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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />The National Weather Service issues flood advisories, watches and warnings16 <br />• Flood Advisory: Thunderstorms have produced heavy rainfall that may result in ponding of water <br />on roadways and in low-lying areas, as well as rises in small stream levels, none of which pose an <br />immediate threat to life and property. <br />• Flood Watch: Atmospheric and Hydrologic conditions are favorable for long duration areal or river <br />flooding. <br />• Flood Warning: Long duration areal or river flooding is occurring or is imminent, which may result <br />from excessive rainfall, rapid snow met, ice jams on rivers or other similar causes. <br />4.2.2.11. Critical Values and Thresholds <br />The National Weather Service uses flood categories to communicate/categorize the severity of flood <br />impacts in the corresponding river/stream reach. The severity of flooding at a given stage is not necessarily <br />the same at all locations along a river reach due to varying channel/bank characteristics or presence of <br />levees on portions of the reach. Therefore, the upper and lower stages for a given flood category are <br />usually associated with water levels corresponding to the most significant flood impacts somewhere in <br />the reach. <br />The flood categories used by the National Weather Service are: <br />• Minor Flooding - minimal or no property damage, but possibly some public threat (e.g., <br />inundation of roads). <br />• Moderate Flooding - some inundation of structures and roads near stream. Some evacuations of <br />people and/or transfer of property to higher elevations. <br />• Major Flooding - extensive inundation of structures and roads. Significant evacuations of people <br />and/or transfer of property to higher elevations. <br />• Record Flooding - flooding which equals or exceeds the highest stage or discharge observed at a <br />given site during the period of record. The highest stage on record is not necessarily above the <br />other three flood categories, it may be within any of them or even less than the lowest, <br />particularly if the period of record is short (e.g., a few years). It is also important to note that <br />minor, moderate, major flood categories do not necessarily exist for all forecast points. For <br />example, a location with a permanent levee may begin to experience impacts at moderate <br />flooding level. <br />4.2.2.12. Prevention <br />Most prevention methods of river flooding fall under mitigation actions. See Mitigation below for <br />methods of prevention. <br />4.2.2.13. Mitigation <br />There are many ways to mitigate flooding hazards. Two techniques are hard and soft engineering <br />mitigation techniques. Hard engineering techniques include building dams, levees, wing dykes, and <br />diversion spillways. Soft engineering techniques include floodplain zoning, afforestation, wet plain <br />restoration, river restoration, and removal of properties in flood prone areas. <br />42 <br />