In the case of floods, GDACS alerts mainly utilises source information from authoritative institutions, media and scientific organisations, rather than automated procedures.

GDACS then issues an alert depending on this information:

1. Red alert for more than 1,000 dead or 800,000 displaced.
2. Orange alert if there are more than 100 dead or 80,000 displaced.
3. Green alert for all other floods.

GDACS alerts published online and sent by e-mail and SMS also include information on the severity of the flood, as calculated by the Global Flood Detection System. This data includes the magnitude, duration and severity, and estimates the significance of the detected extent and duration of the flood with respect to previous years. For the exact definitions and details, please refer to the Flood Detection page on the GDACS website. At the time of writing, these parameters do not affect the level of the alert.

Researchers at the Dartmouth Flood Observatory release a list of significant floods worldwide two or three times a month, including information from official government sources or media on the number of people killed of displaced.