#Runoff news: Low-head dam hazards

From The Fort Collins Coloradoan (Alicia Stice):

In the past 10 years, more than 15 people have drowned on the river from various accidents. Lopez’s death was the first in years involving a low-head dam.

These [weirs] dot rivers across the country, including in Fort Collins, where there is one low-head dam every 1.5 to 2 miles along much of the Poudre.

In Colorado, there is no agency in charge of overseeing safety at these dams. Instead, the Division of Water Resources has a team charged with overseeing the risks associated with large dams at sites such as Horsetooth Reservoir that could pose a hazard if they failed, Colorado Division of Water Resources Dam Safety Chief Bill McCormick said.

“I think these are some of the most dangerous type of structures we have in the country because most people are unaware of the dangers,” said Bruce Tschantz, a Knoxville, Tennessee, water resources engineer who has studied low-head dams extensively. “People tend to overestimate their ability to overcome the current and underestimate the dangers.”

General currents upstream and downstream from a low-head dam. Graphic via Bruce a. Tschantz

In much of the country, low-head dams have been in place for more than 100 years to serve now defunct mills. In Colorado, many of these dams are still active, diverting water into irrigation ditches for agricultural use. While the structures are old, the danger is relatively new.

“The problem of safety around them is more a recent phenomenon as people are using the rivers more,” McCormick said.

The dams slow water upstream and divert it away from the main channel. The water that flows over them creates a rapid on the downriver side that mimics the hydraulics of a washing machine. The water can force victims underwater and spin them around, making it nearly impossible to swim back up to the surface.

“These structures are often very deceiving,” said Kenneth Smith, Indiana Department of Natural Resources assistant director…

Simple engineering solutions can make low-head dams built today much safer by breaking up the flow of water as it moves over the dam. Solutions could include a set of concrete stairs or large rocks on the downstream side of the structure. In many cases, those solutions could be added to existing dams, but that can be costly, and it can be difficult to track down the owners of these century-old structures.

Poudre Fire Authority has been in discussions about what might be done to make sure people know about the dangers of the dams, including the possibility of installing signs along the river warning people of where they are.

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