Assessing the interplay of topography and urbanization on surface runoff: Modelling overland flow in synthetic urban structures as affected by terrain shape and steepness

Urbanization is a global phenomenon characterized by the rapid expansion of urban areas, particularly into steeper terrain, affecting the hydrological cycle by increasing paved areas and surface runoff. The risk of occurrences and severity of flash flooding in both urban and surrounding rural regions may therefore also be increased. While river discharge data may reveal the large-scale effect of urbanization, detailed information on the sensitivity of small-scale changes to the hydrological cycle is generally unavailable. Spatial and temporal changes to catchment properties at multiple scales are necessary to better understand flood dynamics and risk. The objective of this study is twofold: (i) develop a method to assess the impact of the interplay between urbanization and topography on surface runoff and (ii) provide numerical case studies of urban surface runoff focussing on the effect of slope shape and steepness.

We define urban structures by the arrangement of road networks, buildings, and green space distribution at the macroscopic scale complemented by microscale features such as sidewalks and the spatial variability of infiltration properties. Urban structures have been generated by representing those spatial features as digital elevation models (DEM) on flat terrain coded in R. This DEM is then merged with landscape DEMS by overlaying both.  Different urbanization scenarios are being assessed by modeling surface runoff using the 2D shallow water equations under uniform rainfall events. Because global urban expansion is showing an increasing trend to develop over mild to steep slopes, we focus our analysis on the effect of slope shape and steepness over different spatial scales on spatial dynamics of surface runoff. This approach enables us to provide quantitative insights into the sensitivity of local and global runoff dynamics. The effect of urbanization is being described by gradually increasing the fraction of urban land coverage. The effect of large-scale (urban fraction, slope steepness, and shape) and small-scale changes (urban forms arrangement, presence and absence of sidewalks, spatial variability of infiltration properties) are analyzed by integrated spatial indicators such as the distribution of velocity and water depths, and hot spots maps of high velocity and depth, which are related to large scale indicators such as peak flow and time to peak of the discharge hydrograph.

Findings of this research point to the critical role of spatial scale in urbanization together with topography features and its profound impacts on runoff dynamics and infiltration. The interplay between large-scale and micro-scale factors helps to identify how the adjustment of small-scale features affects peak flow and high-risk hot spots. Slope shape analysis has indicated that concave slopes behave differently from uniform and convex slopes, with maximum velocities occurring on midslope depending on average steepness and curvature. Implications for urbanization are being outlined.