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Physical Properties of Bioretention Soils

Physical Properties of Bioretention Soils. Scott Wikstrom City of Walnut Creek. Stormwater Treatment. Rainfall x Tributary Area x Soil Infiltration (Biotreatment) = Clean Water. Why 5? (in/hr). Compromise between treatment and loading rate

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Physical Properties of Bioretention Soils

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  1. Physical Properties of Bioretention Soils Scott Wikstrom City of Walnut Creek

  2. Stormwater Treatment Rainfall x Tributary Area x Soil Infiltration (Biotreatment) = Clean Water

  3. Why 5? (in/hr) Compromise between treatment and loading rate Treatment area: 4% of tributary impervious area Equivalent Rainfall of 400-600 inches per year Seems to work!!! Adopted into Municipal Regional Permit

  4. Soil Infiltration Rate • Gravel = high infiltration • Sandy Loam = Just Right! • Clays = low infiltration

  5. Sandy Loam or is it Loamy Sand?

  6. Soil Mechanics 101 • Three types of soil: Sand, Silt, and Clay • Silt is just very small Sand • Finer than 0.0029 inches (#200 sieve) • Clays are different • Finer than 0.0029 inches (#200 sieve) • Thin, flat plate-like shape • Negative electrical charge attracts and bonds to water

  7. Soil Infiltration (Permeability) Ranges Gravel ~ 10,000 inches / hr Clean Sand ~ 50 inches / hr Fine Sand / Silt ~ 1 inch / hr Silty Sandy Clay ~ 0.05 inches / hr Clay ~ 0.001 inches / hr

  8. Stormwater Treatment Rainfall x Tributary Area x Soil Infiltration = Clean Water Rainfall +/- 100% Area +/- 20% Infiltration +/- 10,000%

  9. Soil Variability • Vertical Variation • Horizontal Variation • Parent Rock Material • Deposition Method

  10. Contra Costa Soil Spec – Part I • Several installations using on-site soils failed • Soil Supplier mix worked well • Well graded mix of two local soils • Some “Topsoil” • Amended with Compost • How to Verify? • Hard to replicate throughout Bay Area

  11. Contra Costa Soil Spec - Part II • Target: 8-12 inches per hour • Goal: 5-50 inches per hour • Limit fines content, especially clay • Readily available – not a proprietary soil blend • Consistent mix • Testing and verification considerations

  12. Soil Gradation Curves

  13. Contra Costa Soil Spec – Part III • Performance Spec • Minimum 5 inches per hour infiltration rate • Support Healthy Plant Growth • Two Mixes provided: Fine Sand with Compost • ASTM C33 Fine Aggregate “Concrete Sand” • Contra Costa Clean Water Program Verification • Inspectors pull tags at jobsite.

  14. Questions? • Comments! • Discussion… Scott Wikstrom (925) 943-5899 x2246 wikstrom@walnut-creek.org

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