Wind Erosion | mysimulator.uk

Three ways a grain travels

Wind-blown sediment moves in three distinct modes depending on grain size. Fine dust stays aloft in suspension, carried for kilometres or continents by turbulence. Sand-sized grains move by saltation - short, low, ballistic hops - which is the dominant mode by mass, typically responsible for around 90-95% of total sediment transport on a sandy surface. Coarser grains too heavy to lift are instead nudged and rolled along the surface by the impacts of saltating grains, a slower mode called creep or reptation.

The threshold friction velocity

A grain resting on the surface won't budge until the wind's shear stress, expressed as a friction velocity u*, exceeds a threshold u*t set by the grain's weight, size and any interparticle cohesion from moisture or crusting. Ralph Bagnold's original wind-tunnel measurements in the 1930s established that for dry, cohesionless sand the threshold rises roughly with the square root of grain diameter - very fine dust actually needs a higher threshold to start moving than medium sand, because cohesive and aerodynamic forces work against it at that small scale.

Q ∝ (ρ / g) · u*³ · (1 − (u*t / u*)²)     Bagnold-type transport law

Q   = sediment transport rate (mass per unit width per unit time)
u*  = friction velocity (wind shear at the surface)
u*t = threshold friction velocity

transport switches on abruptly at u* = u*t, then scales ~u*³

live demo · grains launching into saltation once threshold is crossed● LIVE

Why the impact matters more than the wind

Once a saltating grain lands, its impact can eject several more grains from the bed via a "splash" process - even in places where the wind's direct lift alone would be too weak to move them. This self-sustaining chain lets saltation continue at wind speeds below the speed that originally started it, so the effective dynamic (or impact) threshold is lower than the static (or fluid) threshold needed to lift a grain from an undisturbed bed at rest.

What controls how fast a dune moves

Because sediment flux scales roughly with the cube of the friction velocity above threshold, small changes in wind speed produce disproportionately large changes in how much sand moves - a modest gust can transport far more material than the average wind speed alone would suggest. Moisture works the opposite direction: even a thin film of water between grains adds capillary cohesion that must be overcome before lift-off, sharply raising the threshold, which is why wet or vegetated surfaces resist wind erosion so effectively even under strong wind.

Frequently asked questions

Why does sand transport increase so sharply with wind speed?

Sediment flux scales roughly with the cube of the friction velocity above threshold, so doubling the excess wind speed above threshold can increase transport roughly eightfold - a small gust makes a disproportionately large difference to how much sand moves.

What is the difference between the static and dynamic threshold?

The static, or fluid, threshold is the wind speed needed to lift a grain from a bed at rest. Once saltation is underway, grain impacts can eject others at a lower wind speed than that, called the dynamic or impact threshold, which is why sand transport can continue even if the wind briefly drops below the speed that started it.

Why does wet sand resist wind erosion so much better than dry sand?

A thin film of water between grains creates capillary cohesion that must be overcome before a grain can be lifted, sharply raising the threshold friction velocity - which is why beaches and riverbeds only start blowing once the surface has dried out.

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Wind Erosion: Saltation and the Threshold of Motion