1. Stormwater Infrastructure for Water Quality Management Dr. Larry A. Roesner, P.E. CE 394K.2 Surface Water Hydrology University of Texas, Austin April 8, 1999

2. Urban Runoff Quality Management Practices The Next Generation of Urban Storm Water Management Larry Roesner, Ph.D., P.E. Camp Dresser & McKee Larry Roesner, Ph.D., P.E. Camp Dresser & McKee

3. The Urban Stormwater Problem FlowQuality Receiving Water Regulated Principally by EPA & TNRCC Regulated by Local Agencies

4. Increasing Imperviousness Increases Runoff Return Period, yrs Flow Rate

5. Stormflow Impacts 100 year peak flow increases 2 X 15 year peak flow increases 3 X 2 year peak flow increases 57 X (Denver) 2-yr peak flow occurs –3 X per year (residential development) –6 X per year (commercial development)

6. The STORM Model (Storage Treatment Overflow Runoff Model) Treatment Wet-Weather Storage Q=CIA Runoff Overflow Precipitation

7. Typical Capture Curves 50% Impervious 100% Impervious Treatment Volume Required, inches 0.5 1.0 Percent Capture of Annual Runoff 20 0 50 100 90 The Design Storm

8. So What’s the Problem BMPs target the control of the quality of runoff Conventional drainage facilities control downstream flooding Neither of these activities has as its objective protection of the aquatic environment - If it occurs, it is incidental

9. Stormwater Mgmt Must Address the Entire Flow Frequency Curve Return Period, yrs Flow Rate Frequency Curve with Flood & Water Quality Controls

10. The Fact Is: Simply reducing pollutants in the runoff to the Maximum Extent Practicable will probably not result in significant improvement to the ecological condition of the receiving waters Flow management is also required

11. Urban Runoff Hydrology Small storms account for most of the runoff and are affected most by urbanization. 85 percent of the storms in east Texas are less than 1 inch of rainfall 85 percent of the storms in west Texas are less than 0.65 inches The Design Storm

12. Maximized Water Quality Capture Volume P o = a  C  P A where: P o = Maximized Water Quality Capture Volume (in.) a = Capture Volume Coefficient C = Watershed Runoff Coefficient P A = Mean Storm Precipitation Volume (in.) The Design Storm

13. Maximized Volume for Texas P o = a  C  P A a = 1.3 - 1.6 for 85% capture of annual runoff P A = 0.6 inches in east Texas (wet) = 0.4 inches in west Texas (dry) P o (wet) = 0.12 - 0.14 inches (residential, C=0.15) = 0.70 - 0.86 inches (commercial, C=0.9) P o (dry) = 0.08 - 0.10 inches (residential, C=0.15) = 0.50 - 0.58 inches (commercial, C=0.9) The Design Storm

14. The Stormwater Treatment Train Public Education Spill Prevention Used Oil Recycling Lawn Chemical Mgmt Filter Strips Swales Modular Pavement Infiltration Trenches Pollution Prevention Treatment Controls Source Controls Extended Detention Retention Ponds Wetlands

15. Design of Source Controls

16. Minimize Directly Connected Impervious Area Drain Hard Lot Surfaces onto Pervious Areas Use Modular Pavement where Feasible Drains Streets to Swales

17. Lot/Site Drainage Modular Pavement Grassed Parking Area Reinforced with Geotextile Fabric Depressed Grassed Area

18. Lot/Site Drainage Percolation Trench

19. Basic Design Criteria Percolation Trench Seasonal High groundwater or bedrock more than 4 ft below trench bottom Do not locate in fill material or recompacted soils Soil should be type A or B with minimum hydraulic conductivity of 6.5 x 10 5 ft/sec Po based on lot size and %I

20. Use Swales for Road and Parking Lot Drainage

21. Design Criteria Swales Provide 1-2% slope Max V < 1 ft/sec Max bottom width, 8 ft Min bottom width, 2 ft Minimum length 100 ft Maximum water height for maximized storm than 1/2 the height of standing vegetation Po sized for road runoff plus the portion of maximized storm not captured on building site

22. Infiltration Basins

23. Design Criteria Infiltration Basins Seasonal groundwater or bedrock > 4 ft below basin bottom Do not locate on fill or compacted soils Soil must be type A or B with saturated surface infiltration rate > 0.3 in/hr Size to drain Po in 12 hour Use point system in book for rating suitability of a site

24. Design of Treatment Controls

25. Extended Detention

26. Extended Detention Design Criteria Size to detain Po for 12 to 24 hours, then add 20% for sediment storage Use two stage design (empty less than 50% of volume in first 1/3 of detention time Sediment forebay recommended Clogging outlets are most common failure Emergency spillway Sideslopes 1:4

27. Extended Detention (cont)

28. Detention with Filtration

29. Detention with Filtration Classic Application

30. Detention with Filtration Design Criteria Capture Po or 1/2 inch of runoff from impervious area 24 - 40 hour drawdown time Minimum sand bed = 18 inches Seal bottom of filter chamber Underdrain the sand filter Provide smooth flow transition from presedi- mentation chamber to filter chamber

31. Retention

32. Retention - Design Criteria Design by one of two methods - Solids-settling theory - Lake eutrophication theory Both facilities are larger than an extended detention basin for the same drainage area For biochemical design, size to hold runoff from wettest two weeks for 14 days Design as regional facilities as landscape amenity

33. Constructed Wetland

34. Constructed Wetland Design Criteria Use same guidelines as biological retention, but detention time is 14 days during wettest month Open water is less that 50% of total facility surface area Use a wetland biologist for developing planting program

35. Where Can I Learn More?

36. North Central Texas Council of Governments (1995)

37. The Joint ASCE/WEF Manual of Practice Pragmatic Broadly Based (1998)

38. Targeting Highway Runoff (1997)

39. The Internet www.txnpsbook.org (1999)

40. Summary Design for the small storm Minimize Directly Connected Impervious Area Use the treatment train concept Design outlet controls as multi-stage to reproduce natural flow frequency curve