LAND SUBSIDENCE - WHAT IS IT AND WHY IS IT IMPORTANT?
Land subsidence is a condition in which there is a loss of support from water pressure below the ground. As we withdraw ground water and the water table drops, the soil collapses, compacts, and drops resulting in the decrease of land-surface elevation. Subsidence causes permanent inundation of land, aggravates flooding, changes topographic gradients, ruptures the land surface, and reduces the capacity of aquifers to store water. Land subsidence is a global problem. This situation occurs throughout the United States, but has had more impact in California, Texas, and Arizona. According to a recent U. S. Geological Survey (USGS) report, more than 17,000 square miles in 45 states in the United States, an area roughly the size of New Hampshire and Vermont combined, have been directly affected by subsidence.
WHAT’S CAUSING LAND SUBSIDENCE
Land subsidence can occur naturally or through human activity. Common causes of land subsidence from human activity are: •Excessive pumping of water, oil, and gas from underground reservoirs. For example, withdrawal of oil from the field at Long Beach, California beginning in 1936 resulted in subsidence at rates ranging from 0.5 to 2.0 ft per year in the center of the field. By 1962, the center of the oil field had subsided slightly over 27 ft, caused by the removal of fluid from the pore spaces in the underground rock, which allowed the grains to compact. Similarly, withdrawal of groundwater through well pumping has resulted in subsidence in such cities as Mexico City, Houston, Texas, and Venice, Italy.
•Underground salt, ore, and coal mining. However, where mining activity is planned, mining-induced subsidence can be successfully managed if there is co-operation from all of the stakeholders. This is accomplished through a combination of careful mine planning, the taking of preventative measures, and the...