Particle size distribution and hydrodynamic characteristics of solid matter carried by runoff from motorways

This paper studies solid matter from a section of the A9 motorway, Kerault Region, France, used in a study of the effects of pollution. This study analyzed both settled sediments from collecting basin and characteristics of sediments in the water column during and after eight storm events between October 12, 1993, and February 6, 1994. Settled sediments were used to measure particle sizes, mineral content, and related characteristics, whereas water samples were used to document total suspended solids, mineral content, and heavy metals. Solid particles larger than 100 mu m in diameter are easily separated by simple settling, unlike particles smaller than 100 mu m in diameter, a which remain in suspension in runoff. To treat runoff, particles smaller than 50 mu m in diameter, which represent approximately three-quarters of the weight of the solids, must be studied. On average, 50% v/v of particles have a diameter less than 15.2 mu m whereas the geometric mean is 12.6 mu m. Regarding sedimentation speed, from the eight rainfalls studied, 50% w/w of particles smaller than 50 mu m in diameter fall at a speed less than 2.98 m/h on average (results for the eight rainfalls vary from 2.5 to 3.3 m/h), and 50% of particles between 50 and 100 mu m in diameter fall at a speed less than 9.8 m/h on average (results of the eight rainfalls vary from 5.7 to 13.1 m/h). Thus, treatment of runoff entails separating or eliminating solid waste. Furthermore, measuring sedimentation speed of particles with diameters smaller than 100 mu m seems to be an essential stage in the design of treatment plants (Vignoles, 1991).