Laserfiche WebLink
2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />quality of our drinking water sources and the health of our aquatic ecosystems. <br />4.2.1.4. Potential for Cascading Effects <br />Structures that encroach on the floodplain, such as bridges, can increase upstream urban flooding by <br />narrowing the width of the channel which can cause sediment and debris carried by floodwaters further <br />because the flow is occurring at a higher stage past the obstructions. This can cause channels to become <br />filled with sediment or become clogged with debris causing issues farther upstream from where the initial <br />flooding occurred. <br />Depending on the extent of the flooding, water quality becomes an issue because it becomes necessary <br />to treat contaminated runoff, but depending on the contaminants present this process can be very costly <br />especially when compared to its benefits. In addition to water quality in the runoff poses issues, if any <br />sewer or water treatment plants have been flooded, homes may now not have access to clean water or <br />working restrooms. <br />4.2.1.5. Geographic Scope of Hazard Blc <br />The extent of urban flooding in Hennepin County really depends on an extremely complex set of <br />interactions between the surface and sub -surface drainage networks and features of the environment. <br />Urban flooding can be small in geographic scope as in just a few streets or neighborhoods with minor <br />flooding damage, to large areas of entire cities being under water. <br />4.2.1.6. Chronologic Patterns <br />Urban flooding in Hennepin County typically occurs in the spring and summer months associated with <br />thunderstorms. Springtime urban flooding can come from both snowfall melt and runoff during the spring, <br />a spring thunderstorm that comes before the ground has had time to that completely preventing <br />infiltration, or just a normal thunderstorm (or multiple thunderstorms within a smaller period) with <br />excessive rainfall rates. <br />4.2.1.7. Historical Data Bld <br />Floods have been documented all the way back to 1776 in Minnesota. However official American records <br />don't begin until 1873. As mentioned in river flooding, of the 24 State of Minnesota Flood Declarations, <br />Hennepin County has been included in six, with all having urban flooding issues with road and bridge <br />closures. There have been no other naturally caused incidents that are within the scope of this plan. <br />• 1965 Flooding (DR-188) <br />• 1969 Flooding (DR-255) <br />• 1997 Severe Flooding, High Winds, Severe Storms (DR-1175) <br />• 2001 Severe Winter Storms, Flooding, and Tornadoes (DR-1370) <br />• 2010 Flooding (DR-3310) <br />• 2014 Severe Storms, Straight -Line Winds, Flooding, Landslides and Mudslides (DR-4182) <br />• 2016 Severe Storms & Flooding (DR-4290) <br />34 <br />