Sediment Transport

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Topic 3 Sediment Transport Forest Land Use

• Introduction
• Definitions
• Mechanics
• Sediment Yield
• Channel Dynamics
• Conceptual Model of
• Sediment Transport
• VII. Forest Land Use and Erosion
• VIII. Preventing Accelerated Erosion

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Readings

• Beschta (1987)
• Beschta (1978), Grant Wolf (1991)
• Lewis (1996), Gomi et al. (2005), Ward Jackson
  (2004), Keim Schoenholtz (1999) - Discussions
• General reference Brooks et al 2003

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• Introduction
• sedimentlargest source of
• water pollution (mainly ag, but
• biggest wq problem for forestry)
• B. Sediment effects
• Domestic water supplies
• Fish production/diversity
• Recreation
• Flood storage
• Decreased forest productivity
• Nutrient/chemical transport

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• C. Erosion and sedimentation factors
• Runoff
• Soil properties
• Texture, structure, infiltration
• Vegetation
• ET, interception, f floor, roots
• Climate
• Precip - amt, intensity, form

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• II. Definitions
• Erosion soil/mineral detachment
• and transport
• Water, wind, ice, gravity, human
• Natural/geologic
• Accelerated
• Cultural
• Mass movement (debris flows, debris torrents,
  slumps
• earthflows, creeps, debris avalanches)
• Surface (splash, rill, overland)
• Provides variety of particle sizes can
• modify channel

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B. Suspended Sediment Inorganic material
suspended in streamflow mg/L x 1/ha x L/sec
mg/ha/sec convert to kg/ha/yr (1x106 mg/kg,
3.1536x107 sec/yr)
C. Sedimentation Deposition of suspended
materials carried in water
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III. Mechanics of erosion A. 3 Steps
1. Detachment
2. Transport
3. Deposition
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1. Soil detachment
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2. Soil transport
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3. Deposition
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B. Splash erosion Energy of raindrops
transports soil Potential energy ? Kinetic
energy (Position) (Motion)
Ke 0.5 M V2
Mraindrop mass Vraindrop velocity Kekinetic
energy
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More splash erosion..
raindrop
Terminal velocity reached in 7 m Role of Lower
vegetation Forest floor
Ke
Change soil aggregates
Change soil structure
Puddling
Splashing of particles
Puddling
Less infiltration, more runoff
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• C. Sheet erosion
• Splash erosion surface runoff
• D. Rill erosion
• Runoff into depressions
• Gains depth and velocity
• Transports soil
• E. Gully erosion
• Channelized
• Advanced stages of erosion
• Mass soil movement

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IV. Sediment Yield
Noxubee River, MS
An integration of many processes..
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IV. Sediment Yield
A. Total sediment outflow at a location over time
-Measure sediment load stream discharge
(concentration)

• -Large yields
• Active geologic erosion
• Improper land use
• Degree of aridity

-sediment load/discharge ratio High ratio bad
news!!! high erosion
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B. Sediment movement measurement

• Sediment discharge transported mass
• through given x-section (mg/L)
• 2. Suspended load suspended sand, silt, clay
• settling velocityltbuoyant velocity
• dep on particle size density
• sediment lowest near surface
• silt clay vs. sand
• correlation betw suspended load Q..

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Modes of Sediment Transport

• Suspended Load (clay silt, lt0.05mm)
• Bedload (gravel, cobble, boulders (gt2mm)
• Transition (sand-sized, 0.05-2mm)

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Distribution of suspended sediments
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3. Wash load clay silt component (lt0.05mm) 4.
Bed load sand, gravel, rocks transported along
bottom but not in suspension
Saltation
Rolling sliding

• Stream competence largest grain size a
• stream can move as bed load

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b. Critical tractive force force required to
entrain given grain size c. Erosion velocity
velocity at which entrainment occurs d. Stream
power ability of stream to transport bed
load e. Stream capacity max amt sediment of
given size or smaller that stream can carry as
bed load
Amt of bed load f(Q and slope)
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• 5. How much sediment is carried by stream?
• Supply of sediment to channel (var. source)
• Sediment properties
• Characteristics of channel
• Stream discharge
• Climate, topography, geology,
• vegetation, land use

f (sediment, energy)
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6. Channel degradation Stream energy gt Sediment
supply Results in channel scouring 7. Channel
aggradation Sediment supply gt Stream energy
Suspended sediment usually limited by sediment
supply Bed load usually limited by stream energy
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C. Sediment Budget
Inputs
Storage
Transfers
(channel reach or entire stream)
Outputs
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D. Sediment Delivery Ratio, Dr
-erosion rate vs. sediment transport
Dr Ys/Te
Ys sediment yield at a pt (kg/ha/yr) Te total
erosion within wshed above pt (kg/ha/yr)
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Factors affecting Dr

• Soil texture
• Land use
• Climate
• Stream conditions
• Physiographic position
• Dr decreases w increasing area

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E. Cumulative Impacts on Sediment Yield

• Non-point source
• Annual variation
• Diversity of land use changes
• Reduction in riparian veg
• Streambank alterations
• Increased streamflow
• Channel erosion-lateral extension of
• channelnew sediment source
• Reduced streamflow
• Cause-effect????

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V. Channel Dynamics and Processes A. Channels as
conduits for sediment

• Aggradation accumulate sediment
• -convex cross-sections
• Degradation loss of sediment
• -v-shaped cross-sections

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C. Streams in dynamic equilibrium

• Fluvial processes vs. hydraulic variables
• Driven by energy relationships
• Q AV channel morphology effects
• Shallowsmall A large V
• Deeplarge A small V
• Critical, subcritcal, supercritical flow

stable
unstable
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D. Laminar and Turbulent Flow

• Laminar smooth, parallel flow,
• Uniform velocity
• Turbulent irregular, chaotic flow
• paths-random velocity fluctuations
• in all directions

Slowest layer along bed
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VI. Conceptual Model of Sediment Transport
(Beschta 87)

• How do we account for variations in sediment and
  bed load supplies?
• Streams as integrated systems
• Processes in one reach affect those downstream

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Example of integrated processes

• Active erosion in headwater reach
• Rapid sediment routing downstream
• Steep gradient
• Middle reaches-transition
• Less gradient
• Deposition/aggradation
• Stream shifts laterally, cuts into fluvial
  deposits
• Eventually incises and mobilizes recent
  depositions
• End result rapid aggradation then degradation

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Example (continued)

• Dampens changes in stream channel downstream
• Sediments from headwaters are deposited in
  transition zones then slowly released without
  changing channel morphology downstream

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Sources of Variability

• Variable source areas of sediment
• Particle size gradations
• Non-uniform channel morphology
• Dynamic channel
• Transient flows
• Therefore.difficult to predict sediment
  transport

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Klingemans double condition

• Sediment availability
• Flow capability

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Suspended Sediment Transport Model

• Sediment Rating Curve
• C aQb

CTSS or SSC Qdischarge a,bcoefficients from
linear regression log C vs. log Q
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Modified Suspended Sediment Transport Model

• Incorporation of sediment supply var.
• C aQb g(S)

CTSS or SSC Qdischarge a,bcoefficients from
linear regression log C vs. log Q g(S)supply
depletion or washout
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Problem?

• Sediment hysteresis (rising limbgtfalling limb)
• Slope of rising limb directly related to TSS
  additional sources of sediment are being accessed
• Each succeeding event has more TSS as long as
  runoff exceeds preceding event
• Seasonal decline

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More sediment on rising limb
Changing Sediment Supply
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Distributed Supply Model

• Sediment distributed among many storage
  compartments
• Q determines which compartments are accessed
• Rising limbnew supply compartments progressively
  accessed
• Falling limbfewer compartments accessed
• Think of stratified sediment storage compartments
  along channel

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Distributed Supply Model
Q3
S3
Q2
S2
Q1
S1
Q0
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Simple Bedload ModelPhase I

• Increase Q, turbulence, velocity
• First move sand particles (from pools, channel
  margins, obstructions)
• No disruption of armour layer
• Shear stress turbulence f (channel form)

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Simple Bedload ModelPhase II

• Continued increase Q, turbulence, velocity
• Removal of fine sediment in interstices of armour
  layer
• Disruption of armour layer entrainment of bed
  material from riffle sections
• Sufficient velocity/turbulence to transport
  riffle bedload through pools to next riffle
• No hysteresis!

Armour layer breaking up
bed
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• VII. Forest operations and erosion
• Logging roads 80
• Harvesting tree felling min effect
• Main effect through change in hydrology
• C. Skidding 10-60 of harvest area
• Soil conditions
• Drainage patterns
• Mineral soil exposure
• Variable recovery rates

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Mapping and quantifying the problem
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• D. Site preparation
• Soil disturbance
• Removal of vegetation forest floor
• Compaction
• Disking and bedding
• E. Prescribed burning
• Severity
• Soil type
• Slope
• Precip
• Cover

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Beschta (1978) - paired watersheds
25 patch cut 4 roads 15-30 m buffers Light burn
control
82 cc 5 roads No buffer Burn
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Alsea annual sediment yield comparison
Two storms (1/28/65 and 1/11/72) 36 of total
15-yr sediment yield in Flynn Cr.
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More sediment in Fall than Spring
Flynn Cr.
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Alsea Watersheds
Pre-treatment early fall sedimentgtearly spring
sediment
seasonal shift in sediment rating curves
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Deer Cr. patch cut increased sed. yield
Road construction mass failure
Mass failure
2 sediment
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Discharge-weighted susp. sediment conc.
Deer Creek patch cut
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Needle Branch
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Needle Branch CC increased sed yield
Burning logging along channel
Low response
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Discharge-weighted susp. sediment conc.
Needle Branch clearcut
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• Alsea Summary
• Annual Q increased 3 for Deer Cr.
• Annual Q increased 26 for Needle Br.
• Annual sediment yield increases assoc
• with road construction, mass failure (Deer),
• surface erosion (Needle)
• Hysteresis for sediment yield
• More sediment on rising limb and with
• early season storms

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Grant Wolf (1991)
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Grant Wolf Summary

• Trying to study harvesting effects in unstable,
  environment
• Local mass movement events vs. logging effects
• Use of sediment rating curves (model
  development) annual sediment yields based on
  annual hydrograph

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Table 2. HJA Landslide Chronology
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HJA Sediment Response
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• VII. Controlling erosion-BMPs
• Susceptible situations
• Long, steep slopes, shallow soils
• Low infiltration capacity
• Loss of vegetation
• B. Prevention
• Protect soil surface
• Increase surface roughness
• Shorten slope length
• Maintain vegetation

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Vegetation-the key

• Interception
• ET reduces soil moisture
• Litter and OM
• Roughness
• Soil structure infiltration

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• B. Prevention (continued)
• Revegetation of disturbed areas
• Low-impact logging systems
• Cable logging
• Low-pressure tires
• Maintain litter operate on slash
• Dry-weather logging
• Road SYSTEM
• Minimum area
• Avoid high-hazard areas
• Apply erosion control
• Reduce sediment delivery to streams

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Effect of planning
Planned Skidtrails
Loggers Choice