Offshore Sediment Transport Research Articles

River-derived sediment suspension and transport in the Bohai, Yellow, and East China Seas: A preliminary modeling study

Coastal circulation and transport of sediment derived from the Huanghe and Changjiang Rivers in the Bohai, Yellow, and East China Seas (BYECS) over the past 48 years (1958–2005) were simulated and analyzed using the Coupled Ocean–Atmosphere–Wave–Sediment Transport modeling system. Model skill assessments against in situ wave and hydrographical observations indicate the model simulation can reasonably well reproduce the hydrodynamic environment of the BYECS. Model-simulated regions of high fine sediment accumulation rate correlate well with the observed regions, which are known as “muddy patches.” Bottom stress analysis further indicates that the formation of muddy patches near river mouths is largely due to their proximity to the sediment source. Muddy patches formed in regions farther away from river mouths are results of local weak bottom stress and associated circulation pattern. Simulated seabed sediment distribution reveals that most of the Huanghe-derived sediment stays inside the Bohai Sea, whereas the Changjiang-derived sediment can spread into both the Yellow and East China Seas. Strong seasonal variations exist in the river-derived sediment transport with stronger (weaker) offshore sediment transport occurring in the winter (summer).

EFFECTS OF SEASONAL VARIATIONS ON SANDY BEACH GROUNDWATER TABLE AND SWASH ZONE SEDIMENT TRANSPORT

The hydrodynamics in the swash zone significantly affected the sediment transport mechanisms that mostly control beach face morphology especially under different weather conditions. Rainfall distribution patterns during dry and wet seasons in Peninsular Malaysia will influence groundwater table elevation and the beach profile. This study is aimed at investigating the effects of seasonal variations to beach groundwater elevations and surface profile changes. This work was undertaken at the Desaru Beach, Johor. Rainfall depth, groundwater table, tides, beach profiles, swash depth and swash velocity data were monitored and investigated at the study area. The results showed that the groundwater table was affected by rainfall patterns; higher during the wet season and lower during the dry season. The beach profile also showed erosive condition due to increasing of offshore sediment transport during the wet season, whereas in the dry season the beach profile showed accretion condition due to the increasing of onshore sediment transport. Swash properties like swash water depth and velocity were also monitored and analysed during this study in order to get a clear view about the saturation level effect due to seasonal variations into a infiltration processes at the swash zone. Finally the data showed that there is a lag time between rising and falling of groundwater tables and tides due to the lower hydraulic conductivity effect.

PILE FENCE TO ENHANCE DUNE RESILIENCY

A beach with a berm and dune provides storm protection but wave overtopping and overwash of the dune may lead to dune breaching and inundation. Dune erosion and overwash may be reduced by piles placed linearly along the toe of the dune. This study will explore whether piles can be used to make dunes more resilient against storm surge and waves. Six laboratory tests were conducted to observe the effect of pile fence porosity, location and pile toppling. It is recommended that a pile fence porosity of 0.5 or less be used to obtain significant increase in dune resiliency. Presence of the pile fence may increase offshore sediment transport resulting in more deposition in the surf zone. The pile fence should be placed near the dune toe to reduce wave uprush on the dune foreslope and to minimize pile toppling due to wave action.

A wave-resolving model for nearshore suspended sediment transport

This paper presents a wave-resolving sediment transport model, which is capable of simulating sediment suspension in the field-scale surf zone. The surf zone hydrodynamics is modeled by the non-hydrostatic model NHWAVE (Ma et al., 2012). The turbulent flow and suspended sediment are simulated in a coupled manner. Three effects of suspended sediment on turbulent flow field are considered: (1) baroclinic forcing effect; (2) turbulence damping effect and (3) bottom boundary layer effect. Through the validation with the laboratory measurements of suspended sediment under nonbreaking skewed waves and surfzone breaking waves, we demonstrate that the model can reasonably predict wave-averaged sediment profiles. The model is then utilized to simulate a rip current field experiment (RCEX) and nearshore suspended sediment transport. The offshore sediment transport by rip currents is captured by the model. The effects of suspended sediment on self-suspension are also investigated. The turbulence damping and bottom boundary layer effects are significant on sediment suspension. The suspended sediment creates a stably stratified water column, damping fluid turbulence and reducing turbulent diffusivity. The suspension of sediment also produces a stably stratified bottom boundary layer. Thus, the drag coefficient and bottom shear stress are reduced, causing less sediment pickup from the bottom. The cross-shore suspended sediment flux is analyzed as well. The mean Eulerian suspended sediment flux is shoreward outside the surf zone, while it is seaward in the surf zone.

Suspended sediment transport in the Deepwater Navigation Channel, Yangtze River Estuary, China, in the dry season 2009: 1. Observations over spring and neap tidal cycles

[1] The in situ data in the Deepwater Navigation Channel (DNC), Yangtze River Estuary (YRE), China, in the dry season 2009, shows spring tides associated with greater maximum velocities, more mixing, less stratification, and diffused fluid mud; whereas neap tides are associated with smaller maximum velocities, greater stratification, inhibited mixing, and stratified fluid muds. The balance of salt flux indicates the seaward salt transport is dominated by fluvial flows, and the landward salt transport is generated by compensation flows during spring tides, but shear effects during neap tidal cycles. The balance of suspended sediment flux illustrates the offshore sediment transport is dominated by fluvial flows as well, but the onshore transport is induced by tidal-pumping effects on spring tides, and shear effects on neaps. The suspended sediment transport is strongly affected by the salinity distribution and salinity-gradient-induced stratification in the DNC. The spring-neap asymmetry is generated by the estuarine gravitational circulation during low-flow conditions; while the flood-ebb asymmetric stratification within a tidal cycle is due to the semidiurnal tidally movement of the salt front.

Modeling the morphodynamic response of tidal embayments to sea-level rise

Sea-level rise has a strong influence on tidal systems, and a major focus of climate change effect studies is to predict the future state of these environmental systems. Here, we used a model to simulate the morphological evolution of tidal embayments and to explore their response to a rising sea level. The model was first used to reproduce the formation of channels and intertidal flats under a stable mean water level in an idealised and initially unchannelled tidal basin. A gradual rise in sea level was imposed once a well-developed channel network had formed. Simulations were conducted with different sea-level rise rates and tidal ranges. Sea-level rise forced headward erosion of the tidal channels, driving a landward expansion of the channel network and channel development in the previously non-inundated part of the basin. Simultaneously, an increase in channel drainage width in the lower part of the basin occurred and a decrease in the overall fraction of the basin occupied by channels could be observed. Sea-level rise thus altered important characteristics of the tidal channel network. Some intertidal areas were maintained despite a rising sea level. However, the size, shape, and location of the intertidal areas changed. In addition, sea-level rise affected the exchange of sediment between the different morphological elements. A shift from exporting to importing sediment as well as a reinforcement of the existing sediment export was observed for the simulations performed here. Sediment erosion in the inlet and the offshore transport of sediment was enhanced, resulting in the expansion of the ebb-tidal delta. Our model results further emphasise that tidal embayments can exhibit contrasting responses to sea-level rise.

Suspended Sediment Transport in Rip Currents on a Macrotidal Beach

Thorpe, A., Miles, J., Masselink, G., Russell, P., Scott, T., Austin, M., 2013. Sediment Transport In Rip Currents on a Macrotidal BeachRip currents are offshore-directed flows in the surf zone that may be responsible for significant offshore transport of sediment. In this study high frequency surface elevation and current velocity data were collected alongside suspended sediment concentrations in a rip channel on a high energy, macro-tidal beach. At low tide (water depth over the rip channel (h) = 0.9 m), when the rip current was active (maximum offshore directed flow = ~ 0.4 m/s), net suspended sediment transport was directed offshore, and 88% of the flux was due to the mean flow component. The rip current was found to pulse at very low frequencies (VLF) and this contributed significantly to offshore sediment transport, accounting for 48% of total oscillatory flux (not considering direction). Infra-gravity (IG) frequency oscillations had a marginal contribution to offshore sediment transport within the rip (10% of total oscillatory flux) and incident wave transport was onshore (42% of total oscillatory flux). The relative influence of the wave component increased as tidal elevations increased and the rip current became inactive. For comparison, at mid-tide (h = 3.5 m) the total suspended sediment flux was in the offshore direction and four times less than during low tide. The mean flux accounted for 34% of the total transport at mid-tide. At high tide (h = 6.1 m) the total sediment transport was in the onshore direction due to the incident waves, with a 0% contribution from the mean flux.

放射性炭素年代測定を用いた海底土砂の長期的移動・堆積履歴の推定

放射性炭素年代測定を用いた海底土砂の長期的移動・堆積履歴の推定

Net beach change in the swash zone: A numerical investigation

A range of bed-load sediment transport formulae are used to run fully coupled, morphodynamic simulations of one [1] swash cycle on an erodible plane beach. A system comprising shallow water equations and Exner equation is solved, in which sediment transport rate q is either dependent only on depth averaged velocity (u), or on u and water depth (h). The results are in agreement with equivalent uncoupled results [2] in that all sediment transport formulae considered applied to the event of [1] yield net erosion in the whole of the swash. Consistent with [3], however, full coupling yields significantly less erosion for all the q=q(u) formulae compared to the equivalent uncoupled results. It is shown that differences between uncoupled and coupled approaches (for most formulae) accumulate over the course of the swash event. The main reason for the reduced net erosion is the smaller maximum inundation. It is also shown that including a dependence on h in the bed load sediment transport formula for fully coupled simulations can result in net deposition in the upper swash.Bed shear stress described by a Chezy law is further included in fully coupled simulations to examine net beach change. Much reduced maximum inundation and net offshore sediment transport are predicted both for q=q(u) and q=q(h,u). It is shown that although the net sediment flux at the base of the swash under one [1] swash event is still offshore, deposition in the middle or upper swash may be predicted when bed shear stress is included, particularly when the drag coefficient in the backwash is reduced compared to that in the uprush, consistent with some in-situ measurements. The implication is that bed shear stress must be included not just to obtain correct quantitative beach change, but also to obtain correct qualitative beach behaviour.

Resuspension by fish facilitates the transport and redistribution of coastal sediments

Oxygen availability restricts groundfish to the oxygenated, shallow margins of Saanich Inlet, an intermittently anoxic fjord in British Columbia, Canada. New and previously reported 210Pb measurements in sediment cores compared with flux data from sediment traps indicate major focusing of sediments from the oxygenated margins to the anoxic basin seafloor. We present environmental and experimental evidence that groundfish activity in the margins is the major contributor to this focusing. Fine particles resuspended by groundfish are advected offshore by weak bottom currents, eventually settling in the anoxic basin. Transmittance and sediment trap data from the water column show that this transport process maintains an intermediate nepheloid layer (INL) in the center of the Inlet. This INL is located above the redox interface and is unrelated to water density shifts in the water column. We propose that this INL is shaped by the distribution of groundfish (as resuspension sources) along the slope and hence by oxygen availability to these fish. We support this conclusion with a conceptual model of the resuspension and offshore transport of sediment. This fish‐induced transport mechanism for sediments is likely to enhance organic matter decomposition in oxygenated sediments and its sequestration in anoxic seafloors.

Storms, shoreface morphodynamics, sand supply, and the accretion and erosion of coastal dune barriers in the southern North Sea

The coast of the southern North Sea is bound by dune barriers that have developed adjacent to a shallow storm- and tide-dominated shoreface comprising numerous shore-parallel to sub-shore-parallel tidal sand banks. The banks evolve under the joint control of tide-, wave- and wind-induced shore-parallel currents, which tend to ‘stretch’ them, eventually leading to bank division, and to shoaling and breaking storm waves, which tend to drive them ashore. The banks, thus, modulate the delivery of storm wave energy to the coast, redirect currents alongshore and are the sand sources for the accretion of coastal dunes. Foredune accretion occurs where major sand banks have migrated shoreward over the last centuries to be finally driven ashore and weld under the impact of storm waves. Morphological changes in the bank field can impact on shoreline stability through dissipation or enhanced shoreward transmission of storm wave energy and effects on radiation stress, particularly when waves are breaking over the banks. Where banks are close to the shore, mitigation of offshore sediment transport, especially during storms, can occur because of gradients in radiation stress generated by the complex 3D bank structure. These macro-scale mechanisms involve embedded meso-scale interactions that revolve around the mobility of sand waves, mobility of beach bars and troughs and foredune mobility, and micro-scale processes of bedform mobility in the subaqueous and intertidal domains, and of swash and aeolian beach–dune sand transport. These embedded interactions and the morphodynamic feedback loops illustrate the importance of synchroneity of sand transport from shoreface to dune on this coast.Large stretches of the foredunes show either signs of stability, or mild but chronic erosion. Furthermore, a demonstrated lack of a clear relationship occurs between storminess and coastal response over the second half of the 20th century. The present situation may be indicative of conditions of rather limited sand supply from offshore, notwithstanding the abundance of sand on the nearby shallow shoreface, except in areas where a nearshore storm-driven tidal sand bank has become shore-attached. Apart from the important influence of shoreface sand banks and of wave–bank interactions, foredune accretion and erosion also depend on various context controls that include individual storm characteristics, wind speed and incidence relative to the shore, tidal stage during storms, and direct human intervention on the shore through foredune and beach management. The bewildering variability inherent in these intricately related parameters may also explain the poor relationship between storminess and barrier shoreline change and will still continue to render unpredictable the response of shores to individual storms.

An experimental study on sediment transport and bed evolution under different swash zone morphological conditions

In this study, new large‐scale experimental data are presented showing evidences of a link between the swash zone dynamics and the surf zone morphodynamics. Two sets of large‐scale experiments are presented. The first set of experiments investigates the swash and surf dynamics under the same hydrodynamic forcing but with two different swash zone morphological conditions, one of which was created by manual reshaping of the sub-aerial beach face. It is shown that more dissipative swash zone conditions (a more mildly sloping beach face) significantly reduce the rate of seaward sandbar migration such that it almost ceases during the performed experimentation time. This behavior is compared to a second set of experiments which shows the natural sand bar offshore migration under almost identical hydrodynamic forcing. The changes in the sandbar migration are investigated in terms of the differences in the swash zone dynamics. The more reflective swash zone is characterized by more intense backwashes that, in turn, promote significant quantities of sediment suspension during the interaction of the backwash with the subsequent bore. The dissipative swash zone leads to a larger number of wave–swash interactions, reducing the backwash magnitudes and the magnitude of suspended sediment concentration events in the swash zone, with a corresponding reduction in the offshore suspended sediment transport. The total offshore sediment transport rates are also shown to reduce, leading to the observed modification of the sandbar migration.

Large-scale laboratory investigation into the effect of varying back-barrier lagoon water levels on gravel beach morphology and swash zone sediment transport

It is generally accepted that the effect on beach morphology of the interactions between swash motion and the beach groundwater table depends on the elevation of the latter relative to that of the sea. Specifically, a low beach groundwater table is considered to enhance onshore sediment transport and beach accretion, while the reverse is thought to hold for a high beach groundwater table. Here, we use the results of a carefully designed series of tests conducted in the Delta Flume, the Netherlands, as part of the Barrier Dynamics Experiment (BARDEX), to unequivocally demonstrate the validity of this assertion. During the experiment, a 4-m high and 50-m long gravel barrier (D50=11mm) was constructed within the central region of the 250-m long and 7-m deep wave flume, enabling a back-barrier ‘lagoon’ to be located at its landward side. The groundwater table in the beach was manipulated by lowering and raising the lagoon level, and the gravel beach was subjected to variable wave conditions and sea/lagoon levels. Using as the initial beach profile the morphology resulting from 90min of wave forcing with the sea and lagoon at the same level, the identical wave forcing with a low lagoon level consistently resulted in onshore sediment transport in the swash zone and beach accretion, whereas the identical wave forcing with a high lagoon level consistently caused offshore sediment transport in the swash zone and beach erosion. The divergent beach morphological responses for the low and high lagoon levels could not be attributed to changes in the ‘bulk’ swash hydrodynamics (i.e., based on single-point velocity measurements) arising from a difference in the amount of swash infiltration into the unsaturated beach. Instead, application of a sediment transport model that accounts for the effects of in/exfiltration suggested that the onshore sediment transport and beach accretion for the low lagoon tests mainly resulted from enhanced bed shear stresses during the wave uprush due to infiltration into the saturated part of the beachface. Due to the large hydraulic conductivity of gravel (K=O(0.1ms−1)), the effect of altered shear stresses due to in/exfiltration may be an important factor in sediment transport in the swash zone of gravel beaches.

Coastal profile response to sea level rise: a process‐based approach

ABSTRACTPrevailing ideas and calculations of coastal response to sea level rise (SLR) are often based on the Bruun model (Bruun P., Sea‐level rise as a cause of shore erosion, Journal Waterways Harbors Division, ASCE 88: 117–130, 1962) that predicts upward and landward transfer of an equilibrium profile during SLR through offshore sediment transport on the shoreface. The model is based on a number of assumptions of questionable validity as well as outdated concepts on how sediment is transported across the shoreface. This contribution takes a numerical modelling approach that is based on first‐order processes contributing to the movement of sediment across the shoreface. Using a wave transformation model that predicts hydrodynamic processes driving cross‐shore sediment transport and an energetics‐based model for the coupling between hydrodynamics and sediment transport, we show that cross‐shore sediment transport is mainly onshore directed at the boundary between the lower and the upper shoreface, in agreement with the model proposed by Davidson‐Arnott (Conceptual model of the effects of sea level rise on sandy coasts, Journal of Coastal Research 21: 1166–1172, 2005). The transition from onshore to offshore directed transport is located well within the surf zone and with a rising sea level this transition point becomes displaced landward and upward. Tests also show that substrate slope is of fundamental importance to the manner in which beaches react to rising sea level. Copyright © 2012 John Wiley & Sons, Ltd.

Extreme erosion on high-energy embayed beaches: Influence of megarips and storm grouping

Megarips have long been recognised as an important, yet poorly documented, mechanism of beach erosion on high-energy embayed beaches. The persistence and cumulative effect of megarips during storm groups, are described from three embayed beaches (Arrifana, Mt. Clérigo and Amoreira) exposed to high-energy wave conditions in the mesotidal, bedrock-dominated southwestern coast of Portugal. Morphological changes, determined by topographic monitoring and supplemented by interpretation of digital imagery over two years, revealed the development of storm-induced megarips, which exerted a major influence on beach erosion. Differences in megarip influence within and between beaches are related to embayment geometry and orientation, which significantly influence the nearshore wave field. Rip location is topographically controlled, being determined by alongshore variations in breaking wave height and obliquity, along with interaction of wave-driven circulation patterns and embayment nearshore topography. Moderate beach sand loss during individual storms was linked to the development of megarips with associated rip-neck and feeder channels. Extreme erosion, however, occurred when megarips and feeder channels persisted during successive storms, promoting continued erosion and seaward sediment export. Observations show that once initiated, megarip channels persist for several months and continue to act as conduits for offshore sediment transport under non-storm conditions. The maintenance of such rip circulation systems, driven by morphodynamic feedback, reduces beach recovery ability until the rip-neck and feeder channels are infilled.

Sediment transport associated with morphological beach changes forced by irregular asymmetric, skewed waves

[1] The waveshape effects on sediment transport are investigated for cross-shore beach profile changes. This study is based on experiments performed in the Laboratoire des Ecoulements Geophysiques et Industriels wave flume for irregular waves. The interest of such experiments resides in presenting complex combinations of wave skewness and asymmetry in bed load, ripple, and sheet flow regimes. Net sediment transport rates on typical beach morphodynamics are analyzed in regard to wave skewness and asymmetry, undertow, and ripple occurrence. Onshore bar migration is mainly associated with onshore-directed sediment transport, whereas terrace profile and offshore bar formation correspond to offshore sediment transport. As for natural beaches, energetic (moderate) wave climates mostly induce offshore (onshore) sediment fluxes. For a given significant wave height, an increase (decrease) in the wave climate peak period is associated with an increase (decrease) in wave skewness and leads mostly to offshore (onshore) sediment fluxes. The experiments are fully characterized by unsteady behavior. Consequently, several conditions exhibit phase-lag effects where the sediment is mobilized by the wave crest and transported by the following trough, which produces a net offshore transport even for a weak undertow. The presence of ripples clearly contributes to enhance this behavior. An original concept, due to its application to skewed asymmetric irregular waves, presents the important interaction between wave nonlinearities driving the sediment fluxes. The net sediment transport rate under strongly skewed waves is either offshore directed due to phase-lag effects or onshore directed when the wave asymmetry is large enough. Both these mechanisms probably largely contribute to bar formation and migration.

Along-Strike and Down-Dip Variations in Shallow-Marine Sequence Stratigraphic Architecture: Upper Cretaceous Star Point Sandstone, Wasatch Plateau, Central Utah, U.S.A.

Abstract The sequence stratigraphic architectures of shallow-marine deposits in the upper Cretaceous Star Point Sandstone are analyzed over a large (c. 100 km), nearly continuous outcrop section aligned oblique to depositional strike. The unit consists of five parasequences that predominantly comprise wave-dominated shoreface–shelf deposits. Two parasequences contain riverdominated delta-front deposits locally. Within each parasequence, wave-dominated shoreface–shelf deposits record 7–45 km of ESE- to ENE-directed progradation of a linear to moderately lobate shoreline that was supplied with sediment by longshore drift and subjected to strong offshore sediment transport by storms. Wave-dominated shoreface sandstones in each parasequence thin and wedge out over short distances ( 800 km2) river-dominated delta-front complex (Panther Tongue). This highly focused fluvial sediment flux probably occurred via a structurally controlled sediment entry point between two active thrusts.

SEDIMENT BUDGETS BASED ON THE MASS OF SILT AND CLAY ON INTERTIDAL FLAT ADJACENT TO RIVER MOUTH

Fine-grained sediment budgets based on the monthly bed level and net sediment flux monitoring during October 2006 to October 2007 are estimated to examine the relative significance of tides and river discharges on mud transport on an intertidal flat adjacent to a river mouth in the semi-closed sound. The intertidal flat accreted during normal discharge conditions mainly due to the alongshore sediment flux toward the river mouth. However, the flat eroded during large discharge conditions at the time of low tides. The offshore sediment transport during the large discharge was four times larger than that during the normal discharge. The spatial variations of tidal currents cause alongshore sediment fluxes toward the river mouth on the intertidal flat adjacent to the river mouth. The proposed fine-grained sediment budgets are useful to understand and assess the transport pathways for silt and clay on the intertidal flat.

Changes in monsoon and ocean circulation and the vegetation cover of southwest Sumatra through the last 83,000 years: The record from marine core BAR94-42

We present the palynological record from deep-sea core BAR94-42 taken offshore of southwest Sumatra in the Indian Ocean. Refinement of a previous age model for the core has been made using 7 additional radiocarbon ages and 29 additional oxygen-isotope measurements. These analyses have substantially improved the previously published chronology, which has enabled revision of the results of sedimentological studies of the core. The pollen and re-interpreted biogenic and terrigeneous data were used to reconstruct monsoon circulation and vegetation of southwest Sumatra over the last 83,000 years (83 kyr). During marine isotope stage (MIS) 5a, southwest Sumatra was covered by rainforest, with open herbaceous swamps lining river courses and surrounding lakes. The SE component of the monsoon was stronger than the NW component, resulting in a humid climate with a short, dry season. During MIS 4, conditions were drier, fire activity increased and the monsoon was generally weaker. This latter pattern persisted until MIS1. The vegetation was most open during MIS 3, between ~ 52 and 43 kyr; identifying this phase as the driest of the last glacial. An increase in montane trees from ~ 52 kyr indicates the onset of cooler conditions, which lasted until the transition to MIS 1 at about 11.9 kyr. After ~ 43 kyr, an everwet climate gradually developed as monsoonal circulation intensified and the SE monsoon component became stronger. During this time, closed-canopy rainforest became dominant regionally over southwest Sumatra. Increased monsoonal activity during the early Holocene resulted in increased precipitation, river runoff, sediment discharge and offshore sediment transport from the continental shelf.

Wave Overtopping and Overwash of Dunes

Wave-induced overwash of sand dunes may cause rapid profile changes and possible dune destruction, thus increasing the flood risk landward of dunes. Accurate modeling of the wave overwash processes is therefore essential in predicting dune profile evolution during overwash events. However, available field and laboratory data are very limited. We measured dune profile evolution and overwash transport rates for three tests with different initial dune geometries in front of a low-crested vertical wall exposed to the same wave conditions in a wave flume. In all three tests, the transition from minor to major overwash occurred rapidly but the resilience of the dune against destruction depended on its initial geometry. During minor overwash, offshore sediment transport caused most of the dune crest lowering. These tests are used to assess a numerical model for wave overtopping and overwash of a dune. The numerical model is shown to be in agreement with the measurements of the free surface elevations, fluid velocities, and dune profiles. However, beach erosion in front of the exposed wall is underpredicted.