HYDRO 2016 Paper 4A1

Comparison and characteristics of oceanographic in situ measurements and simulations above submerged sand waves in a tidal inlet

Ingo Hennings, Dagmar Herbers


Ocean colour and its transparency are related to turbidity caused by substances in water like organic and inorganic material. One of the essential climate variables (ECV) is ocean colour. However, this implies the correct interpretation of observed water quality parameters. Acoustic Doppler Current Profiler (ADCP) data of the three-dimensional current-field, echo intensity, modulation of suspended sediment concentration (SSC), and related water levels and wind velocities have been analysed as a function of water depth above submerged asymmetric compound sand waves during a tidal cycle in the Lister Tief of the German Bight in the North Sea. Signatures of vertical current component, echo intensities and calculated SSC modulations in the water column depend strongly on wind and current velocity. Bursts of vertical current component and echo intensity are triggered by sand waves itself as well as by superimposed megaripples due to current wave interaction at high current ≥ 1.0 m s-1 and wind speeds ≥ 10.0 m s-1, preferably of opposite directions, measured at high spatial resolution. The magnitude of currents and SSC modulations during ebb and flood tidal current phases are only weakly time dependent, whereas the local magnitudes of these parameters are variable in space above the sand waves. Intense ejections caused by tidal current velocity transport higher SSC near the bottom boundary layer at the sand waves superimposed by megaripples towards the free water surface. Such typical upwelling mechanism above sand waves creates distinct SSC signatures of remote sensing data visible in air- and space-borne optical imagery. Hydrodynamic parameters such as dynamic buoyancy density, total energy density and action density due to semi-diurnal M2 tide motion which are associated with sand waves are investigated and analysed. Results and characteristics of simulated hydrodynamic parameters in coastal waters above sand waves are presented. It is shown that ADCP measurements are to be consistent with simulations based on the applied theory.

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