Planning nature-based solutions for urban flood reduction and thermal comfort enhancement

As a consequence of climate change and urbanization, many cities will have to deal with more flooding and extreme heat stress. This paper presents a framework to maximize the effectiveness of Nature-Based Solutions (NBS) for flood risk reduction and thermal comfort enhancement. The framework involves an assessment of hazards with the use of models and field measurements. It also detects suitable implementation sites for NBS and quantifies their effectiveness for thermal comfort enhancement and flood risk reduction. The framework was applied in a densely urbanized study area, for which different small-scale urban NBS and their potential locations for implementation were assessed. The overall results show that the most effective performance in terms of flood mitigation and thermal comfort enhancement is likely achieved by applying a range of different measures at different locations. Therefore, the work presented here shows the potential of the framework to achieve an effective combination of measures and their locations, which was demonstrated on the case of the Sukhumvit area in Bangkok (Thailand). This can be particularly suitable for assessing and planning flood mitigation measures in combination with heat stress reduction.

There is an increasing awareness that the interplay between the supposed effects of climate change and global warming combined with rapid and uncontrolled urbanization can lead to serious challenges to urban water managers and city planners. Since the vegetation coverage and green areas are decreasing significantly, the imperviousness rate in different urban areas is increasing . As a consequence, many cities will deal with less reliable drainage systems, more flooding, extreme heat stress and droughts. Urban flooding leads to numerous direct and indirect impacts, and it causes high social, environmental and financial damages to the more vulnerable and less prepared cities around the world. Heat stress is considered to be a phenomenon induced by a hot atmospheric condition, implying an increase of heat-related mortality and morbidity. The increase in urban air temperature can affect human well-being and energy consumption due to the need for extra cooling. Therefore, in order to reduce the vulnerability and increase the capacity of cities to cope with these effects, a paradigm shift in the management and design of urban water systems is required. In this new management approach, multifunctional designs will deal with multiple hazards, meaning that the hazards are not targeted individually, and therefore that urban water systems will now deal with multiple challenges at the same time. In urban drainage management, similar structures are named differently. For instance, green infrastructure (GI), best management practices (BMP), low impact development (LID), water sensitive urban design (WSUD), sustainable drainage systems (SuDS), ecosystem-based adaptation (EbA) and nature-based solutions (NBS), are broadly used. In this work we use the term nature-based solutions. NBS is a relatively new concept; it comprises solutions inspired and supported by nature, which provide multiple benefits and help society to adapt to climate change. Several studies have investigated the effectiveness of NBS for different aspects in urban areas. In particular, several works focus upon the application of NBS to achieve multiple benefits at the same time, e.g.,. Furthermore, several studies have assessed the effectiveness of NBS measures separately on either urban flooding, e.g., or on heat stress, e.g. However, urban flooding and heat stress frequently occur simultaneously, and NBS have the potential to be effective in mitigating both. To the best of our knowledge and the literature review to date, there are no reports of an integrated (combined) assessment using quantitative effectiveness of NBS measures for both flooding and heat stress mitigation. Moreover, a limited number of works studied the effectiveness of these measures in a highly dense urban area of a tropical environment. Further to the above, there is a need to undertake more studies towards the understanding of interactions between different hazards and how they shape vulnerabilities and risk. This can help city planners to make better decisions, and to gain a better understanding of how urban development on one site can influence vulnerability on the other site, and how both of them can happen within the same urban area. Such understanding can lead towards a better identification of locations where mitigation strategies can contribute more efficiently in achieving sustainable urban conditions. The present work provides a contribution in this direction, and it presents a novel framework for the selection and location of NBS to achieve both urban flood reduction and heat stress mitigation. This framework was applied in a case study area in Bangkok (Thailand) through the application of a macro scale model for urban flooding, and a micro scale microclimatic model for human thermal comfort.