Timothy G. Milligan

Controls on effective settling velocity of suspended sediment in the Eel River flood plume

Abstract Bulk effective settling velocities required to explain sinking losses from the Eel River flood plume off the coast of northern California are of order 0.1 mm s −1 for five different helicopter-based sampling surveys conducted in January and February 1998. These effective settling velocities exceed those expected for single-grain sinking and implicate flocculation as an important mechanism for speeding the removal of sediment from the Eel River plume. The relative constancy of effective settling velocities despite widely varying winds, waves, and currents is consistent with photographs in the plume that show little variability in floc size with total suspended sediment mass concentration, turbulent-kinetic-energy dissipation rate, elapsed time since sediment within flocs left the river mouth, or depth. These observations of floc size contrast with those made in winter 1997 during the exceptionally large New Years flood. During that event, increases of floc size with depth are evident. In 1997, higher sediment concentrations associated with the significantly larger discharge likely allowed flocs to grow substantially as they sank through the plume, whereas in 1998 low concentrations precluded significant increases in floc size with depth. These observations do not support the hypothesis that concentration controls maximal floc size; rather they indicate that the growth rate of flocs is a function of concentration. Using a published relationship between floc size and settling velocity for the Eel shelf suggests that approximately three fourths of the sediment in the plume was packaged as flocs during the 1998 floods.

A laboratory assessment of the relative importance of turbulence, particle composition, and concentration in limiting maximal floc size and settling behaviour

Abstract The fate of fine particulate material in aquatic environments is closely linked to aggregation and disaggregation processes. Understanding the mechanisms controlling these processes is fundamental to the development of predictive models of fate and effects for particulate discharges in the coastal zone from such sources as offshore hydrocarbon exploration and development. One of the variables required for the development of these models is maximal floc size. Using a non-invasive imaging technique, the significance of turbulence, composition, and concentration on maximal floc size in an inverting column flocculator was determined for materials commonly discharged during offshore hydrocarbon development. The settling velocity of the suspension was determined from volume concentrations of samples obtained by pipette during still water settling in a manner similar to that of Owen tubes. After 20 h, both maximal floc size and settling velocity showed a highly significant dependence on turbulence and type of material in suspension, but showed no effect from concentration.

Flocculation and sedimentation on the Po River Delta

With the goal of improving understanding of the effect of flocculation on the formation of fine-grained deposits on continental shelves, hydrographic profiling, in situ imaging of suspended matter, and collection of surficial sediment samples were conducted at the Po River Delta in June 2001. These data show that during medium flow conditions (1920 m3/s), sedimentation occurs rapidly immediately offshore of the main distributary, Po della Pila. Rapid sedimentation is promoted by large rapidly sinking flocs forming in the river well upstream of the mouth. The delivery of fine sediment to the seabed at the mouth of the Po is sufficient to overwhelm the erosive effects of waves and currents, leading to accumulation of mud in water depths as shallow as 4 m. On cross-shelf transects 2 km north and south of the mouth, however, suspended sediment supply from the river is reduced to the point that mud accumulates only seaward of the 8-m isobath. Along the central transect, suspended sediment concentration decreases rapidly seaward of the 6-m isobath where the emergence of a more organic-rich population of flocs along a mid-water density interface is suggested. Energetic activity along the 15-m isobath likely promotes resuspension with the potential for removal of material from the delta. Further investigation of floc properties, namely the relationship of floc size to settling velocity, is necessary to establish the degree to which the suspension is flocculated during transport and deposition.

In situ feeding and absorption responses of sea scallops Placopecten magellanicus (Gmelin) to storm-induced changes in the quantity and composition of the seston

Time-series of hourly clearance, ingestion and absorption rates and absorption efficiency were measured over 48 h for adult sea scallops (Placopecten magellanicus) held in situ in a coastal embayment in Nova Scotia, Canada, during a wind-induced resuspension event. Temporal variations in oceanographic variables, and seston quantity and composition (organic matter, organic carbon, nitrogen, chlorophyll a, and inorganic particle size spectra) were monitored during the study with moored instruments and hourly water sampling. Resuspension of bottom materials during the storm resulted in large changes in the amount (1 to 30 mg l−1 total particulate matter) and nutritional quality (25 to 50% organic content) of seston. High sedimentation rates after the storm were accelerated by flocculation, resulting in the rapid settling of resuspended particles and an increase in seston quality. Observed short-term (hourly) fluctuations in clearance rate were not related to storm- or tide-induced changes in seston characteristics but were directly related to flow velocity. Significantly lower clearance rates were observed at relatively low ( 9 cm s−1) flow speeds. The overall reduction in ingestion rates after the storm resulted from decreased food availability. Hourly absorption efficiency (AE) measurements were closely related to seston quality (total organic, organic C and N content) and AE declined exponentially with decreasing seston quality. Reductions in AE during the resuspension event were offset by the increased ingestion rate, resulting in no significant changes in absorption rates for organic matter, C, or N over the sampling period. As the low food quality of the resuspended matter was balanced by increased availability, any physiological regulation of food acquisition (i.e. clearance rate regulation) would have been irrelevant to maintaining food intake constant.

The variability of suspended aggregates on the Amazon Continental Shelf

Abstract Underwater photographs taken as part of the AmasSeds project from 1989 to 1992 reveal the abundance of suspended aggregates on the Amazon Continental Shelf. Computerized image analysis of photographs from specific layers within the water column at inner shelf, midshelf, outer shelf, and rivermouth anchor stations allowed description of thein situ characteristics of the aggregates, or flocs. Floc sizes varied from the lower limit of detection (125 μm) to a maximum greater than 2 mm, with a mean diameter of 518 μm and a modal diameter of 646 μm for the complete data set. Additionally, large, elongate, comet-shaped aggregates (stringers) as large as 2.5 cm were observed in individual photographs. Results of floc-size analyses (in terms of the maximum floc size,dMAX) were compared to salinity, particle concentration, and current velocity. Additionally,dMAX was also compared to shear (G) and the Kolmogorov microscale (λ). Results suggest that salinity has little effect ondMAX (r2 = 0.06), while particle volume concentration demonstrates a strong relationship withdMAX (r2 = 0.92). The maximum floc size increased with increased current velocity andG up to a critical value (floc growth stage) and then decreased as the current velocity andG continued to increase (floc breakup stage). The Kolmogorov microscale, which describes the size of the smallest turbulent eddies (and, thus, should limit thedMAX observed) also demonstrates a relationship with the observed maximum floc size. The results suggest thatdMAX is approximately one-half the magnitude of the Kolmogorov microscale (λ).

Fine-grained suspended sediment dynamics in the Eel River flood plume

Abstract Small rivers (drainage basins 4 km 2 ) discharging runoff from mountainous terrain are major contributors of mud to the marine environment. However, little understanding of the dispersal mechanisms and fate of discharged fine-grained sediments to the continental shelf is known due to the episodic and unpredictable nature of discharge from these rivers. This study used a helicopter-based sampling program to capture unprecedented measures of Eel River, northern California, flood plume events during 1997, 1998, and 1999. In situ measures of floc size and estimated floc fraction show no relationship with concentration, turbulent-kinetic-energy, time from river mouth, wind speed, wave height, or discharge. A relationship apparently does exist between effective settling velocity (bulk mean settling velocity) of plume sediments and wind speed/direction, as well as with tides. Results implicate the energetic nearshore as a source of suspended sediment resupply to the offshore region of the plume. Future studies focusing on surf zone suspended fine-grained sediment dynamics are needed in order to understand the fate of flood plume sediments discharged to exposed, energetic shelves, such as the Eel River margin.

In situ particle (floc) size measurements with the benthos 373 plankton silhouette camera

Abstract The Benthos 373 plankton silhouette camera was one of the first instruments used to study the in situ particle size spectra of flocculated material. Based on a concept originally developed by H. Edgerton for the study of live zooplankton samples, the relatively non-invasive nature of the camera has made it well suited for the study of flocculated suspensions in the coastal zone. Results from studies using the camera showed clearly the dynamic nature of particle flocculation in both fresh and salt water environments. Originally limited by the time required to carry out image analysis of the photographs obtained, recent advances in the digitization of photo negatives and image analysis software have increased the usefulness of this camera for studies of particulate material in the coastal zone.

Fine-grained sediment flocculation below the Hubbard Glacier meltwater plume, Disenchantment Bay, Alaska

A study in Disenchantment Bay, Alaska, demonstrates that fine sediment beneath a meltwater plume is flocculated and that floc sizes and fraction of mass bound within flocs exhibit a pronounced increase with depth rather than down fjord. This spatial pattern of variability likely is due to the longer depositional timescale of flocs compared to their horizontal advection timescale within the meltwater plume. The flux of mass within flocs also increases with depth. These observations have implications for sedimentation models as sedimentation rates estimated from surface waters underestimate those at depth, and could result in the inaccurate prediction of the position of suspension depocenters. The results also may explain the behavior of fine sediment in more complex environments where floc properties are difficult to observe.

The role of particle aggregation in size-dependent deposition of drill mud

Petroleum exploration on continental shelves raises concerns over potential impacts it may have on ecologically sensitive and valuable fishing grounds. Several studies demonstrate that drill mud discharged from platforms exhibit negative impacts on surrounding benthic communities. However, the dispersal and deposition mechanisms of such mud are poorly understood. This study investigates the evolution of disaggregated particle size distributions of bentonite enriched water-based drill mud in a suspension flowing over a flat sand bed using a laboratory flume. Observed size-specific decay rates of suspended sediment concentrations indicate that suspended grains deposit as single grains and flocs. A simple model that considers deposition of particles by gravitational settling of single grains and flocs yields estimates of decay rates in good agreement with the measurements. The proposed model provides a grain size-specific analysis of parameters characteristic of a poorly sorted aging suspension that have previously been undocumented.

Sediment trapping and transport in the ACE Basin, South Carolina

A study took place during May 1998 and May 1999 to examine the processes controlling localized accumulation of fine-grained sediments in the lower Ashepoo River. This region, referred to as the Mud Reach, is an area of muddy bottom sediments bounded by fine sands. The Mud Reach is located downstream of the landward extent of the salt intrusion where an estuarine turbidity maximum commonly occurs. Tidal time-series measurements made in the Mud Reach during May 1998, when river discharge was at a 10-yr high, showed high concentrations of suspended sediment (0.05–1 g I−1) during maximum tidal current velocity with concentrations in the bottom 30 cm exceeding 70 g I−1 (fluid mud). A correlation between salinity stratification and increased suspended sediment concentration suggests that inhibited vertical mixing enhances the settling of flocculated sediments to the bed. Measurements made during May 1999 show a two-order-of-magnitude decrease in the concentration of near-bed sediments. A decrease in river discharge during the 1999 observation period of more than 100 m3 s−1 suggests that changes in the hydrography and in the supply of sediments to the system both may be important factors in the trapping of fine-grained sediments in the region. The source of sediments is likely from muddy deposits in the Fenwick Cut, a man-made section of the Atlantic Intracoastal Waterway about 2 km north of the Mud Reach that connects the Ashepoo and Edisto Rivers. The Fenwick Cut appears to be an effective area for trapping sediments where shoaling has increased by 130% in the last decade. Current measurements show that flow velocities decrease through the Cut, likely allowing for the settling of suspended particles that form the thick deposits of unconsolidated mud observed during both years.