FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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EFFECTS OF EROSION AT EDGEWOOD By Kathy Korbholz Little drops of water The seemingly minimal forces exerted by drops of water and grains of sand, when combined with the element of time, can level a mountain. Erosion is the removal of the surface layer of the earth's crust by the natural agents of wind, water and ice. Wind causes erosion in two ways. Abrasion, or the scouring action of sand/soil particles against the softer areas of rocks, plants and man-made structures, is the first component. This action is usually limited to a height of 18 inches above the ground but can extend to three feet. The second component, deflation (from the Latin to blow away), is the erosive action of the wind carrying off unconsolidated material. During wet periods, water loosens material and leaves it lying on the surface. During dry periods, winds carry off these loosened products of weathering. Deflation moves only sand and dust, leaving behind larger particles the size of pebbles or cobbles. With enough time and wind, only stones remain to form a surface cover. Water in stream beds or rivulets originates as runoff from neighboring slopes. Runoff flows as a sheet of water or in closely spaced shallow channels called rills. This slope wash (or sheet erosion) is sometimes powerful enough to overcome the soil's resistance and manages to carry a great deal of surface material down slope. Disturbed or denuded soils are less resistant to slope wash. Moving water can gouge its own water channel, transport debris, and deposit sediments (including exotic plant seeds) at various points along its course. The nature and extent of water-caused erosion depends on the kinetic energy of the stream, which is a function of the amount of water and the gradient of the channel. The faster the flow, the greater the resultant erosion. The diameter of particles which moving water can carry actually varies as the square of the velocity of the stream. Human trampling promotes erosion by loosening soil materials and destroying plant cover. The trampling damage caused by humans is about eight times that caused by deer. Deer native to San Mateo County have weight ranges similar to humans: 94 lbs. for small does to 200 lbs. for large bucks. When deer walk, their weight is distributed over their four feet and their back feet land on or very near their front tracks. Each deer print, even for a large buck, covers less than six square inches, while a single human boot track can exceed fifty square inches! Each human boot step loosens soil, and breaks or destroys plant cover under foot. The unintentional erosive effects of human trampling are seen at shortcuts across switchbacks. In serpentine areas where the soil depth is thin, and plant cover naturally sparse, trampling-caused erosion of precious soil nutrients is especially devastating. At Edgewood Park further evidence of human-caused erosion exists on the two hillsides facing the Serpentine Trail. There the trampling from off-road vehicles caused plants and soil to be displaced and to be more susceptible to the erosive effects of wind and water. These off-road tracks are still barren after more than 20 years! The presence of plant material, both living, withering, and in mulch form, mitigates the effects of water and wind erosion. First the tree canopy "softens" the pelting force of rain during a storm. All above-ground plant material impedes the flow (velocity) of both wind and water, thereby reducing their carrying capacity to remove soil particles. Above-ground plant residue increases surface roughness, thereby slowing surface runoff and filtering out sediment as rain water percolates. Finally, the root systems of plants physically bind soil particles. On slopes, tap roots penetrate through the soil mantle and help prevent erosion by anchoring to firmer strata below. Erosion is a part of the natural ecology of Edgewood Park and Preserve. New gullies, washed out culverts, and fresh bare spots with little or no plant cover are all evident after last year's heavier-than-normal rainfall. Because erosion can never be reversed, those who care about the Preserve's future must help visitors understand the importance of staying on sanctioned trails and leaving plant material undisturbed. We must all work together to protect our park from excessive erosion. (1) Robert E. Horton, "Sheet Erosion-Present and Past," Trans. Am. Geophys. Union, XXII (1941), 300 |
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