Drowning Machine: The Dangers of Low Head Dams

Dams serve a wide variety of purposes from hydropower to flood control to storage of water for municipal and industrials uses. But when a dam’s useful purpose fades away, the structure itself still remains. Dams come in all shapes and sizes, but contrary to what you might think, the most dangerous dams are often the smallest.

A low head dam, sometimes simply called a weir, is a small structure that impounds a small amount of water and spans the width of river or stream. Usually made from concrete, the purpose of a low head dams is to raise the water level upstream on a river. This can assist with navigation of the channel by boats, create a drop for generating hydropower, and make water available at intakes for water supply and irrigation. Thousands of these structures have been constructed over the years to take advantage of natural watercourses and rivers.

The heyday of low head dam construction was actually in the 1800s when mills and factories often relied on waterpower to drive grinding wheels and other equipment. This was at a time when moving water was the most consistent source of power available in large quantities before widespread adoption of electricity. Most of these old mills and factories are long gone, and the ones that still survive certainly don’t depend on water for power anymore. That means many property owners are forced to maintain these old structures that no longer have any practical use. Or more commonly and much worse, these dams are abandoned by their owner and gradually fall into disrepair.

In the U.S., dam safety regulations focus primarily on the possibility of a dam breaching and causing a flood wave downstream. But, because low head dams are relatively short, a breach poses minimal danger, so most states don’t keep track of these small structures. And, especially if they’ve been abandoned, it can be difficult to enforce maintenance requirements on the owners. But, even though they pose little danger in the event of a breach, low head dams create a public safety issue that has caused more fatalities in the U.S. than all dam failures in the past 20 years. To understand why, we first need to know a little bit about open channel hydraulics.

If you haven’t seen my video about hydraulic jumps, I’ll summarize it here. Go back and check out that video if you want to learn more. Open channel flow - that’s flow not confined within a pipe - has a very important property related to its velocity that governs its behavior. Slow, tranquil flowing water is called subcritical because waves propagate faster than the flow velocity. Fast moving water is supercritical because waves move slower than the flow velocity. Any time a supercritical flow encounters subcritical flow, an interesting phenomenon called a hydraulic jump is formed.

Low head dams almost always have subcritical flow upstream. The flow is deep, slow, and tranquil as it makes its way to the dam. But as the flow passes over the weir, it picks up speed and becomes supercritical. When this supercritical flow transitions back to subcritical flow in the slower moving water downstream, it creates a hydraulic jump as you can see here in my model flume. It’s easy to see why these types of structures could pose a threat to those using the waterway for recreation. Any location with fast moving water and high turbulence can be dangerous to swimmers or kayakers, but the location of this hydraulic jump can turn a manageable risk into an almost surefire way to drown.

The depth of the flow downstream of a dam is called the tailwater, and it controls the location of the hydraulic jump. In my model, I can adjust the elevation of the tailwater by adding or removing these stoplogs. When tailwater is low, the hydraulic jump forms away from the dam. This is a fully developed jump that follows the traditional shape and flow patterns. If I send down this piece of wood as a kayaker surrogate, it experiences some turbulence as it passes over the weir and through the jump but, it doesn’t have much trouble escaping downstream.