Michael Nicolos
3/6/12
GEOL
Geology Midterm
Question 1:
Describe the boundary conditions (depth, velocity, density) necessary for turbulence to occur in a fluid, the “periodic coherent structures” that are characteristic of it, and their role in the initiation of bedforms (ripples and dunes,) and the role of bedforms in organizing turbulence in a flow. Make sure to describe how the geometry of turbulence changes upon the initiation of bedforms?
Two types of flow exist in which a fluid can transport its contents; Laminar and Turbulent. In Laminar flow, a fluid flows in parallel laters with no interaction between the layers. They simply just slide past one another. A Turbulent flow, however, can be characterized by a continuous mixing of the layers brought about by rapid variation in the pressure of the layers in the fluid. Whether or not a flow will be Laminar or Turbulent in nature is determined by Reynalds Number1; which breaks down to contributions to turbulence divided by oppositions of turbulence. Turbulent flow is caused by several key factors; the depth of the channel the fluid is moving through, the velocity at which the fluid is traveling and the density of the fluid itslef. In Turbulent flow, like that of oceans or streams, the velocity of the water pushes its contents forward. However, due to the different sizes, density and shapes of sediments that may be in suspension, the pressure of the water becomes disrupted and chaotic. This causes the current to try and equal itself out, which in turn causes turbulence. Some of the heavier particles traveling may bounce and roll along side one another (known as Saltation) while other lighter particles may stay in suspension. Eventually these heavier rolling particles will come to a rest because they have settled in a position that has a decrease in flow pressure. Often times other heavier particles will join them in this location and orient themselves in a similar fashion, known as Imbrication, and it becomes a bedform. As more sediments pile on the upstream Stross side of this bedform, creating ripples or dunes, they eventually hit a limit we call the Angle of Repose (which is ~30º). With this limit, the downstream Lee side avalances down and is able to avoid suspension in the current becuase of the Flow Seperation Bubble. As this whole process occurs time and time again, the ripples appear to be moving along with the current when, in actuality, the original sediments are staying in the same location as new sediments are arriving and taking their place.
Question 2:
The vast majority of depositional environments experience “unsteady flow” at some scale. Consider the experience in velocity vs time of an individual sediment grains in the following two environments (both at 3 meters depth below mean sea level) at Limantour spit:
(a) Open shoreface
(b) Tidal estuary channel
Relate the differences in flow rate and direction over time at each site (at scales between seconds and weeks) to the diagnostic types of sedimentary structures found in each environment. Make sure to discuss the specific hydrodynamic origin of a diagnostic bedforms from each environment.
As water makes its way from the mountains to the ocean via rivers and streams, the flow of water absorbs foreign particles along the way. These particles are called Sediments and are taken from a varienty of sources including the erosional force of water, the weathering and breaking down of rocks and animal attack. These sediments make their way downstream until it reaches the ocean. This meeting point between the river or stream and the tidal zone is called an estuary. An estuary is often times filled with the sediment of the upstream surrounding location because it is deposited as the flow of the river runs into the tides of the ocean. This deposition allows for the crashing waves and tides of the ocean to transport the sediment. With every rise and fall of the tide, sediment is being suspended and deposited in another location. When the tide begins to lower, the estuary containing all that loose sediment becomes exposed and the sand dunes, ripples, channels and mudflats are revealed. Once the tidal cycle circles back and sea level begins to rise again, the estuary starts to fill with water and is no longer exposed. This process occurs about once every 6 hours due to the fact that we have two high tides and two low tides a day. In open shoreface, however, weathering is much more effective as an erosional force because the surface is constantly exposed with the advancement and recession of each wave. Exposed bedrock and outcrops are subjected to weathering by wind, water and animal attack. This process takes only a few seconds to repeat itself because waves are constantly crashing on the shoreline. Because there are so many ways in which to break down and erode the rocks happening over a much faster time interval, the sediment on shoreface sites are genereally much more fine than sediment found in estuaries.s