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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />4.2.1.10. Detection & Warning <br />The National Weather Service issues flash flood advisories, watches, and warnings. <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 />• Flash Flood Watch: Atmospheric and hydrologic conditions are favorable for short duration flash <br />flooding and/or dam break is possible. <br />• Flash Flood Warning: Excessive rainfall producing thunderstorms have developed, lead to short <br />duration flash flooding. A warning may also be issued if a dam break has occurred. <br />4.2.1.11. Critical Values and Thresholds <br />Using thresholds for flooding indicators can be intellectual traps for the uneducated and what constitutes <br />an important threshold in one situation may be unimportant in another. In broad terms, moderately high <br />rainfall rates begin at about 1 inch per hour, and moderately long durations begin at about one hour, but <br />these should be considered only as the crudest of guidelines. <br />Conversation with the local National Weather Service in Chanhassen, MN has concluded that local <br />forecasters tend to look at the rainfall rate and return period more than any amount threshold. It also <br />depends on antecedent conditions. Consensus between the hydrologist and an operation warning <br />forecaster is they look for model outputs to show them at least a 10-year event as a starting point to get <br />flash flooding. In addition, using one particular source, they use a return period for precipitation to have <br />at least a 20-50-year event to get flash urban flooding in the Twin Cities Metro area. <br />4.2.1.12. Prevention <br />To improve water management and protect the sewage system from damage, cities can revamp their <br />underground pipe and drainage systems by separating rainwater from the sewage system. The separation <br />enables the wastewater treatment plant to function properly, without it being overburdened by large <br />quantities of storm water. <br />Other more obvious methods are to keep sewer systems clean of clog up with waste, debris, sediment, <br />tree roots and leaves. <br />4.2.1.13. Mitigation <br />Areas that have been identified as flood prone areas can be turned into parks, or playgrounds, buildings <br />and bridges can be lifted, floodwalls and levees, drainage systems, permeable pavement, soil <br />amendments, and reducing impermeable surfaces. Reducing impervious surfaces could include the <br />addition of green roofs, rain gardens, grass paver parking lots, or infiltration trenches. <br />Other mitigation strategies include developing a floodplain management plan, form partnerships to <br />support floodplain management, limit or restrict development in floodplain areas, adopt and enforce <br />building codes and development standards, improve storm water management planning, adopt policies <br />to reduce storm water runoff, and improve the flood risk assessment. <br />36 <br />