All issues
Volume 117, Issue 3
Displaying 1-12 of 12 articles from this issue
- |<
- <
- 1
- >
- >|
SPECIAL ISSUE Dynamic geomorphology and stratigraphy explored by model experiments
-
Naofumi Yamaguchi, Fujio Masuda2011 Volume 117 Issue 3 Pages 107-115
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSIn this study, rice grains were used as the bed material in flume experiments conducted to analyze three-dimensional grain fabrics in microdelta deposits, developed under three sets of hydraulic conditions. Rice grains are nearly uniform ellipsoids, making it possible to interpret not only patterns of imbrication, but also the orientations of individual grains based on the apparent lengths of their long axes projected onto a vertical section parallel to the current direction. Two predominant fabrics were observed at the center of experimental microdelta deposits: (1) grains oriented parallel to the current direction with an imbrication angle close to the foreset dip angle, and (2) grains with random orientations and horizontal imbrication. The first type of fabric appears in grains that have first settled on the uppermost lee slope and have subsequently been relocated and reoriented by discontinuous grain flows (avalanches). The second type of fabric appears in grains that have dropped over the full length of the lee slope and have not been relocated or reoriented by later grain-flow events. The results of this study suggest that the characteristics of three-dimensional grain fabrics may contribute toward an understanding of the depositional conditions of deposits formed by slip-face progression.View full abstractDownload PDF (3377K) -
Noriko K. Mitani, Hans-Georg Matuttis, Toshihiko Kadono2011 Volume 117 Issue 3 Pages 116-121
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSThe main parts of pyroclastic flow deposits show various characteristic features, such as density and size segregation of large fragments in the vertical direction. To understand the mechanisms that produce these features, we numerically simulated granular flows on an inclined rough surface with compositional impurities differing in size and density from matrix particles. The simulations show that, in flows with heterogeneous size and density distributions, the impurities rise or sink in the matrix according to their density. With increasing size of impurities and increasing density difference between impurities and matrix particles, the rate of rising or sinking of impurities increases. Observations of density and size segregation in agitated granular systems indicate that impurities are subject to buoyancy and granular viscous forces. The behavior of impurities in the central parts of flows obeys the laws governing the flow of viscous fluids.View full abstractDownload PDF (16786K) -
A review of the autosuspension of turbidity currents: its significance and gaps in our understandingHajime Naruse2011 Volume 117 Issue 3 Pages 122-132
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSAutosuspension is defined as a condition whereby turbidity currents can persist indefinitely if the external conditions are constant. The criteria for achieving autosuspension include erosion and an increase in flow density. Theoretically, most of the turbidity currents flowing down through submarine canyons are considered to be autosuspended currents. However, the theory of autosuspension has not been sufficiently verified through experiments, despite it being essential for understanding the origin of turbidity currents and the evolution of submarine fans. It is necessary to improve the theory by incorporating many other natural conditions, such as the cohesiveness of mud. Experimental investigations of autosuspension would provide advances in our understanding of the sedimentology of turbidity currents.View full abstractDownload PDF (878K) -
Yuichiro Miyata, Chikako Tanaka2011 Volume 117 Issue 3 Pages 133-140
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSUnder shallow flow conditions with a Froude number of about 1.4, the bedform and the direction of migration are affected by grain concentration in an experimental flume. With a decreasing feeding rate, the bedform changed sequentially from a plane bed to antidunes steadily migrating upstream, antidunes with smaller wavelengths showing occasional breakers, standing antidunes, antidunes migrating downstream, and finally fast-migrating “ripples” with an inharmonious water surface in the upper flow regime. Under a higher feeding rate, antidunes are stable with longer wavelengths and with a lower frequency of breaking waves (hydraulic jump). Wave breaking, which is more frequent at lower grain concentrations, also produces a local flat surface via downward filling of the deepened trough. The sheet flow layer, which has a high sediment concentration and is identified from a transmitted light image, is thicker and denser over upstream-migrating antidunes than over downstream-migrating antidunes. Therefore, the sediment concentration is likely to influence bedforms under shallow supercritical flow. The fact that larger wavelengths and less-frequent wave breaking are associated with higher sediment concentrations suggests that the sediment load acts to stabilize the flow. The velocity gradient near the bottom and the turbulent component (velocity deviation) of the vertical and horizontal velocities of the flow are large compared with those of the sediment grains. The grain velocity structure does not change within a range of sediment concentrations between 0.55 and 3.3 wt%. Therefore, the concentration of sediment at the lower boundary can give rise to a steeper density gradient and a shallower velocity gradient for the flow, both resulting in a higher Richardson number (i.e. more stable boundary flow). The sediment concentration can suppress flow oscillation or wave breaking of the flow and thereby produce upstream-migrating stable antidunes or a flat bed.View full abstractDownload PDF (901K) -
Tomohiro Sekiguchi2011 Volume 117 Issue 3 Pages 141-147
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSWave ripples forming under stable oscillatory-flow conditions have a particular set of morphologies, dimensions, and crest orientations. With any significant change in the hydraulic conditions, the wave ripples lose their original equilibrium configuration, take transient forms, and eventually attain a new equilibrium state. This transition process depends on (1) the ratio of the wavelengths of the original ripples to the renewed equilibrium ripples (λe/λi), (2) the degree of asymmetry in oscillatory flow, and (3) sediment grain size when a new oscillatory flow is perpendicular to the crest lines of pre-existing ripples. Distinctive transient ripples, which tentatively appear (and then disappear) during the period of modification, are likely to be preserved in the geological record when λe/λi < 1. The analysis of such ripples can benefit estimations of paleohydraulic conditions.View full abstractDownload PDF (1733K) -
Keisuke Taniguchi, Noritaka Endo, Hideo Sekiguchi2011 Volume 117 Issue 3 Pages 148-154
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSWe used a hydraulic flume to conduct analog model experiments to elucidate the effects of alternating orientations of bidirectional flows (intersection angle θ) on the morphology and evolution of isolated sand dunes. An initial cone of fine-grained quartz sand was exposed to bidirectional water flows with a range of intersection angles (45°, 90°, 135°, and 180°) for 20 cycles of flow alternation, with a cycle duration of 2 min. Experimental trials showed two types of deformation of the crest line: (1) “independence type”, in which a new crest line formed at a different location from the existing one; and (2) “reverse type”, in which an existing crest line reversed its migration direction and a new slip face was generated on the new stoss side of the existing crest. The independence type was observed at θ = 45° and the reverse type was observed at θ = 180°. At intermediate values of θ, the two types occurred contemporaneously. Due to the cumulative effect of deformation of the crest line, three types of topographies developed, with features characteristic of dome, longitudinal (seif), and reversing dunes. These results yield insights into the processes and conditions required for the formation of sand dune types.View full abstractDownload PDF (2065K) -
Atsunari Katsuki, Hiraku Nishimori, Noritaka Endo, Keisuke Taniguchi2011 Volume 117 Issue 3 Pages 155-162
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSBarchans are crescent-shaped dunes that form in areas where unidirectional winds predominate and where the availability of sand is insufficient to completely cover the bedrock surface. Barchans can exist in any region where granular or powdered sediments interact with moving fluids, such as air or water. They occur not only in sand deserts, but in environments such as the deep-sea floor, snowfields, and Martian surfaces. Although barchans have been extensively researched, previous studies have examined only the discrete formation of isolated barchans. Here, we present an overview of studies on barchan development focusing on the interaction dynamics of multiple barchans, based on recent research combining flume experiments and quantitative coarse-grained cell modeling. Both the numerical modeling and the flume experiments show three types of collisions between adjacent barchans, depending on the mass ratio and the locations of the dunes. Coalescence is a type of collision in which two barchans merge to form a single dune. Ejection represents the formation a new, small barchan from a larger “parent” barchan. A third type, called a split barchan, represents the division of a single barchan into two smaller dunes. These results contribute to a basic understanding of not only inter-barchan interactions, but the variety of dynamics involved in dune formation; e.g., the formation of barchan corridors and the birth of barchans from transverse dunes. A synergistic research program involving a combination of well-controlled experiments and corresponding mathematical models is critical if dune research is to evolve from a qualitative to a quantitative science.View full abstractDownload PDF (1037K) -
Shunji Ouchi2011 Volume 117 Issue 3 Pages 163-171
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSA miniature experimental landform developing with rainfall erosion on a flat-topped square mound of a mixture of fine sand and kaolinite was uplifted at a low and constant rate by the device set beneath it, after a low-relief surface had developed. Another run without uplift was also performed to compare and examine the effect of uplift on the development or evolution of experimental landforms. Both runs lasted for about 800 hours. Erosion of the mound started with rapid development of valley systems following the initial collapse of mound-edge cliffs. It continued on intermediate slopes and then reached to ridges. This process of erosion was characterized by an exponential decrease in the average height of the surface without uplift. Uplift raised the whole mound uniformly, but relief increased with uplift. The uplift constantly created height difference across the faults, and this promoted the erosion with knick-points migrating upstream, which resulted in relief increase. The relief increase showed a leveling-off trend during the period of uplift. The relief would have stabilized at a constant value corresponding to the uplift rate, if the uplift had continued longer beyond the limit of device. The development of alluvial fans around the mound had an effect to elevate the local base-level of erosion. As far as sufficient conditions exist for fan development around the mound, the elevation of the mound can increase with uplift while relief remains constant. If alluvial fans reach a limit to their development, the local base-level of erosion and the average height of the mound will stop increasing. Erosion and uplift will possibly show a dynamic equilibrium in the event of prolonged uplift at this rate. These physical analog model experiments do not reproduce real landform development; however, they can provide insights that assist the interpretation of unobservable real landform evolution.View full abstractDownload PDF (972K) -
Tetsuji Muto2011 Volume 117 Issue 3 Pages 172-182
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSA graded river conveys its sediment load without net deposition or net erosion. A correct understanding of grade is fundamental in genetic stratigraphy, because it represents the critical condition that discriminates between aggradational and degradational regimes in river systems. Conventional wisdom regarding fluvial grade in the alluvial lower reaches of a river emptying into the sea is that global (or large-scale) grade controls the final stable state of alluvial systems, representing an equilibrium response to steady external forcing (i.e., stationary sea level). However, results of recent experimental and numerical modeling of alluvial-deltaic depositional systems conducted by the author’s research group have shown that (1) the alluvial lower reaches of a river are capable of attaining grade only under conditions of falling sea level (not sea level standstill); (2) for grade to be attained and sustained, the falling sea level must follow a particular pattern that depends in part upon the alluvial and subaqueous basal slopes (α and φ respectively); and (3) there exist two different kinds of grade: ‘allogenic grade’ attained by the non-equilibrium response to a decelerating fall in sea level (α < φ), and ‘autogenic grade’ attained via the equilibrium response to a steady fall in sea level at any constant rate (α = φ). Where α > φ, the river never attains grade but continues to aggrade under all conditions of relative sea-level fall. The next generation of genetic stratigraphy models, particularly of alluvial-deltaic depositional systems, should take into account this new view of fluvial grade.View full abstractDownload PDF (578K) -
Akihiro Nishimoto, Tsuyoshi Mizuguchi, So Kitsunezaki2011 Volume 117 Issue 3 Pages 183-191
Published: 2011
Released on J-STAGE: July 07, 2011
JOURNAL FREE ACCESSThe formation of three-dimensional prismatic cracks during the drying process of starch-water mixtures is investigated numerically. We assume that the mixture is an elastic porous medium which possesses a stress field and a water content field; the evolution of these fields is represented by, respectively, a spring network and a phenomenological model based on the water potential. As a result of numerical simulations, we reproduce the water content distribution where the drying front propagates from the surface to the interior, and the prismatic structure of cracks grows as the front progresses. The particle-diameter dependence of the columns’ scale and the downward coarsening process of the columnar structure are investigated. We find that the non-uniform fracture process is a consequenice of breaking points propagating along the sides of polygonal cross sections. We also examine the effect of the crack networks on the dynamics of the water content field. The contrast between starch columns and geological columnar joints is discussed.View full abstractDownload PDF (8688K)
Pictorial
-
Noritaka Endo, Keisuke Taniguchi, Atsunari Katsuki2011 Volume 117 Issue 3 Pages V
Published: March 15, 2011
Released on J-STAGE: February 23, 2013
JOURNAL FREE ACCESSDownload PDF (1175K) -
Tomohiro Sekiguchi2011 Volume 117 Issue 3 Pages VI
Published: March 15, 2011
Released on J-STAGE: February 23, 2013
JOURNAL FREE ACCESSDownload PDF (890K)
- |<
- <
- 1
- >
- >|