Best Technical Knowledge is the application of grey AND blue-green technological studies, in combination, to existing infrastructure to find the best solution. The EPA publication "Green Infrastructure Factsheet" [GAI08] states (note "green" in the US is termed "blue-green" in the UK"):
"If, for example, a community does engineering analyses to plan grey infrastructure, sized to achieve high levels of control, and then adds green infrastructure as a layer near the end of the planning process, the community may conclude that green infrastructure does not appreciably increase the level of control. However, if planning specifically encompasses green and grey infrastructure together throughout the process, it is likely the planning will reveal many opportunities to use green infrastructure to keep water out of the system in some or all sewersheds."
An optimum solution for a particular city/region/street requires a cost benefit study which may require grey and blue-green investment with sustainability being a Best Practice guiding principle. Separation of hard infrastructure from "greener" solutions may not be the best answer compared to a combination of the two.