Hydrograph Modification Management Using Simplified Low Impact Development Design

1. Hydrograph Modification Management Using Simplified Low Impact Development Design Christie Beeman, P.E. Philip Williams & Associates

2. precipitation runoff infiltration Hydrograph Modification

3. Pre-Development Time Hydrograph Modification Urbanization tends to increase stormwater runoff: • volume • peak flows • frequency Post-Development Runoff

4. Hydrograph Modification: Flow Peak

5. Hydrograph Modification: Flow Duration

6. Hydrograph Modification Management • SFRWQCB adding C.3 to Bay Area NPDES permits • C.3.f requires Hydrograph Modification Management Plan (HMP) • Control runoff peak flows & durations to pre-project levels • Prevent erosion impacts to receiving streams

7. Contra Costa HMP - approach • Encourage LID for Hydrograph Modification Management • Guidelines already in place for water quality treatment • Typical details • Sizing spreadsheet • Expand for hydrograph modification management

8. Contra Costa HMP: Compliance Options • Compare Pre- and Post-project Site Conditions • Implement LID using IMP Gallery • Design details, specifications and sizing factors • Demonstrate flow peak and duration mitigation • Continuous hydrologic model • Flow peak and duration control • Assess risk of erosion impacts (receiving stream) • Demonstrate low risk of erosion • Propose alternative mitigation

9. LID Integrated Management Practices • Mimic natural hydrologic conditions • Retention, infiltration • Can fit in site landscaping • Practical for small projects precipitation runoff infiltration

10. Contra Costa HMP Challenge: • Hydrograph Modification Management IMP design guidance • Make IMP design and sizing easy • Demonstrate peak and duration control Solution: • Develop IMP sizing factors using continuous simulation hydrologic model

11. IMP Design Guidance – sizing factors • Develop Pre-Project and Post-Project Runoff Time Series using HPSF • Flow peak and duration plots • Build IMPs into the HSPF model • Adjust IMP size until peak and duration plots below pre-project • Sizing factor = (facility size)/(area managed) • Most sizing factors between 0.05 and 0.13

12. LID Integrated Management Practices • Planter box • Infiltration trench/basin • Vegetated swale • Bioretention basin • Dry well

13. Flow-Through Planter Example

14. Flow-Through Planter: Flow Peak Control

15. Flow-Through Planter: Flow Duration Control

16. IMP Sizing Worksheet Define Project DMA Input IMP Sizing

17. A soils D soils IMP Sizing Worksheet - example

18. 1 2 3a 4a 5 4b 3b 6 3c IMP Sizing Worksheet – Delineate DMAs

19. IMP Sizing Worksheet – DMA inputs 1 2 3a 4a 5 4b 3b 6 3c

20. IMP Sizing Worksheet – DMA inputs 1 2 3a 4a 5 4b 3b 6 3c

21. 1 2 3a 4a 5 B C 4b 3b C B 6 3c B A IMP Sizing Worksheet – DMA inputs

22. C 1 2 B 3a 4a 5 4b 3b C 6 B 3c A B IMP Sizing Worksheet – Select IMPs 1 2

23. C 1 2 B 3a 4a 5 4b 3b C 6 B 1 3c 2 A B IMP Sizing Worksheet –IMP dimensions

24. C 1 2 B 3a 4a 5 4b 3b C 6 B 1 3c 2 A B IMP Sizing Worksheet –IMP size