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854 (3) The size and extent of the structures are the minimum necessary to correct the erosion <br />855 problem and are not larger than the following, unless a professional engineer determines <br />856 that a larger structure is needed to correct the erosion problem: <br />857 (i) Retaining walls must not exceed five feet in height and must be placed a minimum <br />858 horizontal distance of ten feet apart; and <br />859 (ii) Riprap must not exceed the height of the regulatory flood protection elevation. <br />860 c.Repair of existing rock riprap, retaining walls, and other erosion control structures above the OHWL <br />861 does not require a permit provided it does not involve any land alterationunder Section 117- <br />862 148(j)2 (Land alteration permit). <br />863 4. Stormwater management. <br />864 a. In the bluff impact zone, storm water management facilities are prohibited, except by permit if: <br />865 (1) There are no alternatives for storm water treatment outside the bluff impact zone on the <br />866 subject site; <br />867 (2) The site generating runoff is designed so that the amount of runoff reaching the bluff impact <br />868 zone is reduced to the greatest extent practicable; <br />869 (3) The construction and operation of the facility does not affect slope stability on the subject <br />870 property or adjacent properties; and <br />871 (4) Mitigation based on the best available engineering and geological practices is required and <br />872 applied to eliminate or minimize the risk of slope failure. <br />873 b. In the water quality impact zone, development that creates new impervious surface, as allowed <br />874 by exemption in Section 117-148(l) (Exemptions), or fully reconstructs existing impervious <br />875 surface of more than 10,000 square feet requires a storm water permit or approved storm water <br />876 plan. Multipurpose trails and sidewalks are exempt if there is down gradient vegetation or a filter <br />877 strip that is at least five feet wide. <br />878 c. In all other areas, storm water runoff must be directed away from the bluff impact zones or <br />879 unstable areas. <br />880 5. Development on steep slopes. Construction of structures, impervious surfaces, land alteration, <br />881 vegetation removal, or other construction activities are allowed on steep slopes if: <br />882 a.The development can be accomplished without increasing erosion or storm water runoff; <br />883 b. The soil types and geology are suitable for the proposed development; and <br />884 c. Vegetation is managed according to the requirements of Section 117-148(i) (Vegetation <br />885 management). <br />886 6. Conditions of land alteration permit approval. <br />887 a. Temporary and permanent erosion and sediment control measures retain sediment onsite consistent <br />888 with best management practices in the Minnesota Stormwater Manual; <br />889 b. Natural site topography, soil, and vegetation conditions are used to control runoff and reduce <br />890 erosion and sedimentation; <br />891 c. Construction activity is phased when possible; <br />892 d. All erosion and sediment controls are installed before starting any land disturbance activity; <br />25 12/30/2020 <br /> <br /> <br />