By Professor Peter Steen Mikkelsen and Nadia Schou Vorndran Lund

Peter: For me, the story of this paper started some 12 years ago. This was around the time where pluvial flooding started occurring in Europe much more frequently than in the previous decades, and where water professionals in Denmark started using the term “cloudburst” for short, intense rainfall events that cause flooding and disrupt critical functions in the city. I was teaching a course on Environmental Management and Ethics at DTU with Dr. Govert Geldof, who with his colleague Dr. Jeroen Kluck in their consultancy work in The Netherlands had discovered what seemed to be different “thinking systems” influencing the actions of different citizens and professionals involved in flood management projects. Dr. Chiara Fratini, initially a student in our course, refined these thoughts on the “Three-Points Approach”, which acknowledges three distinct domains: (a) day-to-day values, enhancing awareness, acceptance and participation among stakeholders; (b) technical optimization, dealing with standards and guidelines for urban drainage systems; and (c) spatial planning, making an area more resilient to future changing conditions. My personal take-home message from this early work was that water solutions must “provide value” in a manner that is understandable to people across all these three domains, if we want water to be appreciated at an appropriate level in urban planning. My colleague Prof. Karsten Arnbjerg-Nielsen included similar thoughts in his work on cost-benefit assessments of climate change adaptation measures.

Peter: In parallel, I had started working with other colleagues on real-time modelling, forecasting and control of urban drainage and wastewater systems. This work was also driven by the increased occurrence of pluvial flooding but in addition by a need to decrease the emissions of pollution from combined sewer systems to receiving waters. All this work focused on optimization of below-ground, piped infrastructure. At one point, I was invited for a conversation with anthropologists and architects as part of an innovation-programme funded by the Danish philanthropist association Realdania. We initially discussed different ways of providing visual flood warnings for citizens, including things like electronic displays and manhole covers with dynamically changing colors. In the process of designing new control software solutions with my colleague Dr. Morten Borup as well as Dr. Henrik Madsen, Dr. Ole Mark and colleagues from DHI, we however had difficulties developing realistic control scenarios that involved stormwater above ground, and to connect this with the broader smart cities agenda. We clearly needed input from someone young and energetic to help pushing the boundaries of our work at this stage.

Nadia: I was involved in the innovation-programme during my Master’s studies in 2015 and wrote my thesis about real-time control (more exactly model predictive control) of urban drainage systems. The aim was to minimize the emissions from a combined urban drainage system to receiving water bodies. Even though real-time control has the potential of reducing this overflow, it became clear that the effect of this control is limited by the capacity of the underground infrastructure. When the underground system capacity is used, there is nothing more to do and an overflow will inevitably happen (Figure A).

Nadia: I started a PhD immediately after my Master’s thesis and began developing the concept of keeping stormwater above ground by controlling the inflow of stormwater from urban surfaces to the sewer system. The question remained what above-ground infrastructure was suitable for this purpose. Currently, many cities are implementing green infrastructure and other forms of local stormwater control measures, and controlling them has also been proposed (Figure B). Like the underground sewer system, they however only have a limited capacity.

Nadia: Living in Copenhagen, I see the huge projects aimed at making the city cloudburst resilient, and I know there are more underway. We thus started considering the idea of using the cloudburst infrastructure more often than originally intended, in this case for mitigating overflows during rain events much smaller than what the cloudburst infrastructure is designed for (Figure C). There already exists sketches of the cloudburst management plans, which enabled us to carry out a proof-of-concept study, showing that overflow could indeed be reduced or entirely eliminated by means of integrated stormwater inflow control. Throughout the project and the writing of the paper, many fruitful discussions led to the insight that this concept could not only be used to improve the status of the local environment, but also to decrease the cost and global environmental impacts related to construction work. Furthermore, experiences from Danish cities has shown that management of stormwater in the urban landscape can also increase the liveability and provide citizens with knowledge about stormwater – hereby counteracting the “out-of-sight out-of-mind” thinking that is so evident in the water sector.

Peter: It is often argued in literature that multifunctionality and transdisciplinary collaboration is needed to develop future cities that are more liveable, smart, resilient and sustainable. The presented concept of integrated stormwater inflow control aims at exactly this. We are aware that the idea goes against traditional thinking in urban drainage and discuss many of the pros and cons in the paper. We hope that our paper will initiate fruitful debate within the wider community concerned with urban water.