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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
<br />Volume 2 — Hazard Inventory
<br />4.3.6.2. Range of magnitude
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<br />Official: 10.00 inches, MSPJuly
<br />Official: 15.10 inches, Hokah,
<br />23-24, 1987
<br />Aug 18-19, 2007
<br />Unofficial: 12.75 inches,
<br />Unofficial, La Crescent, 17.21
<br />Bloomington, July 23-24, 1987
<br />inches, August 18-19, 2007
<br />13.80" MSP JulY 20-24, 1987
<br />17.45 inches, Hokah, August 18-
<br />22, 2007
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<br />17.90 inches, MSP, July 1987
<br />23.86 inches, Hokah, August
<br />2007
<br />4.3.6.3. Spectrum of consequences (damage scale, common impacts and disruptions, response needs)
<br />2
<br />The most dangerous result of extreme rainfall is flash flooding, which has numerous consequences, arises
<br />from a combination of factors, and is covered in greater depth as its own chapter within this assessment.
<br />Other severe hazards are not related to directly flooding. Following is a brief annotated list of common
<br />consequences resulting from extreme rainfall:
<br />• Injury, drowning, death: those unable to get to higher ground, and those stuck in vehicles that
<br />either failed to navigate or are unaware of high water are at significant risk. Flooded roads,
<br />particularly at night, are especially dangerous.
<br />• Infrastructure damage: roads, culverts, drainage basins, bridges, and even dams can succumb to
<br />the direct force of heavy flowing water, and to erosion from the ground below. Sewer and
<br />wastewater systems may overflow.
<br />• Stalled, stranded, or damaged vehicles. Many vehicle batteries die in high water, causing vehicles
<br />to stall. Parked vehicles in low-lying areas may also be inundated and stranded. Water frequently
<br />gets inside the vehicles, damaging the electronics and the interior.
<br />• Structural failure: eroding soils from a heavy rain may undermine the structural integrity of houses
<br />and buildings, resulting in complete or partial collapse.
<br />• Water damage. Water enters sub -grade floors through small openings and in extreme events can
<br />accumulate to inches or even feet on the lowest levels, as municipal sewer systems exceed
<br />capacity and water backs up into residential lines. Electrical equipment becomes susceptible to
<br />damage, and interior materials may be compromised and may develop dangerous mold or mildew.
<br />• Crop damage: it is common for major extreme rainfall events to damage agricultural fields, often
<br />wiping out an entire season's worth of crops.
<br />• Water quality: extreme rainfall washes high level of compounds into area waterways, which may
<br />exceed allowable contaminant thresholds for days or even weeks after a major event.
<br />• Recreational loss: extreme rainfall events target the lowest areas first, meaning that lakes and
<br />rivers are susceptible to overflow. No -wake laws impede water sports, and overflowing streams
<br />and rivers can produce dangerous conditions for canoeing and other human -powered water
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