Storm water is an increasingly important factor in sizing sewer systems. Artelia studies and builds mathematical models of separate networks and has completed numerous schemes to collect, store, treat and redistribute storm water. Today, temporary storage structures are used to avoid costly over-sizing of networks and limit pollution due to runoff. These large underground storage tanks, which are complex structures from both the civil works and hydraulic standpoints, are one of Artelia’s specialities (Lille, Le Havre, etc.). Such extensive techniques, based on technologies with low operating costs, are also a major area of innovation in the framework of sustainable development.