Until a few years ago, there was not much research in France into Combined Sewer Overflow phenomena in storm weather. The water of urban runoff has always been considered “clean” and one considered that the dilution of dry weather flows in storm water decreased the impact of the pollution generated by overflows.

Now, with increased urban development and realization of the importance of pollution caused by urban runoff, the problem can be considered differently. Indeed, some quality studies of receiving waters show that combined sewer networks represent an important pollution source for the natural environment, due to the increasing relative effect of combined sewer overflow discharge into receiving waters.

Thus, combined sewer overflows have until recently been the least known part of the sewer system.

In this present communication, methodology to estimate combined overflows has been elaborated after exploitation of data resulted from ten French real case studies where such problems were observed.

This study has been realized in four steps:

  1. - A bibliography study to discover the actual state of the problem in terms of existent methods concerning both experimentation and modelling and to define the needs of the research.

  2. - Ten French studies have been selected, analysed, and used to define the different methods used, and to show methodological lacunas from the observations and results realized. Elements of improvement have been proposed.

  3. - Methods and new propositions have been defined and a coherent methodological diagram has been realized to compare and test these methods.

  4. - Computer tools have been conceived and tested in the ten study cases.

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