A case study of runoff coefficients for urban areas with different drainage systems

According to the United Nations, half of humanity now lives in cities. In recent years, many countries have seen a rise in the number of and intensity of heavy rainfall events due to the effects of climate change. The consequences of urban flooding in such a context can vary between just causing mild discomfort to tremendous devastation. Building cities that can resist flood is quickly becoming an important part development plans across the world. One of the upcoming techniques to handle storm water is to install sustainable drainage systems (SuDS). The primary goal of SuDS is to reduce to strain on the storm sewer network by retaining storm water on the surface as long as possible. While the water is retained at the surface, it is utilized to improve the local biodiversity and for its aesthetic value. Typical SuDS include vegetated swales, green roofs, detention ponds and open channels. The city of Malmo, Sweden is that has borne the brunt of several unusually high rainfall events in recent years. To build the resilience of the city against future heavy rainfall events and prevent basement flooding, a number of residential areas and parks across the city have incorporated SuDS along with traditional pipe systems. One such project in Malmo is Eco-city Augustenborg, which is the study site the current project. The project focuses on the northern part (NS) of the study area with implemented SuDS and one part that uses a traditional pipe system (the PS). Using rainfall and discharge data measured on-site, the total runoff volume from the PS and NS for several rainfall events was calculated. Measured data from the PS and NS were used compute two values of runoff coefficient (φ) for the study area – one for all pervious surfaces and one for all impervious surfaces (roofs, asphalt and concrete), based on the rational method approach. Results from calculated runoff coefficients were then applied to a 1D MIKE URBAN model of the site to simulate runoff during non-uniform rainfall events. Four historical observed rainfall events were used in this project. The results show that, although the methodology proposed above is very simple, calibration curves were obtained with volume errors as low as 5.8% and peak error as low as 1.07%. The model appears to be more effective for heavier rainfall events. Furthermore, through a literature review portion of the project it is revealed that a widespread confusion exists in scientific articles between the terms Effective Impervious Area (EIA) and Directly Connected Impervious Area (DCIA). These terms are very important in runoff studies but they are often used interchangeably in literature. A better understanding of their definitions would make results from different runoff studies more consistent and comparable. A distinction between the definitions of the two terms is sought out and articulated in this project.