Cascading failure in levee systems: A two-stage material point method framework

Cascading levee failures, typically involving slope sliding followed by overtopping, occur frequently worldwide, yet conventional flood risk assessments treat these two failure modes separately with limited consideration of their interactions. Here, we address this problem from the perspective of post-sliding levee geometry, establishing its correlation with overtopping-induced flood risk. To this end, a novel two-stage Material Point Method (MPM) approach is developed to sequentially simulate sliding (Stage 1) and ensuing overtopping (Stage 2). This closes a methodological gap in modelling cascading failures. The unified MPM framework integrates single-point and double-point formulations in Anura3D, enhanced by tailored inflow boundary capabilities, and was rigorously validated against the physical model experiment. Simulations of the Shijiao Levee at full scale under 16 future hydrological scenarios demonstrate that post-sliding geometry governs the subsequent overtopping-induced flood risk, with the remaining crest width emerging as a tipping-point metric. In the modelled scenarios for this levee section, rapid drawdown yields more hazardous residual geometries than heavy rainfall cases under comparable forcing. Correlation analyses using Pearson and GeoDetector methods reveal that overtopping-induced flood severity is dominated by coupled interactions among residual geometric parameters. Most parameter combinations exhibit nonlinear risk amplification, yet some show compensating weakening. This duality challenges conventional single-factor parametric analyses and motivates the explicit treatment of coupled effects in levee reliability assessment.