I’ve never heard of the Subsidence District. When was it created and what does it do?
The Harris-Galveston Subsidence District (District) is a special purpose district created by the Texas Legislature in 1975. The District was created to provide for the regulation of groundwater withdrawal throughout Harris and Galveston counties for the purpose of preventing land subsidence, which leads to increased flooding. The District’s enabling legislation is found in Chapter 8801 of the Special Districts Code.
What is land subsidence and why is it something that must be prevented?
Subsidence is the permanent and irreversible lowering of the elevation of the land surface in relation to mean sea level. Subsidence in this area is caused by the compaction of subsurface clay layers due to the loss of support pressure resulting from water level declines. Prior to the District’s creation, prolonged and increased pumping of groundwater for municipal, industrial, and irrigation supplies in Harris and Galveston counties had caused the water level of the aquifer system beneath the area to decline over a broad region, which resulted in one of the largest subsidence bowls in the United States. Approximately 4,700 square miles of land subsided more than 0.5 feet from 1943 to 1973, with the maximum subsidence in the area approximated at 10 feet. More than 31 square miles of low lying coastal land was permanently inundated, and the potential for tidal flooding, particularly from storm surges, increased in the coastal region because of loss of elevation.
I don’t live near the coast. Why should I be concerned with subsidence-related flooding?
Subsidence can also lead to increased inland flooding along streams and waterways due to changes in stream gradient and also due to “ponding” caused by localized subsidence in the vicinity of major water production areas. Inland flooding has become a greater concern over the years because the center of maximum groundwater pumping in the District has shifted from the Ship Channel to West and Northwest Houston and Harris County.
Although the District was created for the specific purpose of preventing subsidence, the lowering of water levels in the aquifers beneath the District also results in a number of other problems. Declining water levels can activate or accelerate the movement of geologic faults, which results in significant damage to buildings and other structures. Lower water levels also require deeper wells and increased pumping costs, and in some cases, water levels have fallen below the depths of existing wells requiring the drilling of replacement wells. Another type of problem related to water level declines involves the issue of groundwater quality and can include such problems as saltwater intrusion, radon contamination, and contaminant transport.
How does the Subsidence District accomplish its task of preventing subsidence?
The primary means by which the District accomplishes its statutory purpose of preventing subsidence is by regulating the amount of groundwater that may be withdrawn from aquifers, which means converting some portion of the groundwater demand within the District over to surface water supplies.
The goal is not to prevent all use of groundwater, but instead to use it wisely in a manner that doesn’t cause subsidence or deplete the aquifer, threatening future water supplies. The document
that outlines this regulatory program is the District’s Regulatory Plan, which works in conjunction with the District’s well permitting program.