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 Leah

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

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 Leah

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

Watershed 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.

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

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 Janice

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 Janice

Hydromodification Criteria Development Lower Flow Threshold – 0.42Q2 (ODOT) Upper Flow Threshold – Q10 (ODOT) HSPF Modeling Parameters (2006 SWMP MP, PWR) Rainfall Record Flow thresholds will be a part of WES’s adaptive management strategy.

Sizing 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 Flow Lower Threshold Flow

Sizing 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 Flow Lower Threshold Flow

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 Janice

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

LID Sizing Tool Computational Methods: Facility Geometry Rain Garden Example Janice

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 Janice with intro to Andy

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

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

Detention Pond Interface

Stage-Storage-Discharge Table

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

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

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

Questions?

Sensitivity Analysis Slope Thresholds Time steps

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.

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).

Flow Duration Comparison