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
Leah

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

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
Leah

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
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.

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
Janice

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
Janice

9. 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.

10. 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

11. 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

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
Janice

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

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

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
Janice with intro to Andy

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 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

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