Linking rainfall-driven accumulation to staged warnings: A rapid flood forecasting framework for the Sangabeol underpass, South Korea

Study region The Sangabeol underpass in Gwacheon, South Korea, receives road network runoff at a single low point, examined using a local rainfall record and two additional records from Taean and Naju. Study focus A process-based module couples runoff, routing, effective drainage, and storage-depth conversion with 5-min warning updates, using an effective drainage parameter (η = 0.85) fitted to one approximate depth–time observation and absorbing multiple uncertainties. Rolling forecasts and full-record reference simulations separate accumulation dynamics from rainfall-coverage and warning-rule effects; a hydraulic diagnostic estimates approach-flow depth and velocity from simulated discharge. New hydrological insights for the region Early shallow accumulation alone did not determine subsequent depth-rise rates. Simulated 10–15 cm and 15–20 cm transitions had 60-min/31-min (Gwacheon), 14-min/13-min (Taean), and 23-min/11-min (Naju) durations. The Taean–Naju ranking reversal links evolving rainfall and routed inflow to changing accumulation rates, so an initially slower rise need not imply more time at higher depths. Approach-flow velocities (0.33–0.89 m/s) reversed similarly between the two intervals, indicating discharge contrasts originating upstream of the low point. Predicted remaining times at first Danger were 26–27 min, reflecting the 30-min decision window and 5-min update cycle; limited future rainfall caused 21-, 2-, and 2-min delays in first-Alert 20-cm crossings relative to the full-record reference. These results support interpreting depth, velocity, crossing-time updates, and rainfall coverage for this configuration and retrospective inputs.