Sinkholes: What are They, Why do They Occur, and Why Be Concerned?
By Kimberly Arsenault
Contributor, EDM Digest
Grass Valley, California. Hall County, Georgia. Fraser, Michigan. Philadelphia, Pennsylvania. Bethlehem, Pennsylvania. What do these locations have in common? Sinkholes. Each of the sinkholes occurred within the last three weeks. These sinkholes collapsed roads, sidewalks, and houses, and swallowed vehicles, causing millions of dollars in damages, and many will take weeks, if not months to repair.
Over 80 percent of the land subsidence across the nation can be contributed to human interference – through development and underground water exploitation. Land subsidence has directly impacted 45 states and over 17,000 square miles. A size that is nearly equivalent to New Hampshire and Vermont combined.
So What is a Sinkhole and why does it Occur?
The U.S. Geological Survey (USGS) indicates that sinkholes occur when the rock material below the surface, including carbonate rock such as limestone and dolomite, or evaporite rocks such as salt and gypsum, are able to be dissolved by the groundwater that flows in and around the material. This usually occurs naturally, over time, and with the dissolving of the rocks, underground caverns and spaces may develop, but this occurs slowly, causing the land to gradually sink.
The gradual occurrence of sinkholes results from surface land that is more permeable and has high amounts of sand in the sediments covering the carbonate and evaporite rocks. These types of sinkholes are known as [linkurl=”https://water.usgs.gov/edu/sinkholes.html” title=”cover-subsidence sinkholes”].
In contrast, if surface sediments contain large amounts of clay, which is less permeable, underground openings might form more quickly, becoming large enough that the open spaces are unable to support the weight of the land above, causing a sudden collapse of the surface. These are known as cover-collapse sinkholes.
In Florida, the most common type of sinkhole is known as a dissolution, or limestone solution sinkhole. These sinkholes are formed because the surface land over the carbonate rocks is thin, allowing for easy dissolving of limestone as weak acids in rainfall (caused by carbon dioxide in the air) and surface water continually percolate through rock joints, fractures, and bedding planes. These materials are then carried away by surface runoff, allowing a small depression to form over time in a hydrogeological process that is known a suffosion.
Natural weather events may also contribute to the creation of sinkholes, including excessive rainfall over a short time period or a lack of rainfall that could lead to a reduction in underground water levels. Majority of these processes occur gradually, creating dips in the land surface as the rocks dissolve underneath.
Why Be Concerned about Sinkholes?
The sinkholes that have occurred most recently across the nation have largely been human-induced sinkholes. Human-induced sinkholes often occur due to land-use practices. These include groundwater pumping for irrigation or urban water supplies, and the altering of natural water-drainage patterns such as through development and construction practices and the creation of retention ponds. Altering the earth disrupts natural run-off patterns. The added weight of new materials, along with the water in retention ponds, can cause surface materials to collapse, especially if underlying support (underground material) has already been compromised.
Since underground water helps support surface lands, the lowering of groundwater levels through pumping may result in a loss of the supporting structure below the surface, causing a sudden collapse. Other causes include hidden infrastructure failures such as those that occur from water and sewage pipe leaks, storm-water culverts that are unlined, or from other leaks, including leaks from swimming pools. Construction and the use of heavy equipment also creates vibrations that could destabilized underground structures, resulting in a collapse. The occurrence of any of these events in close relation to one another is likely to increase the possibility of a sinkhole.
The size of a sinkhole can range from less than a foot deep and wide, to one that includes hundreds of acres and to depths that exceed one hundred feet.
Currently, a national tracking system for the cost of sinkholes annually does not exist. In late 2016, the USGS estimated that the cost of sinkholes was at least $300 million per year on average, but believes this figure to be lower than the actual costs sustained.
Hidden infrastructure that is aging and long overdue for repairs and upgrades, including water and sewer pipes buried underground, also plays a large role in the danger of sinkhole causes. The Fraser, Michigan sinkhole, which is 100 feet wide and 250 feet long, caused enough damage to three homes to make them uninhabitable, has shut down a main access road for more than 100,000 residents, and was caused by what officials believe was a sewer pipe 55 feet underground and 11 feet in diameter that burst.
In Grass Valley, although weather is likely the cause – at least in part for the seven stories deep and 250 feet wide sinkhole in California – the repair price far exceeds what the city can afford, causing the city to declare a state of emergency.
Clearly, the human impact on land subsidence, and thus sinkholes, is cause for concern among emergency and disaster management officials, especially as urbanization continues and more homes and structures are vulnerable and at risk to sinkholes. This is even more concerning in the states of Alabama, Florida, Kentucky, Missouri, Pennsylvania, Tennessee, and Texas due to the rock types present and their susceptibility to dissolution by water.
Finding a way to upgrade hidden infrastructure (buried water/sewer pipes), conserving underground water resources, and using mitigation during construction and development are likely to be critical factors moving forward for professionals within the emergency and disaster management field.