In the urban environment, stormwater detention basins are a powerful means to limit the frequency of sewer system failures and consecutive urban flooding. To design such waterworks or to check their efficiency, it is possible to carry out continuous rainfall-runoff modelling. A long-term discharge series obtained from a long-term rainfall series is used as input for a storage model describing the detention basin behaviour: the basin behaviour may be consequently studied over a long period. The provided statistical information on the working state frequency, failure frequency, … of the detention basin is of high interest for the basin diagnostic or for its design. This paper presents the whole methodology which leads to production of such statistical information and especially: the models used to generate long term rainfall series with a short time step, the rainfall-runoff model used to transform the later series into a long term discharge series, and the model used to describe the behaviour of the detention basin. This methodology was applied to evaluate the efficiency of 4 detention basins built for stormwater control and flood mitigation. They are situated on a Swiss urban catchment (Chamberonne catchment – 40 km2) collecting water from the Mèbre and Sorge rivers.