Sediment Gravity Flow We have discussed two classes of flows that can move particles: Simple dry mass wasting; and conventional fluid flow. A third flow class, wet or fluid-assisted mass wasting, has properties of both.

Four types of Sedimentary gravity flows

 Grain flows-occurs when cohesionless sediments (ex. Dry sand) moves downward under the pull of gravity.  Although there is air or water trapped between the grains, it merely acts as a lubricant and does not actually propel the grains.

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

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

Fluidized Sediment Flows  Concentrated dispersions of grains supported by pore water in between.  They begin to flow when something increases the pressure on the interstitial pore water, turning the once-firm sand into a soupy liquid, ex. quicksand.  Quicksand - grain shift and settle past one another; liquefaction. A person should not continue to sink (as in a swimming pool) beyond your point of buoyancy. To get out swim as you would in a pool. Lie flat.

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

Mudflows and Debris flows  Mudflows are composed of a slury-like mass of liquefied mud that move downhill under the force of gravity.  If there are large particles, ranging up to boulder size, they are known as Debris flows.  Mudflows and Debris flows are most common in steep mountain canyons during heavy rainstorm.

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

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

2 meters high Bouldery front At 1.3 m/s

Mudflows and Debris flows  Some beds exhibit reverse (inverse) grading, with the coarsest material at the top and the finer at the bottom.  The pressure push the largest grains to the top, when stop they settle.

Turbidity Currents  Turbidity Currents are gravity flows in which the sediment is supported by upward turbulence of the fluid within the flow.  It is trigger by gravity.  The mass of suspended sediment is denser than water.  Typically produces normal graded bedding.

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