6708 CADY RD 2025-03-31 - Laserfiche WebLink

meet this BMP be appropriate and beneficial to the plant cover to be established. Likewise, it is important that imported topsoils improve soil conditions and do not have an excessive percent of clay fines. This BMP can be considered infeasible on till soil slopes greater than 33 percent. Design Guidelines Soil Retention Retain, in an undisturbed state,the duff layer and native topsoil to the maximum extent practicable. In any areas requiring grading, remove and stockpile the duff layer and topsoil on site in a des- ignated, controlled area, not adjacent to public resources and critical areas,to be reapplied to other portions of the site where feasible. Soil Quality All areas subject to clearing and grading that have not been covered by impervious surface, incor- porated into a drainage facility or engineered as structural fill or slope shall, at project completion, demonstrate the following: 1. A topsoil layer with a minimum organic matter content of 10%dry weight in planting beds, and 5%organic matter content in turf areas, and a pH from 6.0 to 8.0 or matching the pH of the undisturbed soil.The topsoil layer shall have a minimum depth of eight inches except where tree roots limit the depth of incorporation of amendments needed to meet the criteria. Subsoils below the topsoil layer should be scarified at least 4 inches with some incorporation of the upper material to avoid stratified layers,where feasible. 2. Mulch planting beds with 2 inches of organic material. 3. Use compost and other materials that meet the following organic content requirements: a. The organic content for"pre-approved"amendment rates can be met only using com- post meeting the compost specification for BMP T7.30: Bioretention,with the exception that the compost may have up to 35% biosolids or manure. The compost must also have an organic matter content of 40%to 65%, and a carbon to nitrogen ratio below 25:1. The carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region. b. Calculated amendment rates may be met through use of composted material meeting (a.)above;or other organic materials amended to meet the carbon to nitrogen ratio requirements, and not exceeding the contaminant limits identified in Table 220-B, Test- ing Parameters, in WAC 173-350-220. The resulting soil should be conducive to the type of vegetation to be established. 2019 Stormwater Management Manual for Western Washington Volume V-Chapter 11 -Page 928