Soils and The Erosion Process:Understanding SoilsErosion Control vs. Sediment Control)

Understanding Soil Characteristics

Soil Surveys

• Published by USDA and ISU Extension

• Available in published reports or on USDA website as Web Soil Survey

• See SoilWeb for iphone app

• 94% accurate

• On-site soil evaluations are necessary for rain gardens

Web Soil Survey

Soil

• The natural medium for the growth of plants on the surface of the earth, composed of organic and mineral materials.

• Soils and Men, 1938

Within a soil profile there can be a wide variety of:

• parent materials

• soil colors

• textures

• structures

• minerals

• pH levels

• organic matter content

• and thickness of horizons

http://prescriptionsoilanalysis.com/

Soil Composition

• Soil particles - mineral matter (sand, silt, clay) clumped together by organic matter.

• Pore space - gaps between soil particles

• Water and air occupy the pore space, as well as roots and organisms.

Soil Composition

• Composition of urban soils is often altered due to land disturbing activities (grading)

• Pore space is often reduced due to compaction (less water can infiltrate)

• Not uncommon to find <2% OM in modern soils (less water holding capacity, degraded soil structure)

Soil composition and urban soils

Size of Soil Particles

Sand (0.05-2.00 mm)

Silt (0.002-0.05 mm)

Clay (<0.002 mm)

What is going to be

the most difficult

particle size to settle

out of a sediment

basin?

Soil texture is a measure of:• % Sand sized particles

• % Silt sized particles

• % Clay sized particles

Soil Texture

• Loam - an even mixture of sand, silt, clay

• Sandy loam - more sand

• Silt loam - more silt

• Clay loam – more clay

• Rarely will you find pure sand, pure silt, pure clay

LoamsMost Iowa soils are a blend of sand, silt and clay – or loams

• The physical features of the landscape

• The lay of the land – the contour

Topography

• Position on the landscape influences the development of soil.

• Topography is the primary influence on drainage, surface runoff, erosion, and infiltration rates.

Soil Quality: Healthy vs. Unhealthy

Healthy Soil:•Plenty of worm and root channels for infiltration•Microorganism activity improves nutrient availability to plants•High organic matter content; high water holding•Good soil structure•No compaction

Unhealthy Soil:•Minimal worm and root channels•Very little microorganism activity•Low organic matter content •Poor soil structure•High bulk density (compaction)

Soils for Salmon, King County Dept. of Nat. Res. Soils for Salmon, King County Dept. of Nat. Res.

• The sequence of horizons or layers from the surface downward make up what is called a soil profile.

The Soil Profile

• O-Organic, dark leafy material, thin if present

• A-Topsoil, darker than subsoil, loose crumbly, organic matter, provides nutrients

• B-Subsoil lighter colored, denser, accumulation of minerals and clay, most structure, holds moisture

• C-Transition soil and/or parent material, typically densest and can be restrictive

Soil HorizonsFive soil forming factors create soil layers or horizons

A horizon

Bt or Bw

horizon

C horizon

• Stripped off A- horizon, darker soils

• Left with B-horizon, high clay content, lighter soils, compacted

Horizons and urban soils

7.5YR page

Soil Color

Munsell Soil Color Book

Soil Structure

•The shape that soil takes based on its

chemical and physical properties

•Impacts water movement

•Can have a more clayey soil that you think

might not perc. but healthy soil structure

provides good perc. rates

Soil Structure

http://soil.gsfc.nasa.gov/pvg/prop1.htm

Single-Grained: usually sands

Granular: resembles crumbs,

usually surface soils where roots

grow

Platy: thin flat plates of soil, usually

found in compacted soil-natural or

not

Angular: Irregular blocks, usually

found below surface soils, higher

clay content

Soil Structure

Granular soils (typical A horizon or topsoil)

Subangular blocky (typical B horizon or subsoil)

Single Grain (sand)

Platy - looks like stack of paper plates (timber or wet soils)

Angular blocky (subsoils high in clay)

Prismatic - strong structure, mainly deeper than 48 inches

Compacted – Structureless soil. Difficult to probe under

moist conditions

Soil Structure Kit

Soil Consistency

• Friable

• Loam, silt loam, silty clay loam

• Loose

• Sandy loam, loamy sand, sand

• Firm

• Clay loam

• Very Firm

• Clay loam, silty clay, clay

Soil Structure, Consistency

• Good granular soil structure yields friable soils, topsoil

• Soils with loose consistency will typically be sandy

• Soils that are sticky and firm lack these characteristics and are not topsoil, heavy clay content

• Inspect before adding sod or seed

• Color: Darker A-horizon, higher organic matter content

• Texture: Lower clay content, not sticky, won’t ribbon out long length, <30% clay

• Structure: Granular

• Consistency: Loose, friable, no clods

• Free of rocks and debris

• Minimum 3% Organic Matter

• pH 6-8

So What is Topsoil?What should you spread?

Compaction Reduction

How to? Compaction Reduction

• Till or rip subsurface soils before adding topsoil

• Don’t till wet soils

• Add topsoil, don’t compact topsoil—intentionally drive over it with heavy equipment many times

Why the Topsoil andSoil Quality Restoration?

• Less runoff, more infiltration and percolation

• Less impact on streams

• More sustainable lawns

Class Exercise

• Soil texture

• Soil profile

Construction and Stormwater Runoff• After vegetation is removed, the soil is left unprotected

and is susceptible to erosion.

• Erosion from construction activities contributes sediment and pollutants to stormwater runoff.

• Construction materials such as fuel, concrete wash water, hydraulic fluids, oil and grease can be washed into streams by stormwater.

• When BMPs are properly installed, sediment and pollution generated by construction sites are greatly reduced.

Construction and Stormwater Runoff• Highly disturbed soils have also lost much of the organic matter that

glues them together.

• Heavy equipment compacts the soil, reducing the infiltration of rainfall and increasing runoff.

Source: M. Mamo, Department of Agronomy and Horticulture at University of Nebraska-Lincoln

Construction and Soil Erosion•

Construction site erosion is often 4 to 5 times higher than agricultural land erosion…10 times higher is not uncommon. Much more sediment is delivered to receiving waters.

It is a common misperception that stormwater is treated as at a wastewater treatment plant. Most stormwater is discharged directly to streams and lakes. This can have a negative impact on aquatic life and stream and lake water quality.

Erosion and Sedimentation

Detachment

Transportation

Deposition

Types of Soil Erosion

• Raindrop Erosion dislodges soil particles and splashes them into the air.

• The detaches particles are washed over the land in shallow surface flow.

Source: USDA Natural Resources Conservation Service

Types of Soil Erosion• Sheet Erosion transports the soil particles in

shallow flow of water as it runs over the land.

• Shallow overland flow rarely moves uniformly over the land for more than a few feet before concentrating in the contours of the land

Source : Dept. of Civil Engineering, Ryerson University

Tip:

• Soil stabilization practices control erosion from raindrop splash and sheet erosion.

• By protecting the surface, soil particles are not dislodged and transported.

• Temporary and permanent vegetation

• Sodding

• Mulching

• Compost blankets

• Rolled erosion control products

Types of Soil Erosion• Rill Erosion is the erosion that occurs as shallow

surface flow begins to concentrate in low spots in the surface of the land.

• Flow velocity and turbulence increases

• The energy of the concentrated flow detaches more soil particles and causes small, well-defined channels to form

Source: USDA Natural Resources Conservation Service

Types of Soil Erosion

• Soil erosion can occur in a vegetated area, which was previously seeded and mulched.

Source: University of Minnesota

Tip:

• Repair rill erosion prior to seeding!

• When there are high backslopes with large drainage areas, installing an intercept ditch at the top of a slope in conjunction with seeding and mulching will control stormwater runoff and promote the establishment of vegetation.

Types of Soil Erosion• Gully Erosion occurs as the flow in rills come

together in larger and larger channels.

• Gully erosion usually requires heavy equipment and special techniques for stabilization.

Source: USDA Natural Resources Conservation Service

TIP:

• Try to prevent this by addressing and repairing rills!

Types of Soil Erosion• Streambank Erosion is the scouring away of the stream

bank and bed, as a result of the increased volume and velocity of stormwater flow.

• Increased runoff, removal of natural vegetation and channel alteration are the three main causes of streambankerosion.

Source: USDA Natural Resources Conservation Service

Types of Soil Erosion• Wind Erosion is similar to sheet erosion in that

detachment, transportation and deposition of soil particles occur.• The three types of wind erosion are: saltation, suspension and

surface creep.

• The vast majority of wind erosion is a result of the saltationprocess: the movement of fine silts up to medium size sand particles with the wind.

Source: USDA Natural Resources Conservation Service

Benefits of Reducing Erosion and Sedimentation

• Being proactive, aware of site conditions and properly installing erosion and sediment control measures saves time and money

• Reduces the amount of sediment removal

• Re-grading

• Potential fines

• The need to reinstall practices

Best Management Practices• Erosion Control: to keep soil in place

• Sediment Control: to collect or allow soil to settle, preventing it from traveling offsite

• Velocity Control: reduce flows

• Pollution Prevention: good housekeeping

• Post Construction Stormwater Mgmt: permanent controls for water quality and flood minimization

Erosion Control

Preventing detachment

Reducing transportation of detached particles

Minimizing the deposition of sediment at a down-gradient or off-site location

Sediment Control

Trapping sediment that is moving in the erosion process and retaining it on-site

Temporarily impounding sediment laden runoff

Slowing flows and filtering with vegetative buffers

Sediment

Control Erosion

Control

Erosion

Control

Sediment

Control

Erosion Control Practices

Sediment Control Practices

Velocity ControlPractices

Pollution PreventionPractices

Maintain existing vegetation

Compost filter berms Check Dam Porta-Potty locations

Vegetative filter strips Filter socks Diversion structure Construction debris containment and handling

Compost Blankets Wattles Level spreader Concrete washout areas

Dust control Flocculants Rock chutes and flumes Chemical storage and handling areas

Grassed channel Floating silt curtain Rock oultlet protection

Mulching Inlet protection Flow transition mat

Temporary , permanentseeding

Sediment basins and traps

Temporary slope drain

Sodding Silt fencing and similarpractices

Surface roughening, grading strategies

Stabilized exits

Turf reinforcement mats Storm drain inlet protection

Rolled erosion control products

Prioritization of Erosion and Sediment Control Practices

Practice Cost Effectiveness

Maintaining existing vegetation $

Protecting disturbed areas through mulching and re-vegetation $$

Installing diversions around disturbed areas $$$

Sediment removal throughdetention of all site drainage $$$$

Other structural controls to treat sediment-laden flow $$$$$

Design and Specifications

Find Design Standards and Specifications for Iowa Erosion, Sediment, and Velocity Controls at:

www.iowasudas.org

SUDAS Specifications

Vegetation and Soil Stabilizationare the Best Erosion Control

Retain on-site

vegetation

• Temporary

• Permanent

Maintaining On-Site Vegetation

• Maintaining as much existing vegetation as possible during construction, including grass, shrubs, trees, and other ground cover can reduce the need for new vegetation.

• Phasing construction disturbs smaller areas for shorter periods, preserves vegetation, and reduces site exposure to soil erosion.

• Phasing also reduces the need for and difficulties in stabilizing a large area of new vegetation at one time.

Minimize Mass Grading

•Compaction reduction

•Better soils

•Less runoff…More infiltration

Created a building envelope protected

a lot of the existing vegetation