soil erosion by water

Soil Erosion by Water

MM HASAN,LECTURER,AIE,HSTU

Types of Erosion

Two major types of erosion Geological erosion Accelerated erosionGeological 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 and2. Resisting forces which retard erosion.

Soil erosion by water

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 radiationTemperature 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

Vegetation

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

TopographyFeatures that influence erosion are: degree of slope Length of slope Size and shape of the watershed

Straight Complex Concave Convex.

Raindrop Erosion• 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

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.

Interrill Erosion

Splash and sheet erosion are sometimes combined and called interrill erosion.

Rill 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• 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).

Rill erosion• The hydraulic shear τ is defined as

τ = γ R Swhere

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-1

sec-1) and B = a transport coefficient based on soil and water properties generally between 0.01 and 0.1

Example 7.1Determine 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) = 0.07hydraulic radius (R) = 0.01 msoil transport coefficient (B) = 0.1 kg Pa-1.5

soil critical shear (τc) = 2 Pa

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

Gully erosion

• 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.

Principles of gully erosionThe rate of gully erosion depends primarily

on the runoff producing characteristics of the watershedthe drainage areasoil characteristics the alignment size and shape of gully the slope in the channel.

Gully development processes

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.

Four stages of gully development

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• 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.

Sediment movement in channels

Sediments in streams is transported by :

1. Suspension2. Siltation3. 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.

Sediment movement in channels

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.

Universal soil loss equation• 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).

Universal soil loss equationThe 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.

Universal soil loss equationThe 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%,

Erosion Control PracticesContouring 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%)


Strip CroppingStrip 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 PracticesTillage 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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