ON THE DEPOSITIONAL CHARACTERISTICS OF NATURAL COHESIVE & MIXED SEDIMENTS: “FLOCCIN’ ACROSS THE USA!”

Abstract:

Many coastal and inland waterways are dominated by muddy sediments; typically a mixture of clay minerals and various types of organic matter. When cohesive sediment is entrained into suspension, the particles tend to flocculate. Flocs are less dense, but faster settling than their constituent particles thus affecting their depositional characteristics. As flocs grow their effective densities generally decrease, but their settling rates rise due to a Stokes’ Law relationship. Flocculation effects become even more complex when purely cohesive sediments are mixed with different ratios of non-cohesive sediments, and the amount of biological activity changes, i.e. affecting the resultant cohesion. Developing instrumentation that can provide key physical and dynamical data on depositional rates of flocculating sediments is extremely important in advancing our understanding of natural flocculation processes. The data need to be both qualitative and quantitative, as the latter improves our understanding of the depositional and aggregational physical processes through parameterization.

This presentation will demonstrate recent advances in the study of the flocculation process through the use of video image technology. One such device pioneered at HR Wallingford, and implemented with co-authors, is the high resolution floc video camera, LabSFLOC – Laboratory Spectral Flocculation Characteristics (developed by Prof. Manning). LabSFLOC can observe floc spectral physical properties, including: floc size, settling velocity, effective density, porosity, shape, mass, and settling flux (using controlled volume referencing). These data are highly desirable for sediment transport modelers. Examples of floc measurements from locations in estuaries, tidal lagoons, river deltas, and lakes from locations across the USA will be presented. In addition, we will demonstrate how video floc data can be used to parameterize floc settling characteristics for use in modeling.