fluvial systems - general

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.

Photo by W. W. Little

Discharge

Discharge is a measure of the amount of water carried by a stream.

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.

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

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.

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 VelocityA 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.

In a curved channel, velocity is greatest near the outside of a bend just below the surface and least along the bed and banks.

MeanderingBecause 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.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

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).

Photo by W. W. Little

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

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.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Nature of Load

The load includes all material being carried by the discharge through traction, suspension, or dissolution.

Photo by W. W. Little

Bed Load

Photo by W. W. Little

Suspended Load

Photo by W. W. Little

FloodsFloods 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.

Flood Stage Flow Pattern

During flooding, the portion of flow above bank-full travels as a sheet in a more or less straight path.

Mississippi RiverPrairie du Chien, WI (1969) Photo by: Louis J. Maher

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

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.

Fluvial Style (Channel Morphology)

Anastomosed

Meandering Braided

Straight

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.

Fluvial Style vs. Architecture

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 gradientVegetation

Meandering vs. anastomosed:

Base-level fluctuation (rage, amount) – accommodation production

Large-scale Architecture (stacking patterns)

Sand:mud ratiosSand body connectivitySand body geometry (lense vs. sheet)Accommodation space and Preservation potential

Transgressive deposits

Lowstand deposit

Early (slow) rise deposits

Moderate risedeposits

Rapid rise deposits

Highstand deposits

Coastal depositsTidally-influenced fluvial deposits

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GEOL 553 - Introduction to Facies Models - Kendall