factors that influence erosion...thermal degradation / freeze thaw erosion •freezing & thawing...

Factors that Influence Erosion

• Soils • Precipitation • Vegetation • Surface Area • Slope Length • Slope Gradient • Surface Texture Grayling

Most Sites Have Various Soils

How Deep will you be Digging? How will the Soil Change?

Differing Soil Types

Sand

Gravel

Loamy sand

Clay Loam

Clay

Clay

Soil Surveys: Ministry of Environment

http://sis.agr.gc.ca/cansis/taxa/landscape/slc_bcsouth.html

Most sites have specific Geotechnical reports and Bore Logs available

My Next Job Site

Woeisme Acres

This report indicates that

80% of the soil is .01mm

or smaller

200 Sieve

Sandy Soils Erode & Settle Quickly

Compacted Soils

Slow infiltration rates, impedes downward movement of water.

Clay Soils

Low Erosion Potential, but… Once Suspended it’s Very Difficult to Settle!

Organic Matter

Root Zone

Sub Grade

Graphic by USDA, NRCS

Soil Horizons

Erodibility Increases as the % of Silt & Sand Increases

Erodibility Decreases

as the % of Clay & Organic Material

Increases

Soil Texture & Erodibility

Surface Texture

Track Walking Increases Roughness

Imprinting Slopes Increases Texture & Assists Vegetation

Composition of a Compacted Soil

SolidAirWater

Note the reduced air space in a compacted soil.

Before After

70% 50%

Courtesy Dwayne Stenlund CPESC MNDOT

Soil Ripping

Reduces Compaction Increases Infiltration

Thermal Degradation / Freeze Thaw Erosion

•Freezing & Thawing detaches soil particles and causes down-slope movement •Frost loosened soil is more susceptible to erosion by rain and snowmelt •Eroded soil can become saturated with spring breakup resulting in mass movement

Precipitation

Climate

Rainfall factors that influence erosion

http://atlas.nrcan.gc.ca/site/english/maps/environment/climate/precipitation/precip

Mean Annual precip mm per year

Rainfall Data On

the Internet

www.climate.weatheroffice.gc.ca

Benefits of Vegetation

Reduces Runoff Volume

Reduces Flow Velocity

Sediment Filtration

Energy Absorption

Pollution Reduction

Soil Retention

Photo by USDA - NRCS

Trees

Shrubs

Moss

Duff

Roots

http://plants.alaska.gov/

Soil Rehabilitation Handbook Forest Practices Code

DFO Guidance

Revegetation Guidance

TOPOGRAPHY

Doubling the slope length increases erosion potential by 4 times.

Doubling the slope gradient increases erosion potential by 5 times.

Slope Length & Gradient

Surface Area

Larger bare areas contribute larger volumes & greater velocities of runoff

Soil Characterization

Slope Length & Gradient

Surface Texture

Vegetation

Precipitation Basin Area

= Erosion Losses

Factors that Influence Erosion

Revised Universal Soil Loss Equation A=R x K x LS x C x P

R= Rainfall Runoff Erosivity K= Soil Erodibility L= Length of Slope S= Slope Steepness C= Cover Practice P= Support Practice

A= Soil Loss In

Tons / Acre / Year

Soil loss equations can assist contractors in quantifying soil loss and the effects of BMP’s for risk management decisions.

Location

Location

Location

Erosion Risk Assessment

How much risk does your site pose? Consider: • Soil type- above and below the surface • Climate- frequency, intensity, and duration of rainfall • Vegetation • Surface Area • Slope Length & Gradient • Surface Texture