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Leah Johanson, Water Environment Services Andy Thayumanavan, Brown and Caldwell

LID & Detention Pond Sizing Tool to Address Hydro-modification and Water Quality in Clackamas County . Leah Johanson, Water Environment Services Andy Thayumanavan, Brown and Caldwell Janice Keeley, Brown and Caldwell. Agenda. Background Hydro-modification Tool Criteria Development

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Leah Johanson, Water Environment Services Andy Thayumanavan, Brown and Caldwell

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  1. LID & Detention PondSizing Tool to Address Hydro-modification and Water Quality in Clackamas County Leah Johanson, Water Environment Services Andy Thayumanavan, Brown and Caldwell Janice Keeley, Brown and Caldwell

  2. Agenda • Background • Hydro-modification • Tool Criteria Development • Low Impact Development (LID) Sizing Tool • Detention Pond Sizing Tool • User Interface • Next Steps • Questions

  3. WES Surface Water Program Evolution • Past emphasis on meeting regulatory requirements • Stormwater management at the “site level” • Movement toward goal of improving overall health of the watershed • WES can best achieve this through effective and efficient stormwater management • Partnering with others on non-stormwater-related activities that will help WES meet this goal

  4. Watershed Action Plans Project Purpose • To develop basin specific plans that prioritize District activities and future investments for watershed management

  5. Watershed Action Plans Study Area

  6. Key Outcomes of WAP • Better understanding of the conditions in the watersheds • Identification of high priority areas • Prioritized CIP/ programmatic changes • Early action items • Update Stormwater Design Standards

  7. Purpose of Sizing Tool • Stormwater design standards updated to promote the use of Low Impact Development (LID) techniques, to promote infiltration and reduce hydro-modification impacts to streams • Develop a simplified tool for development engineers to easily size LID and Detention BMPs to implement the new design standards

  8. What is Hydro-modification? The frequency and duration of geomorphically significant flows are the primary factors that control channel stability or instability • Geomorphically significant flows range between: • Lower Threshold – bed material is mobilized • Upper Threshold – channel bank over-topping event

  9. Hydromodification Criteria Development • Lower Flow Threshold – 0.42Q2 (ODOT) • Upper Flow Threshold – Q10 (ODOT) • HSPF Modeling Parameters (2006 SWMP MP, PWR) • Rainfall Record

  10. Sizing Strategy – Flow Duration Upper Threshold Flow Lower Threshold Flow

  11. Sizing Strategy – Peak Flow Upper Threshold Flow Lower Threshold Flow

  12. Sizing ToolComputational Methods: Site Hydrology • Developer Enters: • Site specific infiltration rate • Planned impervious area • Pre-developed land use • Drainage area hydrologic soil group – B, C, D

  13. LID Sizing Tool • Facility Types (7 options) • Planter (Infil & Fil) • Rain Garden (Infil & Fil) • Vegetated Swale (Infil & Fil) • Infiltrator

  14. LID Sizing ToolComputational Methods: Facility Geometry • Rain Garden Example

  15. Sizing Tool Results • Finished Product – Web-based interface with underlying look-up table containing sizes for each possible iteration of drainage area land use, soil type, facility type and site specific infiltration rate

  16. Detention Pond Sizing Approach • The detention pond sizing tool performs the following functions • Configure pond geometry • Design outlet structure • Route post-project runoff • Compare pre-project and mitigated flow duration • Size the facility automatically to meet the WES HMP requirements

  17. Computational Methods: Pond Geometry • User inputs initial pond configuration including area, depth and side slopes • Alternatively, the tool automatically provides initial pond geometry

  18. Detention Pond Interface

  19. Stage-Storage-Discharge Table

  20. Flow Duration Analysis • Flow duration comparison is performed for pre- and mitigated time series hydrograph • Complete time series is analyzed • Range of interest is between lower and upper threshold flows

  21. Resizing the Pond • If flow duration criteria is not met then the pond is resized iteratively • The user can use the option to resize by either adjusting the area or depth • Pond sizing is completed when hydro-modification performance requirements are met

  22. Next Steps • Beta testing • Meetingswith co-permittees • Testing and training • ODEQ 319 Grant Funding • Web-based tool rolled out with new design standards • Continued monitoring and adaptive management

  23. Questions?

  24. Sensitivity Analysis • Slope • Thresholds • Time steps

  25. Computational Methods: Outlet Structure • Use pre-defined outlet configuration that include two orifices and an overflow weir. • The bottom orifice is sized to discharge lower threshold flow, QLT • The orifice is sized to discharge the upper threshold flow, QUT.

  26. Compare Histograms • Direct comparison between pre- and post-project durations for each flow bin is performed. • A “Pass” or “Fail” result is generated for each flow bin within the lower and upper threshold flows. • Results take into account the allowable 10% variance over the length of the curve (0.42Q2 to Q10).

  27. Flow Duration Comparison

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