Resolution 7478 hazard mitigation

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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan Volume 2 — Hazard Inventory d111114 Hazard Assessment: FLOODING, URBAN 4.2.1.1. Definition Urban flooding occurs when rain overwhelms drainage systems and waterways and makes its way into the basements, backyards, and streets of homes, businesses, and other properties. As land is converted from fields or woodlands to roads or parking lots, it loses its ability to absorb rainfall. Because of this, densely populated areas are at a high risk for flash floods. The construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. 4.2.1.2. Range of magnitude The 10-year average of recent flood damages is about $20 billion. However, some years have run as high as $40 billion. • Deadliest Flash Flood (Dam Collapse): 1889, Johnstown Pennsylvania: 2,200 people died. • Deadliest torrential rain flood: July 31, 1976, Big Thompson Canyon, Colorado: 143 people died • Longest duration: 1993 61 days; The Great Midwest Flood • Greatest USD Damage: $12 Billion 1993; The Great Midwest Flood 4.2.1.3. Spectrum of Consequences B211b There are several ways in which storm water can cause the flooding: overflow from rivers and streams, sewage pipe backup into buildings, seepage through building wall and floors, and the accumulation of storm water on property and in public rights -of -way. Sometimes, streams through cities and towns are routed underground into storm drains. During heavy rain, the storm drains can become overwhelmed and flood roads and buildings. Low spots, such as underpasses, underground parking garages, and basements can become dangerous. The economic, social, and environmental consequences of urban flooding can be considerable. Water quality issues can arise from sewer overflow's debris contamination, fertilizer runoff from agriculture etc.... which affect public health with possible contaminated drinking water and water borne illnesses. The cost of removal of soil from landslides, or sediment deposits from flooding can be high, as well as wildlife habitat reconstruction as wildlife habitat can be ruined by wash out, water contaminates, oxygen loss, or loss of access to food sources. Chronically wet houses are linked to an increase in respiratory problems, and insurance rates and deductibles may rise to compensate for repeated basement flooding claims. Industry experts estimate that wet basements can lower property values by 10-25 percent and are citied among the top reasons for not purchasing a home. According to FEMA, almost 40 percent of small businesses never reopen their doors following a flooding disaster. Between 2006-2010 the average commercial flood claim made to the NFIP amounted to just over $85,000. Urban flooding also erodes streams and riverbeds and degrades the 33