What are the best detection devices to warn people of an already-moving debris flow?

Question

Quizplus · Accepted Answer

An acoustic flow monitor can detect the sound waves generated by a moving debris flow and send an alarm to alert people downstream to evacuate. This method is more reliable than a tripwire or visual film monitor, as it can detect debris flows even in low visibility conditions. Natural sounds created by the flow may not always be loud or distinct enough to serve as a reliable warning, and a natural barrier may not be effective in stopping the flow or providing sufficient warning to people downstream.

What Are the Best Detection Devices to Warn People of an Already-Moving