This paper studies solid matter from a section of the A9 motorway, Kérault 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 μm in diameter are easily separated by simple settling, unlike particles smaller than 100 μm in diameter, which remain in suspension in runoff. To treat runoff, particles smaller than 50 μm 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 μm, whereas the geometric mean is 12.6 μm. Regarding sedimentation speed, from the eight rainfalls studied, 50% w/w of particles smaller than 50 μm 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 μm 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 μm seems to be an essential stage in the design of treatment plants (Vignoles, 1991).
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