FastFlood

FastFlood runs a simplified steady-state flood solver directly in the browser on global terrain data: pick a region, set a rainfall or discharge scenario, and get an inundation estimate in seconds rather than hours. Developed at Utrecht University, it uses a clever accelerated approximation of the shallow-water solution that benchmarks surprisingly well against full 2D models for screening purposes.

It will not produce design-grade results — no timing, limited structure representation — but for rapid screening, emergency response, teaching and communicating flood risk to non-modellers, nothing free is faster.