New York Flash Flood v0.3 · rain, tide and terrain over a hex grid, and what a model with worse data would have missed

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what happens

0.0

km² under water

people in it

Storm rain over the city

The rain is a real record at one point, Central Park. What falls elsewhere is the first thing nobody measures well.

Coast the head at the edge

Every sea cell holds one water level: the tide at The Battery plus whatever surge you add. A drain whose outfall sits under that level backs up, and the sea comes over the bulkhead once it stands 0.8 m above the ground behind it.

Storm surge on top of the tide

Sandy in 2012 put the water at The Battery about 3.4 m above NAVD88 (NOAA station 8518750), tide and surge together. Ida in 2021 arrived near low water, so the harm came from rain.

City what the ground does with it

Drain capacity, as built

Runoff share (impervious ground)

New York's sewers are built to a DEP design standard of 1.75 inches an hour (44 mm). Ida delivered 80 mm in one hour at Central Park. Everything over capacity runs downhill.

Data risk what the model sees

Each toggle takes one input away from a second simulation and paints where the two disagree. Red is water the model never saw.

One gauge for the whole cityCentral Park's rain, everywhere. The real band moves across the city, same total, different placement. Terrain too coarse to hold a low spotHeights averaged over about a kilometre. Basements, underpasses and the Gowanus bowl vanish into the average. Tide gauge went quietHolds the last level it heard; surge and the rising tide never reach it. Drains assumed cleanRuns the sewer at its design number; the real one is clogged by the share below.

Real clogging

No toggle is on, so both simulations see the same world and the map shows one flood.

Sources what each layer is

The routing is a shallow cascade on 240 m hexes, one height per hex, so a sunken roadway or a basement stair is below what it can resolve. The live forecast run forecasts the rain; the flood it paints is a demonstration. Read the map for the gap between the two runs.