04-04-2012 Planning Commission Work Session

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The statements of need describing the definitions relevant to the above discussion of bluff setbacks and bluff impact zones are discussed below. Subp lb. Bluff: The definition of a bluff is needed because of the new proposal to manage bluff areas in shoreland areas, as described in the Statement of Need for bluff top setbacks and bluff impact zones. The topographic features of a bluff need to be described in detail so shoreland managers, surveyors,property owners and others can have a common understanding of the conditions that constitute a bluff. These descriptions are contained in items A through D of subp. lb.The provision that a bluff slope rise at least 25 feet above the ordinary high water level was developed by the Shoreland Management Committee after a presentation by DNR staff and subsequent discussion about what the appropriate height limitation should be for these regulations. A consensus was reached that 25 feet was appropriate because areas less than 25 feet in height do not have as great a potential for significant slope erosion and failure problems. This value is supported by research indicating a strong relationship between erosion and bluff height when the bluff height is greater than 18'for historic bluff recession on Lake Ontario (1)*. Recognizing that these rules are intended for substantially smaller lake systems than Lake Ontario,the intent is not to extrapolate from that research. Instead,the Lake Ontario research is mentioned to indicate that the 25'height value as derived by shoreland committee members and based on their professional experience is realistic and reflective of documented research. Additionally,committee members agreed that shoreland topographic features of less than 25 feet in elevation are not as visually significant as those areas that are 25 feet or more above the waterbody. An average slope gradient of 300 or more to define a bluff is reasonable since slopes in this category generally begin to exhibit significant problems of erosion,mass wasting, slumping or instability if they are altered by vegetative removal and disruption or grading and filling. Communication with researchers at the University of Wisconsin(2)* indicated that on a general basis, slopes ranging from 25% to 32% are slopes that should be considered as approaching the ultimate angle of stability. In Douglas County,Wisconsin, a generalized stable slope angle of 33%has been suggested for regulatory purposes in predominantly clayey soils on Lake Superior(3)*. Several factors contribute to the stability or instability of a slope,among them the variation of the soil profile,the height of the bluff or slope, soil moisture and groundwater conditions, surface drainage and vegetation cover(4)*. Since these factors can vary significantly from site to site, an average slope gradient of 30%is reasonable and will serve to caution shoreland managers when shoreland development is proposed in bluff areas. Where soil erosion and stability are not issues, as would be the case if rack outcroppings comprise a bluff, a 30% slope combined with a 25 foot or more vertical height does constitute a prominent topographical feature that requires management to preserve the natural scenic values of shoreland areas. It is also reasonable to require that a bluff slope must drain to the waterbody, thereby excluding areas that meet the slope and height requirements of a bluff within shoreland areas, but effectively may have no potential for negative impacts on shoreland values. Finally,it is necessary to exclude from the definition of a bluff any areas that include lands that have average slopes of 18% or less for 50 feet or more between the top of the bluff and the toe of the bluff(as defined later in this document), since these areas could qualify for building sites if the ordinary high water level setback can be met. 9 3