Catchment-scale hydrological performance of sustainable urban drainage systems in a cold maritime climate undergoing soil freeze-thaw cycles and rain-on-snow events

Sustainable urban drainage systems (SUDS) have been increasingly implemented as a low-impact, cost-effective stormwater control measure (SCM). SUDS include a diverse set of infiltration-based measures designed to maintain the pre-development hydrological cycle, reduce runoff, and enhance water quality via infiltration. However, these functions are prone to deterioration during winter, especially when soil frost is present. Unlike inland cold regions, maritime cities are particularly vulnerable to the negative winter impacts due to frequent freeze-thaw cycles, rain-on-snow events, and intermittent midwinter snowmelt. To date, the hydrological efficacy of SUDS at catchment-scale under cyclical cold conditions is still lacking. The goal of this study, therefore, was to evaluate the runoff and volume reduction achieved by a SUDS network in a small, 1.5 ha catchment in Garðabær, Iceland. In addition to assessing the seasonal and spatial variability of infiltration performance of different SUDS elements with varied soil properties and vegetation covers in an urban area. To that end, a total of 18 soil water content reflectometers to measure soil temperature and moisture were implemented in three SUDS components (i.e., densely vegetated rain garden, sparsely vegetated rain garden, and a front lawn with a grass vegetation cover receiving stormwater from a roof through a drain into a soakaway) in the study area at different depths (5–20 cm). An area-velocity flowmeter was installed at the outfall of the catchment to monitor runoff from the SUDS system as well as from the impervious surfaces that include streets, parking lots, and walking paths. Preliminary assessment at the beginning of the freezing period (i.e., November and December) showed that the densely vegetated rain garden was less susceptible to frost formation (frost reached 15 cm depth; min. -1.6 °C) compared to the sparsely vegetated rain garden (20 cm frost depth; min. -3.8 °C at 15 cm). In the front lawn, on the other hand, frost penetrated down to 10 cm depth (the depth at which soil was monitored and the minimum soil temperature dropped to -5.4 °C). The preliminary results show that the SUDS system was very effective during summer/fall and successfully infiltrated a total of 58% (n=14) of the storm events, especially small events (< 2 mm). The runoff coefficient for the events that produced surface runoff ranged between 0.011 and 0.19 (n=24) with an average volume reduction of 92% of the incoming runoff. However, further assessment of the system’s efficiency in terms of volume and runoff reduction during winter is still needed.