fluvial geomorphology1

8/9/2019 Fluvial Geomorphology1

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Fluvial Geomorphology

and Its use in RiverStabilization – Part 1

Instructor:

David T. Williams, Ph.D., P.E.David T. Williams and Associates, Engineers

[email protected]


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What is Fluvial Geomorphology?

Fluvial – Fluvius = River

Geo – of or relating to earth, ground or soil

Morphology – Study of form and structureGeomorphology - the study of landforms, theprocesses that created them, and the history of theirdevelopment.

Fluvial geomorphology - the examination of theprocesses that operate in river systems and thelandforms which they create or have created.


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Things to Remember

A River is part of a System:

System is:

• Dynamic

• Complex• Thresholds Do Exist• Geomorphology Provides Historical Perspective• Size / Power of Stream is Important


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A River is Part of System

The entire watershed forms a systemRainfall => Runoff (Land Use)

Runoff Transports Sediment (LandUse/Cover)Slope / Meanders Impact TransportSediment Size Impacts Transport and

Erosion/DepositionManmade Features Impact EverythingNature has its own plans and goals!!!

• It’s not always what we expect!


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River Sizing/Shape Relationships

LaneLeopold & WolmannSchummKennedyRegime Theory

Channel Forming Discharge

Rosgen/ThorneLots of Uncertainty and Discussion


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Fluvial Geomorphology

Streams are part of watershedChanges that impact streams:

Changes in land useChanges in hydrology

• Reservoirs, M&I Outfalls, Irrigation

Diversions into/out of watershed

Timing of delivery – M&I outfalls• Hydropower / Detention Basins

Flood Control Features


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Lanes Equation

Sediment Load x Bed Sediment Size (D 50 )is proportional to

Stream Discharge x Stream Slope

Q s D50 α Q S


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Lanes Balance


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Watershed Location

Upper Watershed – ErosionProduces sediment

Middle Watershed – TransportationTransports most of sediment produced

Lower Watershed – DepositionDelivers sediment to sink

Ocean, lake, delta, broad plain


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Watershed Location


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Sediment Transport

IT’S NOT THAT SIMPLE

Every river/stream reach has

erosion, transport and depositionoccurring at the same time!


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Transport Reach

Point or mid-channel bars are always changing

Outer banks erode

Inner banks deposit

Sediment moves from outer bank to innerbanks and bars

Meanders move down valley


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What is Stable?

Absolute Stability – Doesn’t Change in EngineeringTime Scale

Think Concrete / Bed Rock / Etc.

Dynamic Stability – Changes but Relationships areConstant in Engineering Time Scale

Unstable – Major Changes in Width, Depth, Flow,Sediment Transport, Sinuosity, Planform or all of Theseare Occurring over a Period of Days, Months or Years


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Absolutely Stable


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Absolutely Stable

Little Cottonwood Creek, Salt Lake City, UT


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Absolutely Stable

Who says concrete channels aren’t green?


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Dynamically Stable


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Unstable

WES Stream Invest.


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Unstable?


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Stability

Rivers want to be Dynamically StableNot Necessarily Constant / The Same

• Bank Location• Meander Locations• Sand / Gravel Bars• Anything having to do with location of features

Remember Locations Move under DynamicStability!


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Stability

Some Streams are more Stable

Clay Bed & Banks

Rock Outcrops or Banks• Mountain Streams / Torrents• Steep Sediment Starved Systems

Concrete / Designed Channels


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Stability

Less Stable Systems

Silt Bed & BanksSand or Gravel Banks

Any Non-cohesive / Uncemented Banks

Braided Systems• High Sediment Loads Fill Flow Areas• Channels Constantly Moving

Channels without Bedrock Controls


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Stability Depends on Perspective

How fast does an unstable rivermove?

How long does change take to beclassed stable?

If a river moves at 10’ per year it’sprobably stable until it gets to 50’ ofyour house!


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Stream Stabilization

Rivers are Mobile

Hard Points• Protect Permanently (Engineering Time)• Concrete• Riprap

Soft Points• Protect for a While then Fail (Melt Away)• Bio-Engineering


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System Instability Features

Headcutschannel bottom erosion progressing upstreamindicating a readjustment of slope, discharge,and sediment.

Knick Pointlocation in the thalweg where there is an abrupt

change in elevation.

Incised ChannelChannel that is not hydraulically connected with

its floodplain due to erosion.


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Causes of System Instability

Upstream CausesChanges in discharge and sediment supply dueto dams or diversions.

Downstream CausesBase level lowering due to cutoffs orchannelization.

Basin wide CausesLand use change such as urbanization thatalters discharge and sediment.

Complex/ Multiple Causes


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System Instability Causes(Profile Adjustment)

AGGRADATIONUpstream increase in

sediment (construction)Downstream rise in thebase level (sea levelrise, deposition in

dams)Basin-wide increase insediment yield (soilerosion)

DEGRADATIONUpstream reduction in

sediment load (dams)Downstream increasein stream power(base level change)

Basin-wide reductionin sediment yield (soilconservation)


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Local Instability

Definition: Localized instability such as bankerosion that is not caused due to systemic dis-

equilibrium in the watershed, but results from site-specific factors or processes. (example, erodingouter bank in a meandering channel)

Note: Stream bank erosion may also be a symptomof system instability.


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Causes of Local InstabilityStream Bank Erosion

Parallel FlowImpinging Flow

PipingFreeze/Thaw (tension cracks)Sheet Erosion

Rilling/GullyingWind WavesOthers


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Types of Local InstabilityStream Bank Failure

Rotational Slip (Slumps)Shallow SlidesPiping FailurePop-out FailureBlock Failure (Slab Failure)Soil/Rock FallWet Earth FlowOthers


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Hard Points in Mobile Rivers

What is your PurposeRestoring River (allows adjustment)Fixing One Bank in PlaceFixing River Reach in Place

• Does it Matter if Concrete or Bio-engineering?Fixed in Place = Fixed in Place!

Be Careful Who You Criticize!• Biologists & Single Purpose Refuges• Engineers & Single Purpose Projects• Stream Restoration & Fixed Designs


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Channel Evolution Model

What Happens to Channels OverTime?

(It’s a “Natural” Process!)


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Simon’s Modification ofSchumm’s Model (courtesy, G.

Athanasakes, Stantec)


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Stage I


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Stage II


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Stage III


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Stage IV

WES Stream Invest.


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Stage V

WES Stream Invest.


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Stage V

Coalville, Utah


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River Mechanics

Branch of fluvial geomorphology thatquantifies the relationship between processand form in rivers.

Channel Characteristics and DefinitionsHydrologic ClassificationChannel Pattern/Planform

Channel Geometry (cross-section)Channel Profile (slope)

Channel Process-Form RelationshipsChannel Classification


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Hydrologic Classification

Arid ZoneFlow Occupancy: 0-50 percentEphemeral StreamsIntermittent Streams

Humid ZoneFlow Occupancy: 50-100 percentIntermittent StreamsPerennial Streams


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Channel Geometry

Pools and Riffles (Crossings)Riffle-Pool channels (sand and gravel)Step-Pool channels (boulders and cobbles,steep slopes > 3%-5%)

Cross-Section Shape

Channel BarsPoint bars

Alternate bars

Mid-channel bars


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Terrace

Bankfull Depth

2nd Terrace1st Terrace

Geomorphic Floodplain

Bankfull Width

Natural Channel Geometry(courtesy, G. Athanasakes, Stantec)


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Channel Profile (Slope)

Product of discharge (Q) and slope (S) isdefined as STREAM POWER.

Stream power is the ability of the channel todo work.

Channel slope is defined as the watersurface slope or the stream bed slope.

Ch l P


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Channel Process-Form Relationships

W = Ca x QaD = Cb x QbV = Cc x Qc

Qs = Cd x Qdwhere:

W = width, Q = water discharge, D = meandepth,

V = mean velocity, Qs = suspended sedimentload

Ca, Cb, Cc, Cd, a, b, c, d are numerical constants

f


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Copeland Method for Sand BedsLess than 50% cover on banks

l d h d f d d


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Copeland Method for Sand BedsMore than 50% cover on banks


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Copeland Method


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Copeland Range of Solutions

fluvial geomorphology1

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