CHAPTER 3

SOIL EROSION AND SEDIMENTATION

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3.1 SOIL EROSION AND SEDIMENTATION PROCESSES

Erosion is a natural process, where energy provided
by water, wind, and gravity drives the detachment, transport, and deposition of
soil particles (Daniel,
Karl, Williard , Pamela & Jon, 2015). There are two main types of soil
erosion: geological and accelerated soil erosion. Geological soil erosion happens
at the same rate as soil is formed. Accelerated soil erosion is the loss of
soil at a much faster rate than it is formed (Mehlhorn & Alqusaireen, 2013).
Detachment occurs when the forces
holding a soil particle in place are overcome by the forces of raindrop impact,
moving water, or win. A portion of the energy from raindrop impact is first
spent to deform peds (i.e., aggregrates of soil particles) and detach soil
particles from the surface. Remaining energy activates the second step of the
erosion process, particle transport. Deposition is the third and final step in
the erosion process and occurs simultaneously with the first two steps. When
the sediment load of moving water is greater than its transport capacity,
deposition occurs. Recently-deposited soil is more vulnerable for re-detachment
and transport than residual soil because the original bonding forces have been
broken. However, a layer of recently deposited loose soil can help prevent
detachment of underlying soil. Individual soil particles can be detached,
transported, and deposited several times during a single storm event (Daniel,
Karl, Williard , Pamela & Jon, 2015).

Sediment
is detached from the soil surface both by raindrop impact and by the shearing
force of flowing water. The detached sediment is transported downslope
primarily by flowing water, although there is also a small amount of downslope
transport by raindrop splash. Once runoff starts over the surface areas and in
the streams, the quantity and size of material transported increases with the
velocity of the runoff. At some point, the slope may decrease, resulting in a
decreased velocity and hence a decreased transport capacity. The sediment is
then deposited, starting with the large primary particles and aggregates.
Smaller particles are transported further downslope, resulting in the
enrichment of fines. The amount of sediment load passing the outlet of a
catchment forms its sediment yield (Manoj & Umesh, 2000).

Water
is an immensely strong element which has the ability to move large amounts of
soil particles and other substances across a large expanse of land. Soil
erosion caused by water is more likely to happen in areas where there is a
downwards slope in the landscape. Figure 1 shows the types of erosion occurring
on an exposure slope (Soil Erosion and Sedimentation, 2018).

 

 

Figure 1: Types of soil
erosion on an exposed slope (Fact
Sheet 1, n.d)

 

(a)  
Splash
erosion

Splash
erosion is the first stage of the erosion process. It occurs when raindrops hit
bare soil. The explosive impact breaks up soil aggregates so that individual
soil particles are ‘splashed’ onto the soil surface. The splashed particles can
rise as high 60cm above the ground and move up to 1.5 metres from the point of
impact. The particles block the paces between soil aggregates, so that the soil
forms a crust that reduces infiltration and increases runoff. (Fact Sheet 1,
n.d)

 

(b)  
Sheet
Erosion

Sheet
erosion is the removal of soil in thin layers by raindrop impact and shallow
surface flow. It results in loss of the finest soil particles that contain most
of the available nutrients and organic matter in the soil. Soil loss is so
gradual that the erosion usually goes unnoticed, but the cumulative impact
accounts for large soil losses. Soils most vulnerable to sheet erosion are
overgrazed and cultivated soils where there is little vegetation to protect and
hold the soil. Early signs of sheet erosion include bare areas, water puddling
as soon as rain falls, visible grass roots, exposed tree roots, and exposed
subsoil or stony soils. Soil deposits on the high side of obstructions such as
fences may indicate active sheet erosion. Vegetation cover is vital to prevent
sheet erosion because it protects the soil, impedes waterflow and encourages
water to infiltrate into the soil. The surface water flows that cause sheet
erosion rarely flow for more than a few metres before concentrating into rills.
(Fact Sheet 1, n.d)

 

(c)   
Rill
Erosion

Rills are shallow drainage lines less than 30cm deep. They develop when
surface water concentrates in depressions or low points through paddocks and
erodes the soil. Rill erosion is common in bare agricultural land, particularly
overgrazed land, and in freshly cultivated soil where the soil structure has
been loosened. The rills can usually be removed with farm machinery. Rill
erosion can be reduced by reducing the volume and speed of surface water with
grassed waterways and filter strips, ripped mulch lines, and contour drains.
Rill erosion is often described as the intermediate stage between sheet erosion
and gully erosion. (Fact
Sheet 1, n.d)

(d)  
Gully
Erosion

Gullies
are channels deeper than 30cm that cannot be removed by normal cultivation.
They can be spectacular to look at but over time actually lose less soil than
sheet and rill erosion. Gullies occur when smaller water flows concentrate and
cut a channel through the soil. Most gullies extend upslope as a result of the
head of the gully being continually undercut and collapsing. However, collapse
and slumping of sidewalls usually contribute a greater proportion of soil loss.
(Fact Sheet 1, n.d)

 

(e)   
Riverbank
Erosion

Riverbank erosion occurs
primarily through a combination of three mechanisms: sub-aerial weakening and
weathering, fluvial erosion, and mass failure. Sub-aerial processes are often
viewed as ‘preparatory’ processes, weakening the bank prior to fluvial erosion.
Sub-aerial processes dominate in the upper reaches, fluvial erosion in the
middle, and mass failure in the lower reaches of a river. Fluvial erosion is
the detachment of particles from the bank surface by the direct action of the
flowing water; mass failure is the collapse of bank material under the action
of gravity; weakening processes are modifications of soil characteristics that
increase bank erodibility, and thus induce bank erosion. (Rana & Nessa, 2017)

(f)   
Tunnel
Erosion

Tunnel
erosion occurs when surface water moves into and through dispersive subsoils. Dispersive
soils are poorly structured so they erode easily when wet. The tunnel starts
when surface water moves into the soil along cracks or channels or through
rabbit burrows and old tree root cavities. Dispersive clays are the first to be
removed by the water flow. As the space enlarges, more water can pour in and
further erode the soil. As the tunnel expands, parts of the tunnel roof
collapse leading to potholes and gullies. Indications of tunnel erosion include
water seepage at the foot of a slope and fine sediment fans downhill of a
tunnel outlet. Remediation actions include breaking open existing tunnels,
revegetation, and increasing soil organic matter. Extensive earthworks may be
required. (Fact Sheet 1, n.d)

 

(g)  
Wind
Erosion

Wind
erosion is the detachment and movement of soil particles by air moving at least
20km per hour. Wind moves the soil in two ways, suspension and saltation.
Suspension occurs when the wind lifts finer particles into the air leading to
dust storms. Saltation occurs when the wind lifts larger particles off the
ground for short distances, leading to sanddrifts. Wind erosion tends to occur
most in low rainfall areas when soil moisture content is at wilting point or
below, but all drought-stricken soils are at risk. Often the only evidence of
wind erosion is an atmospheric haze of dust comprising fine mineral and organic
soil particles that contain wind speed at the ground surface; and planting
windbreaks to reduce wind speed. Also, be ready for severe wind erosion seasons
which tend to be the summers following dry autumns and winters. (Fact Sheet 1,
n.d)

 

 

 

3. 2 UNIVERSAL SOIL LOSS EQUATION (USLE)

The
Universal Soil Loss Equation (USLE) is a mathematical model developed to
predict the soil erosion by rainfall and surface runoff on a field. The
empirical result of the USLE corresponds to a long-term average annual rate of
soil losses under a variety of climatic conditions, soil types, topographic
characteristics, crop systems, and conservation practices. However, USLE only
predicts the amount of soil loss resulting from sheet or rill erosion on a
single slope and does not account for additional soil losses that might occur (LaRocque
A, 2013). 

RUSLE
(Revised Universal Soil Loss Equation) model is only ones of many modification
of USLE, especially for more complex situations of rill and interfile erosion
in conservation planning and land uses. Both erosion-prone models calculate
detachment capacity and soil loss.

(1)

RUSLE model
predicts soil degradation and sediment concentrations better using another soil
erodibility factor (F-soil factor, based on soil texture). Additionally,
cover-management factor is different. A new approach also is connected with
conservation-practice values and slope length and steepness. Revised Universal
Soil Loss Equation (RUSLE) expressed in equation 1 as:

Where:

A
= Average annual soil loss in ton per hectare per year (tn/ha/yr) or (t ha -1/yr)

R
= Rainfall-runoff erosivity factor in MJ · mm · ha –1 · hr –1
· yr –1

K
= Soil erodibility factor in tn · ha · hr · ha –1 · MJ –1·
mm -1

LS
= Topographic factor

L
= Slope length (dimensionless)

S
= Steepness factor (dimensionless)

C
= Cover management factor (dimensionless)

P
= Support practise factor (dimensionless)

 

3.3
SOIL LOSS AND SEDIMENT YIELD PARAMETERS

Rainfall erosivity factor (R),
Soil erodibility factor (K), Slope Length, and Steepness Factor (LS), Cover
management (C) and Support practise (P) factor are the major parameters in the
application of USLE which calculations and values recommended are to be in
standard S.I. unit (Kamaludin, Lihan, Rahman, Mustapha, Idris & Rahim,
2013).

 

3.3.1       
Rainfall erosivity factor (R)

 

In a study conducted by Bol (1978)
in Indone­sia generated an empirical model as show Equa­tion (2) relating
R-factor to rainfall P (mm). (Sholagberu, Mustafa, Yusof & Ahmad, 2016). The
rainfall erosivity factor (R) in USLE is a measure of the erosion force of rainfalls
for the specific area. R-value is greatly affected by the volume, intensity,
duration and pattern of rainfall, whether for single storms or a series of
storms, and by the amount and rate of the resulting runoff. Areas with low
slope degree have low erosivity R values which imply that flat areas would
increase the water ponding on the surface, thus protecting soil particles from
being eroded by rain drops. Large numbers of R factor indicate more erosive
weather conditions (Rahaman, Aruchamy, Jegankumar, & Ajeez, 2015).  According to Teh (2011), the annual
precipitation data in Malaysia is easier to obtain than pluviographic data, R
is in MJ*mm/ (ha*hr) and P is annual precipitation in millimetres (Kurt
Copper,2011).

(2)

 

 

Where:

R
= rainfall erosivity factor (MJ mm/ha-1/ h-1/ hr-1)

P
= annual rainfall (mm)

 

3.3.2       
Soil Erodibility Factor (K)

 

Soil erodibility depends on soil
and, or geological characteristics, such as parent material, texture,
structure, organic matter content, porosity, catena and many more. (Rahaman,
Aruchamy, Jegankumar, & Ajeez, 2015). Soil erodibility is the manifestation
of the inherent resistance of soil particles for the detaching and transporting
power of rainfall (Gelagay & Minale, 2016). The K-factor is empirically
determined or a particular soil type and reflects the physical and chemical
properties of the soil, which contribute to its erodibility potential. The data
are obtained from K values were reflected by the rate of soil loss per rainfall
(R) erosion index (ton.ha) (ha.hr/MJ.mm). For the Soil erodibility factor K,
extensive work has been carried by Tew (1999) to produce a condition a soil
erodibility nomographs, based on unmodified nomograph (Wischmeier et al,
1978). The soil erodibility factor (K) was determined by using Equation 3 (Tew,
1999), as follow (Agele, Lihan, Sahibin & Rahman, 2013).

 

(3)

 

 

Where:

K
        = Soil Erodability Factor,
(ton/ac.)*(100ft.in/ac.hr) for SI unit (ton/ha)(ha.hr/MJ.mm). The conversion
factor is 0.13175,1/7.59.

            M
        = (% silt + % very fine sand) ×
(100 – % clay)

            OM
     = % of organic matter

            S          = Soil structure code

            P
         = Permeability class

The equivalent soil structure classes used in the
USLE in accordance to the reported Malaysian soil by the Department of
Agriculture, Malaysia are given in Table 1.( Abdalla, Elsheikh, & Elhag,
2015)

 

Table 1: Soil structure classes
used in the USLE (DOA)

Soil structure code

Definition of Soil Structure
(Nomograph)

Soil Structure Classes (Soil
Report – DOA)

1

Very
Fine Granular

Very
Fine Granular, Crumb

2

Fine
Granular

Fine
Granular / Crumb

3

Medium
or Coarse Granular

Medium
Granular/ Crumb or Very Fine to Fine Subangular Blocky or Very Fine to Fine
Angular Blocky

4

Blocky,
Platy or Massive

Medium
to Coarse Subangular Blocky, Angular Blocky, Prismatic,
Columnar
or Massive

 

The drainage classes used in
mapping Malaysian soils were used as equivalent to the soil hydraulic
permeability that was used in the USLE Nomograph recommended by Department of
Agriculture, Malaysia and shown in Table 2. From this table, Soil Hydraulic
Permeability Parameter was readily determined. ( Abdalla, Elsheikh, &
Elhag, 2015)

 

Table
2: Soil hydraulic permeability class recommended by Department Of Agriculture
(DOA)

 

Soil
Permeability Value

Soil
Profile Hydraulic Permeability
(USLE
Nomograph)

Soil
Profile Drainage Classes (Soil Report – Depart. Of Agriculture Malaysia)

1

Rapid

Class
8 – 9 : Excessive to Very Excessive
Drained

2

Moderate
To Rapid

Class
5 – 7 : Somewhat Imperfect to Well
Drained

3

Moderate

Class
4 : Imperfectly Drained

4

Slow
To Moderate

Class
3 : Somewhat Poorly Drained

5

Slow

Class
2 : Poorly Drained

6

Very
Slow

Class
0 – 1 : Very Poorly to Somewhat Poorly

Figure 3.1: Malaysian soil erodobility for
calculation of Soil erodibility factor, K, (ton/ac.)*(100ft.ton.in/ac.hr)
(Tew, 1999) (Yusof,
Azamathulla, Abdullah & Zakaria, 2011)
 

 

Figure
3.2: Soil Map for Peninsular Malaysia Soil Series, DOA 2010 (Yusof, Azamathulla, Abdullah &
Zakaria, 2011)

 

Figure
3.3:  Soil Structure Code based on
textural classification (Yusof,
Azamathulla, Abdullah & Zakaria, 2011)

 

Table 3 shows the range Soil Erodibility for Soil Type
of Peninsular Malaysia Soil Series. 

 

Table
3: Range of Soil Erodibility for Soil Type of Peninsular Malaysia Soil Series

Soil
Type

K
Factor
(ton.ha)(ha.hr/MJ.mm)

Clay

0.042 – 0.065

Clay loam

0.030 – 0.047

Sandy Clay

0.031 – 0.043

Sandy Clay Loam

0.028 – 0.059

Sandy Loam

0.004 – 0.036

Silt Loam

0.014 – 0.027

Silty clay Loam

0.032

 

3.3.3       
LS Factor

 

The LS factor (topographic
factor) accounts for the effect of topography on erosion in RUSLE. The slope length
factor (L) represents the effect of slope length on erosion, and the slope
steepness factor (S) reflects the influence of slope gradient on erosion. L is
the flow length and S is slope steepness which is given by meter and percent
respectively. It is the ratio of soil loss from a specific site to that from a
unit site having the same soil and slope but with a length of 22.13m. LS factor
is calculated using the mathematical equation 4: (Agele, Lihan, Sahibin, & Rahman,
2013)

 

 

(4)

            Where:

l           = sheet flow path length (m or feet)

s           = average slope gradient (%)

m         = 0.2 for s < 1 = 0.3 for 1? s <3 = 0.4 for 3? s <5 = 0.5 for 5? s <12      and = 0.6 for s?12%             ********************************(MANUAL)*********************************         3.3.4        Cover management (C)       3.3.5        Support practise (P) factor           REFERENCES   1.        Abdalla, R. F., Elsheikh, O., S., & Elhag, A. R. (2015). SOIL EROSION RISK MAP BASED ON GEOGRAPHIC INFORMATION SYSTEM AND UNIVERSAL SOIL LOSS EQUATION (CASE STUDY: TERENGGANU, MALAYSIA). Indian Journal of Science and Technology, 3(2), 38-43. Retrieved from http://www.indjsrt.com 2.        Agele, D., Lihan, T.,  Sahibin, A.R. & Rahman, Z.A (2013). Application of the RUSLE model inforecasting Soil Erosion atdownstreamof the Pahang river basin, Malaysia. Journal of Applied Sciences Research, 9(1), 413-424. 3.        Daniel J. Holz, Karl W.J. Williard , Pamela J. Edwards , & Jon E. Schoonover. (2015). "Soil Erosion in Humid Regions: A Review." Journal of Contemporary Water Research & Education, no. 154, Apr. 2015, pp. 48–59. 4.        Fact sheet 1: Types of erosion - dpi.nsw.gov.au. (n.d.). Retrieved January 26, 2018, from https://www.bing.com/cr?IG=6ECDD0267AF24575AFA0206302976093&CID=0C12EC61A12A64A734B8E7E6A08565C1&rd=1&h=Xf7F_FVDbdA2zbFzKVrjSCjcqfc7WgvqNaJJzkfxxAM&v=1&r=https%3a%2f%2fwww.dpi.nsw.gov.au%2f__data%2fassets%2fpdf_file%2f0003%2f255153%2ffact-sheet-1-types-of-erosion.pdf&p=DevEx,5062.1 5.        Ganasri, B., & Ramesh, H. (2016). Assessment of soil erosion by RUSLE model using remote sensing and GIS - A case study of Nethravathi Basin. Geoscience Frontiers, 7(6), 953-961. doi:10.1016/j.gsf.2015.10.007 6.        Gelagay, H. S., & Minale, A. S. (2016). Soil loss estimation using GIS and Remote sensing techniques: A case of Koga watershed, Northwestern Ethiopia. International Soil and Water Conservation Research, 4(2), 126-136. doi:10.1016/j.iswcr.2016.01.002 7.        Kamaludin, H., Lihan, T., Rahman, Z. A., Mustapha, M. A., Idris, W. M., & Rahim, S. A. (2013). Integration of remote sensing, RUSLE and GIS to model potential soil loss and sediment yield (SY). Hydrology and Earth System Sciences Discussions, 10(4), 4567-4596. doi:10.5194/hessd-10-4567-2013 8.        Kurt Copper(2011, December 1). Evolution of the Relationship Between the RUSLE R-Factor and Mean Annual Precipitation. 9.        LaRocque A. (2013) Universal Soil Loss Equation (USLE). In: Bobrowsky P.T. (eds) Encyclopedia of Natural Hazards. Encyclopedia of Earth Sciences Series. Springer, Dordrecht 10.    Laflen, J. M., & Flanagan, D. C. (2013). The development of U. S. soil erosion prediction and modeling. International Soil and Water Conservation Research, 1(2), 1-11. doi:10.1016/s2095-6339(15)30034-4 11.    Manoj, K., Jain., & Umesh, C., Kothyari. (2000) Estimation of soil erosion and sediment yield using GIS, Hydrological Sciences Journal, 45:5, 771-786, DOI: 10.1080/02626660009492376 12.    Mehlhorn, S. A., & Alqusaireen, E. (2013). Comparison of Three Soil Erosion Control Treatments. 2013 Kansas City, Missouri, July 21 - July 24, 2013. doi:10.13031/aim.20131620629 13.    (n.d.). Retrieved January 25, 2018, from http://www.hawaiioceanscience.org/soil-erosion-and-sedimentation.html 14.    Rahaman, S. A., Aruchamy, S., Jegankumar, R., & Ajeez, S. A. (2015). Estimation Of Annual Average Soil Loss, Based On Rusle Model In Kallar Watershed, Bhavani Basin, Tamil Nadu, India. ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences,II-2/W2, 207-214. doi:10.5194/isprsannals-ii-2-w2-207-2015 15.    Rana, M. S., & Nessa, A. M. (2017). Impact of Riverbank Erosion on Population Migration and Resettlement of Bangladesh. Science Journal of Applied Mathematics and Statistics, 5(2), 60-69. doi:doi: 10.11648/j.sjams.20170502.11 16.    Sholagberu, A., Mustafa, M. U., Yusof, K. W., & Ahmad, M. (2016). Evaluation Of Rainfall-Runoff Erosivity Factor For Cameron Highland, Pahang, Malaysia. Journal of Ecological Engineering, 17(3), 1-8. doi:10.12911/22998993/63338 17.    Soil Erosion and Sedimentation, (2018). Retrieved from www.hawaiioceanscience.org/soil-erosion-and-sedimentation.html. 18.    Yusof, M. F., Azamathulla, H. M., Abdullah, R., & Zakaria, A. N. (2011). Modified soil erodobility factor, K for Peninsular Malaysia soil series. 3rd International Conference of Managing Rivers in the 21st Century.

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