Consider a ring of radius R with the total charge Q spread uniformly over its perimeter. What is the potential difference between the point at the center of the ring and a point on its axis a distance 2R from the center?
CHAPTER 8 SUBSISTENCE AND SOILS A particular event that is an example is the subsidence aftermath of Hurricane Katrina in Louisiana. NASA investigated this particular state to investigate the soil and land from this state, and prevent and predict possible hazards. Venice is Shaking •Beautiful and famous city in Italy –Subsiding (sinking) at rate of 1.5 mm (~0.06 in) per year in some areas –Built on 118 small islands in a coastal lagoon –Extremely prone to flooding •Has been happening naturally for millions of years –However, over pumping of groundwater significantly increased rate of subsidence –Human response has been to raise buildings and streets •Frequency of floods has increased SOLUTION Mose System, which is building huge engineering barriers when there is an increase of tide Soil and Hazards •Soil –Solid earth material that has been altered such that it can support rooted plant life –Any solid earth material that can be removed without blasting Helps evaluate natural hazards •Soil is produced through weathering –Physical and chemical breakdown of rocks –Changed by residual or transported activity of soil organisms Residual/ Transported Soil and Hazards cont. •Soil development depends on: –Climate –Topography –Parent material •The rock or alluvium from which the soil is formed –Time •Age of the soil –Organic processes •Activity of soil organisms Soil Horizons •Soil profile –Created from vertical and horizontal movements –Distinct layers parallel to the surface •Layers in a profile are soil horizons –O: organic materials –A: mineral and organic materials –E: forms zone of leaching with the A layer –B: enriched in clay, iron oxides, etc., resulting from leaching •Bt enriched with clay materials •Bk: accumulation of calcium carbonate (kjust CaCO3), (caliche CaCO3 layers/lenses/ chunks) –C: partially altered (weathered) parent material –R: unweathered parent material HardpanImpermeable, clayCaCO3/ Iron Oxide/ Silica •Can be an important diagnostic tool for analyzing a soil profile, but can be misleading –O and A horizons are dark Soil Color Continued –E horizon is white –B horizon varies from yellowbrown to light redbrown to dark red –K horizon may be almost white •Soil color can also indicate drainage –Welldrained are aerated: red color –Poorly drained are wet: yellow color Soil Texture •Defined by proportions of sand, silt, and claysized particles –Clay: less than 0.004 mm –Silt: 0.004 to 0.074 mm –Sand: 0.074 to 2.0 mm –Gravel, cobbles or boulders: greater than 2.0 mm •Estimated in the field and refined in laboratory •Particles cling together in peds or aggregates Soil analyses help to recognize hazards (This image is used to analyze the texture of soil ) (This visual describes the description of each soil horizon ) Relative Soil Profile Development •Soils differ in development –Weakly developed soil •A horizon directly over a C horizon (without B) •Few hundred to several thousand years old –Moderately developed soil •A overlying an argillic B ttat overlies the C horizon •More than 10,000 years old (at least Pleistocene) –Welldeveloped soil •B redder, more translocation of clay to B, and stronger structure t t •Between 40,000 and several hundred thousand years and older •Soil chronosequence: youngest to oldest –Give information about the recent history of an area Water in Soils Properties of Soil •Saturated –All the pore spaces in a block of soil are completely filled with water –Unsaturated otherwise •Moisture content –Amount of water in a soil –Important to strength of soil and potential to shrink and swell •Water flow –Saturated flow if all the pores are filled with water –Unsaturated flow otherwise (more common) (this image shows the air relationships when there are solids near each other along with water ) Classifying Soils •Soil taxonomy –Used by soil scientists –Based on physical and chemical properties of soil profile Classifying Soils (cont. ) –Useful for agricultural and land use •Engineering classification of soils –Used by engineers (and for hazards) –Based on particle size or the abundance of organic material Soil Erosion as a Hazard •Can agricultural systems maintain and improve soil fertility while minimizing erosion –Appears many practices are mining the soil –Could erode foundation of our civilization •Soil Erosion –Grainbygrain removal of mineral and organic material by wind and/or water –Removal of soil material at an unacceptable rate –Removal of soil material at a rate faster than it is being produced Cont. •Problem in urban environments –Vegetation often removed prior to development –Persists where protection is not a high priority •Rates of soil erosion –Most concerned with top, organically rich soil •Takes about 500 to 1000 years to form 50 mm (~2 in) of soil •Rate of soil for agricultural land is 0.05 to 0.1 mm per year •Accelerated erosion can remove centuries of soil in less than a decade –Rates measured as volume, mass, or weight •Amount removed from a location within a specified time and area •Can predict soil moved from original location through Universal Soil Loss Equation Introduction to Subsidence and Soil Volume Change •Subsidence –Ground failure characterized by sinking or vertical deformation of land associated with •Dissolution of rocks beneath the surface: karst topography •Thawing of frozen ground •Compaction of sediment •Earthquakes and drainage of magma •Soil volume change –Result from natural processes •Changes in water content of soil •Frost heaving –These are probably not life threatening, but is one of the most widespread and costly natural hazards Kru st •Rocks are dissolved by surface or groundwater –Evaporites: rock salt and gypsum, dissolved by water –Carbonates: limestone and dolostone and marble, dissolved by slightly acidic water •Acid comes from carbon dioxide from plant and animal decay •Common in humid climates •Rocks are dissolved and groundwater level drops, leaving behind caverns and sinkholes –Pits in that are near surface •Sinkholes in large numbers form a karst plain (This image shows a formation of Karst Topography) Krast (cont.) •Sinkholes –Vary in size from one to several hundred meters in diameter –Can open up extremely rapidly •Two Basic types –Solutional sinkholes •Acidic groundwater becomes concentrated in holes in joints and fractures in the rock •Water is drawn into a cone above the hole in the limestone –Collapse sinkholes •Develop by the collapse of material into an underground cavern