Low-head dams: A not-so clear and present danger
by Virgil Chambers, Executive Director, National Safe Boating Council
Rivers can be treacherous, not only because of the tremendous power they possess, through the movement of flowing water, but because of structures they flow over, around and through.
Hazards like "strainers," fallen trees and debris collecting between rocks and bridge piers that can trap floating objects, are generally conspicuous. And those familiar with the dynamics of moving water know how the force of the water, as it comes in contact with different obstructions, can actually hold objects.
Bridge piers and rocks are potentially dangerous basically because they don't move and the water does. If the water is moving fast enough, anything or anyone coming in contact with these structures can be held tight against their upstream side. Once pinned in this way, escape is difficult.
We can easily understand the danger and what is happening when water holds an object against an obstacle.However, the distinction is not so clear with the dangers associated with the river's most perilous obstruction, the low-head dam. It is a man-made structure, typically built to back up water in a reservoir for a variety of reasons.
This wall-like structure pools the water as it flows over the crest and drops to the lower level.This drop creates a hydraulic, which is a backwash that traps and recirculates anything that floats. Boats and people have been caught in this backwash.
A person caught in the backwash of a low-head dam will be carried to the face of the dam, where the water pouring over it will wash him down under to a point downstream called the boil. The boil is that position where the water from below surfaces and moves either downstream or back toward the dam. A person who is caught in a low head dam struggles to the surface, where the backwash once again carries him to the face of the dam, thus continuing the cycle.
To complicate matters, these dams are usually loaded with debris, such as tires and logs on the surface and rocks and steel bars just below, posing additional problems should a person get trapped in this dangerous structure.
Dams do not need to have a deep drop to create a dangerous backwash. During periods of high water and heavy rains, the backwash current problems get worse, and the reach of the backwash current is extended downstream. Small low-head dams that may have provided a refreshing wading spot at low water can become a brutal death trap when river levels are up. Simply put, it is not the drop of the dam which is the lethal danger, but the backwash current. This backwash current is governed by volume of water and flow.
From downstream, you may not realize the danger until its too late. From upstream, low-head dams are difficult to detect. In most instances, a low-head dam does not look dangerous, yet can create a life-threatening situation. You should always pay attention to warning signs, markers or buoys and keep well clear of low-head dams.
Thanks to Virgil Chambers for this article.