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Environmental impact of NBS: assessing environmental performance of NBS & potential trade-offs

Dernière mise à jour : 26 févr. 2020

Nature4Cities project aims to deliver a holistic approach for assessment of Nature Based Solutions (NBS) which is comprised of environmental and socio-economic evaluation of NBS cases supported by a platform to assist urban decision makers.

The environmental platform module will be complementary to Nature4Cities monetary value scale and the quality of life scale.

Nature4Cities Environmental Assessment aims to:

- help decision support during planning phase of NBS projects for selection of NBS alternatives with the highest potential benefit and the smallest environmental footprint.

- evaluate the performance of NBS during their implementation phase as well as adverse (negative) environmental impacts during their whole life cycle

In order to accomplish these objectives, a two stages evaluation strategy was developed. It involved the modelling of the urban ecosystems and (1) a quantitative analysis of benefits, accompanied by performance monitoring of indicators and (2) a quantitative assessment of environmental impacts.

An approach based on Urban Metabolism

NBS can reduce consumption of resources, improve the degree of self-sufficiency and resilience of the cities in multiple ways affecting a variety of urban sub-systems. To be able to assess their effective impact, we needed to find an appropriate approach to model, analyze and quantify the benefits of NBS.

New governance, business, financing models and economic impact assessment tools for sustainable cities are needed. We developed and validated analytical frameworks and methodologies to characterize nature-based solutions and assess their effectiveness (accounting for both environmental benefits and possible negative impacts). This answers also to the need to strengthen knowledge of the multiple benefits of nature-based solutions and trade-offs. Our methodologies ensure a comprehensive assessment of environmental impacts of NBS systems (including possible negative impacts), and the identification of potential shifting of burdens on their life cycle.

To be able to analyze the complex interactions of various urban systems, an urban metabolism approach was adopted. Urban metabolism is a broad range of quantitative methods that attempt to conceptualize urban areas as organisms, requiring goods and energy to maintain functionality and support growth, while emitting waste as a by-product. It includes operating and investment cycles continuously interacting not only with each other but also with environmental and societal systems.

These interactions are measured through urban metabolic flows of materials and energy that are either continuous (i.e. operating flows) or accumulating/disappearing (i.e. stock flows of investment cycles) in nature.

Although urban metabolism has already been employed to assess sustainability and resource efficiency of cities prior to this project, Nature4Cities is the first project to connect urban metabolism and NBS. In this sense, not only Nature4Cities creates an extension to the field of applicability for urban metabolism, it also introduces a new approach for assessment of NBS in a systematic way.

18 indicators calculated through urban flows to help decision makers

We worked to draw the system boundaries of NBS related urban metabolism models (urban flows linking metabolic processes associated with operating and investment cycles).

These urban metabolism models will be used during quantitative assessment of material and energy balances through Material Flow Analysis (MFA) to estimate urban flow indicators. .

Headline urban indicators were provided for each NBS, to optimize the data acquisition phase and quantification of urban flow indicators thus streamlining the environmental assessment to identify the smallest set of indicators capable of providing enough information to the decision makers. Overall, 18 indicators were determined to form the list of headline indicators for NBS within the environmental framework.

Next, list of urban flows involving all the material and energy flows necessary to calculate the complete set of urban flow indicators for environmental assessment of NBS was obtained. All flows are found to be valid for object, neighborhood and city scales with varying relevance. They are consisting of different streams including input flows such as energy demand, output flows such as air pollutants released and finally a special set of flows avoided in terms of resource and energy consumption or emissions and waste.

The list covers a total 64 distinct yet complementary flows, which make up the first component of system boundaries.

Helping decision-makers choosing the most environmentally friendly solution to tackle the urban challenges they are facing

The urban metabolism assessment was then combined with Life Cycle Assessment (LCA) to balance the benefits of NBS during their implementation phase with their “environmental cost” during their whole life cycle.

LCA captures the ‘‘upstream’’ and ‘‘downstream’’ environmental impacts associated with resource flows and emissions that extend beyond urban borders and allows to identify pollution transfers from one life-cycle stage of a system to another, or from one environmental impact category to another. For instance, whenever transportation activities were relevant for an NBS, the processes starting from the cradle phase of fuel extraction, refining and processing were also included in the system boundaries in addition to immediate emissions created by the vehicles.

The literature review showed that although LCA studies have been already conducted on one type of NBS or another, no one ever proposed to adapt the LCA framework to the assessment of multiple types of NBS, making possible the comparison between very different NBS. In this sense, Nature4Cities is a pioneer in proposing to set up a LCA framework for the assessment of NBS in a systematic way for the first time, with the ultimate purpose of helping decision-makers to choose the most suitable solution to tackle the urban challenges they are facing, while taking in consideration possible environmental burden shiftings.

The LCA methodology was also completed with an environmental performance evaluation methodology based on Material Input per Unit of Service (MIPS), which assesses the material input(s) for a product from cradle to grave and per unit of service delivered by this product.

As a proof of concept, the whole environmental methodology was applied on a green façade case study. The successful application to a real case study demonstrated the applicability and the relevance of the developed environmental methodology. The calculated LCA indicators and additional KPIs provided insight on the environmental performance of this NBS implementation as well as potential trade-offs.

In conclusion, the urban flow indicators quantified through MFA along with the life cycle indicators quantified through LCA provide insight on multiple benefits of NBS as well as potential trade-offs. Entire set of indicators sheds light on the Pressure-State-Impact/Benefit and Response relationship of NBS deployment

A methodology leading to a dynamic assessment tool

This method not only helps addressing operation and investment cycles of urban metabolism, but also facilitates dynamic assessment methodology developed Nature4Cities. Dynamic environmental assessment of NBS is envisioned to involve successive execution of the urban assessment for the NBS cases modelled through urban metabolism and climate modelling with respect to appropriate time resolution. This approach will enable monitoring of the Nature4Cities environmental indicators over-time, which would be useful during monitoring stage after NBS deployment.

The environmental assessment module of the Nature4Cities platform will allow evaluating the environmental impacts of NBS through scenarios defined by the platform user.

Discover more publications and results on our publications and results page.

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