Some examples of BGI include rain-gardens, bioretention cells, green and blue-green roofs, permeable pavements, Sustainable Drainage ponds and managed wetlands. While these features are developed at different scales, from the individual building through to city scale, what is common among these infrastructural options is the fact that they leverage the natural processes of attenuation and infiltration. This is the opposite approach to what we are used to seeing in cities, i.e. impermealizing the cityscape and building underground pipes that collect stormwater to drain the city as fast as possible.

While BGI clearly appears attractive and delivers multiple socio-environmental benefits, the paradigm shift from the drained city to the sponge city will not become a reality until we demonstrate that BGI works and actually works better than traditional grey infrastructure with regards to flood risk reduction. To date, engineers feel very comfortable modelling the hydraulic performance of pipe networks in drained cities (see Figure 1); this is not yet the case for sponge city BGI.

Robust evidence base is needed for both (a) retrofit environments where the option to upgrade pipes is compared against the option of introducing BGI features, and (b) new-built environments where planners and developers design and construct from scratch.

Blue-Green roofs potential contribution

Against this background, this article focuses on the assessment of the hydraulic performance of Blue-Green roofs in the City of Amsterdam, as a state-of-the-art BGI. It aims to answer questions relating to the level of flood risk mitigation they offer. Based on this discussion, it is easy to draw parallels to other decentralised BGI options which can be used as alternatives or in combination to BG roofs.

Understanding flood risk mitigation potential offered by the roofs becomes easier if we breakdown the problem to some simple questions: 

• What is the baseline flood risk? 
• How does climate change affect baseline flood risk?
• What are the suitable sites for blue-green roofs? (new or retrofit)    
• How do we measure the contribution of each roof or network of roofs?
• Going beyond static flood risk management, how do we dynamically manage networks of blue-green roofs?

Understanding the baseline flood risk

The first step to understanding baseline flood risk is collecting historical flood information. Where flooding happens is a useful information to have. We also need to establish the associated storm intensity (i.e. precipitation depth for a specific duration) and return period, i.e. frequency of event occurrence.  

One then has to understand how water behaves within the sub-catchment under evaluation. A few possibilities exist:

•  One possibility is that water just didn’t have enough time to infiltrate and created localised flooding. 
• Alternatively, as happens in many cases, water arises to the surface at the location that is most prone to flooding: as precipitation converts to runoff upstream, it is routed overland or in pipes towards that vulnerable spot. As runoff is too much for those pipes to carry, water will surface through manholes and storm drains (see Figure 2). Typically, this will happen at the flatter areas of the catchment.
• Another case is when runoff in pipes find a constriction or bottleneck and water backs-up. 

These different possibilities for flood occurrence can be modelled in urban flood simulation software. The choice of software varies depending on the degree of detail required, type of problem, computational effort, area covered, spatial and temporal resolution of available data among other factors. Typically, a distinction is made between 1D models of the underground system, and 2D models for the surface component. 1D models are simpler, and can be used to identify capacity problems for instance for manhole locations. The US Environmental Protection Agency provides a (free) solution engine on which several other simulators are based.  Coupling 1D/2D allows for modelling the resulting surface flood from this capacity exceedance. For instance, Infoworks ICM is used by Waternet, the water authority in Amsterdam and partner in the RESILIO project, as in the example shown in Figure 3.

Accounting for the impact of climate change on the baseline

While an area might be at low flood risk today, as demonstrated through modelling and absence of historical flooding, this is not necessarily the case for the future. Running simulations for the impact of future climate scenarios (low/medium/high) on precipitation patterns can shed light on potential future flood prone areas.

Identifying suitable spaces for blue-green roofs

The next stage is identifying suitable sites for the implementation of blue-green roofs. The selection is based on a number of criteria, e.g. the size and slope of the roof, the location of the building to make sure retention would make sense there (see next step), or even the alternative BGI options in the vicinity of the building. Cost of retrofitting as well as maintenance issues/costs are also critical. A multi-criteria GIS-based analysis can yield quick assessment results. An example of such a GIS-based quick assessment is shown in Figure 4, where slope, size, year of build, and ownership yields different suitable areas. Such a quick scan should always be followed by a detailed building-level assessment, but provides a first indication of suitability of BGI.

Quantifying the contribution of blue-green roofs

This is the most critical part of the assessment: against the baseline, RESILIO explores what is the potential impact of the Blue-Green roofs. Without even performing any modelling, it is clear that there is direct benefit from water retention at the rooftops. Through retention the building “does its part” with respect to net contribution, also known as release at the greenfield rate (i.e. contributing stormwater to the network no more than if the building was not there at all). It is noted that a BG roof performs much better than a green roof; this is because it has higher storage/retention capacity, and is more responsive (i.e. infiltration rate is not a limiting factor). Figure 5 shows an analysis of how extensive BG roof coverage could contribute to flood volume reductions in the Slotermeer neighborhood in Amsterdam, one of the focus areas of the RESILIO project.

However, it is likely that a few roofs combined together can do more than just have zero net contribution: they can potentially protect the neighborhood from flooding by reducing the pressure to the sewer network. For this broader benefit one needs to carry out more detailed modelling analysis. The integrated modelling of BGI and Grey infrastructure has been demonstrated recently for a sub-catchment in South London [2] but also the US EPA provides clear guidance on how BGI components can be modelled [3].

Smart real-time management of the BG roof network 

Beyond the multiple benefits of the RESILIO roofs, the project’s approach goes beyond state-of-the-art by supporting the hydrological cycle through real time control. Within the RESILIO project, it is investigated how precipitation forecast can be used to drain water from the roof to create buffer capacity for the incoming rainfall, without compromising water availability for the green layer. After capturing the peak of the rainfall event, and when sensors in the drainage system indicate sufficient capacity, the water can be slowly released from the roof into the drainage pipes. Compared to conventional (green) roofs, this distinct smart feature of blue-green roofs boosts the effectiveness in capturing the peak of the rainfall event.

In dry periods it is crucial to keep the blue-layer closed and save all water on the roof to increase the chance of the roofs’ vegetation to survive the drought.

Click here for the full article with images

Source: UIA - Assessing the hydraulic performance of Blue-Green Infrastructure

Why engage communities in NBS development?

Unlike Gray Infrastructure which is designed to address a single technical challenge, Nature Based Solutions that are deployed to serve the very same purpose can – in principle - offer multiple other social and environmental benefits. However, for citizens to harness these benefits, their interaction with NBS needs to be carefully planned. For example, how can we be sure that a park meets the needs of the local residents?

It is, therefore, critical to have engagement methodologies in place as early as possible in project design in order to address issues of problem awareness, understand citizen priorities and maximize the acceptance and the usability of NBS.

The Blue Green (BG) roofs RESILIO project of Amsterdam has been truly proactive in this: it has developed and is currently implementing a multilevel engagement strategy (also adapted to pandemic restrictions). The activities and lessons learnt can support other Blue-Green Infrastructure projects with similar citizen engagement needs. Here we briefly present our engagement and communication strategy as well as some limitations and challenges faced that can be relevant to other project NBS implementers and practitioners.     

How does RESILIO engage and communicate with citizens?

• Develops sense of collective efficacy

The engagement strategy follows the main principles for ‘climate change learning’ to achieve a sense of ‘collective efficacy’, as described by Allen and Crowley (2017). This refers to the sense that one’s actions, in combination with the actions of the community, have the capacity to make an impact. In order to achieve this, it is important that the engagement strategy is relevantly framed by focusing on local needs and interests, should go beyond informing and include hands-on interactive activities within communities that enable learning and established a meaningful dialogue that can facilitate acceptance and consensus building.

• Identifies neighborhood characteristics

Amsterdam’s strategy includes targeted communication and participation approaches tailored to the specific neighborhood characteristics and is based on a stakeholder mapping. Targeting makes use of readily available data sources and tools that provide information on the type of lifestyles that are dominant in the specific BG roofs neighborhoods. Through such an analysis one gets an overview of the flooding hotspots in a neighborhood, the important topics concerning citizens in the neighborhood and active networks and outlets that can be used when reaching out to citizens.

• Develops creative engaging activities

The activities followed so far included presenting the BG roof through a “blue-green roof” bike tour around different neighborhoods where blue-green roofs are installed, but also approaching residents to distribute information material at the street level and organizing tailored events for citizens in the RESILIO Innovation Lab. Other activities involve art and drawing workshops with children and workshops with residents concerning urban biodiversity.

• Brings experts close to citizens

In addition, inviting experts that are involved in the project to the neighborhood events has worked well to inform residents about the multiple benefits of the BG roof approach and create a sense of trust in the project and its goals.

• Reaches local people through their own networks

In addition, through interviews with local people and writing personal articles, the ‘neighborhood stories’, we engaged on a deeper level with residents and through that we expect to reach local people in their networks, to tell them about the RESILIO project. Involving people with what happens “above their heads” is also done by creating time-lapse videos of the work in progress that we share through RESILIO and the City of Amsterdam social media accounts and local online platforms.

• Uses local neighborhood organizations in your strategy

Besides, when developing engagement activities, it is wise to involve local neighborhood organizations and organize activities together. They often have a better understanding of how to reach citizens and communities and already have a significant network in the neighborhood. It is also key for harvesting local knowledge on local needs and motivations of residents (see citizen priorities below).

Engagement challenges and limitations

The communication and participation strategy had to be adapted to the COVID-19 safety measures (social distancing, number of people in events, community centers being closed). Further meetings were moved to online, however the turnout was low. This suggests a need for additional effort and creativity when engaging virtually. Yet, those attending were actively engaged.

The project is investing extra in online recruitment and information distribution through paid social media posts and the use of the project partner’s online network. The project also makes use of the digital resources and resident letters from the housing corporations that have become even more important in the times of Coronavirus.  

Understanding citizen priorities

The RESILIO project is funded by Urban Innovative Actions Initiative (European Regional Development Fund) as a climate adaptation project with emphasis on its flood risk reduction potential as well as for improving water and energy efficiency. Yet, when consulting with residents, it appears their main driver and self-perceived benefit is urban biodiversity. This has suggested the importance of knowing your audience’s nudges. It is hoped that residents will demonstrate an increased awareness for climate action by the end of the project. On the basis of these findings the participation plans were adapted and have included more activities related to urban biodiversity.

Amsterdam’s lessons and advice to other NBS projects exploring whether and how to engage

It is not uncommon that some citizens will be more skeptical than others when it comes to the development of Blue-Green Infrastructure in the neighborhoods. This does not mean that we should avoid discussions with the community during the development of the project. On the contrary, our project so far has shown there is higher risk involved by not engaging than by engaging: involving people with what is happening during construction, creates understanding for possible nuisances during construction.

The project is working closely together with the building owners and their contractors to receive and share all relevant information about building activities and have a feedback loop to the asset owner when residents and neighbors pose questions; thus, the project has built and maintained a high level of mutual understanding and mutual respect. The responses from attendees of the neighborhood events at the Innovation Lab have been very positive. People are quite proud that their building will get such an innovative solution. Any concerns they might have regarding the effect of “water above their heads” or costs that are involved are eliminated through providing expert information.

In summary:

• expert opinion is communicated directly to citizens,
• careful two-way communication takes place through multiple means,
• understanding citizens self-perceived value
• building a sense of ownership and pride can contribute to improved awareness and acceptance of BG roofs.

Source: UIA - Citizen engagement

An unexpected opportunity

As the RESILIO project had picked up in pace on the procurement and development of the new smart Blue-Green systems, the Covid19 crisis arrived and slowed down activities. However, the social and psychological impacts of confinement precisely due to this crisis, have highlighted the urgency for more accessible space. Blue-green roofs can really contribute to this. 
Given that streets and parks as public spaces have not been designed to accommodate social distancing, many municipalities are now trying to find new accessible spaces for citizens. Private building roofs can be part of the solution spectrum when it comes to recreational space requirements.
This applies also to RESILIO whose BG roofs are on social housing corporations buildings, yet the accessibility aspect did not hold a central part in the agenda – partially due to safety and maintenance concerns. However, the InnovationLab of the RESILIO project on the Benno Premselahuis, a building of the Amsterdam University of Applied Science (HvA), is accessible to the public and used as practical example to show residents and other stakeholders in Amsterdam the benefits of BG roofs.
The questions that now arise are "How do we practically leverage this new green space?"; "Where are the best opportunities and how do we strategically plan for new green space?"

Space reassignment opportunity

Many cities have a target for maximum distance from green for all residents. The City of Amsterdam, together with Nijmegen and Utrecht, are involved in mapping projects to identify neighbourhoods that lack green spaces. Based on this information the municipalities identify the higher marginal positive effect of adding green spaces in these neighbourhoods, compared to others that already have such space.

In the Netherlands the areas that lack accessible green spaces are quite often in the historic areas and city centres. These areas have been build up over the ages, leaving very little space for green. These areas are many times also densely populated. This is an issue in the oldest part of Amsterdam, but also in, for example, Utrecht and Nijmegen.

The opportunity for space reassignment might not lie on eye level and therefore not be easy to spot. However we might find space that can be easily reassigned on roof level.

An opportunity for well-being

Due to Covid19, a healthy relationship between humans and their home or office environment needs to be enhanced. In this context, blue-green roofs can offer:

• Socialisation opportunities to combat isolation and loneliness
• Urban gardening opportunities which build community
• Exercise space at a time that our living rooms are too small for this
• Safe playgrounds for children, whose emotional wellbeing can suffer from confinement and school closures
• Space to cool down on hot summer days. If citizens are left without options to cool down, it will put an additional burden on healthcare systems as people can suffer from heat-related symptoms.

Good practices for public utilisation and citizen engagement

There are some great examples of accessible green roofs that enhance social interaction in a safe way, while also offering the opportunity to enjoy green spaces while maintaining social distancing. See below the De Boel, FLOOR, CIRCL and DakAkker Roofs - some of the finest examples for public utilisation.

De Boel roof garden in Amsterdam Zuid is a great example (see photo below). The owner of the building, Vesteda, chose to create this park on top of the flat with rental apartments when renovating. The roof is redesigned using a smart blue-green roof. The area is accessible for all tenants.

Another view of the De Boel Blue-Green Roof (photo credits: Vesteda)

FLOOR is another great example (an artist impression below). The renovation of the entire building (2017) consisted of the transformation from an office / school building to an apartment building. Two floors are added on top, with roof terraces on the top floor. In the original plans these terraces were planned on pebble roof. RESILIOS’s partner Rooftop Revolution convinced the owner to turn this into a blue-green roof and, later on, a smart function was added. The roof is accessible for the tenants on the top floor which is a huge benefit for them (adding also to the value of the property). 

A small but important detail towards the success of the FLOOR project: in the original design the fencing was planned around the individual terraces and when adding the green roof, the fencing was moved to the outside, creating the idea of a park, rather than individual terraces.

An artist impression of the FLOOR roof (Photo credits: the FLOOR project, ASR real estate)

CIRCL roof garden in Amsterdam Zuid is another success story. This is the roof of the circular pavilion of ABN AMRO Bank in Amsterdam’s financial district. The building is built on a square, a public space, but the roof itself is also publicly accessible, thus the amount of public space that was lost due to the building's construction is limited. The quality of the public space is enhanced, since the roof has a beautiful roof garden, planted only with plants meeting ecological criteria, creating a true oasis in this bustling area next to the railway station and in between high rise office buildings.

The CIRCL green roof seen from inside creating a connection with the natural and human external environment.

DakAkker in Rotterdam operates a community-run permaculture garden with a small restaurant and a smart blue-green roof. The DakAkker is about 1000 sqm rooftop farm and is the largest open-air roof farm in the Netherlands and one of the largest in Europe.  The owner’s moto is “How to create a Sustainable City together”. For this reason it showcases how vegetables, edible flowers and fruit are grown and bees are kept. With the smart roof system, it has become a test-site for smart water storage and management.

People relaxing at the café on the DakAkker blue-green roof

Greening as a strategy for social distancing and addressing isolation

A communal space on the roof can provide an easy way to get some fresh air and meet people. In these times of social distancing, some people are anxious to go outside and people who already live in relative isolation face the risk to be isolated even more. A roof provides a safe space, very close to home, where one can relax without meeting other people than their neighbors. Also for people who don’t have a garden, green roofs can provide an alternative space to start gardening or grow their own crops.

We should start seeing roofs as potential gardens and stimulate usage and greening as much as possible. There is a clear need for more green spaces in the city and this need has become even more urgent now that we need to keep social distancing regulations. Stress on space in public parks is high and sometimes parks need to be closed in order to be able to maintain the social distance regulations. If people who do not have a garden could relax on their roofs, stress on public spaces would turn a lot less critical.

In brief, perhaps it is the time to rethink how we manage our roofs and start providing those conditions (investment on safety, accessibility, landscaping) that render them accessible. This applies to our own RESILIO project and beyond.

Information: This is the second web article of a series to follow between 2020-2022 presenting ideas and news about RESILIO. Stay tuned!

Authors: Leon Kapetas is the UIA Expert for the RESILIO project (leonkapetas@gmail.com) & Jan Henk Tigelaar is the Director of Rooftop Revolution

Contact: Mr Age Niels Holstein  (a.holstein@amsterdam.nl) - project manager of RESILIO), Leon Kapetas (leonkapetas@gmail.com) and Jan Henk Tigelaar (janhenk@rooftoprevolution.nl)

1. Introduction

Unlike mono-functional Grey infrastructure, NBS are multi-functional, and as such they deliver benefits across a series of urban challenges for a number of stakeholders. In fact, alternative NBS that can be implemented in an urban context will have a different set of beneficiaries. So this raises the following question: which Grey or NBS option we should choose to capture the maximum value potential and who will benefit most from these interventions? In addition, would the selected option have the highest cost-benefit?

While valuation methodologies do exist, projects typically give limited attention to this activity or have limited capacity to apply them. This results in a limited awareness of decision and policy makers of the Total Economic Value (TEV) (Plottu & Plottu, 2007) of NBS.

In this Zoom-In article, we explore how methods of economic valuation and cost-benefit analysis can be applied to assess the value proposition of Blue-Green roofs of Amsterdam. First, some basic concepts are described to explain the rationale behind the analysis. In particular, we discuss how climate uncertainty (how different impacts arise for a low or high emissions scenarios) can have an impact on the cost-benefit assessment. Lastly, we discuss the implications CBAs can have on the upscaling of blue-green roofs across the city of Amsterdam.

2. Concepts and Methods

2.1. Total Economic Value (TEV) framework

Environmental economists will tell you that before you make your final infrastructure selection you should perform a thorough economic analysis across option. Total Economic Value is a well suited framework for this sort of analysis. Essentially, TEV is used for classifying ecosystem goods and services in order establish their value. TEV encompasses use values and non-use values (see Figure 1 - World Bank, 2004). Use value is comprised by three different value categories (not necessarily all present in all NBS projects):

• Direct use value: good and services that are used directly by humans, including consumptive uses (e.g. food products, material for construction) and non-consumptive uses (e.g. enjoyment for recreation and cultural activities)
• Indirect use value: the values derived from the ecosystem services that benefit outside the ecosystem itself, e.g. flood protection, water filtration, carbon sequestration
• Option value: derived from preserving the option to use in the future ecosystem goods and services that may not be used at present; these can be provisioning, regulating or cultural services as described in the previous categories

Non-use value, also known as existence value, is the value that people enjoy merely by knowing the resource exists even if they never intent to use it themselves.

Figure 1. Total Economic Value – reproduced by IUCN, TNC, World Bank (2004)

2.2. Economic valuations

A typical economic valuation would follow a series of steps for each alternative infrastructure option:

• Step 1. The identification of the full spectrum of benefits that the NBS can provide
• Step 2. The quantification of these benefits, i.e. how much of each benefit is actually ‘delivered’?
• Step 3. The performance of the valuation to express them in monetary terms

It is worth noting that the supply of an NBS service does not necessarily have an equal demand and the benefit is only potential, e.g. the availability of recreation areas does not translate in value if there is no direct use of it.

Two primary methods exist for a monetization exercise: firstly, the revealed preference methods which use observable choices on markets, and, secondly, stated preference methods which are used when no market of goods exist or consumer choices cannot be directly observed. In the latter case, the ‘willingness to pay’ has to be elicited from a hypothetical market. There are several techniques to actually give a price tag on benefits for either of the methods and most of them coming with their own pros and cons. These are not discussed here but useful information can be found in Horvathova (2019). Some of these techniques are employed in the economic valuation of Amsterdam’s blue-green roofs (see below).

2.3. Cost-Benefit Assessments

Once costs and benefits have been expressed in monetary terms, a cost-benefit analysis can be performed and be used for the comparison of different sort of decisions, e.g. alternative grey and nature based solutions. For projects with long-horizons as NBS infrastructure, CBAs take into account the present value of money: money in the present is more ‘valuable’ than in the future. This is to reflect the ability to have a rate of return if money was invested elsewhere. For this reason, CBAs apply a discount rate. Along this line of thinking, spending on capital or operation expenditure tomorrow is preferred. Similarly, some benefits that will be delivered in the future (e.g. carbon sequestration by trees planted today but growing over long periods in the future) are deemed ‘less’ valuable.

As CBAs are calculation intensive and introduce many assumptions, users need to take a critical look to identify where their uncertainty sources lie. This can allow them to perform sensitivity tests to obtain a range of possible results; in this way, their findings become more robust. Lastly, cost and benefits can be reported in terms of a ratio to obtain a metric that is useful to compare across options.

2.4. Accounting for Climate Risk in CBA

One of the primary sources of uncertainty in CBAs that are performed for climate resilience infrastructure is climate change itself. A fast changing climate generating more extremes is expected to make investment in a piece of infrastructure more attractive. For instance, in the case of increasing flood frequency or intensity, damage avoided (seen from a benefit perspective) thanks to the infrastructure will be more significant/frequent than in a slow changing climate, i.e. the infrastructure will do its work more often. Conversely, in a slow changing climate, the same piece of infrastructure (identical design) would have to deal with lower frequency or intensity.

Given climate uncertainty it is not possible to identify the ideal design capacity that would minimize costs and maximize benefits. This type of nuances need to be captured by CBAs through multiple scenario testing. This is relevant in the CBA performed for Amsterdam’s blue-green roofs as shown below.

3. Performing a CBA for Amsterdam’s blue-green roofs.

Against the methodological background presented, RESILIO’s CBA for the blue-green roof systems involves the following steps (see Figure 2).

Figure 2. Benefit identification, their economic valuation and cost benefit analysis framework for Amsterdam’s blue-green roofs (source: Floor Borstlap, Vrije Universiteit Amsterdam)

1. Identification and quantification of relevant costs and benefits.

The project has made significant effort in the collection of biophysical data and execution of simulation models to make a robust quantification of costs and benefits.

 

2. Valuation of relevant benefits.

Costs can be calculated with high accuracy using relevant information from the infrastructure development. On the other hand, benefits cannot be calculated using revealed preference methods as no real-life market directly captures them. The following valuations were performed:

• Direct pricing has been applied, where direct and indirect prevented damages are obtained for climate impact models. While direct and indirect damage assessments are directly related to the benefit that blue-green roofs can offer, the assessment has considerable uncertainty.
• Shadow Pricing has been used to overcome the uncertainties of direct and indirect damage assessments. Through shadow-pricing, the avoided costs (i.e., the benefit) of implementing blue-green roofs can be assessed when they are compared to the costly alternative of water storage in the public urban environment.
• Willingness-To-Pay (WTP) has been applied using surveys to establish the value of biodiversity increase, i.e., this reflects the price citizens are willing to pay for this ecosystem service.
• Benefit transfer methods, i.e., the use of appropriate values based on literature.

It is worth noting that shadow pricing is typically used for intangible assets, i.e., something not normally priced or sold in the market, but also to put a price tag on externalities. It is therefore a suitable method for the valuation of water retention and heat stress reduction. Specifically, the hypothetical market value of these benefit is derived from the cost of alternative infrastructure that could perform the same role, i.e., a grey system holding this much water.

 

3. Cost Benefits Analysis

In this final step, the cost and benefits are compared following the approach outlined above. The assessment can be performed for a middle and high-emission scenarios to establish in which possible future climate condition the benefits will outweigh the costs. In the ongoing analysis, results are established for the high-emissions scenario (see first findings below).

Also, although one might want to compare the cost benefit ratio of two alternative options (e.g., a grey option vs a NBS), the shadow pricing approach followed above implicitly takes this comparison into account.

 

4. First findings and implications

Figure 3 shows initial findings from the cost-benefit analysis performed in RESILIO. The range for the NPV shows the results ranging from low-cost/high-benefits to high-costs/low-benefits. While the technology used in RESILIO is still innovative and under development the discounted NPV shows the potential for a positive cost-benefit analysis. Reducing maintenance costs, achieving economy of scale, and using the roof in a multi-functional manner can tip the scale to a positive NPV under all scenarios.  

Figure 3: Discounted costs and benefits, and discounted Net Present Value

 

RESILIO has developed a good understanding into how sustainability criteria can be introduced in the valuation of this particular NBS intervention. It has also developed a methodology for considering climate uncertainty in a CBA. This has implications also in upscaling: as blue-green roofs form decentralized systems, the rate of their deployment can be planned to match future rate of climate change through slow or fast deployment of blue-green roofs. In essence, there is an option value in Amsterdam’s roofs which underground grey systems cannot offer.

Finally, RESILIO identified transfer mechanisms for costs and benefits. This is crucial, as the costs often lie with the roof owner, while the benefits are for society. Examples of transfer mechanisms are for instance Co-investments by stakeholders that benefit from the blue-green roofs, direct payments for maintenance by stakeholder who benefit from blue-green roofs, subsidies to stimulate the uptake of blue-green roofs This information is essential for Amsterdam in its ambition to build a business case for the RESILIO roofs.

 

References

Horvathova, 2019, An assessment of costs and benefits of nature-based solutions in cities, LIFE Tree Check

Plottu, E., Plottu, B., 2007. The concept of Total Economic Value of environment: A reconsideration within a hierarchical rationality. Ecological Economics 61, 52–61. https://doi.org/10.1016/j.ecolecon.2006.09.027

IUCN, NTC & World Bank, How much is an Ecosystem worth? Assessing the economic value of conservation, Washington, 2004