Optimisation de la décantation des rejets urbains de temps de pluie dans les bassins de retenue contre les inondations

¹ GARIH/CTIA, Lyonnaise des Eaux
² ISIM, Université de Montpellier
³ Communauté Urbaine de Bordeaux

Abstract

Detention tanks in combined or separate sewer systems have been used since the last three decades in order to reduce the floods due to the urban runoff. In addition to their hydraulic efficiency, it appears that they reduce the polluant discharges by settling. The main objective of our study is the optimisation of the settling process in this type of tank by the improvement of the operational rules. The first stage of the study is the characterization of the suspended solids and of their senling process in a combined underground tank and in separate detention tank with grassy bottom.

The grain size of sediments sampled in the combined tank and solids from the superficial layer (0-10 cm) of the separate tank is fine and close to the grain size distribution of suspended solids in stormwater discharges. A grain size segregation of the solids is observed: the filling and emptying process and the tank geometry influence their distribution. For the tank with grassy bottom, the flooding frequency is also a very important parameter. The pollutant concentrations (heavy metals and hydrocarbons) are correlated with the grain size distribution: the finer the solids are, the more polluting they are. These concentrations are high in the combined tank and in the part of the separate tank where the settling process is frequent.

On the other hand, in the separate tank, the pollutant concentrations of the layers 10-30 cm and 30-50 cm are light and decrease with depth up to values corresponding to unpolluted natural soils.