Smart Nature-Based Solutions for Stormwater Management in Urban Areas—An Analysis of Pilot Cases ^†

Abstract

Nature-based solutions (NBSs) have been shown to be effective at addressing urban challenges such as flooding, poor water quality, biodiversity loss, and promoting public health and well-being. The European Commission mandates that cities implement NBSs for stormwater management by 2026 to deal with changing climate conditions like heavy rainfall events. Recent flooding events in Germany, France, and Belgium have demonstrated that the rainfall regime has shifted considerably over the last several decades. This study demonstrates, using 7 pilot areas near Baltic Sea region, that the transition from no- and low-tech to high-tech NBSs can have significant positive impacts on flood protection and water quality, as well co-benefits such as public health, highlighting NBSs’ multifaceted advantages.

1. Introduction

Today, 55% of the world’s population lives in cities, and this figure is expected to rise to 68% globally and to more than 80% in Europe by 2050 [1]. This causes an increase in permeable surfaces at the expense of green areas, which leads to an estimated 45% increase in stormwater runoff volume, a 15% decrease in ground infiltration capacity, and a 10% decrease in evaporation [2]. This has a direct impact on our living environment because it increases the risk of flooding, degrades water quality due to potential CSO activation, and raises average air temperature. As a result, numerous studies [3] and regulations have proposed to use nature-based solutions (NBSs) as a cure-all solution. Several multidimensional approaches have been developed to assess the potential impact of interventions, enabling the analysis and evaluation of the economic and social usefulness and profitability of implementing NBSs [4]. NBSs should be considered more in urban designing and planning to provide more sustainable, livable, and healthy urban spaces [5]. This study analyzes the use of a multidimensional approach to ponds, wetlands, and stormwater reuse in 7 pilot areas around the Baltic Sea. In addition to the multidimensional approach, this study investigated NBSs’ smartening potential by collecting data on stormwater flows and quality in real time. Such smartening allows one to identify the sources of pollution and to prevent problems associated with extreme weather events such as floods, droughts, and the resulting economic and social losses. This study’s goal was to test the multidimensional-analysis approach on multiple smaller-scale NBSs in urban areas through evaluating and visualizing how the smartening and impacts associated with primary and co-benefits are distributed in urban spaces.

2. Methodology

2.1. Determining the Primary and Secondary Benefits of NBSs

The diverse range of primary and secondary benefits established through various studies and applied projects have competing approaches for their quantification and valuation with challenges posed by the drive to valuate non-market properties. Furthermore, the interactions, conflicts, and synergies between different co-benefits and NBS measures complicate the estimation of the full range of impacts. The spatial and temporal variability in the effectiveness of NBSs, along with the uncertainties in projecting long-term benefits and accounting for subjective judgments, complicates the benefit estimation process. Overcoming these challenges requires robust methodologies, interdisciplinary collaboration, and detailed understanding of the numerous dimensions of benefits associated with NBS implementation in urban stormwater management.

Therefore, a survey of 23 NBS-based pilots built in four countries around the Baltic Sea was conducted, and interviews with the stakeholders were carried out to determine the reasons for NBS implementation. Based on the surveys and interviews, eight parameters were identified as primary benefits and nine as secondary benefits. The most important primary benefits were flood risk reduction (69.6% of all 23 pilots) and environmental protection (43.5%). The most reported co-benefits were public health and well-being (78.3%), urban heat (78.3%), social use and cohesion (65.2%), and biodiversity and green space provision (60.9%). Based on the survey results, the most considered primary and secondary benefits were used in this study to analyze the impact of 7 NBS designs around the Baltic Sea region.

2.2. Determining the Impact of Primary and Co-Benefits

Two primary and two secondary benefits were considered when estimating the overall impact of the NBS implementation. The area of each pilot site was divided into a 10 × 10 m grid (Figure 1), and the potential impact was calculated for each cell in the grid as well for each benefit. The impact of each benefit was normalized to a 0–1 scale, with a score of 1 (0–0.3) representing no impact, a score of 2 (0.3–0.6) representing medium impact, and a score of 4 (0.6–1) representing high impact. Two primary benefits were considered: flood reduction and improved water quality. Secondary benefits included urban heat mitigation and improved public health and well-being.

The flood reduction rate was calculated using SWMM models of each pilot area. The scoring ratio shows the percentage of total flood reduction achieved by the NBS compared to the status quo. The water quality improvement was calculated as a purification ratio by comparing the measured total suspended solids concentration (TSS) prior to the intervention with the expected TSS values after the NBS implementation. The urban heat reduction was calculated for each cell in the grid in degrees Celsius using the reduction rates presented in [3]. The values were normalized using the maximum temperature reduction rates for each land use type in each pilot area. Public health and well-being were calculated based on the changes in the landscape, population density, and fun factors of the area. Landscape changes took into account the increased functionality of public spaces (recreation, parks, benches, accessibility). Additional fun factors of the intervention increased the overall score of the NBS’s impact. Each cell in the grid was scored for each primary and secondary benefit, and an overall score for the NBS intervention was calculated to compare the potential impact of each pilot area.

3. Results

3.1. Impact of Benefits

The methodology for estimating the spatial multidimensional benefits of NBSs was implemented and presented in seven pilot areas across four Baltic Sea countries (Estonia, Finland, Sweden, and Latvia). All pilot areas are located in the temperate climate zone. The methodology’s results and visualization are based on two Estonian pilot areas (Figure 1).

In the Viimsi pilot area, the entire catchment area’s runoff passes through the NBS, resulting in 100% retention volume for design rainfall events. In the Tallinn pilot case, the flow reduction is one-third of the total flow. Based on this, Tallinn’s NBS has a lower flooding reduction score than Viimsi’s.

The effect of the NBS on the water quality was determined through the comparison of the measured TSS and expected TSS. For both of the pilot sites, the expected degree of purification is between 50 and 61%. Given the size of the NBS area and the length of the flow path, Viimsi’s NBSs are expected to have a lower water treatment capacity than Tallinn’s solution. This is primarily due to the fact that Tallinn’s pilot area flow path is four times longer than Viimsi’s.

When compared to the existing and planned NBS land use types, the solution in Tallinn will decrease the surrounding temperature, while the Viimsi solution will increase it. The decrease in the cooling effect in Viimsi’s pilot area is caused by an increase in hard and impermeable surface area. Tallinn has a higher impact score than Viimsi based on the estimated well-being value and population density of the pilot location.

3.2. Smart Multidimensional Aspect

Adding smart elements to urban drainage and stormwater systems provides an opportunity to mitigate damage caused by extreme weather events in urban areas. In both Tallinn and Viimsi, NBSs will use a smart valve to automatically regulate flow rates during extreme weather conditions. The valve is connected to the remote-control system. The system consists of both software and hardware components, and it allows for data collection from industrial equipment both remotely and locally. In Tallinn pilot’s case, the flooded area is a four-road intersection with 27 lines and a high traffic volume. The flooded area is located 450 m downstream from the NBS area. In most extreme cases, the flooding will stop the traffic in all 27 lanes of the intersection, resulting in a 650 m interruption in each direction of travel. In the Viimsi pilot case, the flooded area is an underground parking lot for four three- to five-story residential buildings. The flooded area is located 315 m downstream of the NBS area. A smart multifunctional NBS solution allows for the regulation of rainwater flows in such a way that the effect of extreme weather conditions can be mitigated and the economic and welfare losses kept at a minimum.

4. Conclusions

The multidimensional analysis approach used in this study demonstrates the importance of accounting for various factors when assessing the impact of NBSs in urban areas. By quantifying and visualizing the distribution of benefits and co-benefits across different NBS designs, the research contributes to a better understanding of how these solutions can enhance urban resilience and sustainability.

Overall, the findings underscore the need for robust methodologies, interdisciplinary collaboration, and a comprehensive understanding of the multiple dimensions of benefits associated with NBS implementation. By leveraging the insights from successful pilot cases and stakeholder interviews, cities can effectively adopt nature-based solutions to mitigate the impacts of extreme weather events and promote a more resilient and livable urban environment.