摘要:

ABSTRACTIt is now generally recognized that the mathematical description of sediment dynamics will be improved through an understanding of geophysical turbulence. In this study, therefore, the turbulence characteristics of the benthic boundary layer over coarse sediments are examined as a step towards the realization of such a description.Examination of the turbulence has been conducted using spectral, correlation, quadrant and statistical analyses, and a number of features have been identified. Mean eddy length scales obtained using spectral and autocorrelation techniques were found to be similar. The interpretation of the duration and interval between the intermittent ‘bursting’ events, scaled using outer flow variables, was found to be dependent upon the sampling interval. Autocorrelation analysis revealed a high correlation between bedload transport and the instantaneous horizontal flow component rather than instantaneous kinematic stress. From the standpoint of developing prognostic transport models based on physically realistic benthic flow processes, it is considered that turbulence measurements will lead to such formulat

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01533.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTContinuous, detailed records of marine gravel transport have been obtained acoustically and compared with bedload transport rates (qb) predicted by five bedload transport equations using measurements of the near‐bed turbulent current flow. When mean flow data are used in these equations, totalqbestimates are similar to those measured. However, when instantaneous flow data are used, totalqbis over‐estimated by approximately one order‐of‐magnitude. Based on the acoustic measurements, an empirical equation has been obtained that gives accurate estimates of totalqbover a tidal cycle and simulates well the intermittent characteristics of marine bedload tr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01534.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTDuke (1987) argues that the structure reported by Greenwood&Sherman (1986) as hummocky cross‐stratification from a shallow lacustrine environment is, instead, bedding formed by simple post‐vortex oscillation ripples. His primary concern is with the length scale of the structures identified in box cores. However, using measured flow data, it is shown here that existing models of bedform generation based upon wave orbital velocity, orbital diameter and grain diameter do not support a simple post‐vortex ripple interpretation. In particular, the relative steepness of the structure refutes such an origin. The structural characteristics of the lacustrine cores most closely match those associated with classic hummocky cross‐stratification and were produced under a combined oscillatory/quasi‐steady f

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01535.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe distribution of suspended particle size and concentration were measured along the Gironde Estuary, France, from the river seaward to the ocean. The suspended particle size and volume concentration were measured usingin situholography and onboard optical techniques utilizing special procedures in order to avoid floc breakage.Sediments discharged by the rivers coagulate upon encountering the very low salinities (0.2‰ of the upper estuary (confirmed with laboratory experiments), and are then transported and deposited by currents in the remainder of the estuary. This coagulation, coupled with estuarine circulation, produces a turbidity maximum which is offset between the surface and bottom waters. The floc size maximum is oceanward of the turbidity maximum and is, likewise, offset along the estuary by about 30 km. The estuary can be subdivided into the following zones: (1) coagulation; (2) hydrodynamic, landward of the null point; and (3) hydrodynamic, seaward of the null point. Initial coagulation appears to be completed in coagulation zone (1), and particles are transported and settled (with very little floc breakage and recoagulation) in zones (2) and (3) only. The floc settling velocities, coupled with estuarine circulation, control the concentration and size distributions of flocs in the water column, and eventually control the deposition of sediment

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01536.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe grain fabric of a laboratory antidune composed of coarse‐ to medium‐grained sand shows a high‐angle upcurrent imbrication both on the upstream and downstream sides. Such a grain fabric differs from a normal dune or a megaripple, where grain alignment displays an upcurrent imbrication relative to the depositional slope in the foreset lamina and/or near parallelism with the slope surface in the toe‐set lamina. Recognition of antidunes in ancient rocks is difficult, but detailed examination of grain fabric of cross‐stratification makes it possible to distinguish antidunes or ‘backset’ bedding from normal dunes o

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01537.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTWhile extensive aeolian deposits are associated with glaciation, little is known of the mechanisms which produced them. Observation of glaciofluvial sediments in Pangnirtung Pass, N.W.T., indicates that peak deflation occurs in the fall and winter months so that the nature of sediment transport involving frozen surfaces greatly differs from warm environments. Laboratory experiments indicate that such surfaces are mobilized through abrasion, and are most susceptible at low volumetric water content (less than 20%) and low temperature (–20°C). Stable surfaces have volumetric water contents in excess of 30% and temperatures of –15°C. Surface stability also increases with decreasing particle size, density and, possibly, transport load. Collisions with frozen surfaces are relatively elastic with significantly greater kinetic energy transfer to ejected surface grains than observed for loose dry surfaces. Saltation transport in cold environments could therefore be sustained at lower windspeeds than for dry surfaces provided that some loose particles are available initially but do not accumulate at the su

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01538.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTWithin aeolian dune systems spatial patterns of grain‐size variation have been recognized, but little has been said about temporal changes. Increasingly it is becoming clear that linear dunes are associated with bi‐directional wind regimes which are often seasonal. In the Namib Sand Sea, where linear dunes are aligned roughly north‐south, winds blow from the west in summer and from the east in winter. In response to this regime, sand is eroded from the west slopes and deposited on the east slopes in summer, and eroded from the east slopes and deposited on the west slopes in winter. Preliminary evidence from a study of a single Namib linear dune reported here confirms that this seasonal aeolian regime induces seasonal responses in some grain‐size measurements due to the dynamics of sand transport on the dune, the characteristics of the sand source immediately upwind of the sample point and the nature of the deposit. Thus, time of sampling is crucial to the results o

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01539.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTTwo modern fluvial‐aeolian depositional systems (Great Sand Dunes National Monument, Colorado and the Mojave River Wash, California) are remarkably similar in spite of different climates, sizes, fluvial sediment textures, and relative directions of aeolian and fluvial transport. Dune growth and migration, and deflation of blowouts create 8–10 m of local relief in unflooded aeolian landscapes.There are six prominent fluvial‐aeolian interactions. (1) Fluvial flow extends into the aeolian system until it is dammed by aeolian landforms; (2) interdune areas (overbank‐interdunes) upstream of aeolian dams, and alongside channels are flooded; (3) water erodes dunes alongside channels and interdunes; (4) flood waters deposit sediment in interdune areas; (5) fluvially derived groundwater floods interdunes (interdune‐playas); (6) wind erodes fluvial sediment and redeposits it in the aeolian system.Unique and characteristic sediments are deposited in overbank‐interdunes and in interdune‐playas, reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions. These fluvial‐aeolian interdune deposits are characterized by irregular, concave‐up bases and flat upper surfaces containing mudcracks or evaporite cement.Characteristic low‐relief surfaces form in aeolian systems as an effect of flooding. Fluvial deposits are resistant to aeolian deflation. Aeolian sand is preserved when flood sediments are deposited around the bases of the dunes. Thus repetitive fluvial and aeolian aggradation tends to be ‘stepwise’ as interdune floors are suddenly raised during floods. The effects of flooding should be easy to recognize in ancient aeolianites, even beyond the area cov

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01540.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTAn understanding of fluvial‐aeolian deposition derived from modern case‐examples in a previous study is applied to the Permian Cutler Formation and Cedar Mesa Sandstone on the Colorado Plateau. These formations supply an excellent three‐dimensional exposure of intertonguing fluvial and aeolian strata. Four distinct facies associations form the bulk of the Cutler Formation and Cedar Mesa Sandstone: (1) aeolian dune deposits; (2) wet interdune deposits; (3) fluvial channel deposits; and (4) overbank‐interdune deposits. In addition, two distinctive types of erosion surfaces are found within the Cutler Formation and Cedar Mesa Sandstone: pebble‐ to granule‐rich erosion surfaces (aeolian deflation surfaces) and flood surfaces.Fluvial and aeolian intertonguing result in extensive tabular sheets of aeolian sandstone separated by flood surfaces and overbank‐interdune deposits. Fluvial channels are associated with the deposits overlying flood surfaces and are incised into the underlying aeolian sandstones. Overbank‐interdune deposits and wet interdune deposits cover flood surfaces and intertongue with overlying aeolian sandstones.The primary characteristics of ancient fluvial‐aeolian deposition are overbank‐interdune deposits and pronounced extensive erosion surfaces (flood surfaces), which are parallel to underlying fluvial sandstones and thus trend parallel to the palaeoslope and palaeohy

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01541.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe submarine morphology, sediments, and three‐dimensional geometry of a developing fan delta are described using data from acoustic surveys, bottom sampling, and observations from a manned submersible. The fan system is being built in a British Columbian fjord (water depth 410 m) supplied with coarse‐grained sediments from a fjord‐side river.Construction of the subaqueous fan began about 10–12,000 yr BP and is ongoing. The system is analogous to part of one fault‐uplift sedimentation cycle in ancient fan deltas. Initially, when offshore relief was at a maximum, acoustically chaotic sediment wedges were emplaced over fjord‐bottom glaciomarine deposits. Subsequent aggradation/progradation resulted in moderately dipping sequences interrupted by local chaotic units. The present fan surface (average slope 13°) is divided into six zones arranged concentrically from the fan apex, on the basis of form, sediment and process interpretations. Continued subaqueous fan growth results from settling of river‐derived sediments from suspension and downslope sediment dispersal by episodic gravity flows, apparently fed by underflows

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb01542.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY