Risk-based design of grass swales: assessing the impact of soil hydraulic conductivity on swale overflow mitigation

ABSTRACT To assess the reliability of green stormwater infrastructure (GSI), including grass swales, it is required to characterize the frequency of system failures. This involves identifying the conditions when the system operates in failure mode, potentially causing downstream flooding. The study utilized three 23-year meteorological time series from three Swedish locations, characterized by frontal, convective, or orographic rainfall events. These time series served as inputs for the simulation of runoff flows using the SWMM model, representing the physical characteristics of a grass swale (GS) located in Luleå, Sweden. Results showed a trend of reduced overflow occurrences and flood risks at higher ksat values of 25 and 31 mm/h. Exceedance curves indicated that ksat values of 2 or 4 mm/h resulted in the swale operating in failure mode 100% of the time. A minimum ksat of 31 mm/h was required to achieve acceptable operation for more than 50% of the time in Gothenburg and Luleå. Östersund was the only location where the studied GS did not operate in failure mode under a high ksat of 31 mm/h. Local climate – especially rainfall distribution and frequency – sets performance thresholds and underscores the need to integrate technical performance with flood risk management in GSI design.