Subsidence is the sinking of the land surface caused by the withdrawal of groundwater. The land elevation lost to subsidence is permanent and irreversible. In the Harris- Galveston region of Texas, subsidence poses the greatest threat in the coastal areas susceptible to flooding due to high tides, heavy rainfall and hurricane storm surge. Because of low elevation, any additional subsidence in the coastal areas results in a significant increase in potential tidal flooding or permanent inundation.