The importance of inherent uncertainties in state-of-the-art urban storm drainage modelling for ungauged small catchments

A Monte Carlo study has shown that assessment of detrimental effects from urban storm drainage is very uncertain, even when state-of-the-art modelling techniques are used. A method to assess the relative importance of the individual contributions to the overall uncertainty is proposed, based on an analysis of the Monte Carlo simulations. The method can assess the importance of both continuous and discrete variables. Based on an analysis of two small ungauged catchments, it was found that the uncertainty in the description of the rainfall was generally the most important contributor to the overall uncertainty, followed by the uncertainty in the description of the surface runoff. In the evaluation of the extreme discharges of chemical oxygen demand (GOD) from combined sewer overflows the uncertainty in the concentration of COD was found to be the dominant factor. If the sewer system is poorly maintained the uncertainty in the hydraulic resistance may become important.