Fluvial Systems - General
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A fluvial system consists of channels that transport sediment from drainage basins to depositional basins. It can be divided into channel belts, floodplains, and splay deposits. Discharge is the volume of water passing through a stream over time and controls the sediment transport capacity and competence. Meandering streams migrate laterally through erosion on the outside bend and deposition on the inside bend, eventually forming oxbow lakes when neck cut-offs occur. Flooding occurs when discharge exceeds channel capacity.
Fluvial Systems - General
- 1. Photo by W. W. Little Fluvial Systems A fluvial system consists of a network of channels and associated environments that transport sediment from a drainage basin to a depositional basin.
- 2. Photo by W. W. Little Discharge Discharge is a measure of the amount of water carried by a stream.
- 3. Major Divisions of Fluvial Deposits For purposes of large-scale architecture, fluvial systems can be divided into two primary components, channel belt and floodplain. Splay deposits are also significant. Each of these major sub-systems can be further divided into a number of environments and sub-environments, such as channel floor, point bar, abandoned channel fill, levee, overbank, swamp, etc.
- 4. Discharge is the volume of water that passes through a given cross- sectional area of a stream during a specified period of time. Measuring Discharge Q = V/t = vA = vwd P = w + 2d
- 5. Photo by W. W. Little Capacity vs. Competence Capacity: reflects the potential volume of sediment transported by a stream and is controlled by discharge and flow velocity. Competence: measures the maximum size of clast that can be transported by a stream and is related to flow velocity and discharge.
- 6. In a straight channel, due to hydraulic shear, velocity is greatest in the center just below the surface and least along the bed and banks. Flow Velocity A stream obtains its velocity from a conversion of potential energy to kinetic energy under the force of gravity and is, therefore, controlled by stream gradient. There is a limit to the maximum velocity and momentum of a given discharge flowing down a specified slope due to friction between the water and channel boundaries and between molecules within the flow.
- 7. In a curved channel, velocity is greatest near the outside of a bend just below the surface and least along the bed and banks. Meandering Because of turbulence, the focus of maximum velocity does not maintain a straight course and the channel begins to bend. This concentrates velocity toward the outside of the bend, increasing its curvature. The increase in velocity along the outside of a channel causes erosion. A corresponding decrease in velocity on the inside of the channel results in deposition.
- 8. Photo by W. W. Little
- 9. Photo by W. W. Little
- 10. Photo by W. W. Little
- 11. Photo by W. W. Little
- 13. Meander Evolution Because of internal turbulence, all stream channels meander. The degree of meandering is controlled primarily by discharge (energy) and nature of the sediment (bank stability).
- 14. Photo by W. W. Little
- 17. Oxbow Lakes Oxbow lakes form as reaches of a channel become abandoned through meander-neck cut-off or chute entrenchment. These then become filled with fine-grained sediment during flooding
- 18. Lateral Accretion To maintain constant width, as a stream erodes along the cutbank, it must deposit on the point bar. This results in lateral migration of channel facies.
- 19. Photo by W. W. Little
- 20. Photo by W. W. Little
- 21. Photo by W. W. Little
- 23. Nature of Load The load includes all material being carried by the discharge through traction, suspension, or dissolution.
- 24. Photo by W. W. Little Bed Load
- 25. Photo by W. W. Little Suspended Load
- 26. Photo by W. W. Little
- 27. Floods Floods occur when discharge exceeds a channel’s capacity to contain it and stage height rises above bank elevation. • Under natural conditions, flooding typically occurs every two to three years in response to heavy precipitation (duration or intensity) and/or rapid snow melt. • Extremely large floods can be caused by natural phenomena, such as glacial outbursts and hurricanes, or human-related evens, like dam failures.
- 28. Flood Stage Flow Pattern During flooding, the portion of flow above bank-full travels as a sheet in a more or less straight path.
- 29. Mississippi River Prairie du Chien, WI (1969) Photo by: Louis J. Maher
- 30. Photo by W. W. Little
- 31. Photo by W. W. Little
- 32. Photo by W. W. Little
- 33. Photo by W. W. Little Swamps Swamps develop in portions of the floodplain that remain below the water table for extended periods of time. Deposits tend to be rich in plant debris. Anaerobic conditions can lead to preservation in the form of peat, which is converted to coal upon burial. Coal quality is affected by the amount of silt and clay that accumulates during flooding.
- 34. Fluvial Style (Channel Morphology) Anastomosed Meandering Braided Straight
- 35. River Classification Modern rivers are often classified according to channel morphology, referred to as fluvial style. Major considerations include sinuosity (ratio of channel length to a straight line), number of channels (single or multiple), relative channel depth (depth/width ratio), nature of load (bed/suspended load ratio), and bank cohesion.
- 36. Fluvial Style vs. Architecture
- 37. Photo by N.D. Smith (Williams River, Alaska ) Factors Controlling Fluvial Style Meandering vs. braided: Nature of sediment (erodibility, abundance, size) Discharge (volume, variability) Stream gradient Vegetation Meandering vs. anastomosed: Base-level fluctuation (rage, amount) – accommodation production
- 38. Large-scale Architecture (stacking patterns) Sand:mud ratios Sand body connectivity Sand body geometry (lense vs. sheet) Accommodation space and Preservation potential Transgressive deposits Lowstand deposit Early (slow) rise deposits Moderate rise deposits Rapid rise deposits Highstand deposits Coastal depositsTidally-influenced fluvial deposits B
- 39. GEOL 553 - Introduction to Facies Models - Kendall
Editor's Notes
- #40: Bounding surface heterogeneity Ranking (orders) 1st laminae to 8th major unconformity – grad school stuff