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

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

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

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

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

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

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

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

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

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

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

Hydrologic Modeling Use of mathematical equations to estimate runoff as a result of: Source: weather patterns, land use and topography.

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

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

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

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

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

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

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

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

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

Sized BMPs for range site conditions 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: in/hr Developed Relationship Developed Relationship –Contributing Imp Area and BMP Area –Linear, typical R 2 at least 0.99 Sizing Equations Developed Using Continuous Modeling

Sizing Relationships for Sites with 52” Annual Rainfall

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

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

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

Sizing Equations BMP Design Infilt. Rate (in/hr) Forest Standard Treatment Standard MBMB Bioretention Cell 6 inch ponding depth inch ponding depth BR Bottom Area (sf) = Impervious Area (sf) x [M x Mean Annual Precipitation (in) + B]

Sizing Equations BMP Design Infilt. Rate (in/hr) Forest Standard Treatment Standard MBMB Bioretention Cell 6 inch ponding depth inch ponding depth Site Precip. = 49 inches/yr BR Bottom Area (sf) = Impervious Area (sf) x [ x ] = Imp. Area (sf) x

Sizing Equations BMP Design Infilt. Rate (in/hr) Forest Standard Treatment Standard MBMB Bioretention Cell 6 inch ponding depth inch ponding depth Site Precip. = 49 inches/yr BR Bottom Area (sf) = Impervious Area (sf) x [ x ] = Imp. Area (sf) x

Sizing Equations PP Area (sf) = Impervious Area (sf) x [M x Mean Annual Precipitation (in) + B] BMP Design Infilt. Rate (in/hr) Forest Standard MB Permeable Pavement Surface depth calculated 0.13 – ≥ Permeable Pavement Facility 6 inch ponding depth PP Aggregate Depth (in) = M x Mean Annual Precipitation (in)

Sizing Equations BMP Design Infilt. Rate (in/hr) Forest Standard MB Infiltration Trench 18 inchdepth Gravelless Chamber Void space of 2.6 cf/ft Length (ft) = Impervious Area (sf) x [M x Mean Annual Precipitation (in) + B]

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

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

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

Flow Control Credits Facility Type Design Variable Flow Control Credit (Forest Standard) Retained Trees Evergreen 20% canopy area (min 100 sf/tree) Deciduous 10% canopy area (min 80 sf/tree) New Trees Evergreen 50 sf / tree Deciduous 20 sf / tree Green Roof 4 inch depth 42% 8 inch depth 46% DispersionNA77% Permeable Pavement Surface (3” subbase) Slope ≤ 2% 100% Slope 2% - 5% 41%

Flow Control Credits Facility Type Design Variable Flow Control Credit (Forest Standard) Retained Trees Evergreen 20% canopy area (min 100 sf/tree) Deciduous 10% canopy area (min 80 sf/tree) New Trees Evergreen 50 sf / tree Deciduous 20 sf / tree Green Roof 4 inch depth 42% 8 inch depth 46% DispersionNA77% Permeable Pavement Surface (3” subbase) Slope ≤ 2% 100% Slope 2% - 5% 41% Impervious Area Mitigated = Canopy Area x 20%

Flow Control Credits Facility Type Design Variable Flow Control Credit (Forest Standard) Retained Trees Evergreen 20% canopy area (min 100 sf/tree) Deciduous 10% canopy area (min 80 sf/tree) New Trees Evergreen 50 sf / tree Deciduous 20 sf / tree Green Roof 4 inch depth 42% 8 inch depth 46% DispersionNA77% Permeable Pavement Surface (3” subbase) Slope ≤ 2% 100% Slope 2% - 5% 41% Impervious Area Mitigated = Green Roof Area x 42% Downstream BMP Sized for Green Roof Area x 58%

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

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

Questions? Alice Lancaster, PE Herrera Environmental Consultants