Sinkhole Flooding/Subsidence
Sinkholes, sometimes referred to as dolines, are the most common surficial landforms found in karst landscapes. The three main morphological types of sinkholes are bowl-shaped, funnel-shaped, or well-shaped (Sweeting, 1974). The size of individual sinkholes may range from a few meters to in their dimension, several hundred meters in horizontal dimension, and over 100 meters in depth. Sinkholes can occur in close proximity to each other or completely isolated, it all depends on the porosity and permeability of the underlying karst bedrock. The five major types of sinkholes are solution, subsidence, collapse, alluvial stream sink, and subjacent karst collapse sinkholes. They are all are named for the processes leading to their formation (Jennings, 1985).
Aerial view of a sinkhole in an urban area in southern Florida.
Image from: sofia.usgs.gov/.../ofr/01-180/ opandaction.html
Solution sinkholes form in places such as joint intersections. Within the intersections, surface runoff accumulates, leading to the solution of the soluble bedrock. The solutes then move downward through the bedrock joints and bedding planes. The movement of the solute through the bedrock inhibits the solution process to reach deeper into the underlying strata. The result is a funnel-shaped hole on the surface. Water often fills the sinkholes, creating marshes and ponds (Jennings, 1985).
Solution Sinkhole Formation Diagram (Jennings, 1985)
Solution Sinkhole on a farm in rural Kentucky. (Taken by Matt Below)
Subsidence sinkholes occur in karst areas covered by soils or other unconsolidated materials. They appear to occur very quickly , due to the fast collapse of the overlying materials. The reasoning behind this is that most of the solution process takes place under the surface. Upon formation, a cylindrical hole is created. The hole is then eroded over a period of time into more of a bowl shape. These sinkholes are eroded and widened very easily sue to the presence of soils and unconsolidated materials located on the rims of the hole. Subsidence sinkholes are also prone to become ponds of varied sizes (Jennings, 1985).
Subsidence Sinkhole Formation Diagram (Jennings, 1985)
Subsidence Sinkhole in Florida engulfing a private residence
Collapse sinkholes form either from sudden mass movements of karst bedrock due to sudden drops in the water table or when the roof of an underground cavern or cave collapses, or both. Unlike solution sinkholes, collapse sinkholes can be angular in shape with vertical walls. Over time, abrasion will erode the walls and create a round or oval-shaped sinkhole. One common reason for a sudden drop in the water table is excessive groundwater pumping. Over time, collapse sinkholes often fill with water (Jennings, 1985).
Cover Collapse Sinkhole Formation Diagram (Jennings, 1985)
Cover Collape Sinkhole - www.sinkhole.org/ facts3.htm
Alluvial stream sinkholes are created by streams sinking through deposits of alluvium on the surface of the landscape into the underlying soluble karst bedrock. The same natural processes that lead to the formation of subsidence sinkholes can also lead to alluvial stream sinkhole formation. These sinkholes are often prone to filling of water (Jennings, 1985).
Alluvial Stream Sinkhole Formation Diagram (Jennings, 1985)
Alluvial Stream disappearing into the sub-surface. http://www.geosolgroup.com/images/karst.jpg
The formation of subjacent karst collapse sinkholes can be due to cave or cavern roof collapse as well as the continual removal of overlying rocks near widened joints in the karst bedrock. They are formed by processes very similar to those responsible for subsidence sinkholes (Jennings, 1985).
Subjacent Karst Collapse Sinkhole Formation Diagram (Jennings, 1985)
Subjacent Karst Collapse Sinkhole- student.aasdcat.com/.../imageMapHtm/ sinkhole.htm
Subsidence
Subsidence can be defined as "the mass movement of soils, weathering mantles, and superficial deposits that is often gradual." This usually occurs over extended periods of time. The phenomenon referred to as subsidence occuring in sinkholes is an everpresent environmental hazard in areas of karst landscapes. It can occur naturally through the collapse of surface materials due to solution or by way of human interference. Human-induced subsidence most commonly occurs as a lowering of the ground surface due to the withdrawl of massive amounts of groundwater (Ruthven, et. al., 2002). The removal of large amounts of groundwater in karst areas reduces the water pressure in the pore spaces of the bedrock, as well as the bedrock strength. This allows for the weakened areas to collapes.
Sinkhole subsidence have been known to inflict heavy amounts of property damage within both rural and urban settings. Buildings and roads have been documented as collapsing into sinkholes that have formed in the karst bedrock under the infrastructure, as well as in remote areas of the countyside. To have the best understanding of subsidence associated with karst areas, it is essential to understand the area's topology. Due to the variety of karst bedrock, subsidence hazards vary substantially from one area to the next (Ruthven, et.al., 2002).
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USGS photo of a residence in Florida being overtaken by subsidence
Sinkhole underneath roadway near Bowling Green, KY- www.stoptranspark.org/ sinkholes.html
Sinkhole Flooding
Because sinkholes are internally drained and many are filled with water, sinkhole flooding can become a common environmental hazard of karst landscapes as a result of intense or prolonged precipitation. After landing on the land surface, water can either drain through the soil and then on through enlarged joints and cracks in the underlying bedrock or run directly into a sinkhole or other reservoir. Sinkholes drains can become clogged or obstructed with debris, causing the water to back up onto the landscape. Clogging can occur due to the collection of falling debris from the rims of the sinkhole or due to the accumulation of trash at the drain. (Ruthven, et.al., 2002)
Comparison of sinkholes before and after an intense rain eventyunus.hacettepe.edu.tr/ ~harun/karst.htm
Being a nuisance to areas of karst topography, sinkhole flooding can sometimes be prevented. If a sinkhole has a plugged drain, an attempt to unclog it should be made. If successful, it could lead to cleaner water and increased land value (www.caves.org). Any trash of debris removed from a clogged sinkhole must be disposed of properly.
During the event of an intense or prolonged rain event, flooding again becomes an issue. A phenomenon known as ponding may occur, which is an accumulation of excess water in sinkholes. This can be avoided by drilling drainage wells into the aquifer of problem sinkholes or establishing sinkhole flood plain easements (www.dyetracing.com).
