Impacts of Urbanization on Drainage System Health and Sustainable Drainage Recommendations for Future Scenarios—A Small City Case in China

China is urbanizing at an unprecedented rate, but also accelerating the use of water resources and overloading of urban drainage systems. To analyze the impact of urbanization on the drainage-system health in Jinxi, a typical small case area in China, this study proposed an innovative methodological framework for evaluation and prediction based on statistical and modeling methods, which provides a demonstration and reference for urban development and drainage-system construction in developing countries. The result shows that the comprehensive urbanization index (CUI) of Jinxi shows an overall upward trend between 2009 and 2020. The drainage-system health index (DHI) shows a U-shaped trend of decreasing and then increasing, with the threshold in 2016. The years when the DHI and CUI are in balanced development occurred in 2014 and 2018. The impact of urbanization on the drainage-system health is divided into positive and negative aspects, depending on the drainage demands of the urban development. According to the predicted results, it is suggested that the next drainage upgrading measures will be favorable for sustainable urban development when the urbanization rate reaches 60%, the gross industrial output increases by 10%, or the total retail sales of consumer goods increase by 40%.