Sirens are obsolete and cellular coverage is unreliable. This is why $30 NOAA weather radios exist. But even with SAME there are so many alerts that people ignore them.
So let’s start with “What will it take for true emergency warnings not to be ignored”?
Sirens are obsolete and cellular coverage is unreliable.
No, sirens are not obsolete. And they are the premier system in heavily populated areas, like where the camps were.
NOAA radios give you weather and generic warnings about heavy rain, in certain areas, etc.
But they are not to the accuracy level of River gauges and are not tied to those. Gauges slaved to sirens downriver would have saved most of the lives lost.
A flash flood is basically like someone coming to kill you from 10 miles away, in a golf cart. If you are notified when they start rolling, you have a very good chance of getting to safety.