Bottom Boundary Layer Wave Measurements for Particle Studies

Abstract

Suspended particles, which attenuate light and restrict visibility in moderate depths of the ocean, are commonly resuspended from the bottom by waves. This turbid layer initially consists of fine particles with settling rates of 0.01 mm/s that require weeks to settle out. However, the relaxation of the turbid conditions is often only a day or less implying particle settling rates of 1 mm/s, a rate requiring that the small particles aggregate into large particles that can settle more rapidly. Conditions that suspend and possibly disaggregate the fine sediment on the bottom and the aggregated particles in suspension are wave dominated in most continental shelf conditions. Thus formation of a turbid layer requires a shear stress exceeding a threshold. Relaxation of a turbid layer conversely requires a shear stress that remains always or nearly always below another threshold. This dependence of turbidity and the lifetime of turbid layers on shear stress focuses attention on what the shear stress is and what it depends upon. Shear stress at the bottom is a combination of Reynolds stresses from the mean current and the wave boundary layer. The thickness of the wave boundary layer is too small for direct Reynolds stress determinations in the boundary layer but the wave effects can be included into the model in other ways. Thus wave velocities in the wave boundary layer are very important in estimating the mean Reynolds stress. Even more important for the determination of the peak boundary layer stress felt by the bottom interface where the sediment may be eroded and the large particles disaggregated into smaller particles are the instantaneous velocities consisting of the sum of the mean current and the wave velocity. Determination of wave velocity should not be difficult for a rapidly sampling current meter near the bottom. The information required is present in the time series recorded by such an instrument. Yet the waves responsible for this sediment interaction at shelf depths are generally not revealed in conventional directional wave measurement instruments and wave spectra processing programs. Wave measuring instruments that are physically located on the surface or that sense the sea surface elevation acoustically or by wires are even more restricted by the dominance of short period, high amplitude waves when wind is present. Surface wave measurements rarely detect the low amplitude, very long period waves that are the dominant influence on the bottom in depths exceeding 10 meters because these waves are swamped by short period waves in all conditions except glassy calm. In order to adequately measure the waves that are significant for resuspension of benthic sediment, measurements of orbital wave velocity near the bottom in the depths of interest are preferred. While such near-bottom measurements present a problem for extrapolation back to the surface in generation of conventional directional wave spectra, the depth imposes a natural filter that attenuates the signal from short period surface waves yet permits the long period swell to be detected and measured. Direct velocity and pressure measurements from a fixed platform on the bottom are ideal for this purpose. Prefiltering of the data before extrapolating to the surface with a cutoff frequency to reject short period waves can permit conventional directional wave spectral software to reveal these long period waves. No changes need to be made to the directional wave spectrum program when short period waves are naturally attenuated. However, the cutoff must be set to a frequency that prevents the attenuated short period waves from being extrapolated back to the surface where they are dominant. Such a modification of a program will reveal the information needed for studies of turbidity and the relaxation of turbidity on the shelf from depths between 10 meters and 120 meters where long period waves penetrate to the bottom and short period waves are attenuated.