Transcript Chapter 13

Chapter 13
Erosion and Sedimentation
Soil Erosion
• The biggest threat to agricultural and
forestry production worldwide.
• Soil is the basis of much of the wealth on
this planet;
– if we don't take care of it - treat it as a
renewable resource, rather than use it up as we
are doing now - there may be difficult problems
with soil productivity in the future.
Soil Loss in the United States
Each dot represents 250,000 tons. Total US soil loss in 1997 was 2 billion tons.
The worst erosion occurs in the Mississippi Valley and the Midwestern corn belt.
These areas have silty soils, rolling topography, and intensive farming.
Plato on Soil Erosion - 400 BC
“The soil which kept breaking away from the highlands keeps continually sliding
away and disappearing into the sea. What now remains, compared with what
existed earlier, is like the skeleton of a sick man, all the fat and soft earth having
wasted away and only the base framework of the land being left.
“What are now mountains were lofty soil-clad hills; the stony plains of the present
day were full of rich soil, the mountains were heavily wooded - a fact of which
there are still visible traces. There are mountains in Attica which can support
nothing but bees but which once were clothed, not so very long ago, with fine
trees suitable for roofing the largest buildings - and roofs hewn from the timber
are still in existence. The country produced boundless pastures for cattle.
“The annual supply of rainfall was not lost, as it is at present, through being allowed
to flow over the denuded surface into the sea, but was received by the country,
into her bosom, where she stored it in her impervious clay and so was able to
discharge the drainage of the heights into the hollows in the form of springs and
rivers with an abundant volume and a wide territorial distribution. The shrines that
survive to the present day on the sites of extinct water supplies are evidence for
the correctness of my present hypothesis.”
• Piedmont streams have not always run red
and brown from clay.
• The Southeast suffered tremendous erosion
losses during the cotton era (1830-1930).
• Up to 12 inches of soil was lost from many
areas, especially in the Piedmont.
• Much of this soil ended up in the streams,
rivers, and valley bottoms of the Piedmont.
• The effects of this sediment in the river
systems are still evident today.
Average annual loads of
suspended sediment carried
by rivers of Atlantic drainage
of the United States during
years near 1910 and 1970
Soil Erosion in the Southeastern Piedmont
Level of Protection
High
Moderate
Low
Very Low
< 25 mg/L
25 - 80
80 - 200
> 200
Relationship Between Soil
Erosion and Crop Productivity
• Georgia Soil and Water Conservation Commission
– Formed to protect, conserve and improve the soil and water
resources of the State of Georgia. The Commission's goal is to
make Georgia a better place for its citizens through the wise use
and protection of basic soil and water resources and to achieve
practical water quality goals.
• Georgia Forestry Commission
– Provides leadership, service, and education in protection,
management, and wise use of Georgia's forest resources.
• U.S. Natural Resources Conservation Service
– Provides leadership in a partnership effort to help people conserve,
maintain, and improve our natural resources and environment.
• U.N. Food and Agriculture Organization
– Has a mandate to raise levels of nutrition and standards of living,
to improve agricultural productivity, and to better the condition of
rural populations.
Suspended sediment in three Georgia rivers
Wind Erosion
• Suspension
– When very fine particles, silt and clay, are
picked up by the wind and carried in the
atmosphere.
– These particles essentially float on the wind and
are carried high in the atmosphere.
– They may be deposited hundreds and even
thousands of miles away from where they were
picked up.
– Deposition areas of wind blown soils may
eventually build up layers of loess soils.
• Saltation
– The bouncing of medium and fine sand over the
ground surface, usually about 0.5 to 3 feet in
the air.
– When the particles fall back to the ground, their
impact lifts other particles which begin to
saltate.
– Because of these chain reactions, saltation
becomes more severe the longer the high winds
blow.
– If you have ever been to the beach on a day
with strong winds, you have probably
experienced saltation of stinging sand.
Saltation of Sand Particles in Wind Erosion
• Creep
– The rolling of coarse sands along the ground
surface.
– Creep is responsible for the formation and
movement of sand dunes in bare deserts.
– Creeping soils can be trapped with soil fences,
and the fences at the beach are meant to hold
sand on the dunes.
Downwind Effect of a Windbreak
Fluvial Erosion
• Raindrop Impact
– Causes detachment of fine particles from soil
aggregates, and it is also the initiator of
transport.
– Most energy is transferred rapidly to soil
particles when the raindrop crashes into the
ground.
– Raindrop impact is the major detaching
mechanism on bare soils.
• Concentrated Flow
– If the rainfall rate exceeds the infiltration rate,
surface flow will commence over the soil
surface.
– This runoff collects in micro-depressions and
forms channels.
– These small channels then merge into larger
channels which causes both detachment and
transport of soil particles.
• Sheet Erosion
– Movement of the soil surface that does not
involve channel flow.
– Sheet erosion mostly consists of soil
detachment from raindrop impact.
– Subsequent transport is caused by raindrop
splash and a very thin layer of overland flow.
– Sheet erosion uniformly removes soil from a
planar area, and it causes relatively low rates of
erosion.
– The thin film of flow delivers sediment to rills.
• Rill Erosion
– When the contributing area becomes large enough,
the thin layer of overland flow starts to cut small
channels (1-6" deep), called rills or rilles, into the
soil surface.
– Rills are formed when the velocity of the flow on the
soil is large enough to create shear stresses sufficient
to detach and entrain soil particles.
– Rills transport the sediment dislodged by sheet
erosion and carry it off the eroding surface.
– Rills also pick up and transport additional sediment
from the walls and bottoms of the rills themselves.
• Gully Erosion
– Rill erosion on a larger scale, gullies can
become enormous - an example is Providence
Canyon in Southwest Georgia.
– The basic definition of a gully is a rill that is
too deep to cross with farm machinery.
– One way they form is when rills come together
and concentrate even more flow
– A second way is when ground water seeps out
near a spring and washes out a channel below
the spring. Streambanks and stream bottoms
also erode during high flows due to the shear
stress of fast moving water.
• Point-Source Discharge
– Water discharged into a stream from a pipe or structure,
usually associated with a city or industry.
• Nonpoint-Source Discharge
– Water discharged over a wide area, not coming from a
pipe, usually associated with farms, homes, forests, etc.
• Detachment
– The removal of fine particles from aggregates. This is a
necessary step in erosion because the aggregates are too
big to move.
• Transport
– After detachment has occurred, transport is the
movement of detached particles off the source area
(field, construction project, bare-soiled clearcut) and,
eventually to surface waters.
Channel Erosion
• Erosion rates are usually expressed as
inches of topsoil per year or tons per acre
per year.
– An acre-furrow slice weighs two million
pounds if the soil bulk density is 1.4 kg/L.
– Erosion rates of 10-50 t/ac/yr are common on
steep, cleared lands, and this translates into a
loss of 1 inch of topsoil in 3-15 years.
– In other words, the field loses the Ap layer in
20-100 years.
– A tolerable rate of erosion, according to the
NRCS, is 3-5 t/ac/yr, which is the approximate
rate of new topsoil formation (B horizon
turning into A with humus addition ).
Universal Soil Loss Equation
• A = R · K · LS · C · P
• R = Rainfall Erosivity Factor
– A combination measure of climate factors such
as typical rainfall intensities, probability of
extended periods of wet weather, and types of
precipitation (convective, cyclonic, snow, etc.).
The USDA developed maps of R values around
the country.
Rainfall Factor
• K = Soil Erodibility Factor
– Accounts for factors such as texture, organic
content, and aggregate stability. The Soil
Survey maps list the K factors for each soil.
• LS = Length-Slope) Factor
– Accounts for both the length and steepness of
the slopes. Erosion increases as the slope length
increases because the depth and velocity of
water increases. Erosion also increases as the
slope gradient (steepness increases) because
overland flow moves faster on steeper slopes.
• C = Cropping Factor
– Accounts for the type of vegetative cover. C
factors are very low for forests and very high
for bare soils.
• P = Conservation Practice Factor
– Accounts for any soil conservation measures
applied to the land to reduce erosion rates.
Types of Sediment Measurements
• Turbidity
– A measure of the clarity of the water sample.
– Increasing turbidity is an indication of
dissolved or suspended solids present in the
water column.
– Substances which increase turbidity include
particles of suspended sand, silt or clay, organic
substances, coagulated organic colloids
containing iron and aluminum hydroxides, and
microorganisms including phytoplankton and
zooplankton.
• NTU vs. JTU
– Before the advent of modern light scattering
devices, turbidity was measured using the
Jackson candle turbidimeter in Jackson
Turbidity Units (JTU).
– The NTU measure is not exactly equivalent to
the JTU, but is approximately the same, i.e., 40
NTU  40 JTU.
– Turbidity can be determined from grab samples
using Hellig, Hach or Askania turbidimeters.
• Secchi disk
– Used to determine the optical clarity of deep
water bodies such as lakes, reservoirs, estuaries
and oceans.
– A standard disk, generally 20 cm in diameter, is
lowered by rope to a depth where it is no longer
visible and raised until the disk is discernable.
• Suspended Solids Concentration
– Suspended solids are mineral and organic
particles supported by turbulence within the
fluid column.
– The total suspended solids concentration is
determined by extracting and weighing the
suspended solids, reported in units of mass per
unit volume, typically milligrams per liter
(mg/L).
• Hydrometer
– Used to measure the density of the sediment
solution.
– The density, or specific weight, increases as the
sediment concentration increases.
– Because the larger particles settle very quickly,
only the smaller particle classes can be
successfully determined.
– An additional problem with the hydrometer
method results assuming the particle density for
the suspended sediment fraction.
• Bedload Transport
– Bedload solids are those sediments that are
transported along or near the bed of a stream.
– These sediments are generally larger than
suspended solids and either roll or bounce
along the stream bed.
– Bedload solids may comprise the bulk of the
total load transported by the stream because of
their high concentrations (generally higher than
10 g/L, and frequently higher than 100 mg/L).
Measuring Water Erosion
• Sampling method
– grab samples
– automated samplers
• Effect of location
– depth, bank, bend
• Effect of stage
– low vs high
• Comparing turbidity to suspended solids
Rising
Stage
Sampler
Sediment
Sampling
Tool
Deadfall
Sampler
Coshocton Sampler
Preventing Soil Erosion
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Vegetative cover
Surface stabilization
Velocity reduction
Peak flow reduction
Inspection and maintenance
General
Terrace
Design
Types of Graded Terraces
Forest Management
Chap 13 Quiz
1. The streams most commonly degraded in Georgia by sediment today
are: (choose one)
a. agriculture
b. forestry
c. urban
d. mountain
2. Why does Georgia have such high erosion? (choose any/all/none)
a. steep slopes b. erodible soils c. intense rainfall d. land-disturbing activities
3. Match:
a. Suspended Solids
b. Turbidity
c. Bedload
d. Secchi Depth
_____ Clarity of lake water
_____ Sands on streambed
_____ Filterable solids
_____Clarity of river water
4. Give two reasons why we are concerned about erosion:
5. (True/False) Soil mulch and seeding is less effective than silt fences
and hay bales for preventing erosion.