Land Subsidence in Arizona

Arizona Interactive Land Subsidence Map

 Land subsidence has been occurring across Arizona since the early 1900s. Millions of people around the world live in active land subsidence areas, many of whom may not even realize it. Most of the time, there is no clear and identifiable sign that land subsidence has occurred in an area. Areas in Maricopa and Pinal Counties have subsided more than eighteen feet since the early 20th Century.

Land subsidence in the basins of Arizona is generally due to compaction of alluvium caused by lowering of the water table. As the water table declines, pores in the alluvium once held open by water pressure are no longer supported and collapse. Collapse and subsequent lowering in elevation of the land surface is defined as land subsidence. This subsidence is generally not recoverable. If this subsidence occurs over areas of bedrock, differential subsidence can occur.

Differential subsidence is when adjacent areas subside at different rates. Bedrock will not compress like the surrounding alluvium, creating a subsurface platform. Differential subsidence occurs where shallow bedrock and deep bedrock are adjacent to each other, creating a zone of differential change in surface elevation. Because of these different amounts of subsidence, tension can build in the alluvium layer at this differential subsidence zone, forming an earth fissure.

Arizona Land Subsidence Areas

 

Earth Fissures

 

Y fissure int he Queen Creek area after a monsoon storm in August 2005

Earth fissures are cracks at or near the earth’s surface that are the result of differential subsidence. Earth fissures start out as small cracks and may not be visible on the surface. They grow and widen from surface water flowing into the crack, eroding material from the sides.

Earth fissures have caused millions of dollars in property and infrastructure damage, damaging pipelines, roads, canals, flood retention structures, bridges, buildings, and private property. There are unanswered questions about how earth fissures interact with groundwater pollution.

Impacts Of Land Subsidence & Earth Fissures

There are many consequences of land subsidence and earth fissures. The associated elevation and slope change can affect the natural floodplain drainage and the flow within canals and drains. It can also damage intermodal infrastructure (roads, bridges, railways, etc.), water-retention and retarding structures (dams, levees, floodways, etc.) and private properties (homes, driveways, fences, etc.).

Earth fissures are identified by using the ground and aerial monitoring techniques. The Arizona Geological Survey (ASGS) started an intensive earth fissure mapping program in 2006. The goal of this program is to survey and record each known earth fissure around the State and provide this data to the public.

Land Subsidence Monitoring

 

Hawk Rock Subsidence Feature 2004 - 2008 Interferogram

Land subsidence has been detected over the years using surveying techniques such as differential leveling and high accuracy Global Positioning System (GPS) surveying. In the early 1990s, scientists began to use Synthetic Aperture Radar (SAR) and interferometric processing (InSAR) to detect land surface elevation changes. InSAR has been developed into a highly reliable land subsidence monitoring technique that has been utilized by the Arizona Department of Water Resources since 2002. ADWR has identified numerous subsidence features around the State and continues to monitor the extent and rates of these features on an annual basis.

Land subsidence maps were developed by ADWR using archived and current InSAR data with the goal of providing hydrologists, geologists, water managers, and the public the most accurate land subsidence data.

ADWR is also part of the Arizona Land Subsidence Group which was jointly created by the geological and engineering communities as a venue for discussing the state of knowledge regarding subsidence of Arizona’s alluvial basins.