BACKGROUND

PHUSICOS is a Horizon 2020 Innovation Action to demonstrate and evaluate the effectiveness of Nature-Based Solutions (NBS) in mitigating risks posed by extreme hydrometeorological events, particularly in rural and mountainous landscapes across Europe.

The primary objectives include: Risk Reduction: NBS interventions in PHUSICOS focus on modifying vegetation cover and managing surface water runoff to mitigate hazards like landslides and floods, protecting rural areas and critical infrastructure while preventing these events from escalating into large-scale disasters in urban areas downstream. Need for Evidence and Policy Shift: Research and documented evidence are essential to prove the effectiveness of NBS on a larger scale.

The lack of site-specific investigations comparing NBS with traditional alternatives hinders policy change. The PHUSICOS project aims to address this by providing data to support the broader adoption of NBS in policy frameworks. PHUSICOS also aims to identify and discuss various barriers—technical, practical, and policy-related—that hinder the implementation of NBS. By addressing these challenges, the project intends to make future NBS projects more feasible and widely accepted. Lastly, by focusing on these objectives, PHUSICOS contributes to advancing the practical implementation and mainstream adoption of NBS for environmental resilience.

METHODOLOGY OF NBS

The NBS are classified into (i) Green approaches (use of vegetation, forests and urban forestry as NBS), (ii) Blue approaches (full components of water bodies which help with flood mitigation) and (iii) Hybrid approaches, based on the classification used in other review articles for different climates (Debele et al. 2019; Enu et al. 2022). Hybrid approaches in infrastructure and environmental management refer to the integration of natural elements with engineered or artificial structures. (Capobianco et al., 2022)

In essence, a hybrid approach leverages both natural elements, such as vegetation, and man-made structures to mitigate environmental risks, harnessing the strengths of each to provide a balanced and effective solution. The primary goal is to enhance the resilience of infrastructure to environmental challenges, such as flooding or erosion, while promoting sustainability and minimizing the environmental impact.

As for example, in the case of Santa Elena, Spain, the main concern was the erosion and rockfall from a steep slope along a roadway. The intervention was terracing and re-vegetation, in which biological material was proposed to play a role in soil stability. A developed feature, through proposed Nature-based Solution comprised various structures designed of wood and/or local stones.

Figure 1 presents key elements of this case study. The image depicts the unstable slope prior to intervention, showing visible signs of erosion and rockfall risks.

Figure 1. Site before NBS Interventions. (Solheim et al., 2021）

Figure 2 demonstrates the construction of timber structures using locally sourced logs, illustrating the implementation process of NBS while Figure 3 shows completed wooden barriers installed on a steep terrain, reinforcing their role in stabilizing the slope and mitigating rockfall hazards.

Figure 2. Timber Construction (Solheim et al., 2021)

Figure 3. Wooden Barrier Installation (Solheim et al., 2021)

The solutions rest on active measures to stabilize the source areas, and passive measures to slow down and/or divert rocks in their trajectories toward something enhancing the protective role of the forest, such as drainage systems and careful selection of plant species for optimization of slope stabilization without ecological disruption. The promise in-situ includes an active strategy in which the preferential application or timber structures will manual stabilize the source areas and passive measure mix wood or stone structures to slow down and/or divert rocks in their trajectories enhancing the protective role of the forest, such as drainage systems and the careful selection of plant species to optimize slope stabilization while avoiding ecological disruptions.

The combination of these natural and engineered solutions constitutes a hybrid approach, blending ecological processes with structural interventions to achieve sustainable and resilient outcomes.

Figure 4 demonstrates various protective structures applied to slope stabilization and rockfall mitigation, encompassing both active and passive protection measures. The image show timber barriers, which act as deflectors to control rockfall trajectories. Figure 5 illustrates the same timber barrier system under winter snow conditions, confirming its consistent protective performance despite seasonal climatic variations. The final image presents a combination of wooden and stone structures, demonstrating the hybrid approach used to stabilize slopes and reduce erosion risks.

Figure 4. Seasonal Performance of Timber Structures in Snow Conditions(Capobianco et al., 2022)

Figure 5. Slope Stabilization with Wooden and Stone Hybrid Structures (Capobianco et al., 2022)

Figure 6. Hybrid aAproach Used to Stabilize Slopes and Reduce Erosion Risks (Capobianco et al., 2022)

BENEFITS FROM NBS APPLICATIONS

Through slope protection and afforestation, the problems related to erosion has not only been mitigated but also curbed, reducing chances of rock falls.

The process of making terraces and encouraging vegetation on slopes brought about greater stability in structure; evidence suggests that plant roots aid in improving soil's hydro-mechanical properties.

The drainage systems put in place for these terraces along with judicious choices regarding which plants to use optimal stabilization was achieved without introducing foreign species accidentally.

Constructing things like wooden barricades could prevent rocks from tumbling down or something, all the while safeguarding the plants till they become self-sustaining and provide sufficient shelter themselves.

CHALLENGES IN IMPLEMENTATION

A common denominator for the experienced and reported barriers to NBS implementation is a lack of knowledge leading to skepticism. The benefits that Nature-Based Solutions (NBS) offer, particularly their capacity to provide multiple advantages, are insufficiently recognized among all pertinent stakeholder groups.

The concept of Nature-based Solutions (NBS) is frequently overlooked during the design phase for various reasons, particularly the initial implementation costs, which tend to be significantly higher in comparison to established concrete structures (Van Zanten et al. 2023). Constraints associated with NBS include their necessity to be executed alongside grey solutions (hybrid solutions) as supplementary measures. Additionally, challenges arise from the financing of NBS, which must be addressed during annual budget negotiations and are subject to prioritization.

The unpredictability regarding the performance and upkeep of Nature-based Solutions (NBS) in extreme climates constitutes a significant barrier to their widespread implementation along linear infrastructure globally, given that the majority of roads and railways navigate through demanding terrains.

A further obstacle that can arise in road infrastructure pertains to the spatial limitations associated with the implementation of Nature-based Solutions (NBS). Typically, hazards along linear infrastructure are addressed by focusing on isolated problematic points and applying localized mitigation strategies. In contrast, NBS often necessitate more space than traditional solutions. Consequently, it may be more feasible to incorporate NBS elements into new infrastructure projects during the planning, design, and construction phases rather than retrofitting them into pre-existing infrastructure, particularly in urban settings (Ershad Sarabi et al. 2019).

In conclusion, there is a notable absence of regulations and guidelines pertaining to cost–benefit analysis, design, implementation, and monitoring, which are essential for overcoming the entrenched reliance on conventional 'grey' infrastructure and promoting the adoption of Nature-based Solutions (NBS).

OUTCOMES

Nature-based Solutions (NBS) are classified into three categories: green, blue, and hybrid approaches, which are supported by case studies from different countries. These case studies from past and current research initiatives on NBS, give valuable information for the design and implementation of these measures, helping for those managing infrastructure.

A significant number of Nature-based Solutions (NBS) is having an indirect impact on what we call linear infrastructure. They are not implemented directly on the structures like roads or tracks themselves, but they actually help in risk reduction offsite. For example, NBS methods dealing with landslides or debris flows often include afforestation efforts. These aren't usually done right on the specific road in danger; instead, they're applied to slopes or mountains nearby which influences outcomes.

DISCUSSIONS

When nature-based solutions (NBS) are carried out on a broad scale, approaches that are more inclusive and bring together the dialogue between infrastructure owners and other stakeholders, such as forest and farm owners, have been proven to be very effective.

Financial incentives might be needed to make landowners allocate some of their lands for NBS implementation since often NBS occupy big spaces. It is critically important to bear in mind the fact that NBS may not always be efficient for every particular case. This emphasizes the need to develop specific NBS approaches to coping with the hazards and to build a certain type of infrastructure, and in some cases, to apply hybrid solutions combining green and above.

One of the common issues relates to doubting the implementation of NBS owing to the general lack of knowledge about NBS and the linking of societal benefits. This information void causes adverse idleness to many stakeholders, such as being for authorities, landowners, politicians, and contractors in the building and construction industry without the transfer of knowledge. The complexness and time frame of the impact brings the need for the long-term planning that stems from recognizing the full spectrum of co-benefits of NBS to complete the work. Also, Continual close monitoring of all impacts and of the perceptions related to the affected communities aside, as the latter would change with time toward the co-benefits if they would appear.

Fostering the exchange of experiences and knowledge among groups with different areas of expertise is essential to address skepticism. This type of approach is in concordance with the research by Wamsler et al. (2020) who stated the fact that sharing of knowledge is the main tool in order to get rid of barriers and shift to NBS. They stressed the key role of collective knowledge transfer, especially in conditions where the majority of institutions are unfamiliar with NBS (Wamsler et al., 2020).

One way to push for the widespread adoption of NBS is to develop comprehensive guidelines that will offer support to the infrastructure managers in designing, implementing, and maintaining these solutions.

REFERENCES

1. Debele, Sisay E., et al. "Nature-based solutions for hydro-meteorological hazards: Revised concepts, classification schemes and databases." Environmental Research179 (2019): 108799.
2. Enu, Kirk B., et al. "Potential of nature-based solutions to mitigate hydro-meteorological risks in sub-Saharan Africa." Natural Hazards and Earth System Sciences2 (2023): 481-505.
3. Capobianco, V., Palau, R.M., Solheim, A. et al.The potential use of nature-based solutions as natural hazard mitigation measure for linear infrastructure in the Nordic Countries. Geoenviron Disasters 11, 27 (2024). https://doi.org/10.1186/s40677-024-00287-4
4. Solheim, A.; Capobianco, V.; Oen, A.; Kalsnes, B.; Wullf-Knutsen, T.; Olsen, M.; Del Seppia, N.; Arauzo, I.; Garcia Balaguer, E.; Strout, J.M. Implementing Nature-Based Solutions in Rural Landscapes: Barriers Experienced in the PHUSICOS Project. Sustainability 2021, 13, 1461. https://doi.org/10.3390/su13031461
5. Van Zanten BT, Gutierrez Goizueta G, Brander LM, Gonzalez Reguero B, Griffin R, Macleod KK, Jongman B (2023) Assessing the Benefits and Costs of Nature-Based Solutions for Climate Resilience: A Guideline for Project Developers. World Bank, Washington, DC. License: Creative Commons Attribution CC BY 3.0 IGO
6. Ershad Sarabi S, Han Q, Romme L (2019) Key enablers of and barriers to the uptake and implementation of nature-based solutions in urban settings: a review. Resources 8(3):121. https://doi.org/10.3390/resources8030121
7. Wamsler, C.; Wickenberg, B.; Hanson, H.; Alkan-Olsson, J.; Stålhammar, S.; Björn, H.; Falck, H.; Gerell, D.; Oskarsson, T.; Simonsson, E.; et al. Environmental and climate policy integration: Targeted strategies for overcoming barriers to nature-based solutions and climate change adaptation.  Clean. Prod.2020, 247, 119154.

ABOUT AUTHORS

Zhang Yan is Master's degree student in Rail and Urban Transport at the Technical University of Munich (Asia), Singapore. He holds a Bachelor's degree in Mechanical Design, Manufacturing, and Automation, which he earned through joint studies at Southwest Jiaotong University, China, and the University of Leeds, UK, from 2020 to 2024.

Ma Zhongyuan is an MSc candidate in Rail and Urban Transport at Technical University of Munich (Asia), Singapore. He holds a Bachelor’s degree in Transportation Engineering from Southwest Jiaotong University, China, awarded in June 2023.

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