Flowing Water: Sediment Transport and Landforms

Medium-term Plan

10/27 Lecture 13. The Sediment Factory: Source to Sink

11/01 Lecture 14. Flowing Water: Sediment Transport and Landforms  

11/03 Lecture 15. Depositional Systems(HW #4 assigned) 11/08 Lecture 16. Dating the Sedimentary Record (Thompson)

(HW #4 due) 11/10 Lecture 17. Ice Age Cycles (Thompson)

(HW #5?) 11/15 Lecture 18: Waves and Coastal Morphodynamics 11/17 Lecture 19: Shorelines (HW #6 assigned)

11/22 The Anthropocene: Humans as Geomorphic Agents

Reynolds number (laminar vs. turbulent flow)

• u=flow velocity; l=characteristic length (flow depth); =kinematicviscosity (dynamic

viscosity/fluid density)(water ~ 1.5x10-6 m2/s)

• Turbulence is promoted by high flow velocities and flow depths, and low viscosities (Re>2000); laminar flow occurs at Re<500

• Air and water are nearly always turbulent

ul

Re

River Transport

Transport of sediment depends upon

Settling Velocity and Cohesion

Play bdld.mov

Rivers: Sediment transport

• Three modes:• Dissolved load/wash load (ions in solution - pollution)

• Suspended load

– Fine particles (sand, silt & clay)

– Turbulent eddies pick up, carry upward if vel. > settling vel.

• Bedload

– On/near bed; rolling, bouncing (‘saltating’), etc.

• Suspended and bedload increase rapidly with flow strength (nonlinear relationship)

Rivers: Two main kinds

• Alluvial rivers; bed consists of sediment (‘alluvium’ = river-associated sediment)– Downstream reaches

• Bedrock rivers; part of the bed is bare rock, where river cutting down – generally in upper reaches of rivers

Bedrock Rivers

• Erosion rate depends on slope

• Presence of sediment (‘tools’) increases erosion

Photo by Duncan Heron

Alluvial Rivers

Landform: Floodplain

Landform: Floodplain

Braided stream

• Braided streams are bedload dominated

• Nonlinear sediment transport laws result in dynamic feedbacks

Meandering stream; Point Bar and Cut Bank

Photo by Duncan Heron

Point Bars and Cutbanks along river meanders

Santee River, SC

Neuse River, NC

Note point bars

Photo by Duncan Heron

Oxbow lake formation

Play meander.mov, sm1.mov

Incised Meanders

Natural Levee formation

Photo by Duncan Heron

Artificial Levees

Levee Failures

Crevasse Splay Deposits, Mississippi River

Natural River - 1948

1964

Drainage Basins

Graded Stream Profile• Flow increases downstream (tributaries)• Velocity Increases• Equilibrium slope reduces as flow increases

Graded Stream Profile

• Each stretch of alluvial river tends to have slope adjusted to transport sediment delivered to it

• Slope too low, sediment piles up at upstream end

-> slope increases• Slope too high, erosion (less in than out) at

upstream end

-> slope decreases

• Need steeper slope with• Less flow• Larger grains

Base Level Changes

Dam cuts off sediment flux

Shelf Transport SystemShelf Transport System

Gravity FlowsGravity Flows

• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous• Can occur on land or underwater (Pratson.mov)

• Turbidity currents have a higher proportion of water, are always subaqueous, and move due to density contrasts

Pore PressurePore Pressure

• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous

Pore PressurePore Pressure

• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous

• Terrestrial flows: initial sediment packing affects type of flow

Pratson.mov

Pdfmod (weak debris flow)

Pdfst6.mov (Strong debris flow)

Gravity FlowsGravity Flows

• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous• Can occur on land or underwater (Pratson.mov)

• Turbidity currents have a higher proportion of water, are always subaqueous, and move due to density contrasts

• The presence of a dilute suspension of sediment in the water of a turbidity current renders it slightly heavier than the ambient water.• This results in downslope movement of both the sediment and entrained water (or vice versa).

• Sediment suspension can be from:• catastrophic event (earthquake) • flow-generated turbulence (autosuspension).• wave stirring

Turbidity CurrentsTurbidity Currents

turbwg.mov (turbidity current)

Undf.mov (unconfined tc)

Turbidity CurrentsTurbidity Currents

• Turbidity currents also create levees, but can overtop them frequently

TURBIDITY CURRENTS – constructional and erosional

Passive (NJ/NY) Shelf

Monterey Submarine Canyon