Retrofitting urban drainage infrastructure: green or grey?

This paper explores the approaches that London, Glasgow, Washington DC and Philadelphia have taken in responding to urban stormwater and combined sewer overflows challenges. In brief, London has begun construction of a large interceptor tunnel with relatively little attention yet paid to green infrastructure, Philadelphia is pursuing green infrastructure based approaches, and Washington DC and Glasgow are implementing a combination of solutions. The cases illustrate that a diversity of responses are emerging to a common environmental problem in cities around the world.