Ronald L. Shreve

Role of Near-Bed Turbulence Structure in Bed Load Transport and Bed Form Mechanics

The interactions between turbulence events and sediment motions during bed load transport were studied by means of laser-Doppler velocimetry and high-speed cinematography. Sweeps (u′ > 0, w′ 0 w′ > 0) which contribute negatively to the bed shear stress and are relatively rare, individually move as much sediment as sweeps of comparable magnitude and duration, however, and much more than bursts (u′ 0) and inward interactions (u′ < 0, w′ < 0). When the magnitude of the outward interactions increases relative to the other events, therefore, the sediment flux increases even though the bed shear stress decreases. Thus, although bed shear stress can be used to estimate bed load transport by flows with well-developed boundary layers, in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity, it is not accurate for flows with developing boundary layers, such as those over sufficiently nonuniform topography or roughness, in which significant spatial variations in the magnitudes and durations of the sweeps, bursts, outward interactions, and inward interactions occur. These variations produce significant peaks in bed load transport downstream of separation points, thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms.

Subduction-channel model of prism accretion, melange formation, sediment subduction, and subduction erosion at convergent plate margins: 2. Implications and discussion

Many geological and geophysical investigations, particularly the Deep Sea Drilling Project, have shown that convergent plate margins are highly diverse features. For example, at some sites of subduction, such as the Lesser Antilles, the bedded sediment atop the incoming oceanic plate is extensively offscraped, whereas at others, such as Mariana, not only is the incoming sediment completely subducted beneath crystalline rock but portions of the overriding plate are undergoing subduction erosion. Earthquakes indicate wide variations in stress distribution within and between sites of plate convergence. Many ancient accretionary complexes include tracts of intensely-deformed subduction melange that contain blocks of mafic greenstones. Some contain bodies of thoroughly recrystallized blueschist that were uplifted from depths of 20 to 30 km. A comprehensive model for convergent plate margins must explain these and numerous other observations. Although the still widely cited imbricatethrust model for prism accretion qualitatively explains some observations at subduction zones, it does not account for many others, such as deep sediment subduction and subduction erosion.The subduction-channel model postulates essentially the same basic mechanics for all convergent plate margins that have attained a quasi-steady state (typically reached after about 20 Ma of subduction at speeds of 10 to 20 km Ma−1). It assumes that the subducting sediment deforms approximately as a viscous material once it is dragged into a relatively thin shear zone, or subduction channel, between the downgoing plate and the overriding one. It predicts the overall movement patterns of the sediment deforming within the channel and near its inlet, accounts for most of the observed features at convergent plate margins, and quantifies the processes of sediment subduction, offscraping, and underplating, and the formation of subduction melange. The predicted variations in tectonic behavior depend upon such site-specific variables as the speed of subduction, the supply of sediment, the geometry of the descending plate, and the topography and structure of the overriding block.

Bedload transport of fine gravel observed by motion-picture photography

Motion pictures taken at Duck Creek, a clear stream 6.5 m wide and 35 cm deep near Pinedale, Wyoming, provide detailed, quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them. Bed shear stress was approximately 6 Pa (60 dynes cm −2 ), which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material; and transport was almost entirely as bedload. The displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones, and rarely by brief sliding of large, angular ones. Entrainment was principally by rollover of the larger particles and liftoff of the smaller ones, and infrequently by ejection caused by impacts, whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones. The displacement times averaged about 0.2−0.4 s and generally were much shorter than the intervening repose times. The collective motions of the particles were characterized by frequent, brief, localized, random sweep-transport events of very high rates of entrainment and transport, which in the aggregate transported approximately 70% of the total load moved. These events occurred 9% of the time at any particular point of the bed, lasted 1–2 s, affected areas typically 20–50 cm long by 10–20 cm wide, and involved bedload concentrations approximately 10 times greater than background. The distances travelled during displacements averaged about 15 times the particle diameter. Despite the differences in their dominant modes of movement, the 8–16 mm particles typically travelled only about 30% slower during displacement than the 2–4 mm ones, whose speeds averaged 21 cm s −1 . Particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely, with diffusivities of 4.6 and 0.26 cm 2 s −1 , respectively. The bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler. The 2–4 mm particles were entrained 6 times faster on finer areas of the bed, where 8–16 mm particles covered 6% of the surface area, than on coarser ones, where they covered 12%, even though 2–4 and 4–8 mm particles covered practically the same percentage areas in both cases. The 4–8 and 8–16 mm particles, in contrast, were entrained at the same rates in both cases. To within the statistical uncertainty, the rates of distrainment balanced the rates of entrainment for all three sizes, and were approximately proportional to the corresponding concentrations of bedload.

Bedload sheets in heterogeneous sediment

Field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin, migrating accumulations of sediment, and coarse grains cluster at their leading edge. These accumulations are one or two coarse grains high and are much longer (0.2-0.6 m long in sand; 0.5-2.0 m in fine gravel) than their height. The authors propose the term bedload sheet for these features, and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment. In essence, coarse particles in the bedload slow or stop each other, trap finer particles in their interstices, and thus cause the coarse particles to become mobile again. Bedload sheets develop on the stoss side of dunes, causing the dune to advance incrementally with the arrival of each sheet. Successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain-size sorting that distinguishes cross-bedding. Available flume experiments and field observations indicate that bedload sheets are a common, but generally unrecognized, feature of heterogeneous sediment transport.

Sherman landslide, Alaska

Triggered by the earthquake of 27 March 1964, 3 x 107 cubic meters of rock fell 600 meters, then slid at high speed 5 kilometers across the nearly level Sherman glacier near Cordova. The landslide has a number of significant new features in addition to those typical of other large landslides that may have slid on a layer of trapped and compressed air.

Esker characteristics in terms of glacier physics, Katahdin esker system, Maine

The characteristics of large, subglacially formed eskers, such as the Katahdin system, are closely related to two special peculiarities of water-filled tunnels along the beds of ice sheets: (1) the water pressure approximates the weight of the overlying ice; and (2) in tunnels that descend and those that ascend less steeply than ∼1.7 times the ice-surface gradient, the walls melt, producing a sharply arched tunnel cross section, whereas in those that ascend more steeply, they freeze, producing a wide, low one. The first peculiarity primarily governs the paths of these eskers. It causes the tunnels to follow the paths ordinary rivers would follow were the land tipped downglacier ∼11 times the local ice-surface gradient. The paths therefore trend in the general direction of the former ice flow but tend to deviate so as to follow valleys and to cross divides at the lowest passes, as observed. Ice-surface gradients calculated from path deviations at two localities on the Katahdin esker system indicate relatively thin, sluggish ice the surface of which lay ∼200 m below the summit of Mount Katahdin, in agreement with independent geologic evidence. The second peculiarity primarily governs the form, composition, and structure of these eskers. Strong melting causes a large inflow of basal ice and entrained debris to the tunnel and produces sharp-crested eskers of poorly sorted, poorly bedded sand, gravel, and boulders with lithologies like the adjacent till, whereas weaker melting produces multiple-crested ones of similar composition. Freezing precludes inflow and produces broad-crested eskers of fairly well-sorted, well-bedded, more water-worn, coarse sand with few large clasts. Ice-surface gradients calculated from transitions from the multiple-crested type to small areas of broad-crested type on the Katahdin system agree closely with those computed from the paths at nearby localities. An anomalously low gradient calculated from a transition to an area of broad-crested type approximately twice as wide and long as the probable ice depth apparently confirms that, as expected, the basal ice was supported by water pressure over most, if not all, of the width of the esker.

Leakage and Fluidization in Air-Layer Lubricated Avalanches

Air-layer lubrication, that is, nearly frictionless support on a layer of trapped and compressed air, has been suggested for the Blackhawk-type landslides and for some nuees ardentes, snow avalanches, and cratering fallback. To ensure a sufficiently small rate of air loss by leakage through a typical Blackhawk-type landslide, the harmonic mean permeability must be less than about 1 darcy, a value that is reasonable for the extremely poorly sorted debris involved. A further necessary requirement for air-layer lubrication to occur, rather than simultaneous deposition and fluidization, is that the product of the permeability and the bulk density of the basal debris be less than 0.7 times the product of the harmonic mean permeability and the arithmetic mean density for the debris as a whole, as is probably in fact the case in these landslides.

Shear-zone thickness and the seismicity of Chilean- and Marianas-type subduction zones

Chilean-type convergent margins have many large (M > 7.6) earthquakes, whereas Marianas-type ones do not. This dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement, whereas it becomes understandable if it is viewed as a relatively thick, sediment-filled shear zone, which thins or thickens arcward depending on subduction speed and sediment supply. Chilean-type margins have thick trench fills, and their shear zones generally thin arcward from inlets as much as several thousand metres high, the most pronounced thinning being located near backstops. Tall (up to several kilometres) seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities. Their near-basal ruptures can generate large thrust-type earthquakes, mainly concentrated in seismic fronts near backstops. Marianas-type margins, in contrast, have thin trench fills, and their shear zones generally thicken arcward from inlets that can be as little as 300 m high. Seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes. After passing the inlet, they do not touch the roof and therefore cannot generate large earthquakes. A similar mechanism may explain seismic gaps at sediment-poor regions of subduction zones.

The probabilistic-topologic approach to drainage-basin geomorphology

Although the probabilistic-topologic approach to drainage-basin geomorphology that I initiated unifies a wide variety of quantitative empirical geomorphic relationships, some geomorphologists have objected variously that it abandons the scientific method, that its emphasis on topologic properties causes it to miss the geomorphic components of drainage basins, that it lacks physical content, and that it is too complicated to be of practical value. In fact, however, it gives results that are generally simpler, better, and more practical than those given by other methods. It has physical content because it is founded on postulates that are observational statements about actual drainage basins. It emphasizes topologic properties because they dominate the orientation-free planimetric aspects of drainage basins. Finally, it is necessarily probabilistic because of the prominent random element in natural landscapes, which may result from instabilities that amplify small perturbations into large ones.

Late Wisconsin ice-surface profile calculated from esker paths and types, Katahdin esker system, maine

Abstract Values of the gradient of the former ice surface can be inferred at points along a flow line from deviations of esker paths or transitions in esker type and numerically integrated to give the profile. A profile calculated in this way shows that during formation of the Katahdin esker system about 12,700 yr ago the ice thickness at distances of 10, 20, 50, 100, and 140 km from the terminus, which is about two thirds of the distance to the ice divide, was approximately 200, 300, 600, 750, and 900 m. The terminal reach was computed by assuming an unknown uniform basal drag and matching the profile to its known elevation at the terminus and known gradient 10 km upglacier. Correction for isostatic rebound based on the elevations of contemporaneous deltas and of the marine limit proved unnecessary, because the tilt due to the difference in uplift at the two ends of the profile is only 0.1 m km −1 . With other plausible assumptions as to sea levels in the past, elevations of the marine limit, or exact location of the terminus the profile could be as much as roughly 100 m higher. It hits Mount Katahdin about 500 m below its summit, which is at 1600 m, in agreement with the geological evidence farther west. The steepening of the upper part of the profile suggests that the mountain dammed and diverted the ice. Basal drag computed from the profile varies from about 20 kPa (0.2 bar) near the terminus to 30 kPa (0.3 bar) at 100 km to 70 kPa (0.7 bar) at 140 km. The relatively low values away from the influence of Mount Katahdin agree with independent evidence from deep-sea cores of substantial late Wisconsin ice-sheet thinning without comparable areal reduction. The method has potential for application over wide areas that were occupied by the Laurentide and Scandinavian ice sheets.