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Combined sewer overflow (CSO) control strategies are commonly selected relying on optimization or multicriteria analysis. A methodology is proposed in this paper for selecting and evaluating the solutions to mitigate stormwater problems in combined sewer systems (CSSs) based on the understanding and rigorous diagnosis of the causes of the observed problems using hydrological/hydraulic simulations. The methodology was applied to a sewer catchment near Montreal (Canada) in order to reduce the magnitude and recurrence of sewer surcharge and local flooding without increasing the recurrence of CSOs. Two types of control strategies were considered: (1) large-scale solutions, i.e., redirection of stormwater inflows and separation of the CSS; and (2) source control solutions including both green and gray infrastructure. These solutions were evaluated through a comparative analysis of nine scenarios. Based on the results, source control solutions could be considered as the most effective interventions on the studied catchment for eliminating or significantly reducing the occurrence of sewer surcharge (from an average of 6.7 to 2.6) and surface flooding on local streets (from an average of 3.5 to 0.4 per year). The developed methodology is a valuable tool to be applied to other urban drainage catchments that encounter problems similar to those observed on the studied catchment. [ABSTRACT FROM AUTHOR]