Among the many times I‘ve been to historic Würzburg, Germany, one visit stands out in particular. It was a walk across the Alte Mainbrücke, a stone-built structure across the river Main. A complex system of locks and low dams has the eastern third of the river occupied by a flow-driven power plant. Between the thick stone pillars of the bridge, one segment has a tall weir, also called low-head dam, handling the outflow from upstream. On that day, following a week of heavy rain, it was scary to stand on the bridge. Looking down revealed a curved burst of water, 12 feet tall and occupying the entire 40-foot width of the weir. It plunged into the calmer tailwater with brutal force. One could see how the gush continued below the water surface, doubling back into itself. In essence it formed a horizontal cylinder, rotating like the drum of a front-loading washing machine. Anyone falling in there, or somehow making it across the safety barriers in a kayak, would be trapped in a deadly circle of rushing water. I had been looking at what’s called a submerged hydraulic jump.
Countless low-head dams exist in the U.S. South also, many more than a century old and lacking the sophisticated safety features of a functioning power plant. Old grist or cotton mills were abandoned and dismantled and the dams left in place, now without a function. Some are still in use, like the one on the Soque River near Demorest. Upstream of the dam, on a calm, warm day in August, the headwater looked smooth and benign.
After clearing what I was guessing to be about 12 feet of height difference, the resulting hydraulic jump draining into the tailwaters appeared busy but not life-threatening. The Soque River was low that day. But when there’s strong flow, “low-head dams become drowning machines”, say Park Rangers and hydrologists. Once the tailwater gets high enough, it gives the hydraulic jump the recirculating rotation that can trap unsuspecting paddlers. Even a 5-foot dam, looking harmless from upstream, will be a potential death trap.
In July, 2018, the University of Georgia removed the 110-year old, disused White Dam from the Oconee River, eliminating a fish barrier and a potential drowning risk. Many more such useless hazards exist. Until they are gone, kayakers are well advised to stay away from low-head dams, and never try to navigate one.
Rudi Kiefer, Ph.D., is a professor emeritus of physical science at Brenau University. His column appears Sundays and at gainesvilletimes.com.