ERT 349 SOIL AND WATER

ENGINEERING

SOIL EROSION

PREPARED BY: CIK SAMERA SAMSUDDIN SAH

NO. TEL: 04-9798835/013-7004537

INTRODUCTION

Erosion is natural process that occur daily, on all

land, as the result of wind, and water.

Disturbance of the soil surface, including activities

like construction, farming, or logging, greatly

increases the amount of sediment loss from the site

due to erosion (Price and Karesh, 2000).

SOIL EROSION AND SEDIMENTATION PROCESSES

Soil erosion is the detachment, entrainment, and

transport of soil particles from their place of origin by the

agents of erosion, such as water, wind, and gravity.

It is a form of land degradation and can be categorised

as either geological or accelerated surface soil erosion.

Aa result from human activities that expose the soil

surface and thus enabling erosive agents such as rain

to wash away topsoil.

The amount of silt or sediment delivered into water systems is a function of changes in surface drainage patterns, terrain roughness, vegetation, and climatic conditions.

Water is the most significant agent of soil erosion.

The amount and sizes of soil particles transported as sediment increase as the volume and velocity of runoff increase.

There are two main types of soil erosion agent.Soil erosion by wind

Soil erosion by water

SOIL EROSION BY WIND

Wind erosion occurs on dry surfaces, particularly where loose fine material is abundant.

Although not usually a problem in Malaysia, it may be a nuisance where bare ground on construction sites dries out yielding dust, which is blown onto neighbouring premises.

The majority of dust generated and emitted is related to earth moving demolition, construction traffic on unpaved surfaces, and wind over disturbed uncompacted soil (DID, 2000).

Wind erosion not only remove soil, but also damage crops, fences, buildings, and highways.

The areas most subjected to damage are sandy soils along streams, lakes, and coastal plains and organic soils.

Dust particles can travel far, even crossing oceans.

SOIL EROSION BY WATER

Soil erosion by water is the result of rain detaching and transporting bare soil, either directly by means of rainsplash or indirectly by rill and gully erosion.

It reduces soil productivity and is a primary source of sediment that pollutes streams and fills reservoirs.

TYPES OF SOIL EROSION1. Rainsplash Erosion

The force of falling raindrops can dislodge soil particles, which are then available for entrainment by slope runoff.

Bare soil surfaces in Malaysia are extremely susceptible to rainsplash erosion during high intensity rainstorms.

2. Sheet Erosion occurs when loose or detached

soil is transported downhill in a uniform layer.

occurs rapidly during heavy rain but is readily interrupted by vegetation.

The amount of soil loss depends on the depth and velocity of flow, soil structure, and terrain.

A serious consequence of sheet erosion is the very noticeable subsoil layer that is exposed at the surface after the topsoil is removed.

Vegetation is particularly hard to reestablish in such layers.

3. Rill Erosion Entrainment of soil particles over

an exposed terrain causes rill formations.

Rills are shallow channels usually no more than 30 cm deep but can be metres long.

They may be widespread on compacted, exposed surfaces, which are devoid of vegetation.

Water flows more quickly in a rill because it is concentrated and this increases the detachment and transportation of soil particles.

Vegetation plays an important role in dissipating runoff velocity and encourages deposition on-site.

4. Gully Erosion Gullies are incised channels,

which often began as rills. The headwall of a gully tends to

cut back upslope and the sidewalls retreat through slumping associated with subsurface water altering the stability of the gully sides, or undercutting by surface water flowing over the head or sides of the gully.

Gullies are highly effective conveyors of sediment to rivers and their density and depth are indicators of the severity of erosion.

SOIL EROSION PROCESS

Soil erosion due to water is caused by two distinct yet complementary processes namely;rainfall impact, and sediment transport due to overland flow.

Rainfall Impact

Rain is undoubtedly the largest cause of erosion.

Water is about 800 times heavier than air, half to one third the weight of rock and about equal in weight to loose topsoil.

When it flows, it can move loose substances with ease.

Surprisingly, rain's most damaging moment is when a water drop hits the ground.

The characteristics of raindrops affecting erosion are drop size and velocity.

Raindrop erosion is higher on slopes as the runoff moves further downhill due to gravitational influence and the angle of raindrops impact.

Sediment transport

Sediment transport is the movement of solid particles, typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained.

The particulates will be transported when velocity and the turbulence of the overland flow (runoff) when detached by raindrops

Differences in the size, shape and density of the soil particles will affect the rate of transport and distance transported before deposition.

Some of the significant damaging effects of soil detachment include:Disintegration of surface soil strength thus

decreasing the sustainability of slope stability.Easier for soil particles to be transported by

water.Organic matter, fine material and plant

nutrients necessary for revegetation are subsequently removed by water.

FACTORS INFLUENCING SOIL EROSION

The factors influencing soil erosion failures due to water are:climate, topography, soil characteristics, andvegetation.

1. Climate Climatic variables affecting erosion include

rainfall, wind and temperature with rainfall as the most potent factor.

The strength of wind affects the angle of impact and the velocity of raindrops.

Atmospheric temperature levels and wind affect the rate of evaporation and transpiration at the spoil dump site.

Although rainfall intensity and its amount affects soil loss, it has a higher impact on the rate of soil erosion.

2. Topography

Topographic feature affecting the rate of erosion include slope gradient, length of slope, elevation as well as the size and shape of the watershed.

Steeper and longer slope increase runoff and erosion.

Doubling the slope gradient means increasing soil quantity that can be transported by 4 times (IEAust-Old, 1996)

3. Soil Characteristics

The physical properties of soil affect the detachment and transportation of soil particles due to erosion are:Soil texture,

Organic matter,

Moisture content,

Structure,

Density (due to compaction), and

Chemical and biological characteristics of soil

Soil erosion is also influenced by soil properties (include bulk density and shear strength).

4. Vegetation Surface vegetation cover reduces erosion as it

acts as a buffer zone to dissipate the energy of falling raindrops, protecting the soils affected and reducing soil detachment and maximizing water infiltration.

Vegetation roots assist in reducing the velocity of overland flow by increasing surface roughness.

Vegetation transpiration reduces soil moisture thus encouraging infiltration of rainwater which aids reducing surface runoffs and rate of erosion at potential unstable sites.

SOIL LOSSES PREDICTION Soil losses or erosion rate are estimate to

assist farmers and government agencies:to evaluate existing farming systemsto compare alternative management

strategies to reduce soil loss. In 1959, a method of estimating losses

based on statistical analyses of field plot data which resulted in Universal Soil Loss Equation (USLE), and was modified and revised become Modified universal Soil Loss Equation (MUSLE) and Revised Universal Soil Loss Equation (RUSLE).

1. Universal Soil Loss Equation (USLE) USLE has been the most widely used

prediction tool for erosion potential analyses worldwide.

Widely used for predicting soil loss from landscape sites with moderate slopes, normally less than 50% (Wischmeier and Smith, 1978).

The USLE average rate of erosion, A are:

A=R*K*LS*C*P

A = R K LS C P A

A = Tons of soil lost per acre each year R = Rainfall Erosivity factor K = Soil erodibility factor LS = Slope factor C = Cover and management factor P = Support practice factor

2. Modified Universal Soil Loss Equation (MUSLE)

MUSLE predicts soil erosion during individual storms for a selected soil material, compared to USLE which estimates average annual soil losses.

More applicable to regions with rainfall events that are short-duration and high intensity.

Suited to the design of stable spoil dumps in areas with irregular high-intensity rainstorms.

MUSLE empirical erosion model is as follows:

E= {[0.5(EI30) + 0.349 (RO)Qp0.333]*K*LS*C*P}

E : rate of erosion (t/ha) EI30 : integrated rainfall erosivity factor (MJ.mm/ha/h) RO : total runoff per event (m3) Qp : peak runoff rate per event (mm/h) LS : slope factor C : cover and management factor P : support practice factor

In 1985, it was decided that original USLE should be revised to include additional update research information to increase its applicability to different sites worldwide (Renard et al., 1994).

RUSLE model is computer based, normally used for lands that exhibit a variety of vegetation types and on steeper and longer slope.

3. Revised Universal Soil Loss Equation (RUSLE)

IMPACT OF SOIL EROSION

Possible loss of lives and destruction of property due to slope failure/landslide.

Difficulty in the implementation of post rehabilitation site programmes.

Reducing the capability of future post-industrial land use option.

Reduction of soil structure stability due to the continuous loss of soil particles and vegetative cover at site.

Economic losses due to delay in the transportation of goods possibly because of highway/roads being obstructed by debris/mudflows.

EXAMPLE:

A study was conducted at paddy agricultural area at Kuala Perlis. The detail of soil properties as shown below:

Very fine sand (30%), Sand (20%), Silt (10%), Clay (45%), Organic content (7%).

The bare soil area have slope length, λ is 17m with 33% steepness (s).

Estimate the soil erosion rate using USLE.

Solution:

1. R value

= 10500MJ.mm/ha.hr. yr

2. K value

K = [1.0x10-4(12-OM)M1.14+4.5(s-3) + 8.0(p-2)] x 1 100

7.59

M = (% silt + % very fine sand) x (100 - % clay)

OM = % of organic matter

s = soil structure code (refer figure 3.14)

p = permeability class (refer table 3.2)

M = (10+30) X (100-45) = 2200

OM = 7, s = 4, p =6

K = [1.0x10-4(12-7)22001.14 + 4.5(4-3) + 8.0(6-2)] x 1 100

7.59

= (3.2310 + 36.5) x 1 100 7.59

= 0.052 (ton/ha)(ha.hr/MJ.mm)

3. LS value

λ = 17 m, s % = 33% From table 3.5, LS = 7.4731

4. C value From table 3.7, paddy with water, C =

0.01

5. P value From table 3.9, bare soil, P = 1.0

A = R * K * LS * C * P

= 10500 * 0.052 *7.4731 * 0.01 * 1.0

= 40.803 ton/ha/yr