Lecture8 soil erosion dung

THE SOIL WATER EROSION PROCESS DETACHMENT Soil Sediment Load Sediment Transport Detachment DEPOSITION Soil Sediment Load Deposition... Erosion  Erosion: displacement of soil particles;

VFU, 30-September-2014

Principles of

Watershed management

Lecture #8

Soil erosion and Control of Erosion

Dr Bui Xuan Dung- Department of Environment Management

What is Soil

Surface Erosion?

It is the movement of soil

particles from one place to another under the influence of

water or wind

What are the types of soil erosion?

• Water Erosion

• Wind Erosion

THE SOIL WATER EROSION

PROCESS

DETACHMENT

Soil

Sediment Load Sediment Transport

Detachment

DEPOSITION

Soil Sediment Load

Deposition

Erosion

 Erosion: displacement of soil particles;

 Soil loss or sediment production: mass transferred off of a hillslope or plot

Types of

water erosion

Water erosion Gully erosion Rill erosion Sheet erosion Splash erosion

Greater potential for sediment movement

Raindrop impact and splash

splash

Raindrop

Rainsplash

 Two processes involved in erosion:

 Detachment; and

 Transport;

 Rainsplash is the detachment and

movement of particles resulting from the impact of a raindrop on the ground

surface;

Raindrop impact when there is a surface

film of water;

 Drop size increases with rainfall intensity;

 Velocity increases with drop size until ~20

mm hr-1) (moderate intensity);

 Terminal velocity reached after ~10 m;

Raindrop size, velocity, and energy

for different rainfall intensities

Rainsplash

preferentially detaches

small particles

No net transport

on flat areas;

Rainsplash on a slope results in net downslope transport;

Can lead to a

coarsening of the soil

surface;

Rainsplash from multiple drops

Rainsplash: Implications

 High intensity storms are most erosive due

to more and larger raindrops rather than

greater velocity;

 Tree canopy may have little effect or even increase rainsplash erosion because drops are larger and distance to ground may still result in near terminal velocity;

 Silty soils are most susceptible to rainsplash erosion because silt particles are small,

easily detached and transported, and not

cohesive;

Rainsplash: Effect on infiltration

 Raindrop impact can compact the surface

layer and thereby reduce infiltration;

 By detaching and transporting small particles, raindrop impacts can lead to soil sealing (i.e., small particles can form a thin layer and clog the entrance to soil pores), thereby greatly

reducing infiltration;

 Ground cover is critical to preventing sealing and reducing rainsplash erosion!

Discussion: Bare soil

• The soil surface is compacted by raindrop impact;

• Soil particles are washed between the coarser grains to form a structural soil seal;

Sealing on a bare granitic soil

1 mm seal

• It is happens when rainwater flows into lower

elevations, carrying sediments with it

• The water loses some of its energy of motion and it drains into the soil or slowly evaporates

 Detachment and movement of soil particles due to a relatively smooth, thin sheet of

water flowing across the ground surface;

Sheetwash

 Shallow depth and slow velocity means

that sheetwash is less effective at

detaching particles than transporting them;

Rilling

 Coalescing or concentration of surface

runoff into small rivulets as compared to sheetflow (Shear stress (T) = density x

depth x slope);

 Rill detachment and transport capacity can

be much greater than sheetwash due to

deeper flow and faster velocity;

 Rills are defined as small enough to be

removed by ploughing, or less than ~0.1

m2;

Rill erosion

Rills formed on raked hillslopes

Gullying

 Further concentration of flow leading to much deeper incision;

Gully erosion

Why don’t we see rills and gullies

everywhere?

Why don’t we see rills and gullies

overland flow and between rills (interrill areas);

 As the interrill areas erode below the surrounding area, they will become the new pathways for

overland flow, and the areas that formerly had

sheetwash and rill flow will now be subject to

rainsplash;

 Sequential lowering results in an even, downward lowering of the land surface with no gullies

Continuities of water erosion

• Erosion by water is

caused by

raindrops, surface flow and gully flow

• Water erosion is a

selective process in which the organic

matter and finer

soil particles are

removed first

Wind Erosion

Erosion by wind is common is

dry areas where soils are

often bare of vegetation and

high wind velocities are

common

WIND EROSION

CREEP

SUSPENSION

SALTATION

 SALTATION DETACHES PARTICLES

 SMALLER PARTICLES SUSPENDED

 LARGER PARTICLES CREEP

 SANDY AND SILTY SOILS MOST SUSCEPTIBLE

 SOIL ACCUMULATION IN DITCHES AND FENCE ROWS

Erosion: On-site and off-site effects

 On-site effects:

 Loss of mineral soil, nutrients, organic matter;

 Reduction in soil moisture storage capacity

results in more runoff;

 Reduction in on-site productivity not

consistent with long-term sustainability;

Erosion: On-site and off-site effects

 Off-site effects have more direct impact on

people and property:

 Sediment deposition in the stream channel results in:

 More overbank flow and higher floods;

 Habitat loss (fewer fish and other aquatic organisms);

 Loss of reservoir storage capacity;

 Increase in phosphorus and eutrophication (more algal growth, decreased dissolved oxygen, etc.);

Erosion

Impacts on streams

Not all erosion is bad

 Deposition of sediment creates flat, fertile areas that are very productive (e.g.,

floodplains; Red River and Mekong River deltas);

Not all erosion is bad

 Key is how much our activities increase

erosion and sedimentation rates;

 Human activities can cause much greater changes in erosion rates than runoff rates;

 In many cases the change in erosion is of primary concern, not the change in runoff

Some Erosion is Absolutely

Beautiful …

Controls on Water-driven

surface erosion

 Conceptually surface erosion a function of:

 Climate (amount, type, and intensity of

precipitation);

 Soil properties;

 Topography;

 Vegetation cover;

Soil erosion processes

Soil erosion is a complex

process that depends on

soil properties, ground

slope, vegetation, and

rainfall amount and

intensity (Selby, 1993)

Detachment

Transport

Deposition

Quantifying Soil Erosion

Scales of Erosion Measurements

Measuring rill erosion, Hayman fire

Upper Saloon Gulch: 10 July 2002

17 mm rain in 2 hours

Rill erosion in Swale 4: Storm on 21 August 2003

-40 -30 -20 -10 0

0 10 20 30 40 50 60 70 80 90 100

cm

15-Aug-02 23-Aug-02

-40 -35 -30 -25 -20 -15 -10 -5 0

0 10 20 30 40 50 60 70 80 90 100

cm

15-Aug-02 23-Aug-02

8 mm rainfall

I30 = 15.6 mm/hr 27.4 MJ mm/ha yr

Erosion Bridges

Main problems are:

1 Accuracy of data (litter, vegetation, etc.);

2 Precision of the data (small change is large

erosion rate);

3 Confounding factors (freeze-thaw, compaction

by rainfall, etc.);

4 Spatial variability

 Hard to collect accurate and representative

data unless erosion or deposition rates are

very large

Other Plot Scale Techniques

1 Gerlach troughs;

2 Bounded plots with runoff containers;

Erosion Plot in forest

Sediment Production at the

Hillslope Scale

Sediment Fences

We can apply erosion fence But…

Soil erosion estimation using fallout radionuclides

• Cesium-137 (Half-life: 30.2 years, Origin:

Weapons Testing)

Long-term trends of Cs-137 fallout observed in

Tsukuba, Japan (Hirose et al., 2001)

–Sediment tracers to document surface soil erosion

[Ritchie and McHenry, 1975; Walling and He, 1999]

–High affinity for soil particles at the ground surface

Coweeta Hydrologic Lab paired watershed research

Coweeta Hydrologic Lab

USLE Universal Soil Loss Equation

 Wischmeier, W.H and D.D Smith 1978

Predicting rainfall erosion losses USDA

Agriculture Handbook 537, U.S Department of Agriculture

Prediction of soil erosion

Standard USLE plot:

– 22.1m long

– 9% slope gradient

– 4m wide

Universal Soil Loss Equation

Soil erosion plot that is

the standard length of

22 m and standard

slope of 9% near

Pullman, Washington;

Original USLE plots

were only 1.8 m wide;

The equation:

A = R x K x LS x C x P

– A = average annual soil loss (tons/ha year) – R = rainfall and runoff erosivity index

– K = soil erodibility factor

– L = slope length factor

– S = slope steepness factor

– C= crop/management factor

– P = conservation or support practice factor

R (rainfall and runoff erosivity index)

• Erosion index (EI) for a given storm:

– Product of the kinetic energy of the falling raindrops and its maximum 30 minute

intensity

A =R x K x LS x C x P

Soil erodibility factor (K)

 Calculated with an empirical procedure

using percent silt and very fine sand,

percent organic matter, permeability class, and structure class;

 Based on data from about 70 standard

USLE plots with bare soil;

Length-slope factor (LS)

 Slope length and slope steepness are

usually combined into one factor, as

steepness and slope length interact (longer slope has more runoff and can have same

LS factor as a shorter but slightly steeper slope);

 Slope steepness is in percent;

 Slope length defined as the horizontal

distance from where overland flow begins

to the point of channelized flow or

deposition;

LS (slope length-gradient)

• Ratio of soil loss under given conditions to that

at a site with the "standard" slope and slope

length

A =R x K x LS x C x P



Original nomograph for determining LS factor

C (crop/management)

• Ratio of soil loss from land use under specified

conditions to that from continuously fallow and tilled land

Vegetation cover

Plant litter Soil surface Land management

A =R x K x LS x C x P

Sediment yield vs percent bare soil

Percent bare soil

Erosion control practices factor (P)

P =

 P =1.0 if no erosion control practices employed;

 P values generally regarded as being among the least reliable;

Soil loss with bare soil and ploughing up and down slope Soil loss using one or more erosion control practices

P (Erosion control conservation practices)

• Ratio of soil loss by a support practice

Strip cropping, cross slope 0.37

Strip cropping, contour 0.25

MUSLE Modified Universal Soil Loss

Equation

Modified USLE for use in rangeland and forest

environment

C (crop management) and p (erosion

control practice) factors are replaced by vegetation management (VM) factor

A = R x K x LS x VM

– A = average annual soil loss (tons/ha year) – R = rainfall and runoff erosivity index

– K = soil erodibility factor

– L = slope length factor

– S = slope steepness factor

– VM = vegetation management factor

VM factor can be determined by

(1) Canopy cover effects

(2) Effects of low-growing vegetation cover

(3) Bare ground with fine root

Ground cover condition and

VM factor

Erosion in Forestlands

Some typical rates:

tons/ac/yr

(Washington Cascades);

higher?

Controlling Erosion

 Susceptible situations (delineate areas of concern)

 Long, steep slopes, shallow soils;

 Low infiltration capacity;

 Loss of vegetation;

 Prevention

 Protect soil surface

 Increase surface roughness

 Shorten slope length

 Maintain vegetation

Schoenholtz, 2004

Vegetation – the key to erosion

Practices for the Control of Soil

Erosion

Controlling soil erosion by water

Reducing rain drop

impact

1 Soil management, Reducing slope

2 Providing Crop Cover

Increase Soil Resistance 1 Improving soil structure

2 Organic matter and cohesive properties

Reducing shear strength 1 Decreasing runoff velocity

Vegetation practices

1.Vegetarian Cover 2.Contouring

3.Strip Cropping 4.Tillage Operation

Use thick-growing

CROPS:

Crops which cover the ground surface and

fill the surface soil with fibrous roots tend to hold the soil in place and reduce erosion

Cultivate On The

Contour

This is the practice of

planting and cultivating of crops

following the contours of the land

Strip Cropping

This is the practice of planting

two crops in alternating strips or alternately planting a strip and leaving a strip fallow on land

that would otherwise be erodible

CONTOUR STRIP CROPPING

Crawford Co

Tillage Operation

erosion

No-Till (NT)

Strip-Till (ST) Deep Till (DT)

Mechanical practices

1.Terracing 2.Ponds and Dams 3.Vegetated Buffer Strip

Ponds and Dams

Artificial ponds hold or

impound water which

otherwise would be lost

as runoff, and which in

the process of runoff,

would carry soil with it

Vegetated Buffer Strip

in the area that links to stream

How Do Buffers Protect from erosion?

Soil (also pollution)