Floods are waters that frequently cover an area of land that is usually dry. In parts of the world where floods occur frequently, people use sand bags to divert water from dry land and their homes. It is usually used as a foundation for higher levees, or other water-control structures. Sandbags are usually made of burlap or polypropylene as they do not deteriorate unless there is wet sand inside the bag.
A trench is first dug, sandbags are then placed on top of one another in a row. Sandbags should be placed in the direction of the water flow, and there shouldn't be any space in between. The sandbags should be "staggered" on top of one another like bricks, this adds protection and security to stabilise the sandbags. Sandbags should be limited three to four layers, since it may result in it being insecure.
The material frequently used in sandbags is sand, hence the name. It is generally used as it is readily available and traps most of the water, on account of it being denser than water. Density is the measure of a quantity, such as mass. Denser objects are heavier than less dense ones of the same size in the same volume. Denser objects are also heavier than less denser ones of the same size in the same gravitational field. Therefore, 1L of water is lighter than 1L of sand; sand is denser than water. This may possibly be the reason sand is a popular material in sandbags.
Gravel is loose material consisting of rock and mineral fragments, they are larger than 2mm in dimater but smaller than 256mm in diameter. Gravel is produced by the weathering and erosion of rocks. Strong river currents tend to transport gravel great distances before it is deposited, rock fragments that are transported by water are rounded while those carried by ice glaciers possess sharp, angular edges. Gravel is not compact and does not absorb the water so it may be less effective in preventing water leaking from sandbags.
Woodchips are produced from wood residue, manufacturing waste and energy crops. For efficient handling and combustion, woodchips should be around 2-25mm in their largest dimension. They are not compact but do absorb some water, therefore less water leaks through the sandbags. Woodchips are often used for landscaping.
Bibliography for the above information:
- Wikipedia Foundation Inc., 'Sand', 27/04/10, http://en.wikipedia.org/wiki/Sand
- Chemistry Daily.com, 'Density', 27/04/10, http://www.chemistrydaily.com/chemistry/Density
- eHowInc, 'How to make a sandbag', '25/04/10, http://www.ehow.com/how_4224_make-sandbag.html
- Energy Saving Wales, 'Requirements of wood chips used', 27/04/10, http://www.energysavingwales.org.uk/index.cfm?fuseaction=TM.viewTip&tipID=107&aspect=2§or=1¤tTip=107direction=view
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