SOILS10
Erosion Chp 7
Erosion is a natural process that takes place concurrently with soil formation.
Soils accumulate when the rate of erosion is less than the rate of soil formation.
Erosion is a three step process: detachment, transport and deposition.
The source of energy for erosion is either wind, or rain (and subsequent runoff).
Also glaciers but…
From a land use standpoint agriculture produces the largest % of total sediment
load but construction causes the most concentrated loads of erosion. Land
under construction has an increase of erosion between 2,000 and 40,000 X the
pre-construction rate. These rate are typically 10 to 20 X the rates from
agricultural fields of the same size and can be as high as 100X…
A study in Maryland on a construction site found sediment yields of 220
tons /acre compared to 1.7 tons per acre in a nearby stabilized urban area.
Impacts of Erosion
Environmental Eroded soil contains nitrogen, phosphirus, etc. which when
carried into surface waters can cause blooms
 Erosion of streambanks destroy aquatic and riparian habitat
 Deposition of sediment on bottoms of streams blankets benthic
life, essentially paves the bottom of the stream, destroys
spawning sites
 Increases turbidity which reduces photosynthesis (reduced food
supply)
 Suspended sediment abraids and coats aquatic organisms
 Erosion removes the smaller less dense constituents of topsoil:
clay and silt, organic matter which hold nutrients and water
Economic May be hard to quantify. What is value of aquatic habitat?,
diminished water clarity?
 Excessive sedimentation requires dredging, reduces reservoir
storage capacity, impacts shipping channels (dredging costs
from $2.50 to $16.00 per cubic yard)
 Cost of replacement of reservoirs
 Reduced stream capacity may have flooding implications
 Reduced tilth or agricultural capacity of farmland
TYPES OF EROSION
Splash Erosion – related to drop size, splashed particles move downhill..
Sheet erosion – shallow sheets of water move across the surface, not
usually the agent of detachment but an effective mechanism of transport of
particles removed by splash.
Rill Erosion – water rarely moves in sheets for long before it begins to
concentrate in irregularities. Flow deepens as it concentrates and velocity
increases. Rills are small, only a few inches deep but are well defined.
Gully Erosion – gully formation is complex; some occur as rills deepen and
grow wider but gullies enlarge uphill and downhill. Water flowing over the
“headwall” undercuts the headwall and advances the gully uphill…large chunks
can fall away in gully formation , a process called mass wasting. Once a gully
has formed it is very difficult to repair and to stop it from growing.
Channel Erosion – associated with stream channels where bank
vegetation is disturbed…it occurs when stream volume or velocity increases.
Natural streams adjust to normal range of flows, but when normal range changes
channel erosion can occur…
EROSION FACTORS
Climate, Soil Characteristics, Topography, Ground cover.
Climate – effects erosion directly and indirectly. Directly in the form of rain.
Erosive power of rain is determined by rainfall intensity and droplet size. (Use
Chart from Erosion and Sediment Control Handbook, pg 1.8)
Soil Characteristics – texture, organic matter content, structure and
permeability
Topography – slope length and slope steepness. Effect erosion: energy
(erosive energy) of flowing water is the square of the velocity. Long continuous
slopes allow the water to build up energy. As velocity increases the water tends
to form channels… therefore the base of a slope is more prone to erosion than
the top because water is more concentrated and has more momentum when it
reaches the bottom…concave slopes erode more quickly than convex slopes.
Aspect or orientation of slope: south facing slopes are hotter and drier (in
northern hemisphere) more difficult to establish plants, but northern slopes tend
to be cooler and moist but receive less sun…
Ground Cover - principally vegetation…most effective form or erosion
control…shields surface from falling rain, slows rate of runoff, holds soil in place,
maintains soil’s ability to absorb water
Universal Soil Loss Equation –
Estimating Soil loss has 3 applications:
1. To identify erosion prone areas
2. To compare effectiveness of management techniques
3. To estimate volume of sediment storage needed in sediment basins
USLE – A = R x K x LS x C x P where
A=
R=
K=
LS=
C=
P=
soil loss in tons/ (acre)(year)
rainfall erosion index in 100ft tons/acre x in/hr
soil erodability, tons/acre per unit of R
slope length and steepness factor (dimensionless)
vegetative cover factor (dimensionless)
erosion control practice factor (dimensionless)
R= Rainfall Factor
Measure of the erosive force of a specific rainfall – related to the quantity
and intensity of rain. Rainfall factor is the product of the total kinetic energy of a
storm times the 30 minute intensity of fall. Note the R-factors on page 102 of
book.
K= Soil erodability factor
Affected by infiltration rate, permeabiltiy, water retention capacity and
ability of soil aggregate to resist breaking. Factors have been determined
experimentally on sort of a worst case agricultural soil (9% slope!), with bad
practice )tillage up and down!)
LS = Slope length and grade
Remember the erosive energy of flowing water is equal to the square of
the velocity, so that as grade increases the velocity of runoff increases. Increase
the velocity two x and the erosive power increases four x!
C= Management Factor
Role of vegetation in interception, retention of rainfall. Different crops
have different values
P= Practice Factor
Effect of conservation practices
Soil Loss Tolerance Factor (T)
No consensus among scientists – described in two ways –
1.
the maximum soil erosion loss that is offset by the theoretical
maximum rate of soil development, an equilibrium between loss
and development
2.
maximum average annual soil loss that will allow continuous
cropping and maintain soil productivity without requiring additional
management inputs (using fertilizer to overcome reduced
productivity)
Wind Erosion EROSION CONTROL PRACTICES
Ten aspects of erosion controls
1.
fit development to the terrain
2.
time grading and construction to minimize exposure
3.
retain existing vegetation whenever feasible
4.
vegetate and mulch bare areas
5.
divert runoff from exposed areas
6.
minimize slope length and grade
7.
keep runoff velocities low
8.
design/build channels and outfalls to handle concentrated flow
9.
trap sediment onsite
10.
inspect and maintain controls