Study of continuous simulation supporting multiple design criteria for sustainable drainage systems

Traditional (piped) drainage systems are designed to drain quickly. It is therefore reasonable to design a system assuming that it will be empty at the start of the rainfall event. However, the restoration of full capacity in sustainable drainage systems (SuDS) or low impact development is dependent on slower processes (e.g., evapotranspiration, infiltration, or the consumption of harvested rainwater). Current design guidance often does not advise on reasonable assumptions to make regarding SuDS retention capacity. In addition, SuDS have the capacity to control both runoff volumes and flow rates during both routine and extreme storm events. This presents two further interlinked challenges: first, to identify relevant metrics to define SuDS performance; and, second, to define appropriate performance criteria for system design. Using rainwater harvesting as an example, it is argued that continuous simulation supports the calculation of a full range of performance metrics, properly accounting for retention, and empowering users to set design targets that are appropriate for a desired level of protection. Six independent metrics are considered to characterize performance in response to both routine and extreme rainfall events, and a scatter pie plot is introduced as a clear visual indicator of system performance across multiple targets. While current UK guidance for SuDS prioritizes flood risk mitigation and aims to provide protection up to the 1 in 100 year event, it is argued that such stringent expectations may be acting as a deterrent to SuDS uptake, particularly at the domestic scale. Here, lower design thresholds (for household SuDS) in the region of the 1 in 2 year event and 95% of annual runoff are recommended.