I. Four key physical aspects of sedimentation: gravity, shear stress, normal stress, friction

gravity (g) is a force that acts to atttract two masses. In this case it is acting between a grain and the Earth. It pulls the grain straight down into the earth.

shear stress (t) is force acting per unit surface area parallel the surface. It is that component of gravity acting along the surface.

normal stress (s) is force acting per unit area at right angles to the surface. It is that component of gravity acting into the surface.

friction (fr) acts parallel, but opposite, to t. Coefficient of friction is a constant for given surface material. f(grain roughness, roundness, etc) and, as such, does not vary with g or any of its components.

II. Critical angle (a_c) is the slope where t = s (fr). Any increase in slope leads to movement of the grain. Any decrease in slope leads the grain to stop moving.

In other words, at any slope (a) there is a critical shear stress (t_c) beyond which the grain will move and below which the grain will not move.

III. What factors influence the shear stress along the bed under a fluid?

t = rw g h sina since water density (rw) and g are close to constant on Earth and because at very low slopes sin a Å a then you can see that shear stress (t) is approximately Å depth * slope i.e. t Å h(a)

A. t differs from velocity in that velocity is f(slope, friction, g) and does not include r_w or h

B. So, to get a grain to move: the t at the bed of a flow ³ t_c

C. If now we place a grain under a fluid we facilitate its downslope movement by:

generating bouyancy which reduces normal stress increase drag on the grain by allowing moving water to push the grain generate Bernoulli lift which reduces normal stress

IV. Grain settling f(grain diameter (D), grain density (r_s ), fluid (usually water) density (r_f ), viscosity (m))

A. First consider there are two types of flow: laminar vs. turbulent

1. in laminar flow, water particles are all moving nearly parallel to each other downflow

2. in turbulent flow, water particles are moving more chaotically, where in local areas water might be moving up, down, in, out, or even upstream.

B, Even in cases where water is turbulent (which is usually the case), if a grain is small enough in diameter, relative to the characteristic diameter of the turbulence, the grain acts as if it is locally in laminar flow (or, if you prefer, the grain thinks it is in laminar flow). This ability of a grain to be thought of as moving through a laminar flow, even when the flow is turbulent depends on the grain's diameter, its settling velocity, and the apparent viscosity ('kinematic viscosity') of the fluid. If the grain is small and/or settles slowly and/or the fluid behaves very viscously then the grain settles as it were moving down through a laminar flow. In equation form we can say the same thing as: Grain Reynolds Number (Re) = (D * settling velocity)/ kinematic viscosity) ² 1,then it can viewed as having laminar behavior.

Regardless if you follow that explanation or not, just realize that in this discussion we will look only at the case of laminar flow.

C. Stokes' Law: Settling Velocity = 1/18 [((r_s-r_f) g D²)/m]

1. look at the equation and realize:

a slight increase in grain size leads to normal grading

an increase in viscosity and/or an increase in r_f can lead to a matrix-supported deposit (i.e. large clasts appear to be floating in a matrix of fine grained mud) as opposed to grain-supported deposit where each large clast is resting on another large clast.

D. Suspended Load vs. Bed Load: grains will stay in suspension if the upward motion of water, due to turbulence, is larger than the grain's settling velocity.

Therefore, the smaller or less dense grains will tend to by carried in a flow up in suspension and settle down to the bed, to become bedload, once the flow's turbulence becomes less (i.e. the flow slows down).

CLICK HERE to go to movie of bedload transport

CLICK HERE to go to movie of a migrating bedload sheet

V. Sediment Continuity Equation:

A. Hjulstöm's Curve: concerns the minimum shear stress to get a grain to move in a flow (i.e. t_c). In general larger grains require higher t_c

The curve is a bit fuzzy because interial effects are involved (i.e. t to get the grain to move vs. to keep it moving can be a bit different).

Flow Competence: faster moving flows can carry larger particles

Flow Capacity: faster moving flows can also carry more particles

B. Sediment Continuity Equation: is a statement of the conservation of mass, that says that you get deposition vs. erosion (or if you prefer aggradation vs. degradation due to changes in the flow shear stress (or velocity) over time or over space.

If you prefer it said mathematically:

1. Note: grain size has nothing to do with the Sediment Continuity Equation . . . it just says sediment will deposit. As a rule coarser grains will tend to deposit before finer grains (because the former tend to travel low in the flows, and from Hjulstöm's observations, but all sorts of grain sizes if available will settle out. If a flow suddenly stops moving, all the grains will fall out (get normal grading), but if you slow a flow down more gradually, there is a tendency for the coarsest material to come out of the flow, although other grain sizes come out too, especially if they are moving along the bed along with the coarse grains. Ultimately, the faster flows slow down, the more poorly sorted the deposit. The more slowly a flow slows down, the better sorted the deposit. However, bear in mind that sorting is also controlled by what grain sizes are being carried and by the process of transport as well.

VI. Heirarchy of Bedforms as a function of flow velocity

A. Bedform- bed configurations that naturally form by a moving traction carpet of sediment

Define:

ripples, dunes	CLICK HERE to go to movie of subaqueous dune migration
plane bed
antidunes	CLICK HERE to go to movie of antidune formation in a flume

B. Experimental results of bedform

CLICK DIAGRAM TO ENLARGE click next page to return - Hujlström Diagram from Harms, J.C., Southard, J.B., and Walker, R.G., 1982, Structures and Sequences in Clastic Rocks: Lecture Notes for Short Course No. 9, Society of Economic Paleontologists and Mineralogists, 279 p.

Define: 2-D (straight crested) vs. 3-D (sinuous crested) ripples (dunes)

CLICK HERE for animation showing migration of 2-D dunes (ripples)

CLICK HERE for animation showing migration of 3-D dunes (ripples)

C. Note that without aggradation there would be no preservation.

CLICK HERE for slide show of bedforms and related features.

D. Processes over a dune:

Define: entrainment, avalanche face, saltation, bedload, suspended load, flow separation, scour pit, trough cross-bedding

E. Importance of rate of deposition on bedform climb

1.Climbing ripples

CLICK HERE for movie showing climbing ripples and graded bedding formed in a turbidity current

VII. Tool marks vs. sole marks vs. bedforms

VIII. Wave ripples (aka oscillation ripples aka symmetric ripples)

CLICK HERE for movie showing formation of wave ripples

IX. Gravity flows (aka density flows, aka mass flows)

down slope movement of dense fluid (typically made dense by incorporation of suspended sediment)

e.g. mudflow, debris flow, turbidity current, landslide, avalanche

CLICK HERE for movie showing subaerial debris flow

CLICK HERE for movie showing turbidity current

X. What is bedding?

Episodic, discontinuous, deposition

 

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