Stormwater modelling tools : a comparison and evaluation

Abstract: Stormwater is rain, melt and rinse water that temporarily runs off the ground surface. In cities with many impermeable surfaces large amounts of stormwater may be formed. Stormwater is diverted through conduits and ditches to recipients. If the diversion of stormwater is insufficient flooding can occur. Stormwater contains contaminants from the catchment surfaces such as nutrients, heavy metals, bacteria, organic compounds and oils. To study stormwater pollution and flows various stormwater modelling tools are used. The purpose of this thesis is to test, compare and evaluate four stormwater modelling tools and conclude their strengths and weaknesses. This was done by a literature study, a market research, definition of evaluation criteria and by testing the tools on a case study of an industrial and residential area in Scotland. The main developed evaluation criteria was model characteristics, required input, user friendliness, output and model application. The four selected tools were StormTac, Infoworks CS, SuDS Studio and MUSIC. The case study included both flow and pollutants on a yearly basis and for a rainfall event. The results from the study were compared against observed values both before and after calibration. No model gave good model predictions for all parameters. The observed volumes of water were larger than modelled for all four models even after calibration. An intercomparison between the models however gave a better fit. Regarding pollutants, the model predictions compared with observed values varied between the tools and substances. Strengths with StormTac are that it includes over 70 pollutants, includes the recipient and gave good model predictions for pollutant concentrations when compared with observed pollutants. A weakness is that StormTacs automatically calculated base flow was much smaller than observed in the case study and the tool is not suited for studies of single event rainfalls.Strengths with Infoworks CS are that it is fully distributed, a complete network is built up and the tool can be used for design of networks, flood predictions and flows over time. It is time consuming to build a model in Infoworks CS, few default pollutants are included and it gave poor model predictions for nitrogen concentrations in the case study. SuDS Studio is a unique tool for defining suitable options for retrofitting SuDS solutions and this is a strength as it is the only tool of its kind. SuDS studio gave large intervals for pollutant loads in the case study which can be difficult to interpret. A strength with MUSIC is that it is user friendly and can be used for comparing water quality pre and post treatment. It does however only include three default pollutants in the model, and only three pollutants can be modelled at a time.