USING LIDAR TO MAP STABLE SLOPE SETBACKS ON LAKE SUPERIOR SHORE BLUFFS IN IRON AND DOUGLAS COUNTIES, WISCONSIN

Primarily because of its proximity to the Minneapolis-St. Paul population center, there is increasing development pressure on this part of the Lake Superior shoreline. Much of the shoreline of Iron and Douglas counties has bluffs from about 30 to almost 100 feet (10-30m) high. Our setback line is based on stable slope angle, rate of past recession, and a facility setback. The bluffs consist almost entirely of clayey till, sandy, stony till or sand and gravel. The geology of the bluff has been described in two earlier studies, and we use that vertical distribution of sediment, modified by field observations in 2011, as the basis for interpretation of sediment type. Stable slope angles for each sediment were established by measuring natural slopes in the area and determining what angle appears to separate stable from unstable slopes. The stable slope component of setback is the horizontal distance from the base of the bluff to where the stable slope angle intersects the bluff top. This is calculated in a GIS. Most past shoreline recession rates range from almost zero to about 6 feet (2 m) per year. Past annual recession rates, determined in a separate study by comparison of orthophotos taken at least two different times in the past, are multiplied by 50 years. These are added to the stable slope setback and a 75 foot facility setback to produce a total setback line.