Waterfall Rescues and Water Dynamics


CarolinaFireJournal - Bob Twomey
Bob Twomey
08/07/2015 -

For those rescuers lucky enough to live in the mountainous regions of the state where waterfalls commonly exist, you have the added responsibilities and challenges of having to perform rescues from cascading waterfalls and/or the pools below them. In western North Carolina, typically each year, several rescues, or body recoveries, can be expected to occur. It almost never fails that we will have rescues to perform at a set of falls.

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Substantial flow and consistently wet rocks.

Likewise, across the state water rescues from low-head dams, rapids and areas of swiftly flowing water are common. Basically, a waterfall is a beautiful, natural thing to see. They are powerful, awesome in splendor and beauty, many times serene and seemingly docile — and VERY deadly.

Stream and river flows are measured in cubic feet per second, as in the number of cubic feet of water flowing past a certain point in the channel in a given amount of time. One cubic foot of water contains 7.48 gallons of water. Use 7.5 gallons per cubic foot for ease of figuring. A creek flow of 1 CFS (1 cubic foot/sec) means that across the width of that channel, 7.5 gallons of water is flowing past a certain point in one second. If you were to look at that channel for one minute, 7.5 gallons/second x 60 seconds/minute = 450 gallons of water that flowed past that point in one minute. Also, one cubic foot of water weighs 62.4 pounds, so for every second of time passing, 62.4 pounds of water is flowing down that channel. So, a small river, flowing at 500 CFS is flowing at 3740 gal/sec, or 224,400 gal/minute.

The two main dangers around a waterfall is the volume and velocity of water flowing over the falls, and the slickness of the creek and adjacent rocks. First, volume of flows as discussed above gives you an idea about shear volumes of water going over a falls. But a critical factor in water flows is the VELOCITY at which water is traveling in the channel, measured in feet per second. Any given volume of water is flowing at a certain velocity. It is the sum of these two forces that can be deadly to hikers and recreationalists — and to their rescuers.

A human body immersed in water displaces a certain volume of water due to the mass of the body in the water. This is called displacement. If you were to walk into a stream of flowing water, your body gets deeper into that water. Said another way, the water gets higher and higher on your body. As more of your body displaces the water, you become lighter in weight. For every one cubic foot of water displaced by your body, you become 62.4 pounds lighter, because you have displaced that one cubic foot of water. As you walk further out into the channel, your body now displaces even more water. If the average rescuer weighs 200 pounds on dry ground, and you have proceeded into the water to a point where your body now is displacing 2.5 cubic feet of water — approximately waist to lower chest deep — you will have essentially reduced your body weight by 156 pounds! As such, the actual weight of the human body — as it is weighed on dry land — is reduced when submersed in water; thus, you “weigh less” in water than on land due to displacement.

So, how much flow or current is required to sweep you off your feet now? Additionally, you recall from your high school physics classes that for every force there is an opposite and equal force. A 50,000 ton super tanker floats after it displaces an amount of water equal to the tankers weight. Likewise, a PFD (personal flotation device) providing 20 pounds of buoyancy can float a 200 pound rescuer because of the rescuer’s displaced weight in water. Now, if you as a rescuer get into water, you will weigh less. Add the flow velocity to the water you are in, and it should be clear that not much force is required to carry you along in the current.

So, how does all this relate to rescuers and waterfall rescues?

If you are required to enter water above a set of falls, it is critical that you understand how easily even a small flow of water can sweep you off your feet. It is standard protocol to securely be tied in to a rescuer belay or safety rope. Even though you are tied in to prevent you from being swept over the falls, the rocks in and around the channel are usually VERY slick due to the presence of algae, moss and wet soils. Often overlooked as a great danger are the dry rocks close to the water’s edge. They may have dried algae on them. If your boots are wet and you step upon these seemingly dry rocks, the presence of water on your boots is enough to wet and lubricate the dry algae, allowing you to easily slip and fall. Therefore, it should be standard protocol that any rescuer or support staff within a certain distance of the waterline and top of the falls be securely tied-in to a safety rope, dedicated only to that purpose.

If rescues are required to be performed at night, the safety issue is increased. Heavy foliage around the top of a set of falls can greatly complicate access to the top as well as required rope rigging to effect a rescue. Thus, your rescue spotter(s) first deployed to the top of the falls to ascertain the best rope location placements to perform a rescue must be extremely careful approaching the falls due to limited visibility. Should the rescue need to be performed in cold weather, night or day, the presence of ice on the rocks and “spray ice” — that ice formed at, around, and under a set of falls — must be carefully observed, as the presence of ice will have a great bearing on the type of rescue to be performed and where and how rigging will be done to do the rescue.

Another danger to be aware of is the potential for a mass of “root mat” to shear from the surface of the rock under your weight. If you have shallow moss and vegetation on thin, very thin layers of soil atop the rock near the falls, this wet, slick soil mat can, under some conditions, shear or break free of its hold on the rock, usually as a result of your weight and movement on the mat.

Top of the Fall Rescues

Rescues performed from atop a set of falls are indicative of a victim caught at or near the top of the falls, or a victim caught on a shelf or protrusion of some type — log, tree, rock, etc. — along the face of the waterfall. However, most fall victims usually fall to the bottom of the waterfall due to the speed at which the slip occurs, leaving no time to grab a tree or other object. Once acceleration begins in flowing water over slick rocks, there is little a victim — or unsuspecting rescuer —can do but fall.

Cascading type waterfall with pools between falls.

Considerations for recovering a deceased victim of a fall in adverse conditions, such as high water, ice present all around the waterfall, floodwater conditions, night time, extremely slick rocks, or any combination thereof, require very careful evaluation. In rescue, we are taught to carefully consider risk verses benefit to the patient, or victim, and you the rescuer(s). How willing would you be, as Incident Commander (IC), to placing rescuers in great danger or risk, to recover the body of a fall victim? I’ll say it another way: how bad must fire conditions be in a structure fire before the IC decides upon an “exterior only” attack, due to danger to firemen? There will come a time in your rescue career where you are faced with this difficult decision.

Rope Selection

Knowing the dangers discussed in this article, each call on the rescue scene is a judgment call. The dangers discussed notwithstanding, there are also complications associated with water on your ropes and rigging equipment. Have any readers here ever had to rappel on an ice-covered static kernmantle rope before? If not, I can assure you you’re in for a treat. Wet rescue ropes generally lack “friction” in standard braking systems. At the very least, wet ropes, and the wet braking systems through which they are rigged, will not generate the usual amounts of friction found in dry-rigged systems.

“Dry-weave” rescue ropes — woven to help the exit of water off of and out of the rope core and its mantle — offer benefits in water-type rescue work by allowing for more friction to be evident even when the rope is moist. But saturated ropes will usually still be so wet that you must consider the limited, available friction relative to the type rescue you are performing.

For example, if you are rigging a nine to one “Z Drag” hauling system to retrieve your patient and rescuer, and you are using prusik cords as the “rope grabs” on you Z Drag, you may consider using more than two, tandem, triple-wrapped prisiks to grip each section of the Z Drag System. Another consideration here might be the use of mechanical rope grabs that do not slide when under tension. However, the Rescue Technician Ropes Rescue Curriculum teaches that these type grabs can, under great loads, pinch and damage the lifeline rope. If loaded beyond a limit, they can pinch the rope so severely — because they are doing their job — that your lifeline becomes in danger of breaking.

It is a local judgment call on what you use and how you use it, but this article seeks to inform, and hopefully just remind you that knowing the dangers of wet ropes and equipment in wet environments will cause you to carefully consider how you will perform your rescue. And if you have only a body recovery, these considerations will cause you to carefully weigh rescuer risk to victim recovery — the benefit.

Anchor Placement

Anchor selection atop a rocky waterfall with shallow soils deserves great consideration. Trees or suitable shrubs for anchors may have a very weak root system. Placing heavy loads on trees/shrubs with shallow root systems can be catastrophic, especially if you select only one or two trees/shrubs as anchor points. Consider use of multiple anchors to allow for greater safety margins aimed at reducing the chances of anchor failure. You should take the time to evaluate the risks and assure the safety of your rescuers and victim(s).

Usually victims slipping off a waterfall go all the way to the bottom, making access easier and safer — sometimes. Again, the depth of water, its volume and velocity, and the ever-present danger of a “hydraulic” present in the pool below the falling water can be very dangerous. Remember that a hydraulic is a recirculation of water as a result of weight, volume and velocity as the water enters a pool of water. Victims, and rescuers, too, can easily become trapped in a hydraulic, constantly being vertically “recirculated” in the roll of water at the bottom of the falls.

In winter, thick “spray ice” can form onto every surface at and below the falls as the water mist sprays out onto these objects. Some years ago, we had a body trapped in a hydraulic in the pool below a well-known waterfall in January, and everything was coated with clear, thick ice. One could hardly stand up, much less perform a rescue. We eventually retrieved the body, but I can assure you, that recovery was “one for the books.”

A braided flow near vertical waterfall with wet and dry rocks around it.

Summary

In summary, know that there are unique dangers associated with rescues or recoveries at waterfalls. Moving water is very powerful. At the very least, it will cause you to consider additional safety measures to assure a safe outcome. Physical surroundings around waterfalls offer great dangers and limitations to use in your rescue. A careful risk benefit analysis must be performed relative to the circumstances at hand. There is no substitute for good and continuous training, nor for equipment less than the best available. Think and use good judgment, and remember that you are there because the waterfall has already claimed one victim. It CAN happen to you, too!

Bob Twomey is the founder and past chief of the North Carolina High Level Extraction Rescue Team, Inc. a volunteer helicopter search and rescue support team based in Transylvania County, North Carolina. He is the senior helicopter pilot for Wolf Tree Aviation, LLC operating out of Transylvania Community Airport. He has served in numerous officer and training positions in the Transylvania County Rescue Squad. He has been active in SAR for 41 years. He can be reached at 828-884-7174 or [email protected].
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Issue 34.1 | Summer 2019

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