Fine Sediment Resuspension Research Articles

Concentration of butyltin species in sediments associated with shipyard activity

Tributyltin (TBT) and dibutyltin (DBT) were analyzed in sediment samples collected from intertidal locations in Portland and Boothbay Harbor, Maine (USA) in 1990 and 1992. Surface sediment TBT concentrations ranged from 24 to 12 400 ng gm −1 (dry wt basis). Sediments with the highest TBT concentrations were associated with shipyard hull washing/refinishing activities. Analysis of different layers in core samples found that butyltin concentrations decreased with depth at the Boothbay site and remained relatively constant with depth at the Portland site. Elutriate analysis showed that soluble TBT was released from a heavily contaminated sediment. The resulting TBT seawater concentration 1400 ng liter −1 was < 0.03% of reported seawater solubilities of TBT and was only 0.14% of the total TBT in the sediment sample. This suggests that the TBT in the sediments analyzed is in a bound matrix form, such as paint particles, that releases the biocide slowly. The results indicate that there is a potential for future release of TBT from the resuspension of fine sediments at certain locations in Maine.

Resuspension processes and seston dynamics, southern North Sea

Coupling of physical, biological and chemical processes associated with particle resuspension and seston flux was investigated at three sites in the North Sea with contrasting water column (mixed/stratified) and seabed (cohesive/non-cohesive) characteristics. Seston concentration was determined by a combination of local resuspension and advection of a regional horizontal concentration gradient. Model simulations of observations show that fair weather, the bed erosion rate was limited by the availability of suitable bed material. The resuspended particles were derived from a surficial veneer of material (fluff) that was relatively enriched in organic carbon. Sediment from the bed itself was therefore not resuspended by tidal currents even at a shallow water, sandy site. Bioturbation of the seabed by infauna significantly modified the properties of muddy sands at a deep water site in summer, but this was insufficient to cause tidal entrainment of the bed sediment. Resuspension increased under combined wave/current flows during storms. However, model simulations predict that self-stratification of the boundary layer by resuspended fine sediment during storms reduces bed stress and limits further resuspension, so that storm resuspension of fine sediments may be self-limiting. Seston was a mixture of: (1) particles relatively rich in organic carbon, with low settling velocities, in long-term suspension; (2) particles with less organic carbon (though still greater than that of the bed material), faster settling velocities, periodically resuspended; (3) particles that were very rich in organic carbon, with fast settling velocities, produced during plankton blooms. Particles in category 3 scavenged those in category 1 as they settled, so that seston concentrations diminished and deposition rates increased after blooms. In stratified waters during blooms, deposition of organic-rich detritus gave rise to seabed anoxia and efflux of trace metals (Fe and Mn) from pore waters. Differential rates of metal exchange altered the particulate Fe/Mn ratio below the thermocline. Settling, deposition, and resuspension of fluff were therefore important controls of metal exchanges in the boundary layer.

Resuspension in the Kattegat: Impact of variation in wind climate and fishery

The recurrence of various agents of fine sediment resuspension in the Kattegat Sea, notably wind-induced wave action and demersal trawling, and their seasonal variation and long-term trends have been estimated. A comparison between the sediment-water interface and the spatial distribution of theoretical wind/wave impact indicated that the sediment is resuspended by further agents at depths below the permanent halocline between 10 and 20 m depth. The climatic deterioration in 1940–1970 had a significant influence on the recurrence of wind-induced resuspension, although this was mostly limited to a 10–30% shortening of the recurrence period. On the other hand, a quantification of the effect of demersal trawling in the same period suggests a total shortening of recurrence at bottoms below the halocline by 75–85% in the autumn and winter, and with one order of magnitude (90%) in spring and summer, compared with the effect of waves only in the 1930s; resuspension caused by near-bottom uni-directional currents was not quantified. At these deeper bottoms, resuspension conditions have approached those prevailing at shallow bottoms above the halocline.

Studies of fine-grained sediment transport processes and products in the California Continental Borderland

Summary The California Continental Borderland contains a series of margin basins which are sediment traps at increasing distance from the principal terrigenous and biological sediment sources located in the northern coastal areas. Terrigenous sediments are contributed during a short winter season from small steep coastal drainages. Sand content can be as much as 30–40% of the total contributed load. Silts and clays are sorted out of the river input by coastal wave action and distributed offshore by the seasonal ocean circulation systems. Suspended sediment moves to the basin floors via nepheloid plumes that are generated in the well mixed coastal waters and move offshore with the currents as surface, mid-depth and bottom turbid plumes. During times of low suspensate content in the dry season, lateral transport is dominant. At times of high sediment input and higher concentrations of fine suspensates in the coastal waters, cascading to successively deeper pycnoclines becomes an important process. Resuspension of fine sediments from the outer shelf substrate and the axes of submarine canyons by internal waves and tidally generated currents also contributes to the load of the bottom nepheloid plumes. Large semi-fixed seasonal eddies in the Borderland surface circulation hold the suspended load in the northern sector of the borderland for several days to weeks. Zooplankters have ample opportunity to filter out this material and aggregate it into faecal pellets which rapidly fall to the local basin floors. The removal is sufficiently effective so as to deposit a minimum of 60% of the continental contribution within the northern inner zone of the borderland. High sedimentation rates in these basins promote mass-movement and also decrease the already low oxygen contents of basin floor water as a result of oxidation of the organic fraction of the suspended load. Primary sedimentation structures are preserved in these anaerobic basin floor environments. Mass-movements initiate mud turbidity currents that move to the central basin floors. Mass-movements include creep and sliding of thin sheets from low angle slopes. These effects are amplified following exceptional floods at intervals of around 30 years.

Sediment resuspension by surface-wave action: An examination of possible mechanisms

Detailed spectral and coherence analyses of both nephelometer data and near-bottom acoustic concentration profiles support the conclusion of Lesht et al. (1980) that surfacewave activity is the most important input to the sediment resuspension process. The sand-sized fraction is suspended primarily by bursts of turbulence which are related to peak values of the envelope of surface waves. Two mechanisms for the resuspension of fine sediment are considered. When the bed is bimodal sand and silt, pore-water motion induced by wave-pressure fluctuations may carry fine particles into the boundary layer where they are suspended. Motion of the sand matrix as bedload will also expose fine particles to the flow so that they become suspended. The combination of these two mechanisms accounts for the observed quasi-linear relation between wave orbital velocity and suspended sediment concentration for the Long Island data. The second mechanism is also consistent with observations of near-bottom fluid velocity and suspended sediment concentration in Norton Sound, Alaska.