Why are Weirs so Dangerous?

Dams are used for a variety of uses and range massively in their scale, design, and materials. While dams can provide electricity, water supply, and shipping possibilities, they are also potentially dangerous. You will have no doubt heard about the tragic dam collapse in Brazil earlier this year. 

However, it isn’t a large dams that pose the most risk. In this educative video from the Practical Engineering channel, host Grady explains why low head dams or weirs are the most dangerous. Commonly called weirs, low head dams are usually made of concrete are used to raise the water level upstream on a river.

This can help transportation by making a deeper river for larger boats, it also makes more water available for industry or farming. A low head dam can also be part of a hydroelectric scheme. The heyday for low head dams was in the 1880s when flowing water was the most consistent power source for industry. 

While these industries have long gone the dams remain and are needed to be maintained by the now current landowners. So how are they dangerous? Because they are so low, breach of the dam isn’t such a big deal, they aren’t holding back much water. However low head dams have caused more fatalities in the United States than all dam breaches in the last twenty years. 

How? From people drowning in what's’ known as a ‘hydraulic jump’, when slow-moving water meets fast moving water. This turbulent water can pose a massive risk to unsuspecting recreators.