08-12-2019 Council Packet - Laserfiche WebLink

Home Search

Anoka County 2019 Multi-Jurisdictional All Hazards Mitigation Plan 76 81B4.2.1.3 Landslides/Mudslides Landslides (rockslides, mudslides, etc.) are among the most common natural hazards. Unlike most natural hazards, however, most damage is not caused by extreme events, but by uncounted (and often unreported) minor events. Slumps usually damage utilities within or below the slide mass, but seldom cause a threat to life. Flows, in addition to the above hazards can flow around well-built structures, preserving them but causing damage from water and mud. Translational slides can be the most catastrophic. In addition to presenting a hazard to structures and utilities, they can cause damage and death both far from and only slightly below the source. The hazards associated with landslides are as diverse as the types of failure. Falls may damage roads or buildings at the base of a steep slope, injure climbers, or remain on a road as a hazard to transportation. In addition to the direct hazards of a landslide moving out from under or onto structures or utilities, there is a major indirect hazard. Large slides generally do not stop moving until they reach the bottom of a valley where they block streams, usually resulting in flooding and damage to the system ecology (e.g. sediment). 82B4.2.1.4 Land Subsidence Subsidence is the formation of depressions, cracks, and sinkholes in the earth's surface, which normally occurs over many days to a few years. Karst topography develops when beds of relatively soft limestone and dolomite are present. The diluted organic acids present in water percolates downward and dissolves these formations. In such places, rock is honeycombed with cracks, fissures and potentially sizable caverns, which can collapse. In some areas, natural drainage occurs primarily below ground rather than surface streams. These underground passages are commonly connected to the surface by funnel-shaped depressions called sinkholes. The formation of these sinkholes often leads to ground subsidence or collapse. This results from the settlement of collapse of overlying materials into solution openings beneath the surface, such as caves or enlarged joints. Sinkhole development is usually a slow process; however, they may occur suddenly, without warning.