One of the greatest influences on the volume and quality of influent flows to wastewater treatment plants (WWTPs) is rainfall in the drainage area. These wet weather flows (WWFs) can lead to significant environmental impacts due to sewer overflows and affect the efficiency and resource cost of treatment in WWTPs. Previous studies have considered the impacts of WWFs on the WWTP and treatment efficiency, however few have considered the impacts of WWFs on the sewer network that serve these treatment plants. This thesis investigates the influence of drainage area rainfall on the hydraulic flows through an urban drainage system in Northern Ireland using available data. The contribution of the electricity used at pumping stations in two drainage areas is calculated, assessed and compared to the electricity consumed for treatment alone. A method of predicting the increased volumetric flows due to rainfall in the drainage area using the non-linear reservoir model is also presented. Using the developed model, it is possible to predict potential flows through the network, although the quality of the results will depend on the quality of the data available. Using single point measurements of rainfall in the catchment and only data already measured by the water utility, an interquartile range of percentage errors between -10% and 14% of inlet flows was achieved for a one-day time horizon. This range increases to 2% to 48% of annual inlet flows over a four-day horizon. While these error ranges are far from desirable, with better measurement regimes and data availability the method can be refined further, and increased accuracy provided.