THE UNUSUAL HYDRODYNAMICAL BEHAVIOR OF FRESH-WATER ONCOLITES

Algal concretions are frequent in calcareous environments, occurring a s individual particles (oncolites) that are easily transported during floods. Several parameters have been measured and calculated for a pop ulation of 127 particles from French streams. These oncolites have a h igh porosity (mean 42.7%) and a low density (mean 1.65), in respect to quartz and calcite. Oncolites have a settling velocity 1.5 times lowe r than that of quartz grains with the same nominal diameter. The compu tation of multiple correlations results in a classification in terms o f decreasing efficiency: shape coefficients (Corey, psi, and S/I of Sn eed and Folk), nominal diameter, density, edge sharpness and roughness . A C-d (drag coefficient) versus Re (Reynolds number) plot of these o ncolites displays two different clusters with 1000 < Re < 10, 000. One cluster, with 0.6 < C-d < 2.4 is named 'oncolites'. The other, with a C-d around 0.1, represents 'encrusted snails'. In the oncolite cluste r, no preferential arrangement of points appears and the regression li ne is horizontal (parallel to the x-axis), In this part of the diagram , the equation has the form C-d = 0Re + b. This b coefficient is an im portant parameter, because it varies with the values of nominal diamet er and density. The various shape coefficients tested (Corey shape fac tor, sphericity and the Sneed and Folk triangle), the roundness (expre ssed by the Powers index) and the roughness also exert a slight influe nce. Multiple correlation calculations allow them to be ranked in func tion of their influence on the drag coefficient (in decreasing order): density, edge sharpness, nominal diameter, shape coefficients, and ro ughness. This study is a preliminary approach and many additional meas urements on living material are needed to obtain a more pertinent set of data.