Soil erosion by water

Two major types of erosion
 Geological erosion
 Accelerated erosion
Geological erosion: includes soil-forming as well as soil eroding processes
which maintain the soil in a favorable balance.
Accelerated erosion: includes the deterioration and loss of soil as a result of
man’s activities. Although, soil removal are recognized in both cases,
only accelerated erosion is considered in conservation activities.
The forces involved in accelerated erosion are:
1. Attacking forces which remove and transport the soil particles and
2. Resisting forces which retard erosion.

Water erosion is the removal of soil from the lands surface by running water
including runoff from melted snow and ice. Water erosion is sub-divided into
raindrop, sheet, rill, gully and stream channel erosion.
Major Factors Affecting Erosion by Water
1. Climate, 2. Soil, 3. Vegetation and 4. Topography
Climate: - Precipitation, temperature, wind, humidity and solar radiation
Temperature and wind: - evident through their effect on evaporation and
transpiration. However, wind also changes raindrop velocities and angle of
impact. Humidity and solar radiation are less directly involved since they are
associated with temperature.
Soil: Physical properties of soil affects the infiltration capacity of the soil. The
extend to which it can be dispersed and transported. These properties which
influence soil include:

- Soil structure
- Texture
- Organic matter
- Moisture content
- Density or compactness
- Chemical and biological characteristics

Major effect of vegetation in reducing erosion are:
 Interception of rainfall
 Retardation of erosion by decrease of surface velocity
 Physical restraint of soil movement
 Improvement of aggregation and porosity of the soil by roots
and plants residue
 Increase biological activities
 Transpiration – decrease soil moisture resulting in increased
storage capacity.
These vegetative influences vary with the season, crops, degree of
maturity, soil & climate as well as with kind of vegetative
materials namely: roots, plant tops, plant residue

Features that influence erosion are:
 degree of slope
 Length of slope
 Size and shape of the watershed
 Straight
 Complex
 Concave
 Convex.

• Raindrop erosion is soil detachment and transport resulting
from the impact of water drops directly on soil particles or on
thin water surfaces.
• Tremendous quantities of soil are splashed into the air, most
particles more than once.
• Factors affecting the direction and distance of soil splash are
• slope, wind, surface condition, and
• impediments to splash such as vegetative cover and mulches.

Sheet erosion is considered to be a uniform removal of soil in thin
layers from sloping land, resulting from sheet or overland flow.
This type of erosion rarely occurs because minute channels (rills)
form almost simultaneously with the first detachment and
movement of soil particles.
Splash and sheet erosion are sometimes combined and called
interrill erosion.

• Removal of soil by water from small but well defined channels
or streamlets where there is a concentration of overland flow.
• Obviously, rill erosion occurs when these channels have become
sufficiently large and stable to be readily seen.
• Rill erosion is the detachment and transport of soil by a
concentrated flow of water.
• Rills are eroded channels that are small enough to be removed
by normal tillage operations.
• Rill erosion is the predominant form of surface erosion under
most conditions.

• Rill erosion is a function of
• the flow rate or hydraulic shear τ of the water flowing in the rill,
• the soil’s rill erodibility Kr, and
• critical shear τ, the shear below which soil detachment is negligible
• The relationship between rill erosion and the hydraulic shear of
water in the rill is

where
Dr = rill detachment rate (kg m-2 s-1),
Kr = rill erodibility, due to shear (s/m) (Table 7.1),
τ = hydraulic shear of flowing water (Pa) (Equation 7.3),
τc = critical shear below which no rill erosion occurs (Pa)
(Table 7.1),
qs = rate of sediment flow in the rill (kg m-1 s-1),
Tc = sediment transport capacity of rill (kg m-1 s-1).

• The hydraulic shear τ is defined as
τ = γ R S
where
γ = specific weight of water (N m-3), about 9810 N m-3,
R = hydraulic radius of the rill (m),
S = hydraulic gradient of rill flow (m/m).
The sediment transport capacity can be estimated from the
relationship
Tc =Bτ1.5
where
Tc = transport capacity per unit width (kg m-1sec-1) and
B = a transport coefficient based on soil and water properties generally
between 0.01 and 0.1

Determine the rill erodibility (Kr) from the following
observations:
observed runoff rate = 1.0 L s-1
sediment concentration in runoff = 0.12 kg L-1
rill length = 20 m
width = 0.15 m
gradient (S) = 0.07
hydraulic radius (R) = 0.01 m
soil transport coefficient (B) = 0.1 kg Pa-1.5
soil critical shear (τc) = 2 Pa

sediment delivery = runoff × concentration = 1.0 × 0.12 = 0.12
kg/s
qs = sediment delivery/rill width = 0.12/0.15 =0.8 kg m-1 s-1
Dr = sediment delivery/(rill length) = 0.8/20 = 0.04 kg m-2 s-1
τ = 9810 × 0.01 × 0.07 =6.87 Pa
Tc = B τ 1.5 = 0.1 (6.87)1.5 = 1.80 kg m-1 s-1

• Gully erosion produces channels larger then
rills.
• These Channels carry water during and
immediately after rain.
• Gullies are distinguished from rills in that gullies
cannot be obliterated by tillage.

The rate of gully erosion depends primarily
on the runoff producing characteristics of the
watershed
the drainage area
soil characteristics
the alignment
size and shape of gully
the slope in the channel.

1. Water fall erosion at the gully head.
2. Channel erosion caused by water flowing through the gully or
by raindrop splash on unprotected soil.
3. Alternate freezing and thawing of exposed soil banks.
4. Slides or mass movement of soil in the gully.

Stage1 Channel erosion by down ward scour of the topsoil. This
stage normally proceeds slowly where the topsoil is fairly
resistant to erosion
Stage2: upstream movement of the gully head and enlargement
of the gully in width and depth. The gully cuts to the horizon
and the weak parent material is rapidly removed.
stage3: Healing stage with vegetation to grow in the channel.
Stage 4: Stabilization of the gully. The channel reaches a stable
gradient, gully walls reach and stable slope and vegetation
begins to grow in sufficient abundance to anchor the soil
and permit development of new topsoil

• Stream channel erosion and gully erosion are distinguished
primarily in that
• stream channel erosion applies to the lower end of headwater
tributaries and to streams that have nearly continuous flow and
relatively flat gradients,
• whereas gully erosion generally occurs in intermittent or ephemeral
streams or channels near the upper ends of headwater tributaries.
• Stream channel erosion includes soil removal from stream banks
and soil scour of the channel bed.
• Bank erosion can also lead to stream meandering and
rechannelization, resulting in major erosion and deposition
within the floodplain.

Sediments in streams is transported by :
1. Suspension
2. Siltation
3. Bad load movement.
Suspension: suspended sediment is that which remains in
suspension in flowing water for a considerable period of
time without contact with the stream bed.
saltation: sediment movement by saltation occurs where the
particle skip or bounce along the stream bed. In comparison
to total sediment transported ,saltation is considered
relatively unimportant.

Bed load: Bed load is sediment that moves in almost continous
contact with the stream bed being rolled or pushed along the
bottom by the force of the water.
Mavis (1935),developed an equation for unigranular
materials ranging in diameter from 0.35 to 0.57 millimeters and
specifically from 1.83 to 2.64.

• Smith and wisehmeier (1957,1962) developed an equation for
estimating the average annual soil loss.
A=RKLSCP
• Am=2.24RKLSCP (metric unit)
• where
• A = average annual soil loss (Mg/ha),
• R = rainfall and runoff erosivity index for the geographic
location (Figure 7.3),
• K = soil erodibility factor (Equation 7.6, Table 7.1),
• L = slope length factor (Equation 7.7),
• S = slope steepness factor (Equation 7.8),
• C = cover management factor (Table 7.2),
• P = conservation practice factor (estimate with RUSLE).

The topographic factors, L and S, adjust the
predicted erosion rates to give greater erosion
rates on longer and/or steeper slopes, when
compared to the USLE “standard” slope
steepness of 9% and length of 22 m.

The L-factor can be calculated from the equation
where L = slope length factor,
l = slope length (m),
b = dimensionless exponent.
For conditions where rill and interrill erosion are about equal on a
9%, 22-m long slope, then b is:
Where θ = field slope angle = tan-1 (s) and s = slope steepness(m/m).

The S-factor depends on the length and steepness category of the
slope.
For slopes less than 4 m long,
For slopes greater than 4 m long and steepness less than 9%,
For slopes greater than 4 m long and steepness greater than or
equal to 9%,

Contouring is the practice of performing field operations, such
as plowing, planting, cultivating, and harvesting, parallel to
elevation contours.
It reduces surface runoff by impounding water in small
depressions, and decreases the development of rills.
The relative effectiveness of contouring for controlling erosion
can range from preventing all erosion to increasing hillside
erosion by concentrating runoff that may initiate gullying.
Contouring is more likely to fail in climates with high intensity
spring and summer storms, or on sites with steeper slopes (over
about 4%)
MM HASAN,LECTURER,AIE,HSTU

Strip cropping is the practice of growing
alternate strips of different crops in the same
field.
For controlling water erosion, the strips are on
the contour, but in dry regions, strips are placed
normal to the prevailing wind direction for wind
erosion control.

Figure 7.5–Contour strip-cropping in northeast Iowa. (Photograph by Tim McCabe, USDA-NRCS.)

Figure 7.6–Three types of strip cropping: (a) contour, (b) field, and (c) buffer

Tillage management is an important conservation tool.
Tillage should provide an adequate soil and water environment
for the plant.
Its role as a means of weed control has diminished with
increased use of herbicides and improved timing of operations.
The effect of tillage on erosion depends on such factors as
surface residue, aggregation, surface sealing, infiltration, and
resistance to wind and water movement.
Excessive tillage destroys structure, increasing the susceptibility
of the soil to erosion.

Soil erosion by water

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Soil erosion by water