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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
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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