Soil Erosion by Water

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AGSM 335 Water and Soil Management

AGSM 335 Water and Soil Management

SOIL EROSION BY WATER

READING MATERIAL CHAPTER 6 OF TEXT

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dot = 5,000 acres

red = HEL yellow = non- HEL

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Sediment Sources and Sinks
• Sources: natural erosion, ag lands, construction sites, roadway embankments, lumbered areas, surface mines • Sinks: pools and reservoirs, concave slopes, vegetation, flood plains

Principal Processes
• Detachment - dislodging of soil particles from the soil mass by erosive forces (raindrop impact, water flow, wind) • Transport - entrainment and movement of sediment from its original location • Sedimentation - deposition of transported sediment- permanent or temporary

rills => see small channels interrill area => area between rills

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Detachment
Interrill areas  raindrop impact and overland sheet flow  independent of slope length  linear function of slope steepness

soil detachment -- rainfall impact or the shearing force of water flow

Soil Erosion and Control Practices
Erosion => sediment yield => deposition  soil detachment rainfall impact / shearing force of water flow  sediment transport => flowing water  sediment yield => surface runoff velocity Decreases sediment deposited => large particles first

Sediment Transport
Transport Capacity - amount of sediment moving water is able to carry (depends on energy available-- velocity) Sediment Load - amount of sediment actually carried

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Upland SOIL Erosion
Factors • Hydrology (rainfall and runoff) •Topography • Soil type (composition, organic matter) • Soil Particle size •Land cover (interception, roughness, subsurface structure) • Land management (organic matter content, subsurface structure) • Surface roughness (depressions, tillage)

Dominant Types of Erosion

Terminology
Upland areas near watershed divide
 rills => small channels in soil



Interrill (Sheet) - removal of a thin layer of soil particles (overland flow) Rill - erosion in very small channels can be tilled over

location of rills is controlled by microrelief of the land flow concentrated in micro-channels erosion due to shearing forces of channelized flow tilling can remove rills new rills not in the same location



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Rill Erosion

Downslope away from watershed divide
 land micro-relief control ceases

interrill area => area between rills 1. 2. sheet flow overland erosion due to raindrop energy on soil

 land marco-relief starts to control erosion  ephemeral gullies tillage can remove ephemeral gullies but new gullies will occur in the same locations  classic gullies tillage can not remove classic gullies  erosion due to => shearing forces in surface runoff flow

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Dominant Types of Erosion


Farther downslope from divide channels => stable banks => permanent land feature •

Gully - concentrated erosion in surface channels (not streams). Potentially very large. Cannot be removed by tillage. Channel - erosion by water movement in a stream - scour of bottom and bank erosion.

Channel Erosion
• • Detachment and transport - flow in stream Factors - flow characteristics (magnitude and sediment load), soil characteristics, vegetative cover, sidewall stability, alignment

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Gully Erosion
• • • Severe erosion in intermittent channels Detachment and transport - flow in gully Factors - drainage area, soil characteristics, alignment (cutting in outer curves), size (typical flow relative to cross sectional area), shape (depth of flow for low discharges), slope.

Sediment Transport
• Transport capacity > sediment load ----

entrainment equilibrium deposition



Transport capacity = sediment load ----



Transport capacity < sediment load ----

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Deposition
Can happen anywhere downslope when:
 Surface runoff transport capacity < soil available for transport  direct function of flow velocity reduce velocity => increase deposition vegetative filters / terrace...
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