What is silt in diving?
Silt is the fine sediment lying on the bottom and on the ledges of every lake, wreck and cave — clay, mineral dust, rotting organic matter and rust. Disturb it and visibility drops from ten metres to zero in seconds, which is why it is the defining hazard of overhead diving.
"Silt" is a diver's catch-all for anything on the bottom fine enough to go back into the water when you touch it. Geologically it is a grain size — particles between 0.004 and 0.06 mm — but underwater the word covers clay, mineral dust, decaying vegetation and rust flakes alike. What they share is that they are light enough to be lifted by a fin wash, a hand or a stream of bubbles, and slow enough to come back down that the dive is over long before the water clears.
What are the different types of silt?
Not all of it behaves the same way, and knowing what you are floating over tells you how much room for error you have.
- Sand. Coarse and heavy. It billows and then drops out within a minute or two. The forgiving one.
- Clay. The cave diver's least favourite. It goes into suspension at the slightest touch, disperses through the whole passage and stays there for hours. Once it is up, that section is finished for the day.
- Organic detritus ("mung"). Decayed leaves, wood and biological matter — the black or brown fluff you find in sinkholes and lowland caves. Nearly neutrally buoyant, so it barely settles at all; it just drifts with whatever flow there is.
- Rust and scale. The wreck version. It lives on the ceiling of a compartment rather than the floor, and it comes down when exhaled bubbles hit the overhead.
How long does silt take to settle?
Long enough that the answer is always "plan not to disturb it". As a rough guide, for mineral grains in still water:
| What it is | Grain size | Sinks 1 m in |
|---|---|---|
| Sand | 0.06–2 mm | seconds to a few minutes |
| Coarse silt | 0.02–0.06 mm | 5 minutes to an hour |
| Fine silt | 0.004–0.02 mm | an hour to a day |
| Clay | under 0.004 mm | days to months |
| Organic detritus | any | may never settle — it is nearly neutral |
Those figures come from Stokes' law for quartz grains in still water at 10 °C, and they are orders of magnitude rather than a schedule: flow carries suspended sediment downstream instead of letting it fall, clay flocculates unpredictably, and organic material is too light for the physics to apply at all. The practical reading is simple — sand costs you a minute, clay costs you the dive.
What causes a silt out?
- Fin wash. By far the commonest cause. A flutter kick aims a jet of water down and behind you, and a diver in feet-down trim aims it straight at the bottom.
- Bubble percolation. In a wreck or a low cave, exhaled bubbles gather against the ceiling and knock loose whatever is stuck to it, which then rains down on the team behind you.
- Contact. A hand, a knee, a dangling gauge, a light head. Anything hanging off you that touches the bottom lifts it.
- The team. Diver one sees clear water; diver four swims into everybody else's cloud. Silt is cumulative, and the last diver in the line pays for the first three.
How do divers avoid it?
- Buoyancy and trim first. Stable neutral buoyancy and a flat, horizontal body position are what keep the wash behind you instead of underneath you. Everything else is secondary.
- Silt-friendly kicks — frog kick, modified flutter, helicopter turn, back kick, plus finger-walking and pull-and-glide where the passage is too tight for any of them.
- Nothing dangling. Clip off hoses, gauges and lights.
- Stay high. Swim in the upper part of the passage as far as the ceiling allows.
What do you do when it happens?
Stop. Do not swim into it, and do not swim blind looking for a way out. In open water, hold your depth and ascend calmly on your instruments. In an overhead environment the guideline is the entire answer: get a hand on the line in a loose OK grip, take touch contact with your team, and follow the line and its arrows out by feel. That is precisely the emergency the line was laid for.