport is computed for the four grain-size classes of silts , very fine sands , fine sands and medium sands . Predicted times histories of wave variables and the near-bottom mean current and total suspended sediment concentration (SSC) are compared with field data collected off Merlimont Beach, in the south of Boulogne-sur-Mer. Mappings of the growth of the near-bottom total SSC during the storm are produced. The total SSC increases by four at the Merlimont site. The multiplying factor reaches 40 along the exposed northern French coastline revealing the Flandres bed features. This increase is limited to five in deep waters of the southern Dover Strait with local strengthening on the top of the Vergoyer and the Bassure de Baas sandbanks. The storm modifies the granulometric composition of suspended materials by increasing the contribution of medium sands. Main modifications appear at the south-western entrance of the Dover Strait and in the exposed southern North Sea. The near-bottom resuspensions remain elsewhere dominated by the local resuspensions of fine sands added to remote resuspended silts advected by currents. [Copyright &y& Elsevier]
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- 2011
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142. Waves and tides responsible for the intermittent closure of the entrance of a small, sheltered tidal wetland at San Francisco, CA
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Hanes, Daniel M., Ward, Kristen, and Erikson, Li H.
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OCEAN waves , *SALT marshes , *WETLANDS , *HYDRAULICS , *SEDIMENT transport , *MEASUREMENT , *SHORELINES - Abstract
Abstract: Crissy Field Marsh (CFM; http://www.nps.gov/prsf/planyourvisit/crissy-field-marsh-and-beach.htm) is a small, restored tidal wetland located in the entrance to San Francisco Bay just east of the Golden Gate. The marsh is small but otherwise fairly typical of many such restored wetlands worldwide. The marsh is hydraulically connected to the bay and the adjacent Pacific Ocean by a narrow sandy channel. The channel often migrates and sometimes closes completely, which effectively blocks the tidal connection to the ocean and disrupts the hydraulics and ecology of the marsh. Field measurements of waves and tides have been examined in order to evaluate the conditions responsible for the intermittent closure of the marsh entrance. The most important factor found to bring about the entrance channel closure is the occurrence of large ocean waves. However, there were also a few closure events during times with relatively small offshore waves. Examination of the deep-water directional wave spectra during these times indicates the presence of a small secondary peak corresponding to long period swell from the southern hemisphere, indicating that CFM and San Francisco Bay in general may be more susceptible to long period ocean swell emanating from the south or southwest than the more common ocean waves coming from the northwest. The tidal records during closure events show no strong relationship between closures and tides, other than that closures tend to occur during multi-day periods with successively increasing high tides. It can be inferred from these findings that the most important process to the intermittent closure of the entrance to CFM is littoral sediment transport driven by the influence of ocean swell waves breaking along the CFM shoreline at oblique angles. During periods of large, oblique waves the littoral transport of sand likely overwhelms the scour potential of the tidal flow in the entrance channel. [Copyright &y& Elsevier]
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- 2011
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143. Dispersal of Mississippi and Atchafalaya sediment on the Texas–Louisiana shelf: Model estimates for the year 1993
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Xu, Kehui, Harris, Courtney K., Hetland, Robert D., and Kaihatu, James M.
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SEDIMENT transport , *SEDIMENTATION & deposition , *CONTINENTAL shelf , *OCEAN currents - Abstract
Abstract: A three-dimensional coupled hydrodynamic-sediment transport model for the Texas–Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed. At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20–40km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (<10m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the “Dead Zone” (low-oxygen area) within a seasonal–annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and “stepwise-like” compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave–current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf. [Copyright &y& Elsevier]
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- 2011
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144. Shoreline change at an infinite jetty for wave time series
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Walton, Todd L. and Dean, Robert G.
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SHORELINES , *JETTIES , *WATER waves , *TIME series analysis , *PARTIAL differential equations , *SEDIMENT transport , *MATHEMATICAL models , *BEACH erosion , *SHORE protection - Abstract
Abstract: Future shoreline changes on a sandy beach with a structure such as a jetty or groin can be estimated when wave time series is known (i.e. sequence of wave height, period, and direction). This paper presents an extension of an existing solution () for the linearized partial differential equation for shoreline change at an infinite jetty where waves are time varying and when the angle of the shoreline is small with respect to the waves breaking at the shoreline. The novel solution provided in this paper allows the previous constant wave condition solution to be extended to the case where wave properties (i.e. wave direction, wave height, and wave period) are time varying. Example usage of the method presented shows that shorelines may be of different final plan form shape for time varying wave conditions even though the sediment transport along adjacent beaches is not spatially varying (i.e. spatially constant) from time step to time step. Although this difference in shape may have been known previously using numerical models, it could not be proved analytically. Reversals of wave height, period, and direction time series are shown to provide different final shoreline shapes even though the time series consists of the same waves although in different ordered time. The solution provided will allow one line numerical shoreline models to be tested using an analytic solution. [Copyright &y& Elsevier]
- Published
- 2011
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145. Effects of waves on the initiation of headland-associated sandbanks
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Guillou, Nicolas and Chapalain, Georges
- Subjects
- *
SHEAR waves , *CAPES (Coasts) , *HYDRODYNAMICS , *TIDAL currents , *LINEAR statistical models , *SAND , *SEDIMENT transport , *NUMERICAL analysis , *MATHEMATICAL models - Abstract
Abstract: Linear sandbanks appear in the lee of coastal headlands where the hydrodynamics are dominated by strong tidal currents and the seabed is characterized by an abundance of sands. They may develop as symmetrical sandbanks on either sides of the headland or as an unique banner bank. The present study numerically investigates the combined effects of waves and tide on the initial development of headland-associated sandbanks. A morphological model based on the coupling of the wave propagation module SWAN (Simulating WAves Nearshore) with the three-dimensional circulation module COHERENS (COupled Hydrodynamical-Ecological model for REgioNal and Shelf seas) is applied to an idealized Gaussian shaped headland for waves conditions varying in heights and directions at the offshore boundary. The coupling considers the effects of the interactions between the wave and current bottom boundary layers, namely the enhanced levels of turbulence near the bottom and the increase of the total bottom shear stress. Waves substantially modify the initial development of sandbanks formed by suspension narrowing their width and reorienting them along the side of the headland. They weakly impact the morphogenesis of sandbanks by bedload favoring on a short-time scale the growth of symmetric circular-shaped features and a central depositional spit prolonging the headland tip. Waves of transverse directions toward the tip of the headland contribute to the initiation by suspension of a well-developed feature in the headland side of low energy limiting the seabed evolution in the exposed area. [Copyright &y& Elsevier]
- Published
- 2011
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146. Neural-network predictability experiments for nearshore sandbar migration
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Pape, L. and Ruessink, B.G.
- Subjects
- *
ARTIFICIAL neural networks , *SAND bars , *MATHEMATICAL models , *HYDRODYNAMICS , *SEDIMENT transport , *NONLINEAR theories , *OCEAN waves , *GEOGRAPHY , *EXPERIMENTS - Abstract
Abstract: Cross-shore migratory behavior of nearshore sandbars is commonly studied with nearshore bathymetric-evolution models that represent underlying processes of hydrodynamics and sediment transport. These models, however, struggle to reproduce natural cross-shore sandbar behavior on timescales of a few days to weeks and have uncertain skill on longer scales of months to years. One particular concern for the use of models on prediction timescales that far exceed the timescale of the modeled processes is the exponential accumulation of errors in the nonlinear model equations. The relation between cross-shore sandbar migration, sandbar location and wave height has previously been demonstrated to be weakly nonlinear on timescales of several days, but it is unknown how this nonlinearity affects the predictability of long-term (months to years) cross-shore sandbar behavior. Here we study the role of nonlinearity in the predictability of sandbar behavior on timescales of a few days to several months with data-driven neural network models. Our analyses are based on over 5600 daily-observed cross-shore sandbar locations and daily-averaged wave forcings from the Gold Coast, Australia, and Hasaki, Japan. We find that neural network models are able to hindcast many aspects of cross-shore sandbar behavior, such as rapid offshore migration during storms, slower onshore return during quiet periods, seasonal cycles and annual to interannual offshore-directed trends. Although the relation between sandbar migration, sandbar location and wave height is nonlinear, sandbar behavior can be hindcasted accurately over the entire lifespan of the sandbars at the Gold Coast. Contrastingly, it is difficult to hindcast the long-term offshore-directed trends in sandbar behavior at Hasaki because of exponential accumulation of errors over time. Our results further reveal that during periods with low-wave conditions it becomes increasingly difficult to predict sandbar locations, while during high waves predictions become increasingly accurate. [Copyright &y& Elsevier]
- Published
- 2011
- Full Text
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147. Impact of sediment-induced stratification and turbulence closures on sediment transport and morphological modelling
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Amoudry, Laurent O. and Souza, Alejandro J.
- Subjects
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SEDIMENT transport , *TURBULENCE , *SEDIMENTS , *MATHEMATICAL models , *OCEAN bottom , *EROSION , *COMPUTER simulation , *SUSPENDED sediments - Abstract
Abstract: Appropriate descriptions of turbulence are important in predicting sediment transport and seabed evolution. We use here a three-dimensional model to investigate the impacts of sediment-induced stratification, erosion parameterization, and turbulence closures on sediment transport and morphological simulations. The model is implemented in the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), which couples a hydrodynamic model to the General Ocean Turbulence Model (GOTM) and to a sediment transport model. This sediment transport model calculates both suspended sediment concentrations and bed evolution. A large number of turbulence closures can be tested and we focus on the methods used to calculate the turbulent length scale and the stability functions. Different modelling scenarios are assessed against experimental data of velocity profiles, suspended sediment concentrations, and trench migration in a laboratory flume. Numerical results show that the processes investigated have little impact on the flow velocity profiles, in spite of some differences on the bed shear stress. The only exception is the one-equation turbulence closure, which cannot reproduce velocity profiles appropriately. Sediment-induced stratification, erosion parameterization, and turbulence closures can all have significant impacts on the suspended sediment concentration and the bed evolution. Model-data comparisons are found to be particularly sensitive to the method used to calculate the turbulent length scale. [Copyright &y& Elsevier]
- Published
- 2011
- Full Text
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148. Simulating the fate of mechanically eroded masses in the Thermaikos Gulf
- Author
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Kombiadou, Katerina and Krestenitis, Yannis Ν.
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EROSION , *OCEAN bottom , *SEDIMENT transport , *MARINE sediments , *MATHEMATICAL models , *TRAWLING , *MARINE resources - Abstract
Abstract: Benthic trawlers, through the scraping of the upper layers of the seabed sediment, are responsible for major alterations to the distribution of suspended matter in coastal areas. A major trawling site in Greece is that of the Gulf of Thermaikos, located in the NW Aegean Sea (Eastern Mediterranean). In an attempt to assess, for the first time, the quantities of the mobilized sedimentary masses and the properties of their motion in the aquatic domain of the Gulf of Thermaikos, a mathematical model has been formulated. The modeling approach includes the determination of the mechanical erosion rate, the production of turbulence due to the flow through the mesh of the net and the physical processes that control the propagation of the generated sedimentary plume in the marine environment. In the absence of information regarding the haul paths in the gulf, their determination was made stochastically, based on the existing legislative restrictions and trawlers'' fishing practices. The trawling-activated masses thereby defined for the duration of the trawling period in the gulf were very high, indeed several times greater than the related contribution from the major rivers of the area, emphasizing the significance of the activity to the sedimentary input of the local marine environment. The total annual erosion by trawls in the gulf was calculated at 3.38×106 t/yr. Τhe mean mechanical erosion rate per trawled surface (parameter free from the bias raised by the stochastic simulation of the trawl trajectories) was estimated at 430g/m2, values directly comparable to ones defined for similar coastal areas. Results from a simulation of the 2001–02 trawling period were well correlated with in-situ measurements for the same period, while the investigation of the matter''s suspension level and times revealed that the matter exhibits low residence times in the water column and at the same time moves in the vicinity of the bed, contributing to the formation and distribution of benthic nepheloid layers in the gulf. [Copyright &y& Elsevier]
- Published
- 2011
- Full Text
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149. Coherence of river and ocean conditions along the US West Coast during storms
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Kniskern, Tara A., Warrick, Jonathan A., Farnsworth, Katherine L., Wheatcroft, Robert A., and Goñi, Miguel A.
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WINTER storms , *OCEANOGRAPHY , *FLOODS , *MARINE sediments , *WATERSHEDS , *WAVE energy - Abstract
Abstract: The majority of water and sediment discharge from the small, mountainous watersheds of the US West Coast occurs during and immediately following winter storms. The physical conditions (waves, currents, and winds) within and acting upon the proximal coastal ocean during these winter storms strongly influence dispersal patterns. We examined this river–ocean temporal coherence for four coastal river–shelf systems of the US West Coast (Umpqua, Eel, Salinas, and Santa Clara) to evaluate whether specific ocean conditions occur during floods that may influence coastal dispersal of sediment. Eleven years of corresponding river discharge, wind, and wave data were obtained for each river–shelf system from USGS and NOAA historical records, and each record was evaluated for seasonal and event-based patterns. Because near-bed shear stresses due to waves influence sediment resuspension and transport, we used spectral wave data to compute and evaluate wave-generated bottom-orbital velocities. The highest values of wave energy and discharge for all four systems were consistently observed between October 15 and March 15, and there were strong latitudinal patterns observed in these data with lower discharge and wave energies in the southernmost systems. During floods we observed patterns of river–ocean coherence that differed from the overall seasonal patterns. For example, downwelling winds generally prevailed during floods in the northern two systems (Umpqua and Eel), whereas winds in the southern systems (Salinas and Santa Clara) were generally downwelling before peak discharge and upwelling after peak discharge. Winds not associated with floods were generally upwelling on all four river–shelf systems. Although there are seasonal variations in river–ocean coherence, waves generally led floods in the three northern systems, while they lagged floods in the Santa Clara. Combined, these observations suggest that there are consistent river–ocean coherence patterns along the US West Coast during winter storms and that these patterns vary substantially with latitude. These results should assist with future evaluations of flood plume formation and sediment fate along this coast. [Copyright &y& Elsevier]
- Published
- 2011
- Full Text
- View/download PDF
150. Hydrodynamics and sediment-transport in the nearshore of Poverty Bay, New Zealand: Observations of nearshore sediment segregation and oceanic storms
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Bever, Aaron J., McNinch, Jesse E., and Harris, Courtney K.
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HYDRODYNAMICS , *SEDIMENT transport , *MARINE sediments , *STORMS , *CYCLONES , *TURBIDITY - Abstract
Abstract: Nearshore regions act as an interface between the terrestrial environment and deeper waters. As such, they play important roles in the dispersal of fluvial sediment and the transport of sand to and from the shoreline. This study focused on the nearshore of Poverty Bay, New Zealand, and the processes controlling the dispersal of sediment from the main source, the Waipaoa River. Hydrodynamics and sediment-transport in water shallower than 15m were observed from April through mid-September 2006. This deployment afforded observations during 3–4 periods of elevated river discharge and 5 dry storms. Similar wind, river discharge, wave, current, and turbidity patterns were characterized during three of the wet storms. At the beginning of each event, winds blew shoreward, increasing wave heights to 2–3m within Poverty Bay. As the cyclonic storms moved through the system the winds reversed direction and became seaward, reducing the local wave height and orbital velocity while river discharge remained elevated. At these times, high river discharge and relatively small waves enabled fluvially derived suspended sediment to deposit in shallow water. Altimetry measurements indicated that at least 7cm was deposited at a 15m deep site during a single discharge event. Turbidity and seabed observations showed this deposition to be removed, however, as large swell waves from the Southern Ocean triggered resuspension of the material within three weeks of deposition. Consequently, two periods of dispersal were associated with each discharge pulse, one coinciding with fluvial delivery, and a second driven by wave resuspension a few weeks later. These observations of nearfield sediment deposition contradict current hypotheses of very limited sediment deposition in shallow water offshore of small mountainous rivers when floods and high-energy, large wave and fast current, oceanic conditions coincide. Consistently shoreward near-bed currents, observed along the 10m isobath of Poverty Bay, were attributed to a combination of estuarine circulation, Stokes drift, and wind driven upwelling. Velocities measured at the 15m isobath, however, were directed more alongshore and diverged from those at the 10m isobath. The divergence in the currents observed at the 10 and 15m locations seemed to facilitate segregation of coarse and fine sediment, with sand transported near-bed toward the beach, while suspended silts and clays were exported to deeper water. [Copyright &y& Elsevier]
- Published
- 2011
- Full Text
- View/download PDF
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