Threshold Behaviour and Design Controls on Tree Uprooting during Pluvial Flood Events: A Hybrid AI and Experimental Flume-Based Study

Urban and pluvial flooding increasingly threaten green infrastructure, particularly trees, which play a critical role in urban resilience, drainage, and ecosystem services [1]. Despite their importance, the physical mechanisms governing tree uprooting under pluvial flood conditions remain poorly quantified, especially with respect to flow characteristics, soil properties, and planting design [2].

This study presents a comprehensive experimental investigation into the impact of pluvial flooding on tree stability, with a specific focus on uprooting processes under controlled hydraulic conditions. More than 200 flume experiments were conducted using bonsai trees as scaled physical analogues of urban trees. The experiments were designed to systematically examine the influence of flow intensity, planting configuration, and soil depth on tree uprooting. Tests were carried out with both single-tree and two-tree arrangements to assess the effects of interaction between neighbouring trees. A wide range of flow conditions was imposed, representing low, medium, and high-intensity pluvial flooding scenarios, while soil depth was varied to simulate different urban planting constraints.

The results demonstrate that tree uprooting is strongly governed by design layout parameters, particularly tree height and the spacing between trees. Closely spaced trees exhibited altered flow patterns and load distributions, leading to either increased stability due to flow shielding or enhanced vulnerability due to soil disturbance, depending on the configuration. Soil depth was found to be a critical controlling factor, with shallower soils significantly reducing root anchorage capacity and increasing the likelihood of uprooting under flood conditions.

Analysis of flow intensity revealed the existence of a threshold behaviour. While low-intensity flows generally resulted in negligible structural response, medium- and high-intensity flows produced comparable levels of hydrodynamic loading, with no substantial increase in uprooting probability beyond a critical flow threshold. This indicates that once a certain hydraulic forcing is exceeded, additional increases in flow intensity do not proportionally amplify adverse impacts. Instead, the transition across this threshold marks the onset of significant instability and uprooting risk.

These findings highlight the non-linear nature of tree–flow–soil interactions during pluvial flooding and underscore the importance of considering layout design and subsurface constraints in urban tree planting strategies. The identification of critical thresholds for adverse impacts has practical implications for flood-resilient urban planning, suggesting that appropriate spacing, height selection, and soil depth provision can substantially enhance tree stability under extreme rainfall events.

[1] Piadeh, F., Bakhtiari, V., Piadeh, F. (2026). Automated novel real-time framework for rainfall data imputation in flood early warning systems, Engineering Applications of Artificial Intelligence, 1164(B), p.113348. https://doi.org/10.1016/j.engappai.2025.113348

[2] Défossez, P., Veylon, G., Yang, M., Bonnefond, J.M., Garrigou, D., Trichet, P., Danjon, F. (2021). Impact of soil water content on the overturning resistance of young Pinus Pinaster in sandy soil, Forest Ecology and Management, 480, p.118614.