EROSION

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EROSION SEDIMENTATION RELATIONSHIPS
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Precipitation VegetationErosion Rates
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Sources of Sediment in Streamflow

• Surface soil erosion
• Gully erosion
• Soil mass movement
• Channel stream bank erosion

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Definitions

• Erosion removal of soil material
• Sedimentation Deposition of soil material

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Surface Soil Erosion
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Soil Erosion from Overgrazing
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Surface Erosion

• Energy required to dislodge transport soil
  particles
• Ke ½ M V2
• Ke kinetic energy in ergs or ft-lb
• M mass fo falling raindrop in g or lb
• V velocity of fall in cm/sec or ft/sec
• Terminal velocity of raindrop rate of fall when
  air resistance prevents further acceleration

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Capacity of flowing water to pick transport
sediment

• Function of energy determined by
• (1) velocity and (2) turbulence of flow
• V 1.49/n Rh2/3 S1/2
• Overland flow sheet flow rills gullies
  into channels

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Surface Soil Erosion

• Universal Soil Loss Equation
• A R K (LS) C P
• A Soil loss (t/acre/yr)
• R Rainfall erosivity factor (t/acre)
• K Soil erodibility factor (fig 7.3) soil loss
  per unit of rainfall erosivity
• (LS) topographic factor (dimensionless)
• C cropping factor (dimensionless)
• P erosion control factor (dimensionless)

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USLE Revised Modified

• Modified USLE
• A R K (LS) (VM)
• VM Vegetative management factor
• Can be determined as in Fig. 7.4
• for forested areas with required data, or
  from generalized information (Fig. 7.5 Table
  7.4)

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K Factor
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Nomograph to determine LS Factor
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VM Forested Conditions

• Figure 7.4 VM
• Product of A x B x C
• Must have data to calculate
• Other tables can be used tables 7.3, 7.4,

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VM for non-forested (Fig. 7.5)
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Changes in Surface Erosion over Time

• Natural processes fines erode first , leaving
  coarser material (desert pavement) permeability
  K change
• Human induced
• Modify K
• Modify C or VM

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Total Soil Erosion

• Usually a combination of surface erosion, gully
  erosion, in some cases soil mass movement
  (e.g., hillslope failures, stream bank failures)
• Revised USLE WEPP models attempt to better
  predict surface soil erosion from croplands
• None work well for natural watersheds with steep,
  irregular slopes

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Loss of Vegetative Cover Leads to Serious Soil
Erosion
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Surface Gully Erosion Degraded Watersheds
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Gully formation
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Gully Erosion Loss of Land Productivity
Increased sediment Loads
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Functions of Check Dams

• Reduce velocity of water flow
• Trap sediment
• Allow for vegetation establishment
• Management after establishment is KEY

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Gully Control Check Dams
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Gully Control Haiti
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Agroforestry -- Gully Controlin Haiti
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Soil Mass Movement
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When does slope failure occur?

• Expressed as a factor of safety
• F Shear Strength
• Shear Stress
• F 1, failure is imminent
• F gtgt 1, there is little risk

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Forces involved in Soil Mass Movement
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Hillslope failure above the municipal reservoir
for Katmandu, Nepal
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Soil Mass Movement Nemadji Watershed, MN
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Timber Harvesting Landslides
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Debris Flows in Taiwan

• Excessive rainfall of 2500-4500 mm/yr with an
  average of three typhoons each year
• Landslides occur on steep slopes
• Add debris to channels
• Debris flows result from high streamflow
• in debris charged, steep channels

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Landslides Add Material for Debris Flows
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Engineering Hillslopes Channels
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Consequences of Soil Erosion

• Lost productivity of uplands
• Increased sediment in streams rivers
  impacting water quality and aquatic ecosystems
• Increased sediment to downstream areas impacts
  on reservoirs, stream channels

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Accelerated soil erosion and sediment delivery
threatens reservoirs
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Determine Sediment Delivery to Reservoirs

• Sediment delivery ratio
• Dr Ys / Te
• Ys sediment yield from watershed at some
  point in a channel
• Te Total soil erosion above that point
• Dr some fraction, e.g., 0.45, 0.20, etc.

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What happens when the sediment reaches the
reservoir?

• Depending on sediment size, etc., a certain
  fraction will settle behind the dam and finer
  sediment passes through this is determined as a
  percentage of total incoming sediment load (e.g.,
  90)
• Dead Storage allocated space in reservoir pool
  to store expected sediment deposition over the
  life of the reservoir

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Allocation of Storage in a Reservoir
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Reservoir design

• Structural components spillway, etc. based on
  hydrology
• Total volume of reservoir storage
• volume of storage to meet demands e.g.
  irrigation, flood control, hydropower, etc.
• dead storage

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When underestimate dead storage or have
excessive sediment (Morocco reservoir)
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Erosion/sediment controlduring construction
operation
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Engineering structures to trap sediment reduce
flow velocity in channels COSTLY
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Many are ineffective without corresponding
changes in LAND USE
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AGOROFRESTRY An attractive Alternative for
Uplands

• Combination of agricultural crops (annual and
  perennial) and trees on the same piece of land

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Agroforestry helps stabilize hillslopes
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Integrated Watershed Management

• Manage the watershed to sustain life of
  reservoirs beyond the construction phase
• Management must achieve erosion control
  objectives AND meet resource needs of watershed
  inhabitants society
• Reducing excessive sediment in streamflow also
  needed to meet TMDLs, improving aquatic habitat
  channel stability