LANDSLIDE INVENTORY AND CATEGORIZATION FOR HENNEPIN COUNTY, MINNESOTA

After recent precipitation-induced landslides in the region, Hennepin County Emergency Management funded a historical inventory of failures based primarily on newspaper reports. This work is the next step, a geomorphic landslide inventory wherein the historical landslide locations were plotted on a LiDAR-derived hillshade DEM to learn the expression of landslides. Three main types of slope movement occur, two of which can be identified on the hillshade DEM:

1) Rotational—downslope movement of sediment on a curvilinear surface rupture resulting in a headscarp and lobate deposit that were blocky and sometimes involved liquefaction. Large rotational landslides estimated to be more than 4.5 m deep were separated from small failures.

2) Debris flow—a mixture of water, sediment, rock and debris that has become a slurry and moves downslope, commonly with some speed. This category was difficult to distinguish from areas of multiple small slides, dry failure of sand and colluvial or alluvial fans.

3) Topple or falls of rock—defined by a rapid, nearly vertical movement of masses of bedrock or boulders are known to occur but are not distinguishable on the DEM.

Areas of concentrated deep-seated slide activity are primarily located along the Minnesota River and its major tributaries and associated with springlines. The largest appear to predate development of the area because roads and homes are built on the slide deposits. However, reactivation of small portions of these slides is visible, especially where water discharges.

There were 203 scarps mapped, 134 of which had clear, lobate deposits. There were 1146 small slide areas scattered along steep slopes in glacial sediment, primarily till and modified till. These occurred along shorelines and areas of focused overland flow like ravines. In these settings, ongoing erosional processes continue to steepen slopes, priming them for failure. Human alteration to these slopes that mimic erosion, steepen them, or saturate them can also cause failure, e.g., focused discharge, cutting away at the toe, or infiltration of water at the top.

Two notable areas without large rotational failures along the Minnesota River are associated with a previously mapped bedrock valley system which is filled with a sequence of sediment layers with different hydrologic properties.