Evaluating the Influence of Urbanization on Slope Stability: A Case Study on Ischia Island

Physically-based models used to assess slope stability are typically based on simplifying assumptions, such as the absence of anthropogenic structures, which are, in reality, often present on slopes. However, in many urbanized areas, the presence of buildings, roads, and other infrastructures can significantly affect slope stability, both in terms of load and alteration of water flow dynamics. Built structures modify surface water flow, hindering proper infiltration and increasing the occurrence of landslides. Furthermore, an additional factor to consider is the management of water that accumulates on buildings. The malfunction or damage of the sewer system, or in some cases, the absence of an appropriate drainage system, can further influence slope stability [1]. Moreover, the added weight of these structures, particularly when not properly founded on resistant layers, increases the forces acting on the slope, further compromising its stability. Despite this, most slope stability models do not account for the effects of these infrastructures, limiting their applicability in urbanized, sloped areas [2].
This study aims to address this gap by examining how urbanized areas influence slope stability, with a particular focus on small constructions such as houses and buildings located on steep terrains. The research explores the role of these structures in altering rainfall runoff and the ground's drainage capacity—both crucial factors for assessing slope stability. We propose an application of the physically-based model TRIGRS [3], which simulates changes in safety factors due to water infiltration, to analyse the effect of the presence of buildings on surface water flow and infiltration, and to identify areas most prone to shallow landslide triggering in built-up areas.
We applied the model on Ischia Island, a densely populated location Southern Italy, prone to different types of landslides [4]. A procedure was developed to consider the effects of the buildings on water runoff. To simulate the spatial distribution of rainfall, flow directions were modified to account for the presence of buildings, preventing excess water—unable to be absorbed by the ground—from accumulating where the buildings are located. Instead, water falling directly on the buildings is collected at a specific point at the boundary of the structure, simulating its discharge onto the ground.
The results demonstrate that this procedure effectively captures the effects of the building on water runoff, showing a significant increase in slope instability where water discharged from buildings accumulates.  We expect the workflow outlined here to be most effective in areas with informal housing [5], in which additional factors such as weight of the buildings and water leaks may play a relevant role.

References  
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[2] Bozzolan, E. et al. (2022). Sci. Tot. Env. 858, 159412. http://dx.doi.org/10.1016/j.scitotenv.2022.159412
[3] Alvioli, M., Baum, R. L. (2016). Env. Mod. Softw. 81, 122. https://doi.org/10.1016/j.envsoft.2016.04.002
[4] del Prete, S., Mele, R., (2006). Rend. Soc. Geol. It., 2, 29-47. https://api.semanticscholar.org/CorpusID:133382086
[5] Alvioli, M., et al. (2022). Geomatics, Natural Hazards and Risk, 13, 2712-2736. https://doi.org/10.1080/19475705.2022.2131472
[6] Bozzolan, E. et al. (2020). Natural Hazards and Earth System Sciences, 20, 3161-3177. https://doi.org/10.5194/nhess-20-3161-2020