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  • 1
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    Flow structure and resistance over subaquaeous high- and low-angle dunes (2016)
    Wiley
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    Publication Date: 2016-02-20
    Description: A prominent control on the flow over subaqueous dunes is the slope of the downstream leeside. While previous work has focused on steep (~30°), asymmetric dunes with permanent flow separation, little is known about dunes with lower lee-slope angles for which flow separation is absent or intermittent. Here, we present a laboratory investigation where we systematically varied the dune lee-slope, holding other geometric parameters and flow hydraulics constant, to explore effects on the turbulent flow field and flow resistance. Three sets of fixed dunes (lee-slopes of 10°, 20° and 30°) were separately installed in a 15 m long and 1 m wide flume and subjected to 0.20 m deep flow. Measurements consisted of high-frequency, vertical profiles collected with a Laser Doppler Velocimeter (LDV). We show that the temporal and spatial occurrence of flow separation decreases with dune lee-slope. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30° dunes and a weaker shear layer closer to the bed for the 20° and 10° dunes. The decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee-slopes. Aperiodic, strong ejection events dominate the shear layer, but decrease in strength and frequency for low-angle dunes. Flow resistance of dunes decreases with lee-slope; the transition being non-linear. Over the 10°, 20° and 30° dunes, shear stress is 8%, 33% and 90 % greater than a flat bed, respectively. Our results demonstrate that dune lee-slope plays an important, but often ignored role in flow resistance.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Nearshore sandbar rotation at single-barred embayed beaches (2016)
    Wiley
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    Publication Date: 2016-01-19
    Description: The location of a shore-parallel nearshore sandbar derived from seven years of video imagery data at the single-barred embayed Tairua Beach (NZ) is investigated to assess the contribution of barline rotation to the overall morphodynamics of sandbars in embayed environments and to characterise the process of rotation in relation to external conditions. Rotation induces cross-shore barline variations at the embayment extremities on the order of magnitude of those induced by alongshore uniform cross-shore migration of the bar. Two semi-empirical models have been developed to relate the barline cross-shore migration and rotation to external wave forcing conditions. The rotation model is directly derived from the cross-shore migration model. Therefore its formulation advocates for a primary role of cross-shore processes in the rotation of sandbars at embayed beaches. The orientation evolves towards an equilibrium angle directly related to the alongshore wave energy gradient due to two different mechanisms. Either the bar extremities migrate in opposite directions with no overall cross-shore bar migration (pivotal rotation) or the rotation relates to an overall migration of the barline which is not uniform along the beach (migration-driven rotation). Migration and rotation characteristic response times are similar, ranging from 10 to 30 days for mild and energetic wave conditions and above 200 days during very calm conditions or when the bar is located far offshore. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    With or against the tide: The influence of bed form asymmetry on the formation of macroturbulence and suspended sediment patterns (2014)
    Wiley
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    Publication Date: 2014-09-10
    Description: This study examines tide-dependent variations in the formation and dynamics of suspended sediment patterns coupled to mean flow and turbulence above asymmetric bedforms. In the Danish Knudedyb inlet, very large primary bedforms remain ebb-oriented during a tidal cycle while smaller superimposed bedforms reverse direction with each tidal phase. Hydro-acoustic in-situ observations reveal pronounced differences in suspended sediment transport patterns between tidal phases caused by the relative orientation of primary bedforms and the mean tidal flow and flow unsteadiness during a single tidal phase. When flow and primary bedform orientation are aligned, water-depth-scale macroturbulence develops in the bedform lee-sides in the presence of flow separation. Macroturbulent flow structures occur at high flow stages and are coupled to increased amounts of sediment in suspension. When flow and bedform orientation are opposed no evidence of flow separation associated with primary bedforms is found. Sediment-laden macroturbulence at high flow velocities is of a smaller scale and attributed to the superimposed secondary bedforms. The flow structures are advected along the primary bedform stoss-side (temporary hydraulic lee-side). The steep primary bedform lee-side (temporary hydraulic stoss-side) however, limits transport capabilities beyond the scale of primary bedforms.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Shore and bar cross-shore migration, rotation and breathing processes at an embayed beach. (2017)
    Wiley
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    Publication Date: 2017-08-12
    Description: A Principal Component Analysis (PCA) is used to decompose data on the coupled morphodynamics of the shoreline and nearshore sandbar of a typical single-barred embayed beach (Tairua Beach, New Zealand). Dynamic patterns are classified into simultaneous modes, where the bar and shoreline move at the same time, and non-simultaneous modes, where the shore moves independently from the bar, and vice versa. Two simultaneous modes accounting for 65 % of the variance of the shoreline and barline dominate the system. One mode describes inverse shoreline and sandbar cross-shore migrations (alongshore-averaged), occurring with simultaneous rotations in the same direction. The other mode accounts for migration in the same direction accompanied by variations of the barline curvature (similar to 'breathing modes' previously described in embayed beach shoreline modelling studies). Two non-simultaneous modes of lesser importance account separately for independent shoreline and barline rotations (10 to 15 % of the variance explained). A PCA applied to the shore and sandbar behaviours modelled by four standard equilibrium models simulating shore and sandbar cross-shore migrations and rotations show that these are interrelated because of a correlation between wave energy and direction. Shore and bar rotations are coupled partially because the shape of the bay induces a correlation of their respective drivers, the wave angle of incidence and the alongshore gradient of wave energy. However this correlation depends on the wave energy. This, in combination with different shore and sandbar response times (quantified using the models), also explains the independent rotations reflected by the non-simultaneous modes.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Observations of Coherent flow structures over Sub-aqueous high- and low-angle Dunes (2017)
    Wiley
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    Publication Date: 2017-10-25
    Description: Large-scale coherent flow structures (CFS) above dunes are the dominant source of flow resistance and constitute the principal mechanism for sediment transport and mixing in sand bed river and estuarine systems. Based on laboratory observations, CFS formation has been previously linked to flow separation downstream of high-angle dunes with lee-slopes of ~30°. How CFS form in natural, deep rivers and estuaries where dunes exhibit lower lee-slopes and intermittent flow separation is not well understood. Here, we present Particle Image Velocimetry (PIV) measurements from an experiment where dune lee-slope was systematically varied (30°, 20°, 10°), while other geometric and hydraulic parameters were held constant. We show that CFS form downstream of all three dune geometries from shear layer vortices in the dune lee. The mode of CFS formation undergoes a low-frequency oscillation with periods of intense vortex shedding interspersed with periods of rare vortex shedding. Streamwise alignment of several vortices during periods of intense shedding results in wedge-shaped CFS that are advected above the dune stoss-side. Streamwise length-scales of wedge-shaped CFS correspond to large-scale motions (LSM). We hypothesize that the advection of LSM over the dune crest triggers the periods of intense shedding in the dune lee. LSM are weaker and smaller above low-angle dunes; however the low-frequency oscillation in CFS formation periods persists. The formation of smaller and weaker CFS results in a reduction of flow resistance over low-angle dunes.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Wind extremes in the North Sea basin under climate change: an ensemble study of 12 CMIP5 GCMs (2013)
    Wiley
    Details
    Publication Date: 2013-01-18
    Description: [1]  Coastal safety may be influenced by climate change, as changes in extreme surge levels and wave extremes may increase the vulnerability of dunes and other coastal defenses. In the North Sea, an area already prone to severe flooding, these high surge levels and waves are generated by low atmospheric pressure and severe wind speeds during storm events. As a result of the geometry of the North Sea, not only the maximum wind speed is relevant, but also wind direction. Climate change could change maximum wind conditions, with potentially negative effects for coastal safety. Here, we use an ensemble of 12 CMIP5 GCMs and diagnose the effect of two climate scenarios (rcp4.5 and rcp8.5) on annual maximum wind speed, wind speeds with lower return frequencies and the direction of these annual maximum wind speeds. The 12 selected CMIP5 models do not project changes in annual maximum wind speed and in wind speeds with lower return frequencies; however, we do find an indication that the annual extreme wind events are coming more often from western directions. Our results are in line with the studies based on CMIP3 models and do not confirm the statement based on some reanalysis studies that there is a climate-change related upward trend in storminess in the North Sea area.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Biphasic expression of slow myosin light chains and slow tropomyosin isoforms during the development of the human quadriceps muscle (1991)
    Wiley
    Details
    Publication Date: 1991-03-25
    Print ISSN: 0014-5793
    Electronic ISSN: 1873-3468
    Topics: Biology , Chemistry and Pharmacology
    Published by Wiley on behalf of Federation of European Biochemical Societies.
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  • 8
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    A simple model of bedform migration (2010)
    Wiley
    Details
    Publication Date: 2010-06-10
    Print ISSN: 0197-9337
    Electronic ISSN: 1096-9837
    Topics: Geography , Geosciences
    Published by Wiley on behalf of British Society for Geomorphology.
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