What is Erosion and Sedimentation?
Bill Laflamme CPESC, Office of the Commissioner
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION
Protecting Maine’s Air, Land and Water
EROSION AND SEDIMENTATION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Erosion = Movement of soil by action of water or wind.
• Erosion is natural• Accelerated Erosion is not• Sedimentation = “settling out” of soil
particle from the water
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
WHY DO SOILS ERODE?
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
SOILS ERODE based on a combination of factors including:
•The size, shape and cohesion factor of the soil
•The intensity, duration and frequency of rainfall (or wind events)
•The size of the watershed, slope and cover of the area where the soil is located
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
EROSION FACTOR
• Size• Shape• Cohesion
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
Relative Size of Soil Particles
SURFACE AREA OF ONE GRAM OF
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
Sand
16 in2
Silt
10 ft2
Clay
¼ acre
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
MORE SURFACE AREAEQUALS
MORE PHOSPHOROUS
SHAPE
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Sand – Rounded, blocky• Silt – Same as sand, just
smaller• Clay – Plate-like
Why Become Certified ?
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Drains well, • Compacts well but will not stay
together if exposed• No Cohesion (stickiness)
SAND
Why Become Certified ?
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
SILT
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Water moves through
slowly • Compacts at the right
moisture content • Little cohesion -
– Sloppy when wet, – Talcum powder when dry
Why Become Certified ?
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
CLAY
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Moisture content constant,• Difficult to handle and compact if
wet (Rock hard when dry)• High Cohesion - Sticky Feel –
(plates stick together like wet plates of glass)
Why Become Certified ?
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
Soil Texture
Percent Sand
Percent Silt
Percent Clay
Sand 85 – 100 0 – 15 0 -10
Sandy Loam 45 – 85 0 – 50 0 – 20
Loam 23 – 53 27 – 50 7 – 28
Silt Loam 0 – 50 50 -85 0 – 28
Silty Clay 0 – 20 40 -72 28 - 40
Find Out More
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
RAIN
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Intensity– Raindrops, How big and fast
• Duration– How long
• Frequency– How much volume, how often
– 10 year storm, 100 year storm
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
RUNOFF
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
Runoff begins when rain has no place else to go
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
RUNOFF
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Watershed• Slope of ground• Cover (vegetation)• Soils (infiltration)
SLOPE
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
The steeper the faster
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
COVER
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Stop the Raindrop• Store the Raindrop• Canopy (leaves)• Surface Roughness
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
SOILS
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Infiltration Rate• Soil Texture• Surface Sealing• Roughness
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
EROSION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Rain drop• Sheet • Rill• Gully
RAIN DROP EROSION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
RAIN DROP EROSION• The harder the rain and the finer the soil
texture, the more raindrop erosion will occur.
• Consider that a large raindrop will fall at a rate of 30 ft/sec and may be up to 250 time larger than a silt particle.
• Sand on the other-hand may be the same or only half the size of that raindrop
• Soil Sealing - Pounding the Surface
SHEET EROSION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Sheet flow over bare soil picks up soil• Plus soil that was splashed up by the raindrop
is now in suspension and will move with the sheet of water.
• Sheet flow can be up to ½ inch deep• 1/8th of and inch of soil loss per acre will fill a
10 wheel dump truck (15 cu.yds. or 25 tons). • How fast and far this “sheet” of water flows
depends on how deep the water is, the texture of the soil, and the steepness and smoothness of the land.
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
RILL EROSION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• When sheet flow begins to concentrate depth, increases power and begins to cut.
• Rills are technically no more than 1.0 inches deep and will be spread out across a slope.
• Rills pull together and are short lived
GULLY EROSION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Rills coming together on a slope• Flow volume increases – Increase
Depth• Velocity = Slope (Critical)• Power increases to a point that the
bottom cuts down
GULLY EROSION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• They will continue to erode from the bottom up, or head cut.
• The side slopes are usually vertical and will then begin to collapse under their own weight.
• On Construction Sites they usually form in bottom of channels and move upstream
• A gully can be as small as 2 inches by 2 inches to as big as the Grand Canyon
EROSION CONTROL
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
SEDIMENTATION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Sediment is soil suspended in water
• Sedimentation is the settling out of sediment
• Decrease Velocity = Sedimentation– Decrease Velocity – Ponding
• Usually occurs by grade change
SEDIMENTATION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Sands? – Always
• Silts ?– Sometimes
• Clays ?– Almost Never
SEDIMENTATION
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Sheet Flow and Shallow Concentrated– Flatten grade – Roughen surface - filter strip– Barriers – Silt Fence
• Channel Flow – Must Stop velocity – Ponding– Can put back in sheet flow – filter
strips, buffers
Courtesy – Peter G. KnightCourtesy – Peter G. Knight
SEDIMENT CONTROL
MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION www.maine.gov/dep
• Last Line of Defense• Decrease Power = Sedimentation
– Decrease Velocity – Ponding• Silt Fence, Sediment ponds
– Decrease Velocity (and Depth) – Filtering Go from Concentrated Flow to Sheet Flow
• Level Lip Spreaders, Filter Strips, Buffers
www.maine.gov/dep