The Oxford Flood Network uses a network of ultrasonic sensors in a flood-prone area to detect water levels and broadcast their data over short distances to Raspberry Pi-based gateways; these aggregate the data and pass it to an open online database.
The detection devices are Arduino-based. They then communicate to the Raspberry Pi via RF and the data is shared via ethernet to a house or business router via ethernet cable.
They are looking for people to contribute to the project by hosting devices on river overhangs so that the ultrasonic sensor can point downwards and detect the water level. If you are able to help, fill in this online form.
Thanks to the Raspberry Pi Foundation for spotting this one.