Resonant internal waves and their role in transport and accumulation of fine sediment in Eckernfِrde Bay, Baltic Sea

In spring 1993, an instrumented tetrapod was deployed in Eckernförde Bay, Baltic Sea, with the purpose of characterizing physical processes most relevant to sediment resuspension, transport and deposition. Results suggest that sediment transport events in Eckernforde Bay are associated with resonant internal waves. Observed turbidity events were associated with marked along-bay current oscillations and spectral analyses of these currents are suggestive of baroclinic resonance. Furthermore, the peak in the current spectra is close to the previously reported 26–28 h Baltic-wide seich. A one-dimensional, two-layer analytic model is applied which explains the generation of resonant internal waves within Eckernförde Bay by periodic, barotropic flows across a sill near the mouth of the bay. The analytic solution accounts for velocities much larger than those otherwise predicted by barotropic processes or by progressive internal waves, and also explains an observed sign reversal in the correlation between barotropic forcing and the internal wave response. Despite this enhancement of near-bottom currents, estimates of shear velocity suggest that bottom stress never reached the critical magnitude necessary to locally resuspend sediment and was only rarely sufficient to prevent deposition of sediment that may have been suspended elsewhere. During turbidity events, observed suspended sediment concentration did not increase with proximity to the bottom, suggesting sediment advection rather than local resuspension. Bottom photographs support this inference. Although internal wave resonance may commonly produce velocities sufficient to advect fine sediment into Eckernförde Bay, maximum currents are constrained by the waveʹs phase velocity, which is only in the order of 30 cm s−1.