Sediment Gravity FlowSediment Gravity Flow

We have discussed two classes of flows We have discussed two classes of flows that can move particles:that can move particles:

Simple dry Simple dry mass wastingmass wasting; and ; and conventional conventional fluid flow.fluid flow.

A third flow class, wet or fluid-assisted A third flow class, wet or fluid-assisted mass wasting, has properties of both.mass wasting, has properties of both.

Four types of Sedimentary gravity flowsFour types of Sedimentary gravity flows

Four types of Sedimentary gravity flowsFour types of Sedimentary gravity flows Grain flows-occurs Grain flows-occurs

when cohesionless when cohesionless sediments (ex. Dry sediments (ex. Dry sand) moves sand) moves downward under downward under the pull of gravity.the pull of gravity.

Although there is air Although there is air or water trapped or water trapped between the grains, between the grains, it merely acts as a it merely acts as a lubricant and does lubricant and does not actually propel not actually propel the grains.the grains.

Grain flowGrain flow

The most familiar The most familiar examples of grain examples of grain flow are sand flow are sand avalanches. They avalanches. They occur whenever a occur whenever a sand dune sand dune becomes steeper becomes steeper than the angle of than the angle of repose (25°-30 °).repose (25°-30 °).

Grain flowGrain flow

Grain flows also occurs in the deep sea, Grain flows also occurs in the deep sea, where sand avalanches down the sides of where sand avalanches down the sides of steep submarine canyons.steep submarine canyons.

Fluidized Sediment FlowsFluidized Sediment Flows Concentrated dispersions of grains Concentrated dispersions of grains

supported by pore water in between.supported by pore water in between. They begin to flow when something They begin to flow when something

increases the pressure on the interstitial increases the pressure on the interstitial pore water, turning the once-firm sand into pore water, turning the once-firm sand into a soupy liquid, ex. quicksand.a soupy liquid, ex. quicksand.

Quicksand - grain shift and settle past one Quicksand - grain shift and settle past one another; liquefaction. A person should not another; liquefaction. A person should not continue to sink (as in a swimming pool) continue to sink (as in a swimming pool) beyond your point of buoyancy. To get out beyond your point of buoyancy. To get out swim as you would in a pool. Lie flat.swim as you would in a pool. Lie flat.

Fluidized Sediment FlowsFluidized Sediment Flows

Liquefaction occurs in others instances, for Liquefaction occurs in others instances, for example in shockwaves sent through example in shockwaves sent through saturated soil by an earthquake.saturated soil by an earthquake.

Mudflows and Debris flowsMudflows and Debris flows

MudflowsMudflows are composed of a slury-like are composed of a slury-like mass of liquefied mud that move downhill mass of liquefied mud that move downhill under the force of gravity. under the force of gravity.

If there are large particles, ranging up to If there are large particles, ranging up to boulder size, they are known as boulder size, they are known as Debris Debris flowsflows..

Mudflows and Debris flows Mudflows and Debris flows are most are most common in steep mountain canyons common in steep mountain canyons during heavy rainstorm.during heavy rainstorm.

Mudflows and Debris flowsMudflows and Debris flows

These flows These flows have the have the consistency of consistency of wet cement and wet cement and can move as can move as fast as water in fast as water in a flash flood.a flash flood.

Sequence of aerial photograph of a debris Sequence of aerial photograph of a debris flow moving along a canyon bottom.flow moving along a canyon bottom.

2 meters highBouldery frontAt 1.3 m/s

Mudflows and Debris flowsMudflows and Debris flows

Some beds exhibit Some beds exhibit reverse (inverse) reverse (inverse) grading, with the grading, with the coarsest material coarsest material at the top and the at the top and the finer at the bottom.finer at the bottom.

The pressure push The pressure push the largest grains the largest grains to the top, when to the top, when stop they settle.stop they settle.

Turbidity CurrentsTurbidity Currents

Turbidity Currents are gravity flows in Turbidity Currents are gravity flows in which the sediment is supported by which the sediment is supported by upward turbulence of the fluid within the upward turbulence of the fluid within the flow.flow.

It is trigger by gravity.It is trigger by gravity. The mass of suspended sediment is The mass of suspended sediment is

denser than water. denser than water. Typically produces normal graded Typically produces normal graded

bedding.bedding.

Sedimentary Sedimentary structures of structures of the four major the four major types of types of sedimentary sedimentary gravity flow gravity flow deposits.deposits.