Water Quality Problems Impacts of Land Use: Soil Erosion and Sedimentation
WHAT ARE EROSION AND SEDIMENTATION? Erosion and sedimentation are two separate, but inter-related processes. Both processes cause different types of environmental damage, and require different control measures to minimize the impacts.
Erosion is the process by which the land surface is worn away by the action of wind, water, ice, or gravity. In simple terms, it is the process where soil particles are dislodged or detached and put in motion.
Sedimentation is the process whereby the detached particles generated by erosion are deposited elsewhere on the land or in our lakes, streams and wetlands. Together, the two processes result in soil being detached, carried away and eventually deposited elsewhere.
It is estimated that from all sources, over 4.5 billion tons of sediment pollute the rivers of this country each year. This is the equivalent to a volume the size of 25,000 football fields, 100 feet high. It is estimated that 6-13 billion dollars per year are spent in the U.S. to correct the effects of erosion and sediment.
Damage from erosion and sediment affect nearly every citizen. Erosion and sediment result in:
- loss of fertile top soil
- clogged ditches, culverts, and storm sewers that increase flooding
- muddy or turbid streams
- damaged plant and animal life
- filled-in ponds, lakes, and reservoirs
- damaged aquatic habitats and reduced recreational value and use
- structural damage to buildings, roads, and other structures.
TYPES OF EROSION Geologic Erosion or "natural" erosion is the action of the wind, water, ice and gravity in wearing away rock to form soil and shape the ground surface. Except for some stream and shore erosion, it a relatively slow, continuous process that often goes unnoticed. Geologic erosion is reported to account for about 30 percent of all sediment in the United States each year.
Accelerated erosion is the speeding up of erosion due to human activity. Whenever we destroy the natural vegetation or alter the contour of the ground without providing some sort of surface protection, we greatly increase the rate of erosion. This type of erosion is reported to account for about 70 percent of all sediment generated in this country each year. Accelerated erosion can be minimized through careful planning and by implementing appropriate control measures. Farming, construction, logging, and mining are the principle causes of accelerated erosion. These activities radically upset the delicate balance that nature has developed between rainfall and runoff.
FACTORS THAT INFLUENCE EROSION The climatic factors that influence erosion include rainfall amount, intensity, and frequency. During periods of frequent rainfall, a greater percentage of the rainfall will become runoff. Temperature is another climatic factor influencing erosion. While frozen soil is highly resistant to erosion, rapid thawing of the soil surface brought on by warm rains can lead to serious erosion.
Vegetation is probably the most important physical factor influencing soil erosion. A good cover of vegetation shields the soil from the impact of raindrops. It also binds the soil together, making it more resistant to runoff. A vegetative cover provides organic matter, slows runoff, and filters sediment. On a graded slope, the condition of vegetative cover will determine whether erosion will be stopped or only slightly halted. A dense, robust cover of vegetation is one of the best protections against soil erosion.
Physical characteristics of soil have a bearing on erodibility. One soil property that influences erodibility is soil texture. Texture refers to the size or combination of sizes of the individual soil particles. Three broad size classifications, ranging from small to large, are clay, silt, and sand. Soil having a large amount of silt-sized particles are most susceptible to erosion from both wind and water. Soil with clay or sand-sized particles are less prone to erosion.
Slope length, steepness and roughness affect erodibility. Generally, the longer the slope, the greater the potential for erosion. The greatest erosion potential is at the base of the slope, where runoff velocity is greatest and runoff concentrates. Slope steepness, along with surface roughness, and the amount and intensity of rainfall control the speed at which runoff flows down a slope. The steeper the slope, the faster the water will flow. The faster it flows, the more likely it will cause erosion and increase sedimentation.