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Soil Eros ion TSM 352 Land and Water Management Systems

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Soil Erosion. TSM 352 Land and Water Management Systems. Soil Erosion. Most important problem associated with agricultural and other land use practices. Soil Erosion. According to 1987 National Resources Inventory, USDA - PowerPoint PPT Presentation

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Page 1: Soil Erosion

Soil Erosion

TSM 352Land and Water Management Systems

Page 2: Soil Erosion

Soil Erosion

Soil Erosion

Most important problem associated with agricultural and other land use practices

Page 3: Soil Erosion

Soil Erosion

Soil Erosion

According to 1987 National Resources Inventory, USDAo 4 billion metric tons of soil are lost every year due to soil erosion

(wind and water) in 1970so 2 billion metric tons in 1997 due to increased used of

conservation and best management practices (BMPs)o 70% of the total soil loss is from agricultural lando Economics of soil erosion: 44 billion dollars/year, $100/acre

It takes 100-600 years to form one inch topsoil

In Illinois,o 40% (9.6 million acres) cropland suffer from erosiono Average soil erosion rate: 6.18 tons in 1987. (max 50 tons/acre)

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Soil Erosion

Soil Erosion

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Soil Erosion

Soil Erosion by Water

T - maximum average annual permissible soil loss without decreasing productivity T-values ranging from 4.5 to 11.2 metric tons/ha per year (2 to 5 tons/acre per year).

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Soil Erosion

Soil Erosion

Mississippi River The river carries roughly 550-500 million tons of sediment

into the Gulf of Mexico each year. It brings enough sediment to extend the coast of Louisiana

by 91 m (300 ft) each year.

Page 7: Soil Erosion

Soil Erosion

Soil Transport

Mechanism Micro-scale soil transport: Surface erosion Macro-scale soil transport: Mass movements (Landslides)

Erosion is the detachment and transport of soil particles (natural and accelerated)

Sediment is relocated from the source (= soil) to streams and eventually into reservoirs or the sea

Sedimentation: deposition of sediments in streams or on fans and floodplains

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Soil Erosion

Effects of Soil Erosion

On site Loss of top fertile soil Loss of nutrients, OM Decreased productivity

Off site Non-point source pollution Filling of reservoirs and dams Air quality problem Effects on aquatic organisms Effects on drinking water quality Redistribution of pollutants and toxics

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Soil Erosion

Soil Erosion

Erosion is caused by: Water

Primary water erosion (Splash Erosion) Secondary water erosion (Surface flow

induced erosion)

Wind erosion The Dust Bowl of 1930s (1934 -1936) in the

Great Plains

Frost erosion Particularly effective in mountai nous areas Example: NW USA

Page 10: Soil Erosion

Soil Erosion

Factors Affecting Soil Erosion

Climate Soil Topography – slope length and steepness Vegetation Land-use practices

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Soil Erosion

Water Erosion Processes

Raindrop/splash Erosion Sheet Erosion Rill Erosion Gully Erosion Stream channel Erosion Shore line Erosion

Interrill Erosion

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Soil Erosion

Water Erosion processes - Hillslope View

Hillel, 1998

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Soil Erosion

Splash Erosion - Raindrop Impact

Erosive power = S kinetic energy (EK = mv2/2) and momentum (M = mv)

Fog ~ 0.05 mm/h Light rain ~ 1.02 mm/h Heavy rain ~ 15.24 mm/h Torrential rain ~ 101.60 mm/h

P. Gary White

Hillel, 1998; Selby, 1993

Brook et al, Fig 7.2Notes:

- Soil aggregates destroyed- Saltation- Erosivity threshold ~25 mm/h

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Soil Erosion

Surface flow induced erosion

Requires overland flow (thin surface films or concentrated) and often is intensified by raindrop impact

The generation of surface flow depends on:o Rain intensityo Water content of the soilo Density of the soilo Surface roughness of the soil

SCS CN method: “Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these…After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow”

Important in range and agricultural systems

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Soil Erosion

Overland flow

Exceeds detention storage Begins with detachment: requires a force (shear)

created by small eddies in flow or raindrop impactt0 = f (density, water depth, slope)

Sheetflow GullyRill

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Soil Erosion

Rill and Gully Formation

Brooks et al., Fig 8.1

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Soil Erosion

Erosion and Transport Processes

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Soil Erosion

Interrill Erosion

Di = Ki i q Sf Cv

Di = Interrill erosion rate (kg/m2-s)Ki = Interrill erodibility of soil (kg-s/m4) (Table 7.1)i = Intensity of rainfall (m/s)q = Runoff rate (m/s)Sf = Interrill slope factor = 1.05 – 0.85 e(-4sinθ) where θ = slope angle (degree)Cv = Cover adjustment factor (0 - 1)

(Equation 7.1)

Page 19: Soil Erosion

Soil Erosion

Rill Erosion Rate

1 sr r c

c

qD KT

t t

Dr = Rill detachment rate (Kg/m2-s)Kr = Rill erodibility, due to shear (s/m) = Hydraulic shear of flowing water (Pa) = γ R S (Equation 7.3)

γ = Specific weight of water (9810 N/m3)R = Hydraulic radius of rill (m)S = Hydraulic gradient or slope (m/m)

c = Critical shear below which no erosion occur (Pa)qs = Rate of sediment flow in rill (Kg/m-s)Tc = Sediment transport capacity of rill (Kg/m-s) = B 1.5 (Equation 7.4)

B = Transport coefficient based on soil and water properties generally between 0.01 and 0.1 (dimensionless)

(Equation 7.2)

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Soil Erosion

Rill Erosion Rate

If Tc > qs : erosion takes place If Tc < qs : deposition takes place

If > c : erosion takes place If < c : no erosion

1 sr r c

c

qD KT

t t

: defines erosion or deposition

The interrill and rill erosion processes are used in several process-based erosionprediction computer models, including the Water Erosion Prediction Project (WEPP) model

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Soil Erosion

Example: Rill Erosion

A rill channel is observed at a location 20 m from the top of the hill during a storm. The slope is 5%, the rill width is 150 mm, and the hydraulic radius is estimated to be 0.01 m, respectively. The sediment transport rate (qs) is measured and found to be 0.2 kg/m-s. The rill erodibility (Kr) is estimated to be 0.004 s/m, critical shear (c) to be 2.5 Pa, and transport coefficient (B) to be 0.1. Assume that the specific weight of water is 9810 N/m3. Calculate the sediment transport capacity and erosion rate for the location. If the hillslope is 120 m long and rainfall duration is 1 hour, how much soil will be eroded from the hillslope by rill erosion?

= γ R S (Pa) Tc = B 1.5 (Kg/m-s)

(Kg/m2-s) 1 sr r c

c

qD KT

t t

= 4.905 PaTc = 1.086 Kg/m-s Dr = 0.00785 Kg/m2-s

= 0.1629 Kg/s

= 423.8 Kg

= 0.001177 Kg/m-s

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Soil Erosion

Wind erosion

Important especially in arid regions

Dependent on o Wind speed and exposureo Soil particle- and aggregate sizeso Surface roughnesso Tillage

Surface roughness creates turbulence in the surface-near air layer →Under pressure sucks particles in the air

Form of movement:< 0.1 mm → Suspension (“dust storm”)< 1 mm → Saltation (“jumping”)> 1mm → Creep (“rolling”)

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Soil Erosion