WHAT IS LAND SUBSIDENCE?
Land subsidence is the settling or lowering of the land surface resulting from the excessive withdrawal of groundwater. Large-scale pumping of groundwater, at rates far in excess of the rate of recharge, causes the long-term lowering of water levels in the aquifer. Dewatering within the aquifer reduces the hydrostatic pressure (i.e., the buoyancy) supporting the porous sediments and may lead to compaction of the particles, particularly in fine-grained silt and clay layers. The compaction and compression of dewatered sediments causes settling of the overlying sediments and sinking of the surface of the land. Where there are differential amounts of compaction of the subsurface sediments, earth cracks may form and extend to the land surface.
Land subsidence is the lowering of the ground relative to a fixed reference point. The rate of subsidence varies from place to place but has been measured to be as high as 0.8 feet per decade in some parts of Louisiana. Subsidence increases flood risk by allowing storm surge and larger waves to propagate further inland.
