Presentation on theme: "Leah Johanson, Water Environment Services"— Presentation transcript:
1LID & Detention Pond Sizing Tool to Address Hydro-modification and Water Quality in Clackamas County Leah Johanson, Water Environment ServicesAndy Thayumanavan, Brown and CaldwellJanice Keeley, Brown and CaldwellLeah
2Agenda Background Hydro-modification Tool Criteria Development Low Impact Development (LID) Sizing ToolDetention Pond Sizing ToolUser InterfaceNext StepsQuestionsLeah
3WES Surface Water Program Evolution Past emphasis on meeting regulatory requirementsStormwater management at the “site level”Movement toward goal of improving overall health of the watershedWES can best achieve this through effective and efficient stormwater managementPartnering with others on non-stormwater-related activities that will help WES meet this goalLeah
4Watershed Action Plans Project Purpose To develop basin specific plans that prioritize District activities and future investments for watershed management
5Watershed Action Plans Study Area In Kellogg-Mt. Scott and other highly developed watersheds, hydromodification has likely had a significant impact on water quality, stream habitat, and channel conditions during the last 100 years. Hydromodification impacts have occurred to a lesser extent in Rock Creek and other watersheds that have less development. However the impacts could increase dramatically in the future as these watersheds are urbanized further unless mitigation of new impervious surfaces is effective at maintaining a hydrologic equilibrium with current conditions.
6Key Outcomes of WAPBetter understanding of the conditions in the watershedsIdentification of high priority areasPrioritized CIP/ programmatic changesEarly action itemsUpdate Stormwater Design Standards
7Purpose of Sizing ToolStormwater design standards updated to promote the use of Low Impact Development (LID) techniques, to promote infiltration and reduce hydro-modification impacts to streamsDevelop a simplified tool for development engineers to easily size LID and Detention BMPs to implement the new design standardsJanice
8What is Hydro-modification? The frequency and duration of geomorphically significant flows are the primary factors that control channel stability or instabilityGeomorphically significant flows range between:Lower Threshold – bed material is mobilizedUpper Threshold – channel bank over-topping eventJanice
9Hydromodification Criteria Development Lower Flow Threshold – 0.42Q2 (ODOT)Upper Flow Threshold – Q10 (ODOT)HSPF Modeling Parameters (2006 SWMP MP, PWR)Rainfall RecordFlow thresholds will be a part of WES’s adaptive management strategy.
10Sizing Strategy – Flow Duration For flow rates ranging from 42% of the pre-project 2-year runoff event (0.42Q2) to the pre-project 10-year runoff event (Q10), the post-project discharge rates and durations shall not deviate above the pre-project rates and durations by more than 10% over and more than 10% of the length of the flow duration curve.Upper Threshold FlowLower Threshold Flow
11Sizing Strategy – Peak Flow For flow rates ranging from the lower flow threshold (0.42Q2) to Q5, the post-project peak flows shall not exceed pre-project peak flows. For flow rates from Q5 to Q10, post-project peak flows may exceed pre-project flows by up to 10% for a 1-year frequency interval. For example, post-project flows could exceed pre-project flows by up to 10% for the interval from Q9 to Q10 or from Q5.5 to Q6.5, but not from Q8 to Q10.Upper Threshold FlowLower Threshold Flow
12Sizing Tool Computational Methods: Site Hydrology Developer Enters:Site specific infiltration ratePlanned impervious areaPre-developed land useDrainage area hydrologic soil group – B, C, DJanice
15Sizing Tool ResultsFinished 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 rateJanice with intro to Andy
16Detention Pond Sizing Approach The detention pond sizing tool performs the following functionsConfigure pond geometryDesign outlet structureRoute post-project runoffCompare pre-project and mitigated flow durationSize the facility automatically to meet the WES HMP requirements
17Computational Methods: Pond Geometry User inputs initial pond configuration including area, depth and side slopesAlternatively, the tool automatically provides initial pond geometry
20Flow Duration Analysis Flow duration comparison is performed for pre- and mitigated time series hydrographComplete time series is analyzedRange of interest is between lower and upper threshold flows
21Resizing the PondIf flow duration criteria is not met then the pond is resized iterativelyThe user can use the option to resize by either adjusting the area or depthPond sizing is completed when hydro-modification performance requirements are met
22Next Steps Beta testing Meetings with co-permittees Testing and trainingODEQ 319 Grant FundingWeb-based tool rolled out with new design standardsContinued monitoring and adaptive management
25Computational 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, QLTThe orifice is sized to discharge the upper threshold flow, QUT.
26Compare HistogramsDirect 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).