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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />4.1.2.4. Potential for Cascading Effects <br />In Minnesota, most sinkholes are in rural areas and develop very slowly. These sinkholes are not <br />dangerous, and they do not cause much destruction except for the loss of crop land. When sinkholes <br />happen in developed urban areas however, they have the potential to be much more costly and, in some <br />cases, even dangerous. The active karst areas in southeast Hennepin County are in places with <br />concentrated developments of housing, businesses, schools and infrastructure. The potential for <br />destructive sinkhole events in Hennepin County has not been adequately assessed. Hennepin County <br />Emergency Management is initiating a study of sinkhole hazards in the county that is expected to be <br />complete by 2020. <br />4.1.2.5. Geographic Scope of Hazard Blc <br />The southeastern three-quarters of Hennepin County is underlain by carbonate bedrock and is karst <br />terrain. The western and northern limits of this area begin in the south around Excelsior and extend <br />northward into Medina, then eastward into Brooklyn Center. Most of this area is comprised of covered <br />karstwhich has overlying glacial material more than 100 feet in depth. An area with pockets of transitional <br />karst which has overlying glacial material between 50 and 100 feet thick is roughly bounded in the south <br />by Edina, west to Wayzata, and northeast to Brooklyn Center. Active karst is found in mostly along the <br />Mississippi River from North Minneapolis south to Fort Snelling. Scattered outlying pockets of active karst <br />can be found westward from Golden Valley south to St. Louis Park. Active karst areas have less than 50 <br />feet of overlying material covering them. <br />Note: Other types of land subsidence are directly caused by human activities and are dealt with in the <br />human -caused, industrial/technological section of this hazard assessment. These include water or sewer <br />system breaks that cause sinkholes or collapse of underground tunnels. <br />4.1.2.6. Chronologic Patterns <br />Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole <br />hazard assessment in 2020. <br />4.1.2.7. Historical Data Bld <br />The Seven Oaks Park in south Minneapolis is a sinkhole. The surface depression is approximately 300 feet <br />wide and over 20 feet deep. The time of formation of the sinkhole is unknown but predates the <br />construction of the structures around it. Seven Oaks Park is located between E 341h Street and E 351h Street <br />at 471h Avenue South in Minneapolis (USNG 15T VK 83754 76384). Other possible sinkholes are nearby but <br />await more definitive confirmation. <br />There have been no other naturally caused incidents that are within the scope of this plan. <br />4.1.2.8. Future Trends Ble <br />Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole <br />hazard assessment. <br />4.1.2.9. Indications and Forecasting <br />20 <br />