  Social media can be used for disaster risk reduction as a complement to
traditional information sources, and the literature has suggested numerous ways
to achieve this. In the case of floods, for instance, data collection from
social media can be triggered by a severe weather forecast and/or a flood
prediction. By way of contrast, in this paper we explore the possibility of
having an entirely independent flood monitoring system which is based
completely on social media, and which is completely self-activated. This
independence and self-activation would bring increased robustness, as the
system would not depend on other mechanisms for forecasting. We observe that
social media can indeed help in the early detection of some flood events that
would otherwise not be detected until later, albeit at the cost of many false
positives. Overall, our experiments suggest that social media signals should
only be used to complement existing monitoring systems, and we provide various
explanations to support this argument.
