Flood Alarm Systems for Critial Sites – With Dual or Triple Redundancy and Autonomy from the Internet

Key to the reliability of any flood alarm system is its sensor. First developed in 2010, the BDT was originally designed to overcome the shortcomings of traditional flood sensor technologies from float switches to pressure and time of flight level transducers in sewer flooding applications.

Since then, the BDT has won several industry awards and has been used in numerous critical flood alarm systems for industrial and commercial applications including the NHS and since 2020 Network Rail for track flood monitoring.

The BDT takes full advantages of the laws of physics

Uniquely the BDT detects a flood by looking for the presence of water in a unique way, the change in the surrounding electrical dielectric constant. Where the dielectric constant of air is nominally 1 and that of water 78. This change in dielectric constant is converted into a numeric number which enables the BDT to determine if it is submerged, partially submerged, in air (dry) or importantly its cable is damaged or it has developed a fault, making the BDT highly suitable for high safety integrity (SIL) applications such as rail.

By focusing on dielectric change there are no moving parts, noting to get jammed and if the BDT gets covered in debris unlike a float switch or radar sensor it will still report a flood. By measuring bulk dielectric, the BDT is also capable of ignoring other factors that can disable or damage other flood alarm technologies too, such as such as spiders’ webs, damp, ice and snow.

No Mains Required

Water and mains electricity are not a good combination and should be avoided. Not only is the provision of mains expensive but also poses some serious H&S issues near water. Furthermore, often during flooding power fails necessitating a battery backup.

The BDT is ultra-low power, consuming on average just a few uA, making it perfect for battery operation. This enables the BDT sensors to be placed anywhere,  enhancing safety, speeds up installation, while the BDT is capable of achieving up to two decades of operation from a single lithium cell.

Wireless and BDT Sensor Technology the perfect combination for flood monitoring

BDT transducers may be used with a variety of wireless technologies, from RDN’s own RDNET1000 radio telemetry to the cellular or Iridium Satellite networks. The type of system required, and the communication media will depend on the criticality of the application and if the system is local, remote, or ultra-remote

Radio Telemetry Based Flood Alarm System

For local flood monitoring and total autonomy of operation from third parties, radio telemetry is the preferred medium, such as RDN’s own RDNET1000 system/s. This is by far the most popular method of interfacing the BDT, with circa 300-million BDT messages delivered this way annually, over distances of a few meters to 5km.

Radio telemetry signals derived from BDTs can be used to control evacuation beacons, operate sirens, and operate flood gates enabling the creation of smart food intervention systems that are off internet making them innately cyber secure, plus are not subject to the investment uncertainty seen in the cellular industry with its migration from G to G as technology emerges.

RDN offer two radio telemetry services, short range systems on licence free radio and long range highly resilient and legally protected systems on Ofcom licenced radio spectrum. Licence free systems are perfect for in building or near building use where the likelihood of interference is minimal, whereas the licenced services are recommended for any system that operates in public space or over a distance exceeding 100m. Licences fees are modest starting from under £ 70.00 per annum.  A LoraWAN variant of the BDT is planned for the summer of 2023.

Cellular Flood Warning Systems

Where cellular coverage exists then SMS and the emerging new technologies such CAT-M1 and NB-IoT can be used. Cellular enables messages to be sent to individuals such as Flood Wardens using SMS text, emails or displayed on the internet.

As with any cellular service there is need of a contract, SIM card and always the risk of network outages or long-term obsolescence and the networks evolve and migrate from 3G to 4G, 5G and beyond.

Satellite Based Flood Warning Systems

For remote locations and where cellular coverage is patchy, the Iridium satellite network has been used with tremendous success and has been successfully trialled in combination with the BDT and adopted by the likes of Network Rail.

Iridium uses LEO (low earth orbit satellites) meaning that every part of the globe is covered and unlike geostationary satellites that sit at the equator, Iridium antenna are small, there are no shadows to contend with and alignment is not required, making installation easy.

Iridium will even work in remote steep sided valleys, in forests and in deep ravines, only subject to a short delay and increased battery consumption as it waits for a satellite to come overhead.

As with cellular, Iridium enables messages to be sent to a dashboard and then on to individuals such as Flood Wardens using SMS text, emails or displayed on the internet.

Hybrid Flood Warning Systems

Where both control or local warnings are required plus remote warnings by for example SMS text or MS Teams then hybrid systems may be created, using a mixture of communication media such as radio telemetry plus Iridium.

Alternatively, where dual redundancy is required Iridium and cellular may be used in parallel.


The BDT has been successfully deployed in flood warning applications for over a decade. It has been used with radio telemetry to provide simple in building alarms, combined with cellular to deliver  SMS text messages and with iridium to deliver critical messages concerning track flooding for rail networks.

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