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 ana lyzed both settled sediments from collecting basin and characteristics of s ediments in the water column during and after eight storm events between Oc tober 12, 1993, and February 6, 1994. Settled sediments were used to measur e particle sizes, mineral content, and related characteristics, whereas wat er samples were used to document total suspended solids, mineral content, a nd heavy metals. Solid particles larger than 100 mu m in diameter are easil y separated by simple settling, unlike particles smaller than 100 mu m in d iameter, a which remain in suspension in runoff. To treat runoff, particles smaller than 50 mu m in diameter, which represent approximately three-quar ters of the weight of the solids, must be studied. On average, 50% v/v of p articles 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 elim inating solid waste. Furthermore, measuring sedimentation speed of particle s with diameters smaller than 100 mu m seems to be an essential stage in th e design of treatment plants (Vignoles, 1991).