Effects of urban structures on spatial and temporal flood distribution

Urban pluvial flooding is a modern, growing global disaster, particularly in developing countries with inadequate infrastructure. It remains a challenge to accurately model the runoff behavior in urban areas with a complex topography and to quantify the impact of spatial urban patterns on changing urban rainfall-runoff response. The question to be addressed is how varying the urban spatial configurations can quantitatively influence the overland flow response in relation to the spatiotemporal hydrodynamic variables such as water depth, velocity, and outflow discharge. We use a 2D shallow water model to indicate the influence of changing spatial urban factors (such as the orientation of streets and buildings, and adding sidewalks) in small idealized (synthetic) urban catchments during a single pluvial flood event. The domain layout extends over a size of 267.5m*267.5m with a 3% longitude slope. We differentiate mainly between two street networks: i) the two-way main street with of 14-m width with sidewalks, and ii) side streets of 10m width (Fig.1). We then define novel spatially integrated indicators over the domain at the steady state to analyze quantitatively runoff variables in correlation with the urban features (Fig.1). Additionally, local hotspot maps were created to assess the flood-risk thresholds, such as human stability and failure of buildings. Hotspots are defined as the places with the highest flow velocity magnitudes and water depths (> 90%). The results of the modeling showed that, with respect to the flow velocities in small-scale urban catchments, the main street layout is the dominant urban factor, followed by the side street widths, which were decisively determined by the geometry of the sidewalks. The comparison with real flood risk thresholds shows that the lower part of the main road is the most sensitive to flood risk in the domain with a high-risk hazard for human stability. However, the riskiest case is not corresponding to the fastest hydrograph response. Varying the spatial urban configurations, especially the rotation of the main roads, changes the flood risk thresholds and delays runoff. On the other hand, spatially integrated indicators of the flow variables in the domain are showing low sensitivity to the spatial urban features. Our findings offer a new important perspective on the development of urban flood risk assessment, especially for rapidly urbanizing cities, and provide a better understanding of the spatiotemporal rainfall-runoff generation in a small urban catchment considering the spatial layout of the urban structures.

Fig.1 Overview of the modelling approach and evaluation of the runoff data