This project presents a method to predict the rating curves of combined sewer overflow (CSO)structures based on the geometry of the structure and the connected sewer system. 3 importantparameters have been found to influence the rating curve; the weir coefficient, the capacity of thedischarge pipe and the resistance induced by potential screens. Methods to predict the weir coefficienthave been proposed for two types of CSO structures being side weir and stilling pond CSO structures.Furthermore the location for installing a water level sensor giving the best prediction has beendetermined. Based on CFD modelling of close to a thousand randomly generated CSO structures, itwas found that the weir coefficient can be predicted based on the Froude number and the geometryof the weir crest. A method to relate the hydraulic resistance of a screen to the flow over a weir wereproposed, and the method has been validated and found accurate based on laboratory experimentsand CFD modelling. Based on the laboratory test it was further shown how the hydraulic resistanceof the screen increased as the screen clogged. A simple model of the CSO discharge pipe has beenproposed and validated against the results of CFD models.Rating curves of 5 real life CSO structures have been predicted and compared with the results fromCFD modelling and real-life measurements. The predicted rating curves have been deemed accuratefor structures which falls within one of the two types of CSO structures.