Sedimentation and Remote Sensing
Introduction: A certain amount of released earth materials into water or the atmosphere
is a natural occurrence; however, excessive sedimentation is of concern when
environmental or commercial problems arise from the pr
ocess. It is also of concern when
the sedimentation is clearly the result of erosion, possibly due to urbanization, poor
farming practices and industries such as mining.
The tie to physical science is in the physics of soil processes such as infiltrati
and permeability. Infiltration is the rate of entry of precipitation into the surface of soil,
runoff is the percentage of precipitation which does not enter the soil and permeability is
the ease with which water can travel through the soil pr
ofile, usually measured as a rate
Infiltration rate is dependent upon several factors which are difficult to quantify,
including soil type (textural), soil structure and slope. There is a dual nature to soil on a
gradient and all of it is inde
ed “on a slippery slope.” If infiltration is not sufficient,
runoff occurs and pulls soil particles with it in a steady stream. However, if infiltration is
too great, massive earth movement called mudslides can occur.
blic Affairs Office, Menlo Park, CA.
Affairs Office Menlo Park, California
Infiltration rate is lowest for clay particles in a massive structure on a steep slope.
profiles are characterized by a larger volume of pore space than
ut all pores are
much smaller than those for sand. Movement through the soil is not due to a
gravitational gradient (as in sand) but is due to electrostatic attraction between hydrogen
in water and electronegative elements in the soil (oxygen, silicon, et
c.) The overall
process is slow and there is usually not sufficient time for absorption due to rapid water
flow on a steep grade unless there is sufficient vegetation to trap water for a longer time
Clay or sand are soil textures; soil structur
e consists of a secondary organization of soil
particles into “shapes” such as granular, cubic, columnar or one large mass (massive).
The advantage of cubic or columnar shapes for infiltration is that channels exist in the
soil which are larger than the i
ndividual pores and allow more rapid infiltration.
blocky structure of subsoil
There are various equations for fluid flux through a permeable material; Darcy’s law is
probably best known:
where q is fluid flux, k is permeability, Pb is pressure at base of fluid front, Pa is pressure
luid head and
is viscosity of the fluid. Flux is measured as m
/s, which reduces
There are other equations which consider additional criteria, but all are dependent on
permeability. One equation for hydraulic conductivity, which is simi
lar to Darcy’s is:
K = k
where K is hydraulic conductivity, k is permeability,
is specific weight of water, and
is viscosity of water. For “standard” conditions, hydraulic conductivity is very close to
permeability. Hydraulic conductivity te
chnically can vary with conditions of flow while
permeability is a property of the soil itself.
Permeability can be calculated with an equation:
k = C d
where k is permeability, C is a configuration constant and d is average pore
ration constants can be estimated from texture and structure, but
usually permeability is best measured empirically.
Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system.
Name of soil separate
Diameter limits (mm)
Very coarse sand*
Very fine sand
less than 0.002
* Note that the sand separate is split into
five sizes (very coarse sand, coarse sand, etc.). The size range for sands,
considered broadly, comprises the entire range from very coarse sand to very fine sand, i.e., 2.00
We are going to equate infiltration with pe
rmeability although they are technically
different. Infiltration is dependent on permeability but also soil surface conditions.
Permeability is typically measured under saturated flow conditions, which we will
emulate in the laboratory, but infiltration
rate can vary widely due to incoming rate of
In the following laboratory, we will measure permeability for two soils
a clay and a
and look at permeability for these soils on a slope.
Set up a canister with holes
on the bottom, lined with filter paper (thin) and fill
with sand about the one
fourth of the canister height.
Position the canister on a ringstand or other upright apparatus and clamp a hose or
buret above the canister. A ring with a wire gauze between
the hose and soil will
help disperse the water over the soil.
Place another empty canister (without holes) below the soil canister (diagram A).
Slowly saturate the soil, then one student must adjust the faucet or buret flow until
the rate produces ponding
water and then back off to a flow where no ponding
Measure the leached water height in cm after about ten minutes and then divide
this value by 10 to get permeability flow rate in cm/min.
For soil on a slope: Remember the fauce
t speed used in setup A and use this in
setup B. The only difference here is that the soil canister will be put on a slope of
about 10 degrees.
Repeat the same procedure as for setup A (at the same flow rate) and collect
leached water in the lower caniste
r for 10 minutes. Calculate permeability in
Note rate of runoff as well. After 10 minutes, collect the water accumulated on
the downward side of the surface with a pipet and place this into a canister of the
same size as the others and note heigh
t in cm. Divide this by 10 to get runoff rate
in cm/min. (If there is no runoff, increase the steepness of the soil until there is
runoff and run the experiment again with measurements. Note the angle of
inclination in your notebook.)
Repeat the entire
process for a clay soil. If there is time, repeat the process for
clay with plants “planted” in the soil.
A slightly loose hose dispersed water well
A buret was somewhat more precise in
Using a protractor to measure slope
runoff from soil at 10
List permeability for the following:
Clay, tilted, with vegetation:
The rate of
flow for the flat soils actually represents infiltration rate which is just
below the ponding rate. Any type of ponding is considered to be potential runoff
and erosive even when on a “flat” surface, due to imperfections in terrain.
tolerated a higher rate of “precipitation” without ponding?
For the same angle, which type of soil produced more runoff, sand or clay?
What was the nature of the runoff water? (Did it contain soil, etc.)
l particles of soil
See if the following relationship tentatively worked out in class is a good
predictor of runoff rate in cm/min:
permeability rate on 0
is the angle of slope for the canister
Permeability is the rate at 0
degree slope in cm/min
For sand at no slope, permeability was 0.1 cm/min
Calculated runoff for sand at 10
degree slope: Sin(10) x
0.1 = 0.017 cm/min
Actual runoff rate for sand at 10
degree slope: 0.016 cm/min
a. Determine the gravitational acceleration on a discrete particle of water
at the top of a slope which is 14.7 m long at an inclination of
Determine the velocity of this particle of water at the bottom of the
How much time will it take the water to reach the bottom of the slope?
a. For a sphere of water of 0.0042 cm
, calculate its mass and gravitational force
sesses on this slope.
F = 0.0042 x 3.35 = 0.014 N
The sphere of water will only be able to move a soil sphere of equal size
or smaller (due to contact). Assume a coefficient of friction for the soil
sphere of 0.9 and a density of 2.65 g/cm
. What is the radius of the largest
sphere the water will move? What classification is it? (sand, silt, clay).
Assume the gravitational force Fp of the water on this slope is translated
into the lesser horizontal force
Fh = Fp (cos 20)
once it reache
Fp = 0.014 N
Fh = 0.013 N
Assume Fh is equal to frictional force Ff to produce movement of constant
velocity (not acceleration). This corresponds to the largest soil particle
which can be moved.
0.013 = 0.9Fn
where Fn is the weight of the soil particle
Fn = 0.0144 N
The mass of the particle =
0.0144 N/9.8 =
= 0.00147 g/x x = 0.000556 cm
Vol of sphere = 0.000556 = 4/3
r = 5.1 x 10
ion to remote sensing:
Sedimentation can benefit agriculture by depositing nutrients on flood plains and
extending delta land, but also costs humans in terms of flood damage, waterway
clogging, poor water quality, and recreational site damage. Of late
it is of increased
concern due to effects on environments which are fragile: estuaries, wetlands, coral reefs
and continental shelves.
Erosion is increased soil loss and sedimentation due to poor supervision of human
activities. The main causes of erosi
on include lack of vegetation on agricultural land,
overgrazing, deforestation and mining operations.
View the following satellite images and see if you can identify the location and find the
A large sediment plume enters the Mozambique Channel
south of the resort town of Beira.
(Satellite photo courtesy
Conjectured water channels on the red planet.
Here are a few interesting pictures of wind erosion as well:
See if you can identify the
location and the extent of wind
Find erosion statistics for North Dakota: How much soil is lost per year by water erosion and b
wind erosion? What is the tolerable limit set forth by the USDA?
Find satellite images of sedimentation in rivers in the Midwest (North Dakota if possible.) The
1997 Red River flood might be a good case study, if satellite images are available.