Suspended sediment signatures induced by shallow water undulating bottom topography

Near-surface signatures of suspended sediment concentration (SSC) related to submerged bed forms displayed in visible band data gathered by space-borne imaging sensors are described. Detailed Acoustic Doppler Current Profiler (ADCP) data of vertical current velocity component, echo intensity, modulation of SSC and related oceanographic and meteorological observations in a tidal inlet of the North Sea have been analyzed. To understand the associated hydrodynamics of the optical imaging mechanism theoretical considerations based on the Navier-Stokes and sediment continuity equations have been carried out. As a first approximation these theoretical results explain signatures of enhanced SSC of in situ measurements and analyzed space-borne images. Strong currents flowing over steep bottom topography are able to stir up the sediments to form both a general continuum of SSC and localized pulses of higher SSC in the vicinity of the causative bed feature itself. The “strain rate” equivalent to the current gradient is an important parameter describing near water surface oceanographic and meteorological phenomena in the visible and microwave part of the electromagnetic spectrum observed by shore- and ship-based as well as air- and space-borne remote sensing systems. Significant disagreements between theoretical results of the “strain rate” generated by the surface current and in situ observations at the crests of submerged ridges and sand waves were identified. Possible explanations could be: first, the current velocity may not be barotropic near the crests of such bed forms and second, mass conservation may not only be maintained through an acceleration or deceleration of the flow. This implies that active up- and downwelling effects of the three-dimensional current field can also play a significant role for hydrodynamic interaction between current and underwater bed forms.