Causes and effects of soil loss
Soil loss is caused by a variety of factors including erosion from wind and water, mechanical tilling, logging, agricultural practices, and poor water management. Erosion and sedimentation, the major effects of soil loss, are widespread and can be devastating.
Erosion is a natural process on hill slopes. The rate of erosion is determined by several factors including soil type, rainfall, and length and percent of slope. Generally, human induced changes in the landscape lead to higher levels of erosion than would occur naturally. While there are many ways to minimize erosion, vegetative cover is the most effective over the long term. When vegetation is removed, the rate of soil erosion is greatly accelerated, often beyond sustainable levels.
Eroded soil deposited down slope is referred to as sedimentation. When severe rain or wind events occur or soils are disturbed by human activities, soils are moved off site and deposited on land, in lakes, wetlands, and streams. Sediment, the single largest non-point source pollutant, contributes to the decline of surface water quality, imperils aquatic wildlife, and leads to increased stream bank erosion and flooding. Levels of sedimentation increase due to roads; residential and commercial development; agricultural practices; timber harvesting; and any other land disturbing activity.
Soil Loss and Erosion
The Sedimentation Control Act of 1973 requires operators to implement short and long-term mitigation measures to reduce erosion on- and off-site. Forest landowners who wish to harvest trees are exempt from these regulations if they comply with forestry practice guidelines, which include erosion control measures and mitigation. Best management practices (BMPs) are voluntary practices which reduce sources of sedimentation and runoff, confine sediment on site, and trap the movement of sediment so that it settles. Although these practices are not required by law, it is estimated that about 85-90% of landowners and loggers voluntarily comply with BMPs and regulations. If loggers/landowners are found to be out of compliance, the North Carolina Division of Forest Resources (NCDFR) works with operators to correct problems. If not resolved, the operator may forfeit their exemption under the Sedimentation Control Act and must seek a permit to continue the activity.
Forest road erosion
Forest road erosion and eventual sediment delivery to nearby streams largely depends on soils, climate, traffic intensity and topography. Erosion rates from forested roads range from about 0.5 to 100 tons per acre per year, while the geologic or natural erosion rate is estimated at 0.1 tons per acre per year. A sustainable range of erosion from disturbance is estimated at 0.4 to 2.0 tons per acre per year. Sediment control can be significantly improved through appropriate road location, drainage systems and re-establishment of vegetation. These practices effectively trap eroded road sediments and isolate or essentially disconnect roads from stream systems.
In the Southern Appalachians, cut slopes (as opposed to fill slopes) and road beds have been found to account for as much as 75-80 percent of soil loss from the road area, the majority of which occurs during the establishment period for vegetation. These rates decrease significantly, however, following complete reestablishment of vegetative cover.
Steep Slopes and Development
Because building construction was easier and less costly, and crop cultivation better in the fertile valleys, human development historically was limited to broad basins, terraces, and flood plains. However, as land became scarcer in the valleys and the demand for uninterrupted mountain views increased, residents slowly began building uphill toward the intermediate and even high mountains. This preference has led to development on steep slopes with few regulations in place to protect those located down slope from soil erosion and landslides.
Soil Subsidence and Landslides
Causes and triggers of soil and debris slides are omnipresent in the region. Causes are those conditions leading to instability of slope, while triggers are those events that initiate slope movement. Slides are influenced by geology, geomorphology, weather, climate and slope. Mountain side disturbances can result in embankments or slope angles that are too steep or too high and, therefore, inherently unstable. The most common trigger of slides in the region is frequent or high intensity storms. Other triggers include repeated freezing and thawing of the soil, blasting, and earthquakes. Landslides present an ever present danger in the region. Their likelihood is increased by weather events, vegetation removal, and poorly designed access.
Regional soils which are prone to landslides are those high in mica (>30%), especially when used in embankments, as mica is difficult to compact. Most debris flows, however, initiate in soil types made up predominantly of silt and sand, often with gravel to cobble size stones. An increase in water content increases the pore-water pressure which causes a decrease in shear strength. Hence, these soils are more prone to move downhill, especially after large rain events. When a landslide occurs, debris is moved downhill quickly, and includes anything generally larger than coarse sand, often destroying whatever is in its way and moving those materials downhill with it.
Sulfidic rock is common in the region, particularly in certain formations, generally owing to high pyrite content. Unless properly mitigated, excavation of this rock can lead to acidic runoff leading to stream degradation. Furthermore, untreated acidic bedrock used in embankments can become unstable and result in landslides.
A damaging landslide occurs nearly every year in the region. Since 1916, eight major storm events have triggered numerous landslides across western North Carolina. In 1985, a major rockfall crushed a tunnel portal. In 2009, a bedrock landslide, the most destructive in more than a decade, closed Interstate 40 in Haywood County for over six months with a repair cost of $12.9 million. Soil movement, a downhill slide of soil only, can be equally destructive. In 2004, intense rain events from hurricanes Frances and Ivan led to over 400 slides, causing five deaths, wiping out 27 houses, and disrupting transportation throughout the region.
As of April 2010, over 6,000 landslides and landslide deposits have occurred in Western North Carolina. In Buncombe County alone, geologists found evidence of over 1,253 landslide features, 314 landslides and 938 landslide deposits. Over half of these occurred where slopes had been modified by people.
Currently, a seller is not required to tell a potential purchaser if the property under consideration may be in the path of a potential landslide. Legislation is being considered in the North Carolina state legislature which would limit steep slope development. The proposed legislation requires a licensed general contractor to supervise construction on any activity occurring on slopes greater than twenty-two degrees. It would also require retaining walls over eight feet in height be prepared by an engineer.
Conservation and Mitigation of Soil Resources
Techniques that prevent or reduce soil loss include surface water runoff control, vegetative cover, sedimentation catch basins, and buffer zones. Practices that reduce the speed of runoff will reduce erosion significantly and can be used in forestry, agriculture, and construction operations. Plant cover significantly reduces erosion because root systems securely hold soil in place. Buffer zones, designated areas of trees, shrubs, and herbaceous vegetation act as filters to keep sediment from water bodies.
To reduce landslides and soil movement, it is imperative that slope modifications stay within stable limits. Controlling drainage and reducing the slope angle reduces landslide potential. Drains can be constructed to contain runoff and prevent infiltration. Steep slopes can be graded into gentler slopes and a series of 'stair-steps' can be created on very steep slopes. Many engineering techniques are available to prevent water entry and inhibit slope failure. Wire cables and wire fences minimize the danger of rockfall. Correction of some landslides can be accomplished by installing a drainage system, which reduces water pressure in the slope, thereby preventing further movement.
Human-caused acidic deposition of sulfates from the atmosphere have had a negative impact on sensitive soils in the region. Several factors interact to increase this effect. Forests soils associated with pines, spruce, and fir are naturally more acidic than areas dominated by hardwoods such as oak, hickory, maple, or poplar. Elevation is also an important factor when assessing the risk of acidification as some soils, especially high mountain systems, are more sensitive to acidic deposition due to increased deposition levels, decreased soil depth and decreased microbial activity.
Bedrock geology, however, has the greatest influence on soil sensitivity to acidic deposition. In Western North Carolina, soils that develop from bedrock classified as siliceous are more sensitive to acidification then soils that have developed from carbonate rocks.
Under natural conditions, soil acidity remains relatively stable with the ability to balance normal increases in acidity by weathering rocks or deposition of dust from the atmosphere. This balance is essential to maintaining healthy terrestrial and aquatic ecosystems. However, with increased deposition of sulfur, sensitive soils become too acidic, releasing biologically toxic levels of aluminum. Aluminum penetrates the fine roots of vegetation and exacerbates nutrient (especially calcium) deficiencies. At high enough concentrations, aluminum can kill roots, reducing the amount of area from which nutrients and water can be absorbed into the vegetation. Consequently, tree and plant mortality may increase during periods of drought.
High soil acidity and toxic levels of aluminum have been recorded at Shining Rock Wilderness Area where the soil is at near maximum capacity to retain future deposition of sulfates. Therefore, any reduction in sulfur dioxide emissions will have a delayed benefit in improving ecosystem health because it will take a long time for the accumulated sulfur to be removed from the soils. In addition, aluminum can be transported from the soil to streams where high acidity and high concentrations of aluminum can be lethal to sensitive aquatic organisms.
The addition of nitrogen from air pollution is believed to be utilized biologically by forest organisms (with the exception of high elevation old growth spruce-fir forests), since nitrogen is typically a nutrient in short supply and is not currently believed to be negatively impacting the region’s forests.