With or against the tide: The influence of bed form asymmetry on the formation of macroturbulence and suspended sediment patterns

Abstract

This study examines tide-dependent variations in the formation and dynamics of suspended sediment patterns coupled to mean flow and turbulence above asymmetric bed forms. In the Danish Knudedyb inlet, very large primary bed forms remain ebb-oriented during a tidal cycle while smaller superimposed bed forms reverse direction with each tidal phase. Hydroacoustic in situ observations reveal pronounced differences in suspended sediment transport patterns between tidal phases caused by the relative orientation of primary bed forms and the mean tidal flow and flow unsteadiness during a single tidal phase. When flow and primary bed form orientation are aligned, water-depth-scale macroturbulence develops in the bed form 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 bed form orientation are opposed no evidence of flow separation associated with primary bed forms is found. Sediment-laden macroturbulence at high flow velocities is of a smaller scale and attributed to the superimposed secondary bed forms. The flow structures are advected along the primary bed form stoss-side (temporary hydraulic lee-side). The steep primary bed form lee-side (temporary hydraulic stoss-side) however, limits transport capabilities beyond the scale of primary bed forms.

Observed Suspended Sediment Dynamics during a Tidal Cycle above Submerged Asymmetric Compound Sand Waves

“…These observations were in agreement with the basics on dynamics of the ocean circular vortex and wave motions [17,18]. Tide-dependent variations in the formation and dynamics of suspended sediment patterns coupled to mean flow and turbulence above asymmetric bedforms were examined [19]. A decrease in water clarity of the southern and central North Sea was reported [20].…”

“…As a first approximation, roughness effects were neglected for simulations of the different hydrodynamic parameters according to Eqs. (1)- (19). This is a crucial point in the theory because the lift and downforce coefficients were derived by using a flat plate.…”

The Coastal Observing System for Northern and Arctic Seas (COSYNA)

“…These landers bridge the observational gap between longterm monitoring stations, remote sensing applications, and ship-based field campaigns. They are mobile, and can be used to spatially interpolate between monitoring stations and provide data with very high temporal resolution (Kwoll et al, 2013(Kwoll et al, , 2014Oehler et al, 2015a;Ahmerkamp et al, 2017). Lander operations aim at measuring various processes close to the seafloor or in the sediment and are designed to have minimal impact on the environment and quantities that are measured.…”

“…During some parts of the tidal cycle a periodic stratification of the water column has been observed in shallow areas of the German Bight forming distinct layers that move independently with a decoupled tidal ellipticity (Krämer and Winter, 2016;Kwoll et al, 2013Kwoll et al, , 2014Ahmerkap et al, 2017). The difference in sea bed dynamics between fair weather conditions and storms is also investigated in the research area "Seafloor Dynamics" of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Research Center/Cluster of Excellence "The Ocean in the Earth System".…”

Measurements and dynamics of multiple scale bedforms in tidally energetic environments

“…These observations have shown that the larger dunes remain oriented towards and slowly migrate in the direction of the dominant tidal flow, whereas surveys taken during maximum ebb and flood reveal smaller bedforms reverse asymmetry with the tides (Bokuniewicz et al, 1976;Winter et al, 2008;Kwoll et al, 2014). Although these studies provide accurate maps of the bedforms and information of their asymmetry and long timescale migration, they lack detail on bedform dynamics throughout the tidal cycle.…”