Flood footprint modelling to evaluate the flood risk management benefits of blue–green infrastructure

Sustainable development relies on the capacity of communities to adapt to climate change and the natural disasters related to it. Primary among these in many regions of the world is the increasing frequency of prolonged, intense rainfall that drives surface water flooding. Given that more than half of the global population now lives in cities, and that this proportion is increasing, developing new adaptation strategies for urban stormwater infrastructure requires urgent attention. Cities in the UK have long been vulnerable to surface water and coincident flooding, but piped drainage infrastructure built in the Victorian era is well beyond its design life and has insufficient capacity to cope with modern rainstorms. Replacing such grey systems like-for-like, requires ever-larger pipes and underground storage tanks, making this an option affordable only in the largest metropolitan areas. Also, in terms of cost-benefit analysis, grey, single-purpose urban drainage solutions offer only a single benefit, which is the damage cost avoided when a rainstorm occurs. In contrast, blue–green infrastructure (BGI) generates multiple co-benefits during non-flood as well as flood conditions, and may help to relieve pressure on existing grey infrastructure. When widely implemented, blue–green approaches recreate a more natural urban water cycle, which not only mitigates the impact of climate change on flood risk but also reduces water scarcity during droughts and offsets the urban heat island effect. However, the capital costs of creating integrated treatment trains of BGI and grey infrastructure may exceed those of rehabilitating piped systems, and for cost–benefit analyses to support sound investments it is vital that evaluations of both the true costs of flooding and the multiple co-benefits of blue–green approaches are reliable and accurate. In this context, a new method is proposed in this chapter: the flood footprint (FF). To evaluate the true costs of flooding, the FF uses damage functions based on flood simulation models to assess the direct economic impacts of flooding, and a macro-economic model to appraise the indirect impacts based on wider disruption to commerce and manufacturing supply chains. To demonstrate the FF, the method is applied to assess the economic benefits of hypothetical BGI in Newcastle upon Tyne, UK, under six different return period floods. The results show that the implementation of blue–green approaches would provide co-benefits and costs avoided under all six events. The cash value of these benefits ranges from £33 million for a 2-year return period flood to £560 million for a 200-year return period event. Flood risk benefits related to the avoidance of indirect flood damages contribute 23–96% of the total avoided costs. The analysis reveals that most of the indirect benefits are generated in business sectors that are not directly protected from inundation by BGI, but that depend on the continuous functioning of other related sectors during and following a flood event.