First we bring in the Army/Navy/Air force which ever is nearer and reachable. Perhaps a combination of them, as needed. Alert all government officials.
Use the media and radio to alert people and inform the locations which are in danger before flood hits the villages. Move the people from flood affected areas to safe areas in some community centers. Provide them shelter, food, clothes and sanitation facilities. In order to do this many volunteering organizations, government bodies, media need to coordinate and help the people. Collection of donations, clothes, food from people in other cities to donate to them. To provide people shelter, electricity and other facilities is not easy. Especially for children, suitable food and milk to be made available is not simple.
Seeing from the sky if people and animals are stranded due to floods by surveying on a helicopter. Clearing roads removing felled trees, repairing roads and finding alternate routes to reach various villages and towns. Seeing that diseases do not spread due to masses gathering in one place.
Arranging for some ways to clear the flood waters from residential areas. Taking actions to assess the damage on the soil due to flood waters to re-instill the fertility of the lands.