The Fluvial The Fluvial Geomorphic Geomorphic

SystemSystem

Fluvial (Latin fluvius: river)

Streams 85% sediment transport, Streams 85% sediment transport, glaciers 7%, waves 1.5%, wind 1%glaciers 7%, waves 1.5%, wind 1%

Lab Hose, sand in beaker

Conversion Potential energy (elevation) to Kinetic EnergyConversion Potential energy (elevation) to Kinetic Energy(motion). Most energy lost to friction, but 2-4% of total (motion). Most energy lost to friction, but 2-4% of total

potential energy converted to work of erosion and potential energy converted to work of erosion and transportationtransportation

Tributaries

Trunk Stream

Drainage Divide

Distributaries And Delta

Gradient decreases downstreamFactors that increase downstream:

VelocityDischargeChannel size

InfiltrationInfiltration

• Infiltration is controlled byInfiltration is controlled by–Intensity and duration of rainfallIntensity and duration of rainfall–Prior wetted condition of the soilPrior wetted condition of the soil–Soil textureSoil texture–Slope of the landSlope of the land–Nature of the vegetative coverNature of the vegetative cover

http://plantandsoil.unl.edu/croptechnology2005/soil_sci

Stream water feedsStream 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.

•Runoff• Sheetflow = overland flow

thin slow surface layer

• transitions into rill erosion Sheetflow

Rill Erosion, Gully formationRill Erosion, Gully formation

Source: Ramesh Venkatakrishnan

Rill Erosion. Individual Rill Erosion. Individual Channels do not persistChannels do not persist

Gully. Larger Gully. Larger channels,channels,they persist, grow they persist, grow headward and widen.headward and widen.

Rill: a trickle of water, a very small stream

Gully: a small watercourse with steep sides, usually cut into soil

DischargeDischarge

• The amount of water passing through a The amount of water passing through a channel cross-section in unit timechannel cross-section in unit time

• Q = Q = v v * A where * A where v v is theis the average flowaverage flow

velocity [meters / second] through a cross velocity [meters / second] through a cross section A [meterssection A [meters22]]

Thus Q has units metersThus Q has units meters33/second/second

Bedrock ChannelsBedrock Channels

Cut down by: Cut down by:

• Solution Weathering - LimestoneSolution Weathering - Limestone

• Plucking (= quarrying) esp. jointed rocksPlucking (= quarrying) esp. jointed rocks– Fast moving water gets in cracks and pushes Fast moving water gets in cracks and pushes

blocks free. Cycles of wetting and drying, or blocks free. Cycles of wetting and drying, or freezing and thawing, loosen blocksfreezing and thawing, loosen blocks

• Abrasion Abrasion – the impact of rolling and bouncing detritus.the impact of rolling and bouncing detritus.

Notice I skipped page 200 lower right beginning "Soil Erosion" and resumed page 203 right "Bedrock Channels"

Photo by Natalie RomanoffUsed with Permission

WaterfallsWaterfalls• p 205 right "In stratified rocks of contrasting p 205 right "In stratified rocks of contrasting

lithology … each resistant layer forms a lithology … each resistant layer forms a waterfall in a bedrock channel, usually with a waterfall in a bedrock channel, usually with a plunge pool" that scours the underlying beds.plunge pool" that scours the underlying beds.

• p206 left (paraphrased for clarity) As the p206 left (paraphrased for clarity) As the waterfalls migrate headward, if the resistant waterfalls migrate headward, if the resistant layer dips headward, waterfalls will [become layer dips headward, waterfalls will [become smaller]; if layers dip seaward, the waterfall smaller]; if layers dip seaward, the waterfall may grow in height.may grow in height.

Local change in base level affects river profiles: nickpoints

Resistant layer dips headward

Alluvial ChannelsAlluvial Channels

• Channels usually lined with Channels usually lined with alluviumalluvium

• Removed and carried further Removed and carried further downstream during flooddownstream during flood

• Re-deposited during waneRe-deposited during wane

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

Tributaries

Trunk Stream

Drainage Divide

Distributaries And Delta

Gradient decreases downstreamFactors that increase downstream:

VelocityDischargeChannel size

p 207 r para 1

Width, Depth, Current Velocity and Longitudinal Profile

Water flows with less friction in larger channelsmaintains velocity with less slope p209 l

Ultimate Base Level ControlUltimate Base Level Control• At sea level, no further conversion of potential energy to At sea level, no further conversion of potential energy to

stream work is possible. stream work is possible.

• Sea level lowered at least 100 m during peaks of Sea level lowered at least 100 m during peaks of glaciation. Headwaters were higher, stream water had glaciation. Headwaters were higher, stream water had more potential energy, faster streams cut deep canyonsmore potential energy, faster streams cut deep canyons

• All rivers now enter the sea via estuaries (drowned All rivers now enter the sea via estuaries (drowned lower valleys) or deltas (sediment filling lower valleys).lower valleys) or deltas (sediment filling lower valleys).

Overbank deposition during flooding

Flood over banks into wider area, lower velocity, sand drops.(Muddy water over floodplain)

6_6Fine-grained sediment

on floodplain

Flood water

Erosion of uppermostfine-grained sediment

Older sediment

1 Pre-flood

Flood stage2

Post-flood3

Fining-upward flood deposit

Bedding plane is anerosional surface

Floods change the local conditions

We can recognize past floods

Bounders on bottom, sands and muds suspended

Flood: One source of Graded Bedding

skip 211r, 212, 213

Load, CompetenceLoad, Competence• Streams transport sedimentStreams transport sediment

• Transported material is called loadTransported material is called load• Types of loadTypes of load

– Dissolved load – Dissolved load – ions in solution fromions in solution from weathering weathering– Suspended load – fine particles*Suspended load – fine particles*– Bed load Moves during high velocity eventsBed load Moves during high velocity events Sandy Portion – Saltation (bouncing)Sandy Portion – Saltation (bouncing)

Cobbles – Traction (rolling)Cobbles – Traction (rolling)

• Competence – Largest particles it can moveCompetence – Largest particles it can moveProportional to velocity squared KE = 1/2 mVProportional to velocity squared KE = 1/2 mV22

*Our Buckingham Pi studies of grain in fluid*Our Buckingham Pi studies of grain in fluid

Important definitions

• DepositionDeposition of sediment by a stream of sediment by a stream

•Caused by a decrease in Caused by a decrease in velocityvelocity–Competence is reducedCompetence is reduced

–Sediment begins to drop Sediment begins to drop outout

http://www.geo.cornell.edu/geology/classes/Geo101/graphics/AKbraid_stream.gif

SimilitudeSimilitude

• For model studies that teach us about the For model studies that teach us about the real world, Similitude (similarity) is neededreal world, Similitude (similarity) is needed

• Dimensionless numbers such as the Dimensionless numbers such as the

Reynolds Number Re = Reynolds Number Re = vR/vR/ must be the same in model and real worldmust be the same in model and real world

• R is the channel cross sectional area R is the channel cross sectional area divided by the length of the wetted divided by the length of the wetted perimeterperimeter

skip 216, 217, 281 left

Page 215

Dimensionless Numbers for StreamsDimensionless 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 = Reynolds Number Re = vR/vR/laminar or turbulentlaminar or turbulent

• Drag Coefficient CDrag Coefficient CDD = = F F grain suspensiongrain suspension

VV22DD22

These are related to competence and erosion

CapacityCapacity• Maximum mass of sediment a Maximum mass of sediment a

stream can transport.stream can transport.

• Measured by amount of Measured by amount of suspended load.suspended load.

skip 219 220 left

Channel Shape, Habit, , Habit, Solid Load

• Q determines cross-sectionalQ determines cross-sectional

channel shape andchannel shape and• HABITHABIT (shape from space) measured using (shape from space) measured using thalwegthalweg• StraightStraight: Suspended load channels (<3% bedload): Suspended load channels (<3% bedload)• MeanderingMeandering: single sinuous channel, carries mostly : single sinuous channel, carries mostly

suspended load, with point bars,suspended load, with point bars,

loopsloops• BraidedBraided: bedload > 11% total, high channel width to : bedload > 11% total, high channel width to

depth, mid-channel barsdepth, mid-channel bars

Braided StreamBackground: Figure 10-15 from text, p.221, after Schumm, 1981

Erosion and deposition along Erosion and deposition along a meandering streama meandering stream

Meanders get more extreme with time. Note the THALWEG (blue arrows)

Floodplain

MeanderingStream

Oxbow

http://hays.outcrop.org/gallery/rivers/arid_meander?full=1

Channel moves in direction of cutbankPointbar advances as crossbed drapes

Reminder: Point Bar SequenceReminder: Point Bar Sequence

Erosion

Gravel of bed

Crossbeds of Bar

Fines of Floodplain

Braided StreamsBraided Streams

• Braided: high total load; bedload > 11% total, high Braided: high total load; bedload > 11% total, high channel width to depth; mid-channel barschannel width to depth; mid-channel bars

Braided Stream

Graded StreamsGraded Streams• Grade: natural tendency toward self Grade: natural tendency toward self regulationregulation

• Changing conditions cause Changing conditions cause adjustmentadjustment

–Sudden extra sediment narrows channel:Sudden extra sediment narrows channel:

so faster flow and much more erosion during flood, so faster flow and much more erosion during flood,

so blockage removed and previous channel shape so blockage removed and previous channel shape reestablished.reestablished.

Demo: narrow channel of lab hoseDemo: narrow channel of lab hose

http://www.tweed.nsw.gov.au/Education/rm_4_bankerosion.htm