Net Sediment Transport Pathways Research Articles

Sediment distributions and transport patterns in the Pearl River Estuary and its adjacent coastal ocean

Coastal sediment is important for the development of river deltas and for carrying both nutrients and pollutants into estuaries and coasts. Totally 265 surficial sediments collected from the Pearl River Estuary and its adjacent coastal ocean were analyzed by using Gao-Collins method to study the sediment transport path and deposition center. The Pearl River Estuary was an area of sandy mud sediment and showed some muddy patches. There were 5 distinguishable sediment convergence centers, which represent 3 types of sediment transport patterns and hydrodynamic environments. The net sediment transport pathways in the estuary and shelf sea were mainly in the offshore and landward direction, respectively. The analyzed results indicated that the main part of muddy sediment was confined to the mouth of the estuary due to the combined effects of the expansion and salinity intrusion.

The Irish Sea bed load parting zone: Is it a mid-sea hydrodynamic phenomenon or a geological theoretical concept?

On tidally-dominated shelf seas, hydrodynamic flow exerts shear stress on the seabed, inducing sediment transport and seabed evolution. Net sediment transport pathways and associated bed load partings zones (the ‘head’ of sediment transport pathways) are important elements of this sediment dispersal process. A robust comprehension of these dynamics on a local and regional scale are crucial for a range of offshore activities, including the rapidly growing offshore renewable energy industry. Previous studies infer a bedload parting zone (BLP) in the central Irish Sea, yet this predominantly bedform-derived BLP remains to be fully supported by underlying hydrodynamics. This study reviews how sediment transport pathways in the Irish Sea are defined to date, investigates whether a hydrodynamic regime capable of supporting this inferred BLP exists, and if proven, aims to elucidate its origin.Tidal asymmetry indicators, derived from the phase and amplitude relationships of numerically modelled M₂ and M₄ tidal constituents, reveal the existence of a symmetrical tidal zone in the proximity of the inferred BLP. New numerically modelled residual tidal current and bed shear stress vectors reinforce the presence of this distinct tidal character change and further reveal strong opposing tidal flows extending from this region, thus supporting the presence of a tidal flow capable of producing divergent bedload patterns. Analysis of tidal propagation through the Irish Sea Basin concludes that the origin of the BLP is mainly attributed to the intersection of the north and south tidal fronts at an inclined angle due to Coriolis Forcing and coastline interactions. Minor factors impacting the shape and location of the BLP are the change in tidal character at (a) abrupt bathymetry changes, (b) headlands and intricate coastline topography, and (c) large-scale constrictions. Further investigation into localised morphodynamics is needed. These findings are highly valuable to efficient marine spatial planning, underpinning a movement towards a climate resilient economy. The integrated approach used in this body of work can be applied to tidally-dominated continental shelves beyond the Irish Sea.

Grain-size characteristics and net transport patterns of surficial sediments in the Zhejiang nearshore area, East China Sea

Summary Spatial variations in grain-size parameters can reflect sediment transport patterns and depositional dynamic environments. Therefore, 616 surficial sediment samples taken from the Zhejiang nearshore area in the East China Sea were analyzed to better understand the net sediment transport pattern in this region. The study area is generally dominated by clayey silt and has relatively coarse mud sediment in the southeast. The sorting coefficient of surface sediment is higher than 1.4, and sediment is poorly sorted throughout the study area. The skewness has a strong correlation with the mean grain-size diameter. The net sediment transport pathways obtained by the grain-size trend analysis indicate several distinct characteristics of the surficial sediment transport. The sediment is transported southward under the action of the stronger southward Zhejiang–Fujian Coastal Current (ZMCC) in winter in the upper part of the nearshore area. Influenced by the obstruction of the Taiwan Warm Current (TWC) and the tidal current, surficial sediment transport vectors display two areas of grain-size trend convergence and indicate the net deposition environment has a high sedimentation rate. However, the transport is mainly toward the north under the control of the prevailing northward ZMCC and the strong TWC in the summer. The sedimentary rate is closely related to the processes of the sediment transport. On the one hand, sediment transportation affects the depositional rate in a different environment. On the other hand, the modern sedimentary rate can reflect indirectly the sediment source and sediment transportation.

A detrital sediment budget of a Maldivian reef platform

Sediment dynamics are an important control on the morphology and development of reef systems by actively removing and redistributing excess detrital sediment. This study presents quantitative data from direct point measurements of sediment transport on the platform surface and fore-reef slope of Vabbinfaru reef, North Malé Atoll, Maldives. A suite of sediment traps were used to construct actual rates of platform sediment fluxes and off-reef export over different spatial and temporal (seasonal) scales to establish key sediment transport pathways. Findings showed that high sediment fluxes occur on Vabbinfaru platform in the absence of major storm activity (up to 1905gm−1d−1), with 95% of annual transport occurring during the southwest monsoon as a result of increased wave energy. Climate-driven changes in the platform process regime caused a reversal of net sediment transport pathways between each monsoon season. Off-reef export rates were high, reaching a maximum of 12.58kgm−1y−1 for gravel and 407gm−1d−1 for sand-sized sediment. An estimated 127,120kg is exported from the platform annually equating to a significant loss from the reef sediment budget and contributing to the long-term geomorphic development of the fore-reef slope and atoll basin. Detrital sediment reservoirs on Vabbinfaru are not purely depositional carbonate sinks, but rather temporary stores that are important in the transfer of sediment between reef zones.

A Sediment Trend Analysis (STA®) in Support of Dredged Material Management in Lyttelton Harbour, Christchurch, New Zealand

McLaren, P. and Teear, G., 2014. A sediment trend analysis (STA®) in support of dredged material management in Lyttelton Harbour, Christchurch, New Zealand. About 300 sediment samples were collected from outer Lyttelton Harbour for the purpose of carrying out a sediment trend analysis (STA). The purpose was to assess the sediment behaviour of dredged material placed along the harbour's north side, where disposal operations have been occurring yearly since 1969 (43 years ago at the time of sampling for the STA). Prior to 1969, material had been placed on the south side of the harbour. STA is a technique that determines net sediment transport pathways and the behaviour of bottom sediments from the relative changes in grain-size distributions. The STA determined 68 sample sequences (lines of samples) showing that the dominant source for the harbour sediments is from the steep surrounding hillsides. Sediments are moved from the shoreline into the harbour and progress seaward into the Pacific Ocean. There is no longer any evidence for dredged material on the south side; however, material on the north side becomes incorporated into the existing transport regime and can result in temporary deposition in its transport out of the harbour. The conceptual understanding of the transport regime and the dispersal of dredged material did not conflict substantively with previous hydrodynamic observations and modelling.

Surficial sediment distribution and the associated net sediment transport pattern retain-->in the Pearl River Estuary, South China

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.

An integrated GIS for sedimentological and geomorphological analysis of a lagoon environment. Barra de Cananéia inlet region, (Southeastern Brazil)

The aim of this work is to use GIS integration data to characterize sedimentary processes in a SubTropical lagoon environment. The study area was the Cananeia Inlet estuary in the southeastern section of the Cananeia Lagoon Estuarine System (CLES), state of Sao Paulo, Brazil (25°03′S/47°53′W). The area is formed by the confluence of two estuarine channels forming a bay-shaped water body locally called “Trapande Bay”. The region is surrounded by one of the most preserved tracts of Atlantic Rain Forest in Southwestern Brazil and presents well-developed mangroves and marshes. In this study a methodology was developed using integrated a GIS database based on bottom sediment parameters, geomorphological data, remote sensing images, Hidrodynamical Modeling data and geophysical parameters. The sediment grain size parameters and the bottom morphology of the lagoon were also used to develop models of net sediment transport pathways. It was possible to observe that the sediment transport vectors based on the grain size model had a good correlation with the transport model based on the bottom topography features and Hydrodynamic model, especially in areas with stronger energetic conditions, with a minor contribution of finer sediments. This relation is somewhat less evident near shallower banks and depositional features. In these regions the organic matter contents in the sediments was a good complementary tool for inferring the hydrodynamic and depositional conditions (i.e. primary productivity, sedimentation rates, sources, oxi-reduction rates).

Sediment Trend Analysis through the variation of granulometric parameters: A review of theories and applications

Spatial variations in grain-size parameters (i.e. grain-size trends) contain information on sediment transport patterns. Analytical procedures have been proposed using the grain-size trend to determine net sediment transport pathways. In the first part of this study, the fundamentals of the theory are presented through methods for analysing 1D and 2D variations. The methods used are critically discussed, while pointing out some severe problems. So far, these methods suffer from limitations leading to serious interpretational errors, making it necessary to take account of two kinds of uncertainties. Inputs uncertainties are linked to the physical sediment properties as well as procedures of sampling and analysis. Model uncertainties are then discussed for each step of the grain-size trend analysis. The validity of Sediment Trend Analysis under natural conditions is tested against published field studies to determine the most appropriate variation trend to use in a specific environment. Proposals are given for each step of the procedure for optimal use of the method using a Quality Assurance (QA) approach. Further developments are proposed, such as integration into a Geographic Information System.

Impact of offsite sediment transport and toxicity on remediation of a contaminated estuarine bay

Sediment of Ostrich Bay, an arm of Dyes Inlet on Puget Sound, was historically contaminated with ordnance compounds from an onshore US Navy facility. An initial recommendation for a sediment cover to mitigate benthic risks was followed by studies of sediment transport and deposition to determine whether contaminated sediment from Dyes Inlet or other offsite sources in Puget Sound may contribute to Ostrich Bay impacts. A Sediment Trend Analysis (STA ®) identified net sediment transport pathways throughout the bay and inlet by examining changes in grain size distributions in multiple adjacent samples. Results indicated that fine-grained sedimentary material transports into and deposits throughout the Dyes Inlet system, with no erosion or transport out of Ostrich Bay. Echinoderm larvae mortality bioassay results were elevated in fine-grained sediments of both Ostrich Bay and Dyes Inlet. Ordnance compounds were undetected, and although sediment mercury concentrations were elevated at 0.48–1.4 mg/kg in both waterbodies, the relationship with toxicity was weak. Results of the studies and sedimentation modeling indicate that impacted sedimentary material deposits throughout the Dyes Inlet/Ostrich Bay system from unknown sources and will prevent natural recovery of Ostrich Bay as well as negate long-term effectiveness of active remedial measures. Stakeholders have recognized that remediation of the bay can be achieved only after the toxicity of depositing sediment decreases.

The use of grain size trends in marine sediment dynamics: A review

Spatial changes in grain size parameters (i.e. grain size trends) contain information on sediment transport patterns. An analytical procedure has been proposed to transform the grain size trends into an image of trend vectors, which may represent net sediment transport pathways. A fundamental assumption for such an approach is that the frequency of occurrence of the trend adopted is much higher in the transport direction, than in any of other directions. Preliminary studies show agreement between this assumption and observations. However, further investigations into the physical processes and mechanisms for the formation of grain size trends are required to improve the technique, including flume experiments and numerical modeling. Moreover, attention should be paid to the trends associated with finegrained sediment, for the method of grain size trend analysis is so far designed for coarse-grained material only. The processes of flocculation during settling and the wash-load property must be considered. Appropriate interpretation of grain size data will improve our understanding of the physics of granular materials.

Offshore sand bank dynamics

The present paper reports some key results from field investigations and numerical modelling studies of the tide- and wind-induced hydrodynamics and sediment dynamics of Middelkerke Bank (MB) in the southern North Sea of Europe conducted during December 1992 to March 1993. Strong surface current refraction and acceleration effects were observed over MB using the HF radar system OSCR ( Ocean Surface Current Radar). Results suggest that OSCR data may be used remotely to monitor large-scale bathymetry in shallow coastal environments. Spatial variation in tidal propagation characteristics and modification of shoreward propagating waves was not detected at locations around MB during the experiment. Observed residual currents were found to be correlated strongly with wind speed and direction during the period 26 February to 18 March 1993. However, in low wind stress condition, a three-dimensional numerical model (3D-Bank) indicated the presence of a clockwise residual circulation of water around MB consistent with theory. Spatial and temporal variation in the average total drag coefficient ( C d) of MB were investigated and found to correlate strongly with tidal current speed. Fluorescent sand tracers, used to monitor net sediment transport pathways, revealed a net clockwise movement of sediments around MB consistent with predictions by 3D-Bank and with theory.

A FORTRAN program for grain-size trend analysis to define net sediment transport pathways

A FORTRAN program for grain-size trend analysis to define net sediment transport pathways

Grain size trends associated with net sediment transport patterns: An example from the Belgian continental shelf

Grain size trends in relation to net sediment transport pathways are examined, using some commonly-used grain size parameters. The results of the trend analysis (for 15 types of the trends) are compared with a known net sediment transport pathways established on the basis of the sandbank hydro- and sediment-dynamics (i.e. general water and sediment movement patterns and bedform asymmetry). It is shown that: (1) the results for grain size trends associated with “a worsening in sorting along the transport pathways” have little similarity to transport pathways; (2) the results for grain size trends associated with “an improvement in sorting along the transport pathways” have relatively high degree of similarity to the identified pathways; and (3) the residual pattern, derived on the basis of a combined grain size trend used elsewhere (Gao and Collins, 1992), appears to be most similar and suitable for defining transport pathways.

Sediment dynamics of the Severn Estuary and inner Bristol Channel

Net sediment transport pathways have been determined in many environments by studying grain size trends. This approach is extended here to an inner shelf environment, with improved statistical techniques. Patterns of net surficial sediment transport are proposed, indicating that the area is dominated by up-estuary transport. Exceptions exist in the form of radial mud transport at the river mouths and (clockwise) sand circulation around linear sandbanks. Six different sedimentary environments are defined on the basis of the shape of the ‘transfer function’ and its relationship to the grain size distributions. Erosion takes place along the axis of the estuary. Dynamic equilibrium is reached around the linear sandbanks, whilst accretion occurs at the head of the estuary and where there are riverine inputs. The results are compared with numerical model outputs, representing water movement and sediment transport. Over the seaward part of the study area, there is agreement between the two approaches; contradictions increase, however, towards the upper reaches of the estuary. The discrepancies can be attributed to: (1) inherent differences between patterns of water and sediment movement; (2) difficulties in modelling intertidal boundary conditions; (3) inadequate representation of river influences in the models; and (4) time-scale differences, associated with sediment transport processes.