Jeffrey D. Parsons

Experimental Study of the Grain‐Flow, Fluid‐Mud Transition in Debris Flows

We have performed a series of laboratory experiments that clarify the nature of the transition between fluid‐mud and grain‐flow behavior. The surface velocity structure and the speed of the nose of debris flows in channels with semicircular cross sections were measured with several cameras and visual tracers, while the mass flow rate was recorded using a load cell at the exit chamber. Other rheological tests were used to calculate independently the yield strength and matrix viscosity of the debris‐flow mixture. Shear rates were varied by nearly an order of magnitude for each mixture by changing the channel radius and slope. Shear rates were significantly higher than expected (6–55 s−1), given the modest slopes examined (10.7°–15.2°). The large values were primarily a result of the concentration of shear into narrow bands between a central nondeforming plug and the sidewall. As a result, the shear rate of interest was calculated by using the width of the shear band and the plug velocity, as opposed to the flow depth and front velocity. The slurries exhibited predominantly fluid‐mud behavior with finite yield strength and shear‐thinning rheologies in the debris‐flow body, while frictional behavior was often observed at the front, or snout. The addition of sand or small amounts of clay tended to make the body of the flows behave in a more Bingham‐like fashion (i.e., closer to a linear viscous flow for shear stresses exceeding the yield stress). The addition of sand also tended to accentuate the frictional behavior at the snout. Transition to frictional grain‐flow behavior occurred first at the front, for body friction numbers on the order of 100. Similar behavior has been observed in an allied field site in the Italian Alps. In the experiments, it was hypothesized that the snout‐grain‐flow transition was a result of concentration of the coarsest material at the flow front, reduced shear near the snout, and loss of matrix from the snout to the bed. Regardless of the frictional effects at the snout, flow resistance in the body was nearly always regulated by yield‐stress and shear‐thinning properties, with no discernible boundary slip, despite volumetric sand contents in excess of 50%.

Similarity of gravity current fronts

Mixing processes in gravity current fronts have only recently been quantified due to their complex, unsteady nature. The similarity of the mixing processes in these pioneer works, however, has not been explored adequately. Experiments that explore a wide range of fronts have been performed. These experiments have used techniques that exhaustively sample the temporal (using a high-speed conductivity probe) and spatial (using planar laser-induced fluorescence) density field more thoroughly than any previous work. Both types of experiments have confirmed earlier research suggesting that low Reynolds number fronts mix differently and less than higher Reynolds number flows. Similarity appeared to be achieved for Req>1000, where Req is a Reynolds number based upon the cube root of the buoyancy flux into the front and the height of current. It appears that certain secondary mixing processes, seen by other researchers studying stratified mixed layers, are responsible for the earlier changes seen with Reynolds num...

Evidence for superelevation, channel incision, and formation of cyclic steps by turbidity currents in Eel Canyon, California

We performed a multibeam survey of Eel Canyon, offshore northern California. The survey revealed a significant bend in the canyon that appears to be due to the oblique compressional tectonics of the region. A series of steps within a linear depression, similar to 280 m above the canyon floor, extends from the canyon rim at this bend to the subduction zone and a distinct fan-like topographic rise. We hypothesize that the linear depression is a distributary channel and the steps are cyclic-step bedforms created by turbidity currents. Our interpretation indicates that turbidity currents are able to run up and overspill the 280-m-high canyon wall, resulting in a partial avulsion of the canyon and the construction of a fan lobe that is offset from the canyon mouth. Simple hydraulic calculations show that turbidity currents generated in the canyon head from failure of 2-3 m of material would be capable of partially overflowing the canyon at this bend, assuming steady-uniform flow, full conversion of the failed mass into a turbidity current, and a range of friction coefficients. These estimates are consistent with analyses of sediment cores collected in the head of Eel Canyon, which suggest that 2-3 m of material fails on decadal time scales. Our calculations show that the overflowing parts of the currents would have large shear velocities (>10 cm/s) and supercritical Froude numbers, consistent with erosion of the distributary channel and formation of cyclic steps by turbidity currents.

A Preliminary Experimental Study of Turbidite Fan Deposits

We present preliminary laboratory experimental results that demonstrate lobe switching in a turbidite fan. After a significant number of individual event beds (more than eight) in both experimental fans made in this study, the flow began to focus itself into a lobe, at the center of which appeared a subtle channel-form. Initial lobe switching occurred after nearly twenty event beds. Subsequent switching appeared to scale with lobe size and sedimentation rate. The densest, coarsest runs tended to initiate the switching of the depocenter. Instabilities within the turbidity current itself appear to be responsible for initial flow confinement to a lobe and ultimate switching events. Currently untreated scale effects and measurement difficulties limit quantitative extension of the results to natural settings, though some general relationships are proposed.

Mixing at the front of gravity currents

Abstract Observations made with the help of a movable-bed tank designed and operated to freeze the motion of gravity current fronts indicate that both the scale of the apparatus and the magnitude of the current Reynolds number have a strong influence on mixing rates. Low mixing rates are associated with fronts in relatively shallow fresh water depths and low Reynolds numbers. Mixing rates increase more rapidly as the ratio of current thickness to fresh water depth increases. For the deepest fresh water flow conditions, when scale effects can be expected to be negligible, the dimensionless mixing rate is approximately equal to three times the relative current thickness and has an upper limit of about 0.3. Lower mixing rates are observed as the current Reynolds number decreases, suggesting that viscous effects can still exist even for deep fresh water conditions. Application of the experimental results to estimate some parameters of a hypothetical gravity current on the continental shelf yield reasonable values. However, it is clear that, owing to the scale and Reynolds number effects that might be present in laboratory experiments, particular care should be exercised when trying to extrapolate results to gravity currents in nature.

Turbulent structure of high-density suspensions formed under waves

We performed a series of laboratory experiments on the interactions between turbulent wave boundary layers and a predominantly silt-sized sediment bed. Under a wide range of wave conditions similar to those observed on storm-dominated midshelf environments we produced quasi-steady high-density benthic suspensions. These suspensions were turbulent, while containing large near-bed concentrations of suspended sediment (17–80 g/L), and were separated from the upper water column by a lutocline. Detailed measurements of the vertical structure of velocity, turbulence, and sediment concentration revealed that the wave boundary layer, while typically >1 cm thick in sediment-free conditions, was reduced substantially in size, often to <3 mm, with the addition of suspendible sediment. This likely resulted from sediment-induced stratification that limited vertical mixing of momentum. Despite boundary layer reduction the flows were able to support high-density suspensions as thick as 8 cm because turbulent energy was transported upward from this thin but highly energetic near-bed region. Standard formulations of the Richardson number for shear flows are not applicable to our experiments since the suspensions were supported from transported rather than locally produced turbulence.

Enhanced Sediment Scavenging Due to Double-Diffusive Convection

ABSTRACT River water intruding into a lake or ocean typically results in a surface layer of warm, sediment-laden water overriding relatively dense, cold fluid. Though this system is stable with respect to density, instability may arise as a result of the different diffusivities of sediment and heat. A series of experiments were conducted which indicated that double-diffusive convection (DDC) was responsible for rapid sedimentation of slowly settling particles. A simple theory, based upon relevant physical processes, is developed that describes this behavior. It also incorporates the previously neglected effects of ambient stratification on the strength of the DDC. Application of the theory and our results to field data taken from oceanic river plumes demonstrates that the flux due to double-diffusive sedimentation (DDS) can be at least as strong as flocculation-enhanced gravitational settling.

The Sound Generated by Mid-Ocean Ridge Black Smoker Hydrothermal Vents

Hydrothermal flow through seafloor black smoker vents is typically turbulent and vigorous, with speeds often exceeding 1 m/s. Although theory predicts that these flows will generate sound, the prevailing view has been that black smokers are essentially silent. Here we present the first unambiguous field recordings showing that these vents radiate significant acoustic energy. The sounds contain a broadband component and narrowband tones which are indicative of resonance. The amplitude of the broadband component shows tidal modulation which is indicative of discharge rate variations related to the mechanics of tidal loading. Vent sounds will provide researchers with new ways to study flow through sulfide structures, and may provide some local organisms with behavioral or navigational cues.

Are fast‐growing martian dust storms compressible?

A simple scale analysis demonstrates that rapidly-expanding Martian dust storms are driven by extremely large density differences. Thermal gradients appear to be inadequate to produce these density contrasts. When dust produces the densities required, the storms become compressible because of the slowing of the speed of sound due to the addition of particles. Though compressible, particle-laden flows have never been analyzed due to their highly complex nature, pressure waves produced by these flows could be hypothesized to provide a positive feedback capable of producing rapid growth.

Formation of a sandy near‐bed transport layer from a fine‐grained bed under oscillatory flow

Bed surface coarsening was found to be an important effect for the formation of ripples and the dynamics of the boundary layer above a predominantly silt-sized sediment bed (median particle size equal to 26 μm; ~20% fine sand, 70% silt, 10% clay) under oscillatory flow (with orbital velocities of 0.32-0.52 m/s) in a laboratory wave duct. Following bed liquefaction, substantial winnowing of the bed surface occurred due to entrainment of finer material into suspension. Bed surface coarsening was quantified with micro-scale visualization using a CCD (charged-coupled device) camera. Under most wave orbital velocities investigated, the coarse surface particles were mobilized as a near- bed transport layer approximately 4 grain-diameters thick. The transport of these coarse sediments ultimately produced suborbital or anorbital ripples on the bed, except for the highest orbital velocities considered where the bed was planar. Micro-scale visualizations were used to construct a maximum (particle) velocity profile extending through the near-bed transport layers using particle-streak velocimetry (PSV). These profiles had a distinctive kink in log linear space at the height of the transport layer, suggesting that the near-bed sediment transport reduced skin friction and contributed to the boundary roughness through extraction of momentum.