I think that a simple chain of small electronic signals could notice the flooding and spread the information to all the other signals in the chain downstream. People could see a small flashing red light chain and rush to the next safe exit. Providing the system with a few discrete metallic scales leading just up in strategic points would save lives.
This would not be difficult to design, just a sensor placed at 50cm high that would activate automatically when flooded) and could be implement discretely without ruining the landscape.
This type of system would actually be quite expensive. Detecting flooding requires some kind of "flood sensor". Is that a water sensor? What happens when it gets covered in mud? Is it waterproof? How's it powered? How does it transmit data? Where is it placed? What happens if it falls over?
A flood sensor is simple technology. Each WC in the planet has one. You just need a box, a wire, and a buoy attached to a rod. When the buoy starts floating the rod closes an electric circuit conveniently placed much higher than the highest flood.
Could be introduced in a tube drilled directly in rock for minimum visual impact. A system of an oiled hollow cylinder (buoy) inside a major tube, all placed in the drilled tunnel could move smoothly for a long time and return again by gravity after the flood as long as the drill left some empty space below (for mud evacuation purposes)
> How does it transmit data?
standard electric cable (for example), or wifi, radio... whatever appropriate for the area
> Where is placed?
The rod/buoy, At 50cm high or 1m high (or any appropriate value) in the upper parts of the cannon. This would avoid the problem of mud covering electronics also.
> What happens if it fails?
The system returns to the same point as it is currently: "This is dangerous, not guarantee, we recommend to hire a guide, do it at your own risk".
The difference is that in the meanwhile some lives could be saved, so would be an improvement over the current situation.
> How do we know if it fails
A final led at the starting (and end?) of the cannon could be arranged to turn off, for example. Trivial in design of electronic circuits.
This would not be difficult to design, just a sensor placed at 50cm high that would activate automatically when flooded) and could be implement discretely without ruining the landscape.
Just an idea that just occurred to me.