Soil Erosion By Water -Causes and Mitigation

USDA - Natural Resources Conservation Service

Jerry J. DaigleState Soil Scientist

Charles M. GuilloryAsst. State Soil Scientist

E. J. (Ed) Giering III, PEState Conservation Engineer

Bradley A. StickerAsst. State Conservation Engineer

Stormwater Phase II

Soil - Ecological and Hydrological Resourcesupports plant growthprovides waterprovides nutrientsprovides mechanical supportfilters before enteringsurface waterfilters beforeenteringaquifers

Stormwater Phase II

soil FiltersBuffers

Degrades

Detoxifies

Immobilizeswatertable

Soil - Controls the fate of precipitation

controls what part infiltratescontrols what part runs offstores water at depths available for plantstransmits water to underlying materials

Stormwater Phase II

Erosion

Erosion is a process of detachment and transport of soil particles by erosive agents

Ellison, 1944

Stormwater Phase II

Sediment LoadSediment Transport

Detachment

Soil

Rain

RunoffSoil

Infiltration

Deposition:

Soil

Sediment LoadSediment Transport

Deposition

Reduces the sediment loadAdds to the soil massLocal deposition• Surface

roughness depressions

Remote deposition• Concave slope• Strips• Terraces

Stormwater Phase II

ErosionErosive Agents• Raindrop impact

• Overland flow of surface runoff from rainfall

Stormwater Phase II

Stormwater Phase II

Erosion is a ConcernDegrades soil resource• Reduces soil productivity• Reduces soil organic

matter• Removes plant nutrients

Causes downstream sedimentationProduces sediment which is a pollutantProduces sediment that carries pollutants

Types of Erosion

Stormwater Phase II

Geologic

• natural process

• humans can accelerate

Types of Erosion

Stormwater Phase II

Raindrop or Splash

• high detachment capacity

• low capacity for transport

Types of Erosion

Stormwater Phase II

Interrill (sheet)• low detachment

capacity• higher capacity for

transport

Types of Erosion

Stormwater Phase II

Rill• small channels

<1 foot deep• low detachment

capacity• higher capacity

for transport

Types of ErosionStormwater Phase II

Gully• from

concentrated storm water flow

• removes higher volumes per unit area

Types of ErosionStormwater Phase II

Gully• from

concentrated storm water flow

• removes higher volumes per unit area

Types of Erosion

Stormwater Phase II

Stream Channel• high volume peak

flows may erode streambanks and channels

• removes portions of the land surface above and adjacent to the bank

• can deliver large volumes of sediment directly into the stream

Types of Erosion

Stormwater Phase II

Mass movement (Landslip)

• slides - shear along specific surfaces

• flows - viscous,fluid movement

Factors affecting erosion

Stormwater Phase II

Topography• slope gradient• overland flow slope

length• length of eroding

portions• slope shape

Land use• cover management• supporting practices

Climate• rainfall - amount• rainfall - intensity

Soil• texture• moisture• surface

roughness

Simple Uniform Slope

Soil LossSedimentYield

Stormwater Phase II

Sediment Yield = Soil Loss

Stormwater Phase II

Complex Slope

Soil loss

Remote deposition

Sediment yield

Sediment Yield = Soil Loss - Deposition

Complex Slope

Soil loss

Remote deposition

Sediment yield

Soil loss

Stormwater Phase II

Sediment Yield = Soil Loss - Deposition

Local Deposition

Random Roughness:

Ridges-Furrows:

Stormwater Phase II

Sediment Yield = Soil Loss - Deposition

Stormwater Phase II

Deposition

Transport capacity = sediment load

Sediment production less than transport capacity

Deposition because sediment production exceeds transport capacity

Relation of deposition to transport capacity and sediment load on a complex slope

HillslopeTransport capacity

Sediment load

Stormwater Phase II

How Deposition at a Grass Strip Affects Sediment Characteristics

17Primary sand524Large aggreg.

1436Small aggreg.5824Primary silt225Primary clay

After (%)Before (%)Particle class

SDR = 0.2Note how deposition enriches sediment in fines

Stormwater Phase II

Basic PrinciplesSediment load accumulates along the slope because of detachmentTransport capacity is function of distance along slope (runoff), steepness at slope location, cover-management, storm severity (10 yr EI)Deposition occurs where sediment load becomes greater than transport capacity

Stormwater Phase II

Prevention and Reductionfit development to the terraintime disturbance to minimize soil erosionretain existing vegetation whenever feasiblevegetate and mulch denuded areasdivert runoff away from denuded areasminimize length and steepness of slopeskeep runoff velocities lowprepare drainage ways and outlets to handle flowtrap sediment on-siteinspect and maintain control measures

Stormwater Phase II

Soil loss

Remote deposition

Soil loss

Sediment yield

Remote deposition

Soil loss Remote deposition

Terraces

Stormwater Phase II

Strips

Soil lossRemote deposition

Soil loss

Sediment yield

Remote deposition

Stormwater Phase II

Dense grass

Erodible soil surface

Deposition region

Deposition ends where transport capacity = sediment load

Transport capacity

Sediment load

Relationship of Deposition to Transport Capacity and Sediment Load for a Grass Strip

Stormwater Phase II

Why worry about erosion at construction sites?

Stormwater Phase II

Sediment Laden Runoff

Stormwater Phase II

Sediment in Drainage Channels

Stormwater Phase II

Construction Site BMPs

Need to address soil erosion and sediment transportMay be structural and/or non-structural Should be applied according to a plan developed prior to construction start

Stormwater Phase II

Types of Construction Site BMPsErosion control BMPs• prevent the detachment of soil particles

and subsequent suspension and transportation in runoff water

• first and best line of defense

Sediment control BMPs • remove a portion of the suspended soil

particles from runoff

Stormwater Phase II

Erosion Control BMPsMinimize disturbed areas and preserve existing vegetationTemporary and permanent vegetative establishment of disturbed areasMulching disturbed areasCovering stockpilesStabilized construction site entrancesDivert runoff from disturbed areasVarious combinations of all of the above

Stormwater Phase IIMulching

Seed prior to mulching

Hay or straw @ 1.5 to 2 tons per acre

Mechanically anchor with netting or crimper

Blowing Straw Mulch

Stormwater Phase II

Vegetate As Soon As Possible After Construction

Stormwater Phase IICommercial Mulch Matting

Wide variety of types for various applications

Apply in accordance with manufacturer’s recommendations

Stormwater Phase II

Commercial Mulch Matting

Stormwater Phase II

Vegetative EstablishmentEstablish vegetation as soon as practical after disturbanceNRCS FOTG Standard - Critical Area Planting (Code 342)• Use appropriate seed mix for time of year• Test soil• Fertilize and lime according to soil test

Stormwater Phase II

Vegetative Establishment

Stormwater Phase II

Sediment Control BMP’s

Second line of defenseRequire proper installationMaintenance is crucial through out project life

Stormwater Phase II

Sediment Control BMP’s

Silt fencesCheck damsDrain inlet protectionSediment retention basinsOther sediment retention devices

Stormwater Phase II

Improperly Installed Silt Fences

Stormwater Phase IIProperly Installed Silt Fences

Stormwater Phase II

Open Top Gabion Check Dams

Stormwater Phase IIHay Bail Check Dams Working Improperly

Stormwater Phase IITypical Drawing of Hay Bail Check Dams

Stormwater Phase IIHay Bail Check Dams Functioning Properly

Stormwater Phase II

Drain Inlet Protection• Requires monitoring and maintenance

Stormwater Phase IITypical Drawing of Drain Inlet Protection

Stormwater Phase IITypical Drawing of Drain Inlet Protection

Stormwater Phase II

Sediment Basins

Drainage areaRetention timeTemporary storage volume

Stormwater Phase IITypical Drawing of Sediment Retention Basins

Stormwater Phase IISediment Retention Basins

Stormwater Phase IITypical Drawing of Sediment Retention Basins

Stormwater Phase IICommercially Available Sediment Retention Devices

Stormwater Phase II

Available ResourcesUSDA Natural Resources Conservation Service • www.usda.nrcs.gov

International Erosion Control Association• www.ieca.org

Stormwater Phase II

Our Soilis

Our Strength