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Factors that Influence Erosion
•Soil Properties Soil Properties •PrecipitationPrecipitation•Vegetation Vegetation CoverCover•TopographyTopography
Soil Properties
Texture
Structure
Organic Content
Depth of Solum
D S J /E A O /D o c u m e n ts /C o u r s e M a n u a l /S o i l C h a r t
.
S ANDY C LAYL OAM
S ANDYC LAY
C LAY
C LAY L OAM
L OAMS ANDY L OAM
S ILT L OAM
S IL T Y C L A YL O A M
S AND
100
90
80
70
60
50
40
30
20
10
10
20
30
40
50
60
70
80
90
100102030405060708090100
P ERCENT S AND
S OIL T EXTURAL C LASSES
P ERCENTC LAY
P ERCENTS ILT
S ILT
S IL T YC L A Y
50% Sand
31% Clay
19% Silt
Textural Triangle
Hjulstrom Diagram
Soil Thickness
The upper part of the soil is called the Solum (A+B horizons). In undisturbed soils these upper horizons are highly permeable due to well developed soil structure.
Soil Structure
On a construction site soil structure is often destroyed by compaction, either intentionally…
or as an consequence of other construction activities
Or removed completely during mass grading
Sources of Information on Site Specific Soil
Properties
SCS (now NRCS) Soil SurveysSCS (now NRCS) Soil Surveys
NRCS Web Soil Survey
Example Web Soil Survey Map
Geotechnical Reports
Geologic Maps
Examples of Erosion Issues for Some Common Local Soil Series
Alderwood soil
Developed on glacial till. Coarse texture and glacial consolidation render it relatively resistant to erosion, but the presence of a shallow impervious layer at depth often leads to erosion problems related to standind g water and poor drainage.
Indanola Soils
Developed on developed on sandy glacial advance outwash. These soils are readily eroded by flowing water.
Kitsap soils are developed on fine-grained Kitsap soils are developed on fine-grained proglacial lake beds. They have poor drainage, are proglacial lake beds. They have poor drainage, are difficult to work when wet, create highly turbid difficult to work when wet, create highly turbid runoff when disturbed and are often assosiated runoff when disturbed and are often assosiated with areas of unstable slopeswith areas of unstable slopes
Kitsap Kitsap SoilsSoils
A good CESCL knows their dirt!
PrecipitationPrecipitation
RainfallRainfallfactors thatfactors that
influenceinfluenceerosionerosion
Climate
www.wrcc.dri.edu
Rainfall Data on the Internet
http://www.wrcc.dri.edu/summary/Climsmwa.html
SWPPP Source Material
45% probability of ¼ inch in June
Vegetation
o Increase Surface Increase Surface
RoughnessRoughness
o Absorb Rainsplash Absorb Rainsplash
EnergyEnergy
o Provide Soil Provide Soil
Reinforcement Reinforcement
o Promote Good Soil Promote Good Soil
StructureStructure
o Reduce Runoff Reduce Runoff
Volume Volume
o Trap Sediment Trap Sediment
How do Plants stabilize the Soil?How do Plants stabilize the Soil?
Photo by USDA - NRCS
Interception and energy absorption
Trees
Shrubs
Moss
Duff
Roots
Soil Improvement
Photo courtesy Clark County Watershed Stewards
So, What’s the first thing So, What’s the first thing we do at a construction we do at a construction
site??site??
Topography
Elements of Topography:
Slope Steepness
Slope Length
Slope curvature
Aspect
Slope Steepness
The ability of flowing water to carry sediment increases (roughly) with the cube of the slope angle!
Slope LengthWith uniform rainfall on a continuous impervious slope, the depth of flowing water, and therefore its erosive power increases down slope. The longer the slope, the greater the erosivity at the bottom.
Slope curvature
Aspect
North
A = R x K x LS x C x P
R= Rainfall Runoff ErosivityK= Soil ErodibilityL= Length of SlopeS= Slope SteepnessC= Cover PracticeP= Support Practice
A= Soil Loss in
Tons / Acre / Year
Revised Universal Soil Loss Revised Universal Soil Loss EquationEquation
Cover-Management (C)Cover-Management (C)• DefinitionDefinition: this factor represents vegetative, : this factor represents vegetative,
management and erosion-control practice management and erosion-control practice effects that primarily affect the process of effects that primarily affect the process of detachment on soil lossdetachment on soil loss
• Similar to the other factors, the calculated C Similar to the other factors, the calculated C factor is the ratio of soil loss comparing the factor is the ratio of soil loss comparing the defined, existing surface conditions to that defined, existing surface conditions to that of a of a
unit plotunit plot
• Has the greatest possible range of all factorsHas the greatest possible range of all factors
Bare Soil Loose 1.0
Freshly disked or rough irregular surface 0.9
Compact smooth by equipment up and down hill
1.3
Compact smooth by equipment across slope 1.2
Construction Site P Practice Factors
Wind Erosion
Factors Affecting Wind Erosion
Field conditions conducive to wind erosion are:
•Loose, dry, and finely granulated soil
•Smooth soil surface with little or no vegetation present
•Sufficiently large area susceptible to erosion
•Sufficient wind velocity to move soil. Winds are considered erosive when they reach 13 miles per hour at one (1) foot above the ground or about 18 miles per hour at 30 feet (threshold velocity).
(Photo and text from NRCS)
As a rule of thumb, wind speeds greater than 8 m/s (17 mph) at 2 meters (6feet) height are generally required to initiate movement of mineral soils.
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