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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL Steep Slope Areas Existing Steep Slope Areas—On-site steep slope areas that already have native soils and robust native landscapes should be protected from disturbance as a preference to re-grading and augmenting the disturbed soil with soil amendment. Also, steep slope areas may be subject to critical area protection per the City's Critical Areas Code(EMC 19.37),which outlines criteria for classification of geologically hazardous areas. Steep slopes that remain on site and that are not subject to the City's Critical Areas Code which are not constructed as part of the development AND where native soils and vegetation is sparse should be amended by planting deep rooting vegetation. Soil amendments shall be applied via a pit application at least twice as wide as the root ball of the vegetation being planted with a mix of 50 percent compost to 50 percent soil mixture. Constructed Steep Slopes—In lawn and landscaped areas,the slope angle should be minimized to the greatest extent possible for both stability and lawn maintenance concerns and shall be no steeper than the angle of repose of the underlying soils. Terracing is recommended to minimize steep slope angles. Adequate drainage systems must be installed on constructed steep slope areas where it is determined that retained runoff may cause instability. To provide adequate drainage,a professional engineer must determine the drainage pattern of the slope and design/install controlled drainage at the outfall of these areas. A subsurface collection system should be installed at the base of each terrace to redirect water away from any retaining structures. Subsurface collection systems may also be necessary in low depressions of a non-uniform site, although it is recommended that these areas be left undisturbed so as to serve as natural stormwater retention areas. An appropriate receiving area for the water collected and concentrated by the subsurface drainage system must be provided. Additional guidelines for this BMP can be found in Building Soil: Guidelines and Resources for Implementing Soil Quality and Depth BMP T5.13 in Ecology Stormwater Management Manual for Western Washington,which is available at www.soilsforsalmon.org under the"How-To" section,or at <www.buildingsoil.org>. 12.3.3 BMP 12.30 On-site Infiltration Areas On-site infiltration areas are small on-site depressions or infiltration trenches. Only runoff from non- pollutant generating impervious and pervious surfaces may be discharged to on-site infiltration areas designed per this volume. Infiltration for stormwater treatment is covered in Chapter 9. This section provides pre-sizing of on-site infiltration areas and may be used for the design of small infiltration areas if the contributing basin area meets the following criteria: 1. has a contributing impervious area,x,less than 2,000 square feet; and 2. has a contributing pervious area,y,less than 13,000 square feet; and 3. has a combination of impervious and pervious surfaces such that the"equivalent impervious area", as defined by the following equation, is less than 2,000 square feet: equivalent impervious area=x+(y+ 6.5) where x=the square feet of impervious area in the contributing basin y =the square feet of pervious area in the contributing basin Infiltration areas with larger contributing basins must be designed in accordance with the criteria in Chapter 9 of this volume. Volume 111—Flow Control and Treatment 12-7