Objective: Global climate change and rapid urbanization have intensified compound flood risks in coastal cities. A critical research gap persists regarding cascading failures triggered by instability in high-standard flood control infrastructure. Prevailing assumptions often underestimate the failure probability of robust defense systems; however, historical events, such as the 2015 dike overflow along the Jiangnan Canal, demonstrate that extreme events can overwhelm even 50–200-year flood defenses. This study examines the Wuxi Yundong Enclosure, a representative high-standard flood control zone in the Yangtze River Delta. Focusing on the "rainstorm–dam Breach–urban flooding" disaster chain, this study aims to quantitatively reveal the superposed magnification mechanism of economic losses under beyond-standard rainfall, providing a scientific basis for resilient urban flood management. Methods: An integrated "hydrodynamic simulation–loss quantification–attribution analysis" framework was developed. A multiscale coupled hydrodynamic model was established to link a Taihu Basin-scale model (boundary conditions), a 1D river network model (canal dynamics), and a 2D inundation model (surface flooding). Validation against the June–July 2016 flood event yielded a Nash–Sutcliffe efficiency of 0.85 and R² of 0.99. A hydrographically triggered breach mechanism was simulated: failure occurred when water levels exceeded the dike crest by 10 cm for 10 min, forming a trapezoidal breach (top width 12 m, bottom width 8 m, height 3 m). Twelve scenarios were designed across four return periods (50-, 100-, 150-, and 200-year) covering rainstorm-only, breach-only, and compound conditions. Economic losses for seven asset types (agriculture, buildings, equipment, and others) were quantified by spatializing high-resolution land-use data and applying depth–damage curves. Novel indices—the composite increment coefficient and amplification factor (AF)—were proposed to isolate the marginal impact of dike breaching. Results: The simulations revealed significant nonlinear amplification in compound scenarios. (1) Hydrological Effect: At the 200-year return period, the maximum inundation depth reached 2.44 m, a 134.7% increase over the linear sum of single-disaster scenarios. (2) Economic Magnitude: Total direct losses in the compound scenario surged by 221.7% relative to linearly superimposed single disasters. High-loss zones (> 30, 000 CNY per 900 m² pixel) expanded by 179.1%. Critically, over 50% of total losses occurred within a narrow 25 h window coinciding with peak rainfall and dike breach. (3) Asset Heterogeneity: Amplification varied significantly by asset type. Due to "plateauing" depth–damage curves, AF values for plantations, fisheries, and equipment remained below 100%. Conversely, buildings exhibited the highest sensitivity, with AF increasing by ~6.9% per decade of return period. (4) Attribution: The net superposition increment for total assets turned positive near the 85-year return period. Buildings accounted for the vast majority of incremental loss, contributing 94.2% of the total superposition at the 200-year return period. The total asset loss growth rate was approximately 3613.8 million CNY per 10-year increase in return period. Conclusions: This study demonstrates that high-standard flood control zones face catastrophic risks during compound disasters with structural failure. The economic impact of the "rainstorm–dam Breach–urban flooding" chain vastly exceeds the sum of individual hazards. The identification of buildings as the primary driver of loss magnification underscores the necessity for targeted structural reinforcements and risk-transfer strategies. Despite focusing on direct losses and excluding indirect disruptions, the framework provides a robust methodology for extreme flood risk analysis. These findings offer critical insights for optimizing flood storage strategies, refining emergency protocols, and updating engineering standards to incorporate compound disaster chains, thereby enhancing urban flood security in deltaic megacities globally.