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Simplified Sizing Tool for LID Practices in western Washington

Simplified Sizing Tool for LID Practices in western Washington. Alice Lancaster, PE Herrera Environmental Consultants. Presentation Overview. WA NPDES permit Simplified tool: allows sizing of pre-designed LID BMPs without continuous modeling Kitsap County Western WA lowlands.

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Simplified Sizing Tool for LID Practices in western Washington

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  1. Simplified Sizing Tool for LID Practices in western Washington Alice Lancaster, PE Herrera Environmental Consultants

  2. Presentation Overview • WA NPDES permit • Simplified tool: allows sizing of pre-designed LID BMPs without continuous modeling • Kitsap County • Western WA lowlands

  3. NPDES Permit Requirements Oregon Hydromodification Provisions Assessment of hydromod impacts Development strategies National Shift LID, infiltration MEP ->numerical stds Focus on flow duration Western Washington LID to maximum extent feasible Numerical Std: Match pre-developed flow durations Continuous hydrologic modeling

  4. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function

  5. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function Compost Amended Soil

  6. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function Retain Trees and Vegetation

  7. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function Green Roofs

  8. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function Bioretention

  9. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function Permeable Pavement

  10. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function Dispersion

  11. Minimize impervious surfaces Minimize disturbance to soils & vegetation BMPs that reduce runoff using infiltration, evapotranspiration, or stormwater reuse Examples…. BMPs to Reduce Hydromod Mimic predeveloped hydrologic function Rain Water Harvesting

  12. Hydrologic Modeling Use of mathematical equations to estimate runoff as a result of: • weather patterns, • land use and • topography. Source: http://www.und.nodak.edu/

  13. Single-Event Methods Single storm event (typically 24-hours) Simulate corresponding runoff (peak flow) Ex. SCS, SBUH, StormShed, xp-SWMM, HEC-HMS Rainfall (in) Unmit. Runoff (cfs) Mit. Runoff (cfs) Runoff (cfs) Rainfall (inches) Time (hrs)

  14. Continuous Models Rainfall (in) Unmit. Runoff (cfs) Mit. Runoff (cfs) Runoff (cfs) Rainfall (inches) Time (hrs) • Long-term rainfall and evaporation • Simulate long-term runoff • Flow durations -> period of time channels experience elevated flows Ex. HSPF, WWHM, MGS Flood

  15. Challenges for Region 26 inches • New BMPs • New Modeling Requirements • New Standards -> BMP size vary widely across the region • Rainfall patterns • Precipitation depth • Soil characteristics • Predeveloped vegetation cover 68 inches

  16. Objectives of Project • Size BMPs without extensive calculations or continuous modeling • Streamline agency review(“rule of thumb” sizing equations) • Reduce barriers to LID implementation

  17. Project Site • Kitsap County • Interest in integrating LID • Support plan reviewers • Allow design without engineer Coming Soon to all western WA lowlands

  18. Sizing Tool Summary • “Pre-Designed” BMPs: prescribed depths, slopes, porosities, etc. • Sizing Equations: BMPs by contributing impervious area, infiltration rate & precip • Flow Credits: impervious surface reduction credits • Calculator

  19. Sizing Equations • Infiltration BMPs • Function of contributing area, infiltration rate & precip. Seattle Public Utilities

  20. Sizing Equations “Pre-Designed” BMPs • Bioretention Cell • Permeable Pavement Facility • Infiltration Trench • Gravelless Chamber

  21. Sizing Equations Design Configurations • No underdrain • Ponding: 2 to 10” • Side slopes 3H:1V • 12” bioretention soil (18” for treatment) per Seattle specs • Bioretention Cell • Permeable Pavement Facility • Infiltration Trench • Gravelless Chamber

  22. Sizing Equations Developed Using Continuous Modeling • Sized BMPs for range site conditions • Western Washington Hydrology Model • Precipitation: 32, 36, 44, 52 in/yr • Contributing Imp Area: 2,000 – 10,000 sf • Design Infilt. Rates: 0.13 - 2.0 in/hr • Developed Relationship • Contributing Imp Area and BMP Area • Linear, typical R2 at least 0.99

  23. Sizing Relationships for Sites with 52” Annual Rainfall

  24. Sizing Factors for Sites with 52” Annual Rainfall BR Area = 0.21 x Imp. Area

  25. Sizing Factorsfor Sites with 52” Annual Rainfall Sizing Factors 42% 29.3% 21% BR Area = SF x Imp. Area

  26. Sizing Equationsfor Anywhere in Kitsap County Slope (M) Sizing Factors Y-Intercept (B) BR Area = Imp. Area x [(M x Precip) + B] Bioret Area = SF x Imp. Area Sizing Factor

  27. Sizing Equations BR Bottom Area (sf) = Impervious Area (sf) x [M x Mean Annual Precipitation (in) + B]

  28. Sizing Equations Site Precip. = 49 inches/yr BR Bottom Area (sf) = Impervious Area (sf) x [0.0026 x 49 + 0.0269] = Imp. Area (sf) x 0.1543

  29. Sizing Equations Site Precip. = 49 inches/yr BR Bottom Area (sf) = Impervious Area (sf) x [0.0006 x 49 + 0.0002] = Imp. Area (sf) x 0.0296

  30. Sizing Equations PP Aggregate Depth (in) = M x Mean Annual Precipitation (in) PP Area (sf) = Impervious Area (sf) x [M x Mean Annual Precipitation (in) + B]

  31. Sizing Equations Length (ft) = Impervious Area (sf) x [M x Mean Annual Precipitation (in) + B]

  32. Flow Control Credits • Impervious surface reduction credit • Degree standard achieved Seattle Public Utilities

  33. Flow Control Credits“Pre-Designed” BMPs • Retained Trees • Newly Planted Trees • Green Roof • Dispersion • Permeable Pavement Surface

  34. Flow Control CreditsDesign Configurations • Retained Trees • Newly Planted Trees • Green Roof • Dispersion • Permeable Pavement Surface • Evergreen and deciduous trees • Minimum 6” DBH Literature review of infiltration, evapotranspiration, interception

  35. Flow Control Credits

  36. Flow Control Credits Impervious Area Mitigated = Canopy Area x 20%

  37. Flow Control Credits Impervious Area Mitigated = Green Roof Area x 42%Downstream BMP Sized for Green Roof Area x 58%

  38. BMP Sizing Calculator • Guides designer through selecting and sizing BMPs • Documentation for project submittal

  39. Conclusions • Relatively easy method for designers to select and size BMPs • Streamline agency review • Rule of thumb sizing • Standard submittal worksheet • Reduces barriers to LID implementation • Through Ecology grant will expand tool to western Washington lowlands in 2010-2011

  40. Questions? Alice Lancaster, PE Herrera Environmental Consultants www.herrerainc.com alancaster@herrerainc.com 206-441-9080

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