Stormwater – Quality Management

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Stormwater – Quality Management

  1. 1. Stormwater Infrastructure for Water Quality Management Wali Memon Wali Memon 1
  2. 2. The Urban StormwaterProblem Wali Memon Flow QualityRegulated by RegulatedLocal Agencies Principally by EPA & TNRCC Receiving Water 2
  3. 3. Increasing ImperviousnessIncreases Runoff Wali Memon Flow Rate 3 Return Period, yrs
  4. 4. Stormflow Impacts• 100 year peak flow increases 2 X• 15 year peak flow increases 3 X Wali Memon• 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) 4
  5. 5. The STORM Model(Storage Treatment Overflow Runoff Model) Precipitation Wali Memon Wet-Weather Runoff Storage Q=CIA Treatment Overflow 5
  6. 6. Typical Capture Curves The Design Storm 100 Percent Capture of Annual Runoff 90 Wali Memon 50% Impervious 50 100% Impervious 6 0.5 20 0 Treatment Volume Required, inches 1.0
  7. 7. 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 Wali Memon 7
  8. 8. Stormwater Mgmt MustAddress the Entire FlowFrequency Curve Wali Memon Flow Rate Frequency Curve with Flood & Water Quality Controls 8 Return Period, yrs
  9. 9. The Fact Is: Simply reducing pollutants in the runoff to Wali Memon theMaximum Extent Practicable will probablynot result in significant improvement to theecological condition of the receiving waters 9 Flow management is also required
  10. 10. Urban Runoff Hydrology The Design Storm Small storms 85 percent of the storms in east Texas account for Wali Memon are less than 1 inch most of the of rainfall runoff and are affected most 85 percent of the storms in west Texas by urbanization. are less than 0.65 inches 10
  11. 11. Maximized Water QualityCapture Volume The Design Storm Wali Memon Po= a • C • PA where: Po = Maximized Water Quality Capture Volume (in.) a = Capture Volume Coefficient C = Watershed Runoff Coefficient 11 PA= Mean Storm Precipitation Volume (in.)
  12. 12. Maximized Volume for Texas Po= a • C • PA a = 1.3 - 1.6 for 85% capture of annual runoff PA = 0.6 inches in east Texas (wet) Wali Memon = 0.4 inches in west Texas (dry) Po (wet) = 0.12 - 0.14 inches (residential, C=0.15) = 0.70 - 0.86 inches (commercial, C=0.9) Po (dry) = 0.08 - 0.10 inches (residential, C=0.15) = 0.50 - 0.58 inches (commercial, C=0.9) 12 The Design Storm
  13. 13. The Stormwater TreatmentTrain Pollution Prevention • Public Education • Spill Prevention Wali Memon • Used Oil Recycling • Lawn Chemical Mgmt Source Controls • Filter Strips • Swales • Modular Pavement • Infiltration Trenches Treatment Controls • Extended Detention 13 • Retention Ponds • Wetlands
  14. 14. Design of Source Controls Wali Memon 14
  15. 15. Minimize Directly ConnectedImpervious Area • Drain Hard Lot Surfaces onto Pervious Wali Memon Areas •Use Modular Pavement where Feasible •Drains Streets to 15 Swales
  16. 16. Lot/Site Drainage Wali Memon Depressed Grassed Modular Pavement Area Grassed Parking Area 16 Reinforced with Geotextile Fabric
  17. 17. Lot/Site Drainage Percolation Trench Wali Memon 17
  18. 18. Basic Design CriteriaPercolation Trench• Seasonal High groundwater or bedrock more than 4 ft below trench bottom• Do not locate in fill material or recompacted soils Wali Memon• Soil should be type A or B with minimum hydraulic conductivity of 6.5 x 105 ft/sec• Po based on lot size and %I 18
  19. 19. Use Swales for Road andParking Lot Drainage Wali Memon 19
  20. 20. Design Criteria Swales • Provide 1-2% slope • Max V < 1 ft/sec • Max bottom width, 8 ft Wali Memon • 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 20 maximized storm not captured on building site
  21. 21. Infiltration Basins Wali Memon 21
  22. 22. Design CriteriaInfiltration 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 Wali Memon > 0.3 in/hr• Size to drain Po in 12 hour• Use point system in book for rating suitability of a site 22
  23. 23. Design of Treatment Controls Wali Memon 23
  24. 24. Extended Detention Wali Memon 24
  25. 25. Extended DetentionDesign Criteria• Size to detain Po for 12 to 24 hours, then add 20% Wali Memon 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 25• Emergency spillway• Sideslopes 1:4
  26. 26. Extended Detention (cont) Wali Memon 26
  27. 27. Detention with Filtration Wali Memon 27
  28. 28. Detention with FiltrationClassic Application Wali Memon 28
  29. 29. Detention with Filtration Design Criteria • Capture Po or 1/2 inch of runoff from impervious area Wali Memon • 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 29
  30. 30. Retention Wali Memon 30
  31. 31. Retention - Design Criteria • Design by one of two methods - Solids-settling theory - Lake eutrophication theory Wali Memon • 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 31 as landscape amenity
  32. 32. Constructed Wetland Wali Memon 32
  33. 33. Constructed WetlandDesign Criteria • Use same guidelines as biological retention, but Wali Memon 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 33
  34. 34. Summary• Design for the small storm• Minimize Directly Connected Wali Memon Impervious Area• Use the treatment train concept• Design outlet controls as multi- stage to reproduce natural flow 34 frequency curve
  35. 35. Questions! Wali Memon 35

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