  Rapid impact assessment in the immediate aftermath of a natural disaster is
essential to provide adequate information to international organisations, local
authorities, and first responders. Social media can support emergency response
with evidence-based content posted by citizens and organisations during ongoing
events. In the paper, we propose TriggerCit: an early flood alerting tool with
a multilanguage approach focused on timeliness and geolocation. The paper
focuses on assessing the reliability of the approach as a triggering system,
comparing it with alternative sources for alerts, and evaluating the quality
and amount of complementary information gathered. Geolocated visual evidence
extracted from Twitter by TriggerCit was analysed in two case studies on floods
in Thailand and Nepal in 2021.
