MONITORING, MODELLING APPROACHES AND EVALUATION OF THE RESILIENCE OF BIORETENTION SYSTEMS

Abstract :

The imperviousness of urban areas inhibits processes such as infiltration and evapotranspiration which allow stormwater to re-enter the natural water cycle. As a result, the demand on existing sewage systems is increasing due to excessive amounts of stormwater runoff. The upsurge in storm frequency and intensity is only exacerbating this demand. Bioretention systems, a Nature-Based Solution (NBS) for stormwater management, allow for the passive capture and treatment of stormwater. Monitoring and modelling of such systems is important to ensure the success of their design, implementation, and maintenance. In this study, a review of existing literature in the domain of monitoring and modelling of bioretention systems in the context of climate change is presented. Four case studies of bioretention monitoring in different climates and configurations were used to validate the ability of models to simulate performance in various climates. Methods for assessing the resilience of bioretention systems were adapted. Preliminary validation of newly developed infiltration models using data from one case study is shown, as well.

Key words : nature-based solutions, climate change, stormwater management, climate change, urban hydrology, infiltration, field validation

Body :

Urban growth and the resulting increase in impervious surfaces have had a significant impact on the natural hydrological cycle, particularly under climate change with heavier and more frequent precipitations (Seneviratne et al., 2021). This interference has created a pressing need for innovative stormwater management solutions. Impervious surfaces, such as roads, rooftops, and parking lots, disrupt the natural processes of infiltration (where water seeps into the ground) and evapotranspiration (ET, the combined process of water evaporation from the soil and transpiration from plants). Consequently, several issues arise.

Among these issues, one can mention the urban heat island effect, which is characterized by higher temperatures in cities compared to surrounding rural areas. The lack of infiltration and ET reduces the cooling effects that vegetation and natural surfaces provide, leading to increased heat retention in urban environments. This phenomenon not only causes discomfort for residents, but has broader implications for energy consumption and public health (Tong et al., 2021). Decreased infiltration also impacts groundwater resources, since less water is able to recharge groundwater aquifers. Coupled with the greater water demand experienced by urban areas, this can lead to depletion of groundwater resources.

The alteration of the natural water cycle due to urbanization also contributes to generation of significant amounts of stormwater runoff. With limited opportunities for absorption into the ground, rainfall quickly accumulates on impervious surfaces, creating excess runoff. In high quantities, this runoff poses a considerable challenge in the domain of urban stormwater management, as it overwhelms existing drainage systems and increases vulnerability to flooding.

Excess runoff has also an effect on receiving water bodies. From a hydrological perspective, the increased quantity of runoff creates changes in the timing, duration, and intensity of peak flows in rivers and streams (Jacobson, 2011; Seneviratne et al., 2021). Urbanization also contributes to water quality degradation, since stormwater runoff collects various pollutants – such as heavy metals, nutrients, and organic waste – as it comes into contact with impervious surfaces. When this contaminated runoff reaches larger receiving water bodies, serious environmental and sanitary risks are posed (W. Ahmed et al., 2019; Pascaud et al., 2015; Sidhu et al., 2013).

To address these challenges, bioretention has emerged as a component of many NBS systems for sustainable stormwater management, such as bioswales, infiltration basins, and rain gardens. These solutions are known by various names globally, such as Sustainable Stormwater Drainage Systems (SUDS) in Australia and the UK, Low Impact Development (LID) or Best Management Practices (BMP) in North America, and Techniques Alternatives (TA) in France (T. D. Fletcher et al., 2015). Regardless of its terminology, these interventions aim to augment the sustainability and resilience of existing stormwater management infrastructures, thus mitigating the detrimental effects of urbanization.

Bioretention systems typically consist of specially designed vegetated areas – often integrating engineered structures such as underdrains – that capture and passively treat stormwater. Many different configurations of such systems exist and are chosen according to the specific needs of the sites at hand. Through the creation of conditions suitable for infiltration and ET to occur, they are able to re-implement elements of the natural water cycle in urban areas and counteract the aforementioned repercussions of increased urbanization. As runoff flows into bioretention areas, vegetation and soil help filter out pollutants and retain water. The treated water can then slowly infiltrate into the ground, recharging aquifers, or be released at a controlled rate, reducing the strain on conventional drainage systems and decreasing the risk of flooding. The term “bioretention” is based on the way that these systems use biological components such as soil and vegetation for the retention of water and pollutants.

The incorporation of bioretention systems into urban infrastructure is becoming increasingly relevant especially in the context of climate change. With the increase in extreme weather events (Seneviratne et al., 2021) current urban stormwater infrastructure will be subjected to new demands (Berggren et al., 2012; S. Fletcher et al., 2019; Rosenberg et al., 2010; Semadeni-Davies et al., 2008; Willems et al., 2012). Increased storm frequency and intensity will need to be accounted for in order to maintain the integrity of urban areas. In Romania, Pânzaru et al. (2022) found evidence of urbanization’s impact on urban water availability and quality, and recommends NBS water management to be implemented in conjunction with conventional engineering to increase climate resilience. Adu Boateng et al. (2023) also found that there is a need for such measures to be implemented into urban development in Kumasi, Ghana, for climate change mitigation and adaptation.

For successful design, implementation, and maintenance of bioretention systems, monitoring and modelling are essential. Monitoring consists of regular data collection and analysis of different parameters such as infiltration and pollutant removal efficiency, whereas modeling makes use of models to simulate a system’s performance under different situations. For the development of models, monitoring data is used to validate the model’s ability to accurately represent a system’s behavior. There are many existing models for bioretention design, although there is a challenge in finding a balance between accuracy and facilitated operation – many models are highly complex involving many parameters and long computation times.

Therefore, there is a need for accurate and easy-to-operate hydrological models. To account for the effects of climate change and assess the resilience of existing bioretention systems, it is crucial that these models have the capacity to accurately represent the behavior and performance of such systems across varying weather regimes. Furthermore, the validation and calibration of these models using diverse field hydrological monitoring data is essential. The main objectives of this study are to present a review of current research in bioretention modelling and monitoring, prepare case studies of bioretention systems with field data from different climates and with different configurations to be used for the evaluation of models, and to develop a method for testing the resilience of the systems.