River Morphology

8/2/2019 River Morphology

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EPS 50: Lecture 24

Rivers and Streams

Stream flow and transport

Stream morphology

Streams, rivers and human intervention

Water on Planet Earth

Rivers & lakes only 0.009 % of water

Streams are most important agent of landscape modificationand erosion in most environments

Worldwide, streams carry ~16 billion tons of sediment and ~3billion tons of dissolved matter each year

Pre-human transport might have been only ~50% of this


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Studying Rivers and Streams

Fresh water supply Transportation

Agriculture

Renewable, clean energy resource

Production of fertile floodplain soil

Flood hazard to communities

The Waters of California

CA stream flows: (1) Sierra Nevada

(plate tectonics)

(2) W-E winds

(the hydrologic cycle)

CAs most valuable and

contested resource 150 years of damming,

diverting and polluting

Most water goes to CentralValley agriculture


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EPS 50: Lecture 24

Rivers and Streams


Stream morphology


How Stream Waters Flow

Depends on flowvelocity, geometry(depth), viscosity(fluid dynamics)

The viscosity (low)and velocity (high)of stream waterusually results inturbulent flow

Smooth sheet-like flow at alow velocity, streamlines areparallel

Usually confined to edges andtop of stream

Irregular swirling flow

Occurs at most rates ofstream flow

Keeps particles in suspension

Laminar flow Turbulent flow


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Stream Discharge (Q)

Five importantconditions:

Channel width

Channel depth

Velocity

Gradient

Bed roughness

Q = v x A (depth x width)

A

v

Discharge (m3/s) = width (m) depth (m) velocity (m/s)

water cross section x velocity

30 m3/s 180 m3/s

Stream Flow Continuity

Q = v A

Given constant discharge

Reduction in area ==> faster flow

Changes in slope and roughness influence velocity and mustbe compensated by change in area

A balance ofdriving

and resisting forces

determines the natureof river flow :

Driving force = water weight x sin(bed slope)

Resisting force = river-bed area x river-bed shear stress


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Increased Flow Velocity

Increased suspended sedimentIncreased bed load transportIncreased saltation, rolling and sliding

Stream Sediment Transport

suspendedload

bed load

saltation

Particle Size vs. Current Velocity

Cohesion of clay and silt particles


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Bedforms Associated with Velocity

ripples ripples on dunes

structure produced by amigrating nearshorebar, Pliocene terracedeposits, Monterey Bay,California

www.usgs.gov

Stream Erosion Physical weathering, abrasion, plucking & pot holes

Potholes form by pebbles and gravel grinding inside eddies

Waterfall undercutting and headward erosion

Chemical and biological factors also contribute to erosion


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Work and graphics by David Finlayson,

Univ. of Washington

Missoula Flood(s) - 15,000years ago at close of ice age

~100 individualfloods

perhaps 1 every 50to 150 years

evidence: scour,huge-scalesediment deposits,wave-cut platforms

When the ice dam broke,

it sent ~800 meter wall ofwater racing at 100 kmph.

Ripple marks are 15-20meters in scale.


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Catastrophism

vs.

Uniformitarianism

Harlan Bretz first proposedfeatures were due tocatastrophic flooding,initially met with skepticism.

Floating ice dams and

subsequent flooding has nowbeen observed in Greenland,

Alaska and Himalayas (albeitat smaller scale).

EPS 50: Lecture 24

Rivers and Streams


Stream morphology



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The shape of a river

Rivers form channels, valleys andfloodplains

The shape of river flow varies fromstraight to sinuous andmeandering to braided

The longitudinal profile of a streamrepresents an attempt to achievegrade or equilibrium (reduce anddistribute work in natural system)

Adjustments to profile, channelcross section and channel patternsresult from changes in sedimentsupply, discharge and slope

Stream Longitudinal Profile

Base Level

All streams, large and small showthe same concave-up profile

Result of balance oferosion(incision) and deposition

Base level controls the elevationof the longitudinal profile

Erosion-dominant

Deposition-dominant


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Longitudinal Profile and Capacity

A

B

C

Knickpoints

Sudden breaks in slope due to faulting,lithology, tributaries, dams, etc.

Rapids develop at knickpoints

High gradient and high stream powerdownstream => Erosion

Low gradient and low competenceupstream => Deposition

Headward migration results

Mount, 1995


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Dams and Stream Longitudinal Profiles

Dams result in depositionupstream & erosion downstream

The stream depositssediment in the upperpart of the reservoir

The sediment-depleted streambegins to erodedownstream of thedam

Erosion on thecutbank

Deposition onpoint bar

Oxbow Lake

MeanderCutoff

Meander

Neck

Stream Meanders and Floodplains


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Lateral migration of streams creates river flood plain Sinuosity reflects balance of energy efficiency and distribution

as a function of load, gradient and discharge

Local disturbances in flow resistance encourage meandering

Why Meander?

Point Bar Meandering channel

Braided Streams

Some streams have multiple channels with numeroussand bars and repeatedly diverging and joining channelsforming an interlacing network


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Why Braid?

Attempt to dissipate excess energy

Related to steep gradients, highlyvariable water discharge, abundantcoarse load, and easily erodedbank material

Channel Form

Transition between straight,meander and braidedstreams is complexthreshold function ofdischarge, slope andsediment load

Large rivers and low slopestend to form floodplainmorphology in broad valleys

Mount, 1995


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Streams and Tectonics

Antecedent streams Longitudinal profile

adjusts to tectonics

Terraces are uplifted anddeformed by folding

Streams and Pre-existing Structure

Downcutting can cause astream to be superimposed ona pre-existing structure

Left-behind terraces followlongitudinal profile

Delaware water gap


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Alluvial Fans

Form at mountain fronts

Widening from narrowstream valley to broadvalley Loss ofcompetence andcapacity

Often related to

tectonic uplift

Drainage Networks


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Major American River Systems

Drainage divides bound drainage basins

EPS 50: Lecture 24

Rivers and Streams


Stream morphology



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Floods! Historically, many major cities

are built in floodplains

US flood cost is ~$1.5 billion/yr

Our attempts to harness riversare only partly successful

Building a Floodplain, One Flood at a Time

Low Natural LeveeOverbank flow results in theOverbank flow results in the

flooding of the floodplainflooding of the floodplain

Decreased flow velocityDecreased flow velocityresults in deposition ofresults in deposition of

suspended sedimentsuspended sediment

1996 Liuzhou, China


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Discharge Changes and Floods Stream flow

fluctuations can beimmense

Hydrologists oftencharacterize dischargeas X-year floods

The Eel River had adischarge of ~752,000cfs on Dec. 23, 1964 (>1993 Mississippi flood)

?Skytomish River, Washington

1993 Mississippi Flood1992 1993


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1993 Mississippi Flood

500 - year flood (90-day volume)

Dense levee system confinedstream flow and led to sharpincreases in peak flow andcatastrophic flooding

~500 levee breaks allowed fordischarge on flood plains whichreduced impact on major citiesdownstream

Total estimated damage 16 billion

48 people killed

The Problem With Levees Levees protect from flooding onregular basis

BUT flow constriction during floodscauses water to flow fasteranddeeperincreasing stream power

Flooding occurs upstream(backup) and downstream duringbiggest floods


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The Problem With Levees

Levees protect from flooding onregular basis

BUT flow constriction duringfloods causes faster anddeeperflow, increasing stream power

Flooding occurs upstream(backup) and downstream duringbiggest floods

Flood enhancement through floodcontrol: Criss & Shock, Geology, 2001

Flood stages forconstantdischargehave increased 2-4 m over the pastcentury, mostly attributable tochannelization

Build more levees ?

FEMA Land buyouts &levee strengthening andbuilding

European model makemore room for the river

Now counterbalanced bymassive construction inflood plain:

Since 1993 28,000 homes,26% increase in

population, 26.8 sq km(6630 acres) of newcommercial and industrialdevelopment totaling 2.2billion dollars.

Mitigation Strategy

Documents

River Morphology