Soil Erosion

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22 févr. 2014 (il y a 3 années et 7 mois)

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Soil Erosion

TSM 352

Land and Water
Management Systems

Soil Erosion

Soil Erosion


Most important problem associated with agricultural and
other land use practices

Soil Erosion

Soil Erosion


According to 1987 National Resources Inventory, USDA

o
4 billion metric tons of soil are lost every year due to soil
erosion (wind and water) in 1970s

o
2 billion
metric tons in 1997 due to increased used of
conservation and best management practices (BMPs)

o
70% of the total soil loss is from agricultural land

o
Economics of soil erosion: 44 billion dollars/year, $100/acre



It takes 100
-
600 years to form one inch topsoil



In Illinois,

o
40% (9.6 million acres) cropland suffer from erosion

o
Average soil erosion rate: 6.18 tons in 1987. (max 50 tons/acre)


Soil Erosion

Soil Erosion

Soil Erosion

Soil Erosion by Water

T
-

maximum average annual
permissible soil
loss without decreasing
productivity

T
-
values
ranging from
4.5 to
11.2 metric tons/ha per year (2 to 5 tons/acre per year).

Soil Erosion

Soil Erosion


Mississippi River


The river carries
roughly
550
-
500
million
tons
of sediment
into the Gulf of Mexico each year.


It brings
enough sediment
to
extend the coast of Louisiana
by 91
m (300
ft
) each year.

Soil Erosion

Soil Transport


Mechanism


Micro
-
scale soil transport: Surface erosion


Macro
-
scale soil transport: Mass movements (Landslides)



Erosion

is the detachment and transport of soil particles
(natural and accelerated)



Sediment

is relocated from the source (= soil) to streams
and eventually into reservoirs or the sea



Sedimentation
: deposition of sediments in streams or on
fans and floodplains


Soil Erosion

Effects of Soil Erosion


On site


Loss of top fertile soil


Loss of nutrients, OM


Decreased productivity



Off site


Non
-
point source pollution


Filling of reservoirs and dams


Air quality problem


Effects on aquatic organisms


Effects on drinking water quality


Redistribution of pollutants and toxics


Soil Erosion

Soil Erosion

Erosion is caused by:


Water


Primary water erosion (Splash Erosion)


Secondary water erosion
(
Surface flow
induced erosion)



Wind erosion


The Dust Bowl of 1930s
(1934
-
1936) in the
Great
Plains



Frost erosion


Particularly effective in mountai
nous areas


Example: NW USA


Soil Erosion

Factors Affecting Soil Erosion


Climate


Soil


Topography


slope length and steepness


Vegetation


Land
-
use practices

Soil Erosion

Water Erosion Processes


Raindrop/splash Erosion


Sheet Erosion


Rill Erosion


Gully Erosion


Stream channel Erosion


Shore line Erosion

Interrill

Erosion

Soil Erosion

Water Erosion processes
-

Hillslope View

Hillel, 1998

Soil Erosion

Splash Erosion
-

Raindrop Impact


Erosive power =
S

kinetic energy

(E
K

= mv
2
/2)
and
momentum

(M = mv)


Fog ~ 0.05 mm/h


Light rain ~ 1.02 mm/h


Heavy rain ~ 15.24 mm/h


Torrential rain ~ 101.60 mm/h

P. Gary White

Hillel, 1998; Selby, 1993

Brook et al, Fig 7.2

Notes:

-

Soil aggregates destroyed

-

Saltation

-

Erosivity threshold ~25 mm/h

Soil Erosion

Surface flow induced erosion


Requires overland flow (thin surface
films or concentrated) and often is
intensified by raindrop impact



The generation of surface flow
depends on:

o
Rain intensity

o
Water content of the soil

o
Density of the soil

o
Surface roughness of the soil



SCS CN method: “Water moves
through a watershed as sheet flow,
shallow concentrated flow, open
channel flow, or some combination of
these…After a maximum of 300 feet,
sheet flow usually becomes shallow
concentrated flow”




Important in range and agricultural
systems

Soil Erosion

Overland flow


Exceeds detention storage


Begins with
detachment
: requires a force (shear)
created by small eddies in flow or raindrop impact

t
0
= f (density, water depth, slope)

Sheetflow

Gully

Rill

Soil Erosion

Rill and Gully Formation

Brooks et al., Fig 8.1

Soil Erosion

Erosion and Transport Processes

Soil Erosion

Interrill

Erosion

D
i

= K
i

i

q
S
f

C
v

D
i

=
Interrill

erosion rate (kg/m
2
-
s)

K
i

=
Interrill

erodibility

of soil (kg
-
s/m
4
) (Table 7.1)

i

= Intensity of rainfall (m/s)

q =
R
unoff rate (m/s)

S
f

=
Interrill

slope factor = 1.05


0.85 e
(
-
4sin
θ)
where
θ
= slope angle (degree)

C
v

= Cover adjustment factor (0
-

1)

(Equation
7.1)

Soil Erosion

Rill Erosion Rate



1
s
r r c
c
q
D K
T
t t
 
  
 
 
D
r

= Rill
detachment
rate (Kg/m
2
-
s)

K
r

= Rill
erodibility
, due to
shear (s/m)

𝜏

= Hydraulic
shear of flowing water (
Pa) =
γ

R S (Equation 7.3)


γ

= Specific weight of water (9810 N/m
3
)


R = Hydraulic radius of rill (m)


S = Hydraulic gradient or slope (m/m)

𝜏
c

= Critical shear below which no erosion occur (Pa)

q
s

= Rate of sediment flow in rill (Kg/m
-
s)

T
c

= Sediment transport capacity of rill (Kg/m
-
s) = B
𝜏
1.5
(
Equation
7.4)


B =
T
ransport coefficient based
on soil and water properties

generally
between 0.01 and
0.1
(dimensionless
)

(Equation
7.2)

Soil Erosion

Rill Erosion Rate


If
T
c

>
q
s

: erosion takes place


If
T
c

<
q
s

: deposition takes place



If
𝜏

>
𝜏
c

: erosion takes place


If
𝜏

<
𝜏
c

:
no erosion



1
s
r r c
c
q
D K
T
t t
 
  
 
 
: defines erosion or deposition

The
interrill

and rill erosion processes are used in several process
-
based erosion

prediction computer models, including the Water Erosion Prediction Project
(WEPP
) model

Soil Erosion

Example: Rill Erosion

A rill channel is observed at
a location
20 m
from
the top of the
hill
during
a
storm.
The
slope is
5
%,
the rill
width is
150
mm,
and the
hydraulic
radius is
estimated to be 0.01
m
,
respectively. The
sediment
transport
rate (
q
s
)
is measured and found to be 0.2
kg/m
-
s.
The rill
erodibility

(
K
r
) is estimated to be 0.004 s/m, critical
shear (
𝜏
c
)
to be 2.5
Pa, and transport coefficient (
B
) to be 0.1. Assume that the
specific
weight
of water is 9810 N/m
3
.
Calculate
the sediment
transport
capacity and
erosion rate for
the location
.
If the hillslope is 120 m long
and rainfall duration is 1 hour, how much soil will be eroded from the
hillslope by rill erosion?



𝜏

=
γ

R S
(Pa)


T
c

=
B
𝜏
1.5
(
Kg/m
-
s)




(Kg/m
2
-
s
)




1
s
r r c
c
q
D K
T
t t
 
  
 
 
𝜏

= 4.905 Pa

T
c

=
1.086 Kg/m
-
s

D
r

=
0.00785 Kg/m
2
-
s

=
0.1629 Kg/s

=
423.8 Kg

=
0.001177 Kg/m
-
s

Soil Erosion

Wind erosion


Important especially in arid regions



Dependent on

o
Wind speed and exposure

o
Soil particle
-

and aggregate sizes

o
Surface roughness

o
Tillage



Surface roughness creates turbulence in
the surface
-
near air layer →


Under pressure sucks particles in the air



Form of movement:


< 0.1 mm → Suspension (“dust storm”)


< 1 mm → Saltation (“jumping”)


> 1mm → Creep (“rolling”)

Soil Erosion