Multifunctional design of storm-surge basins along motorways

Studies looking at the incidence of motorways on the environment all point to the problem of run-off waters that are loaded with various pollutants : small particles, de-icing salts, heavy metals, petrol byproducts. Most storme-surge basins are designed primarily as buffers to quantitatively handle run-off, when they ought to play a qualitative role in the protection of the streams receiving their effluents, and of ground waters as well. A multifunctional design, with a more global approach of hydraulic aspects, hydrological elements pertaining to the ecological role and integration of a basin into the landscape, has been drafted concerning the storme-surge basin of Cédrogne in Belgium. Designed in accordance to a revised workplan proposed by GIREA, it is made up of three basins for sedimentation, depollution and stocking. After 8 years of operation, our assessment emphasizes the buffering effect of the basin, with nearly 80 % of discharge being absorbed during heavy rains, 93 % of suspended matter being trapped, a dilution of chloride with a reduction in concentrations of nearly 50 % in periods of large inputs (up to 1795 mg/l), as well as the accumulation of heavy metals in sediments and the vegetation. The natural setup of the basins has made it possible for a divesified flora and fauna to establish itself, with a given number of rare species present. At present, the study aims at similarly improving the design of other basins on the E411 motorway, and to analyze various situations where specific problems have been identified : flooding, run-off from other sources than the motorway (agricultural, modern railways, water treatment facilities, sewers, etc.), which are an even bigger risk for ground waters.