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 process. 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 infiltration, runoff
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 profile, usually measured as a rate
as well.
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 indeed “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.
http://www.ent.iastate.edu/imagegal/practices/tillage/conventional/erosion.html
U.S.G.S. Public Affairs Office, Menlo Park, CA.
U.S.G.S Public
Affairs Office Menlo Park, California
Infiltration rate is lowest for clay particles in a massive structure on a steep slope. Clay
profiles are characterized by a larger volume of pore space than sand, but 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, etc.) 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
period.
Clay or sand are soil textures; soil structure 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 individual pores and allow more rapid infiltration.
http://nesoil.com/gloss.htm
blocky structure of subsoil
http://www.evsc.virginia.edu/~alm7d/soils/images/images1.html
There are various equations for fluid flux through a permeable material; Darcy’s law is
probably best known:
q = -k (Pb – Pa)/ 
where q is fluid flux, k is permeability, Pb is pressure at base of fluid front, Pa is pressure
at fluid head and  is viscosity of the fluid. Flux is measured as m3/m2/s, which reduces
to m/s.
There are other equations which consider additional criteria, but all are dependent on
permeability. One equation for hydraulic conductivity, which is similar 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 technically can vary with conditions of flow while
permeability is a property of the soil itself.
Permeability can be calculated with an equation:
k = C d2 where k is permeability, C is a configuration constant and d is average pore
diameter. Configuration 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*
2.00 - 1.00
Coarse sand
1.00 - 0.50
Medium sand
0.50 - 0.25
Fine sand
0.25 - 0.10
Very fine sand
0.10 - 0.05
Silt
0.05 - 0.002
Clay
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., 2.00-0.05 mm.
edis.ifas.ufl.edu/SS169
We are going to equate infiltration with permeability 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
water.
In the following laboratory, we will measure permeability for two soils- a clay and a
sand- and look at permeability for these soils on a slope.
Laboratory procedure:
1. 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.
2. 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.
3. Place another empty canister (without holes) below the soil canister (diagram A).
4. 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
occurs.
5. 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.
6. For soil on a slope: Remember the faucet 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.
7. Repeat the same procedure as for setup A (at the same flow rate) and collect
leached water in the lower canister for 10 minutes. Calculate permeability in
cm/min.
8. 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 height 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.)
9. 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 pinpointing
necessary flow rate.
Using a protractor to measure slope
Questions:
1.
List permeability for the following:
Sand, flat: 0.1 cm/min
Pipetting runoff from soil at 10-degree incline
Sand, tilted: 0.06 cm/min
Clay, flat: 0.02 cm/min
Clay, tilted: 0.005 cm/min
Clay, tilted, with vegetation:
2. 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.
Which soil tolerated a higher rate of “precipitation” without ponding?
sand
3. For the same angle, which type of soil produced more runoff, sand or clay?
clay
4. What was the nature of the runoff water? (Did it contain soil, etc.)
Contained small particles of soil
5. See if the following relationship tentatively worked out in class is a good
predictor of runoff rate in cm/min:
sinx(permeability rate on 0-degree slope)
Where:
 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
6.
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 20 degrees.
a. Determine the velocity of this particle of water at the bottom of the
incline.
b. How much time will it take the water to reach the bottom of the slope?
7. a. For a sphere of water of 0.0042 cm3, calculate its mass and gravitational force
it possesses on this slope.
0.0042 g;
F = 0.0042 x 3.35 = 0.014 N
a. 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/cm3. 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 reaches the
bottom:
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.
Ff= Fn
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
2.65 g/cm3 = 0.00147 g/x x = 0.000556 cm3
Vol of sphere = 0.000556 = 4/3 r3 r = 5.1 x 10-2 cm;
Medium sand
Relating sedimentation 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 erosion 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
sedimentation source:
Image of the Ganges River delta and
the Bay of Bengal acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS). This image
shows the massive amount of sediments delivered to the Bay of Bengal by the Ganges River, sediments
that are derived from erosion of the Himalayan mountain range to the north. Click on this image to see a
large high-resolution version that includes the Himalayan range. Mt. Everest, the highest point in the
world, is located in the upper right corner of the high-resolution image.
http://daac.gsfc.nasa.gov/oceancolor/scifocus/oceanColor/sedimentia.shtml
SeaWiFS image of the U.S.
East Coast acquired one week after the passage of Hurricane Floyd (see image
below). The sediments generated by the flood waters of rivers in North Carolina are
seen entering the Gulf Stream off of Cape Hatteras. Also note the increased
turbidity in the sounds and river estuaries and persistent sediment suspension
southward along the coast.
http://daac.gsfc.nasa.gov/oceancolor/scifocus/oceanColor/sedimentia.shtml
SeaWiFS image of Italy and the
Adriatic Sea. The Balkans to the west and the snow-covered Alps to the north are also visible. The Po
River valley is the hazy brown area just south of the Alps. The plume of sediments carried by the Po River
is seen on the western side of the far northern Adriatic Sea.
http://daac.gsfc.nasa.gov/oceancolor/scifocus/oceanColor/sedimentia.shtml
A large sediment plume enters the Mozambique Channel
south of the resort town of Beira. (Satellite photo courtesy NASA)
http://www.star.le.ac.uk/edu/Probes.shtml
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-blown debris.
http://www.msmedia.homestead.com
Assignment:
1. Find erosion statistics for North Dakota: How much soil is lost per year by water erosion and by
wind erosion? What is the tolerable limit set forth by the USDA?
Source: USDA-NRCS
Most sources list 5 tons/acre per year as the tolerable limit.
2.
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.
The following images show Red River flooding in 1997; although only water levels are shown, the amount
of sedimentation from the event can be surmised.
March 1997 before flood
Red is snow cover; yellow is cloud cover
April 1997 during flood
May 1997 after flood
www.math.montana.edu/.../rrf/flood_pics.html