Moving Water
RESCUE TECHNIQUES
Photo by Bridget Thomson
Dive/Rescue Series:
We all know that water either moves or sits still. Moving water could have a surface current, bottom current, or both. Still water will cause an unconscious victim to pretty much have a direct descent to the bottom and offers no further movement of the body. There will, of course, be an ascending of the corpse after it begins to decay, but then we are no longer dealing with rescue.
Moving water can be rather confusing, and our in-water rescue teams need better diving capabilities and understanding of moving water. Let us begin by looking at the speed of water.
A one-knot current moves at a rate of 100 feet per minute (fpm), which means that a two-knot current moves at a rate of 100 feet in 30 seconds or 200 fpm. That’s pretty fast.
I constantly hear statements such as: Our divers were in a sixknot current the last time they were looking for a drowned victim; and, I think the water was moving at twenty knots the night of the flood.
For our own safety, I believe we should know as close as possible just how fast water is really moving. Another safety precaution should be the mandatory use of personal flotation devices (PFDs) for everyone going closer than 50 feet to the water. When someone falls in the water or if a rescuer is in trouble, everyone wants “in.” Personnel without PFDs will enter the water or find themselves close to the water’s edge without even realizing it. Fast water (2+ knots) can be dangerous to all who are near the water’s edge. Without the use of PFDs, drowning can be imminent.
A well-conditioned diver with top skills could possibly make a slight headway in a 1-knot current. He would stand still in a l’/2-knot current and would be driven backwards with no hope of forward momentum against 2 knots.
Putting a diver in water running at more than 1½ to 2 knots without special training and well-planned tactics could be a major mistake. Two to 2½ knots of moving water will tear off a diver’s mask if he turns sideways to the current. It also makes it almost impossible to work from shore. Boat capabilities are needed.
We also need to know the speed of water in order to correctly set up a safety boat or backup for our inwater rescue team. If the water is moving at 2Vi knots (250 fpm) and our diver is in 40 feet of water, where should our backup boat be downstream?
A diver should ascend at 60 fpm. However, we know that most divers ascend at more like 100 fpm. From a depth of 40 feet, a diver would be at the surface in about 30 seconds. If he were untethered, he would have traveled at a rate of 250 fpm for 30 seconds. He would be no less than 125 feet from his last known underwater location when he surfaces. It would take a safety boat a good 30 seconds after that to even see the strayed diver and begin to proceed in a rescue. (An untethered diver is extremely susceptible to drifting in direct proportion to the current and will be traveling as fast as the water.) Within another 30 seconds, the diver will have traveled a minimum of another 125 feet at the surface.
Since the backup boat needs to come upstream to the diver and not downstream to keep from running over him, the safety boat should be no less than 300 to 350 feet downstream from the farthest point the tender intended to place the diver. This will assure us a good start in planning our backup safety for our in-water team.
When your team arrives on site, you should put a marker on the beach and walk up current for 50 feet along the beach front. Then take a closed styrofoam coffee cup or tennis ball and toss it from shore into the current. As the float is moving, walk back to the marker on the beach, timing the movement of the cup or ball. If the item reaches the marker in 60 seconds, the water is moving at a half knot.
We can now begin to make a plan for diving as well as begin to figure out what might have happened to our victim and how we are going to work our divers to our best advantage.
If the water is 30 feet deep and we use an average drop rate of one foot per second for our victim, in a half-knot current with 30 feet of 25 to 35 feet away from where it went underwater. Now the problem is as good as the information you have on where the victim was last seen.
We know through tests run in 1985 using an in-water rescue mannequin about the size of a three-year-old and weighing 28 pounds, that the body would travel in direct proportion to the speed and depth of the water until it reaches the bottom. Once on the bottom, the mannequin, whose relative weight was only two to three pounds, did not move.
Tests were run to check how far a victim would travel after submersion with immediate retrieval. Additional tests were performed by determining the placement of the victim after submersion and placement of a marker buoy nearby. In 10 out of 12 tests where tidal changes can be up to ten feet and tidal currents are up to two to three knots, the mannequin did not move more than one or two feet once on the bottom.
Mannequins were left on the bottom for 24, 48, and 72 hours. In one 72-hour test, a mannequin moved over 10 feet to the side of an underwater bulkhead during the first 16 hours underwater. It remained there for the duration of the 72-hour test. In another 48hour test, our mannequin traveled over 150 feet down river. The reason and time of movement is unknown.
These tests were conducted in Whitestone, NY, at the end of a 400-foot pier which connects to a 125-foot steel barge. In one test, we placed a diver 100 feet downstream and on the bottom of the river in 29 feet of water at Site 1 (see schematic). With 1¾ to 2 knots current, we threw a mannequin in the water at Site 2. In 30 seconds, our mannequin came to rest at the feet of our diver—a little over 100 feet away.
The mannequin was tossed into the water at various times of the day and night. The following illustrates how far the mannequin traveled (before reaching the bottom) at varying current rates:
Actual work site: 400 ft. off shore Depth at high tide: 34 ft. max. Depth at low tide: 20 ft. max.
Current
0 knot
1/2 knot
1 knot
1 ½ knots
2+ knots
Traveled
2-4 feet
25-50 feet
40-60 feet
60-85 feet
90-100 feet
Recently in Washington, D.C., during a training program, we placed several mannequins on the bottom of a river. One was not found that afternoon and was left on the bottom for over 39 hours. This particular mannequin needed boat rescue, and our trainees were able to retrieve the mannequin in a total time of less than 26 minutes, including gearing up and launching the boat. To their surprise, the doll had not moved from where we placed it. To their dismay, they had missed it with three 40-minute tries two days earlier.
I know that many of you will have charts explaining how currents are dissipated as you go deeper underwater; however, using the one foot per second drop combined with surface speed of the water, your victim should not be outside the parameters you have designated.
Again, safety backup for your divers and surface crew is a must in case of a lost or runaway diver. We need to have a backup safety procedure for our dive crews in case one of our divers is taken downstream by the current.
Victims most often sink right where the accident occurred and move only during their descent to the bottom. And there they stay until the body begins to decay and body gases cause them to begin to rise. I quite often hear “rescue” teams tell me that 90% of all drowned victims come up in the same general area. When running water is involved and the body begins to decay, there is a slow rising of the victim. During that time the body will move.
After the body surfaces, it is usually found in an area where locals frequent, such as a park, good fishing area, or swimming hole. This is not because that is where the body was or where it came up. It is because that is where people congregate and the body has moved along the surface until it reaches a point where it can be most readily seen.
