Cascading floods in multi-level levee protection areas: Case studies of Dongting lake bursting

Study region: Dongting Lake reservoir, the middle reach of Yangtze River, China. Study focus: This paper aims to develop an efficient numerical approach to simulate the entire process of cascading failure in multi-level levees, coupling the multi-level breaches with the multi-level flooding. Two notable cases in 1996 and 2024 in the study region are taken as case studies to reveal the hazard amplification effects from cascading levee failure, quantify the buffering effect of multi-level levees, and assess the effectiveness of the mitigation measures during emergency rescue. New hydrological insights for the region: Comparing with the single levee breach in 2024, the primary reason for the 2-level cascading levee failure in 1996 was the continuous rise in water level in Dongting Lake after breach #1, which accelerated the breach expansion and hastened the flood routing process. The second-level levee failure triggers subsequent erosion and expansion of breach #1, resulting in a 2-times increase in the breach length and a stronger destructive power of the second-level flood. Considering the mitigation measures based on four scenarios, the prompt sealing of breach #1 effectively decreases the length of breach #2 by 2/3 and lowers the maximum inundation rate by 1/3. The timely reinforcement of the second-level levee can prevent or delay the cascading failure of multi-level levees, providing window periods for emergency rescue.