Particle DepositionParticle Deposition

Settling rate -- the speed at which a particle will settle to the bottom of a body of water (deposition)

There are There are 3 factors3 factors which affect the which affect the settling rate of a particlesettling rate of a particle

1) Size – The smaller the particle (clay, silt) the slower it will settle out. Larger sediments (cobbles, boulders) will settle quickly.

As the stream slows down, the larger As the stream slows down, the larger particles settle first…particles settle first…

2) Shape – Rounder, more spherical particles settle out faster than flat, angular or irregularly shaped particles.

Why is this? Friction is decreased when a particle is rounded and therefore it settles faster. It’s like it’s more aerodynamic - like a sports car. But in this case, it’s hydro-dynamic.

Hydrodynamic particles (round) settle Hydrodynamic particles (round) settle faster than flat or irregular shaped faster than flat or irregular shaped

particles particles

3) Density – If two particles have the same basic size and shape, the denser particle will settle faster.

Which will settle faster – a glass marble or a lead marble? (PS – they are the SAME SIZE marble…)

Settling Rate vs. Settling TimeSettling Rate vs. Settling Time

There is an inverse (indirect) relationship between settling rate and settling time. Think about it, sediments that settle at a faster rate (rate increases) require less time to settle (time decreases)

Time to settle (sec)Settl

ing

Rat

e (c

m/s

ec)

Remember what the graph of an inverse relationship looks like…

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Sorting of Deposited Sorting of Deposited ParticlesParticles

Final deposition of particles (sediments) usually occurs at the mouth of a stream. This is due to the faster flowing stream emptying into a slower larger body of water.

Horizontal sortingHorizontal sorting takes place. The sediments that were once carried down the stream are arranged from largest to smallest.

Horizontal SortingHorizontal Sorting

More Horizontal SortingMore Horizontal Sorting

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Vertical SortingVertical Sorting

Happens in QUIET WATER (still – lake, pond) Particles sort out from bottom to top in layers On bottom – largest, roundest, densest particles On top – smallest, flattest, least dense particles Can occur when a depositional event, like a landslide, dumps a large volume of unsorted sediments into a still body of water like a lake It creates “graded beds” of sediments

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Vertical Sorting: Graded BeddingVertical Sorting: Graded Bedding

One depositional event

One depositional event