LAB #7
Link to Wetland DEC Regulations - Useful for Field Trip Lab!
Soil erosion is a major problem on a worldwide basis. Pimental et al. (1995) cited an
estimate of 75 billion metric tons of soil removed from land by erosion worldwide. They
also cited estimates for average rates for soil loss of 17 tons/ha/yr in the US and
Europe, and 30 to 40 tons/ha/yr in Asia, Africa, and South America. Although the magnitude
of those numbers was challanged by Pierre Crosson in subsequent issue of Science, erosion
levels even half of those suggested pose very real problems. A recent report by the EPA
(1995) attributes most of the sediment to agricultural land use.
This lab exercise examines the severity of soil erosion, the mechanics involved, and how
human activity influences erosion. Soil erosion on forestland will be contrasted to that
on agricultural land. The application of the Universal Soil Loss Equation to estimation of
erosion from forestland will be to focus of this exercise.
There are 1.5 billion acres of nonfederal land in the United States. The use of this land
is changing. Cropland and forestland acreages are decreasing while pastureland and
rangeland are increasing. The acreage devoted to urban, built-up and transportation uses
is increasing at a growing rate. Since 1958, 39 million acres have been converted. Once
converted, such land usually cannot be farmed again without intensive and costly
reclamation. About one-third of the land converted each year, nearly one million acres, is
prime farmland. Cropland represents the greatest soil erosion potential. Urban areas also
have high erosion potential.
Soil erosion is usually measured or estimated in terms of tons of soil moved per acre per
year (or, in the metric world, Mg/ha/yr). A ton (2000 lb) of soil is roughly equivalent to
a cubic yard (3' x 3' x 3'). To convert convert ton/ac to Mg/ha, multiply by 2.242. A one
inch layer of soil over one acre weighs about 150 tons. Thus, a soil losing 5 tons per
acre per year would lose an inch in about 30 years. The tolerable level of soil loss,
defined as that loss that would not impair productivity and create gullies, was estimated
to be 4 or 5 tons of soil per acre annually on deep soils. Holding erosion within these
rates permits sustained cropping of the soils UNTIL total soil depth declines. While this
works temporarily if the A horizon formation occurs at one inch in 30 years, slower rates
of soil formation in the deeper layers (1 inch in 300 to 500 years) translate into loss of
soil depth. It should be mentioned that in recent decades, increases in productivity as a
result in improvement in technology have masked the effects of soil erosion on crop
yields. In the long term, the figure of 5 tons/ac/yr is excessive. We will talk more about
soil loss tolerance in lecture.
The Nation's total water erosion is about 11 percent from streambanks, 6 percent
attributable to gullies, 3 percent from roads and roadsides, and 2 percent from
construction sights. The remaining 78 percent is sheet and rill erosion, mostly from
cropland. The average annual rate of sheet and rill erosion on cropland in the United
States is 13 Mg/ha (data for 1992). More than 77 percent of the Nation's cropland is
eroding at about 5 tons or less per acre; soil loss at this rate represents only about 27
percent of all cropland erosion. On more than 90 percent of the cropland, the annual
erosion rate is 10 tons or less per acre; soil loss at this rate represents 46 percent of
all cropland erosion and 11 percent of all excessive erosion. Therefore, it is on a
relatively small acreage of cropland- the remaining 10 percent- that most cropland erosion
occurs, some at rates of 100 tons or more per acre annually. Controlling the erosion on
the most severely eroding 10 percent of the cropland would reduce total cropland erosion
by 54 percent. However, erosion would continue to gradually degrade extensive acreages of
productive cropland now eroding less dramatically, but still seriously, at 5 to 10 tons
per acre.
Using the same report as a data source for forestland, it shows that annual erosion
amounts to about 7 percent of the total 6.42 billion tons, or 0.44 billion tons from
forestland. The average forestland erosion rate per acre was estimated as 1.19 tons. About
89 percent erodes at a rate of less than 2 tons per acre annually, 7 percent at 2 - 5 tons
per acre, 3 percent at 5 - 14 tons and only 1 percent erodes at a rate of more than 14
tons annually. However, James Patric, a competent hydrologist /soil scientist, published
an article in the February 1984 issue of the Journal of Forestry which shows that actual
erosion from forestland is much lower. Based on actual nationwide sediment data, he showed
that eastern forests erode at a rate of 0.139 tons per acre annually, Western forests
(except the Pacific Coast) at 0.165 tons per acre, while the Pacific Coast forests had the
greatest erosion rate of 3.983 tons per acre per year, the latter reaching a maximum value
of nearly 50 tons per acre (high rainfall and erosive volcanic soils contribute to the
high rates). One-third of the Eastern and Western forestland observations did not exceed
0.02 ton per acre and three-quarters did not exceed 0.25 ton. He suggests that a long-term
average of not more than 0.25 ton per acre per year is a good first approximation for the
Eastern and Western forestland. Any model that estimates values larger than this should be
suspect.
It is evident that the major erosion problems occur on crop and range land. Erosion on
forestland is more obvious to the layman accustomed to the undisturbed pristine condition
of the forest, as compared to the annually-tilled cropland; hence, forestland erosion
receives undue attention as compared to cropland erosion, even though it is a problem of
much less seriousness. In some areas, like the Pacific Coast, erosion on forestland must
be dealt with. But we have the technology to address the problem.
Besides the loss of soil from the land, erosion has direct impact on water quality.
Bacteria, nutrients, dissolved salts, suspended solids, and toxic materials are the most
common and most serious nonpoint source pollutants in the Nation's waters. Agriculture is
the most widespread cause of nonpoint source pollution. This pollution comes mainly from
runoff or irrigation return flow. Runoff has carried agricultural chemicals and sediment
into once-pure rivers and lakes. The degraded water in those areas can support only small
numbers of fish and harm the animals that live near the stream.
Pollution from nonpoint sources is receiving wider attention as point sources are
generally being brought under control. Nonpoint source pollutants are generally diffused
and may be more readily assimilated by the receiving waters than are the more concentrated
pollutants from point sources. Because nonpoint source pollutants are generally released
as pulse loads during rainfall, violations of water quality standards may be intermittent
rather than continuous, and therefore difficult to identify and qualify.
Raindrop impact causes soil erosion. Raindrops range in size
from small to large, and fall with low to high intensities.
If the soil is dry, the raindrop is absorbed, and the soil becomes moist. As more
raindrops fall, they hit the soil-water surface and considerable splashing occurs. The
splash is muddy or turbid; the soil particles have been brought into suspension by the
impact of the falling raindrop. Soil aggregates are broken down or particles are detached
from the soil mass. The turbid water enters the soil, clogging the pores. The continued
impact of the raindrops compacts and seals the immediate surface forming a crust and
reducing infiltration. Raindrops falling in the ponded water create turbulence increasing
detachment, and microchannel flow brings about transport of the suspended material through
overland flow. If precipitation continues, the moving turbid water detaches more soil from
the edges of channels and rills, and where large volumes of water are moved with high
velocity, gullies are formed, especially on steep slopes. Where the velocity of the moving
water is reduced by a spreading of the water in a wide channel, or the precipitation ends,
the carrying capacity of the water is reduced and the suspended materials settle out and
become deposited. The coarsest material (sands) are deposited first. Clays may be
transported long distances that may be measured in miles, whereas sands and silts may be
deposited in the same field or at the bottom of the original slope.
Let us now examine some of the results of water erosion. Here is a sandy
loam soil eroded after the removal of the slash pine cover by road construction. Sandy
soils have a low erosion potential, but this soil has an impermeable subsoil which is
illustrated in the next slide.
Under the intense 'gully washers' of Southern Alabama, the surface soil is quickly
saturated and flow begins, removing the upper horizons.
The exposed subsoil has decreased infiltration and exasperates
the erosion condition, as shown here.
Pedestals are formed by the protected soil under the iron
concretions, illustrating the degree of erosion that recently occurred here.
Erosion has formed gullies in this fill material. The slope
angle exceeds the natural angle of repose for this undisturbed glacial till, resulting in
some movement in addition to the erosion of the bare surface.
Here the yard is slowly eroding away. It is true that
vegetative cover will decrease soil erosion, but it has little effect on an unstable
slope.
The water accumulates into larger channels and larger gullies. The water is downcutting into the fill more rapidly than the removal of the
shoulders in the gully. If left unchecked, the gully will widen.
This forest tree nursery shows erosion of bare soil between the raised
seedbeds. The mechanical channelization enhances erosion and the water course should be
interrupted by some means.
Gully erosion in windblown material, or loess, can be severe in
the southeastern United States such as at this site in Tennessee.
Here is similar erosion but in weathered chert soils. Though
gravelly, these soils can erode, too.
This is probably the epitome of a gullied landscape; hence, the Badlands
of South Dakota near Rapid City. These sediments are highly erodible under the very
intense but sporadic thunderstorms of the region.
The eroding away of this once flat terrain or tableland is
exhibited here. This is natural erosion. The earlier slides illustrated what is termed
accelerated erosion- that erosion which is greater than geologic or natural erosion and is
caused by human activity on the landscape.
Earlier I had mentioned the condition of unstable slopes. This phenomenon contributes to
erosion by the transport of material from hillslopes to water channels. It is a slow,
insidious process, but significant in the long term.
Here, soil is slumping on a small scale- a person's backyard.
The longitudinal fissures have formed in deep glacial till fed by a spring, and the slope
has been undercut to provide a level area for commercial
development .
These fissures have formed in deep glacial till fed by a
spring, and the slope has been undercut to provide a level area for commercial
development, as shown in the next slide. The soil is both eroding and slumping into the
rear of this building. The slumping occurs when the soil is
saturated. Water concentrates at the back of the building creating a hydraulic head, which
forces its way through the concrete-block wall. Obviously, not the best condition for any
dry building.
The sliding scurrfis grass results from the trampling,
compaction, and erosion of the vegetation and soil, with channelization of water down the
trail. The fertilization and seeding is an attempt to establish more rugged non-native
vegetation to hold the remaining soil and native vegetation in place. Eventually, it is
hoped that the native vegetation can re-establish itself.
This is an example of the lower portion of a debris avalanche
on Whiteface Mountain near Lake Placid. The mass movement was triggered by an intense
thunderstorm on Labor Day, 1971, dumping 6 inches of rain in a half-hour.
The impermeable anorthosite bedrock is exposed beneath the
shallow soil. It acted as a lubricated slide surface.
The view from the top shows the pathway of the avalanche and
how it blocked the Memorial Highway in two places. A much larger slide occurred on the
east slope above Whiteface Ski Area, knocking out a chair lift tower, a warming hut, and
burying the administration building and parking lot under a thick layer of sand, mud,
boulders, and trees. Fortunately, no one was injured. However, the danger of avalanches
persists in the Olympic Area.
Slide areas are also extensive in the White Mountains of New
Hampshire as shown here.
Mass movement and the creation of slump conditions are hazards encountered on unstable
slopes during road construction and become a major engineering concern.
Wind erosion occurs by the same basic mechanics as water erosion. Detachment must occur,
followed by transport and deposition. However, the material carried by wind is finer-
mostly the fine sands and silts. Clay soils do not generally blow as they have a high
resistance to detachment. Obviously, it occurs on bare soil only but may override
vegetated soils.
This is an example of sand tailings from mine operations being
blown after deposition by hydraulic means. Attempts are now being made to establish
vegetation on these tailings.
When Sphagnum moss dries on freezing, then becomes crushed by
hikers boots, it is very susceptible to blowing. The amount of erosion on the anorthesite
rock is obvious from the light color of the newly exposed rock.
This overview gives a general idea of the very shallow, exposed and fragile ecosystem that
exists on East Peak of Colden Mountain.
This area of Rocky Mountain National Park, along the Trail
Ridge Road, is exposed to the winds for much of the winter (the road is not usually snow
covered). As a result, the wind kills the vegetation on the leading edge, drying it and it
becomes light enough that the wind can lift it as shown here. The gravelly bed in the
foreground has had its alpine tundra vegetation blown away.
Wind erosion on agricultural soils can be catastrophic. The Dust Bowl of the 1930's is
most famous. Recent dust bowl storms carried material from the Redlands of Texas all the
way to Cincinnati, Ohio- a distance of some 2,000 miles! Fortunately, wind erosion does
not occur on forestland except when site preparation procedures are followed for even-age
artificial reproduction. And then, it occurs only once a rotation, or every 30 to 60
years. The amount of erosion is minor because of the extreme roughness and presence of
debris following site preparation .
Several earlier slides showed erosion on hiking trails. These trails have the tendency to
follow easy access routes and traverse steep slopes, collecting and channeling water. The
soil on the trail is bare and has become compacted, enhancing the erosion potential. The
trails become deepened and difficult for hiking, and footholds become hazardous. During
rainy weather they become nearly impossible for safe passage. Further, the material is
washed directly into the very streams the hiker seeks to admire and gain refreshment from.
Slide 27 is a good example. Here soil from the bare, compacted
path is being eroded into Hermit Lake at the bowl of Tuckerman's Ravine on Mount
Washington, New Hampshire. Not only is the soil filling in the lake bank, but the influx
of nutrients is causing the eutrophication of the lake. Other sources of pollution are
probably involved as well.
This trail condition exists on the top of Crane Mountain in the
southeastern Adirondacks. Though not a popular area, it still exhibits the symptoms of
overuse.
This trail is certainly what would be considered a popular area: the Cowlitz Glacier Trail to Camp Muir on the southwestern slope of
Mt. Rainier, Washington. Death of the vegetation, soil compaction, and subsequent erosion
are all in evidence here. The symptoms of too many people at the wrong time. What do you
think might be several solutions to this problem?
Natural erosion from forestland is due to the slow and steady mass movement of soil to
streambanks where it is eroded by the moving waters, especially during the full flow of
spring runoff and snowmelt. Accelerated erosion is caused by road construction through the
mishandling of road surface runoff and drainageway design. Several effective engineering
control measures are available for slope stabilization, but as a rule, they are expensive
and generally applicable to specific occurrences. Current erosion control methods on
forestland in the West have been directed toward identification and characterization of
unstable ground, avoidance of disturbances on slope erosion processes. In the East, the
problem is handled adequately by well-designed roads and drainage systems since the
terrain here is generally less rugged. It is when no design consideration is given to
roads that we get into trouble.
The skidding of harvested material from the woods also can be a
potential erosion hazard. Ruts are formed in the soil by the tires of the skidder,
creating potential channels for water movement. Upon closer examination we find evidence
that water has already moved some soil in the gully formed by
the dragging of the tail-end of the logs behind the skidder. If this is a short slope or
if the flow is interrupted by a decrease in slope or a depression, no great harm is done.
If it is a continuous channel to the drainage way, then we could expect problems of water
pollution.
When we cross streams during the skidding operation we can
expect to find turbid water. Downstream
the brook enters a trout stream. The sediment on the right side of the stream, gradually
becomes dispersed as the distance from the brook increases.
Another area to avoid in skidtrail layout is a wet area. It
would not take very long for this area to become a 'hog wallow', greatly disturbing the
soil and vegetation, but being a hazard for the equipment. Getting a skidder or dozer hung
up can be costly in lost time.
There are two means by which forest managers can minimize erosion damage. Careful
selection of the silviculture system is one. If erosion hazard is unusually high, the
optimum decision may be to forego logging and intensive management and instead, practice
protective management. The manager must select a silvicultural approach which is suitable
to the site. The options available might range from light thinning to clearcutting and
planting. The shelterwood system may provide the surface soil protection and maintenance
of anchoring/stabilizing root systems necessary in unstable areas.
These deep pumice pumice soils of Oregon are examples of
unstable areas where shelterwood or patch clearcuts may be more appropriate.
These patch clearcuts about 30 acres each in Douglas-fir stands
are located on the Gifford Pinchot National Forest in Washington, not too far from Mt. St.
Helens.
A second means of minimizing erosion damage is by careful selection of the logging system.
Tractor or skidder movement of logs drags them over the surface, compacting the soil and
laying bare large areas of the soil surface. Results of many studies show that about 25 to
35 percent of the harvested area is disturbed by tractor logging. The highlead system
where butt ends of the logs are raised off the ground by a cable system reduces the degree
of compaction and the area disturbed to about 15 percent of the area. The skyline system
where the logs are entirely lifted off the ground reduces the area disturbed still
further, to about 12 percent of the area. Balloon logging reduces the area to about 6
percent, and helicopter logging to about 2 percent of the area and virtually no
compaction. It must be remembered, however that helicopter logging is the most expensive
method. No balloon, helicopter or skyline logging is done in the east or south. Some Cable
logging is done in West Virginia and in southern swamps.
Believe it or not, horse logging is seeing a revival. This system has the least impact of
all, but it is very difficult to find a good teamster these days. There are some
landowners in Vermont that will allow only horse-logging on their property.
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