Key Points of Sediment Transport in Water
1) Faster flows have more bed shear stress. Thus, faster flows move larger grains (when considering sand sizes and larger).
2) Sediment is transported as bedload and in suspension. Bedload consists of rolling and saltating grains.
3) Grain size, density and flow strength (Re) determine how grains are transported.
4) As flow strength changes, grains are eroded or deposited. These relationships are represented in the Hjulstrom diagram.
A Few Definitions:
1) "Stratification" - layers in rocks; stratified rocks are those organized into beds
See Grand Canyon Beds: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L1/12GrandCanyon.jpg
2) “Beds” are separated by “bedding planes” - cm to m thick units of sedimentary rock that were deposited approximately horizontally (beds) and are separated by horizontal planes (bedding planes); the rocks typically weather more along these planes. Beds are usually fairly uniform or change gradationally in composition. Bedding planes usually represent breaks in sedimentation or changes in grain size. In other words, they usually represent changes in flow characteristics.
See Cache Creek Turbidite Beds and Bedding Planes: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L1/13tiltedturbidites.jpg
3) "Laminae" are color, composition, or grain size variations defining surfaces within a bed. They typically represent variations in flow velocity, sediment supply, sediment composition, etc. Planar Laminae are parallel to bedding, e.g. planar.
4) "Cross Lamination”, "Cross Stratification" or "Cross Bedding" are laminations or layers that are oriented obliquely to bedding. They truncate older laminae and are truncated by younger laminae. The erosional surfaces that separate “sets” of similarly oriented laminae are called “bounding surfaces”. There are lots of subdivisions of cross stratification; different types represent different types of bedforms and different flow conditions.
See Burns Cliff on Mars observed by Opportunity: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L4/3BurnsCliff.jpg The upper part of the image has planar lamination, and the lower part to the far left has cross lamination or stratification.
See: http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughXStrat3.jpg for trough cross stratification and other examples of dune cross stratification at: http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Dunes.html
See ripple cross lamination on Mars: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L4/5MartianRipples.jpg
Bedforms
When sediments get deposited from turbulent flows, the sediment interacts with the geometry of the flow. Depending on the flow speed, turbulence, and sediment characteristics, different structures or bedforms develop.
See: http://faculty.gg.uwyo.edu/heller/SedMovs/mcbriderips.htm
Bed Geometry and Flow Separation - Until now, we have been implicitly assuming that the bases of beds are flat and smooth, but if sediment is present, they are not. If you start with a smooth bed of sand and increase water speed above it, irregularities form from irregularities in the flow and develop into ripples. First, a few grains pile up. Once the height of the pile is several grains high, there is a flow shadow down stream of them, and the laminar sublayer detaches from the base of the flow. The water has enough momentum that it does not hug the bed surface and instead, goes shooting out over the top. This point is called the separation point. The water flows forward and downward and reconnects with the bed at the attachment point. At the attachment point, water is flowing directly towards the sediment with a lot of force. This force moves the grains and causes erosion. In contrast, the area between the separation point and the attachment point has very low flow. In fact there are back eddies, where the flow is upstream. Thus, sediment transport is very irregular along the bedding surface at a local scale.
Sediment Transport Over a Ripple - Sediment grains are mobilized at the attachment point - more so than in normal flow because the water is shooting directly into the sediment - and the grains are moved downstream by saltation and traction. As the flow becomes parallel to the sediment surface again, its ability to transport sediment decreases. Thus, the grains tend to pile up and a new mound forms. This gives a periodic chain of mounds - the beginnings of ripples. As flow continues, grains roll and saltate up the stoss (upcurrent) side of the ripples. Once they pass the crest, they reach the low flow on the lee side of the ripple. The larger grains settle out and roll partway down the slope; this is the site of net deposition. As the process of deposition on the lee side and erosion on the stoss side continues, the ripples migrate downstream. If there is net deposition of sediment, the ripples leave behind distinctive dipping layers between two erosional surfaces that can be preserved in the rock record. These layers slope downstream and are one type of cross lamination.
A sketch of a ripple or dune like the one in lecture:
http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/duneXStrat.jpg
Watch the USGS bedform movies described at: http://mygeologypage.ucdavis.edu/sumner/gel109/labs/USGSBedforms.html
Bedforms and Flow Velocity- The size and shape of subaqueous bedforms depends on flow strength and grain size and can be used to interpret ancient flow characteristics in a depositional environment from looking at sedimentary rocks. See Nichols (2009, Sedimentology and Stratigraphy, section 4.3) for bedform, flow speed, and grain size relationships.
Ripples (crest-to-crest differences of less than 50 cm and heights of less than 4 cm) – The minimum flow for ripples is determined by the minimum velocity for sediment transport. Once this flow speed is reached, ripples form if the sediment is transported as bedload. The maximum flow speed for ripples depends on the location of the attachment point on the stoss side of the ripples. As flow gets faster, too much erosion occurs at the crests of the ripples - the point of attachment is too far up the stoss side of the ripple- and the ripples flatten out. Dunes develop.
Dunes (60 cm-100’s m wavelength and 10’s of cm to meters in height) - Dunes develop as ripples flatten out because large scale irregularities start to develop. The basic ideas of dune and ripple formation are the same. The difference is that the area of flow separation is much larger (see Fig 4.17, Nichols 2009). Roller vortexes (e.g. upstream flows along the lee sides of dunes) are common, and the upstream flow can be strong enough to form ripples that migrate upstream. As flow speeds increase, the dunes start to flatten out.
Planar/Flat Lamination – Planar lamination forms when the flow is strong enough that the beds flatten out. The momentum of the transported grains and fluid are high enough that they tend to move horizontally, eroding any irregularities in the bed. This zone of planar lamination is called “upper flow regime”. (Why “upper”? - there is a zone of planar lamination in coarse grained sediment at low flow velocities.)
Antidunes - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly (Putah Creek in flood; tidal channels; creeks flowing across beaches - see http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg). Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases.
Other Types of Flows - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/ARipples.html. Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than the stoss slope. Flows can also be irregular due to combinations of currents and waves, etc. Some of these flows are very characteristic of specific environments, for example, storm-influenced beaches. The structures they produce are very useful for interpreting ancient rocks, and we will highlight them as we discuss different sedimentary environments.
Bedforms and Grain Size - Bedforms also vary with grain size (see Figure 4.20, Nichols, 2009). Very fine sand and silt are very easy to transport and erode. They form nice ripples, but do not form dunes when transported by water. Instead, ripples transition into planar laminae. Coarse sand and larger sediment is too hard to transport and erode to get ripples. The erosional force at the reattachment point is not strong enough to erode the coarse grains and produce the erosional surfaces on the backs of ripples. Without this erosion, troughs do not form and without troughs, crests do not form. The sequence of structures in granules with increasing flow is:
1) no transport
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring
3) dunes - the flow is strong enough to erode at the attachment point
4) upper planar lamination
5) antidunes
In contrast, the sequence of structures in silt is:
1) no transport
2) ripples
3) upper planar lamination
4) antidunes
Extra
High Sediment Loads - Sometimes with slope failures on land or under water, much more sediment can be put into motion than the flow would normally erode. Depending on the amount of water mixed with the sediment, the flow characteristics are different. When abundant water is present, the sediment can form a thick slurry with a higher density than sediment-free water, commonly leading to a higher Re and more turbulent flow (Re=u*l*r/µ). Also, collisions between grains become extremely important. Both of these tend to keep the sediment moving. Grain-to-grain collisions also have an important effect on grain sorting. The collisions tend to make sorting much less efficient and the sediment that gets deposited tends to consist of which ever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes.
If there is very little water associated with a clay-rich sediment flow, the flow can be very viscous due to the charge attraction among clay particles. The high viscocity makes the flow laminar (Re=u*l*r/µ). Debris flows with lots of cohesive mud are like this. In laminar flows, there is no mixing of the water or grains (or ice) and there is no sorting of grain sizes. Thus, the sediment remains mixed up with large grains, sometimes boulders, “floating” in mud. They flow down hill pulled by gravity until the flow seizes up and stops. This can be due to too low a slope or loss of water. Underwater debris flows can also be diluted by water that gets incorporated at the edges of the flow and become less viscous and more turbulent.
There also are dry sediment flows in which air is present between grains. For example, rock avalanches and some pyroclastic flows from volcanoes lack water. For these to move significant distances, large amounts of energy from either gravity or explosions are necessary to keep the sediment in motion.
Showing posts with label saltation. Show all posts
Showing posts with label saltation. Show all posts
Wednesday, January 16, 2013
Monday, January 14, 2013
Fluid Flow and Sediment Transport
Key Concepts from Lecture 2
Reynolds Number - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Re = (fluid inertial forces)/(fluid viscous forces) = l*u*r/µ where the variables are flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (r), and fluid viscosity (µ). Turbulent flow has Re is greater than 2000 and laminar flow has Re is less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well.
Boundary Layer and Laminar Sublayer - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer.
Sediment Transport
Bed Shear Stress: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. It also depends on the slope. If the slope is steep, gravity helps pull grains down the slope, increasing bed shear stress. Also, the roughness of the bed is a factor. A rough bed deflects flows and increases turbulence, which increases the bed shear stress, particularly in places where flow is directed into the sediment and the boundary layer is compressed. (See http://en.wikipedia.org/wiki/Sediment_transport#Bed_shear_stress for more detailed information.)
The Bernoulli Effect
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect.
Which Grains Move? Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is the position of a grain relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference as if it is on a flat surface. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average.
Bedload and Suspended Load Transport
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain settles out versus how turbulent the water is (back to Re...). Bedload refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in traction. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be saltating. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water.
Here is a playlist with movies related to sediment transport: Sumnerd’s Sed Transport Playlist and an extra movie at
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb)
Suspended sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow.
Photo of suspended sediment in a Costa Rica River: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg
YouTube video of white clay in a turbulent flow in a flume: http://tinyurl.com/78kg3z The pulsing in the flow is (probably) due to the pump that is making the water flow.
Hjulstrom Diagrams
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams. Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the critical velocity. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the critical threshold for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done!
A copy of the Hjulstrom Diagram can be found on Wikipedia: http://en.wikipedia.org/wiki/Hjulstr%C3%B6m_curve
Deeper flows can move larger grains at the same flow velocity because they are more turbulent: Re=u*l*r/µ and l is larger. This is because deeper flows can have larger variations in flow speed and the laminar flow layers are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than just Hjulstrom diagrams, which summarize a lot of characteristics into two axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess.
Silt and Clay - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clay minerals tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode.
In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix).
A few more words about saltation: Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur.
Watch grains transported by saltation and traction in these movies: http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb)
http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm (14 Mb)
You can also see bedload transport in a movie that describes why we think we have found evidence of a river on mars: http://www.youtube.com/watch?v=HYHc2alzdUk My friend Sanjeev narrates it. He is a professor at Imperial College in London. More information and graphics are at http://mars.jpl.nasa.gov/msl/news/index.cfm?FuseAction=ShowNews&newsid=1360
Deposition: Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and starts to leave grains on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments.
Ripples and Other Bedforms
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser laminae that dip downstream.
Next Time: We will talk about the details about sedimentary structures.
Reynolds Number - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Re = (fluid inertial forces)/(fluid viscous forces) = l*u*r/µ where the variables are flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (r), and fluid viscosity (µ). Turbulent flow has Re is greater than 2000 and laminar flow has Re is less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well.
Boundary Layer and Laminar Sublayer - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer.
Sediment Transport
Bed Shear Stress: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. It also depends on the slope. If the slope is steep, gravity helps pull grains down the slope, increasing bed shear stress. Also, the roughness of the bed is a factor. A rough bed deflects flows and increases turbulence, which increases the bed shear stress, particularly in places where flow is directed into the sediment and the boundary layer is compressed. (See http://en.wikipedia.org/wiki/Sediment_transport#Bed_shear_stress for more detailed information.)
The Bernoulli Effect
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect.
Which Grains Move? Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is the position of a grain relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference as if it is on a flat surface. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average.
Bedload and Suspended Load Transport
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain settles out versus how turbulent the water is (back to Re...). Bedload refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in traction. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be saltating. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water.
Here is a playlist with movies related to sediment transport: Sumnerd’s Sed Transport Playlist and an extra movie at
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb)
Suspended sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow.
Photo of suspended sediment in a Costa Rica River: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg
YouTube video of white clay in a turbulent flow in a flume: http://tinyurl.com/78kg3z The pulsing in the flow is (probably) due to the pump that is making the water flow.
Hjulstrom Diagrams
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams. Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the critical velocity. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the critical threshold for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done!
A copy of the Hjulstrom Diagram can be found on Wikipedia: http://en.wikipedia.org/wiki/Hjulstr%C3%B6m_curve
Deeper flows can move larger grains at the same flow velocity because they are more turbulent: Re=u*l*r/µ and l is larger. This is because deeper flows can have larger variations in flow speed and the laminar flow layers are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than just Hjulstrom diagrams, which summarize a lot of characteristics into two axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess.
Silt and Clay - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clay minerals tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode.
In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix).
A few more words about saltation: Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur.
Watch grains transported by saltation and traction in these movies: http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb)
http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm (14 Mb)
You can also see bedload transport in a movie that describes why we think we have found evidence of a river on mars: http://www.youtube.com/watch?v=HYHc2alzdUk My friend Sanjeev narrates it. He is a professor at Imperial College in London. More information and graphics are at http://mars.jpl.nasa.gov/msl/news/index.cfm?FuseAction=ShowNews&newsid=1360
Deposition: Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and starts to leave grains on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments.
Ripples and Other Bedforms
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser laminae that dip downstream.
Next Time: We will talk about the details about sedimentary structures.
Wednesday, February 15, 2012
Rivers
Transport Capacity
Erosion by water occurs when water is flowing across a surface and the flow is capable of transporting more sediment than is currently moving as bedload. This is called the sediment transport “capacity”. A certain number of grains of a certain size can be picked up by the Bernouli effect for a given flow. If there are too many grains, they start colliding and the characteristics of sediment transport change. Grains are directed back toward the bed and up into the flow. Eventually, more go back to the bed and are deposited, leaving fewer grains in the flow even at high flow speeds because there are more grains than the transport capacity of the flow.* In contrast, if there is a shortage of grains of a size that can be moved by the flow, e.g. the flow is moving all of grains present, any new grains will be eroded off the bed as soon as they are available. The flow then has excess transport capacity.
* Think about dumping a truck load of fine sand into a fast moving river, it takes time to move all that sediment even if the flow speed is theoretically fast enough to erode fine sand.
One of the most common times for a flow to have excess transport capacity is when the flow is speeding up. We know from the Hjulstrom diagram that faster flows transport larger grains. They can also transport more grains. Thus, water flowing downhill commonly speeds up, has excess capacity and erodes sediment. When it slows down, sediment is deposited. In floods, the water speeds up, erodes sediment, and transports it. As the flood ends, the water slows down and deposits the excess sediment. In general, erosion occurs when flows are speeding up or when they go from an environment with low sediment (e.g. a dam spillway) to an environment with more sediment (e.g. a river bed).
Rivers
Rivers are responsible for most sediment transport from mountains to lowlands and the oceans. They do the most to even out the topography that tectonic processes create. Rivers consist of channel, bank and overbank or floodplain deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds.
River Types -
Straight (rare, except for ones humans have modified)
Meandering (high sinuosity)
Braided (many branches within a channel)
Anastomosing (rivers with branching and merging channels)
The form of the river is controlled by the gradient of the river bed (steep = braided, gently dipping = meandering), local vegetation that stabilizes banks and limits the number of channels, sediment grain size, particularly the ratio of suspended versus bedload sediment, and sediment volume. A high bedload gives rise to abundant bars, which promotes formation of braided rivers.
Braided Rivers
Braided rivers develop when the proportion of bed load sediment is high, which produces abundant bedforms and promotes the development of bars, and thus, the braided character of the river. The sediment is commonly coarse, which requires fast flow and steep gradients for the sediment to be transported. Much of the geometry of braided rivers is shaped by the highest flows, e.g. spring floods, when the bars are covered in water. Many braided rivers have exposed bar tops for much of the year.
Flow speeds and transport capacity vary dramatically within a braided river. Friction with the riverbed tends to slow down the flow, particularly where the flow is shallow. Thus, the Reynolds number in shallow areas is relatively low (but still high enough that the flow is turbulent) and the transport capacity is low. In contrast, the transport capacity and Reynolds number are much higher in the deeper middles of channels in the river. Thus, the coarsest sediment is transported here, whereas finer sediment gets deposited in shallow areas. Also, bars block the flow on the upstream sides, and like dunes, the upstream sides tend to erode. Areas of low flow and eddies form on the downstream sides of bars, and they are usually sites of net deposition. Thus, bars migrate downstream through time. If we summarize the processes:
Sediment Transport:
1) The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest.
2) Bars are eroded upstream where the bars deflect the flow. Sediment is deposited on downstream side of bars and some on the flanks of bars where flow is slower, particularly on the insides of bends.
3) Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels.
Sedimentary structures include:
1) trough x-bedding in channels, due to the migration of irregular dunes
2) coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes
3) sediment on the edges of bars fines upward because the flow is shallower and slower, e.g. has a lower Reynolds number. Sedimentary structures can include anything from upper flat to ripple laminations.
Braided River Facies
Channels migrate back and forth leaving a sheet of sand with abundant cross stratification. These sheets of sand tend to fine upward. General characteristics of braided river deposits include:
1) Scoured surface at the base of a channel
2) Gravel lag at base of channel
3) Trough x-bedded sands deposited just off the center of channels
4) Occasional tabular x-stratification from migrating bars
5) Sand deposited at slower speeds (ripple cross lamination possible)
6) Overbank deposits from floods mostly composed of sand and silt, with some mud
The large scale geometry of the deposits includes sheets of sand with various grain sizes representing bar migration separated by floodplain deposits.
Meandering Rivers
Meandering rivers have a low gradient and thus slower flow, and usually have a high proportion of suspended sediment relative to the amount of bedload. A meandering river channel has curves that meander back and forth on a gently sloping floodplain. The flow speed in the channel varies with the geometry of the meanders. Water has to travel faster on the outside of bends than on the insides of bends. We know from the relationships between Reynolds number and bed shear stress that higher flow speeds mean that more and coarser sediment can be transported at higher flow speeds. Thus, we can predict that:
1. there is more erosion on the outsides of bends
2. the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available
3. sediment will accumulate on the insides of bend and this sediment will be finer grained.
If we look at a channel in cross section, it is asymmetric, representing the sites of erosion and deposition. Variation in flow speed also produce different sedimentary structures. Upper planar lamination and dune cross stratification are common where Re is highest, and ripple cross lamination is common where Re is lower.
The main parts of the channel include eroding bank, the thalweg (the deepest point of the flow) and the point bar (on the inside of the bend where most sediment is accumulating). As the channel migrates due to erosion and deposition, a distinctive suite of sedimentary structures accumulate. The deepest part is coarser and has upper planar lamination or dune cross stratification. This is overlain by finer sediment with current ripple lamination.
As meandering rivers migrate, the meanders tend to increase. Eventually, the channel forms almost a circle, and the meander gets cut off, often during a flood. This straightens the channel temporarily and produces an ox bow lake in the abandoned meander. The lake accumulates mud and organic matter.
Watch this cartoon of a meander migration in France: http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm
Levees and Floodplains - When a river floods, it goes from a confined flow in the channel which is very rapid to a widespread flow across the floodplain. It slows down very quickly and the water becomes shallower, both of which cause a decrease in Re. Thus, the water can not transport as much sediment on the floodplain as it does in the channel. Thus, finer sands that may be in suspension during a flood are transported as bedload or rapidly deposited once the river tops its banks. This produces levees. The finer silts and especially clays remain in suspension much longer and settle out on the floodplain as the flood waters dry up.
Watch this model of a meandering river flood: http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm
Over time, the levees build up and provide a higher bank for the channel than the level of the floodplain. Thus, the channel bottom can aggrade (fill in) until the bottom of the channel is as high or higher than the floodplain. When the next flood comes along, the river avulses and does not go back into its old channel which is higher than a new one on the floodplain. This results in the downstream part of the channel being completely abandoned.
Meandering River Channel Facies:
1. Scoured base of flow
2. Lag deposit with mud rip-up clasts and the coarsest grains being transported
3. Fining upward sands with trough cross stratification
4. Rippled sands
5. Sigmoidal cross stratification from migrating point bars
Floodplain Facies
1. Fine sand with climbing ripples
2. Mudstone/shale with mud cracks
3. Soils
4. Root casts
Ox Bow Lake Facies
1. Mudstone/shale without mud cracks
2. Organic-rich deposits, including coal
3. Anoxic water indicators (especially in fossils and absence of trace fossils)
Differences between braided and meandering river deposits:
1. Braided river deposits are commonly coarser grained
2. Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on floodplains.
3. Overbank deposits are better developed and finer grained in meandering river systems.
4. Bar migration is much more regular in direction in meandering rivers because there is a well defined, single thalweg towards which the bars migrate. In contrast, braided river bar migration occurs in multiple directions. Thus, meandering rivers produce a more regular geometry of tabular cross bedding, when preserved.
General Characteristics of Fluvial Sediments:
1) On a large scale, river deposits consist of sheets and lenses of sand deposited in channels associated with flat laminated shales and silts with rare rippled sand beds deposited on floodplains.
2) Fining upward sequences of beds in the sands with sedimentary structures that indicate decreasing flow speeds.
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification.
4) Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop since they are rarely preserved
5) Soil development in associated shales deposited in the floodplain environment.
Look at pictures of fluvial rocks at http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html
Erosion by water occurs when water is flowing across a surface and the flow is capable of transporting more sediment than is currently moving as bedload. This is called the sediment transport “capacity”. A certain number of grains of a certain size can be picked up by the Bernouli effect for a given flow. If there are too many grains, they start colliding and the characteristics of sediment transport change. Grains are directed back toward the bed and up into the flow. Eventually, more go back to the bed and are deposited, leaving fewer grains in the flow even at high flow speeds because there are more grains than the transport capacity of the flow.* In contrast, if there is a shortage of grains of a size that can be moved by the flow, e.g. the flow is moving all of grains present, any new grains will be eroded off the bed as soon as they are available. The flow then has excess transport capacity.
* Think about dumping a truck load of fine sand into a fast moving river, it takes time to move all that sediment even if the flow speed is theoretically fast enough to erode fine sand.
One of the most common times for a flow to have excess transport capacity is when the flow is speeding up. We know from the Hjulstrom diagram that faster flows transport larger grains. They can also transport more grains. Thus, water flowing downhill commonly speeds up, has excess capacity and erodes sediment. When it slows down, sediment is deposited. In floods, the water speeds up, erodes sediment, and transports it. As the flood ends, the water slows down and deposits the excess sediment. In general, erosion occurs when flows are speeding up or when they go from an environment with low sediment (e.g. a dam spillway) to an environment with more sediment (e.g. a river bed).
Rivers
Rivers are responsible for most sediment transport from mountains to lowlands and the oceans. They do the most to even out the topography that tectonic processes create. Rivers consist of channel, bank and overbank or floodplain deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds.
River Types -
Straight (rare, except for ones humans have modified)
Meandering (high sinuosity)
Braided (many branches within a channel)
Anastomosing (rivers with branching and merging channels)
The form of the river is controlled by the gradient of the river bed (steep = braided, gently dipping = meandering), local vegetation that stabilizes banks and limits the number of channels, sediment grain size, particularly the ratio of suspended versus bedload sediment, and sediment volume. A high bedload gives rise to abundant bars, which promotes formation of braided rivers.
Braided Rivers
Braided rivers develop when the proportion of bed load sediment is high, which produces abundant bedforms and promotes the development of bars, and thus, the braided character of the river. The sediment is commonly coarse, which requires fast flow and steep gradients for the sediment to be transported. Much of the geometry of braided rivers is shaped by the highest flows, e.g. spring floods, when the bars are covered in water. Many braided rivers have exposed bar tops for much of the year.
Flow speeds and transport capacity vary dramatically within a braided river. Friction with the riverbed tends to slow down the flow, particularly where the flow is shallow. Thus, the Reynolds number in shallow areas is relatively low (but still high enough that the flow is turbulent) and the transport capacity is low. In contrast, the transport capacity and Reynolds number are much higher in the deeper middles of channels in the river. Thus, the coarsest sediment is transported here, whereas finer sediment gets deposited in shallow areas. Also, bars block the flow on the upstream sides, and like dunes, the upstream sides tend to erode. Areas of low flow and eddies form on the downstream sides of bars, and they are usually sites of net deposition. Thus, bars migrate downstream through time. If we summarize the processes:
Sediment Transport:
1) The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest.
2) Bars are eroded upstream where the bars deflect the flow. Sediment is deposited on downstream side of bars and some on the flanks of bars where flow is slower, particularly on the insides of bends.
3) Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels.
Sedimentary structures include:
1) trough x-bedding in channels, due to the migration of irregular dunes
2) coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes
3) sediment on the edges of bars fines upward because the flow is shallower and slower, e.g. has a lower Reynolds number. Sedimentary structures can include anything from upper flat to ripple laminations.
Braided River Facies
Channels migrate back and forth leaving a sheet of sand with abundant cross stratification. These sheets of sand tend to fine upward. General characteristics of braided river deposits include:
1) Scoured surface at the base of a channel
2) Gravel lag at base of channel
3) Trough x-bedded sands deposited just off the center of channels
4) Occasional tabular x-stratification from migrating bars
5) Sand deposited at slower speeds (ripple cross lamination possible)
6) Overbank deposits from floods mostly composed of sand and silt, with some mud
The large scale geometry of the deposits includes sheets of sand with various grain sizes representing bar migration separated by floodplain deposits.
Example of a braided river in Alaska: http://g.co/maps/wrk9n It is cutting through glacial morraines deposited as a glacier retreated up the valley. Follow the river downstream (to the north and east) to http://g.co/maps/q5kq7. How does the channel geometry change?
Meandering Rivers
Meandering rivers have a low gradient and thus slower flow, and usually have a high proportion of suspended sediment relative to the amount of bedload. A meandering river channel has curves that meander back and forth on a gently sloping floodplain. The flow speed in the channel varies with the geometry of the meanders. Water has to travel faster on the outside of bends than on the insides of bends. We know from the relationships between Reynolds number and bed shear stress that higher flow speeds mean that more and coarser sediment can be transported at higher flow speeds. Thus, we can predict that:
1. there is more erosion on the outsides of bends
2. the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available
3. sediment will accumulate on the insides of bend and this sediment will be finer grained.
If we look at a channel in cross section, it is asymmetric, representing the sites of erosion and deposition. Variation in flow speed also produce different sedimentary structures. Upper planar lamination and dune cross stratification are common where Re is highest, and ripple cross lamination is common where Re is lower.
The main parts of the channel include eroding bank, the thalweg (the deepest point of the flow) and the point bar (on the inside of the bend where most sediment is accumulating). As the channel migrates due to erosion and deposition, a distinctive suite of sedimentary structures accumulate. The deepest part is coarser and has upper planar lamination or dune cross stratification. This is overlain by finer sediment with current ripple lamination.
As meandering rivers migrate, the meanders tend to increase. Eventually, the channel forms almost a circle, and the meander gets cut off, often during a flood. This straightens the channel temporarily and produces an ox bow lake in the abandoned meander. The lake accumulates mud and organic matter.
Watch this cartoon of a meander migration in France: http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm
Levees and Floodplains - When a river floods, it goes from a confined flow in the channel which is very rapid to a widespread flow across the floodplain. It slows down very quickly and the water becomes shallower, both of which cause a decrease in Re. Thus, the water can not transport as much sediment on the floodplain as it does in the channel. Thus, finer sands that may be in suspension during a flood are transported as bedload or rapidly deposited once the river tops its banks. This produces levees. The finer silts and especially clays remain in suspension much longer and settle out on the floodplain as the flood waters dry up.
Watch this model of a meandering river flood: http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm
Over time, the levees build up and provide a higher bank for the channel than the level of the floodplain. Thus, the channel bottom can aggrade (fill in) until the bottom of the channel is as high or higher than the floodplain. When the next flood comes along, the river avulses and does not go back into its old channel which is higher than a new one on the floodplain. This results in the downstream part of the channel being completely abandoned.
Meandering River Channel Facies:
1. Scoured base of flow
2. Lag deposit with mud rip-up clasts and the coarsest grains being transported
3. Fining upward sands with trough cross stratification
4. Rippled sands
5. Sigmoidal cross stratification from migrating point bars
Floodplain Facies
1. Fine sand with climbing ripples
2. Mudstone/shale with mud cracks
3. Soils
4. Root casts
Ox Bow Lake Facies
1. Mudstone/shale without mud cracks
2. Organic-rich deposits, including coal
3. Anoxic water indicators (especially in fossils and absence of trace fossils)
Differences between braided and meandering river deposits:
1. Braided river deposits are commonly coarser grained
2. Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on floodplains.
3. Overbank deposits are better developed and finer grained in meandering river systems.
4. Bar migration is much more regular in direction in meandering rivers because there is a well defined, single thalweg towards which the bars migrate. In contrast, braided river bar migration occurs in multiple directions. Thus, meandering rivers produce a more regular geometry of tabular cross bedding, when preserved.
General Characteristics of Fluvial Sediments:
1) On a large scale, river deposits consist of sheets and lenses of sand deposited in channels associated with flat laminated shales and silts with rare rippled sand beds deposited on floodplains.
2) Fining upward sequences of beds in the sands with sedimentary structures that indicate decreasing flow speeds.
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification.
4) Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop since they are rarely preserved
5) Soil development in associated shales deposited in the floodplain environment.
Look at pictures of fluvial rocks at http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html
Monday, February 6, 2012
Arid Environments
Review of Details of Cross Lamination/Stratification Formation
There are several important details of cross lamination formation that I’d like to emphasize before the midterm. First, the geometry of the laminae within the cross stratification reflects the geometry of the surface during deposition. A layers of sediment is deposited on a surface, and the geometry of the bottom of the layer preserves the geometry of the depositional surface. If there is a change in slope on the surface, there will be a change of slope at the bottom of the laminae that mimics the surface. However, layers can vary in thickness. On the lee size of a ripple or dune crest, there is often more deposition on the steep slope because that is where the sediment lands when it is washed over the crest. It is the high depositional rate here that makes the slope steep. If the slope gets too steep, the grains avalanche downslope, and it is no longer quite as steep. The geometry of the ripple or dune is directly related to the distribution of deposition.
The geometry of laminae on the downstream side, e.g. where they merge with the erosion surface that forms on the stoss side of the next ripple/dune downstream, is also interesting. The laminae thin to zero thickness right where they “downlap” onto this surface. If deposition occurs beyond the steep slope of the ripple/dune, the laminae curve and gradually thin. If deposition only occurs on the steep slope, the laminae end more abruptly again the erosional surface. The geometry of the laminae tells you very precisely where deposition occurred relative to the ripple/dune trough.
These videos review these concepts:
http://youtu.be/r62qIKNBkos
http://youtu.be/0Zanh17ulXs
http://youtu.be/ogM-UqcYIfU
Arid Environments
Aridity - Aridity defines a desert, not temperature. An arid region gets less than 250 mm of rain/year. Our average in Davis is about 480 mm. That puts us barely in a semi-arid climate, which has 250-500 mm of rain/year on average. Arid environments are characterized by little vegetation.
Types of Deposits Typical of Arid Environments - 1) Wind blown sand (well sorted, texturally mature medium or finer sand); 2) Flash flood deposits (poorly sorted breccia, including debris flows); and 3) Playa lake deposits (silt, mud and evaporites). There are also very cold desert environments, such as the McMurdo Dry Valleys, Antarctica. These environments have glacial deposits left by glaciers that flow in from areas with higher precipitation (e.g. higher elevations) or the ice cap.
Flash Flood Deposits - When it rains in deserts, it often floods because there is little vegetation to trap water in soils and slow the runoff. Two environment types dominated by flash flood sediment transport are common: valleys with ephemeral rivers (wadis) and alluvial fans. Alluvial fans form in areas with a steep gradient from a drainage catchment to the basin floor whereas wadis in valleys form where the gradients are much lower. Tectonic activity it typically required to maintain steep slopes because they erode to lower slopes through time. The Basin and Range Province in eastern California and Nevada is an area with abundant examples of alluvial fans. Less structurally active deserts where deposition is dominated by flash floods, such as eastern Egypt, tend to have wadis (which is an Arabic word).
Alluvial fans in Death Valley: http://g.co/maps/n2udd
A wadi in Egypt: http://g.co/maps/369c4
Alluvial fans - Alluvial fans are cone shaped accumulations of coarse sediment deposited at the transition from confined flow in a canyon to unconfined flow in a basin. This also corresponds to a break in slope. As the slope shallows and the flows spread out, the flows slow down and deposit much of the sediment that they were able to transport in the canyon. (Think about the Hjulstrom diagram.) Fan geometry is determined by the rate of deposition. At the canyon mouth, it is steeps (up to 15°) due to rapid deposition of coarse sediment. It shallows to about 5° over the main part of the fan and shallows even more to 1-2° at the toe. Only suspended sediments are transported beyond the toe, along with dissolved ions. If the water can pond, the fine grains settle out and the water evaporates forming minerals like gypsum and halite, and creating playa lake deposits. Deposition on a given alluvial fan is very rare - one event occurs about every 300 years on most fans in the southwestern US.
Wadis - Wadis are similar to braided river deposits, which we will talk about next week. They have a high sediment load for the amount of water.
Flow types - Two types of flows are common: 1) debris flows and 2) sheet flows.
Debris flows are slurries of mud, rock debris, and just enough water to make the sediment into a viscous flow. Due to the high viscosity, the flow is laminar, like a glacier, and like a glacier, there is no significant sorting of grain sizes. Debris flows can transport very large blocks. Debris flows continue to move until the internal friction of the flow due to viscosity exceeds the flow’s momentum when it freezes into place. This can occur due to either the loss of water or lower slope. The resulting deposits show little sorting and would be classified as a mud supported breccia or a diamictite. In most cases, debris flow deposits are unsorted and lack any form of stratification. They are laterally restricted because they do not spread out too much, and they are commonly an even thickness throughout, with steep edges to the flows.
Sheet flows are turbulent flows with significantly more water and less mud than debris flows. Since the flows are turbulent, there is significant grain sorting and normally graded, fining upward deposits are common. Once a flow reaches the mouth of the canyon, the flow spreads out and the coarsest rocks are deposited first. Finer grains are deposited later and farther down the fan and later in time. This produces normally graded beds, but deposition is very rapid and the grading is commonly poor. The suspended load may make it to the toe of the fan if the water doesn’t filter into the fan first. Sheet flood deposits produce broad deposits that are clast supported, with some imbrication of clasts. Unlike a debris flow, sheet flows commonly cover 1/3 to 1/2 of a fan surface.
Other types of flows - A number of other flow types are also common on fans. For example, if there is insufficient rain to produce a sheet flow, ephemeral rivers can flow down the surface of the fan - which is more common. This produces braided river type deposits, which we will talk about later. There is also a significant gradation between debris flows and sheet floods. They represent two end members, and there are lots of variations in mud content and water content which variously affect the viscosity of the flow and thus the resulting sedimentary deposits.
Characteristics of Alluvial Fan Deposits
Eolian Environments:
The viscosity of air is low so it is typically a turbulent flow (Re=ulρ/µ). Density is also low, but viscosity is a larger effect. Think back to the Bernoulli Effect. The lift force has to overcome gravity, but it also depends on the difference in density between the fluid and the grains. The lower the density of the fluid, the harder it is to lift the grains against gravity. Thus, wind speeds must be very high to transport grains, and wind tends to lift only medium sand or smaller grains into saltation or suspension even at 30 m/s (about 60 miles/hour). Larger grains can only roll along the ground, mostly due to impacts from saltating grains.
Saltation - Wind transports sand as bedload (traction and saltation) and in suspension, like water. The traction and saltation transport are slightly different, however, because the impacts between grains are more forceful. Water dampens the impacts by limiting grain speed by friction between the water and grains and the effects of viscosity. Air does much less so because of both lower density and much lower viscosity. Thus, impacts when saltating grains land are very forceful. This has 2 effects: 1) more grains are launched into the saltating layer than the fluid could lift. This leads to a positive feedback - once saltation starts, the number of saltating grains increases rapidly. 2) The landing grains push the grains they hit, leading to surface creep of grains. This processes can push much bigger grains up slopes than could be transported by the wind alone, even with traction transport.
Ripple Formation - As in water ripples, wind ripples form as an initial pile of grains once saltation has started. However, the mechanisms of growth are different. There is no separation of a laminar flow sublayer or back eddy as seen in water ripples, in part because air flow is very turbulent. Rather, the impacts from saltating grains push coarser grains up the back sides of ripples to the crests where they eventually avalanche off the lee slope. Most of the smaller grains get transported off the crests where wind speeds are the greatest. Some small grains may also accumulate in troughs, especially between the larger grains. This leads to one of the rare cases of reverse grading: The larger grains are more concentrated at the tops of ripples and smaller grains are more concentrated at the bases. If there is significant accumulation of sand, thin reversely graded layers can be preserved in the rock record and are indicative of aeolian transport.
Cross Lamination - Migration of ripples occurs due to erosion on the windward side and deposition on the leeward side, as in water ripples. However, cross lamination is rare because the sand in dunes tends to be well sorted, especially the sand that gets transported down the lee slope - which is where sediment accumulates. Sometimes cross lamination is preserved due to fluctuations in wind speed resulting in different grain sizes being deposited at different times.
Dunes and Draas - Larger bedforms also form due to wind transport of sand. Unlike water transported sand, dunes and draas (huge bedforms really observable only from the air) commonly have abundant smaller bedforms developed on them. Basic sand transport on both is the saltation of sand up their windward sides and avalanching down their leeward sides. Cross stratification is common and large scale. Meter-thick beds are common even though the tops of the bedforms are not preserved.
Sand Characteristics - Another aspect of strong collisions between grains is that they are commonly rounded very quickly and commonly have frosted surfaces due to collision damage. The collisions also break down softer grains, particularly lithic fragments. Thus, most dune sand consists of well rounded, well sorted quartz sand. Rare exceptions, such as the gypsum sands at White Sands National Park can persist due to a lack of hard dense grains to abrade the softer gypsum grains.
Characteristics of Eolian Deposits -
There are several important details of cross lamination formation that I’d like to emphasize before the midterm. First, the geometry of the laminae within the cross stratification reflects the geometry of the surface during deposition. A layers of sediment is deposited on a surface, and the geometry of the bottom of the layer preserves the geometry of the depositional surface. If there is a change in slope on the surface, there will be a change of slope at the bottom of the laminae that mimics the surface. However, layers can vary in thickness. On the lee size of a ripple or dune crest, there is often more deposition on the steep slope because that is where the sediment lands when it is washed over the crest. It is the high depositional rate here that makes the slope steep. If the slope gets too steep, the grains avalanche downslope, and it is no longer quite as steep. The geometry of the ripple or dune is directly related to the distribution of deposition.
The geometry of laminae on the downstream side, e.g. where they merge with the erosion surface that forms on the stoss side of the next ripple/dune downstream, is also interesting. The laminae thin to zero thickness right where they “downlap” onto this surface. If deposition occurs beyond the steep slope of the ripple/dune, the laminae curve and gradually thin. If deposition only occurs on the steep slope, the laminae end more abruptly again the erosional surface. The geometry of the laminae tells you very precisely where deposition occurred relative to the ripple/dune trough.
These videos review these concepts:
http://youtu.be/r62qIKNBkos
http://youtu.be/0Zanh17ulXs
http://youtu.be/ogM-UqcYIfU
Arid Environments
Aridity - Aridity defines a desert, not temperature. An arid region gets less than 250 mm of rain/year. Our average in Davis is about 480 mm. That puts us barely in a semi-arid climate, which has 250-500 mm of rain/year on average. Arid environments are characterized by little vegetation.
Types of Deposits Typical of Arid Environments - 1) Wind blown sand (well sorted, texturally mature medium or finer sand); 2) Flash flood deposits (poorly sorted breccia, including debris flows); and 3) Playa lake deposits (silt, mud and evaporites). There are also very cold desert environments, such as the McMurdo Dry Valleys, Antarctica. These environments have glacial deposits left by glaciers that flow in from areas with higher precipitation (e.g. higher elevations) or the ice cap.
Flash Flood Deposits - When it rains in deserts, it often floods because there is little vegetation to trap water in soils and slow the runoff. Two environment types dominated by flash flood sediment transport are common: valleys with ephemeral rivers (wadis) and alluvial fans. Alluvial fans form in areas with a steep gradient from a drainage catchment to the basin floor whereas wadis in valleys form where the gradients are much lower. Tectonic activity it typically required to maintain steep slopes because they erode to lower slopes through time. The Basin and Range Province in eastern California and Nevada is an area with abundant examples of alluvial fans. Less structurally active deserts where deposition is dominated by flash floods, such as eastern Egypt, tend to have wadis (which is an Arabic word).
Alluvial fans in Death Valley: http://g.co/maps/n2udd
A wadi in Egypt: http://g.co/maps/369c4
Alluvial fans - Alluvial fans are cone shaped accumulations of coarse sediment deposited at the transition from confined flow in a canyon to unconfined flow in a basin. This also corresponds to a break in slope. As the slope shallows and the flows spread out, the flows slow down and deposit much of the sediment that they were able to transport in the canyon. (Think about the Hjulstrom diagram.) Fan geometry is determined by the rate of deposition. At the canyon mouth, it is steeps (up to 15°) due to rapid deposition of coarse sediment. It shallows to about 5° over the main part of the fan and shallows even more to 1-2° at the toe. Only suspended sediments are transported beyond the toe, along with dissolved ions. If the water can pond, the fine grains settle out and the water evaporates forming minerals like gypsum and halite, and creating playa lake deposits. Deposition on a given alluvial fan is very rare - one event occurs about every 300 years on most fans in the southwestern US.
Wadis - Wadis are similar to braided river deposits, which we will talk about next week. They have a high sediment load for the amount of water.
Flow types - Two types of flows are common: 1) debris flows and 2) sheet flows.
Debris flows are slurries of mud, rock debris, and just enough water to make the sediment into a viscous flow. Due to the high viscosity, the flow is laminar, like a glacier, and like a glacier, there is no significant sorting of grain sizes. Debris flows can transport very large blocks. Debris flows continue to move until the internal friction of the flow due to viscosity exceeds the flow’s momentum when it freezes into place. This can occur due to either the loss of water or lower slope. The resulting deposits show little sorting and would be classified as a mud supported breccia or a diamictite. In most cases, debris flow deposits are unsorted and lack any form of stratification. They are laterally restricted because they do not spread out too much, and they are commonly an even thickness throughout, with steep edges to the flows.
Sheet flows are turbulent flows with significantly more water and less mud than debris flows. Since the flows are turbulent, there is significant grain sorting and normally graded, fining upward deposits are common. Once a flow reaches the mouth of the canyon, the flow spreads out and the coarsest rocks are deposited first. Finer grains are deposited later and farther down the fan and later in time. This produces normally graded beds, but deposition is very rapid and the grading is commonly poor. The suspended load may make it to the toe of the fan if the water doesn’t filter into the fan first. Sheet flood deposits produce broad deposits that are clast supported, with some imbrication of clasts. Unlike a debris flow, sheet flows commonly cover 1/3 to 1/2 of a fan surface.
Other types of flows - A number of other flow types are also common on fans. For example, if there is insufficient rain to produce a sheet flow, ephemeral rivers can flow down the surface of the fan - which is more common. This produces braided river type deposits, which we will talk about later. There is also a significant gradation between debris flows and sheet floods. They represent two end members, and there are lots of variations in mud content and water content which variously affect the viscosity of the flow and thus the resulting sedimentary deposits.
Characteristics of Alluvial Fan Deposits
- Poorly sorted beds that are of an approximately uniform thickness but of limited lateral extent, deposited by debris flows;
- Moderately to well sorted beds, often normally graded with pebbles at the base deposited in ephemeral channels; these show some cross stratification due to turbulent flow dynamics;
- Normally graded beds that are laterally extensive deposited by sheet flows;
- Average grain size decreases down slope and the abundance of debris flow deposits decreases down slope.
Eolian Environments:
The viscosity of air is low so it is typically a turbulent flow (Re=ulρ/µ). Density is also low, but viscosity is a larger effect. Think back to the Bernoulli Effect. The lift force has to overcome gravity, but it also depends on the difference in density between the fluid and the grains. The lower the density of the fluid, the harder it is to lift the grains against gravity. Thus, wind speeds must be very high to transport grains, and wind tends to lift only medium sand or smaller grains into saltation or suspension even at 30 m/s (about 60 miles/hour). Larger grains can only roll along the ground, mostly due to impacts from saltating grains.
Saltation - Wind transports sand as bedload (traction and saltation) and in suspension, like water. The traction and saltation transport are slightly different, however, because the impacts between grains are more forceful. Water dampens the impacts by limiting grain speed by friction between the water and grains and the effects of viscosity. Air does much less so because of both lower density and much lower viscosity. Thus, impacts when saltating grains land are very forceful. This has 2 effects: 1) more grains are launched into the saltating layer than the fluid could lift. This leads to a positive feedback - once saltation starts, the number of saltating grains increases rapidly. 2) The landing grains push the grains they hit, leading to surface creep of grains. This processes can push much bigger grains up slopes than could be transported by the wind alone, even with traction transport.
Ripple Formation - As in water ripples, wind ripples form as an initial pile of grains once saltation has started. However, the mechanisms of growth are different. There is no separation of a laminar flow sublayer or back eddy as seen in water ripples, in part because air flow is very turbulent. Rather, the impacts from saltating grains push coarser grains up the back sides of ripples to the crests where they eventually avalanche off the lee slope. Most of the smaller grains get transported off the crests where wind speeds are the greatest. Some small grains may also accumulate in troughs, especially between the larger grains. This leads to one of the rare cases of reverse grading: The larger grains are more concentrated at the tops of ripples and smaller grains are more concentrated at the bases. If there is significant accumulation of sand, thin reversely graded layers can be preserved in the rock record and are indicative of aeolian transport.
Cross Lamination - Migration of ripples occurs due to erosion on the windward side and deposition on the leeward side, as in water ripples. However, cross lamination is rare because the sand in dunes tends to be well sorted, especially the sand that gets transported down the lee slope - which is where sediment accumulates. Sometimes cross lamination is preserved due to fluctuations in wind speed resulting in different grain sizes being deposited at different times.
Dunes and Draas - Larger bedforms also form due to wind transport of sand. Unlike water transported sand, dunes and draas (huge bedforms really observable only from the air) commonly have abundant smaller bedforms developed on them. Basic sand transport on both is the saltation of sand up their windward sides and avalanching down their leeward sides. Cross stratification is common and large scale. Meter-thick beds are common even though the tops of the bedforms are not preserved.
Sand Characteristics - Another aspect of strong collisions between grains is that they are commonly rounded very quickly and commonly have frosted surfaces due to collision damage. The collisions also break down softer grains, particularly lithic fragments. Thus, most dune sand consists of well rounded, well sorted quartz sand. Rare exceptions, such as the gypsum sands at White Sands National Park can persist due to a lack of hard dense grains to abrade the softer gypsum grains.
Characteristics of Eolian Deposits -
- Well sorted, rounded grains;
- Little clay or silt sized grains;
- Large bedforms, thus thick sets of cross strata;
- Ripple stratification is rare;
- Some reversely graded laminae (not beds);
Wednesday, January 25, 2012
Sedimentary Structures cont. and Facies
Sedimentary Structures Continued
Homework #2 due today.
Ripples and Dunes (A review with a bit of additional information)
A sketch of a ripple or dune like the one in lecture:
duneXStrat.jpg
Remember where the separation point and attachment point are located. The geometry of the flow tracks these points. Erosion can only occur where there is a bed shear stress sufficient to move sediment. In other words, the the main flow must be near the sediment surface. Sediment accumulates into a deposit in the flow shadow downstream of the ripple or dune crest. In other words, sediment accumulates in the flow detachment zone. Laminae are visible where deposition occurs due to variations in flow speed and thus the grain sizes transported and deposited.
Dunes and ripples behave similarly at the level of detail that I have been describing them. Their cross stratification geometries are similar. However, dunes are larger than ripples. If the distance between erosion surfaces defining cross sets is greater than a few centimeters, the cross stratification has to be from a dune. Ripples are only a few centimeters tall, and they cannot create laminae that are higher than the ripple crest-to-trough distance. Thus, if cross sets are greater than a few centimeters high, the cross stratification must be from dunes. However, if the cross sets are only one centimeter high, the cross stratification could be due to either ripples or dunes. It is possible for ALL sediment to be eroded as a dune migrates, leaving no cross stratification. If only a small amount of sediment accumulates, the cross sets might be only a centimeter high, much like ripples. In the field, grain size variations and changes in cross stratification along an outcrop can help you distinguish between ripples and dunes in a case like this. For example, you could look for an instance where the cross stratification is more than a few centimeters high. If you did not find one, that might suggest ripple cross lamination rather than dune cross stratification.
Variations in Geometry and Bedform
Dunes and ripples are often irregular in plan view. This affects the geometry of the cross stratifcation/lamination. The laminae are always approximately parallel to the dip on the lee sides of the ripples or dunes. If the direction that these dip varies, the orientation of the laminae also varies. When looking at deposited cross stratification/lamination, these variations appear as variable dips in the laminae because you are viewing them at different angles.
Watch the USGS bedform movies described at: USGSBedforms.html
Remember that the structures also change with flow speed, both in terms of their geometry and which ones form. Grain size is also important. The sequence of structures in granules with increasing flow is:
1) no transport
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring
3) dunes - the flow is strong enough to erode at the attachment point
4) upper planar lamination
5) antidunes
In contrast, the sequence of structures in silt is:
1) no transport
2) ripples
3) upper planar lamination
4) antidunes
Antidunes - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly (Putah Creek in flood; tidal channels; creeks flowing across beaches - see ScottAntidunes.jpg below). Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases.
Other Types of Flows - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at ARipples.html Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than the stoss slope. Flows can also be irregular due to combinations of currents and waves, etc. Some of these flows are very characteristic of specific environments, for example, storm-influenced beaches. The structures they produce are very useful for interpreting ancient rocks, and we will highlight them as we discuss different sedimentary environments.
Environments and Facies
Look at the photo of Scott Creek Beach at:
ScottAntidunes.jpg
Note that the antidunes are forming in one part of a creek. The middle of the creek has upper planar lamination flow speeds, and the closest part is very shallow and has some antidunes again. (I know some of this from being there more than from looking at the photo.) Note that there is a faint lamination present in the eroding bench on the far side of the creek. This lamination mimics the beach surface. It is lamination from the waves swashing and transporting sediment on the beach. If all sediment transport stopped immediately, one would see a suite of sedimentary structures: Antidunes and upper planar laminae next to each other in the creek, an erosional surface overlying planar stratification that undulates like a beach. The association of these features would tell you that the sediment was deposited in an environment with a variety of flow conditions.
The suite of structures forms a facies. A facies (Latin for aspect or appearance) is a body of rock (i.e. a sequence of beds, etc.) or sediment marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock/sediment above, below and adjacent to it. A sedimentary facies has a characteristic set of properties that makes it distinctive, which the geologist defines. Usually facies are defined based on a suite of characteristics in rocks/sediment.
From Sediment Transport to Rocks - We have been talking about sediment transport and structures. These are processes that influence sedimentary rocks. What we really need is to be able to use our understanding of the processes to interpret ancient rocks when we can no longer see the processes in action. As I mentioned in the first class, we can use the modern processes as a model for interpreting past processes, which is the Principle of Uniformitarianism. However, it is often very different to see a process going on than it is to look at the ultimate deposited rock and interpret the process. For example, with bed forms, the entire shape of the structure you see as it migrates is rarely preserved. Instead, you only see a small part of it, if you get any sediment accumulation at all. Thus, we can also start the interpretation from the rock end by describing the general characteristics of the rocks and interpret flow from things like grain size, preserved cross stratification, and biogenic components. Then we can evaluate which environments are consistent with those characteristics.
Facies vs Environments - By grouping characteristics of the rocks into facies, the depositional environments can be more easily compared and interpreted. It is important to remember that the sedimentary environment is the combination of physical, chemical and biological processes that influence sediment deposition, whereas sedimentary facies are the characteristics of the rocks/sediments after deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross stratification; the stratification is the result of high flow speed, but they are not the same.
Example Facies
Facies are groupings of rock types based on similar features. We use these groupings to generalize individual properties into useful, genetically related categories. Some examples include:
Facies based on grain size:
Coarse-grained sandstone with 1-5% pebbles
(suggests high flow speeds)
Fine-grained, well-sorted sandstone
(suggests low flow speeds with either only one size sediment source or a consistent flow speed)
Mudstone
(suggests standing water)
Facies based on sedimentary structures:
Fine-grained sandstone with current ripple cross lamination
Fine-grained sandstone with upper planar lamination
Fine-grained sandstone lacking cross stratification, but with abundant burrows
Facies based on grain composition:
Coarse-grained sandstone with 25% lithic fragments, 25% feldspar, and 50% quartz
Coarse-grained sandstone with 80% quartz, 10% mica, and 10% feldspar
Coarse-grained sandstone with 99% quartz and trace gold flakes
Beach Facies What features will we see on the field trip to Bodega Bay beaches? How should we divide those into facies? We can compare them to what we would see in the rock record. Take a look at photos of Scott Creek Beach stratification again: Beach.html. Predict some of the facies.
See you at 8 am Sunday for the field trip!
Homework #2 due today.
Ripples and Dunes (A review with a bit of additional information)
A sketch of a ripple or dune like the one in lecture:
duneXStrat.jpg
Remember where the separation point and attachment point are located. The geometry of the flow tracks these points. Erosion can only occur where there is a bed shear stress sufficient to move sediment. In other words, the the main flow must be near the sediment surface. Sediment accumulates into a deposit in the flow shadow downstream of the ripple or dune crest. In other words, sediment accumulates in the flow detachment zone. Laminae are visible where deposition occurs due to variations in flow speed and thus the grain sizes transported and deposited.
Dunes and ripples behave similarly at the level of detail that I have been describing them. Their cross stratification geometries are similar. However, dunes are larger than ripples. If the distance between erosion surfaces defining cross sets is greater than a few centimeters, the cross stratification has to be from a dune. Ripples are only a few centimeters tall, and they cannot create laminae that are higher than the ripple crest-to-trough distance. Thus, if cross sets are greater than a few centimeters high, the cross stratification must be from dunes. However, if the cross sets are only one centimeter high, the cross stratification could be due to either ripples or dunes. It is possible for ALL sediment to be eroded as a dune migrates, leaving no cross stratification. If only a small amount of sediment accumulates, the cross sets might be only a centimeter high, much like ripples. In the field, grain size variations and changes in cross stratification along an outcrop can help you distinguish between ripples and dunes in a case like this. For example, you could look for an instance where the cross stratification is more than a few centimeters high. If you did not find one, that might suggest ripple cross lamination rather than dune cross stratification.
Variations in Geometry and Bedform
Dunes and ripples are often irregular in plan view. This affects the geometry of the cross stratifcation/lamination. The laminae are always approximately parallel to the dip on the lee sides of the ripples or dunes. If the direction that these dip varies, the orientation of the laminae also varies. When looking at deposited cross stratification/lamination, these variations appear as variable dips in the laminae because you are viewing them at different angles.
Watch the USGS bedform movies described at: USGSBedforms.html
Remember that the structures also change with flow speed, both in terms of their geometry and which ones form. Grain size is also important. The sequence of structures in granules with increasing flow is:
1) no transport
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring
3) dunes - the flow is strong enough to erode at the attachment point
4) upper planar lamination
5) antidunes
In contrast, the sequence of structures in silt is:
1) no transport
2) ripples
3) upper planar lamination
4) antidunes
Antidunes - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly (Putah Creek in flood; tidal channels; creeks flowing across beaches - see ScottAntidunes.jpg below). Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases.
Other Types of Flows - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at ARipples.html Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than the stoss slope. Flows can also be irregular due to combinations of currents and waves, etc. Some of these flows are very characteristic of specific environments, for example, storm-influenced beaches. The structures they produce are very useful for interpreting ancient rocks, and we will highlight them as we discuss different sedimentary environments.
Environments and Facies
Look at the photo of Scott Creek Beach at:
ScottAntidunes.jpg
Note that the antidunes are forming in one part of a creek. The middle of the creek has upper planar lamination flow speeds, and the closest part is very shallow and has some antidunes again. (I know some of this from being there more than from looking at the photo.) Note that there is a faint lamination present in the eroding bench on the far side of the creek. This lamination mimics the beach surface. It is lamination from the waves swashing and transporting sediment on the beach. If all sediment transport stopped immediately, one would see a suite of sedimentary structures: Antidunes and upper planar laminae next to each other in the creek, an erosional surface overlying planar stratification that undulates like a beach. The association of these features would tell you that the sediment was deposited in an environment with a variety of flow conditions.
The suite of structures forms a facies. A facies (Latin for aspect or appearance) is a body of rock (i.e. a sequence of beds, etc.) or sediment marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock/sediment above, below and adjacent to it. A sedimentary facies has a characteristic set of properties that makes it distinctive, which the geologist defines. Usually facies are defined based on a suite of characteristics in rocks/sediment.
From Sediment Transport to Rocks - We have been talking about sediment transport and structures. These are processes that influence sedimentary rocks. What we really need is to be able to use our understanding of the processes to interpret ancient rocks when we can no longer see the processes in action. As I mentioned in the first class, we can use the modern processes as a model for interpreting past processes, which is the Principle of Uniformitarianism. However, it is often very different to see a process going on than it is to look at the ultimate deposited rock and interpret the process. For example, with bed forms, the entire shape of the structure you see as it migrates is rarely preserved. Instead, you only see a small part of it, if you get any sediment accumulation at all. Thus, we can also start the interpretation from the rock end by describing the general characteristics of the rocks and interpret flow from things like grain size, preserved cross stratification, and biogenic components. Then we can evaluate which environments are consistent with those characteristics.
Facies vs Environments - By grouping characteristics of the rocks into facies, the depositional environments can be more easily compared and interpreted. It is important to remember that the sedimentary environment is the combination of physical, chemical and biological processes that influence sediment deposition, whereas sedimentary facies are the characteristics of the rocks/sediments after deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross stratification; the stratification is the result of high flow speed, but they are not the same.
Example Facies
Facies are groupings of rock types based on similar features. We use these groupings to generalize individual properties into useful, genetically related categories. Some examples include:
Facies based on grain size:
Coarse-grained sandstone with 1-5% pebbles
(suggests high flow speeds)
Fine-grained, well-sorted sandstone
(suggests low flow speeds with either only one size sediment source or a consistent flow speed)
Mudstone
(suggests standing water)
Facies based on sedimentary structures:
Fine-grained sandstone with current ripple cross lamination
Fine-grained sandstone with upper planar lamination
Fine-grained sandstone lacking cross stratification, but with abundant burrows
Facies based on grain composition:
Coarse-grained sandstone with 25% lithic fragments, 25% feldspar, and 50% quartz
Coarse-grained sandstone with 80% quartz, 10% mica, and 10% feldspar
Coarse-grained sandstone with 99% quartz and trace gold flakes
Beach Facies What features will we see on the field trip to Bodega Bay beaches? How should we divide those into facies? We can compare them to what we would see in the rock record. Take a look at photos of Scott Creek Beach stratification again: Beach.html. Predict some of the facies.
See you at 8 am Sunday for the field trip!
Monday, January 23, 2012
Cross Stratification
Key Points of Sediment Transport
1) Faster flows have more bed shear stress. Thus, faster flows move larger grains (when considering sand sizes and larger).
2) Sediment is transported as bedload and in suspension. Bedload consists of rolling and saltating grains.
3) Grain size and flow strength (Re) determine how grains are transported.
4) As flow strength changes, grains are eroded or deposited. These relationships are represented in the Hjulstrom diagram.
A Few Definitions:
1) "Stratification" - layers in rocks; stratified rocks are those organized into beds
See Grand Canyon Beds: Grand Canyon Beds
2) “Beds” are separated by “bedding planes” - cm to m thick units of sedimentary rock that were deposited approximately horizontally (beds) and are separated by horizontal planes (bedding planes); the rocks typically weather more along these planes. Beds are usually fairly uniform or change gradationally in composition. Bedding planes usually represent breaks in sedimentation or changes in grain size. In other words, they usually represent changes in flow characteristics.
See Cache Creek Turbidite Beds and Bedding Planes: turbidites
3) "Laminae" are color, composition, or grain size variations defining surfaces within a bed. They typically represent variations in flow velocity, sediment supply, sediment composition, etc. Planar Laminae are parallel to bedding, e.g. planar.
4) "Cross Lamination”, "Cross Stratification" or "Cross Bedding" are laminations or layers that are oriented obliquely to bedding. They truncate older laminae and are truncated by younger laminae. The erosional surfaces that separate “sets” of similarly oriented laminae are called “bounding surfaces”. There are lots of subdivisions of cross stratification; different types represent different types of bedforms and different flow conditions.
See Burns Cliff on Mars: Mars cross stratification
 |
| (Image courtesy of NASA/JPL and the MER mission!) |
The upper part of the image has planar lamination, and the lower part to the far left has cross lamination or stratification.
See: trough cross stratification and other examples at: dune cross stratification
See ripple cross lamination on Mars: martian ripples
 |
| (Image courtesy of NASA/JPL and the MER mission!) |
Bedforms
When sediments get deposited from turbulent flows, the sediment interacts with the geometry of the flow. Depending on the flow speed, turbulence, and sediment characteristics, different structures or bedforms develop.
See: Ripples Movie
Bed Geometry and Flow Separation - Until now, we have been implicitly assuming that the bases of beds are flat and smooth, but if sediment is present, they are not. If you start with a smooth bed of sand and increase water speed above it, irregularities form from irregularities in the flow and develop into ripples. First, a few grains pile up. Once the height of the pile is several grains high, there is a flow shadow down stream of them, and the laminar sublayer detaches from the base of the flow. The water has enough momentum that it does not hug the bed surface and instead, goes shooting out over the top. This point is called the separation point. The water flows forward and downward and reconnects with the bed at the attachment point. At the attachment point, water is flowing directly towards the sediment with a lot of force. This force moves the grains and causes erosion. In contrast, the area between the separation point and the attachment point has very low flow. In fact there are back eddies, where the flow is upstream. Thus, sediment transport is very irregular along the bedding surface at a local scale.
Sediment Transport Over a Ripple - Sediment grains are mobilized at the attachment point - more so than in normal flow because the water is shooting directly into the sediment - and the grains are moved downstream by saltation and traction. As the flow becomes parallel to the sediment surface again, its ability to transport sediment decreases. Thus, the grains tend to pile up and a new mound forms. This gives a periodic chain of mounds - the beginnings of ripples. As flow continues, grains roll and saltate up the stoss (upcurrent) side of the ripples. Once they pass the crest, they reach the low flow on the lee side of the ripple. The larger grains settle out and roll partway down the slope; this is the site of net deposition. As the process of deposition on the lee side and erosion on the stoss side continues, the ripples migrate downstream. If there is net deposition of sediment, the ripples leave behind distinctive dipping layers between two erosional surfaces that can be preserved in the rock record. These layers slope downstream and are one type of cross lamination.
A sketch of a ripple or dune like the one in lecture: ripple sketch
Watch the USGS bedform movies described at: USGS Bedforms page
Bedforms and Flow Velocity- The size and shape of subaqueous bedforms depends on flow strength and grain size and can be used to interpret ancient flow characteristics in a depositional environment from looking at sedimentary rocks. See Nichols (2009, Sedimentology and Stratigraphy, section 4.3) for bedform, flow speed, and grain size relationships.
Ripples (crest-to-crest differences of less than 50 cm and heights of less than 4 cm) – The minimum flow for ripples is determined by the minimum velocity for sediment transport. Once this flow speed is reached, ripples form if the sediment is transported as bedload. The maximum flow speed for ripples depends on the location of the attachment point on the stoss side of the ripples. As flow gets faster, too much erosion occurs at the crests of the ripples - the point of attachment is too far up the stoss side of the ripple- and the ripples flatten out. Dunes develop.
Dunes (60 cm-100’s m wavelength and 10’s of cm to meters in height) - Dunes develop as ripples flatten out because large scale irregularities start to develop. The basic ideas of dune and ripple formation are the same. The difference is that the area of flow separation is much larger (see Fig 4.17, Nichols 2009). Roller vortexes (e.g. upstream flows along the lee sides of dunes) are common, and the upstream flow can be strong enough to form ripples that migrate upstream. As flow speeds increase, the dunes start to flatten out.
Planar/Flat Lamination – Planar lamination forms when the flow is strong enough that the beds flatten out. The momentum of the transported grains and fluid are high enough that they tend to move horizontally, eroding any irregularities in the bed. This zone of planar lamination is called “upper flow regime”. (Why “upper”? - there is a zone of planar lamination in coarse grained sediment at low flow velocities.)
Antidunes - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly (Putah Creek in flood; tidal channels; creeks flowing across beaches - see http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg ). Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases.
Other Types of Flows - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at Dawn’s ripple image page. Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than the stoss slope. Flows can also be irregular due to combinations of currents and waves, etc. Some of these flows are very characteristic of specific environments, for example, storm-influenced beaches. The structures they produce are very useful for interpreting ancient rocks, and we will highlight them as we discuss different sedimentary environments.
Bedforms and Grain Size - Bedforms also vary with grain size (see Figure 4.20, Nichols, 2009). Very fine sand and silt are very easy to transport and erode. They form nice ripples, but do not form dunes when transported by water. Instead, ripples transition into planar laminae. Coarse sand and larger sediment is too hard to transport and erode to get ripples. The erosional force at the reattachment point is not strong enough to erode the coarse grains and produce the erosional surfaces on the backs of ripples. Without this erosion, troughs do not form and without troughs, crests do not form. The sequence of structures in granules with increasing flow is:
1) no transport
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring
3) dunes - the flow is strong enough to erode at the attachment point
4) upper planar lamination
5) antidunes
In contrast, the sequence of structures in silt is:
1) no transport
2) ripples
3) upper planar lamination
4) antidunes
Extra
High Sediment Loads - Sometimes with slope failures on land or under water, much more sediment can be put into motion than the flow would normally erode. Depending on the amount of water mixed with the sediment, the flow characteristics are different. When abundant water is present, the sediment can form a thick slurry with a higher density than sediment-free water, commonly leading to a higher Re and more turbulent flow (Re=u*l*r/µ). Also, collisions between grains become extremely important. Both of these tend to keep the sediment moving. Grain-to-grain collisions also have an important effect on grain sorting. The collisions tend to make sorting much less efficient and the sediment that gets deposited tends to consist of which ever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes.
If there is very little water associated with a clay-rich sediment flow, the flow can be very viscous due to the charge attraction among clay particles. The high viscocity makes the flow laminar (Re=u*l*r/µ). Debris flows with lots of cohesive mud are like this. In laminar flows, there is no mixing of the water or grains (or ice) and there is no sorting of grain sizes. Thus, the sediment remains mixed up with large grains, sometimes boulders, “floating” in mud. They flow down hill pulled by gravity until the flow seizes up and stops. This can be due to too low a slope or loss of water. Underwater debris flows can also be diluted by water that gets incorporated at the edges of the flow and become less viscous and more turbulent.
There also are dry sediment flows in which air is present between grains. For example, rock avalanches and some pyroclastic flows from volcanoes lack water. For these to move significant distances, large amounts of energy from either gravity or explosions are necessary to keep the sediment in motion.
Wednesday, January 18, 2012
Fluid Flow II
Lecture 3: Fluid Flow and Sediment Transport
Key Concepts from Lecture 2
Reynolds Number - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Re = (fluid inertial forces)/(fluid viscous forces) = l*u*r/µ where the variables are flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (r), and fluid viscosity (µ). Turbulent flow has Re is greater than 2000 and laminar flow has Re is less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well.
Boundary Layer and Laminar Sublayer - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer.
Sediment Transport
Bed Shear Stress: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. It also depends on the slope. If the slope is steep, gravity helps pull grains down the slope, increasing bed shear stress. Also, the roughness of the bed is a factor. A rough bed deflects flows and increases turbulence, which increases the bed shear stress, particularly in places where flow is directed into the sediment and the boundary layer is compressed.
The Bernoulli Effect
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect.
Which Grains Move? Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is a grain's position relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average.
Bedload and Suspended Load Transport
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain settles out versus how turbulent the water is (back to Re...). Bedload refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in traction. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be saltating. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water.
Here is a playlist with movies related to sediment transport: Sumnerd’s Sed Transport Playlist and an extra movie at
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb)
Suspended sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow.
Photo of suspended sediment in a Costa Rica River:
http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg
YouTube video of white clay in a turbulent flow in a flume: http://tinyurl.com/78kg3z The pulsing in the flow is (probably) due to the pump that is making the water flow.
Hjulstrom Diagrams
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams. Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the critical velocity. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the critical threshold for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done!
A copy of the Hjulstrom Diagram from the book: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/Hjulstrom.jpg
Deeper flows can move larger grains at the same flow velocity because they are more turbulent: Re=u*l*r/µ and l is larger. This is because deeper flows can have larger variations in flow speed and the laminar flow layers are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than just Hjulstrom diagrams, which summarize a lot of characteristics into two axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess.
Silt and Clay - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clay minerals tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode.
In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix).
A few more words about saltation: Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur.
Watch grains transported by saltation and traction in these movies: http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb)
http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm (14 Mb)
Deposition: Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and starts to leave grains on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments.
Ripples and Other Bedforms
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser laminae that dip downstream.
Next Time: We will talk about the details about sedimentary structures. Homework is due on Monday.
Key Concepts from Lecture 2
Reynolds Number - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Re = (fluid inertial forces)/(fluid viscous forces) = l*u*r/µ where the variables are flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (r), and fluid viscosity (µ). Turbulent flow has Re is greater than 2000 and laminar flow has Re is less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well.
Boundary Layer and Laminar Sublayer - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer.
Sediment Transport
Bed Shear Stress: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. It also depends on the slope. If the slope is steep, gravity helps pull grains down the slope, increasing bed shear stress. Also, the roughness of the bed is a factor. A rough bed deflects flows and increases turbulence, which increases the bed shear stress, particularly in places where flow is directed into the sediment and the boundary layer is compressed.
The Bernoulli Effect
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect.
Which Grains Move? Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is a grain's position relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average.
Bedload and Suspended Load Transport
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain settles out versus how turbulent the water is (back to Re...). Bedload refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in traction. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be saltating. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water.
Here is a playlist with movies related to sediment transport: Sumnerd’s Sed Transport Playlist and an extra movie at
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb)
Suspended sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow.
Photo of suspended sediment in a Costa Rica River:
http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg
YouTube video of white clay in a turbulent flow in a flume: http://tinyurl.com/78kg3z The pulsing in the flow is (probably) due to the pump that is making the water flow.
Hjulstrom Diagrams
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams. Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the critical velocity. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the critical threshold for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done!
A copy of the Hjulstrom Diagram from the book: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/Hjulstrom.jpg
Deeper flows can move larger grains at the same flow velocity because they are more turbulent: Re=u*l*r/µ and l is larger. This is because deeper flows can have larger variations in flow speed and the laminar flow layers are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than just Hjulstrom diagrams, which summarize a lot of characteristics into two axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess.
Silt and Clay - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clay minerals tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode.
In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix).
A few more words about saltation: Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur.
Watch grains transported by saltation and traction in these movies: http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb)
http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm (14 Mb)
Deposition: Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and starts to leave grains on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments.
Ripples and Other Bedforms
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser laminae that dip downstream.
Next Time: We will talk about the details about sedimentary structures. Homework is due on Monday.
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