A new study suggests that urban areas are more prone to rainfall-induced landslides compared to rural regions.
Urban environments have mostly been expanded in flat regions that benefit development and growth. However, the growing population arises additional needs to urbanize regions that are more prone to natural hazards. Cities currently expand to foothills that may present higher inclination and where rainwaters accumulate. The combination of extreme urbanization with the increasing intensity of precipitation events makes rainfall-induced landslides a major issue in such environments. Given that more than 50% of the world's population lives in cities, the ramifications of such landslides are significant and include infrastructure damage, disturbances in the urban networks, or potential fatalities.
Landslides comprise a wide range of ground movements including falls, topples, slides, lateral spreads, flows, or a combination of one or more movements. The majority of landslide phenomena are associated with precipitation rates. The new research suggests that precipitation-triggered landslides are impacted by the surrounding environment however, the impact of man-induced land use has not been taken into consideration in slope stability assessment. The team investigated the landslide sensitivity of urban regions to alteration in rainfall rates by utilizing a large database of ground failures accounting for the United States Pacific coast. The precipitation data referred to daily, 10-day and 30-day periods. The researchers isolated the effect of precipitation on landslide phenomena by using a panel regression technique with fixed effects. Thus, they controlled the impact of additional influential parameters such as slope inclination, wildfire or soil/rock type.
The study's findings suggest that urban areas are 10 times more sensitive to precipitation changes. This means that the probability of a rainfall-induced landslide occurring in an urban area is 10 times higher than the correspondent probability in a rural region. The cause of this difference is related to human activities that have altered the landscape by changing the topography, removing vegetation or constructing impervious surface covers (e.g., roads, parking lots, etc.). These actions are responsible for reducing the slopes' factors of safety and thus making them more prone to instabilities during rainfalls. “I didn’t set out to study urban versus rural hazards, but the urbanization signal was the strongest in preliminary models. I recognized something was there,” Elizabeth C. Johnston, lead author of the study and a Ph.D. candidate in Earth System Science, Stanford University, stated. The fact that urbanized areas are more prone to landsliding during rainfalls has been hypothesized but this is the first study to derive quantitative results and to validate the idea.
The results indicate that landslide hazard assessment should be site-specific and take into consideration the urbanization rates. “If we assume different land-use types have the same relationship between precipitation and landslide hazard, we’re going to end up underestimating landslide hazard,” Johnston, added.
The authors conclude that their work framework can be used to study the effects of additional parameters that impact landslide hazard such as snow accumulation or wildfires.