Thomas F. Gross

Sediment-transport events on the northern California continental shelf during the 1990–1991 STRESS experiment

Abstract Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) “C” transect as part of the 1990–1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds.

Modelling some effects of behavior on larval settlement in a turbulent boundary layer

Abstract A one-dimensional model of larval concentration and settlement flux in a turbulent boundary layer was used to consider how some aspects of larval behavior in the plankton or on the bottom might affect settlement rates. The focus was on behavioral modifications of two terms in the model—the vertical (fall) velocity ( w f ), a composite measure of swimming and gravitational sinking, and the probability of settlement per unit time ( p ) for larvae interacting with the bottom. Depth-independent changes in vertical speed can increase the settlement rate by up to an order of magnitude. Such changes might be produced by negative phototactic responses that induce passive sinking of larvae throughout the water column during daylight, by photonegative swimming responses that may occur in relatively shallow or clear water (where swimming responses are fairly uniform throughout the water column), or by photonegative swimming responses that are of similar magnitude over a broad range of light intensities. In addition to increasing the long-term average settlement rate, negative phototactic responses should increase temporal variability in settlement at time scales of ≦24 h, as long as larvae are responding to changes in light intensity at depth. This prediction, coupled with results from Gross et al . [(1992) Journal of Marine Research , 50 , 611–642], suggests that for larvae that respond to light there may be two sources of substantial (order-of-magnitude) temporal variability in settlement that operate at time scales of 24 h or less— diel periodicity in light intensity and semi-diurnal or diurnal tidal periodicity in boundary shear stress and turbulence intensity. The additional effect of phase shift between tidal and light cycles introduces substantially less variability to the average (>24 h) settlement rate than do individual effects of the two forcings. Behavioral responses that produce a depth dependence in vertical speed can affect the settlement rate only if w f varies roughly by a factor of 2 or more very close to the bottom (i.e. within a few percent of total boundary layer thickness for tidal boundary layers). Phototactic or barokinetic responses of larvae are not likely to produce depth gradients in vertical speed strong enough to affect settlement rates. However, such gradients might be produced by a recent contact with a potential settlement site, by effects of velocity gradients near the bottom on larval orientation, or by larval responses to chemical cues associated with the bottom. Behavioral responses to fluid forces exerted on larvae on the bottom can alter the settlement rate by an order of magnitude or more. However, the settlement rate is predicted to be most sensitive to effects of weak, rather than strong, flow on the settlement probability. Thus, understanding larval responses to weak flows may be especially important to predicting settlement rates.

Estimation of stress and bed roughness during storms on the Northern California Shelf

Abstract Turbulent boundary shear stress depends on a roughness length scale which characterizes momentum transfer to the seabed. The effective roughness length is due to physical roughness geometry such as sediment ripples, grain size and processes affecting momentum transfer, such as bedload transport and wave motions. During storms on the California shelf, wave motions dominate the turbulent boundary layer, although feedback through wave-induced bed forms and sediment transport are important. Several intense storms on the Northern California Shelf were monitored with pressure sensors, acoustic current meters and an optical backscatter sensor within 5 m of the bed at 90 m depth. Wave spectra, velocity profiles, turbulent kinetic energy and suspended sediment concentrations were obtained. At 90 m depth, waves were measured in the band of 0.05-0.08 Hz, with nearbed wave velocities of 5–30 cm s −1 . In spite of wave-induced currents of up to three times the mean speed, 30 min average velocities yielded typical logarithmic profiles. Roughness, as indicated by the zero intercept of the logarithmic profiles, z 0 , varied by a factor of 25 throughout the storms, with a maximum of 18 cm, when mean currents were above 5 cm s −1 and the wave amplitude was maximal. Such large increases in z 0 are predicted by models of wave-current interaction. Effects of sediment transport and bed forms are not easily extracted from the data during storms. But, the fairly large non-storm values of ∼0.5cm indicate the effect of bed ripples.

Bottom boundary layer spectral dissipation estimates in the presence of wave motions

Abstract Turbulence measurements are an essential element of the Sediment TRansport Events on Shelves and Slopes experiment (STRESS). Sediment transport under waves is initiated within the wave boundary layer at the seabed, at most a few tens of centimeters deep. The suspended load is carried by turbulent diffusion above the wave boundary layer. Quantification of the turbulent diffusion active above the wave boundary layer requires estimates of shear stress or energy dissipation in the presence of oscillating flows. Measurements by Benthic Acoustic Stress Sensors of velocity fluctuations were used to derive the dissipation rate from the energy level of the spectral inertial range (the -5/3 spectrum). When the wave orbital velocity is of similar magnitude to the mean flow, kinematic effects on the estimation techniques of stress and dissipation must be included. Throughout the STRESS experiment there was always significant wave energy affecting the turbulent bottom boundary layer. Lumley and Terray [(1983) Journal of Physical Oceanography, 13, 2000–2007] presented a theory describing the effect of orbital motions on kinetic energy spectra. Their model is used here with observations of spectra taken within a turbulent boundary layer which is affected by wave motion. While their method was an explicit solution for circular wave orbits aligned with mean current we extrapolated it to the case of near bed horizontal motions, not aligned with the current. The necessity of accounting for wave orbital motion is demonstrated, but variability within the field setting limited our certainty of the improvement in accuracy the corrections afforded.

In-situ measurements of particle settling velocity in the deep sea

Two instruments, the remote optical settling tube and the benthic autonomous settling tube, have been deployed on the Nova Scotian continental rise at a depth of 4820 m. The few results obtained suggest that optical and gravimetric methods give comparable results. Assumptions about particle density little affect the settling velocity distribution, but do scale the total magnitude. By using a distribution of declining density with increasing size indicating increasing water content, the total concentration inferred from the light attenuation by using Mie theory is quite close to the total found from gravimetric data. The coarser end of the size distribution changes more with overall change in concentration than the fine end. The settling velocity distribution shows the presence of several modes. The two principal ones can be matched with modal sizes of ∼3.3 μm and ∼60 μm in Coulter Counter data allowing inference of Stokes density contrasts of 1.5 and 0.061 g cm−3, respectively. At concentrations of about 200 mg m−3, more than 50% of the material has a settling velocity less than 2 × 10−4 cm s−1 (probably < 3 μm in size), but this percentage appears to decrease with an increase in total concentration.

Sediment concentration profiling in HEBBLE using a 1-MHz acoustic backscatter system

Abstract As part of the High Energy Benthic Boundary Layer Experiment (HEBBLE), a 1-MHz Acoustic BackScatter System (ABSS) was deployed in 4800 m of water on the Nova Scotian continental rise. The purpose of the instrument was to measure non-intrusively a time series of the vertical concentration profile over the course of one year, from September 1985 to September 1986. In this paper, we discuss the details of the ABSS instrument, the results of the time series data analysis, and the implications of the results on sediment transport modeling in HEBBLE. The systematic variation in the ABSS and light transmissometry calibration with changes of particle size distribution caused by environmental variability is discussed. A comparison of the ABSS estimates of particle concentration with those obtained from light transmissometers seems to indicate that the suspended particle size distribution changed surprisingly little throughout the experiment.

Acoustic measurements of the spatial and temporal structure of the near-bottom boundary layer in the 1990-1991 STRESS experiment

Abstract As part of the 1990–1991 Sediment TRansport Events on Shelves and Slopes (STRESS) experiment, a 5 MHz Acoustic BackScatter System (ABSS) was deployed in 90 m of water to measure vertical profiles of near-bottom suspended sediment concentration. By looking at the vertical profile of concentration from 0 to 50 cm above bottom (cmab) with 1 cm vertical resolution, the ABSS was able to examine the detailed structure of the bottom boundary layer created by combined wave and current stresses. The acoustic profiles clearly showed the wave-current boundary layer, which extends to (order) 10 cmab. The profiles also showed evidence of an “intermediate” boundary layer, also influenced by combined wave and current stresses, just above the wave-current boundary layer. This paper examines the boundary-layer structure by comparing acoustic data obtained by the authors to a 1-D eddy viscosity model formulation. Specifically, these data are compared to a simple extension of the Grant-Glenn-Madsen model formulation. Also of interest is the appearance of apparently 3-D “advective plume” structures in these data. This is an interesting feature in a site which was initially chosen to be a good example of (temporally averaged) 1-D bottom boundary-layer dynamics. Computer modeling and sector-scanning sonar images are presented to justify the plausibility of observing 3-D structure at the STRESS site.

Suspended sediment storm modeling

Abstract A variety of sediment-transporting storms was observed in the HEBBLE long deployment indicating several distinct forcing mechanisms; in particular, some storms appear to originate locally (Gross and Williams, this issue). In this paper, a simple model of a locally forced storm is presented to elucidate the time-dependent dynamics of erosion and deposition events in the deep sea. Based on these modeling efforts, we make the following inferences. Storms exhibit peak sediment concentration before peak velocity. This behavior is due to a limited amount of erodible bed sediment. After near-instantaneous erosion of the bed is complete, fine-sediment concentration in the water column drops due to dilution throughout a thickening turbulent mixed layer in the accelerating flow. During and immediately after a storm event, turbulence intensity above the 40-m-thick momentum mixed layer can be sufficient to suspend fine particles in low concentration to elevations of 100 m above bottom or more. Following the storm, the rate of sediment clearing depends on gravitational settling and the potential importance of additional turbulence-driven deposition and aggregation mechanisms. Syn- and post-storm “overthickening” of the suspended sediment mixed layer relative to the Ekman momentum mixed-layer thickness, as well as sensitivity of the post-storm concentration profile to turbulence-driven deposition, can explain some complex features of near-bed nepheloid layers in the HEBBLE region observed by other workers. Validation of suspended sediment transport models is based on field observations obtained with light transmissometers. Transmissometer measurements of suspended sediment concentration depend, in turn, on calibration of instrument response to both mass concentration and particle size distribution. By analyzing the temporal and spatial changes in mass concentrations with the model presented herein, we calculate an operational calibration function. During HEBBLE storms, this calibration function does not change by more than 30% due to particle sorting by turbulent suspension.

Acoustical and optical backscatter measurements of sediment transport in the 1988–1989 STRESS experiment

Abstract During the 1988–1989 Sediment Transport Events on Shelves and Slopes (STRESS) experiment, a 1-MHz acoustic backscatter system (ABSS), deployed in 90 m of water off the California coast measured vertical profiles of suspended sediment concentration from 1.5 to (nominally) 26 meters above bottom (m.a.b.). An 8-week-long time series was obtained, showing major sediment transport events (storms) in late December and early January. Comparison of the acoustics measurements from 1.5 m.a.b. are made with optical backscatter system (OBS) concentration estimates lower in the boundary layer (0.25 m.a.b.). Correlations between ABSS and OBS concentration measurements and the boundary layer forcing functions (waves, currents, and their non-linear interaction) provided a variety of insights into the nature of the sediment transport of the STRESS site. Transport rates and integrated transport are seen to be dominated by the largest storm events.

Residual Circulations Due to Bottom Roughness Variability under Tidal Flows

Abstract Tidal flows over irregular bathymetry are known to produce residual circulation flows due to nonlinear interaction with gradients of depth. Using the depth-averaged vorticity equations, the generation of residual vorticity and residual flows due to variation of the frictional coefficient are examined. The authors find that the contribution due to bottom roughness variations can be as large as that arising from gradients of depth and velocity. Specific cases are considered on the northern California shelf, Georges Bank, and the U.S. South Atlantic Bight. The generation of residual vorticity is a strong function of the length scales at which roughness or depth vary. Length scales of bottom roughness variation are commonly within the range of greatest effect (e.g., sand patchiness, cobbly outcrops, etc.). The site-specific cases show that the bottom roughness variability can generate as much residual circulation as that expected from depth variability. The implication for numerical modeling studies ...