A Comparison of the Relative Sediment Transport of Quartz and Aragonite Sand for Use as Beach Renourishment Materials in South Florida

An evaluation of the relative rates of sediment transport of an oolitic aragonite sand and a quartz quarry sand as possible beach renourishment materials has been conducted. When comparing equal volumes, the aragonite experienced less transport than the quartz in both the longshore and in the onshore-offshore directions. When comparing equal size fractions, in sizes 0.35mm and smaller, aragonite was less transportable. The quartz was less transportable in the sizes greater than 0.35mm. This trend was observed in two separate experiments and is attributed to the effective density ratio of aragonite to quartz, dissimilarities in roundness and sphericity, and to differential entrainment and transport of these materials in suspension and bed load within the confines of the inherent bed roughness. The effective density ratio of aragonite to quartz is highest in the smaller grain sizes and decreases with increasing grain size because the larger aragonitic grains possess fewer oolitic lamellae per grain and resemble their initial biogenic nucleus. In the smaller size fractions where suspension transport is thought to predominate, a larger quartz grain is hydraulically equivalent to a smaller aragonite grain due to the greater density of the aragonite. The aragonite has a higher settling velocity out of suspension and it is less entrainable, due to sheltering effects in the bed matrix allowing a lower position in the velocity profile and a larger reactive angle to the flow. As grain size increases above 0.35mm, the density of the aragonite approaches that of the quartz. The principle of hydraulic equivalence suggests that for two materials of similar density, there should be no difference in the entrainment and transportability between equal size fractions. The preferential transport of the aragonite relative to the quartz in the size fractions greater than 0.35mm is attributed to the difference in their shape, where the rounder aragonite is more easily rolled in traction as the size of both the aragonite and quartz exceed the background bed roughness. The physical characteristics of aragonite indicate that it has a hydraulic behavior similar to a quartz sand of a slightly larger size. If renourishment is undertaken on John U. Lloyd Beach with aragonite, the most probable source material would be a mining stockpile (mean size 0.52mm) from Ocean Cay in the Bahamas. Based on a theoretical (mean size only) method of the U.S. Army Corps of Engineers, utilization of this stockpile material would reduce the erosion rate on Lloyd Beach by 10%. The results of my study indicate that beach losses could be further reduced by using this aragonite due its higher density. Secondary characteristics such as density and shape of the renourishment material manifest themselves differently in the suspension and bed load modes of transport and should be considered when choosing a borrow source. Additional transport studies need to be done utilizing larger volumes of material and monitored over a longer time interval