Factors that Influence Erosion Soil Properties Precipitation Vegetation Cover Topography Cert 3 Replace slide 1, title slide
Soil Properties Texture Structure Organic Content Depth of Solum
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 Surveys Know what you will be dealing with, Most surveys are now available online
NRCS Web Soil Survey
Example Web Soil Survey Map
Geotechnical Reports Many projects have extensive information compiled in previous reports, Get all the information you can.
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. A Group C soils lack of infiltration is usually associated with an impervious layer, such as the hardpan shown in this slide.
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
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
A good CESCL knows their dirt!
Does the rain affect anyone here? Precipitation
Rainfall factors that influence erosion Rainfall effects are driven by Frequency, Intensity, & Duration. The surface area will determine the runoff amount.
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.
www.wrcc.dri.edu
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. http://www.wrcc.dri.edu/summary/Climsmwa.html SWPPP Source Material
45% probability of ¼ inch in June
Vegetation
How do Plants stabilize the Soil? Increase Surface Roughness Absorb Rainsplash Energy Provide Soil Reinforcement Promote Good Soil Structure Reduce Runoff Volume Trap Sediment
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
Soil Improvement Trees Shrubs Moss Duff Roots
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
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 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.
Slope curvature
Aspect North
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
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
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
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.