Survive A Silt-Out!: Diving In Low Visibility
Would you know what to do if you get caught in a silt-out with almost zero visibility?
Silt-outs are what happens when we kick up the fine sediment that we often find in enclosed underwater areas, such as inside wrecks or caves, as well as sometimes on the bottom of open water as well, and in particular in lakes.
Silt is technically a type of granular material that is finer than sand and is often light and dusty in feel when dry, much like the type of flour used for baking, and with a more slippery feel when wet.
As it is very light, it is easily disturbed by movement, either from waves, current, or a diver’s body or equipment.
It is often carried by water currents and accumulates inside enclosed areas that are protected from currents, which is why we very often find it in caves and wrecks.
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The fact that it is easily disturbed is what can create a silt-out.
A silt-out happens when someone touches or kicks up a large amount of silt, degrading the visibility to often zero.
Inside a silt-out, you can literally not see your own hand in front of your eyes.
Dangers of a silt-out
Silt-outs tend to happen near bottoms and inside wrecks or caves.
Because the visibility decreases instantaneously to next to nothing, it can cause uneasiness or panic in divers, even experienced divers.
It can cause uncontrolled ascents, or, if near a drop-off, descents, as well as buddy separation.
Inside enclosed water spaces, it can be fatal, as was demonstrated only too well a few years ago in the Grotto Rosso in Italy, where a group of divers, including their guide, drowned inside the cave after becoming disoriented following a silt-out.
With no visibility, it is hard to find your way out of a wreck or a cave, which can cause panic, which in turn leads to more frantic movement patterns, worsening the silt-out.
The best thing to do about silt-outs is to avoid them altogether.
Your finning technique, buoyancy, and trim are critical in this respect.
The preferred finning technique in areas where silt is present is the bent-knee cave diver kick, and of course, good buoyancy control will mean that you don’t accidentally hit the bottom (or the ceiling of a cave or wreck, where silt also can be lodged).
Trim, the ability to stay level in the water, will help keep your legs and fins off the bottom, where it can disturb the silt.
Whenever you head into a cave or wreck, you should always have adequate training, and make sure you bring a powerful dive torch (which can help cut through the silt, offering some orientation), as well as a line attached outside the entrance to the cave or wreck.
This will allow you, in case of a silt-out, and you are diving in low visibility, to follow the line back to safety.
Get out of a silt-out
Should you find yourself in the midst of a full-on silt-out, there are a number of things you can do to get out safely.
1. Stay calm. Silt-outs are scary, but panicking will only make matters worse. Stop, breathe, think.
2. Maintain your depth. This won’t be a problem if you’re in a fairly small area, such as inside a wreck’s living quarters, but in large cargo rooms, or inside large caves, it can be trickier, and in particular in open water. Notice any pressure changes in your ears from increasing or decreasing pressure, and try to bring your dive computer or depth gauge close enough to your eyes to read it.
3. Make your way our carefully. If you’re in open water or have enough room overhead, try ascending slowly using your BCD until you come out of the silt cloud. Be careful you don’t ascend too fast, as the silt-out can make it hard to discern your ascent rate.
- If you don’t have this option, swim carefully out of the silt-cloud, using the bent-knee cave diver kick to avoid kicking up any more silt.
- If you’re inside a wreck or cave, you should have a line reel with you, and you should follow this line, Hansel and Grethel-style, back to the entrance.
- If you’re in a wreck without a line reel (which you shouldn’t be), try scanning around for anywhere in the cloud where the cloud seems even a little bit lighter than elsewhere, as this is probably the light zone around the entrance (provided you’re still within visual contact of the light zone).
One final tip: keep attention to your bubbles. It is in fact possible to be disoriented enough in a silt-out to not be able to tell the difference between up and down.
But thankfully, bubbles always rise, so noticing which way your bubbles go when you exhale can help you keep your bearings.