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the fluvial geomorphic system

Conversion Potential energy (elevation) to Kinetic Energy

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the fluvial geomorphic system

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    1. The Fluvial Geomorphic System

    2. Conversion Potential energy (elevation) to Kinetic Energy(motion). Most energy lost to friction, but 2-4% of total potential energy converted to work of erosion and transportation

    3. Infiltration Infiltration is controlled by Intensity and duration of rainfall Prior wetted condition of the soil Soil texture Slope of the land Nature of the vegetative cover

    4. Stream water feeds Infiltration (rain and snowmelt soak into soil) water moving in pore spaces Throughflow water moving through voids in soil In cracks, burrows, root casts, etc.

    5. Rill Erosion, Gully formation

    6. Discharge The amount of water passing through a channel cross-section in unit time Q = v * A where v is the average flow velocity [meters / second] through a cross section A [meters2] Thus Q has units meters3/second

    7. Bedrock Channels Cut down by: Solution Weathering - Limestone Plucking (= quarrying) esp. jointed rocks Fast moving water gets in cracks and pushes blocks free. Cycles of wetting and drying, or freezing and thawing, loosen blocks Abrasion the impact of rolling and bouncing detritus.

    8. Waterfalls p 205 right "In stratified rocks of contrasting lithology … each resistant layer forms a waterfall in a bedrock channel, usually with a plunge pool" that scours the underlying beds. p206 left (paraphrased for clarity) As the waterfalls migrate headward, if the resistant layer dips headward, waterfalls will [become smaller]; if layers dip seaward, the waterfall may grow in height.

    10. Alluvial Channels Channels usually lined with alluvium Removed and carried further downstream during flood Re-deposited during wane

    11. "As mean discharge of a river increases downstream, channel width, channel depth, and mean current velocity all increase."

    12. Ultimate Base Level Control At sea level, no further conversion of potential energy to stream work is possible. Sea level lowered at least 100 m during peaks of glaciation. Headwaters were higher, stream water had more potential energy, faster streams cut deep canyons All rivers now enter the sea via estuaries (drowned lower valleys) or deltas (sediment filling lower valleys).

    14. 6_6

    15. Load, Competence Streams transport sediment Transported material is called load Types of load Dissolved load – ions in solution from weathering Suspended load – fine particles* Bed load Moves during high velocity events Sandy Portion – Saltation (bouncing) Cobbles – Traction (rolling) Competence – Largest particles it can move Proportional to velocity squared KE = 1/2 mV2 *Our Buckingham Pi studies of grain in fluid

    16. Deposition of sediment by a stream Caused by a decrease in velocity Competence is reduced Sediment begins to drop out

    17. Similitude For model studies that teach us about the real world, Similitude (similarity) is needed Dimensionless numbers such as the Reynolds Number Re = rvR/m must be the same in model and real world R is the channel cross sectional area divided by the length of the wetted perimeter

    18. Dimensionless Numbers for Streams Froude Number Fr = v type of turbulence (gd)1/2 Tranquil, or supercritical flow. Latter standing wave, resistance to flow Supercritical mostly in narrow bedrock channels in flood Reynolds Number Re = rvR/m laminar or turbulent Drag Coefficient CD = F grain suspension rV2D2

    19. Capacity Maximum mass of sediment a stream can transport. Measured by amount of suspended load.

    20. Channel Shape, Habit, Solid Load Q determines cross-sectional channel shape and HABIT (shape from space) measured using thalweg Straight: Suspended load channels (<3% bedload) Meandering: single sinuous channel, carries mostly suspended load, with point bars, loops Braided: bedload > 11% total, high channel width to depth, mid-channel bars

    21. Erosion and deposition along a meandering stream

    23. Reminder: Point Bar Sequence

    24. Braided Streams Braided: high total load; bedload > 11% total, high channel width to depth; mid-channel bars

    25. Graded Streams Grade: natural tendency toward self regulation Changing conditions cause adjustment Sudden extra sediment narrows channel: so faster flow and much more erosion during flood, so blockage removed and previous channel shape reestablished. Demo: narrow channel of lab hose

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