The Opportunities and Challenges for Urban NBS: Lessons from Implementing the Urban Waterbuffer in Rotterdam

Abstract

In Rotterdam, the proportion of impervious surfaces has increased due to urbanisation and changing land use priorities. Combined with the effects of increasingly frequent extreme rain events and longer dry periods, the current capacity of the urban water system is reaching its limits. In response to the lack of retention capacity in the neighbourhood Spangen, a hybrid Nature-based Solution (NBS) was implemented in the summer of 2018. This NBS, called the Urban Waterbuffer, has been at the heart of the Rotterdam case study, focusing on drivers and barriers for implementation, direct and indirect costs, and (co-)benefits of the system. The case study compared this hybrid approach to dealing with heavy rainfall with grey and green alternatives at a neighbourhood scale. Seven lessons were drawn on the opportunities and challenges for implementing NBS for urban water management in the Dutch context. The smaller scale of this case study, compared to other case studies in the NAIAD project, highlighted the similarities and differences in NBS effectiveness and viability across a wide range of spatial scales. The lessons learned from this Rotterdam case study can facilitate future decision-making on, and implementation of climate-adaptive urban water management strategies across Europe.

FormalPara Highlights

• The UWB Spangen has reduced pluvial flooding, whilst also storing and supplying 15 million litres of water for irrigation, reducing the waste of drinking water for non-consumable purposes.

• The benefits of a realised pilot project include valuable lessons around the various motivations and disincentives for NBS uptake in the Dutch context, and the need for diverse stakeholders to work together.

• The broader neighbourhood assessment indicates that in certain contexts, NBS do appear able to deliver a comparable level of service for an equivalent or lower capital outlay than grey solutions, whilst also delivering a multiplicity of additional benefits, albeit often requiring more space.

16.1 Introduction

16.1.1 Background

In the coming years, The Netherlands will face a range of water challenges, not least of which concern rising sea levels, as well as risks from the large rivers Rhine, Meuse and Scheldt that flow through the country. However, within cities other obstacles are emerging. The onset of climate change is bringing increasingly frequent extreme precipitation, as well as higher temperatures and longer periods of drought (Aerts et al. 2012). In this context, Dutch cities are facing a need for renewed investment in water infrastructure and climate adaptation, if they are to maintain their renowned liveability and high quality of life. As such, cities are increasingly turning to emerging technologies and innovations with keen interest, particularly those that can address multiple issues integrally.

The city of Rotterdam lies in the province of South Holland, within the Rhine-Meuse-Scheldt River delta, near the North Sea. The city has a population of around 645,000, with an average density of 2963 inhabitants per km² (CBS 2019). Like the nation at large, much of Rotterdam’s past and future successes are strongly linked to water; it is home to Europe’s largest port, approximately 36% of the municipal area consists of water and 80% of the city lies below sea level. As such, the city is both highly dependent on, and yet vulnerable to threats from water (Gemeente Rotterdam 2013).

As with many cities, the proportion of impervious surfaces in Rotterdam has increased over time, due to urbanisation and changing land use priorities. Over a century ago, several canals, which previously took care of household water management, were filled up to make space for new infrastructure. Drainage became increasingly reliant on a sewage system, which now appears insufficient to cope with heavy rainfall events (De Greef 2005). With the aforementioned emerging climate change impacts, Rotterdam is now becoming susceptible to not only pluvial flooding, but also higher levels of heat stress, and degradation of building foundations (in case of wooden pilings) due to dropping ground water tables in many parts of the city (Fig. 16.1).

Fig. 16.1

A map of Rotterdam, showing areas vulnerable to flooding, heat stress and subsidence. (Adopted from Gemeente Rotterdam 2013). A detailed characterization of the case study area can be found in Pengal et al. (2017), Section 5.10

Acknowledging that this combination of urbanisation and climate change poses a significant threat to the city’s future prosperity, the city of Rotterdam set the objective of becoming 100% climate-proof by 2025, through the Rotterdam Adaptation Strategy (Dircke and Molenaar 2015). This aims for a robust water system that protects the city and prevents climate-related nuisances, achieved through forming connections across disciplines, programmes and multiple stakeholders. Consequently, the water policy is shifting from a catch-store-dispose approach, towards local infiltration, retention, storage and reuse. As will be elucidated throughout this chapter, this broader policy objective provided an entry point for innovative approaches that aim to break away from the traditional water management paradigm.

16.1.2 The Case Study Area: Spangen Neighbourhood

The Rotterdam case study focuses on Spangen, a low-lying neighbourhood in the west of the city, spanning roughly 65 ha with around 10,000 inhabitants. In Spangen, as with much of the city, the effects of urbanisation are clear: hard surfaces dominate and there are few green areas (Fig. 16.2).

Fig. 16.2

A representative view of the Neighbourhood of Spangen, showing the high percentage of impermeable surfaces surrounding the Sparta Stadium. Photo by FieldFactors, 2017

In recent years, Spangen has been suffering frequent nuisance during heavy rain events. Hydrological analysis confirmed the need for additional retention capacity in the neighbourhood, and the upcoming investments were taken as an opportunity to simultaneously tackle some of the underlying issues, such as the lack of green space. The Rotterdam case study in NAIAD has assessed the implementation of an innovative NBS for rainwater retention and reuse, the so-called Urban Waterbuffer (UWB).

The UWB Spangen presents a solution for the localised pluvial flooding, whilst also providing additional green space, and large-scale seasonal water storage (see Fig. 16.3). In 2018, the first pilot application was built in Spangen. Additionally, the UWB has been assessed as one of the measures in the three neighbourhood wide strategies that complemented the Rotterdam Case Study.

Fig. 16.3

Schematic representation of the Urban Waterbuffer in Spangen, Rottterdam

The objectives in the Rotterdam case study were to perform:

• 1. An empirical assessment of the recently implemented UWB in Spangen. Spanning a 4 ha catchment area, this hybrid solution reduces pluvial flooding using an underground buffer tank for temporary retention, together with biofiltration and aquifer storage and recovery (ASR) techniques to provide long term storage and reuse of the captured rainwater.

• 2. A broader assessment of the impacts of the UWB, when nested within a hybrid water management strategy at the neighbourhood scale, and how this compares with fully green solutions and grey alternative strategies (see Fig. 16.4).

The assessments of the UWB Spangen and the three neighbourhood strategies have been executed in line with the methodologies developed within NAIAD. In Fig. 16.5, an overview of the NAIAD framework applied in the Rotterdam case study is presented. The scale at which the methods were applied (UWB project scope versus neighbourhood strategy) varied to accommodate the methodological approach and to deliver the relevant insights.

Fig. 16.4

Overview of the three neighbourhood-wide strategies that were assessed in the Rotterdam Case Study

Fig. 16.5

An overview of the methodologies applied in the Rotterdam case study

16.1.3 Chapter Outline

Drawing upon an examination of the factors leading to the implementation of the UWB and an assessment of long-term potential future costs and benefits of NBS, important insights have been derived from the Rotterdam case study. This chapter aims to share these insights through seven key lessons that have emerged from the research. These lessons result from utilising the methodologies introduced in this volume. The aim is to support stakeholders in future assessment and decision-making on NBS implementation, by shedding new light on the impact of NBS in urban areas and their potential for wider uptake.

16.2 Lessons from the Rotterdam Case Study

16.2.1 The NBS Implementation Was Driven by Its Ability to Address Multiple Challenges Integrally

The UWB in Spangen benefited from a window of opportunity that arose when multiple initiatives coincided in 2016. The city had just launched the Water Sensitive Rotterdam program, aiming to improve the city’s climate resilience; and the Spangen neighbourhood was already earmarked as in need of additional water retention measures. Simultaneously, residents of the neighbourhood had been requesting additional green space to counteract the dominance of paved surfaces. Additionally, a recently formed consortium of knowledge, market and public partners was searching for locations to pilot urban ASR under the umbrella of a research project (Urban Waterbuffer), subsidised through a national innovation fund. Finally, the local football club emerged as a potential end user for the large volumes of rainwater that could be treated and stored by the hybrid measure. The stored water is used to substitute the use of tap water for non-consumable purposes, like irrigation of the sports field, allowing the football club to reduce their drinking water footprint. The UWB offered an integral approach to these various issues, leading to its successful implementation in the summer of 2018.

16.2.1.1 Drivers of the Implementation Process

Based on interviews with key stakeholders, a timeline was developed to visualise critical milestones in the decision-making process and identify key success factors (Fig. 16.6 and Chap. 9 of this volume for further background on the Adaptive Planning framework). One of the notable insights from this process was the role of various policy programmes within the City of Rotterdam and Water Authority Delfland (e.g. Water Sensitive Rotterdam,¹ Resilient Rotterdam²; Waterbeheerplan 2016–2021, Delfland 2015) in fostering the adoption of measures beyond traditional sewer replacement. The authorities aimed to exploit multi-functional approaches to improve the city’s climate resilience, and to increase the amount of green infrastructure in the city.

Fig. 16.6

Analysis of adaptive planning process around the implementation of the UWB Spangen: Timeline of the decision-making processes. (Source: Dourojeanni Schlotfeldt 2019)

Besides the primary benefit of the UWB solution to reduce the risk of sewer overflow, the ability to use the collected rainwater locally, was considered as a critical co-benefit. This even led to a new business case involving the local football club as end-user. The creation of this new local fresh water source contributes to the city’s resilience to future droughts.

In addition, the spatial improvement, such as added green space, water playing features, and seating elements that the UWB brought to the neighbourhood, were an important motivation to invest in the UWB. Aside from their main objective to resolve the water nuisance in the area, the Water Authority in particular also had a policy objective to stimulate water awareness among inhabitants. This objective was fulfilled through the incorporation of an open and visible water treatment element and the ability for children to play with the harvested water through a water feature.

16.2.1.2 Stakeholder Perception Analysis

The multiple functions of NBS naturally leads to the involvement of many different parties with different interests and responsibilities. If these parties are well aligned, this can create opportunities to benefit from cost-savings through one integral solution. In contrast, the involvement of many stakeholders can also be a burden on the decision-making process, with transaction costs, risk of protracted disputes and missed opportunities. A reflection on the successful alignment of the stakeholders in the UWB project led to the identification of two key drivers. First of all, key individuals within the involved organisations were highly committed to making the project work from the outset and took a leading role in aligning stakeholders and their interests. Secondly, there was a common understanding between stakeholders that action was required. Various interviews were analysed using Fuzzy Cognitive Mapping to identify the different stakeholder perceptions regarding the main problems that needed addressing. The results indicated a large common understanding of the main issues at hand (sewer overflow), the factors that cause sewer overflow, and the potential for the UWB to reduce the risk of pluvial flooding (see Fig. 16.7). For the successful implementation of NBS, it is important that stakeholders share a common understanding that achieving climate resilience in urban areas will require new, cross sectoral approaches. Having to deal with more extreme rain events, longer lasting dry periods and increasing temperatures in urban areas requires measures that go beyond traditional sewer replacement and address these multiple issues integrally.

Fig. 16.7

A Fuzzy Cognitive Map (FCM) showing the relationships between factors related to pluvial flood risk as perceived by stakeholders in Spangen (developed for the Rotterdam case study by Giordano, R. and Pagano, A. in 2018). The thickness of the arrows indicates the relative number of respondents who acknowledged the causal relations between the factors (thicker = mentioned by more stakeholders)

16.2.2 Current Planning Regulations and Policy Making Do Not Facilitate the Uptake of NBS

Despite the political willingness to integrate NBS in the climate adaptation agenda, the current planning regulations and processes in the Netherlands do not facilitate the uptake of NBS. Throughout the implementation process, various challenges arose that nearly blocked the realisation of the UWB in Spangen. For example, the business-as-usual approach was initially preferred by certain departments of the implementing authorities. During the stakeholder workshops, a special emphasis was put on the upscaling perspectives of NBS for climate adaptation in the Dutch urban context. What became evident was that the current frameworks in Dutch planning practices do not facilitate the uptake of NBS at the operational level. One of the barriers that was identified in the Rotterdam case study is the sectoral silos and split responsibilities among and within various public organisations. Typically, departments within organisations have a dedicated budget and responsibility for a “single” task. As NBS typically address multiple elements of urban planning and water management, they require the integration of tasks, responsibilities and budgets across the various silos. Without clear tactical and operational guidelines to assess a measure’s impact across other silos, operational decisions typically remain based on a measure’s mono-functional efficiency. Therefore, in addition to political willingness at strategic level, specific multi-purpose objectives at the tactical level are necessary to transform willingness into action, without having to be dependent on passionate individuals or dedicated cross-silo programmes, like the Water Sensitive Rotterdam.

For the UWB Spangen project, this cross-silo program was a key factor in overcoming barriers in the decision-making process. Additionally, a national research fund helped to lower the required investment and uncertainties for various project commissioners. This pilot approach helped to work cross-sectorally and step outside of some of the more rigid planning regimes and regulations. Lastly, the UWB project benefitted from strong commitment of some specific stakeholders.

16.2.3 NBS Can Compete on Life Cycle Cost with Grey Solutions, Though Strategies Must Be Carefully Designed and Assessed per Location

Although NBS are often claimed to be cost effective, there currently remains limited evidence in support of this (Le Coent et al. 2020). In the Rotterdam case study, uncertainty about the costs of NBS did indeed emerge as a concern for a wide range of stakeholders. These concerns included the potential impact of high upfront costs, uncertainties of long-term maintenance requirements, and even difficulties in internalising many of the acknowledged co-benefits. There was a clear desire amongst stakeholders for more information on how the costs of these new approaches compared to business as usual, not only initially, but also over the long term.

As part of the economic assessment in the Rotterdam case study (approach outlined in Chap. 6), the costs of three neighbourhood-scale water management strategies – grey, hybrid and green – were compared through the application of life cycle costing (LCC). The strategies were specified with similar retention capacity in order to provide equivalent flood risk reduction, which allowed for a comparison between the strategies on their overall costs for a certain level of service (see Dartée et al. 2019 for a more detailed overview of this approach).

The Rotterdam case study was unique as compared to other case studies presented in this volume, in that it was possible to draw upon real implementation costs for the UWB, which was incorporated as a central measure into the hybrid strategy. In order to best align these empirical cost figures with the values sourced from literature for other measures, local Dutch cost data were chosen wherever available. This aimed to improve the compatibility between the UWB and other cost data, but also increased the potential influence of outliers when compared to expressing results as a range. As with any such assessment, the results are therefore to be considered indicative rather than definitive, given the many context specific factors that can influence the costs of implementing public infrastructure. That is also why it is critical to design strategies carefully, as context specific factors can lead to significantly higher or lower costs for certain measures in relation to their hydrological effects on reducing flood risk (e.g. Low permeability of soil would require a bigger area of green infiltration strips to offer a similar retention/infiltration capacity and hence would lead to higher opportunity costs).

The results of the Rotterdam case study showed hybrid and green strategies to be marginally cheaper over a 50-year time period, than the grey strategy for the same level of service, both before and after the application of a 3% discount rate (Fig. 16.8), supporting the notion that NBS can compete with grey solutions on cost.

Fig. 16.8

The Life Cycle Costs (LCC) of the three strategies in the Rotterdam case study over 50 years in Million EUR, showing a breakdown of costs drivers for each strategy. This overview presents summaries of both discounted and undiscounted assessments. Discount rate applied was 3%

The results also highlight the importance of taking a long-term view to cost assessments, as maintenance expenditures represent a significant portion of the overall LCC. Given the long timeframe of the LCC, the reference values for maintenance costs are potentially strongly influential, yet there is a relative scarcity of reliable long-term maintenance cost data, particularly for green infrastructure. It is worth noting that, once discounted, these costs represent a smaller share of the LCC, yet the limited availability of relevant data serves to highlight the importance of further implementation of, and research on, NBS to address knowledge gaps and improve understanding and confidence in their costs over time.

16.2.4 Monitoring Co-benefits Is Critical to Support the Wider Uptake of NBS

Much of the debate around NBS centres on their multifunctionality, and indeed it was one of the key motivators for the implementation of the UWB in Spangen. In Rotterdam, the identification of co-benefits for the UWB began prior to implementation through stakeholder consultation, which identified the following potential co-benefits:

• Local temperature regulation

• Water quality regulation

• Cost reduction of water treatment

• Storm water re-use

• Spatial quality betterment

• Increased green

• Increased water awareness

• Increased social cohesion

The economic assessment of co-benefits was undertaken at the neighbourhood scale, so the initial list of co-benefits was expanded to account for the variety of different measures under consideration and the multiple impacts they can provide. Sixteen (16) co-benefits were identified and considered for assessment (Fig. 16.9), drawing predominantly from the EKLIPSE framework. Direct valuation was the dominant method used, drawing values from peer reviewed literature, except in the case of the UWB, where some data was available directly from the implemented project UWB Spangen.

Fig. 16.9

Overview of the co-benefits assessed in the Rotterdam Demo

Whilst our assessment aimed to monetise as many of the co-benefits as possible so as to capture the full value provided by NBS, some of the co-benefits were not able to be valued monetarily, mainly due to one of two reasons:

• Insufficient data/reference literature

• Risk of double counting

The monetised results from the co-benefit assessment (Fig. 16.10) showed, firstly, that even at a small scale the value of co-benefits can be substantial, and secondly, that the kinds of co-benefits that deliver value in dense urban areas can be very different from those provided by large scale ecosystems. By far, the largest benefits were derived from avoided health care costs and labour loss, as well as property value increases. Other high value co-benefits included revenue from the supply of water from the UWB, and heating savings from improved insulation due to green roofs. Many other co-benefits provided marginal value at this small scale.

Fig. 16.10

The undiscounted value of co-benefits delivered by each strategy in Spangen over 50 years in Million EUR

Given the relatively close LCC results between the strategies, co-benefits are a clear differentiator for NBS compared to grey solutions. However, as some of the NBS measures considered can provide co-benefits that were not monetised, the above results do not represent the full value of additional co-benefits provided. Examples from the case study include mitigation of the urban heat island effect through local temperature regulation, which in the wake of ongoing urbanisation and record temperatures is becoming increasingly central to the discussion on NBS in The Netherlands. Similarly, increased water awareness was not monetised, but was an important potential co-benefit to both the municipality and the Water Authority. This interest is likely linked to the fact that 60% of the land within the municipality is privately owned (De Doelder 2019), meaning that the authorities are partially dependent on private actions to increase the city’s climate resilience.

As can be seen, in the current climate of enthusiasm for NBS, certain potential co-benefits, tied to specific contextual objectives, can prove decisive for project implementation even in the absence of clear quantification of their monetary value. However, evidence that NBS can actually deliver these potential co-benefits is likely to become increasingly important as the initial enthusiasm around NBS begins to fade. As such, it is important that monitoring and assessment of co-benefits will be prioritised, particularly for social co-benefits, to build a better understanding on performance in some of the areas where data is lacking. However, the evidence in support of the delivery of co-benefits need not be exclusively monetary. The multifunctionality of NBS is their key advantage, and the case for them is most compelling when impacts are considered holistically. This means that consideration of a range of environmental indicators alongside economic valuation may be most suited to capturing the full scope of NBS benefits and functions.

16.2.5 If There Is Space, Full NBS Is Ace; if Space Is Tight, Hybrid Might Be Right

The Netherlands is one of the most densely populated countries on earth, and as a consequence, the value of land in urban areas is high. This means that there is always competition for space among potential users, and that water management interventions that require additional space compared to business as usual will involve trade-offs. To fairly compare NBS with grey infrastructure, it is thus important to consider the opportunity costs associated with their implementation, which in the Rotterdam case study was achieved through using the value of the land required for each strategy as a proxy, taken from Levkovich et al. (2018). Whilst this is a widely used approach, it should be applied with caution, as NBS measures located in areas such as sidewalks would be unlikely to limit development potential. Thus, the use of a single land value risks overstating the opportunity cost. In the Rotterdam case study, given the use of a single high land price, the opportunity cost assessment should be considered as an upper bound estimate (Fig. 16.11), imposing significant negative impacts on the NBS strategies.

Fig. 16.11

Opportunity costs for the three strategies, using value of the land required for implementation in Million EUR

Consequently, when bringing together the various results of the economic assessment, it became clear that neither the equivalent avoided damages, nor the relatively small differences in implementation costs between the strategies would prove decisive. As a result, it was opportunity costs and co-benefits that emerged as the key differentiators (see Fig. 16.12).

Fig. 16.12

The integrated economic results for each strategy, showing the large influence of co-benefits and opportunity costs as presented in Le Coent et al. (2020)

Whilst the assessment was subject to substantial variability, particularly given the long timeframe and large number of measures, the order of magnitude difference between the various pillars does allow for the following lesson to be drawn with a relatively high level of confidence. When deciding between NBS and grey infrastructure to mitigate flood risk in dense urban areas, the question can be boiled down to whether the opportunity costs imposed by the space requirements of NBS can be offset by the co-benefits that they can provide. Table 16.1 shows the Benefit Cost Ratios (BCR) of the integrated economic results - the sum of all of the benefits divided by all of the costs for the three strategies, expressed in economic terms.

Table 16.1 The total costs, benefits, Net Present Value (NPV) and Benefit Cost Ratio (BCR) of the three strategies over 50 years

What can be seen is that when considering the economic results alone, no strategies resulted in a BCR above 1 – the point at which an intervention is generally considered cost effective. However, considering the difficulty in including all direct and indirect damages in the assessment and the strong societal relevance, responsible authorities in The Netherlands do at times invest in risk reduction measures with a BCR < 1 (Jonkman et al. 2004). This means that in this case the BCR is more useful in comparing the strategies to one another than in assessing cost effectiveness in absolute terms. When viewed in this light, the results show that the hybrid and green strategies bring significantly more benefits per Euro spent than the grey approach.

The results of the economic assessment support the notion that, at least in the Dutch context, if there is space available, NBS are likely a better choice than standard grey approaches, given that they can potentially provide similar core functionality for an equivalent or slightly lower cost, whilst also delivering an array of co-benefits. However, if the opportunity costs of implementation are high and space is scarce, hybrid solutions may prove a better choice, as they can provide many of the benefits with less space requirements. The single strongest case for grey infrastructure remains familiarity and institutional embeddedness. Nevertheless, in the Dutch context there is widespread and growing understanding that the multi-faceted challenges of the twenty-first century are likely to be best served by multifunctional approaches.

16.2.6 Multi-functionality of NBS Allows for the Development of New Business Cases

As an example of an effective Public-Private-Partnership, the implemented UWB Spangen offered valuable insights on potential business cases for NBS. The partners involved included two public authorities (municipality and Water Authority), a semi-public utility provider, and private companies (see Fig. 16.13).

Fig. 16.13

Distribution of responsibilities between stakeholders in relation to UWB Spangen

The business case in this project could inspire future implementations of similar NBS, even though it was in the context of a pilot project. Through the pilot setup, the technical risk and investment threshold for the various stakeholders to collaborate was reduced, and typical concerns related to innovations were allayed by allowing for intense monitoring and research. The UWB in Rotterdam is the first urban implementation of this solution in the Netherlands, and its pilot status allowed for slightly different legal arrangements to be applied. These advantages might no longer be available in future implementations of the solution.

In order to reflect on the upscaling potential of the UWB, as well as what is needed to build sustainable business cases for NBS, a workshop was held with key stakeholders. One identified driver for the business case of the UWB specifically is the exploitation of the new water source, as this can generate direct cash flows through the water supply, though it must be noted that tap water rates in The Netherlands are currently extremely low. However, since projections show increasing pressure on freshwater availability and security of supply, the drinking water prices are expected to rise, generating a bigger saving to be achieved through the use of treated storm water. The societal value placed on measures that increase water availability is also likely to grow, considering the potential damages droughts might incur on existing green space, infrastructure and human health.

In the case of the implemented solution in the Rotterdam case study, the UWB, as it generates a saleable commodity in the form of the harvested water, there are opportunities for developing business models with private sector partners, creating a return on investment over time. As such, Public-Private-Partnerships are expected to remain the main implementation arrangement for the UWB.

In order to facilitate the cooperation of various stakeholders, it is important that NBS provide a distinct level of service that the relevant parties are willing to invest in. In current practice, the impact of co-benefits is seldom addressed in terms of an increased level of service. As a result, building financial and legal arrangements that are based on the exchange of these benefits is challenging, particularly since often the implementing party is not the same as who will receive the benefits. Another challenge in making the business case for NBS is the fact that there can be a vast difference between the monetary value of co-benefits in an impact assessment, and the tangible contributions to a budget bottom line that these co-benefits can actually provide. This means that for NBS in general, it appears that municipalities and other public bodies are well suited for implementing them, as the broader societal benefits could contribute to achieving other objectives within their responsibilities. Within NAIAD, special emphasis was put on the role of the insurance industry in future business cases for implementing NBS. Following the interaction with the Dutch insurance industry during this research, it is expected their role within funding and financing of Dutch urban water management will remain minimal. However, they do have means to nudge individuals towards more climate-adaptive behaviour through facilitating the development of water awareness among citizens.

What did come forward during stakeholder consultation in Rotterdam is the high willingness to collaborate across disciplines in order to benefit from the multi-functionality of NBS. Even though the assessments of various alternative measures do not always explicitly consider co-benefits, the majority of the larger public investments in The Netherlands require a social-cost benefit analysis to be performed. Stakeholders acknowledge there is a growing understanding of the relevance of the societal and environmental impacts of alternative measures. In order to be able to capitalise on the various co-benefits over time, working with integrated and long-term contracts might create a better incentive for decision-makers to consider their investments in terms of wider impact and potential multi-stakeholder value creation. Also, it was pointed out that better alignment of the various responsibilities of public authorities in The Netherlands could help to make room for more integral and multi-functional measures, allowing for more cost-effective interventions in the long-term.

16.2.7 Implementing NBS Is Needed to Catalyse Wider Acceptance, Interest and Future Uptake

One of the challenges with the implementation of the UWB in Spangen was the management of uncertainties related to the newness of the solution: actual effects on the water system, the water quality achieved, and the need for new operation and maintenance regimes. The best means to overcome these challenges was to implement the solution and consequently intensively monitor system performance. The availability of an innovation fund proved highly valuable as it allowed for some of the aforementioned uncertainties to be addressed in a pilot setting (Fig. 16.14), and the performance of research. Reflecting on the entire process, it was apparent how much impact the implementation of the UWB had on the wider acceptance of and interest in NBS. Being able to showcase the solution and prove the concept works was a critical milestone in order to build capacity for the larger uptake of other similar solutions.

Fig. 16.14

The Biofilter component of the Urban Waterbuffer in Spangen

Particularly in the Dutch water management sector, standardization, robustness and economies of scale have long dominated business as usual practices. Given that NBS offer completely different characteristics, the evidence of NBS effectiveness is often still limited. So long as this remains the case, it will be difficult to convince stakeholders of the efficacy of the investment in NBS. With the emergence of co-benefits as a crucial component of the case for NBS, how these are best to be assessed remains a key question. As discussed throughout this chapter, on several occasions over the course of the implementation of the UWB Spangen, stakeholders identified the difficulties in assessing co-benefits, and concerns about long term costs and reliability as key challenges for the upscaling of NBS. Much of this difficulty stems from a lack of comparable existing cases to draw from, meaning that reasonable estimates of the level of service that can be expected for key co-benefits, or the long-term costs for a given measure over time are not well established.

In the current climate in the Netherlands, there is a willingness to subsidise innovation and assumptions on co-benefits that may result from an implemented NBS appear sufficient, but this can be expected to change as novelty fades and budgetary realities set in. Therefore, building more NBS, monitoring their impact and using that to overcome uncertainties regarding effectiveness, costs and co-benefits is needed in order to provide the necessary evidence base to facilitate their wider uptake (Fig. 16.15). Being able to engage with stakeholders around the actual implementation of an NBS in Spangen and being able to communicate and discuss the NAIAD results with these stakeholders, might result in the beginning of a paradigm shift in which NBS are considered a worthy equivalent of and maybe even a better alternative to hard engineering structures.

Fig. 16.15

Pathway to mainstreaming NBS. Having validated the effectiveness in multiple pilots is needed to be able to take on business as usual practices

16.3 Conclusions

In the 18 months of monitoring, since its implementation, the UWB Spangen has performed strongly, having reduced pluvial flooding, whilst also storing and supplying 15 million litres of water for irrigation, reducing the waste of drinking water for non-consumable purposes. It has gained widespread interest from a range of stakeholders, both in The Netherlands and abroad.

The benefits of a realised pilot project extend beyond raising awareness and proving efficacy, as pilots also offer important research opportunities. With the Rotterdam case study built around this actual piece of hybrid infrastructure, it was possible to gain important insights that may not have been derived from modelling and forecasting alone. These include valuable lessons around the various motivations and disincentives for NBS uptake in the Dutch context, and the need for diverse stakeholders to work together. The smaller scale of the Rotterdam case study compared to other case studies in the NAIAD project, though posing some challenges, also made possible to highlight the similarities and differences in NBS effectiveness and viability across a wide range of spatial scales.

The broader neighbourhood assessment indicates that in certain contexts, NBS do appear able to deliver a comparable level of service for an equivalent or lower capital outlay than grey solutions, whilst also delivering a multiplicity of additional benefits. However, they often require more space, and in order to justify this use of space, it is important that the many co-benefits NBS can provide are valued and properly accounted for, despite the complexity of achieving this. Finally, the relatively high levels of uncertainty still involved with research related to the UWB type of NBS considered in this chapter (and others) can best be improved through a greater number of implemented projects. Which would be just one of many co-benefits of implementing more NBS to resolve pressing urban water challenges and design for climate resilient cities.