Investigating the Multifaceted Benefits of Rain Gardens in the City

When it rains heavily, water accumulations form on roofs and streets, which can flow into buildings and cause damage to both structures and their contents. The conventional method to reduce such risks of accidents and damage is to capture and channel the water into an underground piping system that discharges it into the nearest river or coastal sea. Such infrastructure is expensive, and many systems are aging. Moreover, they cannot handle additional stress – for example, from urban densification or increased precipitation intensity associated with global warming.

“In the past decade, a new approach to blue-green stormwater solutions has been gaining momentum – one based on using water as a resource where it falls rather than disposing of it,” says Hrund Ólöf Andradóttir, professor at the faculty of Civil and Environmental Engineering at the University of Iceland. She is currently leading a study on the multifaceted benefits of blue-green infrastructure in the city.

Rainwater as a Resource

Blue-green stormwater solutions mimic the natural water cycle, in which water is temporarily stored in wetlands and soils. As water percolates through the soil, heavy metals and oil compounds are filtered out. “On the surface, a variety of plants, shrubs, and trees can be cultivated to enhance the aesthetic value and biodiversity of the city while also contributing to improved mental well-being among residents. Furthermore, the vegetation and soil can reduce the city’s carbon footprint by sequestering carbon. Thus, blue-green infrastructure serves both as a countermeasure and as an adaptation strategy in response to climate change,” Hrund explains.

Building Rain Gardens on the University Campus

The study involves monitoring the performance of bioretention cells, or rain gardens, constructed according to internationally recognized methods. Rain gardens are a type of vegetated basin designed to capture, store, and purify water from small building roofs or larger areas such as roads or parking lots during rain events. “We chose to study rain gardens because they are the most common form of blue-green stormwater solution in the city and, due to their versatility, they can range from small installations to very large ones (up to 800 m²), located within private lots or in public spaces along walkways, in parking areas, or along road edges,” Hrund says.

“In constructing the rain gardens, we aimed to maximize the quality of the soil and installed drainage systems to enhance its retention capacity. We are experimenting with different configurations of vegetation on the rain garden surfaces – ranging from a diverse mix of low-growing turf to taller plants such as Icelandic woolly willow, wood cranesbill, and water avens,” Hrund explains. She continues, “We plan to regularly measure both the hydrological and ecological performance over a two-year period, with particular emphasis during winter when the ground freezes and snow covers the surface. In addition, our research team is monitoring the soil’s moisture and temperature with a permanent recording system and keeping track of weather conditions, snow, and frost on the surface.”

Investigating Rain Gardens Through Vegetation and Soil Across Different Seasons

When asked about the inspiration for the study, Hrund explains that as she delved deeper into blue-green infrastructure, her interest began to extend beyond hydrological engineering alone. Hrund was the primary supervisor and project manager for Tarek Zaqout’s doctoral research on the hydrological efficiency of blue-green stormwater solutions in a cold maritime climate. Zaqout is now an expert at the Icelandic Meteorological Office. His research provided strong indications that the ability of vegetative belts to reduce flood peaks is closely related to the choice of plants and surface materials. “In collaboration with Tarek, I decided to investigate which plants would be most suitable for facilitating water flow and purification while also being attractive and capable of sequestering carbon in a cold climate,” she explains.

Hrund and Tarek have also brought on board Ása Lovísa Aradóttir, a professor at the Agricultural University of Iceland and an expert in ecosystem restoration, along with Jóhann Þórsson, a specialist and team leader for climate and soil at Land and Forest Iceland. “With Ása Lovísa’s help, we selected Icelandic plants that can tolerate periodic submersion, and since it is crucial to reduce atmospheric carbon dioxide, Jóhann will assist us in evaluating the carbon sequestration of both the soil and vegetation using methods similar to those applied in rural areas of Iceland,” Hrund explains about the research team.

In addition, three students from the University of Iceland participate in the study: Gonzalo Eldredge Arenas, a doctoral student in environmental engineering, is measuring the rate of water flow through the soil and its efficiency in purifying polluted stormwater; Muhammad Ayesh Muneeb, a master’s student in environmental engineering, is researching the carbon sequestration capabilities of the vegetation and soil; and Freyja Ragnarsdóttir Pedersen, a master’s student in ecosystem restoration, is monitoring the root systems beneath the surface as well as the species composition and condition of the above-ground vegetation.

Interested in Healthy and Sustainable Urban Environments

Hrund’s research interest primarily lies in fostering a healthy and sustainable urban environment. Over the past 18 years, she has studied the hydrological performance of various types of blue-green infrastructure. Her research has focused on the capital region, including the purification performance of detention ponds that receive runoff from streets and rooftops in Grafarholt, and vegetated swales that manage excess water in the BREEAM-certified neighborhood of Urriðaholt in Garðabær. BREEAM is an international standard that assesses and certifies the environmental performance of buildings and development projects. “These studies are particularly aimed at understanding how urban wastewater systems behave in a cold maritime climate – characterized by periodic frost and ice accumulation in winter, as well as rain and snow combined with high winds. In addition to these studies, I have researched water and air quality in Reykjavík and proposed measures for improvement, and I enjoy connecting my research with everyday life and society,” Hrund says. She has also enjoyed successful collaborations with academics at the University of Iceland and various companies and institutions in the country over the past two decades. Furthermore, she has served on the board of the Icelandic Water and Sewage Association since 2012.

Diverse and Long-Term Data

The study has secured funding for two years, but it is crucial to monitor the performance of the vegetated basins over a longer period to gain insights into various weather conditions and how the characteristics of the soil and vegetation change over time. The results will provide indications on how to maximize the quality and services of green spaces in densely populated areas and reduce uncertainty regarding the efficiency of blue-green solutions under frost conditions. “The study will also contribute new knowledge in hydrology, soil science, ecosystem restoration, rehabilitation, and urban resilience to climate change, thereby promoting broader implementation of blue-green infrastructure in urban areas,” Hrund concludes.