The Road-Underpass Flood Trap: Fill Rate, Escape Window, and a Passive Countermeasure Package

A fill-rate and escape model for sunken road underpasses, computed from stated geometry, catchment, runoff and drainage inputs and stated depth thresholds for a medium sedan (0.05 m ponding reference, 0.35 m door-blocking, 0.60 m float). For a typical urban arterial underpass (12.0 m wide, 20.0 m flat floor, 5% grades, 2.5 ha catchment, 0.10 cubic metres per second baseline drainage), a 100 mm/h storm gives inflow of 0.625 cubic metres per second, door-blocking in 216.0 s, float depth in 438.9 s, and an escape window of only 192.0 s; at 150 mm/h the window falls to 120.4 s, and with blocked drains at 100 mm/h to 161.3 s. Basin water looks calm (velocity-depth product 0.052 square metres per second at 100 mm/h, against a stated pedestrian instability value of 0.6). A passive package of drainage sized to balance the 100 mm/h design storm, depth-referenced barriers closing at 0.15 m, raised escape refuges, ladders and depth-gauge signage is priced at $259,785 for the typical underpass, $207,460 for a small local underpass and $384,560 for a large highway underpass, prevents filling at or below the design storm for its own catchment, and widens the typical 150 mm/h window to 322.6 s. The failure envelope is printed in full: where effective catchment or intensity exceeds the design point the package cannot keep the window open, with computed windows down to 23.0 s at 200 mm/h over a 10.0 ha catchment. All results are simulated, not measured; nothing is described as built, installed or field-tested.