A multi-level, multi-scenario perspective on the interplay between urban planning and flood risk management

Worldwide, urban planning has a prevailing influence on the evolution of flood risk. At the municipal and regional levels, bans (or restrictions) on new developments in flood-prone areas enable mitigating increases in vulnerability. At the local level, flood-sensitive urban design contributes to lowering the residual risk, by acting on both the hazard and the vulnerability. The risk-reduction potential of various urban planning policies may strongly depend on the type of flooding (e.g. riverine vs. pluvial). In recent research, we shed light on the effects of contrasting urban planning policies at the regional and local levels, considering both riverine and pluvial floods.
First, at the regional level, we looked at the effect of “sustainable urban planning” compared to a “business-as-usual” approach for the case of riverine floods. While the latter is characterized by substantial urban sprawl, the former tends to promote more compact developments, such as densification of already-urbanized areas. We used a combination of agent-based and cellular automata models to simulate urbanization and densification over the next decades in the southern part of Belgium [1]. For this case study, all scenarios based on “sustainable urban planning” lead to values of future flood risk comparatively higher than in the “business-as-usual” scenarios. This results from the concentration of existing urbanized areas in the lower part of the valleys, given the historical appeal of the rivers for economic activities and transportation.
At the local level, we used procedural modelling to compare thousands of different layouts of buildings in terms of their influence on flood hazard. Focusing on the case of riverine floods typical of lowland rivers (mild slope, relatively long flood waves), we found that maximizing connectivity throughout the urbanized area enables mitigating to some extent the detrimental effects of developments on the upstream areas [2]. Again, this contrasts with overarching principles of sustainable urban planning, which recommend avoiding voids in-between buildings, hence hampering flow connectivity. The analysis was recently extended to pluvial flooding, for which the conclusions drawn for riverine floods are not directly transferable.
Overall, these results suggest that sustainable urban planning is certainly the way to go; but some of the underlying principles need to be modulated when it comes to developments in flood-prone areas. This emphasizes the strongly interdisciplinary nature of sustainable urban design and of building resilience into urban systems. While the results so far were harvested by coupling advanced computational approaches in urbanization modelling and inundation modelling, a next stage in the research will consist in observation-based verification of the findings.

References
[1] Mustafa, A. et al. (2018). Effects of spatial planning on future flood risks in urban environments. J. Environ. Manage. 225, 193–204.
[2] Bruwier, M. et al. (2018). Influence of urban pattern on inundation flow in floodplains of lowland rivers. Sci. Total Environ. 622-623, 446–458.