Things that look and feel like something you've done a thousand times, and may do a thousand times again with no danger. Except that sometimes, things can change very quickly.
There's a creek bed near me that dried out almost completely over the summer; usually there's some water flow and occasional shallow pools where small fish live. While it was completely dry, I wanted to walk as far downstream as I could. I woke up the Saturday morning I planned to do it to the sound of torrential downpour. As recently as that Thursday, there was no rain in the forecast. I can only imagine if the downpour was sudden, and a few hours later.
As it happens, after the downpour I went to see the creek, and it was flowing quite rapidly at a depth of about 3 feet, well more than enough to sweep someone away never to be seen again.
Flash floods 1) develop quickly 2) often in impassable terrain (upstream and downstream are your only options), and 3) often under "blue sky" conditions from distant of hours-old rain rain.
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.
It's not easy to get up at an embankment. Mud and a 10-20mph current of water (water's heavy) pushing you away. Unless a root of a tree to grab on to, or a really solid rock, very hard and getting into the situation best avoided at all costs.
Maybe creeks are different where you're from, but as a young individual in the US northeast the idea of being "swept away never to be seen again" by a creek is a bit comical, torrential rain or not. You'd make it about a few yards before hitting one embankment or other and clamber out. Cold and wet, sure, but certainly to be seen again.
By the way, it is sunny overhead. The water is coming from a torrential downpour some miles away in the mountains. This is normal in New Mexico which experiences highly localized heavy downpours during July/August.
To point out the reason. In the northeast a small creek represents a small drainage area. In the Southwest a small dry creek can represent a large drainage. All the water dropped by a thunder storm will end up in it.
I remember one time flying over the southwest with the light just right and you could see vast drainages with thin thin cracks that are the actual channels cut down into the sand stone.
Indeed, in the South West, flash flooding carries families away occasionally. This year we've had exceptional fires, and the drainage from burn scars often carries what used to be a forest along with it. I don't know about GP's situation, obviously, but there is definitely a spectrum of intensity with regard to sudden water flows.
There's a creek bed near me that dried out almost completely over the summer; usually there's some water flow and occasional shallow pools where small fish live. While it was completely dry, I wanted to walk as far downstream as I could. I woke up the Saturday morning I planned to do it to the sound of torrential downpour. As recently as that Thursday, there was no rain in the forecast. I can only imagine if the downpour was sudden, and a few hours later.
As it happens, after the downpour I went to see the creek, and it was flowing quite rapidly at a depth of about 3 feet, well more than enough to sweep someone away never to be seen again.