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Investigations of continental shelf sedimentology can be separated into small-scale sediment–fluid-interaction studies and large-scale sediment-survey techniques. Small-scale studies rely on time-series observations from benthic tripods using current meters, turbidity sensors, CTDs, cameras, and acoustic altimeters and sonars. Large-scale studies rely on repeated spatial surveys using side-scan sonar, acoustic profiling methods, and coring techniques. Over the past four decades these approaches have been very successful in describing the sediments on continental shelves and interpreting the physical forcing mechanisms and resulting sediment transport pathways, depositional sites, sediment budgets, and strata formation. Results from shelf studies carried out off the west coast of North America and the Amazon River are used to illustrate the many insights gained from small-scale and large-scale investigations. Recent developments of new sensors and observational techniques have greatly improved our ability to investigate the sedimentology of continental shelves. The time and spatial scales of resolution for both small-scale and large-scale investigative approaches have begun to overlap, and the possibility of concurrent investigations defines exciting new opportunities for future research.

Benthic currents, temperature, salinity, and suspended sediment concentration were measured in 1993–1994 on the Bear Island Fan, at water depths of approximately 1700 m, near the wreckage of the Russian nuclear submarine “Komsomolets”. A maximum current speed of 51 cm/s and a mean speed of approximately 10 cm/s was recorded. There was a marked seasonal pattern in flow strength, and the net transport was northward along the slope contours. The strongest flow event of the year, however, was directed to the southwest. The smooth (apart from burrows created by benthic fauna), unscoured appearance of the sediment surface in bottom photographs suggest that the deep currents on the continental slope are weak. Our measurements, however, show that the currents frequently exceeded the erosion threshold speed. Peak concentration values during the energetic winter period were within the range expected for an erosion event. The data indicate a high potential for sediment erosion and transport on the Bear Island Fan, with implications for the possible transport of contaminants from the submarine.

As part of the Sediment TRansport Events on Shelves and Slopes (STRESS) program, a remote optical settling ☐ was deployed on the northern California continental shelf. The device operates by isolating a volume of sediment-laden fluid from the environment and then monitoring its sedimentation behavior with a transmissometer. Results show a bimodal distribution of suspended sediment during low-energy periods on the shelf that reflects the size distribution of bottom sediments. The coarse mode sinks at 0.026 cm s −1 (22 m day −1 ) and the fine mode settles at 0.0025 cm s −1 (2 m day −1 ). Between one-quarter and two-thirds of the total mass resides in the coarse mode. Roughly one-quarter is in the fine mode. The remainder sinks too slowly ( −1 or −1 ) to be resolved during the 18-h measurement cycles. Greatest uncertainty in assigning mass to the various settling velocity classes comes from sensitivity to ill-constrained particle geometry of the conversion from light attenuation to mass. The device failed during higher energy periods, probably due to penetration of fluid into the ☐. Complete isolation of the fluid from the environment would improve the performance of settling ☐es in energetic settings.

TRANSPORT of sediments on continental shelves involves several complex processes, including entrainment and deposition of particles at the sea floor, vertical transport of suspended particles by boundary layer turbulence, water-column evolution of particle size and density by floc aggregation and break-up, and advection of suspended particles by vertically non-uniform, unsteady, horizontal currents (Fig. 1). These processes lead to sediment dispersal and topographical evolution on a side range of spatial scales, which feed back to influence the various transport mechanisms. Shelf sediment transport influences, and is strongly influenced by, the activities of bottom-dwelling organisms, as well as by physical flow-particle and particle-particle interactions. The many processes involved in shelf sediment transport raise an intriguing array of scientific questions with applications related to evolution of bottom roughness and topography, development of stratigraphy, water clarity, and transport of contaminants attached to fine particles. The objective of the Sediment TRansport Events on Shelves and Slopes (STRESS) program (NoWELL et al. , 1987) was to understand the processes controlling sediment transport and microscale topography in the storm-dominated shelf setting, and to develop a tested capability to predict these processes. STRESS was primarily a field measurement program (Fig. 2) designed to provide comprehensive, high-quality measurements sufficient for critical evaluation and improvement of conceptual and mathematical models. The measurement program was a remarkably successful attempt to resolve the vertical distribution of velocity, temperature and salinity throughout the entire continental shelf bottom boundary layer, with corresponding indirect estimates of particle concentration, size and settling velocity, and accompanying observations of small-scale topography. This issue contains 10 articles describing initial results based on STRESS measurements. The first article (SHERWOOD et al . , 1994) provides a description and a classification of sediment transport events during fall and winter storms in the STRESS study area, off the coast of northern California (Fig. 2). A second group of articles addresses the difficult problem of obtaining time-series estimates of particle size and settling velocity, based on a novel laser diffraction device (AG~WAL and PO~SMITH, 1993), a more traditional optical settling tube (Hn~L et al. , 1994), and a combination of acoustical and optical backscatter

Theory and data for turbulent flow in smooth pipes are employed to evaluate von Karman's constant from integral flow parameters. A set of equations for mean velocity and centerline velocity are der...

Particle deposition experiments using mimics of biogenous negative relief (“pits”) and low-excess-density particles in a small annular flume indicate a significantly enhanced deposition rate (number of particles per time) compared to smooth, flat patches of the same diameter. This study included flow visualizations as well as observations of particle residence times, particle concentrations in the pits, and particle fluxes to the pits from the main flow. Experimental conditions of particle concentration, shear velocity, and particle settling velocity mimicked the dynamic characteristics (low excess density and large size) of organic-rich floes and flow conditions in the subtidal and deep sea where biogenous pits are common features. Results suggest that pits provide benthic organisms an important capture mechanism for such floes. Flow visualizations concur qualitatively with previously reported results for twodimensional cavity flow, with unique features due to the conical shape of the pits. When the Rouse number (settling velocity/shear velocity) was much less than 1, pit deposition rate increased with increasing pit aspect ratio (AR = depth/diameter; ranging from 0.25 to 2) and always exceeded deposition to a flat patch of comparable diameter. For the single aspect ratio tested (AR = 0.5) under conditions of increasing turbulence, deposition to the pit increased under transitional flow, but then decreased to near zero when conditions reached fully rough flow. Relative enhancement of deposition to this pit decreased with increased ambient bed roughness since grave1 beds also effectively collect particles. Particle concentration inside pits decreased weakly with pit aspect ratio but greatly increased with increasing roughness Reynolds number. Particle residence time increased somewhat with pit aspect ratio but decreased significantly with increasing roughness Reynolds number. Particle flux into pits from the main flow increased with both increasing aspect ratio and increasing roughness Reynolds number. Enhancement of food supply to pit inhabitants thus depends on the flow regime.

When measured or apparent yield stress of cohesive muds is assumed to reflect average particle-particle bond strength in network structures, critical bed shear stress required for grain-by-grain entrainment can be predicted from an analysis of the forces acting on component grains of geometrically-flat mud beds. Erosion resistance is then defined in terms of cohesive yield stress as well as nominal particle size, shape, relative density, and packing geometry. Predicted values of critical bed shear stress compare favorably with existing data from selected studies in which appropriate yield stress and hydrodynamic parameters have been reported.

The concentration of fine 0(1pm) suspended sediment in deep-sea nepheloid layers is roughly 10 5 particles cm -3 . Given this concentration, aggregation theory dictates that marine snow particles must remove fine particles at a rate of 3.5 x 10 -4 particles cm -3 s -1 for scavenging of small particles by large, fast-sinking ones to play a significant role in deposition from nepheloid layers. Assuming that one in every 10 fine particles that collide with a marine snow particle sticks to it, to achieve the above removal rate given a marine snow concentration of 10 -4 particles cm -3 requires contact efficiencies of 10 -1 to 10 -2 . Such values of contact efficiency are significantly larger than theoretical predictions of contact efficiency, but are supported by evidence from studies of radionuclide fluxes and particle size distributions. Discrepancies between inferred and predicted values of contact efficiency arise from differences in actual and model particle properties. Contact by direct interception potentially is enhanced by roughened particle surfaces and by wake capture. Wake capture is the process whereby fine particles are entrained in the recirculating eddies present behind settling particles with Reynolds numbers greater than one.

Flow velocity required to erode a bed of acid‐washed sand is increased by intergranular adhesion resulting from growth of the benthic marine bacterium Al‐teromonas atlantica. In general, we find that either pure exopolymer alone or exopolymer generated during in situ growth increases erosion resistance of fine quartz sand. Moreover, the degree of erosion resistance increases in proportion to the concentration of exopolymer‐component uronic acids, which in turn is dependent on relative nitrogen content of peptone‐based growth media. Specifically, we observe that approximately 100 nmol of exopolymer or 1.5 nmol of component uronic acids generated by in situ bacterial growth under nitrogen‐rich conditions per gram of dry sediment can effectively double seawater‐flume flow velocity required for initiation of transport of otherwise noncohesive, 125–177 μm quartz grains. This maximal effect corresponds to an estimated adhesive force that exceeds submerged particle weight by an order of magnitude and ex...

In the general study of benthic processes an important set of unknown exists concerning interactions between the bed and the flow field. The local geometry of the bed in part controls the structure of the bottom boundary layer, but the bed geometry itself is also in part controlled by the flow. Benthic organisms complicate the problem further. Our long range research objectives are to understand the various interactions between sediments, flow and organisms in sufficient detail to permit predictions regarding the relative importance of various processes in different environments. Our approach is an iterative one between theory and field measurements. The proximate objectives of this research are to: (1) document the temporal variation in microtopography of a silt-bottom (90 m STRESS site, northern California shelf) that is most often influenced solely by biological processes, but is episodically modified by physical sediment transport events, (2) determine the recovery time required for biological reworking to erase physically generated bedforms, (3) measure absolute erosion and deposition amounts following sediment transport events, and (4) compare measured time-series of biological and physical roughness to predictions based on combined wave-current bottom boundary models that are forced using current velocity profiles and wave data.