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Chapter 13. Erosion and Sedimentation. Soil Erosion. The biggest threat to agricultural and forestry production worldwide. Soil is the basis of much of the wealth on this planet;
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Chapter 13 Erosion and Sedimentation
Soil Erosion • The biggest threat to agricultural and forestry production worldwide. • Soil is the basis of much of the wealth on this planet; • if we don't take care of it - treat it as a renewable resource, rather than use it up as we are doing now - there may be difficult problems with soil productivity in the future.
Soil Loss in the United States Each dot represents 250,000 tons. Total US soil loss in 1997 was 2 billion tons. The worst erosion occurs in the Mississippi Valley and the Midwestern corn belt. These areas have silty soils, rolling topography, and intensive farming.
Plato on Soil Erosion - 400 BC “The soil which kept breaking away from the highlands keeps continually sliding away and disappearing into the sea. What now remains, compared with what existed earlier, is like the skeleton of a sick man, all the fat and soft earth having wasted away and only the base framework of the land being left. “What are now mountains were lofty soil-clad hills; the stony plains of the present day were full of rich soil, the mountains were heavily wooded - a fact of which there are still visible traces. There are mountains in Attica which can support nothing but bees but which once were clothed, not so very long ago, with fine trees suitable for roofing the largest buildings - and roofs hewn from the timber are still in existence. The country produced boundless pastures for cattle. “The annual supply of rainfall was not lost, as it is at present, through being allowed to flow over the denuded surface into the sea, but was received by the country, into her bosom, where she stored it in her impervious clay and so was able to discharge the drainage of the heights into the hollows in the form of springs and rivers with an abundant volume and a wide territorial distribution. The shrines that survive to the present day on the sites of extinct water supplies are evidence for the correctness of my present hypothesis.”
Piedmont streams have not always run red and brown from clay. • The Southeast suffered tremendous erosion losses during the cotton era (1830-1930). • Up to 12 inches of soil was lost from many areas, especially in the Piedmont. • Much of this soil ended up in the streams, rivers, and valley bottoms of the Piedmont. • The effects of this sediment in the river systems are still evident today.
Average annual loads of suspended sediment carried by rivers of Atlantic drainage of the United States during years near 1910 and 1970
Level of Protection High < 25 mg/L Moderate 25 - 80 Low 80 - 200 Very Low > 200
Georgia Soil and Water Conservation Commission • Formed to protect, conserve and improve the soil and water resources of the State of Georgia. The Commission's goal is to make Georgia a better place for its citizens through the wise use and protection of basic soil and water resources and to achieve practical water quality goals. • Georgia Forestry Commission • Provides leadership, service, and education in protection, management, and wise use of Georgia's forest resources. • U.S. Natural Resources Conservation Service • Provides leadership in a partnership effort to help people conserve, maintain, and improve our natural resources and environment. • U.N. Food and Agriculture Organization • Has a mandate to raise levels of nutrition and standards of living, to improve agricultural productivity, and to better the condition of rural populations.
Wind Erosion • Suspension • When very fine particles, silt and clay, are picked up by the wind and carried in the atmosphere. • These particles essentially float on the wind and are carried high in the atmosphere. • They may be deposited hundreds and even thousands of miles away from where they were picked up. • Deposition areas of wind blown soils may eventually build up layers of loess soils.
Saltation • The bouncing of medium and fine sand over the ground surface, usually about 0.5 to 3 feet in the air. • When the particles fall back to the ground, their impact lifts other particles which begin to saltate. • Because of these chain reactions, saltation becomes more severe the longer the high winds blow. • If you have ever been to the beach on a day with strong winds, you have probably experienced saltation of stinging sand.
Creep • The rolling of coarse sands along the ground surface. • Creep is responsible for the formation and movement of sand dunes in bare deserts. • Creeping soils can be trapped with soil fences, and the fences at the beach are meant to hold sand on the dunes.
Fluvial Erosion • Raindrop Impact • Causes detachment of fine particles from soil aggregates, and it is also the initiator of transport. • Most energy is transferred rapidly to soil particles when the raindrop crashes into the ground. • Raindrop impact is the major detaching mechanism on bare soils.
Concentrated Flow • If the rainfall rate exceeds the infiltration rate, surface flow will commence over the soil surface. • This runoff collects in micro-depressions and forms channels. • These small channels then merge into larger channels which causes both detachment and transport of soil particles.
Sheet Erosion • Movement of the soil surface that does not involve channel flow. • Sheet erosion mostly consists of soil detachment from raindrop impact. • Subsequent transport is caused by raindrop splash and a very thin layer of overland flow. • Sheet erosion uniformly removes soil from a planar area, and it causes relatively low rates of erosion. • The thin film of flow delivers sediment to rills.
Rill Erosion • When the contributing area becomes large enough, the thin layer of overland flow starts to cut small channels (1-6" deep), called rills or rilles, into the soil surface. • Rills are formed when the velocity of the flow on the soil is large enough to create shear stresses sufficient to detach and entrain soil particles. • Rills transport the sediment dislodged by sheet erosion and carry it off the eroding surface. • Rills also pick up and transport additional sediment from the walls and bottoms of the rills themselves.
Gully Erosion • Rill erosion on a larger scale, gullies can become enormous - an example is Providence Canyon in Southwest Georgia. • The basic definition of a gully is a rill that is too deep to cross with farm machinery. • One way they form is when rills come together and concentrate even more flow • A second way is when ground water seeps out near a spring and washes out a channel below the spring. Streambanks and stream bottoms also erode during high flows due to the shear stress of fast moving water.
Point-Source Discharge • Water discharged into a stream from a pipe or structure, usually associated with a city or industry. • Nonpoint-Source Discharge • Water discharged over a wide area, not coming from a pipe, usually associated with farms, homes, forests, etc. • Detachment • The removal of fine particles from aggregates. This is a necessary step in erosion because the aggregates are too big to move. • Transport • After detachment has occurred, transport is the movement of detached particles off the source area (field, construction project, bare-soiled clearcut) and, eventually to surface waters.
Erosion rates are usually expressed as inches of topsoil per year or tons per acre per year. • An acre-furrow slice weighs two million pounds if the soil bulk density is 1.4 kg/L. • Erosion rates of 10-50 t/ac/yr are common on steep, cleared lands, and this translates into a loss of 1 inch of topsoil in 3-15 years. • In other words, the field loses the Ap layer in 20-100 years. • A tolerable rate of erosion, according to the NRCS, is 3-5 t/ac/yr, which is the approximate rate of new topsoil formation (B horizon turning into A with humus addition ).
Universal Soil Loss Equation • A = R · K · LS · C · P • R = Rainfall Erosivity Factor • A combination measure of climate factors such as typical rainfall intensities, probability of extended periods of wet weather, and types of precipitation (convective, cyclonic, snow, etc.). The USDA developed maps of R values around the country.
K = Soil Erodibility Factor • Accounts for factors such as texture, organic content, and aggregate stability. The Soil Survey maps list the K factors for each soil. • LS = Length-Slope) Factor • Accounts for both the length and steepness of the slopes. Erosion increases as the slope length increases because the depth and velocity of water increases. Erosion also increases as the slope gradient (steepness increases) because overland flow moves faster on steeper slopes.
C = Cropping Factor • Accounts for the type of vegetative cover. C factors are very low for forests and very high for bare soils. • P = Conservation Practice Factor • Accounts for any soil conservation measures applied to the land to reduce erosion rates.
Types of Sediment Measurements • Turbidity • A measure of the clarity of the water sample. • Increasing turbidity is an indication of dissolved or suspended solids present in the water column. • Substances which increase turbidity include particles of suspended sand, silt or clay, organic substances, coagulated organic colloids containing iron and aluminum hydroxides, and microorganisms including phytoplankton and zooplankton.
NTU vs. JTU • Before the advent of modern light scattering devices, turbidity was measured using the Jackson candle turbidimeter in Jackson Turbidity Units (JTU). • The NTU measure is not exactly equivalent to the JTU, but is approximately the same, i.e., 40 NTU 40 JTU. • Turbidity can be determined from grab samples using Hellig, Hach or Askania turbidimeters.
Secchi disk • Used to determine the optical clarity of deep water bodies such as lakes, reservoirs, estuaries and oceans. • A standard disk, generally 20 cm in diameter, is lowered by rope to a depth where it is no longer visible and raised until the disk is discernable.
Suspended Solids Concentration • Suspended solids are mineral and organic particles supported by turbulence within the fluid column. • The total suspended solids concentration is determined by extracting and weighing the suspended solids, reported in units of mass per unit volume, typically milligrams per liter (mg/L).
Hydrometer • Used to measure the density of the sediment solution. • The density, or specific weight, increases as the sediment concentration increases. • Because the larger particles settle very quickly, only the smaller particle classes can be successfully determined. • An additional problem with the hydrometer method results assuming the particle density for the suspended sediment fraction.
Bedload Transport • Bedload solids are those sediments that are transported along or near the bed of a stream. • These sediments are generally larger than suspended solids and either roll or bounce along the stream bed. • Bedload solids may comprise the bulk of the total load transported by the stream because of their high concentrations (generally higher than 10 g/L, and frequently higher than 100 mg/L).
Measuring Water Erosion • Sampling method • grab samples • automated samplers • Effect of location • depth, bank, bend • Effect of stage • low vs high • Comparing turbidity to suspended solids
Preventing Soil Erosion • Vegetative cover • Surface stabilization • Velocity reduction • Peak flow reduction • Inspection and maintenance
Chap 13 Quiz 1. The streams most commonly degraded in Georgia by sediment today are: (choose one) a. agriculture b. forestry c. urban d. mountain 2. Why does Georgia have such high erosion? (choose any/all/none) a. steep slopes b. erodible soils c. intense rainfall d. land-disturbing activities 3. Match: a. Suspended Solids _____ Clarity of lake water b. Turbidity _____ Sands on streambed c. Bedload _____ Filterable solids d. Secchi Depth _____Clarity of river water 4. Give two reasons why we are concerned about erosion: 5. (True/False) Soil mulch and seeding is less effective than silt fences and hay bales for preventing erosion.
