Combined sewer overflows can in some cases cause critical oxygen concentrations in small rivers. The traditional design criteria for the overflow structures is not specificly related to this effect. By combination of a long historical rain series, a runoff model and a river quality model, it has been possible to achieve operational design diagrams for overflow structures. The diagrams are statistically based and relate the independent parameters for the catchment, the overflow structure and the river to the effect in question, the oxygen concentration.

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