Agenda - Planning Commission - 02/02/2017

Home

Bluff Protection Standards Purpose Protection of bluffs in the MRCCA was a major focus of this rulemaking, and the subject of much debate, input, and analysis. Protection of bluffs is important to reduce erosion and slope failure within the MRCCA, as well as to maintain habitat and the MRCCA's iconic scenic character. The proposed bluff protection standards in these rules prohibit structures, impervious surfaces, vegetation removal, and land alteration on bluffs and in the bluff impact zone, with some exceptions. The proposed standards also require structures to be set back from the top of bluffs. Standards implementing these protections are described in the rule -by -rule analysis. This section establishes the need for these standards and consolidates the key bluff -related definitions and the underlying technical parameters used to develop the standards. Bluff Erosion and Failure Concerns The geology across the MRCCA is variable, with both glacial sediments and bedrock at the surface. These geological features are prone to erosion and natural instability. The northern portion of the MRCCA has more glacial deposits, including outwash, alluvium, colluviums, and terraces. These unconsolidated materials are sensitive to disturbance and susceptible to erosion. Downstream in the gorge area between Minneapolis and St. Paul, bedrock deposits are more prevalent. In the southern portion of the MRCCA, limestone, shale and sandstones form cliffs and outcroppings. These rocks are susceptible to fracturing, sliding, and other stressors. These glacial and bedrock materials can be unstable and are subject to slumping, sliding, creep, and erosion when exposed to stresses such as construction activities, stormwater runoff, structure placement, vegetation removal, and land alteration. Springs and seeps are the natural outlets of ground water in bluff environments, and are common in these bedrock outcroppings and cliff areas, where they contribute to slope creep, erosion and failure. (Figure 2 illustrates a bluff face with seeps.) Slope erosion is a concern throughout the MRCCA. In August 2008, the Mississippi Water Management Organization (MWMO) conducted an inventory of toe, bank, and upland erosion along the east and west banks of the Mississippi River from 1-694 south to the Ford Dam. A number of the inventoried riverbank sub -reaches showed signs of erosion and were identified as highly susceptible to future erosion'. ' Mississippi Watershed Management Organization, 2010. Figure 2. These icicles form at "the weeping wall" and emerge from beds within the Platteville Limestone. This photo was taken just down -river from the 2014 Fairview Hospital landslide in Minneapolis. Photo by Carrie Jennings. 22