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MPhil in Engineering for Sustainable Development

global challenges, engineering solutions

Studying at Cambridge

Ceridwen Salisbury

A Real Options Approach to Balancing Blue-Green and Grey Urban Drainage Solutions

While there is a growing movement away from conventional ‘grey’ drainage infrastructure (i.e. largely concrete, piped networks, combined sewers and centralised treatment), ‘blue-green’ infrastructure/ Sustainable Drainage Systems (focussing on treatment at source and trying to replicate pre-development conditions) is unlikely to replace it entirely – at least within the foreseeable future. Establishing a#balance between the two in both existing and new developments should be an important aim in urban drainage planning, within the wider aim of achieving longterm sustainability.
At the same time, planning for the future depends on a range of uncertainties and risks. In terms of urban drainage planning, these uncertainties include climatechange and the associated impacts, technical uncertainties around infrastructure performance, and socio-political uncertainties such as urbanisation trends, institutional support and public acceptance of alternative approaches.


The Real Options (ROs) approach allows for future uncertainties by considering various future scenarios and developing action-paths or sets of options accordingly.


Using this method to compare different approaches to urban drainage (i.e. grey, blue-green or a balance of the two) allows for both current and possible future conditions to be taken into account. The value of flexibility can thus be determined by comparing the project value of adaptive solutions with that of static solutions.


Depending on the level of uncertainty in the future, and the cost of introducing flexibility into projects, this value might or might not be sufficient to warrant the adoption of adaptive solutions.


A specific site (the UKcric building on the West Cambridge site) is being used to verify the method, but the intention is that ultimately the method will be developed to the point where it can be applied to a wide range and scale of situations, and to help engineers and decision-makers to implement more sustainable and effective solutions to urban drainage.

Key Research Questions
· How much grey and how much blue-green infrastructure should be installed in urban developments?
· To what extent is this influenced by uncertainty in future conditions?
· What is the value of flexibility in planning urban drainage for the cities of the future?