ABSTRACT Experiments were conducted to define equilibrium combined-flow bed configurations developed under a wide range of oscillatory (Uo) and unidirectional (Uu) velocity components, with a constant oscillation period of 8.5 sec and median grain size of 0.09 mm. In the range of flows studied (Uo, 0-0.80 m/sec; Uu, 0-0.25 m/sec) the bed phases are: no movement, small 2D ripples, small 3D ripples, large 3D ripples, and plane bed. Small 2D ripples are stable at low Uo and Uu; by increasing either velocity component the plan-view bed configuration becomes progressively less regular as small 3D ripples develop. Small 3D ripples are stable at low to moderate Uo and a wide r nge of Uu. Under purely oscillatory flow, as Uo increases beyond about 0.40 m/sec, large-scale, hummocky bed forms become stable. When a unidirectional component is applied, these bed forms quickly develop a downstream asymmetry and generally move in the direction of Uu. The dip angles of the lee flanks increase with increasing Uu, although they remain less than the angle of repose. At high oscillatory velocities (Uo, 0.70-0.80 m/sec), plane bed is the stable bed phase when Uu > 0.05 m/sec. Much hummocky cross-stratification shows a preferred dip direction of the coset laminae. Our experiments support the idea that such cross-stratification is produced by combined flow: the large 3D ripples produced at high oscillatory velocities and small to moderate unidirectional velocities would presumably generate what might be called anisotropic hummocky cross-stratification (if we had been able to aggrade the bed during the runs). This would be the case even in dominantly oscillatory flow; the experiments show that even a weak unidirectional current of several centimeters per second superimposed upon a strong oscillatory flow results in bed forms that move predominantly in a single direction and that leave sets of laminae that dip predominantly in that direction. In runs at large Uo, the purely oscillatory large-scale bed forms were quickly planed off as Uu was increased from zero, but a series of almost stationary gentle undulations on the bed surface persisted for indefinitely long times where the original topographic highs were situated, even at the highest unidirectional flow investigated, 0.25 m/sec. This suggests a possible origin for the gentle undulation observed in Cretaceous parallel-laminated shallow-marine sandstones described by Arnott (1987, 1988), with spacings up to a few meters and heights not much more than a centimeter.
Two‐dimensional equilibrium boundary‐layer flows were investigated in an open water channel with a width/depth ratio of 6 for a smooth bed of 0.16 mm quartz grains ( d 50 ) and compared with those of an immobile smooth cemented bed with the same sand roughness. For flows at Reynolds numbers between 20,000, representing onset of erosion, and 28,000, before appearance of rhomboid ripples, quartz grains rolled over an otherwise smooth sand bed with a density of ≤40 grains cm −1 s −1 . Then the universal law of the wall, as obtained for the fixed, smooth sand bed, could not be confirmed by the data. Instead, (1) a logarithmic layer was found that extended further into the wake region and had a reduced value of von Karman's constant κ = 0.32 ± 0.04, (2) the friction diagram indicated Reynolds number dependent drag reduction, and (3) the logarithmic layer extended down to the top of the rolling grains at Re U >25,000. These results are interpreted as a new class of wall‐bounded shear flow with different momentum transfer processes and a velocity‐defect law throughout the flow down to the top of the rolling grains. Some conclusions are discussed for sedimentological and engineering problems in which this type of flow is the rule rather than the exception.
ABSTRACT We have measured the mean magnitude and direction, and the rms fluctuation intensity, of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long. Downstream of reattachment, the mean skin‐friction magnitude is about 20% greater than its free‐stream value along the centre‐line and comparably reduced to either side of it. The horizontal divergence of the skin‐friction vector field is positive (divergent) along the centre‐line and negative (convergent) to either side of it. Neither of these conditions favours development of a ridge longer than the separation length along the centre line. The development of ridges or tails many obstacle heights long, commonly observed in nature, requires considerable modification of the simple sediment‐free wakes we have studied.
ABSTRACT Use of depth and mean velocity to characterize bed configurations in uniform open-channel flow over a loose sediment bed leads to a three-dimensional diagram, with dimensionless measures of depth, mean velocity, and sediment size (or these three variables themselves) as coordinates, with the property of one-to-one correspondence between possible bed configurations and points in the diagram, thus eliminating overlapping of fields in diagrams involving bed shear stress. The diagram is most readily visualized by means of depth-velocity sections for a series of sediment sizes. Depth-velocity diagrams plotted from U. S. Geological Survey flume data for five sediment sizes ranging from fine sand to very coarse sand show contiguous but nonoverlapping fields for ripples, dunes, transition, and flat bed (is the finer sands) and lower flat bed, dunes, transition, and upper flat bed (in the coarser sands), with increasing mean velocity; field boundaries are almost parallel to the depth axis or slightly inclined. Each of these fields is truncated by a field for standing waves and antidunes at smaller depth or higher velocity. A size-velocity section for a depth of 0.2 m constructed from the depth-velocity sections shows more clearly the relations among the bed-configuration fields with varying sediment size. Several lines of evidence indicate that the dune field, which lies between the fields for ripples and flat bed in the fine to medium sand range, wedges out with decreasing sediment size at about 0.08 mm; in finer sediments, ripples pass directly into a flat bed. The relations between the ripple field for finer sands and the lower flat-bed field for coarser sands is unclear. If the densities of fluid and sediment are varied, then there is a different depth-velocity-size diagram for each ratio of sediment density to fluid density. The wide gap in density ratio between the cases of sand in water and sand in air on Earth might be bridged by experiments at intermediate density ratios.
Research Article| May 01, 1978 Erosion of fine-grained marine sediments: Sea-floor and laboratory experiments ROBERT N. YOUNG; ROBERT N. YOUNG 1Massachusetts Institute of Technology–Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, Massachusetts 025433Present address: NOAA, Atlantic Oceanographic and Meteorological Laboratories, Maine Geology and Geophysics Laboratory, 15 Ricken-backer Causeway, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar JOHN B. SOUTHARD JOHN B. SOUTHARD 2Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1978) 89 (5): 663–672. https://doi.org/10.1130/0016-7606(1978)89<663:EOFMSS>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT N. YOUNG, JOHN B. SOUTHARD; Erosion of fine-grained marine sediments: Sea-floor and laboratory experiments. GSA Bulletin 1978;; 89 (5): 663–672. doi: https://doi.org/10.1130/0016-7606(1978)89<663:EOFMSS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Estimates of threshold erosion velocities have been obtained in the field and in the laboratory for marine mud relatively rich in organic matter and having an active infauna. A sea-floor flume was used to erode undisturbed sediments on the sea floor, and a laboratory flume was used to study the effects of sampling, redeposition, bioturbation, organic-matter content, and compaction time on erosion resistance.Critical shear velocity, u*c, for erosion of muds in situ ranged from 0.32 to 0.84 cm/s, and u*c was apparently independent of season or bulk physical properties. Erosion was often but not always initiated in biogenically disturbed parts of the bed. Lee drifts around small bed-roughness elements were the most common constructional bedforms observed during the sea-floor experiments, and narrow and shallow longitudinal furrows were the most commonly observed erosional features. Tidal u*c values measured at the field site were of the same magnitude as the u*c values measured with the sea-floor flume.Mean u*c for muds redeposited and bioturbated for 1 to 60 days in the laboratory flume (0.92 cm/s) was greater by a factor of two than mean u*c for muds eroded in situ (0.45 cm/s). As bioturbation time in the laboratory increased, u*c decreased. Increases in sediment organic content were found to increase u*c systematically in flat, nonbioturbated laboratory beds. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
ABSTRACT Thirteen runs were made in a small recirculating flume to simulate the deposition of the climbing‐ripple sequences commonly present in fine‐grained facies of fluvial and deltaic deposits. These sequences consist of intergradational climbing‐ripple cross laminae and draped laminae. The experiments were based on the assumption that stratification type depends mainly on near‐bottom flow structure and uniform sediment fallout from an overloaded flow. Various combinations of curves of velocity versus time and of sediment feed versus time in runs lasting from 45 to 840 min were used in an exploratory program; conditions for each run were selected on the basis of experience in previous runs. The runs verified that Type A (erosional‐stoss) climbing ripples are produced by aggradation rates that are small relative to ripple migration rate, and Type B (depositional‐stoss) climbing ripples are produced by aggradation rates that are large relative to ripple migration rate. Draped lamination results from continued fallout of sediment from suspension after ripple migration ceases or almost ceases. Comparison of geometric details of the ripple stratification produced in the flume runs with that in natural sequences, supplemented by considerations on maximum and minimum migration rates of ripples, suggests times of no more than a few tens of hours for the deposition of the climbing‐ripple portions of sequences 10‐20 cm thick. Runs in which deposition of a 20 cm sequence took more than 10 h produced such atypical features of ripple geometry as sharp crests, planar lee‐side laminae, and angular toeset‐foreset contacts.
