Pluvial flooding in urban areas drained by storm sewer networks is characterized by surcharge-induced inundation. Urban inundation models need to reproduce the complex interaction between the sewer flow and the surcharge-induced inundation to make reasonable predictions of the likely flood damage in urban areas. In the framework of the present work, the storm sewer model SWMM5 and a newly developed two-dimensional (2D) noninertia overland-flow model have been coupled to simulate the interaction between the sewer system and the urban floodplain. The solution of the 2D model is on the basis of an alternating direction implicit scheme that solves the 2D noninertia free-surface shallow-water equations. For accuracy reasons, the time step is limited and controlled by the use of iteration to home-in on an accurate solution at each sweep. The dynamic interaction between the two models is bidirectional, and the interacting discharges are calculated according to the water level differences between the flows in the...

There is growing evidence that traditional response to floods and flood-related disaster is no longer achieving desirable results. Nature-Based Solutions (NBS) represent a relatively new response towards disaster risk reduction, water security, and resilience to climate change, which has a potential to be more effective and sustainable than traditional measures. However, in practice, these measures are still being applied at a slow rate while traditional grey infrastructure remains as a preferred choice. This can be attributed to several barriers which range from political and governance to social and technological/technical. More generally, there is a lack of sufficient knowledge base to accelerate their wider acceptance and uptake. The present work provides contribution in this direction and addresses the question of effectiveness of different types of NBS (i.e., small- and large-scale NBS) and their hybrid combinations with grey infrastructure. The work has been applied on the case of Ayutthaya, Thailand. The results suggest that the effectiveness of small-scale NBS is limited to smaller rainfall events whereas the larger (or extreme) events necessitate combinations of different kinds of measures with different scales of implementation (i.e., hybrid measures).

Urban flooding has become an increasingly important problem and a growing social issue around the world. Since it continues to be regarded as an almost inevitable danger, the development of cost-effective flood management strategies has become of the utmost importance for many cities, and particularly for those cities in developing countries where the financial resources for recovery from flood-related disasters are almost nonexistent. As a consequence, there is nowadays a range of modelling approaches available for modelling floods and developing flood management strategies. Typically, one-dimensional (1D) hydrodynamic models are used as a standard industry practice. More recently, however, model formulations have included a 1D representation of the main channels and a 2D representation of the floodplains. Since the physical process of describing exchanges of flows with the floodplains can be represented in different ways, the predictive capability of the different modelling approaches can also vary. The present paper explores the difference in predictive capabilities of 1D and 1D-2D modelling approaches for the purpose of urban flood analysis across irregular terrains. It is shown, as expected, that in the case of terrains suited to exclusively 1D models the prediction of flow variables along the channel can be realistic, but that, when it comes to the projection onto a 2D map, the representation of the terrain topography together with the mapping techniques that are employed introduce a limiting factor in their successful application. Practical implications of these two modelling approaches are analysed and illustrated on a case study from St Maarten, NA. The results of this study provide users of numerical models with information that can be used to aid them in determining which tool to use and which aspects to consider in order to make more reliable analyses of flooding processes in urban areas.

This research proposes a holistic approach to flood risk assessment that combines quantitative and qualitative aspects. This approach was developed and applied in the Ayutthaya region in Thailand, which is a UNESCO World Heritage Site. First, flood risk was assessed traditionally as a product of hazard and vulnerability. Both qualitative and quantitative data were gathered from publicly available sources and through interviews, questionnaires, and focus group discussions to assess the vulnerability, using various weights for the different vulnerability dimensions. The hazard was assessed using a coupled 1D-2D flood model, and the resulting vulnerability and risk were mapped. Second, an alternative flood risk map was produced based on group mapping exercises with local residents, which captures the level of perceived risk. The traditional flood risk map was adjusted by varying the vulnerability weights to better match the perceived risk map. The analysis of these two maps revealed that two approaches to flood risk assessment can be used effectively in gaining different insights of the phenomena, and as such, they both should be used in flood risk management planning.

Nature-based solutions (NBS) are inspired and supported by nature but designed by humans. Historically, governmental stakeholders have aimed to control nature using a top-down approach; more recently, environmental governance has shifted to collaborative planning. Polycentric governance and co-creation procedures, which include a large spectrum of stakeholders, are assumed to be more effective in the management of public goods than traditional approaches. In this context, NBS projects should benefit from strong collaborative governance models, and the European Union is facilitating and encouraging such models. While some theoretical approaches exist, setting-up the NBS co-creation process (namely co-design and co-implementation) currently relies mostly on self-organized stakeholders rather than on strategic decisions. As such, systematic methods to identify relevant stakeholders seem to be crucial to enable higher planning efficiency, reduce bottlenecks and time needed for planning, designing, and implementing NBS. In this context, this contribution is based on the analysis of 16 NBS and 359 stakeholders. Real-life constellations are compared to theoretical typologies, and a systematic stakeholder mapping method to support co-creation is presented. Rather than making one-fit-all statements about the “right” stakeholders, the contribution provides insights for those “in charge” to strategically consider who might be involved at each stage of the NBS project.

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.