19.1 BMP L613 - Post-Construction Soil Quality and Depth
Naturally occurring (undisturbed) soil and vegetation provide important stormwater functions including: water infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant biofiltration; water interflow storage and transmission; and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with minimal topsoil and sod. Not only are these important stormwater functions lost, but such landscapes themselves become pollution generating pervious surfaces due to increased use of pesticides, fertilizers and other landscaping and household/industrial chemicals, the concentration of pet wastes, and pollutants that accompany roadside litter.
Soil amendments are required for the disturbed areas of sites subject to Minimum Requirement 5 - Onsite Stormwater Management that will not be covered by an impervious surface, incorporated into a stormwater facility, or engineered as structural fill or slope.
Where Minimum Requirement 5 - Onsite Stormwater Management does not apply, it is recommended to incorporate soil amendments into all disturbed areas of the site.
19.1.3 Infeasibility Criteria for Soil Quality BMPs
Post-Construction soil quality and depth is considered infeasible on slopes greater than 33%. Citation of this infeasibility criterion requires evaluation of site specific conditions.
This BMP is considered infeasible if there are any conflicts with any of the following competing needs criteria:
Requirements of the following federal or state laws, rules, and standards:
Historic Preservation Laws and Archaeology Laws as listed at: https://dahp.wa.gov/project-review/preservation-laws
Federal Superfund or Washington State Model Toxics Control Act
Federal Aviation Administration requirements for airports
American with Disabilities Act
When found to be in conflict with special zoning district design criteria adopted and being implemented pursuant to a community planning process.
Public health and safety standards
Transportation regulations to maintain the option for future expansion or multi-modal use of public rights-of-way.
City of Tacoma Critical Area Ordinances that provides protection of tree species or other critical areas.
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
Retain the duff layer and native topsoil in an undisturbed state to the maximum extent practicable. In any areas requiring grading, remove and stockpile the duff layer and topsoil onsite in a designated, controlled area, not adjacent to public resources and critical areas, to be reapplied to other portions of the site where feasible. Use appropriate BMPs to ensure retained soil does not enter the stormwater system.
All areas subject to clearing and grading that will not be covered by impervious surface, incorporated into a stormwater facility or engineered as structural fill or slope shall, at project completion:
Have a topsoil layer that is 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.
The topsoil layer shall have a minimum organic matter content of 10% dry weight in planting beds, and minimum 5% organic matter content in turf areas, and a pH from 6.0 to 8.0 or matching the pH of the original undisturbed soil.
Planting beds have been mulched with 2 inches of organic material
To meet the organic content requirements above use one of the following options:
Compost applied at the Preapproved Rate (see Table 4 - 23: Amendment Options). Compost shall meet the compost specifications in A900 - Compost.
Compost applied at a Calculated Rate (see Table 4 - 24: Calculated Amendment Rate). Compost shall meet the compost specifications in A900 - Compost.
Other organic matter applied at a Calculated Rate (see Table 4 - 24: Calculated Amendment Rate). Other organic matter shall:
Have a carbon to nitrogen ratio below 25:1. Carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region.
Not exceed contaminant limits identified in Table 220-B, Testing Parameters, in WAC 173-350-220.
The resulting soil shall be conducive to the type of vegetation to be established.
City of Tacoma Standard Plan GSI-01 provides a graphical representation of BMP L613. The
Standard Plan is available at https://www.cityoftacoma.org/government/city_departments/public_works/engineering/standard_plans_and_g_i_s_typical_details.
19.1.4.3 Implementation Options:
Use one of the following options to meet the post construction soil quality and depth requirements. Use the information in this BMP and the most recent version of “Guidelines for Resources for Implementing Soil Quality and Depth BMP T5.13” to meet the requirements of this BMP. This guidance can be found online at: https://www.soilsforsalmon.org/ .
Option 1. Leave native vegetation and soil undisturbed, and protect from compaction during
Option 2. Amend existing site topsoil or subsoil either at default Preapproved Rates (see Table 4 - 23: Amendment Options), or at Calculated Rate (see Table 4 - 24: Calculated Amendment Rate) based on tests of the soil and amendment.
Option 3. Stockpile existing topsoil during grading, and replace it prior to planting. Stockpiled topsoil must also be amended if needed to meet the organic matter or depth requirements, either at a default Preapproved Rates (see Table 4 - 23: Amendment Options) or at a Calculated Rate (see Table 4 - 24: Calculated Amendment Rate).
Option 4. Import topsoil mix of sufficient organic content and depth to meet the requirements.
More than one method may be used on different portions of the same site. Soil that already
meets the depth and organic matter quality standards, and is not compacted, does not need to be amended.
There is a soils management plan worksheet available at www.cityoftacoma.org/stormwatermanual_templates that can be used to help determine the amount of amendments needed for a given area.
Table 4 - 23: Amendment Options OPTION 1: Leave native vegetation and soil undisturbed, and protect from compaction during construction Identify areas of the site that will not be stripped, logged, graded or driven on, and fence off those areas to prevent impacts during construction. If neither soils nor vegetation are disturbed these areas do not require amendments. OPTION 2: Amend existing site topsoil or subsoil either at Preapproved Rate or at Calculated Rate based on the tests of the soil and amendments Scarify or till subgrade to 8 inches depth (or to depth needed to achieve a total depth of 12 inches of uncompacted soil after calculated amount of amendment is added). Entire surface should be disturbed by scarification. Do not scarify within drip line of existing trees to be retained. PREAPPROVED RATE: Place 3 inches of composted material and rototill into 5 inches of existing site soil (a total amended depth of about 9.5 inches, for a settled depth of 8 inches). CALCULATED RATE: Place calculated amount of composted material or approved organic material and rototill into depth of soil needed to achieve 8 inches of settled soil at 10% organic content. Rake beds to smooth and remove surface rocks larger than 2 inches diameter. PREAPPROVED RATE: Place 1.75 inches of composted material and rototill into 6.25 inches of existing site soil (a total amended depth of about 9.5 inches, for a settled depth of 8 inches). CALCULATED RATE: Place calculated amount of composted material or approved organic material and rototill into depth of soil needed to achieve 8 inches of settled soil at 5% organic content. Water or roll to compact soil to 85% of maximum dry density. Rake to level, and remove surface rocks larger than 1 inch diameter. OPTION 3:Stockpile existing topsoil during grading, and replace it prior to planting. Stockpiled topsoil must also be amended if needed to meet the organic matter or depth requirements, either at a default Preapproved Rate or at a Calculated Rate. If placed topsoil plus compost or other organic material will amount to less than 12 inches: Scarify or till subgrade to depth needed to achieve 12 inches of loosened soil after topsoil and amendment are placed. Entire surface should be disturbed by scarification. Do not scarify within drip line of existing trees to be retained. Stockpile and cover soil with weed barrier material that sheds moisture yet allows air transmission, in approved location, prior to grading. Replace stockpiled topsoil prior to planting. Amend if needed to meet required organic content. PREAPPROVED RATE: Place 3 inches of composted material and rototill into 5 inches of replaced soil (a total amended depth of about 9.5 inches, for a settled depth of 8 inches). CALCULATED RATE: Place calculated amount of composted material or approved organic material and rototill into depth of replaced soil needed to achieve 8 inches of settled soil at 10% organic content. Rake beds to smooth and remove surface rocks larger than 2 inches diameter. Mulch planting beds with 2 inches of organic mulch or stockpiled duff. PREAPPROVED RATE: Place 1.75 inches of composted material and rototill into 6.25 inches of replaced soil (a total amended depth of about 9.5 inches, for a settled depth of 8 inches). CALCULATED RATE: Place calculated amount of composted material or approved organic material and rototill into depth of replaced soil needed to achieve 8 inches of settled soil at 5% organic content. Water or roll to compact soil to 85% of maximum dry density. Rake to level, and remove surface rocks larger than 1 inch diameter. OPTION 4: Import topsoil mix of sufficient organic content and depth to meet the requirements. Scarify or till subgrade in two directions to 6 inches depth. Entire surface should be disturbed by scarification. Do not scarify within drip line of existing trees to be retained Use imported topsoil mix containing 10% organic matter (typically around 40% compost). Soil portion must be sand or sandy loam as defined by the USDA. Place 3 inches of imported topsoil mix on surface and till into 2 inches of soil. Place second lift of 3 inches topsoil mix on surface. Rake beds to smooth, and remove surface rocks over 2 inches diameter. Use imported topsoil mix containing 5% organic matter (typically around 25% compost). Soil portion must be sand or sandy loam as defined by the USDA. Place 3 inches of imported topsoil mix on surface and till into 2 inches of soil. Place second lift of 3 inches topsoil mix on surface. Water or roll to compact soil to 85% of maximum. Rake to level, and remove surface rocks larger than 1 inch diameter Table 4 - 24: Calculated Amendment Rate CALCULATING CUSTOM AMENDMENT RATES TO ACHIEVE A TARGET SOIL ORGANIC MATTER CONTENT Where soils already have some organic content, it is often cost-effective to calculate the amount of compost amendment needed to achieve the target 10% soil organic matter for landscape beds or 5% for turf areas, rather than using the preapproved rates. Custom amendment rates can be calculated using either the equation below, or the Compost Amendment Rate Calculator available as an Excel spreadsheet online at www.SoilsforSalmon.org. EQUATION FOR CALCULATING COMPOST APPLICATION RATES: Use this equation to calculate compost application rates to achieve the target final soil organic matter content (10% for landscape beds or 5% for turf areas) for a soil with a given bulk density and initial soil organic matter. CR = Compost application rate (inches) calculated needed to achieve the target final organic matter (FOM) D = Depth of finished incorporation (inches) SBD = Soil bulk density (lb/cubic yard dry weight)* SOM% = Initial soil organic matter (%)*** FOM% = Final target soil organic matter (%)*** CBD = Compost bulk density (lb/cubic yard dry weight)** COM% = Compost organic matter (%)*** Assumption: This equation calculates compost rate using an additive approach. For example, a 3-inch compost rate incorporated to an 8-inch depth will be a final mix containing 3/8 compost and 5/8 soil by volume. * To convert Soil Bulk Density in g/cm3 units to lb/cubic yard, multiply by 1697. ** To convert Compost Bulk Density from lb/cubic yard “as is” to lb/cubic yard dry weight, multiply by solids content. *** All Organic Matter measurements are based on the commonly used “loss-on-combustion” method.
Establish soil quality and depth toward the end of construction and once established, protect from compaction, such as from large machinery use, and from erosion.
Plant vegetation and mulch the amended soil area after installation.
Leave plant debris or its equivalent on the soil surface to replenish organic matter.
Reduce and adjust, where possible, the use of irrigation, fertilizers, herbicides and pesticides, rather than continuing to implement formerly established practices. Follow Integrated Pest Management (IPM) techniques.
19.1.5 Post Construction Soil Quality and Depth Modeling
All areas meeting the soil quality and depth design criteria may be entered into approved runoff models as "Pasture" rather than "Lawn/Landscaping".