Witnesses for Urban Flash-Flood Pockets: A Fill-Time Bound and a Low-Cost Water-Level Warning Network

SIMULATED, NOT MEASURED. A rational-method runoff and fill model for six representative urban flood pockets (vehicle underpass, sunken junction, pedestrian subway, basement district, parking ramp, road dip) computes time to dangerous depth at 50, 100 and 150 mm/h rainfall. At 100 mm/h times range from 8.11 to 68.9 minutes and all six pockets meet a design requirement that warning lead exceed evacuation time by at least 60 seconds, with margins of 115.4 to 3275.0 seconds. At 150 mm/h the vehicle underpass fills in 5.04 minutes, its warning lead of 202.1 seconds falls short of its 240 second evacuation time by 37.9 seconds, and it is named as a no-win pocket for water-level-only warning. A witness network with a level node at each inflow throat and at the sump, a float backup, a gateway and an entry sign costs an average of $2631.7 capital per pocket, $15790 for all six.