David E. Drake

Rapid and widespread dispersal of flood sediment on the northern California margin

The dispersal of flood sediment from small river systems is a poorly studied, yet potentially important aspect of active continental-margin sedimentation. In January 1995, during a flood with a 30 yr return period, the Eel River (northern California) delivered an estimated 25 ± 3 × 106 t (metric tons) of fine-grained (<62 µm) sediment to the ocean. The flood formed a distinct layer on the sea bed that was centered on the 70 m isobath, extended for 30 km along shelf and 8 km across shelf, and was as thick as 8.5 cm, but contained only 6 × 106 t of sediment. Thus, 75% of the flood-derived sediment did not form a recognizable deposit, but was instead rapidly and widely dispersed over the continental margin. Stratigraphic models of, and compilations of sediment flux to, active continental margins need to take the dispersive nature of small river systems into account.

Sediment resuspension and bed armoring during high bottom stress events on the northern California inner continental shelf: measurements and predictions

Abstract Geoprobe bottom tripods were deployed during the winter of 1990–1991 on the northern California inner continental shelf as part of the STRESS field experiment. Transmissometer measurements of light beam attenuation were made at two levels and current velocity was measured at four levels in the bottom 1.2 m of water. Intervals of high measured bottom wave velocity were generally correlated with times of both high attenuation and high attenuation gradient in the bottom meter of the water column. Measured time series of light attenuation and attenuation gradient are compared to values computed using a modified version of the Smith [(1977) The sea, Vol. 6, Wiley-Interscience, New York, pp. 539–577] steady wave-current bottom-boundary-layer model. Size-dependent transmissometer calibrations, which show significantly enhanced attenuation with decreasing grain size, are used to convert calculated suspended sediment concentration to light attenuation. The finest fractions of the bed, which are the most easily suspended and attenuate the most light, dominate the computed attenuation signal although they comprise only about 5–7% of the bed sediment. The calculations indicate that adjusting the value of the coefficient γ0 in the expression for near-bed sediment concentration cannot in itself give both the correct magnitudes of light attenuation and attenuation gradient. To supply the volumes of fine sediment computed to be in suspension during peak events, even with values of γ0 as low as 5 × 10−5, requires suspension of particles from unreasonably large depths in the bed. A limit on the depth of sediment availability is proposed as a correction to suspended sediment calculations. With such a limit, reasonable attenuation values are computed with γ0 ≈ 0.002. The effects of limiting availability and employing a higher γ0 are to reduce the volume of the finest sediment in suspension and to increase the suspended volumes of the coarser fractions. As a consequence, the average size and settling velocity of suspended sediment increases as bottom shear stress increases, with accompanying increases in near-bed concentration gradients. Higher concentration gradients produce larger stratification effects, particularly near the top of the wave boundary layer at times when wave shear velocities are high and current shear velocities are low. These are the conditions under which maximum attenuation gradients are observed.

Rippled Scour Depressions on the Inner Continental Shelf Off Central California

ABSTRACT Side-scan sonar records taken during the recent Coastal Ocean Dynamics Experiment (CODE) show elongate, shore-normal rippled depressions of low relief on the inner continental shelf off central California between Bodega Bay and Point Arena. These features extend up to 2 km from the coast into water depths of up to 65 m. The proposed mechanism for their generation is storm-generated bottom currents associated with coastal downwelling during the late fall and winter which scour the surficial fine-sand sediment and expose the coarser-sand substrate in the depressions. The zones of most intense erosion and the irregular spacing of the features may be controlled by submerged rock ledges and other prominent coastal features. The large straight-crested ripples within the depressions (heights to 40 cm; wavelengths to 1.7 m) are probably formed by large-amplitude, long-period surface waves generated by winter storms.

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.

Nepheloid layers and internal waves over continental shelves and slopes

Theoretical and laboratory results indicate that bottom velocities within shoaling internal gravity waves intensify upslope approximately inversely proportional to the water depth. The elevated velocities (and bottom stresses) caused by shoaling and, possibly, breaking internal waves might explain the generation and maintenance of near-bottom nepheloid zones and attached turbid plumes that have been observed over certain continental shelves and slopes. This process is proposed as an explanation of zones of relatively low transmissibility that emanate from the upper continental slope near Newport submarine canyon off southern California.

Post-depositional alteration and preservation of sedimentary event layers on continental margins, I. The role of episodic sedimentation

The degree of post-depositional alteration and hence the preservation potential of sedimentary event beds and transient signals is determined by the outcome of a ‘race’ between biological (and to a lesser extent physical) processes that conspire to destroy a signal and sediment accumulation which advects the layer or signal out of the surface mixing zone. Preservation potential specifically depends on the relative magnitudes of the (1) biological mixing intensity (sometimes parameterized as a biodiffusivity, Db), (2) mixing-layer thickness, (3) layer or signal thickness, and (4) sediment accumulation rate. These terms control the dissipation time (i.e. time required to destroy a signal) and the transit time (i.e. time necessary to advect a signal through the surface mixing layer). On fine-grained, upper continental margins in general, and the Eel River shelf (northern California) in particular, biological mixing intensity is high (i.e. Db ranges from 10 to 100 cm2/yr), mixing-layer thickness is large (>10 cm), and sediment accumulation rates are rapid (0.1–1 cm/yr). Despite the high sediment accumulation rates, transit times through the surface mixing-layer range from decades to a century. Signal dissipation times are considerably shorter: (1) <3 yr for beds up to 6 cm thick imaged in X-radiographs, and (2) <15 yr for the grain-size signature of beds up to 8 cm thick. Therefore event layers and their corresponding grain-size signature have a low probability of preservation. However, short-lived episodic sedimentation events (e.g. oceanic floods) can instantaneously advect material through the surface mixing layer, thereby preserving event beds and transient signals. On the Eel River shelf the sequential timing of episodic sedimentation events has exerted a first-order control on the resultant stratigraphic record (presence/absence of layers and grain-size fluctuations). Episodic sedimentation – a hallmark of continental shelf settings – is key to understanding strata formation and preservation on margins.

Seasonal variation in sediment transport on the Russian River shelf, California

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MEASUREMENTS IN THE BOTTOM BOUNDARY LAYER ON THE AMAZON SUBAQUEOUS DELTA

Abstract An instrumented bottom tripod ( geoprobe ) recorded flow and suspended sediment data in the bottom boundary layer above the lower foresets of the Amazon subaqueous delta in 65 m mean water depth in February, 1990. After about two weeks of operation the apparent seafloor at the tripod site rapidly elevated over a 14-hour period by about 44 cm. This sudden change, which was detected by an acoustic altimeter and which caused the loss of signals from the lowermost geoprobe current and optical sensors, is thought to have been caused by the incursion of a dense bottom layer of fluid mud that migrated downslope from shallower sections of the foresets. The fluid-mud migration across the outer part of the foresets, if a repetitive and occasional process in this region, could be a major mechanism for episodic seaward growth of the delta. Current velocity profiles are used to estimate shear velocities, u∗, and roughness lengths, zo, during the first two weeks of measurements. The extremely energetic semi-diurnal tidal motion required correction of the estimates of u∗ and zo for acceleration. Average values of u∗ and zo for the two-week period were 1.7 cm/s and 0.3 cm, respectively. The effects of strong stratification due to flocculated, near-bottom suspended sediment on the estimates of u∗ are significant for the lower values of u∗ (u∗

A new instrument system to investigate sediment dynamics on continental shelves

Abstract A new instrumented tripod, the GEOPROBE system, has been constructed and used to collect time-series data on physical and geological parameters that are important in bottom sediment dynamics on continental shelves. Simultaneous in situ digital recording of pressure, temperature, light scattering, and light transmission, in combination with current velocity profiles measured with a near-bottom vertical array of electromagnetic current meters, is used to correlate bottom shear generated by a variety of oceanic processes (waves, tides, mean flow, etc.) with incipient movement and resuspension of bottom sediment. A bottom camera system that is activated when current speeds exceed preset threshold values provides a unique method to identify initial sediment motion and bed form development. Data from a twenty day deployment of the GEOPROBE system in Norton Sound, Alaska, during the period September 24 – October 14, 1976 show that threshold conditions for sediment movement are commonly exceeded, even in calm weather periods, due to the additive effects of tidal currents, mean circulation, and surface waves.

Estimates of the suspended sediment reference concentration (Ca) and resuspension coefficient (γ0) from near-bottom observations on the California shelf

Near-bottom current and suspended sediment measurements above a sandy clayey silt bed on the California continental shelf are used to estimate the near-bed suspended sediment reference concentration, Ca, and the bed shear stress, τ0. A proposed model contends that Ca should be proportional to the normalized excess shear stress acting on the sediment grains and to the availability of erodible grains in the bed. The coefficient of proportionality (γ0) in this model is estimated for 10 erosion events and is found to range from about 1.5 × 10−5 to 3 × 10−4. Moreover, the estimated γ0 values appear to be inversely proportional to the excess bed shear stress. This result suggests that other factors, such as bed armoring and grain cohesion, inhibited the resuspension of particles at the relatively higher shear stresses.