EROSION
This week we will examine harvested sites at Heiberg Foresat. Specifically, we will focus
on BMPs and their role in minimizing soil erosion in managed forests.
Soil erosion is a major problem on a worldwide basis. Pimentel 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 by Pimental et al. are alarming. Toy et al. (2002) point out that all of these
figures are imprecise and that the average erosion rates for large areas misrepresents the
mangitude of the problem.
There are 1.5 billion acres of nonfederal land in the United States. The use of this
land is changing. Since 1982, cropland and rangeland declined by
4.9 and 4.5 million ha, respectively (Toy et al., 2002). Once converted, such land usually cannot
be used for production of food and fiber 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.
A recent report by the EPA (1995) attributes agricultural as the primary source of nonpoint source
pollution.
Soil erosion is usually measured or estimated in terms of mass of soil moved per unit area
per year (ton/ac/yr or Mg/ha/yr). A ton (2000 lb) of soil is roughly equivalent
to a cubic yard (3' x 3' x 3'). To 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 long term productivity,
is estimated to be 4 or 5 ton/ac/yr 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.
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.
James Patric published an article in the February 1984 issue of the Journal of Forestry
documented that eastern forest land erodee at a rate of 0.139 tons per acre annually. The erosion
rate for Western forests (except the Pacific Coast) was 0.165 tons per acre. Pacific Coast
forests had the greatest erosion rate (3.983 tons / acre / year), reaching a maximum
value of nearly 50 tons / 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. Patric suggested
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.
Sediment delivery to surface water degrades water quality and is referred to as nonpoint source
pollution (NPSP) because the exact source cannot be identified (in contrast to an outflow pipe from
a factory referred to as point source). The primary source of sediment to surface water is
agricultural land, where soil is detached by water and sediment inputs begin as overland flow.
Bacteria, nutrients, dissolved salts, suspended solids, and toxic materials. are adsorbed to
sediment, compounding the problems caused by sediment alone.
Pollution from nonpoint sources is receiving wider attention as point sources are generally
being brought under control. Nonpoint source pollutants are generally diffuse 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.
Impacts of water erosion are illustrated below.
Here is a sandy loam soil eroded after the removal of the slash
pine cover by road construction.
Although sandy soils generally have a low erosion potential, this soil has an impermeable subsoil.
Under the intense 'gully washers' of Southern Alabama, the surface soil is quickly saturated and
flow begins, removing the upper horizons.
The decreased infiltration of the exposed subsurface
exacerbates the situation.
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 accelerated erosion.
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. 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.
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.
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
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.
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.
Erosion in Forest Recreation
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 hikers admire.
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?
Erosion in Forest Management
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 removal of trees from the site, not
from tree cutting. Roads and skid trails account for more than 95% of the erosion from harvested
sites. The collection of practices that are used to minimize erosion associated with forest management
are referred to as Best Management Practices (BMPs). New York is one of many states that have
published an illustrated field guide for installation of BMPs. Some of the potential problems associated
with harvesting are illustrated below.
Skidding of harvested material from the woods can generate sediment.
Ruts caused by the skidder create potential channels for water movement. Upon closer
examination we find evidence that water has moved some soil
in the gully created by dragging logs behind the skidder. If this is a short slope or if the flow is
interrupted by a water bar or broad based dip, no great harm is done. If it is a continuous channel
to the directly connected with the stream, NPSP would result.
Crossing streams with a skidder generates sediment and produces
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.
Planning is the first BMP. Avoiding or minimizing stream crossings reduces the potential for
sediment delivery to streams. Wet areas should be avoided
in skidtrail layout.
Timing and intensity of harvesting operations are important considerations. Partial cutting in stands
of shallow rooted trees on thin soils may result in loss of remaining crop trees from blowdown.
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 matching the harvesting system and timing
of harvest (i.e. spring vs. summer) to site conditions. Large feller forwarders are a poor choice for
operations on imperfectly drained silt loam soils.
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. Careful planning of the layout of both haul
roads and skid trails minimizes the area devoted to roads and trails.
Skidding is not the only way to bring trees out of the forest. The highlead system where butt ends
of logs are raised off the ground by a cable system reduces the degree of ground disturbance. The
skyline system where the logs are entirely lifted off the ground further reduces the area disturbed.
Balloon logging and helicopter logging further reduce site impact, but these are expensive and
can only be used profitably where the value of product extracted is high enough to justify the costs.
method.
Literature Cited
Pimental, D., C. Harvey, P, Resosudarmo, K. Sinclair, D. Kurz, M. McNair, S. Crist, L. Shpritz,
L. Fritton, R. Saffouri, and R. Blair. 1995. Environmental and economic costs of erosion
and conservation benefits. Science. 267: 1117-1123.
Toy, T.J., G.R. Foster, and K.G. Renard. 2002. Soil rosion: Processes, prediction, measurement,
and control. John Wiley and Sons, Inc. 338 p.
Link to Wetland DEC Regulations