1. Factors that Influence Erosion
Soil Properties
Precipitation
Vegetation Cover
Topography
Cert 3
Replace slide 1, title slide

2. Soil Properties
Texture
Structure
Organic Content
Depth of Solum

3. 50% Sand
31% Clay
19% Silt
Textural Triangle

4. Hjulstrom Diagram

5. 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.

6. Soil Structure

7. On a construction site soil structure is often destroyed by compaction, either intentionally…
or as an consequence of other construction activities

8. Or removed completely during mass grading

9. Sources of Information on Site Specific Soil Properties

10. SCS (now NRCS) Soil Surveys
Know what you will be dealing with, Most surveys are now available online

11. NRCS Web Soil Survey

12. Example Web Soil Survey Map

13. Geotechnical Reports
Many projects have extensive information compiled in previous reports, Get all the information you can.

14. Geologic Maps

15. Examples of Erosion Issues for Some Common Local Soil Series

16. 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.
A Group C soils lack of infiltration is usually associated with an impervious layer, such as the hardpan shown in this slide.

17. Indanola Soils
Developed on developed on sandy glacial advance outwash. These soils are readily eroded by flowing water.
In GEO Reports know what “TYPE” Means, A – B- C- D
Type A Is great until it blows, or washes away, this soil settles quickly

18. Kitsap Soils
Type D is Tough to work with, can be expansive
Type B & C are in the middle, B is good for farming, C is usually hard, rocky, low infiltration.
Kitsap soils are developed on fine-grained proglacial lake beds. They have poor drainage, are difficult to work when wet, create highly turbid runoff when disturbed and are often assosiated with areas of unstable slopes

19. A good CESCL knows their dirt!

20. Does the rain affect anyone here?
Precipitation

21. Rainfall factors that influence erosion
Rainfall effects are driven by Frequency, Intensity, & Duration.
The surface area will determine the runoff amount.

22. Climate
This slide shows the rainfall patterns across the State of Washington.
There aren’t many other places that have as much variations in rainfall as Washington. This is a good map to start with to determine how much rainfall your construction area can expect.

24. Rainfall Data on the Internet
This slide shows all of the weather stations across the State of Washington. You can use the internet now to obtain rainfall data.
SWPPP Source Material

26. 45% probability of ¼ inch in June

27. Vegetation

28. How do Plants stabilize the Soil?
Increase Surface Roughness
Absorb Rainsplash Energy
Provide Soil Reinforcement
Promote Good Soil Structure
Reduce Runoff Volume
Trap Sediment

29. Interception and energy absorption
Erosion can be reduced by:
Limiting removal of existing vegetation
Decreasing duration of soil exposure to rain
Phasing clearing and grubbing
Preservation of existing vegetation on erodible soils, steep slopes, drainage ways, and stream banks.
Photo by USDA - NRCS

30. Soil Improvement
Trees
Shrubs
Moss
Duff
Roots

31. So, What’s the first thing we do at a construction site??
The first part of construction. Has the soil been affected by compaction already?
Photo courtesy Clark County Watershed Stewards

32. Topography Elements of Topography: Slope Steepness Slope Length
Slope curvature
Aspect

33. Slope Steepness
The ability of flowing water to carry sediment increases (roughly) with the cube of the slope angle!

34. Slope Length
With 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.

35. Slope curvature

36. Aspect
North

37. 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
Good Tool to demonstrate the effects of cover materials and land use practices.
Results described as tons of soil lost per acre per year.
STORY SLIDE: Dewatering pumps below geotextile and rock
This is construction of a giant detention system being used for construction stormwater treatment during construction.
STORY SLIDE: Pump discharge, rock and fabric layers, filtration and discharge of groundwater and stormwater.
A= Soil Loss in
Tons / Acre / Year

38. Cover-Management (C)
Definition: this factor represents vegetative, management and erosion-control practice effects that primarily affect the process of detachment on soil loss
Similar to the other factors, the calculated C factor is the ratio of soil loss comparing the defined, existing surface conditions to that of a
unit plot
Has the greatest possible range of all factors 38

40. Construction Site P Practice 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

41. Wind Erosion

42. 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)

43. 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.