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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />[� 1;2. Hazard Assessment: SINKHOLE <br />4.1.2.1. Definition. <br />A sinkhole is a bowl -shaped depression in the land <br />surface. Sinkholes are also called subsidence, which is a <br />downward settling of the surface without any horizontal <br />movement. Sinkholes result from natural processes <br />where near -surface carbonate bedrock is dissolved by <br />water to form underground spaces, also called voids. <br />These voids typically form along existingjoints or cracks in <br />the rock that aid the movement of water. Some voids <br />grow toward the surface where infiltrating surface waters <br />meet and flow downward into the drain of the void. This <br />action weakens the rock. Eventually, the weight of <br />overlying materials can result in a collapse. Areas <br />favorable for sinkhole development are called karst terrain. Certain human activities may speed up the <br />natural sinkhole processes in karst areas. Human activities outside of normal karst terrain can also trigger <br />unexpected human -caused ground collapses in materials not usually prone to sinkholes. <br />4.1.2.2. Range of magnitude <br />Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole <br />hazard assessment in 2020. <br />4.1.2.3. Spectrum of Consequences B211b <br />4.1.2.3.1. PRIMARY CONSEQUENCES: <br />Sinkholes and other land subsidence can cause significant direct damage to buildings, roads, water <br />supply systems and other infrastructure. The loss of land usable for farming or other development <br />is another consequence of sinkhole activity. Finally, groundwater contamination is a significant <br />consequence of karst and sinkhole activity. Subsurface water flow in karst areas creates a <br />situation where surface water, along with their contaminants, quickly travel deep into aquifers <br />without significant filtration. The problem is worsened when people use sinkholes as garbage <br />dumps, which was formerly a common practice in the United States. <br />4.1.2.3.2. SECONDARY CONSEQUENCES: <br />4.1.2.3.2.1. Disease. Dumping of wastes into sinkholes maybe a source of disease. A <br />disease outbreak in Harmony, Minnesota (Fillmore County) was traced to a sinkhole used <br />as a disposal point for human waste. <br />4.1.2.3.2.2. Dam failures. There have been instances of dams and other water -control <br />infrastructure being undermined by sinkholes and other karst activity. <br />4.1.2.3.2.3. Fires or explosions. When structures, or infrastructure such as pipelines are <br />impacted by sinkholes and gas lines are compromised, fires and explosions are possible. <br />19 <br />