ASCE LID Conference LID Analysis Considerations in Western Washington November 17, 2008 Doug Beyerlein, P.E. Clear Creek Solutions, Inc. This presentation was originally given at the ASCE LID Conference in Seattle, WA, in November 2008 by Doug Beyerlein, P.E. Clear Creek Solutions, Inc. Presentation Introduction Permeable Pavement Modeling Rain Garden/Bioretention Modeling Impervious Runoff Dispersion Modeling Green Roof Modeling Summary Introduction There is nothing magical about LIDs. Water must go somewhere. Water must either: 1. 1.Infiltrate into the soil Evaporate/transpire into the atmosphere Runoff. Introduction Key analysis considerations: 1. 1.Type of soil Meteorological conditions precipitation evapotranspiration (ET) Single-event design storm methodology doesnt work for LID modeling because: Single-event flow frequency standards are based on inappropriate assumptions. Single-event modeling does not include the long-term effects of evapotranspiration and infiltration. Introduction LID hydrologic modeling requires continuous simulation: WWHM (HSPF) Continuous simulation hydrology models the entire hydrologic cycle for multiple years. WWHM: Western Washington Hydrology Model Developed for the State of Washington Department of Ecology. Has the ability to model a full range of LID facilities and practices. Other versions for California. Introduction Permeable Pavement Modeling Permeable Pavement Reduces Runoff Volume: 1.Infiltration to native soil. 2.Evaporation. Permeable Pavement Flow Paths Infiltration to native soil Surface Runoff Rain on pavement Infiltration to gravel subgrade Underdrain Flow Infiltration through pavement Evaporation from pavement Permeable Pavement Flow Paths Infiltration to native soil is dependent on native soil characteristics. Infiltration to native soil (range of rates: zero to 0.01 in/hr) WWHM Permeable Pavement Modeling Table 1. Permeable Pavement Reduction of Total Runoff Volume Site Infiltration (in/hr) Total Runoff (in/yr) Reduction (in/yr) Reduction (%) Impervious % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % Permeable % WWHM Permeable Pavement Modeling Figure 1. Permeable Pavement Reduction of Total Runoff Volume WWHM Permeable Pavement Modeling Reasons why permeable pavement is a good LID option in Western Washington: 1.Ratio of drainage area to infiltration facility area is 1 to 1. 2.Western Washington rainfall volumes and intensities are relatively low. 3.Subsurface storage provides water for long-term slow infiltration and evaporation. Rain Garden/Bioretention Modeling Bioretention includes planter boxes Bioretention and Rain Gardens Reduce Runoff Volume: 1.Evaporation. 2.Transpiration. 3.Infiltration to native soil. WWHM Rain Garden/Bioretention Modeling Downstream control structure: Water infiltrates into the soil before runoff. Bioretention Flow Paths Infiltration to Native Soil Weir Flow Inflow to Bioretention Facility Infiltration to Amended Soil Underdrain Flow Vertical Orifice Flow Bioretention Flow Paths Infiltration to Native Soil Infiltration to native soil is dependent on native soil characteristics. WWHM Bioretention Modeling WWHM Rain Garden/Bioretention Modeling Table 2. Rain Garden Reduction of Total Runoff Volume Ratio of Rain Garden/ImperviousReduction for Till Soil Reduction for Outwash Soil 0%0.0% 2%12.1%37.2% 4%21.8%57.0% 6%31.0%68.7% 8%38.3%75.0% 10%44.7%79.1% 20%66.0%88.2% 40%82.9%92.8% 60%88.8%94.4% 80%91.5%95.2% 100%93.0%95.7% 200%95.5%96.7% WWHM Rain Garden/Bioretention Modeling Figure 2. Rain Garden Reduction of Total Runoff Volume WWHM Rain Garden/Bioretention Modeling Reasons why bioretention is a good LID option in Western Washington: 1.Regardless of volume reduction there are good water quality benefits. 2.Works best with outwash soils; less benefit with till/poor draining soils 3.Soil moisture storage provides water for long-term slow infiltration and evapotranspiration. Impervious Runoff Dispersion Modeling Dispersion of impervious roof runoff on adjacent pervious lawn allowing some water to infiltrate before becoming stormwater runoff. Impervious runoff dispersion reduces runoff volume by slowing the runoff velocity and allowing: 1.Evaporation from adjacent pervious land. 2.Transpiration from adjacent pervious land. 3.Infiltration on adjacent pervious land. Impervious Runoff Dispersion Flow Paths Infiltration to Native Soil Impervious Surface Pervious Surface Surface Runoff Interflow Infiltration to native soil is dependent on native soil characteristics. WWHM Impervious Runoff Dispersion Modeling Table 3. Impervious Area Reduction of Total Runoff Volume Ratio Pervious/ImperviousReduction % 0%0.00% 10%4.50% 20%7.20% 30%9.30% 40%11.00% 50%12.30% 75%14.80% 100%16.50% 150%18.60% 200%19.90% 400%22.30% 600%23.20% 800%23.70% 1000%24.00% 1500%24.40% 2000%24.60% WWHM Impervious Runoff Dispersion Modeling Figure 3. Impervious Area Reduction of Total Runoff Volume WWHM Impervious Runoff Dispersion Modeling Reasons why impervious runoff dispersion is a good LID option in Western Washington: 1.Turns impervious runoff into pervious runoff. 2.Works best with outwash soils; less benefit with till/poor draining soils 3.Soil moisture storage provides water for infiltration and evapotranspiration. Green Roof Modeling Green Roofs Reduce Runoff Volume: 1.Evaporation. 2.Transpiration. Green Roof Flow Paths Green Roof PET Limitations Potential Evapotranspiration (PET) Seattle November-March rainfall = 27 inches Seattle November-March PET = 3 inches (0.02 in/day) Excess runoff = 24 inches WWHM Green Roof Element WWHM Green Roof Modeling Table 4. Green Roof Reduction of Total Runoff Volume Site Soil Depth (in) Total Runoff (in/yr) Reduction (in/yr)Reduction (%) Impervious % Green Roof % Green Roof % Green Roof % Green Roof % Green Roof % Green Roof % WWHM Green Roof Modeling Figure 4. Green Roof Reduction of Total Runoff Volume WWHM Green Roof Modeling Reasons why green roofs are a good LID option in Western Washington: 1.Can be used in highly developed urban areas. 2.Soil moisture storage provides water for evapotranspiration. Summary In Western Washington: 1.Permeable pavement can provide 100% infiltration at a low infiltration rate of 0.01 inches per hour. 2.Rain gardens work best in outwash soils, but have some benefit in till soils. 3.Impervious dispersion can reduce total runoff volume by up to 25%. 4.Green roofs can reduce total runoff volume by 20% to 30%. Summary In Western Washington: 5.Soil infiltration rates play a major role in determining LID effectiveness. 6.Low winter evapotranspiration rates combined with non-stop, continuous, never-ending, will-we-ever-see-the-sun- again?, winter rainfall limit the effectiveness of green roofs in reducing total runoff volume. Acknowledgements Seattle Public Utilities provided much of the funding for the modeling of green roofs and bioretention. Taylor Associates of Seattle provided info and photos of Seattle green roofs. The City of Portland, Oregon, Bureau of Environmental Services (BES) provided the Hamilton green roof monitoring data. Questions? Contact: Doug Beyerlein