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Stumbled into a few videos online about a danger that would be found mostly on rivers. River boaters probably already know about this but I did not. Google "low head dams" or "hydrolic jumps".
Ix

I think these type of dangers are largely a problem for folks in small craft like tubes, canoes and kayaks. Perhaps some of our whitewater enthusiasts can tell us more.

Low head dams or weirs are definitely hazardous. They often look passable for rafts, kayaks or canoes, however on the immediate downstream side of the dam a trough sets up, and it's easy to get trapped in it. Once it grabs you, it's virtually impossible to paddle out. If you flip out of your boat, undertow can easily drown you, even if wearing a PFD. Stream flow makes a big difference. During high water, a weir may just show a slight bulge on the surface, and be safe to pass. At slightly lower stream flows, the trough sets up and it gets hazardous.

hey all ,. just wanted to toss my 2 cents in here.. I was actually a swift water rescue tech. (never Worked in the area, but took the classes).

The first problem with the structures , sometimes called wiers , is that they are build in a uniform manner.. so if you go 5 feet left or 5 feet right, . it still has the same water flow. Unlike most natural features where if you go left or right,. something different is happening.. and that makes it better because it will "kick" you out.

The second issue is that the smooth surface make a very turbulent water flow on the other side.. it is affectionately called a "washer" cause it spins around and around.

Third issue, that not all weirs have is called a trash collector.. it is a space under the lip designed to catch things (sometimes people).

they fall under the category of "just avoid them" . but in the event you end up in one, .. you should know that the top water, is going backward upstream, and then gets pushed down but the new water ...and of course, .. when you need air you head for the surface. so you get trapped in the cycle until you die. The way out is to go as deep as you can... as that is were the water that is exiting the system is, sometimes you even need to crawl along the bottom. So down to the bottom and then out and up.

Kevin.

Would these be related to wing dams like on the Mississippi river Low water....no problem....can be seen along way off. High water big problem.....cant be seen until its too late Ron.

mrron_tx wrote:Would these be related to wing dams like on the Mississippi river Low water....no problem....can be seen along way off. High water big problem.....cant be seen until its too late Ron.

I had to look that one up. Never heard of them before.

Wing dams line both banks along the Mississippi, and they create all kinds of problems for recreational boats. They exist to deflect current and deeper water to the center of the channel. The Mississippi is first and foremost a commercial navigation channel. Wing dams become man made reefs which can be struck if you stray too close to shore, and they cause sand bars and shoals immediately below them. It is possible to anchor in the lee of a wing dam, but it's not a very comfortable place to spend the night, because tow boats with enormous barge trains sometimes put in for the night at such places. Even with an anchor light showing, you feel vulnerable. To top things off, the Corps of Engineers has dammed off virtually all of the side channels, which otherwise would offer good overnight anchoring locations, in the interest of keeping as much water as possible in the main channel. Add to that a huge load of woody debris, almost complete lack of fuel docks between Illinois and Louisiana, and you begin to appreciate why the great Mississippi isn't so great as a cruising route.

Bottom Line is u r sayin u don,t wanna b trying to "wing it" down there !

J

Chinook wrote:Wing dams line both banks along the Mississippi, and they create all kinds of problems for recreational boats. They exist to deflect current and deeper water to the center of the channel. The Mississippi is first and foremost a commercial navigation channel. Wing dams become man made reefs which can be struck if you stray too close to shore, and they cause sand bars and shoals immediately below them. It is possible to anchor in the lee of a wing dam, but it's not a very comfortable place to spend the night, because tow boats with enormous barge trains sometimes put in for the night at such places. Even with an anchor light showing, you feel vulnerable. To top things off, the Corps of Engineers has dammed off virtually all of the side channels, which otherwise would offer good overnight anchoring locations, in the interest of keeping as much water as possible in the main channel. Add to that a huge load of woody debris, almost complete lack of fuel docks between Illinois and Louisiana, and you begin to appreciate why the great Mississippi isn't so great as a cruising route.

I absolutely agree about the Ms river not being a good cruisers route.... unless You have a large crew to take shifts and a very large fuel tank.....take the Tenn-Tombigby (?) much more of a cruiser friendly route . Ron.

Vivid illustration of the hydraulics of a low head dam, and what happens to a boat that drifts over one. It's easy to see how a person in the water, even with a floatation vest on, wouldn't stand a chance.

Kevinem said:

The way out is to go as deep as you can... as that is were the water that is exiting the system is, sometimes you even need to crawl along the bottom. So down to the bottom and then out and up.

After seeing that video, I seriously doubt anyone's ability to overcome the swirling water by diving deep and using the churning forces to escape. A nice theory, but I think the bouncing one would be taking would make holding your breath for any length of time, fantasy thinking.

Ray

Ray
Doe,s it give U reoccurring nightmares about being drunk in a large whirlpool tub cranking up the pressure only to b pulled under water & to resurface in a state of shock & panic & then to suddenly relax with a big smile on ur face after realizing u managed to keep that half full glass of Scotch above the water without spilling a wee dram !!


J

I'm thinking about it John,I'm thinking about it

Ray