L. D. Wright

Effects of ambient currents and waves on gravity-driven sediment transport on continental shelves

Abstract Observations from several shelf environments show that down-slope gravity-driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity-induced sediment flux. The phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment-laden rivers. The field sites examined are the Gulf of Bohai off the mouth of the Yellow River (Huanghe), the northern California shelf off the mouth of the Eel River, and the Louisiana shelf west of the mouths of the Mississippi River. Off the Yellow River, rapid down-slope transport over distances of a few km occurred when frictional resistance, induced by strong along shelf currents, was temporarily relaxed. More prolonged down-slope motion over longer distances occurred following floods of the Eel River, when wave and current agitation provided turbulence to sustain gravity-driven transport of fluid mud. On the Louisiana inner shelf, the down-slope gravity force was much weaker, but observations suggest that thin gravity flows may still have occurred in the presence of waves. A simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity-driven transport of suspended sediment. This theory is quantitatively consistent with the observations from the three sites. If the supply of easily suspended sediment is less than the capacity of ambient currents (including waves) to carry sediment, then intense turbulence limits gravity-induced sediment transport by increasing the drag at the base of the flow. When ambient currents abruptly cease, rapid down-slope transport can then occur over short distances until the sediment settles. Such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow. The maximum sustained rate of gravity-induced sediment transport occurs when ambient currents are strong, but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents. Feedback then leads to values of the gradient Richardson number (Ri) within the flow that are near the critical value of 1/4. This partially damps bottom drag, but still allows the generation of sufficient turbulence to maintain sediment in suspension. Observations also indicate systematic relationships among Ri, the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow.

Hyperpycnal plumes and plume fronts over the Huanghe (Yellow River) delta front

The Huanghe (Yellow River) discharges extremely high suspended sediment concentrations (25 to 220 g/l) which favor sustained hyperpycnal plumes (underflows). Observations of weakly hyperpycnal unchannelized plumes and indirect evidence of strongly hyperpycnal channelized underflows over the delta front indicate the importance of these modes of sediment dispersal. The weakly hyperpycnal plumes occupy the entire water column over the shallow (<5 m) delta top. From a pronounced front near the break in slope at about 5 m depth, they descend over the delta-front slope as wide-spread underflows. Evidence of strongly hyperpycnal underflows was shown from subaqueous valleys partly filled with low-density mud.

Sedimentary framework of the modern Huanghe (Yellow River) delta

The geometry, stratigraphy, and structure of recently deposited Huanghe (Yellow River) Delta sediments were examined by high resolution subbottom profiles and medium-penetration boomer profiles. The results indicate that the active (post-1976) subaqueous delta advances as a single thin localized lobe with a maximum thickness of only 15 m. Calculations of sediment volumes indicate that 90% or more of the sediment supplied by the Huanghe remains within 30 km of the mouth. Sediment on the delta platform near the mouth is fine sand; elsewhere silts and clays prevail.[/p]

BIOLOGICAL MEDIATION OF BOTTOM BOUNDARY LAYER PROCESSES AND SEDIMENT SUSPENSION IN THE LOWER CHESAPEAKE BAY

Abstract In relatively low-energy estuarine environments where bottom skin friction shear stresses do not appreciably exceed the critical shear stress required for sediment transport, benthic biology can profoundly affect the responsiveness of the bed to benthic flows. The bottom boundary layer processes that suspend and transport fine sediments at two sites in the lower Chesapeake Bay, eastern USA were examined in an interdisciplinary study that embraced all seasons. Instrumented boundary layer tetrapods were used to record time varying bed stress and suspended sediment concentrations. Box cores, surface and profile photographs and direct observations provided data on benthic biology. An annular seabed flume was used to obtain in situ estimates of the threshold shear stresses at which sediments were resuspended. Observed bed stresses related to semi-diurnal tidal currents were in the range of 0.10 to 0.14 Pa at the lower-energy site but often exceeded 0.20 Pa at the more energetic site. These stresses were often enhanced by wind stress and waves. Biological effects dominated bed roughness and influenced the susceptibility of bottom sediments to physical entrainment. Wave- or tide-generated bed forms rarely were observed at either site. Epifaunal organisms increased the relief amplitude of the sediment-water interface by more than 8 cm. Temporal and spatial variability in the abundance and species composition of epifauna was high at both sites, making the effects of these organisms on hydraulic roughness difficult to predict. Application of the seabed flume showed that the critical stress at which sediment was suspended varied from 0.10 to 0.14 Pa. This variability is attributed to seasonal and interannual variability in biological activity: the lowest critical stress values coincided with the most intense bioturbation. The floor of the bay stem appears to approximate a quasi-equilibrium surface with normally recurrent physical bed stresses remaining near or slightly below threshold values. The bed remains active because benthic biota mix the sediment column, provide bed relief, and mediate the responsiveness of the bed to physical processes.

Suspended sediment advection by tidal currents off the Huanghe (Yellow River) delta

Studies to date indirectly indicate that only a small percentage of the sediment discharged by the Huanghe (Yellow River) is presently transported from the Gulf of Bohai to the Huanghai (Yellow Sea). Direct measurements in early summer 1985 show low concentrations of suspended sediment east of 119°45′E but high concentrations in Bohai Bay. Stokes drift associated with an amphidrome of the M2 tide may contribute to a northwestward transport of Huanghe sediment.

FIELD MEASUREMENTS OF FAIRWEATHER BOTTOM BOUNDARY LAYER PROCESSES AND SEDIMENT SUSPENSION ON THE LOUISIANA INNER CONTINENTAL SHELF

Field measurements of bottom boundary layer and sediment-transport processes were made on the Louisiana inner continental shelf in spring 1992 at a depth of 15.5 m, and in spring and summer 1993 at a depth of 20.5 m. Two different wave–current boundary layer/sediment-transport models were applied to the measured near-bed flows. In addition, the log-profile method was applied to estimate hydraulic roughness and bed stress. Consistent with the results of others, our measurements show that near-bed flows were very weak under non-storm conditions. Bed stresses were typically too low to resuspend bed sediments. However, the advection of high-turbidity layers or plumes past the instrumentation apparently caused a sustained period of high suspended sediment concentration throughout the log layer in spring 1993. In the absence of wave activity or high suspended sediment concentrations, boundary layer profiles showed the bed to have been hydraulically very smooth with z′o<0.1 cm. However, wave agitation, combined with increased suspended sediment concentration caused hydraulically rough conditions with z′o<0.1 cm.

The subaqueous delta of the modern Huanghe (Yellow River)

The subaqueous delta of the Huanghe (Yellow River) has been studied using high-resolution acoustic systems. There are many subtle variations in sea floor morphology and sediment geometries; smooth, featureless areas are rare. The main components of the subaqueous delta include broad, shallow channels; moderately disturbed areas with near-surface cut and fill structures; heavily disturbed areas with sea floor depressions, pits, and gullies; and a smooth, gently sloping distal delta apron or “rise.” These features are not directly related to sediment settling from dilute surface plumes but are due to gravity-driven hyperpycnal underflows, submarine mass movements, and silt flows.

Resonant internal waves and their role in transport and accumulation of fine sediment in Eckernförde Bay, Baltic Sea

Abstract In spring 1993, an instrumented tetrapod was deployed in Eckernforde Bay, Baltic Sea, with the purpose of characterizing physical processes most relevant to sediment resuspension, transport and deposition. Results suggest that sediment transport events in Eckernforde Bay are associated with resonant internal waves. Observed turbidity events were associated with marked along-bay current oscillations and spectral analyses of these currents are suggestive of baroclinic resonance. Furthermore, the peak in the current spectra is close to the previously reported 26–28 h Baltic-wide seich. A one-dimensional, two-layer analytic model is applied which explains the generation of resonant internal waves within Eckernforde Bay by periodic, barotropic flows across a sill near the mouth of the bay. The analytic solution accounts for velocities much larger than those otherwise predicted by barotropic processes or by progressive internal waves, and also explains an observed sign reversal in the correlation between barotropic forcing and the internal wave response. Despite this enhancement of near-bottom currents, estimates of shear velocity suggest that bottom stress never reached the critical magnitude necessary to locally resuspend sediment and was only rarely sufficient to prevent deposition of sediment that may have been suspended elsewhere. During turbidity events, observed suspended sediment concentration did not increase with proximity to the bottom, suggesting sediment advection rather than local resuspension. Bottom photographs support this inference. Although internal wave resonance may commonly produce velocities sufficient to advect fine sediment into Eckernforde Bay, maximum currents are constrained by the waves phase velocity, which is only in the order of 30 cm s −1 .

Sonar evidence for methane ebullition in Eckernförde Bay

Abstract A bottom-mounted, circularly scanning sonar was used to observe the methane-rich seafloor of Eckernforde Bay during the months of April and May in 1993. Event-like changes in the acoustic signal were observed and are shown to be caused by scatterers in the water column that are interpreted to be gas bubbles rising in columns having transverse dimensions 2–5xa0m. The events do not correlate with seafloor current stress, temperature, or refraction due to stratification, but a strong correlation is seen with pressure at the seafloor, consistent with gas ebullition due to pressure release. It is not possible to definitively exclude scattering from pelagic animals as the cause of these events, but the observed localization at a few spots on the seafloor appears to be inconsistent with the biological explanation. These data are insufficient to determine the flux of free methane, but bounds are estimated and suggestions are made for future measurements that could determine flux.

Short period internal waves over the Huanghe (Yellow River) delta front

Internal waves with periods of about 5 minutes and trough to crest heights of up to 6.2 m were observed acoustically over the actively accreting delta front of the Huanghe (Yellow River) in the western portion of the Gulf of Bohai, Peoples Republic of China. The radian frequency of the internal waves was close to the locally-observed Brunt-Vaisala frequency. Through the relatively short (∼one hour) duration of an internal wave train, the amplitude was observed to decrease progressively with time. These internal waves may cause resuspension of delta-front sediments.