Net Sediment Flux Research Articles

Sediment Delivery to a Tidal Marsh Platform Is Minimized by Source Decoupling and Flux Convergence

AbstractSediment supply is a primary factor in determining marsh response to sea level rise and is typically approximated through high‐resolution measurements of suspended sediment concentrations (SSCs) from adjacent tidal channels. However, understanding sediment transport across the marsh itself remains limited by discontinuous measurements of SSC over individual tidal cycles. Here, we use an array of optical turbidity sensors to build a long‐term, continuous record of SSC across a marsh platform and adjacent tidal channel. We find that channel and marsh concentrations are correlated (i.e., coupled) within tidal cycles but are largely decoupled over longer time scales. We also find that net sediment fluxes decline to near zero within 10 m of the marsh edge. Together, these results suggest that large sections of the marsh platform receive minimal sediment independent of flooding frequency or channel sediment supply. Marsh‐centric, as opposed to channel‐centric, measures of sediment supply may better characterize marsh platform vulnerability.

Automated measurement of eroding streambank volume from high-resolution aerial imagery and terrain analysis

Excessive sediment is an important form of surface water impairment throughout the continental United States. Numerous studies have investigated the role of upland soil erosion as a source of sediment and phosphorus, but contributions of streambank erosion are still poorly understood. Current methods such as delineation and automated channel planform morphometric models are either too time-intensive, or do not provide adequate spatial resolution to measure smaller rivers over large scales. To estimate sediment contributions from river migration on large scales, we have created the Aerial Imagery Migration Model (AIMM), a Python and ArcPy based automated channel migration model designed to estimate volumes of erosion and deposition related to channel migration. AIMM utilizes the Normalized Difference Water Index (NDWI) to derive binary representations of river channels from aerial photography. The location of the channel is then compared between two time periods to identify zones of erosion and deposition and the volume loss related to channel migration is then calculated using a LiDAR-derived DEM. When compared to three delineations and the RivMAP model in the South Fork Iowa River watershed, AIMM was found to have a 98% agreement with RivMAP, 79% agreement with delineations, and predicted net sediment flux that was within one standard deviation of the mean prediction from the delineation analysis. Where public imagery is available, AIMM can be widely applied to estimate volumes of sediment loss in a time and cost-efficient procedure. In particular, the use of AIMM within the project-planning phase of conservation efforts could help focus resources in areas where they can have the most impact.

Assessing the effects of shoreface profile concavity on long-term shoreline changes: an exploratory study

Coastal erosion under the context of global sea-level rise has threatened societies living along the coast. The processes related to decadal- to centennial-scale shoreline changes are much more complex than those suggested by the Bruun effect. This paper derives a one-dimensional model to explore the relationships between shoreline change, profile concavity, and sediment fluxes in wave-dominated coastal systems where alongshore sediment transport dominates the shoreline changes. The model incorporates an exponential approximation of shoreface profiles into the shoreline Exner equation. We found an equilibrium relationship in which a stable morphology profile is maintained during synchronous changes in sediment loss (gain) and shoreline retreat (advance) distances. Complex shoreline retreat behaviours can be classified into two dynamic equilibrium cases that are either above or below this equilibrium relationship. For a given negative net sediment flux, the shoreline retreat slows when the shoreface morphology steepens (i.e. increased profile concavity), and vice versa. Likewise, for a given positive net sediment flux, the shoreline advance slows when the shoreface morphology becomes gentle (i.e. decreased profile concavity), and vice versa. Interestingly, the higher the net sediment flux is, the greater the influence of shoreline changes on shoreface morphology. The short- and long-term processes governing the shoreface morphological changes are also discussed. This study demonstrates the importance of shoreface morphological concavity on the shoreline changes. Furthermore, the presented first-order model might have the potential to be used as a quick assessment for future decadal- to centennial-scale shoreline changes in coastal management and engineering activities.

The Impact of Winter Storms on Sediment Transport Through a Narrow Strait, Bohai, China

AbstractThe Yellow River is one of the most significant sources of terrestrial sediment to the global seas, and the Bohai Strait is the only pathway that delivers Yellow River‐derived sediments from the shallow Bohai Sea to the Yellow Sea. To investigate sediment transport processes through the strait under the influence of storms (strong northerly winds) that frequently occur in winter, we deployed two sets of observing platforms equipped with Acoustic Doppler Current Profilers (ADCP) and a suite of other sensors in the strait in January 2018. Aided by a system of high‐resolution models, we reconstructed sediment dynamics in response to the strong northerly wind of a winter storm. Model results show that the instantaneous suspended sediment flux (SSF) is highly aligned with tidal currents, while the net sediment flux has a clear dependence on variations in exchange flow and sediment resuspension. Enhanced coastal currents, intensified wave motions, and higher suspended sediment concentrations indicate that the through‐strait sediment flux during outflows is greater than during inflow conditions. Such SSF asymmetries are believed responsible for the net sediment export through the Bohai Strait in wintertime. Diagnostic analyses provided insights into the dynamic mechanisms of exchange flow variations influenced by both the strong northerly winds and the wind‐triggered coastal trapped waves in the shallow, narrow strait via geostrophic effects. This study highlights the importance of storm‐induced horizontal exchange processes in a coupled bay‐shelf system.

Sediment Transport and Morphological Changes at the Mouth Delta of Shirakawa River

Intertidal mudflats are important for land conservation, flooding risk alleviation and biological environment. Mudflat profiles have been measuring monthly along some cross-shore lines exceeding 1,000 m in the vicinity of a river mouth since December 2000. For the mud profile evolution, the annual accretion rate was 4.2 cm/year and 1.6 cm/year on the right and left lines, the seasonal variation in the range -5 cm and 5 cm, and the maximum episodic variation 12.1 cm and 22.0 cm. The wind waves could play many contributions for deposition and erosion of sediment. Sediment budgets based on the monthly bed level and net sediment flux monitoring were estimated to examine the relative contributions of tides and river discharge to sediment transport. The estimated sediment budgets are interpreted using the water mass balance equation and the horizontal tidal current pattern. The intertidal flat accreted during normal discharge conditions are primarily attributable to the alongshore sediment flux toward the river mouth. However, the flat was eroded when the large offshore suspended sediment transport occurred on the flat during the large river discharge. The net alongshore tidal current causes alongshore sediment fluxes toward the river mouth on this intertidal flat adjacent to the river mouth. In observation periods, there were two times of a significant flood occurred on July 12, 2012 with the discharge water rates of 2,300 m3/s and on June 21, 2016 with 1,600 m3/s. Also two earthquakes scaled of 7 on the Japanese scale struck Kumamoto Prefecture on April 14 and 16, 2016, and then about the 0.4 m ground subside was occurred at the present field site. A large amount of sediment from Shirakawa River was discharged into the delta due to the flood. The clinoform along the water route approximately propagated 150 m in the offshore direction and the bed level increased by 1.5 m. The estimated sediment discharge was 1.1x105 m3/yr./km2 during 2014 to 2016, corresponds four times during 1978 to 1997.

Was the trend of the net sediment flux in Poyang Lake, China, altered by the Three Gorges Dam or by sand mining?

The net sediment flux in Poyang Lake, the largest freshwater lake in China, converted from positive to negative after 2000. Whether the effect of the Three Gorges Dam (TGD) or sand mining caused this change was disputed. We analyzed the trend of sediment input and output in Poyang Lake from 1956 to 2014, determined the annual numbers of sand dredgers by interpreting Landsat ETM + images, and calculated the changes to the annual net sediment flux (Sedimentdiff) after 2000. The decreased sediment input was mainly caused by decreased sediment concentrations in the five tributaries. The increased sediment output was caused by increased sediment concentrations, which was not related to the operation of the TGD, but strongly related to sand mining activities in the northern parts of the lake. The Sedimentdiff variation in different operational periods of the TGD showed that the TGD was not the main driver of the change to the net sediment flux trend in Poyang Lake. The strong positive correlation between the number of sand dredgers and Sedimentdiff indicated that sand mining activities in the northern parts of the lake were important drivers of the changes in the net sediment flux trend in Poyang Lake after 2000. It was sand mining activities, rather than the TGD, that altered the trend of the net sediment flux in Poyang Lake after 2000.

The coupling of bay hydrodynamics to sediment transport and its implication in micro-tidal wetland sustainability

To investigate bay hydrodynamics and its impacts on the adjacent micro-tidal wetland sustainability, hourly measurements of wave, tidal current, and benthic suspended sediment concentration in summer, winter, and spring of 2015–2016 were conducted in Fourleague Bay, Louisiana, USA. High-temporal resolution data indicate that benthic suspended sediment resuspension had a dominant periodicity of 4.8-d, which was mainly caused by wind-driven waves. Sediment flux reached 28 g·m−2·s−1 during events. Net sediment flux direction is northwestward in summer, and southeastward in winter and spring. Potential depth-integrated sediment flux to surrounding wetland varied within 0–500 g·m−1·s−1. Seasonal variations of river discharge and wind direction (particularly speed >3 m·s−1) dominated potential sediment contribution from the bay to the surrounding wetland. Three sediment transport regimes were delineated: ‘bypassing’ season, resuspension-accumulation season, and combined ‘bypassing’ and resuspension-accumulation season. This study couples bay hydrodynamics to the sediment transport processes and sustainability of adjacent wetlands in a micro tidal environment. It sheds light on the understanding of natural feedback mechanisms and how estuarine-marsh system survive high relative sea level rising scenario in micro tidal environment, which could aid in the design of future ecological engineering restoration strategies.

ĐÁNH GIÁ ẢNH HƯỞNG CỦA MỘT SỐ GIẢI PHÁP CÔNG TRÌNH ĐẾN TRAO ĐỔI NƯỚC VÀ VẬN CHUYỂN BÙN CÁT KHU VỰC ĐẦM NẠI (NINH THUẬN)

With the area of only about 700 ha, Nai lagoon has a great contribution to ecology as well as socioeconomic development of Ninh Hai disctrict and Phan Rang-Thap Cham city (Ninh Thuan province). However, the deposition tendency has occurred recently, making the lagoon shallower. It also causes the decrease in water exchange as well as environmental degradation in this area. Therefore, solutions that increase water exchange and reduce the deposition rate in the Nai lagoon are necessary. Based on the Delft3D model with MORFAC approach, this paper gives some assessments on the impact of coastal engineering solution on water exchange and sediment transport. The simulation scenarios include as extending present jetties to 165 m long, extending present jetties combined with dredging lagoon inlet, extending only northeast jetty to 260 m long combined with dredging lagoon inlet, and dredging all lagoon area. The results show that the change in water exchange is very small (almost below 1%), except in the case of dredging the entire lagoon area, water exchange decreases about 10% (due to the increase of the lagoon water volume) compared with the present. Sediment flux balance at cross-section MCI (Tri Thuy bridge) also changes slightly under influence of coastal engineering works. On the other hand, sediment flux from the coastal zone into the inlet (cross-section 2- MCII) and alongshore to Southwest (cross-section MCIII) have strongly decreased. Especially, in the scenario of extending northeast jetty combined with and dredging the inlet zone, net sediment flux into the inlet decreases from 637 m3/day to 180 m3/day (droughty year) and from 535 m3/day to 80 m3/day in the flood year.

The Variations of Sediment Transport Patterns in the Radial Sand Ridges along the Jiangsu Coast, China over the Last 30 Years

Tao, J.; Xu, F.; Yao, P.; Zhou, Z., and Zhang, C., 2018. The variations of sediment transport patterns in the radial sand ridges along the Jiangsu coast, China over the last 30 years. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 216–220. Coconut Creek (Florida), ISSN 0749-0208.The research objective is to clarify the primary factors lead to the variations of the suspended sediment transport in the radial sand ridges (RSRs) along the Jiangsu Coast over the last 30 years. The suspended sediment concentration (SSC) fields measured in several field surveys from 1980 to 2010 were analyzed. The field data indicate that the SSCs in 1980 are higher than those 2010. A two-dimensional numerical model was developed to simulate the inter-annual variations of the suspended sediment transport. Generally, the magnitude of the SSC is highly relevant with the current speed, which peaks around the Old Yellow River Delta (OYRD) and in the central RSRs. However, the northern part of the RSRs indicates higher SSC than the southern part, which is attributed to their different bed sediment composition. The net suspended sediment flux was calculated. In the north of the RSRs, the sediments are transported in the south direction, which implies a sediment input from the OYRD to the RSRs. In the south of the RSRs, a net sediment loss was observed, which is consistent with the decreasing of the sediment flux from the Yangtze River. The simulated suspended sediment transport in 1980 and 2010 mutually show similar spatial patterns, while SSCs in 1980 are overall larger than those in 2010. It is concluded that the decrease of the SSCs from 1980 to 2010 is attributed to the reduction of the OYRD and the large-scale reclamation along the Jiangsu coast, while the sediment fluxes from the Yangtze River is less important.

Turbidity Currents With Equilibrium Basal Driving Layers: A Mechanism for Long Runout

AbstractTurbidity currents run out over 100 km in lakes and reservoirs, and over 1,000 km in the ocean. They do so without dissipating themselves via excess entrainment of ambient water. Existing layer‐averaged formulations cannot capture this. We use a numerical model to describe the temporal evolution of a turbidity current toward steady state under condition of zero net sediment flux at the bed. The flow self‐partitions itself into two layers. The lower “driving layer” approaches an invariant flow thickness, velocity profile, and suspended sediment concentration profile that sequesters nearly all of the suspended sediment. This layer can continue indefinitely at steady state over a constant bed slope. The upper “driven layer” contains a small fraction of the suspended sediment. The devolution of the flow into these two layers likely allows the driving layer to run out long distances.

Net sediment transport in tidal basins: quantifying the tidal barotropic mechanisms in a unified framework

Net sediment transport in tidal basins is a subtle imbalance between large fluxes produced by the flood/ebb alternation. The imbalance arises from several mechanisms of suspended transport. Lag effects and tidal asymmetries are regarded as dominant, but defined in different frames of reference (Lagrangian and Eulerian, respectively). A quantitative ranking of their effectiveness is therefore missing. Furthermore, although wind waves are recognized as crucial for tidal flats’ morphodynamics, a systematic analysis of the interaction with tidal mechanisms has not been carried out so far. We review the tide-induced barotropic mechanisms and discuss the shortcomings of their current classification for numerical process-based models. Hence, we conceive a unified Eulerian framework accounting for wave-induced resuspension. A new methodology is proposed to decompose the sediment fluxes accordingly, which is applicable without needing (semi-) analytical approximations. The approach is tested with a one-dimensional model of the Vlie basin, Wadden Sea (The Netherlands). Results show that lag-driven transport is dominant for the finer fractions (silt and mud). In absence of waves, net sediment fluxes are landward and spatial (advective) lag effects are dominant. In presence of waves, sediment can be exported from the tidal flats and temporal (local) lag effects are dominant. Conversely, sand transport is dominated by the asymmetry of peak ebb/flood velocities. We show that the direction of lag-driven transport can be estimated by the gradient of hydrodynamic energy. In agreement with previous studies, our results support the conceptualization of tidal flats’ equilibrium as a simplified balance between tidal mechanisms and wave resuspension.

Sediment flux and sediment-induced stratification in the Changjiang Estuary

Sediment deposition in the north passage of the Changjiang Estuary, where the Deep-water Navigation Channel (DNC) is located, has been a major concern in the past decades. To understand the suspended sediment dynamics and the effects of sediment-induced stratification on sediment flux in the navigational channel, field data on tidal flow and suspended sediment concentration (SSC) are collected and analyzed in this study. It is shown that net sediment transport is dominated by ebb currents in the study area. The net sediment flux is generally toward the ocean and the maximum value is found to be in the middle reach of the passage. In the lower reach of the passage, the net sediment flux is landward in the lower layer and seaward in the upper layer of the water column due to the two-layer feature of the estuarine circulation. Advective flux plays a significant role in transport of sediment in upper and middle reaches of the passage by carrying 70–100% of the suspended sediment. However, this amount is reduced to 30–60% in lower reach of the passage, where tidal effects become more important. The suspended sediment-induced stratification in the north passage is examined by calculating eddy viscosity. It is found that suspended sediment can reduce eddy viscosity by 10–30%. The highest depth-averaged SSC is located in the middle reach of the north passage, where the averaged SSC is 4–15 times higher than that in the upper reach. In this region, bed shear stress is larger at ebb, while SSC is higher at flood. It is inferred that suspended sediments in the DNC during flood are partially transported from a neighboring shoal, which plays an important role in sediment dynamics in the north passage.

Responses of water environment to tidal flat reduction in Xiangshan Bay: Part II locally re-suspended sediment dynamics

Xiangshan Bay is a semi-enclosed bay in China, in which tidal flats have been substantially reclaimed to support the development of local economies and society over previous decades. The loss of tidal flats has led to changes of tides and locally suspended sediment in the bay. The effects of tidal flat reduction on locally suspended sediment dynamics was investigated using a numerical model forced by tidal data and calibrated by observed tidal elevation and currents. The model satisfactorily reproduces observed water levels, currents, and suspended sediment concentration in the estuary, and therefore is subsequently applied to analyze the impact of tidal flat reclamation on locally suspended sediment transport. After the loss of the tidal flats from 1963 to 2010, the suspended sediment concentrations (SSC) at the bottom boundary layer were reduced/increased in the outer bay/tidal flat areas due to weakened tidal currents. In the inner bay, the SSC values near the bottom level increased from 1963 to 2003 due to the narrowed bathymetry, and then decreased from 2003 to 2010 because of the reduced tidal prism. The model scenarios suggest that: (1) a reduction of tidal flat areas appears to be the main factor for enhancing the transport of sediments up-estuary, due to the increased Eulerian velocity and tidal pumping; (2) A reduction of tidal flat areas impacts on spatial and temporal SSC distribution: reducing the SSC values in the water areas due to the reduced current; and (3) a tidal flat reduction influences the net sediment fluxes: lessening the erosion and inducing higher/lower landward/seaward sediment transportation.

Measurements of suspended sediment transport and turbulent coherent structures induced by breaking waves using two-phase volumetric three-component velocimetry

Sediment suspension and transport under plunging regular waves was investigated in a laboratory surf zone using the volumetric three-component velocimetry technique. The two-phase flow measurements captured the motions of sediment particles simultaneously with the three-component, three-dimensional velocity fields of turbulent coherent structures (large eddies) induced by plunging breakers. Sediment particles were separated from fluid tracers based on a combination of particle spot size and brightness in the two-phase flow images. The interactions between the large eddies and bottom sediment were investigated in the outer surf zone. The measured data showed that breaker vortices impinging on the bottom was the primary mechanism that lifted sediment particles into suspension. High suspended sediment concentrations were found in the wall-jet region where the impinging flow was deflected outward and upward. Sediment particles were also trapped by counter-rotating vortices behind the down flow. Suspended sediment concentrations were significantly lower in the impingement zone where the fluid velocities were downward, even though the turbulent kinetic energy in the down flow was very high. Suspended sediment concentration was well correlated with vertical velocity and apparent shear stresses in the deflected flow, and with vorticity magnitude in the counter-rotating vortices. A linear relationship was found between net sediment flux and net turbulent kinetic energy flux over one wave cycle. It was found that a strong deflected flow in front of the impingement zone enhanced onshore sediment transport compared to a more symmetrical flow pattern, while counter-rotating vortices kept sediment particles in suspension for transport offshore after flow reversal. Onshore sediment transport was observed in less than 20% of the breaking waves. In most wave cycles, net sediment flux was directed offshore due to advection by predominantly offshore flow velocities.

Flux and its seasonal variation of suspended particulate matter in the Bohai Sea, Yellow Sea and East China Sea

AbstractTransport and flux of suspended particulate matter (SPMs) are important but hotly debated topics in the Bohai Sea (BS), Yellow Sea (YS) and East China Sea (ECS). In this study, we calculated the sediment flux and its seasonal variation combining observed suspended sediment concentration (SSC) with numerically simulated currents. Field observations were conducted in the BS, YS and ECS at 43 stations in summer and 46 stations in winter along five transects. HYCOM/NCODA reanalysis data were used to calculate climatologically monthly averaged currents in January and July from 1998 to 2007. The results indicated that a high concentration of SPM moved out of the BS and moved southward into the YS and ECS in winter, with a net sediment flux of 0.12 t/s at the Bohai Strait and 4.37 t/s south of the Yangtze River. In summer, SSC increased from the surface to the bottom layer due to stratification. A southerly wind and stronger warm currents in summer transported SPM from the ECS to the YS and the BS, with a net northward sediment flux of 4.79 t/s at the boundary of the ECS and the YS, and a net sediment flux of 0.08 t/s input into the BS. This study can help to distinguish the contribution of the Yellow River‐ and Yangtze River‐derived sediments to the mud‐areas' generation on the continental shelves area of the Yellow Sea and East China Sea. Copyright © 2016 John Wiley & Sons, Ltd.

Sediment Transport and Dispersal Pattern from the Bohai Sea to the Yellow Sea

ABSTRACT Wang, A. M.; Wang, H. J.; Bi, N. S., and Wu, X., 2016. Sediment transport and dispersal pattern from the Bohai Sea to the Yellow Sea. Suspended sediment flux, dispersal patterns and its possible mechanisms in the Bohai Sea were investigated based on the observational data in May and November, 2012, data retrieved from MODIS imageries and the ocean current data from HYCOM. The sediment dispersal pattern and the annual net sediment flux presented evident seasonal variability. During wintertime, the prevailing strong northerly winds and the related high wave heights resuspended the sediment along the coast of the Huanghe Delta that was transported into the Laizhou Bay along its western coast and exported to the Bohai Strait through the eastern Laizhou Bay as enhanced by the coastal current. The wintertime contributed approximately 4.84 Mt of suspended sediment flux to the Yellow Sea, as higher than the flux in summertime by one order. In summertime, the less energetic environment together with the s...

Erosion and deposition within Poyang Lake: evidence from a decade of satellite data

Abstract Poyang Lake, an important wetland in the Ramsar Convention List, is the largest freshwater lake in China and an essential component of the Yangtze River system. The lake is increasingly experiencing serious water crises including seasonal desiccation, decreased wetland area, and water shortages, all of which are closely related to progressive changes in the lake's topography over recent years. Atime-series of bottom topography would contribute to our understanding of the lake's evolution during the past several decades. However, quality bathymetric data for Poyang Lake are scarce owing to the highly dynamic and turbid nature of its water. To resolve this limitation, we used a total of 146 medium-resolution satellite images to build annual and quasi-annual bottom topography maps of Poyang Lake during the period from 2000 to 2010 based on the well-established waterline method. Our results show that: (1)the average elevation of the lakebed relative to sea level has decreased by 14.4 cm/yr. from 2000 to 2010; and (2)the observed annual changes in the lakebed elevation were well correlated (r = 0.84) with measured changes in the lake's annual net sediment flux. The observed trends may be attributed to the impacts of human activities, especially the operation of the Three Gorge Dams, frequent sand mining, and the implementation of a large water conservancy project. This decade-long quantitative understanding of the lake's evolution and bottom topography elevations might assist both researchers and local policymakers in ecological management, wetland protection, and lake navigation safety.

Mechanisms underlying the regional morphological differences between the northern and southern radial sand ridges along the Jiangsu Coast, China

Radial sand ridges (RSRs) spread in a fan-shaped pattern over the seabed of the southern Yellow Sea along the Jiangsu Coast (China) with pronounced differences between the northern and southern channel-shoals (indicated as “NCS” and “SCS”, respectively). A depth-averaged nested numerical model is employed to analyze the tidal hydrodynamics and sediment dynamics of the RSRs. Model results show considerable regional differences in terms of tidal current vector ellipses: “eight-shaped” ellipses are typical in the NCS where rotational tidal waves dominate, while the SCS are characterized by “egg-shaped” ellipses because of the dominance of progressive tidal waves. Sand ridges tend to be elongated and straight in the NCS while short and dincontinuous in the SCS. Numerical Lagrange particles released near the ridges in the NCS tend to travel across the crest of the ridges rather than move around them as observed in the SCS. Consistently, the patterns of sediment convergence and divergence indicate that suspended sediments converge over the whole Xiaoyinsha Ridge located in the NCS, while converge to the two sides of the Hetunsha Ridge located in the SCS. Model results also highlight the importance of initial sediment compositions in determining patterns of the net sediment fluxes. The suspended sediment concentration (SSC) time series during one tidal cycle may show single or double peaks depending on the grain size and the related lag effect. Bed load transport is much smaller than suspended load. So sediment compositions should be carefully considered when simulating the SSC field, and the hydrodynamic of the morphological evolution of the RSRs at the regional scale. Overall, this study suggests that the NCS and SCS may be treated as two relatively independent geomorphological systems characterized by different tidal flows and sediment compositions.

Temporal observations of a linear sand dune in the Simpson Desert, central Australia: Testing models for dune formation on planetary surfaces

AbstractLinear dunes are the most common dune form found on planetary surfaces, yet questions remain about their formation. Temporal observations of a linear dune located in the Simpson Desert of central Australia were made to monitor dune movement and to test competing hypotheses regarding linear dune formation. Our observations were collected on three separate occasions from 2006 to 2014. Rebar stakes were placed in a gridded pattern so that multiple measurements of sand thickness, GPS surveys, and photographs could be taken at the same locations over time. We observed widespread reworking of sand on and around the dune crest, with sand accumulation locally exceeding a meter between surveys. Overall, the height of the dune crest increased by several centimeters. We also observed fluctuations in the sand cover in the adjacent swales that often exceeded 2–3 cm between surveys, yet we did not observe any appreciable changes in the position of the dune's downwind terminus. Weather data indicate that the effective sand‐transporting winds in the Simpson are widely unimodal. Net sediment flux (resultant drift direction) is toward the north‐northwest, locally at an oblique angle to dune orientation. Collectively, our results suggest that the linear dune is actively maintained by vertical accretion. The implications from our observations are that linear dunes on other planetary surfaces could form in wind regimes that are widely unimodal, even where the resultant drift direction is locally oblique to dune orientation. In particular, such findings may provide support for global circulation models of Titan.

Settling fluxes of cohesive sediments measured by sediment traps in a semi-enclosed embayment with strong tidal environments

To understand the variations of vertical settling flux in a spring-neap tidal cycle in strong tidal environments, an anchored vessel with a Hydro-Bios Multi-Sediment Trap and a Nortek Acoustic Doppler Velocimeter (ADV) was deployed in the sub-tidal flat in Luoyuan Bay, a typical semi-enclosed embayment, from January 16 to 31, 2010 (from spring to neap to spring again). The observed results reveal that there is a strong relationship between the Signal-Noise Ratio (SNR) recorded by the ADV and the synchronous suspended sediment concentration (SSC) obtained by the filtrated method. The time-series of SSC and settling velocity were then estimated. The measured results indicated that the maximum tidal range during the observation was 7.03m, which occurred during the spring tide, and the minimum value during the neap tide was 3.58m. The near-bottom (1.1m above the bed) current speed, SSC, bed shear stress, and settling velocity decreased from spring to neap tide, and the maximum values were 0.556m/s, 53.40mg/L, 0.806N/m2 and 0.26mm/s, respectively. The horizontal flux varied from 0.04g/m2s to 21.77g/m2s; the horizontal fluxes over a tidal cycle decreased from spring to neap tide, and the net horizontal flux of suspended sediment was 880.66kg/m2 with a net transport direction of 292° during the entire observation time. The daily vertical settling fluxes, calculated based on mass balance, varied from 17.03g/m2d to 56.86g/m2d, which were lower than those obtained by the sediment trap (18.50–332.80g/m2d). According to the analyzed results of the hydrodynamic data, when the current speed exceeds 0.20m/s, the flow can penetrate the whole sediment trap and the bottom quiescent layer no longer exists. The horizontal flux plays an important role in the trap collection efficiency in strong tidal environments, which can lead to overestimation and a maximum value of 585% during the spring tide. A constant settling velocity (mean value during the observation) may underestimate the daily settling fluxes by approximately 22.28–51.80% during the neap tide and overestimate the fluxes by approximately 19.61–30.86% during the spring tide.