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LID & Detention Pond Sizing Tool to Address Hydro-modification and Water Quality in Clackamas County Leah Johanson, Water Environment Services Andy Thayumanavan,

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Presentation on theme: "LID & Detention Pond Sizing Tool to Address Hydro-modification and Water Quality in Clackamas County Leah Johanson, Water Environment Services Andy Thayumanavan,"— Presentation transcript:

1 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

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

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 8

9 Hydromodification Criteria Development »Lower Flow Threshold – 0.42Q 2 (ODOT) »Upper Flow Threshold – Q 10 (ODOT) »HSPF Modeling Parameters (2006 SWMP MP, PWR) »Rainfall Record 9

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

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

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

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

14 LID Sizing Tool Computational Methods: Facility Geometry »Rain Garden Example 14

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 15

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 16

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

18 Detention Pond Interface 18

19 Stage-Storage-Discharge Table 19

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 20

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 21

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

23 Questions?

24 Sensitivity Analysis »Slope »Thresholds »Time steps 24

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, Q LT »The orifice is sized to discharge the upper threshold flow, Q UT. 25

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 (0.42Q 2 to Q 10 ). 26

27 Flow Duration Comparison 27


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