Flood forecasting

FREQUENCIES AT THE HEART OF FLOOD FORECASTING 

Frequencies are extensively used to face flooding. Over 700 sensors, spread over the different rives in continental France are used by the flood forecast services. They measure the water level and transmit data to the prefectures. Normally, the transmissions are sent every hour, but they increase when it rains and can occur every 5 minutes.  If major flooding is rare, its important effects mean that they have long been noted in the national registers.  The most important flooding on the Seine occurred in February 1658: 8.96 m converted to the modern scale (Austerlitz bridge), or 34 cm higher than the 1910 level. Indeed, in Paris, systematic readings have been taken since the mid 19th century using a scale located at the Austerlitz bridge. The torrential flooding in the Gard in 2002 accelerated the redesign of the flood warning system, which changed to a flood forecast system on the main rivers in continental France.  A flood forecast and warning system requires a central operational service to communicate with the decentralised stations and with all people in contact with the service.  Joint expertise between meteorologists and hydrologists was also needed, resulting in the creation:

• of the Central Hydro-Meteorology and Flood Forecast Support Service (SCHAPI) in 2003. It is in charge of displaying the risk at the national level (maps "vigicrues") and is located in Toulouse, as is Météo-France. It reports to the risk prevention general department (DGPR).
• of 19 Flood Forecast Services (SPC) which are in charge of the specific watch for their territories.  

To coordinate operations at a local level, the Regional Environmental, Planning and Housing departments (DREAL) are in charge, for each basin Prefect, of organising and implementing the directives. They are the main information relay to local authorities.

The relays used to monitor floods operate in the 70/80 MHz band and collect the data from sensors installed at the river level: it is these level sensors that can operate using ultrasounds or use pulse radar technology operating in the 24 GHz band.  The relays can either receive alarms sent spontaneously by the sensors or query them following a request from an operator or according to a pre-defined cycle.

The sensors provide rainfall or river level readings which contribute to the overall flood forecast and warning process.  

The degree of interference between the radio relays must be sufficiently low not to put into doubt the reliability of the communications, which requires the assignment of several frequencies for the equipment controlled at the basin level. 

This information is then re-transmitted to the prefectures in order to inform or warn populations. The transmission can use the phone network, optical fibre or even microwaves in the 1.4 GHz band. 

Flood control is also a fundamental part of dam safety, in particular in areas around nuclear power stations. Some additional stations are thus operated by EDF.  

Finally, flood prevention requires international cooperation and alert sharing. Thus, in 1993 in the north of the Aisne department, the Oise floods caused flooding and showed the importance of warning populations of the arrival of floods soon enough, including in the Belgian Hainault region.