There is no such thing as a ‘wall of water’
by Emmett M. Laursen on Sep. 05, 2007, under OpinionIn the Aug. 1 edition of the Tucson Citizen, in a story on a man killed in a flooded wash, the old nonsense phrase “wall of water” was used.
Why do reporters and editors use hackneyed phrases such as this when it is so patently a false concept?
There is no such thing as a “wall of water” on this earth with the possible exception of ice. But I won’t give anyone that technicality. Ice is a quite different breed of cat than liquid water.
Why do I object to the seemingly innocent phrase “wall of water”? Because ignorance can be a dangerous state of mind.
We drink water (and need to); we wash in water; water is a wonderful solvent.
But every so often we read of a baby that drowned in a bucket of wash water, a toddler who can’t swim does likewise in a pool, as does a teen who crosses a small, swift stream and is swept away and is found the next day dead.
Deep water is dangerous unless you can swim (and float). Swift water is dangerous unless you can swim and are strong.
It helps you to make decisions if you understand how water in motion behaves, what flow patterns should be expected, what pressures can be exerted on your body. If the water in your pool is recirculated, don’t go anywhere near the drain hole – it will try to suck you into it and drown you.
What is all this blather about? The answer to the question is “To keep you alive”!
If you think that a “wall of water” is the threat from water to be worried about – and the only threat to be concerned with – then you could well be a hiker who doesn’t come back on his own two feet from a walk in the mountainous woods.
It would be good if an introduction to fluid mechanics course was required for graduation from eighth grade.
It is surprising how forceful water is on your legs when you cross a creek. The bed is usually silt or sand, there is no traction and the soil is scoured away by the water. Even with rocks it is not a firm footing like solid ground.
A car crossing a wash will tend to float downstream and get caught in debris as would a person. As water leaks into the car it will sink.
If there is a pool of water and there is recirculation into a hole in the wall of the pool, your hand or body can get stuck in the hole due to the pressure.
I remember an incident in Tucson in the 1970s when a couple of boys went into a canal to swim. A gate in the main channel was closed, creating a placid pool. A bypass channel had a gate that was partially open.
One boy swam close to the main gate. However the flow pattern brought him close to the bypass gate. The forces were sufficient to bring him down to the high velocity through the partially open gate. His body went through the opening, but his head got caught by the gate. He drowned.
This dangerous situation could have been prevented with a grid placed far enough away from the bypass canal opening to reduce the velocity and therefore the forces of the flow entering the bypass channel.
Other similar examples could be cited. Every geometry has its own flow pattern that cannot be seen and is therefore difficult to understand because of the transparency of the water.
There is no such thing as a “wall of water.” There is just a water surface that is rising quickly, coming from upstream or a dam break, that can cause a flash flood.
Such an occurrence is uncommon and even rapidly rising water is not necessarily dangerous once it becomes a pool of water.
Emmett M. Laursen is a professor emeritus of civil engineering and engineering mechanics of the University of Arizona. His specialties are in fluid mechanics, hydraulic engineering and sediment transportation.
