Scienceguyorg Ramblings: Small Observation Gives Important Water Safety Lesson

Monday, July 6, 2015

Small Observation Gives Important Water Safety Lesson

With all the walking I do, I try to make it more interesting by observing carefully what I see in my surroundings. Recently I was walking along a stream and noticed just below a very small dam probably only about one foot high were several soda bottles somehow trapped just below the dam. The bottles would bob up and down slightly but never moved far from that spot. Anything floating in moving water you would think would drift downstream. I knew there must be something different about the flow of the water just behind the small dam but I wasn’t sure what it was. A friend of mine referred to this as a vortex, which can be defined as whirling or circular motion that tends to form a cavity which tends to draw into the center.

Understanding how aircraft fly and their efficiency is much about how air moves over the surfaces; water is just another fluid so I inquired on a model aviation listserve as to what was going on. I received an interesting link to a video that was a news segment where they did a test to demonstrate the phenomenon I had witnessed. In doing further research using “dangers of low head dams” will give plenty of results. Low head normally refers to a dam that is 10 feet high or less, the danger is the recirculating current found behind the dam. Many people have lost their lives by getting too close to this powerful current and having their boat pulled under, the warning signs below dams are there for a good reason.

In the following diagrams I show how as the water is squeezed going over the dam the speed increases, it then goes downward with great force over the back of the dam. The fast moving current hits the bottom and then pushes upward to the surface but is pulled back towards the fast moving current over the dam; it is a continuous cycle of recirculating water.