A range of solutions to future flood risk are available ranging from blue-green infrastructure (BGI) as commonly incorporated in sustainable drainage systems (SuDS) to traditional grey infrastructure (e.g. pipe networks, storage tanks, flood walls). Each offers a different profile with respect to costs, flexibility of implementation and the ability to deliver a range of wider benefits beyond their flood protection function. An important question that must be addressed when considering these approaches is what is the most suitable mix of grey and blue-green solutions to urban flooding at any location and at any future time? This paper uses an adaptation pathways approach to compare a range of alternative options to deal with current and expected future flood risk in part of a London borough. Solutions considered separately and in combination include grey pipe expansion, bioretention cells, permeable pavements and storage ponds. A methodological framework combines a range of existing tools to develop, assess and characterize each pathway, including a storm water management model (SWMM), a SuDs opportunity selection tool, an adaptation pathway generator and the CIRIA B£ST tool for monetizing multiple benefits. Climate change is represented by the UK Water Industry Research method for establishing future rainfall intensities for sewer and BGI design. The results showed that by extending the way in which adaptation pathways are compared and evaluated through the wider consideration of multiple benefits there is a trade-off between deferring interventions until they are needed for flood risk mitigation and delivering the multiple benefits associated with interventions so that performance thresholds do not need to be met before introducing new options. The relative contribution of each option's capital and operation and maintenance costs has implications on when the option is implemented as well as the rate of implementation. The monetization of the multiple benefits associated with each pathway shows that their economic co-evaluation alongside infrastructure costs can change the preference for one pathway over another. This article is part of the theme issue ‘Urban flood resilience’.

Preliminary results of the UK Urban Flood Resilience research consortium are presented and discussed, with the work being conducted against a background of future uncertainties with respect to changing climate and increasing urbanization. Adopting a whole systems approach, key themes include developing adaptive approaches for flexible engineering design of coupled grey and blue-green flood management assets; exploiting the resource potential of urban stormwater through rainwater harvesting, urban metabolism modelling and interoperability; and investigating the interactions between planners, developers, engineers and communities at multiple scales in managing flood risk. The work is producing new modelling tools and an extensive evidence base to support the case for multifunctional infrastructure that delivers multiple, environmental, societal and economic benefits, while enhancing urban flood resilience by bringing stormwater management and green infrastructure together.

Urban Resilience has recently emerged as a systematic approach to urban sustainability. The malleable definition of resilience has rendered its operationalisation an intriguing task for contemporary cities trying to address their organisational problems and confront uncertainty in a holistic manner. In this article we investigate the implementation challenges emerging for Resilient Strategies by the inattention paid to urban geological risk. We conceptualise urban geological risk as the combination of urban geohazards, geological vulnerability and exposure of the built environment and focus on the case study of Thessaloniki, Greece, a city that joined the 100 Resilient Cities initiative in 2014 and published its “Resilience Strategy 2030” (RS) in 2017. After a review of the RS, historical records of natural hazard events and with evidence gathered through interviews with city officials, we emphasize on earthquakes and surface flooding as the most relevant geohazards for Thessaloniki to tackle in its journey towards urban resilience. First, we examine geological vulnerability to earthquakes in conjunction with exposure of the built environment, as an outcome of ageing building stock, high building densities and the urban configuration, in Acheiropoietos neighbourhood, within the historic centre of the city. Then, we explore geological risk to surface flooding in Perea, in Thermaikos Municipality, with a particular focus on flash floods, by demonstrating how limited consideration of local geomorphology as well as semi-regulated urban expansion and its limited connection with emergency planning increase exposure of the built environment to surface flooding. Finally, we come up with the major implementation challenges Thessaloniki’s RS faces with regard to urban geohazards.