Determining concentration and fall velocity of estuarine particle populations using ADV, OBS and LISST

Abstract

In describing suspended sediment conditions in the lower Chesapeake Bay, VA, USA, this paper reports and develops methods for distinguishing multiple particle populations in the bottom boundary layer of estuaries in general. In addition, a novel application of the acoustic Doppler velocimeter (ADV) is shown to estimate in situ particle fall velocity at a single point without affecting the ambient turbulence. In situ estimates of suspended sediment concentration from ADV, optical backscatter, and laser in situ scattering and transmissometry (LISST) instruments are compared with gravimetrically determined mass concentrations from pumped water samples. In this environment, acoustic backscatter from the ADV proved to be the best estimator of mass concentrations due to its apparent insensitivity to the size or density of muddy aggregates. The concentration estimates and the relative sensitivities of the instruments to particle size and density combined with size distribution information from the LISST reveal the characteristics of multiple particle populations in the bottom boundary layer. Two rapidly settling sediment populations are suggested with similar fall velocities but distinct critical erosion stresses. A slowly settling background population is also identified whose concentration varies over meteorological time scales. Fall velocities are estimated analytically from a balance of settling and diffusive flux gradients using two methods, one employing Reynolds concentration flux, and the other estimating eddy diffusivity using the von-Karman Prandtl equation. Comparison of the local change and advective terms in the solute transport equation to the magnitude of the settling term suggests that a balance between the settling and resuspension term is a good first order approximation at this site, validating the indirect method for estimating settling velocity. Single elevation estimates of fall velocity using the ADV to estimate Reynolds concentration flux produced the best estimates of fall velocity which are on the order of 1mm/s.