Comparison of two models of aggregation in continental‐shelf bottom boundary layers

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Abstract
Comparison of two models of aggregation and disaggregation of fine sediment in continental‐shelf bottom boundary layers delineates conditions under which these processes can be modeled simply. Model 1 predicts the evolution of the particle size distribution based on layer‐averaged mean sediment concentrations. Model 2 avoids the simplifying assumptions of model 1 by representing suspension dynamics with a Monte Carlo simulation of the joint probability density function of number concentration in the various particle size classes. In contrast to model 1, this technique bases aggregation rates on local concentrations and explicitly treats the effect of concentration correlations on aggregation rates. Each model produces a time series of particle size distribution in a steady, horizontally uniform, nondepositing, continental‐shelf bottom boundary layer. Comparisons are made for suspensions that are initially polydisperse and monodisperse, for shear velocities of 0.01 and 0.005 m s−1, for fractal dimensions of 1.92 and 2.4, for initial layer‐averaged sediment concentrations of 1, 2.5, 10, and 25×10−3 kg m−3, and for sticking efficiencies of 0.1 and 1.0. In general, model 1 is accurate for suspensions that are initially polydisperse and do not develop strong vertical structure. Results suggest that as long as maximal settling velocity does not exceed roughly 0.6u*, vertical gradients in sediment concentration do not degrade substantially the accuracy of layer‐averaged mean models for concentrations typical of the continental shelf. When initial suspensions are monodisperse and/or vertical gradients are large, transport‐limited aggregation can slow overall aggregation rates of model 2 relative to model 1, or important interactions can occur between particle types that both show strong vertical structure, speeding overall aggregation rates in model 2. In the future the effects on the accuracy of layer‐averaged mean models of short‐ and long‐term unsteadiness, erosion, and deposition will be explored.