The Resilience Trap: Non-linear Hydraulic Failure and Flood Peak Coincidence in Green-Grey Infrastructure under Extreme Rainfall

Low Impact Development (LID) excels in handling frequent storms but often fails hydraulically under extreme rainfall. This study isolates the non-linear interactions between spatial layout and flow regimes using a generalized SWMM model. We identified a critical regime shift from gravity to surcharged pressure flow during 10-year events. We specifically identified a "Resilience Trap" emerged under 20-year conditions: low-coverage (10%) LID did not attenuate flow but amplified it. The delayed runoff released by LID synchronized with the upstream flood peak, causing a "peak coincidence" that increased outlet discharge by 2.6%. This finding overturns the assumption of monotonic LID benefits. It proves that without hydraulic coordination, improperly placed LID actively degrades system resilience by disrupting runoff phasing.