I know that lowhead dams, aka, "drowning machines" are killers. I also know that any dam shouldn't be approached too closely from the upstream side. I'm not too sure what else I need to know regarding dam safety.
Obviously, if there is a massive volume of water, any dam can be dangerous.
I'm thinking of two dams in particular. There was one that emptied from the bottom, and formed an area of current. It looked safe enough, and I was thinking of jumping in and letting the current carry me downstream a little ways. Not being certain of this though, I decided not to and be safe rather than sorry. The other dam was 20' in height, and in one place sloped, so that water ran down the side. In another place, the water fell like a waterfall. I was thinking of putting a canoe in below this dam to do some fishing. Again, I don't know how safe this would be in regards to currents underneath that would trap someone under water. This second dam looks like the pictures that come up if you do an image search for "Weir Humber River". However, the dam I'm describing is on another river.
How about swimming along in something like this? It looks safe enough:
https://en.wikipedia.org/wiki/Weir#/media/File:Thorp_Gristmill_Weir.jpg
Is the backwash of any dam safe? If below the backwash, are you essentially in the clear? Is there anything else that I should know?
Edit: I found this video that clears things up a little, but I'd still appreciate replies.
https://www.youtube.com/watch?v=XsYgODmmiAM
interesting video
There are so many configurations of low head dams, and changes in volume of flow can change the degree of hazard they pose so much that it is difficult to make generalizations.
Dams such as the Jordan River and the Binghamton, NY dams shown in the video are very clearly lethal. Both produced river-wide, uniform, unbroken hydraulics with a well-defined boil line and strong upstream recirculation.
The dam in the Thorp Gristmill photo does not look too bad to me. I would want to see it in real time before deciding for sure, but it appears that the great majority of flow is going over between the two bridge pilings. There might be a hole with some recirculation at the base of the drop there, but if so it is relatively narrow as the bridge pilings produce eddies on either side. On the bank side of the pilings it does not look as if there is enough flow to create a dangerous recirculation. Of course, at much higher flow volumes the situation could be quite different.
If you have a low head dam with a clearly defined boil line and you make sure you stay downstream of it you will not be sucked back upstream toward the dam. For some dams, the boil line might not be so obvious, or it might not form a straight line across the stream, however.
As an example, quite a few low head dams do not have spillways that extend all the way across the stream bed. The sides of the dam near the banks might be built up sufficiently that no water flows over. In this type of situation there might be recirculating bank eddies on the downstream side of the dam. Sometimes the recirculation in these eddies may extend well downstream of the boil line of the main spillover, and might not be apparent to those who do not look carefully. If you were in one of these eddies and not paying attention, you could get fed back upstream and fed into the hydraulic at the base of the spillover.
thanks
Thank you for posting that video. I had no idea how hazardous those can be. I don’t know of any around me, but for certain now if I ever run into one, I’ll be sure to bypass it.
hindsight
suggests the main danger is not approaching the navigable sluice if there is one or not seeing the dam drop as a physical object in your path.
The canoe/kayak broaches at dam top on that thin water, goes over and prob upside down into the bottom roll.
There was a recent double Delaware drowning of night or twilight kayakers fresh from a bar scene who may have put in directly above a mill dam.
There is or was an impressive igneous rock dam across the Chattooga with an eroded underwater space upstream catching sideways paddlers.
Low head dams trap debris in the roll in turn catching and drowning paddlers sideways or not.
Debris is underwater and you cannot see that so…
It is safest to assume
that all dams have some type of recirculating hydrolic at the base. This is the real danger, not falling water. Paddling up to the base of a dam to see if one gets pulled in or not, is not the thing to do. Unless someone that you trust can verify or there is some or proof of no hydrolic, I would not approach an unknown dam. Just my $00.02 worth.
Pag
Safest approach, yes, but you can see…
… the recirculation. As typical examples, there are two well-known videos of firefighters attempting rescue/recovery below a dam where the viewer can see the exact location which will turn out to be the point of no return for the unfortunate folks in the rescue boat, long before the boat ever gets there (and this would still be true even if there were no boat in the video). On the other hand, I’ve seen a few dams where the outflow looks just like the rapids in a natural chute, where the water at the surface is tearing along in the downstream direction and anything carried by the water will move in that direction too. Your advice makes sense in light of all the people who have misjudged a dangerous recirculating condition, but I wouldn’t go so far as to say that no one should trust their own eyes.
glad you found that video,
was going to suggest it, pretty technical stuff but you get the idea- and its not always just the recirc but broken and breached dams often have debris- like rebar and chunks of concrete. Man made features are more problematic in general. Even under bridges you tend to find more hazards in addition to the visible bridge abutments there is often stuff under the water.
Surprises at Bridges
Yes, man-made obstacles are often so much worse than natural ones. The generally wide, flat and peaceful lower Wisconsin River can be quite cantankerous at some of the railroad bridges, and this is especially true at two of them. At one bridge, there are huge fluctuating circles of fast water, as well as strong eddylines, which though fun, have real potential to for flipping unsuspecting paddlers. That’s an amazing thing, considering how benign the water is at other places. Another bridge used to form a big whirlpool about 50 feet across, and it was about two feet lower in the center than the elevation of the surrounding water. Operators of small motorboats going upstream would pass through this whirlpool and sometimes find that the “uphill climb” to get out would be almost too much for them. Since one abutment of that bridge got demolished (along with part of the superstructure) a few years ago, I’m not sure that the whirlpool still forms when the water is high the way it used to. When I used to see it from shore, it was always a rather scary sight.
whirlpool
was the whirlpool steep enough to capsize a canoe?
Do you think this (https://www.youtube.com/watch?v=EYI67uZkNvQ) is actually a whirlpool, or a phenomenon similar to that seen with dams?
Reply
The whirlpool I described wasn't "steep" enough to capsize a canoe, but the speed at which it rotated would have been enough to put casual canoers in a bad position upon exiting the whirlpool. Also, at the whirlpool's center, differential currents within such a small area might upset a boat, and I think a swimmer could potentially have been pulled under at that location. I'm not sure.
In your video, I'd call that the same kind of recirculation that occurs below many dams. You can see the boil line too, though it's a pretty broad line in this case, relative to the size of the channel. I suppose if you look at the spinning motion horizontally from either shore, you could call it a whirlpool, but by my way of thinking, a whirlpool spins as viewed from above, and has a downward current at the center.
reply
I guess the guy in the vid was disoriented. I wonder how easy it would be for the average person to get out of that situation. It seems that if he were to get to the non-turbulent water toward the camera, everything would be fine.
What I think:
I think he was a rag doll in a washing machine. I doubt if there's much he could have done. That might have been a case where removing the PFD would have caused him to be ejected out the bottom (where the main flow was exiting), but that probably would have eliminated any chance of surfacing again if not successful. He was probably disoriented, yes, but he was probably twirling around enough when below the surface that no attempt to swim would lead anywhere. I can't promise that I'm right about that stuff.
I think that guy prepared himself by taking a big breath before going under. He was underwater for a long time. Perhaps he even had the presence of mind to conserve his energy (and breath-holding ability) by not struggling to swim except when at the surface. I think that a lot of people in that same situation would have ended up dead.
not as exciting as
the French Rally videos but…you have to be there
https://www.google.com/#q=dam++current+recirculation+site:youtube.com&tbm=vid&start=0
recirculating hydraulic
That was a recirculating hydraulic, very similar to what occurs at the base of a low head dam.
The boats that had sufficient momentum to go over the drop and jump past the boil line had no trouble.
As for swimming out of a hydraulic like that, assuming that the swimmer even had his head out of the water long enough to get his bearings (which is doubtful) swimming for the safety of the eddy is much more easily said than done. Those who have had the unpleasant experience of being recirculated will understand.
The pour over water immediately forces you under. As soon as you resurface, the recirculation pushes you back up to the pour over. Furthermore, swimming in that type of water is well nigh impossible. The water is highly aerated to the point that it will barely float a body, even with a PFD. The water affords little purchase for a paddle, or for a persons arms and legs as they try to swim.
Another video
Elkhorn Creek near Frankfort Kentucky is a popular Class II+ whitewater run. It has a low head dam at the Jim Beam Old Grand-Dad distillery that most people portage. The dam can be run, but it is not recommended for reasons made clear by the video:
http://www.youtube.com/watch?v=VqH_v24LMN0
Had the C-1 boater not capsized and been able to take one more forward stroke he would have been past the boil line safely. He wisely choose to punch out and swim vigorously downstream but his boat, which had less sticking down into the deeper current flowing downstream, did not escape.
I mentioned earlier in this post to beware of bank eddies that tended to have recirculation feeding back up into the hydraulic. This dam is a good example. The spillway over this dam does not extend all the way to the river left bank. There is a small landing area on the extreme left where most people get out to portage around the dam. Downstream of this landing is a bank eddy visible in the video. Although it is not dramatic, one can clearly see the upstream recirculation in this eddy feeding right into the hydraulic at the dam pour over.
I have seen people who did not take notice of this recirculation to try to put in this eddy too far upstream so as to avoid carrying over rocks further downstream. While messing around with their spray skirt or whatever, they can be fed up into the hydraulic unaware.
another video
This one is incorporated into the ACA website and was produced by a good friend of mine.
www.youtube.com/watch?v=a5ajFJ4tuoA
My lowhead dam story
There is a short WW run on the North Branch of the Pawtuxet River in RI that only runs when the river is in flood. It’s about 2 miles long with lots of long wave trains, a couple of class II drops and a long nasty ledge (like a low head dam) with relatively safe shoot on the river left. Here is the ledge:
http://www.flickr.com/photos/eckilson/9030450468/in/album-72157634097612538/
I ran this section a couple of years ago with a few friends and a new guy who had just purchased a Mamba. I was in the lead and forgot about the ledge until I was almost in it. It was too late to head left and catch the shoot, so I got up as much speed as I could and ran the drop. I punched through fine.
Unfortunately, the new guy that was following me didn’t make out quite as well. He went over the drop sideways and got stuck in the hole. He bailed out of the boat and was able to swim out over the boil line - lucky. His brand new Mamba got sucked under in the recirculating current and never came back up.
Since then we have called that ledge the Mamba Eater. We were both pretty lucky that day.
thanks
for the warning
No problem
I’ll take you down if you ever come to RI 