Thursday, March 3, 2011

Laminar & Transitional Flows

Re= l*u*ρ/µ

Ice flows are laminar because they have very high viscosity. This can be seen in the ice cliffs along the edges of glaciers in Taylor Valley, Antarctica. The ice is particularly cold and is so viscous that it does not flatten out on the time scale of at least dozens of years. Because the flow is laminar, when the ice melts or sublimates, it dumps all grain sizes into one deposit, called a diamictite. (Diamictites are defined as very poorly sorted sedimentary rocks with no grain size sorting within them. Thus, debris flows also deposit.) If one knows that the diamictite was deposited by ice, it is then called till or tillite.

If the glacier melts on land, it leaves piles of till in moraines. Water from melting of the ice is turbulent (unless it contains lots of mud-sized sediment). Thus, the turbulent water from melting ice can sort sediments, usually associated with braided river facies due to a high proportion of bed load sediment.

If glacial ice melts over water, the debris is deposited into the water, commonly forming a till sheet. If only a few large clasts are deposited in the water, they are called “drop stones”. This is one way to get large grains in very deep, standing water.

Mudflows & Debris Flows
High concentrations of mud-sized grains, particularly if they are clay minerals, increase the viscosity of the flow and decrease the flow speed. Both of the reduce the Reynolds number, leading to laminar or, more commonly, transitional flows. These flows can transport boulders at very low flow speeds compared to a water flow with little mud. Also, the grains are not well sorted by size. All grain sizes are transported together and when the flow stops, they are deposited all at once. This creates a diamictite, like glacier ice melting does. (However, it does NOT produce a tillite, which has to have a glacial origin.)

Mudflows (few large clasts) and debris flows (many large clasts) commonly form when there is a very high supply of mud-sized grains relative to the amount of water. The ratio of mud to water affects viscosity and thus the level of turbulence in the flows. If sediment is deposited or water is added, they can become more turbulent through time. In contrast, if water drains out or more sediment is added to the flow through erosion, they become more viscous. If they become too viscous, they can “freeze” in place, leaving a thick layer of diamictite.

Flash floods and land slides are processes that can initiate mudflows and debris flows.

Here is a play list of laminar and transitional debris flows:

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