Erosion and Sedimentation

Erosion – Detachment , movement and deposition of soil by water, wind, ice or gravity.

Sediment – Particles derived from inorganic or organic material that have been or are currently being transported by a fluid.

Sediment Yield – The quantity of sediment , passing a section per unit time.

Importance 1. Soil Productivity – majority of the nutrients are in the top 3

cm of a semiarid soil. Changes in soil texture, loss of clay and silt.

2. Sedimentation of Channels and Reservoirs – millions of dollars are spent every year on dredging.

3. Impacts on Aquatic Habitat – sediment makes respiration difficult and destroys breeding areas.

4. Increase Water Treatment Costs – dredging ponds and replacing filters.

5. Physical Damage – gulling and channel scour can cause damage to structures, increase flooding potential.

6. Property Values – Can have economic impact on property.

Rain

Raindrop Impact

Infiltration – The controlling factor

Overland Flow

Sources of Sediment Yield

Splash and Sheet is relatively low

Rill and Gully is relatively high

Factors1. Climate – the more rainfall the more potential erosion. High intensity

rainfall is the most sever. 90% of the erosion by water occurs during the summer monsoon in SE Arizona.

2. Soil Type and Disturbance – fine texture soils, without organic material, are more likely to erosion. Soils will high infiltration rates will have less overland flow/erosion. Soil aggregation decreases the erodability of a soil. Surface rock cover also decreases the erodability. Soil disturbance (consolidated un-consolidated) will significantly increase erosion.

3. Topography – Steep slopes have higher potential erosion rates because of increase velocity and overland flow rates.

4. Land Cover – the more the better!!! Most natural systems are sustainable because of natural cover conditions. The most important impact of management is changing the amount and type of cover. Cover during critical periods is the most important.

Sediment Yields, Areas, and Total Amounts from Different Land Use in U.S. (from U.S. EPA

Land Use Sed. Yield

(tons/ac-yr)

Area

(106 ac)

Total Sediment

(106 tons/yr)Construction 125 1.5 188

Surface Mining 24 2.4 58

Forest Mgt 10 16.0 160

Cropland 4.0 482 1930

Pasture/Range 2.3 522 1200

Urban 1.4 15 21

Forest 0.18 522 107

Langbein & Schumm (1958)

Impact of Management

Considered both cover and available energy

Natural

Accelerated

Cropland vs. Rangeland

Cropland Rangeland

intro

Soil vegetation topography management

Raindrop Splash Sheet Flow Rill Erosion

Erosion processes on hillslopes intro

Raindrop Splash Sheet Flow Rill Erosion

Splash and Sheet Erosion, Interrill Erosion

Concentrated Flow Erosion

Interrill vs Rill

• Importance is related to position on hillslope and cover characteristics.

• Interrill (splash and sheet) is higher on the upper slope, but deceases in importance as you move downslope.

• Vegetation can prevent rills from forming.

Erosion processes on cropland vs. rangeland

Interrill – natural good covered rangelandsRill erosion – plant forms and rangeland conditions.

intro

Cropland Rangeland

Role of Vegetation1. Protective Canopy – breaks the impact of raindrops and guards against

splash erosion.

2. Mat of Litter – additional protection against raindrop splash and also slows surface runoff.

3. Roughness – decrease runoff velocities, prevents rills from forming.

4. Roots – holds soil in place and increase infiltration. This is true for both hillslope and riparian vegetation.

5. Soil Organic Material – improves soil structure and soil water holding capacity. Soil aggregation makes a soil less erodible.

6. Increase Interception and Transpiration – less water reaches the soil and low soil water contents will increase infiltration.

7. Other Resource Values – timber, forage, habitat, visual quality, etc.

Cover vs. Erosion

• The research is clear, the more cover the better. Cover can be canopy, ground, rock, pavement, etc.

Erosion Process• Detachment

– Breaking the soil down into individual particles– Erosion in the Interrill area occurs primarily as a result of raindrop

impact. – Rainfall is the most important, flowing water (shear stress), animals and

man are other sources.– Function of rainfall intensity

• S = grams of soil splash in 30 minutes• i = rainfall intensity (inches/hour)• V = drop velocity in ft/sec• D = drop diameter in mm• Ellison (1944) in a lab experiment

– Regulated by soil type, soil organic material, and cover.– % clay is important since it is more likely to move.

65.007.133.4 iDVS

Kinetic energy of rain and runoff

• Kinetic Energy = ½ * mass * (velocity)2

• Rain has α 256 times more kinetic energy then sheet flow.

Rain Runoff

Mass Assume mass of falling rain is R

Assuming only 25% of rain becomes runoff or

Runoff mass = R/4

Velocity Assume terminal velocity of 8 m/s

Assume speed of surface flow in 1 m/s

Kinetic Energy ½ * R * 82 = 32R ½ * R/4 * 12 = R/8

Erosion Process• Transportation

– Flowing water (sheet and rill)– Stream power (based on Manning’s equation has been used to represent

transport capacity (Moore and Burch, 1986)

– where• Q/b = Discharge per unit length• s = Slope gradient• n = Manning’s n• Q/J = Discharge per rill per unit length• W = Rill shape factor

)n/s()b/Q(P 6.03.04.0Sheet

W)n/s()J/Q(P 75.0375.125.0Rill

Slope Geometry

• Only human altered environments have uniform slopes.

• You can have areas of erosion and deposition on a hillslope depending on the gradient changes.

Supply vs Transport