Suspended minerogenic particle distributions in high-energy coastal environments: optical implications

This paper examines suspended minerogenic particle distributions in the near-coastal ocean, Oceanside, California. The environment is dominated by resuspension of particles from a well-sorted sandy sediment. We obtain information on the suspended mineral matter from the relocatable numerical sedimentation model, TRANS98. Inputs for the model include bottom particle size distribution, wind speed, wave period and height, and near-bottom current speed. Model output is a vertical profile of mineral particle concentrations and size distributions at selected depths. The mineral particle size distribution allows the calculation of the total scattering coefficient of suspended minerogenic matter. The particle size distribution for the bed at Oceanside is approximately Gaussian lognormal in character. The predicted size distribution near bottom is a skewed Gaussian lognormal; then the distribution shifts toward the increasing importance of smaller size classes as we approach the water surface. Deeper stations maintain a lognormal character up into the surface layers while the shallower stations switch to a particle size distribution of the smallest size class being the modal class in the surface layers. Hyperbolic size distributions have been proposed for the Oceanside site from optical observations. These distributions would have a slope of 2.1 for coastal water which would decrease to as low as 1.3 for shallow nearshore stations and near the bottom. None of the proposed hyperbolic slopes provides an adequate approximation of the particle distributions predicted by TRANS98. The larger particle size classes are significantly overestimated by hyperbolic models, which causes the total scattering coefficient of the minerogenic matter to be overestimated. These results indicate that hyperbolic slope models cannot be used to retrieve particle size distributions by inversions involving the total scattering coefficient nor are they accurate estimators of the minerogenic scattering coefficient