What a gauge tells you before it goes silent

The gauge at the Alder Reach jetty stopped reporting at 03:12 on the night of the autumn flood, when the river stood 163 centimetres above its warning level. We drove out to it the next morning, before the water had fully dropped, to find out why a policy that depends on a number had lost that number at the moment it mattered.

The housing was open and dry. The pressure sensor was fine. What had failed was the radio link on the pole above it, which had taken a lightning strike two kilometres away and rebooted into a default configuration. This is the most common way a gauge goes silent: not through the water it measures, but through the cheap part that tells us about it.

The last readings are not noise

The interesting part was in the log, not the hardware. For the six hours before it went quiet, the gauge had been reporting a rise of 2.3 centimetres every ten minutes, a steady slope, and then a sudden flattening at 163. A flat line at the top of a rise looks like a peak. It was a stuck value: the sensor was still alive and the logger was repeating its last reading. We now flag any gauge that reports an identical value for 40 minutes during a rise.

What the policy does next

When a gauge stops, the calculation agent does three things in order. It checks whether the downstream gauge saw the same rise a few hours later, which tells it whether the river really peaked. It reads the radar rainfall that fell upstream and compares it with what the river should have done. And it falls back to the next gauge in each policyholder’s schedule, writing the swap into the settlement note so that no one has to guess which instrument paid them.

In this case the downstream gauge reached a peak of 171 centimetres at 05:40, and the three checks agreed within 9 centimetres. The two policies assigned to the Alder Reach gauge were settled at Tier 1 on the downstream value, and each settlement note listed the silent gauge, the stuck reading and the replacement.

What we changed on the pole

We now install every new gauge with two radio links on different carriers and a small battery that holds the last 14 days of readings locally. A silent gauge that was still logging can be read back after the event, and we can show the policyholder the real peak even when the live feed lost it. It costs about $310 more per site. It is the cheapest way we know to avoid an argument.

Why we write these notes

An index policy is only as good as the instrument on the pole, and the instrument is rarely the exciting part. We would rather you read about the stuck value and the second radio than hear about them for the first time in a settlement note. If you hold a Flood level policy, the gauge assigned to your site and its two fallbacks are listed on your schedule.