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LH Dam

                                                                                                                                                   -- Jim Simmons 
 *Work in progress, please bear with us as we have changed sites.

Generally, the public does not perceive whitewater as being all that risky. This can be verified by the numerous river accidents around the country with people being ill prepared for the forces and power of whitewater. The public is also unaware of the dangers of low head dams as substantiated by accidents and deaths that continue on dams.
Note: research has been done my a civil engineering graduate student at Brigham Young University on low head dams.  Go to this site to see the results of the research:

Dams constructed earlier in the 20th century have been called 'killing machines" because water flowing over a smooth face all the way across a stream, creates a backflow and hydraulic that appears rather innocent but in actuality is quite deadly. To compound the risks of the strong hydraulic, builders often constructed concrete abutments at the ends making it virtually impossibile for an entrapped person or boat to escape by exiting the ends.  Other common man-made obstructions that can create a strong hydraulic action after substantial rainfall, and therefore a hazard to the unwary, are concrete low water crossings, low lying bridges, and elevated roadways.

Since 911 and increased emphasis on all types of rescue, more information is being made available to the public about the dangers of low head dams. Some cities have placed signs upstream of urban dams while others promote media reports and provide notices from local emergency agencies about dam dangers. While all this is helpful there is still much misunderstanding about dams, river currents and the powerful nature of rivers, especially in flooded conditions. Recent 2007 summer storms producing heavy rainfall and flooding taxed rescue personnel around the U.S. with these kinds of rescues.

Let's take a look at how low head dams and hydraulics act. As the current falls over the face of the dam, or over a rock/ledge creating a river hydraulic and into a depression below, the water flowing downhill (gravity) tries to reach equilibrium by filling in the void created by the depression. A hydraulic results when the water flowing upstream recirculates behind the object. Also, the churning aerated water that boils back to the surface does not offer an entrapped swimmer any buoyant support and because of the power and turbulence of the water, a swimmer wearing a PFD can easily be drowned if caught in a hyrdaulic. (See Figure 1)
*Work in progress, please bear with us.
Whitewater paddlers might think of how ineffective a paddle brace is in the 'crazy' white water that has a whirlpool kind of frothy, boiling action--it offers little or no support for the bracing action. This aerated water also doesn't have much oxygen and a trapped person would have diffiuclty getting a gulp of air.

Just downstream of the depression and backwash, a boil line forms where the water boils back to the surface in different directions. This is a neutral area but an object or person upstream of this boil line will be pulled back into the hydraulic. The area downstream of the 'boil line' is termed the outwash. Here the water becomes darker and river current again travels downriver. Typically, anyone trying to rescue an entrapped swimmer, or a boat, must not go upstream of the boil line. In the past rescuers lacking knowledge of the hydrology of dams have been sucked into the hydraulic only to become victims themselves.

It is beyond the scope of this discussion to include dam rescue methods, but a Rating System has been developed by Rescue 3 International for assessing the degree of difficulty for outside rescuers trying to rescue a person(s) stuck in the hydraulic. One handy place to find this rating scale is in the Appendix of Fred Ray's SWIFTWATER HANDBOOK FOR RESCUERS (interested readers might want to see the serious risks involved to both rescuers and victims). Information about dam rescue methods can be found in Ray's handbook, as well as in a rescue text by Wayne Sundmacher, Jr.

Important factors about each individual dam are a) the height and angle of the dam face or drop--the higher the drop the greater the backwash will be, b) the volume of water flowing over the dam--the more water the greater the backwash, and c) depth from the highest point of the boil line to the depression at the base of the dam. Of course, the width of a stream where a dam is located is also a critical factor when attempting to rescue someone--the wider the more difficult for rescuers. (Refer to Figure 2 for a profile view)

The Binghamton, NY dam accident of 1975, in which three firefighters died, actually gave a lot of impetus to the beginnings of modern day swiftwater rescue. At this dam location the river was 472 feet wide, very smooth (a classic killing machine) even though it was only four feet high. Watching a video of this tragic event is quite sobering for sure.

The answer for low head dams is recognition and prevention, the same 'cure' as for cold weather outdoor activities and the accompanying threat of hypothermia. A dam, even one that's not very high, from a position upstream, will have a 'horizon' line displaying features downstream that tend to look the same (in the background). The consistent features should alert people that a change in the stream is ahead. If uncertain about what's below whitewater paddlers and fishermen on a lake, should take the time to go to shore and walk down and scout to see what is ahead. If warning signs are posted be sure to heed them and exit the stream or lake to go check. Over a lifetime of observing the phenomenon of low head dams I have known of many people who have unsuspectingly floated, or drifted, over a dam and into the hydraulic below.

In whitewater/river boating hydraulics are formed by river current flowing over the natural formations of rocks and ledges creating the same kind of depression and hydraulic of a low head dam. These hydraulics (or holes) may be rather small and not very wide. As contrasted with structured dams the edges of rocks/ledges are usually unsmooth and create a turbulence that can make river holes irregular and unpredictable. In whitewater boating language, depending on how the deep the depression (hydraulic), the curler formed by the backwash may be unforgiving and described by whitewater boaters as a 'keeper'. The terms friendly or unfriendly have been used to describe how a hole is configured and whether or not it is comfortable for playboating.

An instructor colleague, Tom Jenkins, likes to think of an eddy (in the horizontal plane) as a hydraulic turned on its side. The river flow in the 'eddy' recirculates back upstream behind the rock, or obstruction, that is causing the eddy to form. As the river current blasts by the edge of the rock heading downstream the river current drags water out from behind the obstructing rock and the surface eddy flow in the lower portion of the eddy moves back upstream to get into the void. In fact, positioning your whitewater boat in the lower portion of the eddy will allow the surface eddy flow to help your boat gain momentum to begin a forward ferry or make a peel out.

Whitewater paddlers enjoy 'playing' in river holes, performing a variety of surfing and playboat type maneuvers. A popular aspect of boating today is referred to as "park and play." After a work day paddlers travel to a favorite surfing spot, especially on a dam controlled stream with dependable river flow, park the truck and 'play' at a favorite surfing spot without ever progressing downriver. The water falling over river ledges often creates a consistent surfing wave (similar to ocean waves) that help boaters sharpen paddling skills.

For safety awareness if the ends of the river 'hole' behind a rock or ledge are curved (positioned) upstream of the apex of the hydraulic, much like a half moon in a bright sky, this hole is termed unfriendly. (See Figure 3) It also might be deep and violent. As with a low head dam should a swimmer be caught in this turbulence the water's action would recirculate the swimmer, or boat, to the ends of the hole, back to the middle likely preventing the swimmer from escaping.
*Work in progress, please bear with us.
Figure 3
(Ends curved upriver)
On the other hand, if the ends of the 'hole are curved (positioned) downstream of the apex this hole is termed friendly because the swimmer (or boater) can maneuver to the ends of the hole amd once again get into the current that continues downstream. (See Figure 4) A hole that is exactly perpendicular to the river current direction may or may not be what is called a 'keeper' in whitewater lingo. Paddlers either have to possess local knowledge about one, or else evaluate the dynamics of river hydraulics before venturing into a certain hole.
*Work in progress, please bear with us as we are changing sites
Whitewater paddlers who have been caught in strong hydraulics have reported that the water action tried to suck off their helmets or PFDs, and river shoes have been jerked completely off. When wet exiting in a hole recently, Tim Jones described how the hydraulic action immediately tried to take off his helmet and as he tumbled about on 'rinse cycle' the hydraulic tried to pull off his river shoes. While he stated this hole wasn't really that hard to exit, he commented about the strong pull while he was in it.

In our rescue workshops if we have the necessary river features at a teaching site we like to have participants float over pourover rocks. Although a hydraulic forms below a pourover rock (term for the river current pouring over a smooth flat rock), a swimmer will usually float on through rather than be stopped by the hole. As mentioned above, the aerated water in the turbulence created will not support a swimmer's body weight. In this case a swimmer will submerge under the surface and momentum will carry the swimmer on into the downstream current.

Rescuing someone from a low head dam or a strong river hydraulic is not an easy task, even for trained rescuers who may have the necessary equipment. To adhere to rescue principles, reaching and throwing strategies should be attempted first. Although rescue PFDs are useful tools in modern day swiftwater and whitewater rescue it would not be wise to purposely swim into a very strong and large river hydraulic while tethered to a rescue line. Such a decision would be high risk as a rescuer could easily become wrapped and entangled in the line because of tumbling about in the "washing machine" turbulence of the hydraulic.

American Rivers, a river activist organization formed in 1973, continues to strive to address the matter of dams across America that are no longer needed, have been abandoned, or that are in need of repair and are a threat to the public. AR has recently been involved in the dismantling of several major dams around the nature once again takes its course in natural river environments. There is a boost to wildlife, fish migrations and spawning, and recreational boating as a river is returned to his natural flow.

Refer to related articles on River Hydrology and River Swimming Strategies.

Jim Simmons--11/2/'07. ACA Instructors, Tom Jenkins and Jim Jones contributed to this article.  Updated in 2013 to reference the low head dam research conducted at Brigham Young University.
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