Design of Effective Stormwater Treatment Systems for Water Quality


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Discusses the importance of properly designing stormwater treatment systems to effectively remove pollutants from stormwater and how not to construction bioretention systems

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Design of Effective Stormwater Treatment Systems for Water Quality

  1. 1. The Importance of Assessing Pollutant Loads from Land Development Projects and theDesign of Effective Stormwater Treatment Systems 2010 Watershed Management ASCE/EWRI8/24/2010 Trinkaus Engineering, LLC
  2. 2. Steven Trinkaus Licensed Professional Engineer (CT) Certified Professional in Erosion and Sediment Control (CPESC) Certified Professional in Storm Water Quality (CPSWQ) Over 28 years in the Land Development Field Expertise in the field of Stormwater, Water Quality Issues & Low Impact Development8/24/2010 Trinkaus Engineering, LLC
  3. 3. Current Local Stormwater Focus Peak rate control for large storm events No consideration of small frequent storms No evaluation of pollutants found in stormwater No evaluation of the effectiveness of stormwater systems to remove pollutants8/24/2010 Trinkaus Engineering, LLC
  4. 4. Storm Water Runoff8/24/2010 Trinkaus Engineering, LLC
  5. 5. Storm Water Quality Issues8/24/2010 Trinkaus Engineering, LLC
  6. 6. Water Quality Issues8/24/2010 Trinkaus Engineering, LLC
  7. 7. Water Quality Impacts Lawns to edge of water – Increased nutrient loads Connected Impervious Areas Results of Non-Point Source Pollutants8/24/2010 Trinkaus Engineering, LLC
  8. 8. How we design today Stone berm across level No provisions to ensure long bottom of basin does NOT flow paths in basin create a proper forebay This cell may trap some coarse sediment but will NOT prevent resuspension of sediments Berms of modified riprap do not function well at trapping of fine sediments (void spaces are too large) Flows may “short circuit” parts of basin due to location of inlet & outlet structures Forebay will not trap sediments over long term8/24/2010 Trinkaus Engineering, LLC
  9. 9. What we see today Outlet set too close to inlet – flows will short circuit – inadequate treatment for water quality Riprap Berm Does Not create forebay Inadequate Forebay – No Depressed Sediment Storage - Lack of Maintenance8/24/2010 Trinkaus Engineering, LLC
  10. 10. Why should we Assess Pollutant Loads? Are permit performance standards being met? Will there be short or long term adverse impacts to receiving waterways? Are the treatment systems designed correctly?8/24/2010 Trinkaus Engineering, LLC
  11. 11. How do you Assess Pollutant Loads? SLAMM Source Loading and Management Model P8 Program for Predicting Pollutant Particle Passage through Pits, Puddles and Ponds The Simple Method Estimates pollutant loads for urban areas8/24/2010 Trinkaus Engineering, LLC
  12. 12. The Simple Method Equation developed by Tom Schueler in 1987 to estimate pollutant loads on an annual basis Requires easily obtainable information to use: Annual Precipitation Pollutant Concentrations Percent Impervious Cover per land use type Watershed Area8/24/2010 Trinkaus Engineering, LLC
  13. 13. The Simple MethodL = 0.226(P)(Pj)(Rv)(C)(A) where: L = Pollutant Load in Pounds P = Rainfall Depth (inches) Pj = Factor that corrects P for storms that produce no runoff, use Pj = 0.9 Rv = Runoff Coefficient, fraction of rainfall that turns to runoff, Rv = 0.05 + 0.009(I) I = Site Impervious Coverage (percent) C = Flow weighted mean concentration of pollutant (mg/l) A = Area of Site (acres) 0.226 = Unit Conversion Factor8/24/2010 Trinkaus Engineering, LLC
  14. 14. Best Source of Pollutant Concentrations National Stormwater Quality Database (NSQD), Version 1.1 – (Maestre & Pitt, 2005)Land Use Category TSS TP TN2-8 units/ac 60 0.38 2.18+ units/ac 60 0.38 2.1Commercial 58 0.25 2.6Industrial 80 0.23 2.1Transportation 99 0.25 2.3Forest Deciduous 90 0.1 1.58/24/2010 Trinkaus Engineering, LLC
  15. 15. Pollutant Concentrations for Metals & Total Petroleum Hydrocarbons (TPH)Land Use Category Zn Cu TPH Medium Density Res. 0.176 0.047 0.344 High Density Res. 0.218 0.033 0.344 Commercial 0.156 0.037 0.324 Transportation 0.156 0.037 0.375 Sources: Metals (NURP 1983, Horner 1994, Cave 1994); TPH’s – UNHSC & NY Stormwater Manual 20038/24/2010 Trinkaus Engineering, LLC
  16. 16. How do the Systems Work? LID Systems: Filtration thru soil columns, Uptake by vegetated biota, Settlement due to slow flow velocities, and Infiltration into underlying soils, Biological and chemical reactions within the soil media and plants assimilation.8/24/2010 Trinkaus Engineering, LLC
  17. 17. Pollutant Removal EfficienciesI. Event Mean Concentration (EMC)II. Mass Efficiency EMC gives equal weight to large & small storms and averages incoming & outgoing concentrations for all storms Mass efficiency is affected by the volume of water in the system & losses that occur within the practice, such as evapotranspiration & infiltration8/24/2010 Trinkaus Engineering, LLC
  18. 18. Pollutant Removal Efficiencies2. Mass Efficiency Mass efficiency is affected by the volume of water in the system & losses that occur within the practice, such as evapotranspiration & infiltration Method is based upon a summation of incoming & outgoing loads and is considered the more accurate method Not easily applied for a proposed project due to lack of monitoring data8/24/2010 Trinkaus Engineering, LLC
  19. 19. Caveats of “Removal Efficiencies” Removal efficiency is closely related to influent quality. I.E. the “dirtier” the influent water is higher the pollutant removal rate will likely be as a percent. Removal efficiency rates may encourage designs which do not address ‘source’ control. A system which has a ‘high’ removal efficiency rate may still be discharging high pollutant concentrations in the effluent.8/24/2010 Trinkaus Engineering, LLC
  20. 20. Caveats of “Removal Efficiencies” Many Best Management Practices have not been monitored long enough to establish valid data to determine a supportable removal rate. Removal efficiency does not always account for how much water is treated. If a system is bypassed due to clogging, a stated removal rate is not likely to be valid.8/24/2010 Trinkaus Engineering, LLC
  21. 21. Caveats of “Removal Efficiencies”When using ‘median’ removal rates, it is imperative to design the treatment system fully in accord with the specifications as provided for in the State’s Water Quality or Storm Water Manual.I.E. A forebay for a Wet Extended Detention Pond needs to be 4’ deep and contain 10% of the WQV within the forebay. A 1’ deep forebay does not work.8/24/2010 Trinkaus Engineering, LLC
  22. 22. IRREDUCIBLE CONCENTRATIONS “If pollutant concentrations in the influent approach the “Irreducible” concentrations noted below, then it is not possible to change the effluent concentrations very much (Schueler)” Irreducible Pollutant Concentrations (CWP)Water Quality Parameter Irreducible ConcentrationTotal Suspended Solids 20 to 40 mg/lTotal Phosphorous 0.15 to 0.2 mg/lTotal Nitrogen 1.9 mg/lNitrate-Nitrogen 0.7 mg/lTKN 1.2 mg/l8/24/2010 Trinkaus Engineering, LLC
  23. 23. Pollutant Removal Efficiency Rates – Conventional Systems8/24/2010 Trinkaus Engineering, LLC
  24. 24. Pollutant Removal Efficiency Rates LID Systems8/24/2010 Trinkaus Engineering, LLC
  25. 25. Pollutant Removal LID systems are most effective when used as part of a “Treatment Train”. This is a system when more than one system is used in series to treat runoff. As you can observe on the prior tables, by using multiple treatment systems, significant pollutant removal rates can easily be achieved.8/24/2010 Trinkaus Engineering, LLC
  26. 26. Pollutant Removal After const., the goal is met by the utilization of LID systems to trap sediment as many other pollutants are attached to soil particles. Proper design and construction is very important, especially for infiltration systems8/24/2010 Trinkaus Engineering, LLC
  27. 27. Bioretention-Designed to providegroundwater recharge &water quality- Infiltrate runoff intounderlying soils- Set ponding depth persoil type to fully drainwithin 24 hrs.- Max. drainage area = 5acres- Gravel layer with raisedunderdrain can provideaddl. storage in system 8/24/2010 Trinkaus Engineering, LLC
  28. 28. “Rain Garden/Bioretention” Rain Garden at Sibley Residence - Newtown, CT Designed & Constructed by Homeowner8/24/2010 Trinkaus Engineering, LLC
  29. 29. Residential Rain Garden (dry)8/24/2010 Trinkaus Engineering, LLC
  30. 30. Residential Rain Garden (wet)8/24/2010 Trinkaus Engineering, LLC
  31. 31. Infiltration Systems-3’ separation from bottom ofsystem to SHGW- Native soils must have < 20%& 20-40% silt/clay- Native soils must have in-situ infiltration rate of 0.5”/hr- 25% of WQv to be providedby pretreatment- Must be installed “off-line)- Install on slopes < 15%- Basin to fully infiltrate WQvthrough bottom of basin only 8/24/2010 Trinkaus Engineering, LLC
  32. 32. Infiltration Basin Mulvaney Subdivision – Ridgefield, CT Very sandy soils – has never discharged via overflow pipe8/24/2010 Trinkaus Engineering, LLC
  33. 33. Extended Detention Shallow Wetlands- Min. drainage area = 10 ac.- Maximize flow paths by use ofhigh & low marsh area, islands- Required forebay with 10% WQv- Surface area of system = 1.5% ofdrainage area- 65% of area shall have a depth <18”- 35% of area shall have a depth <6”- Deep water areas (>4’) shallcontain 25% of WQv- Min. L:W ratio = 3:18/24/2010 Trinkaus Engineering, LLC
  34. 34. “Subsurface Horizontal Flow Gravel Wetlands” UNHSCDeep Forebay & Two Treatment Cells8/24/2010 Trinkaus Engineering, LLC
  35. 35. Subsurface Gravel Wetlands Required Design Elements Forebay – 10% WQv, 4-6’ depth Two treatment cells, each holding 45% of WQv Minimum length of treatment cell – 15’ (longer is better) Water quality outlet pipe to maintain saturated condition just below soil surface8/24/2010 Trinkaus Engineering, LLC
  36. 36. Subsurface Gravel Wetlands Required Design Elements Surface layer – 8” wetland soil Filter layer – 3” pea gravel Treatment layer – 24” of 1” clean crushed stone (washed, no fines) Appropriate wetland plants to be used to survive inundation depth to provide WQv. (larger cells, less depth of ponding better)8/24/2010 Trinkaus Engineering, LLC
  37. 37. Subsurface Gravel Wetlands8/24/2010 Trinkaus Engineering, LLC
  38. 38. Pond / Wetland SystemMin. drainage area = 25 ac.Forebay required – 10% WQvCreate long flow paths within systemby using high & low marsh areasSurface area of system must be a min.of 1.5% of drainage areaOutlet pool must contain 10% WQv35% of surface area must be shallowmarsh (<6”)50% of surface area must be less than18” in depth 8/24/2010 Trinkaus Engineering, LLC
  39. 39. Wet SwalesMax. slope = 4%Max. drainage area = 5 ac.Linear applications are bestPretreatment is required andmust contain 10% WQv 8/24/2010 Trinkaus Engineering, LLC
  40. 40. Wet Swale Swale side slopes shall be 3:1, Bottom width shall be min. of 8’, with a maximum ponded depth of 12” Non-erosive velocities must be provided for 1- yr, 24 hr storm event Swale shall handle flow rate from 10-yr, 24- hr storm event on contributing drainage area8/24/2010 Trinkaus Engineering, LLC
  41. 41. Water Quality SwalesWet Swale – G&F Dry Bioswales – HighRentals – Oxford, CT Point – Seattle, WA 8/24/2010 Trinkaus Engineering, LLC
  42. 42. Grass Filter Strip Ledgebrook Lane – Southbury, CT8/24/2010 Trinkaus Engineering, LLC
  43. 43. Filter Strips Maximum slope = 6% Stone trench or raised concrete lip – very Generally – important berms are not to achieve needed or overland desired as flow concentration flow can develop8/24/2010 Trinkaus Engineering, LLC
  44. 44. Sediment Forebays Forebay must hold as a fixed volume – 10% WQv Ideal depth is WQv 4-6’ to promote settlement and sediment storage Forebay needs to have 3:1, L to W ratio to promote residence time and settlement8/24/2010 Trinkaus Engineering, LLC
  45. 45. Wet Extended Detention Pond Most important features: - Forebay - 6-8’ permanent pool - Aquatic shelf around pond - Appropriate plants for hydrologic conditions Pond system must be designed in accord with state manual to be effective8/24/2010 Trinkaus Engineering, LLC
  46. 46. How not to apply LID Failed Bioretention: No sizing calculations, out of date soil mixture, too few plants, soil compaction8/24/2010 Copyright Trinkaus Engineering, LLC Trinkaus Engineering, LLC
  47. 47. How not to apply LID Failed Bioretention: water can enter from one side only, 2’ of soil mix on top of compacted structural fill with no underdrain, overflow grate set flush to bottom of facility, no sizing calculations8/24/2010 Copyright Trinkaus Engineering, LLC Trinkaus Engineering, LLC
  48. 48. How not to apply LID8/24/2010 Copyright Trinkaus Engineering, LLC Trinkaus Engineering, LLC
  49. 49. How not to apply LIDFailedBioretention:Overflowgrate setflush tobottom offacility, nostoragevolume, noplants 8/24/2010 Copyright Trinkaus Engineering, LLC Trinkaus Engineering, LLC
  50. 50. Commercial Site Goodhouse Flooring – Newtown, CT 1+ acre – Open Meadow in Industrial Park Soils consist of Hinckley, excessively well drained sand (Class A) Slight slope (average 3%)8/24/2010 Trinkaus Engineering, LLC
  51. 51. Existing Conditions Ex. Conventional Slight slope – drainage south to north Hinckley Soils8/24/2010 Trinkaus Engineering, LLC
  52. 52. Commercial Site Revised Building Program (LID) Building: 8,000+ with parking/loading for commercial flooring company Stormwater: Grade paved surface direct all runoff to one of eight bioretention facilities Stormwater storage: Bioretention will fully infiltrate all storm events up to 100-yr (7.2”/24hr) Stormwater treatment: 85%+ removal of TSS, TPH & metals, roughly 50% removal of TP8/24/2010 Trinkaus Engineering, LLC
  53. 53. Proposed Conditions8 Bioretention systems will treat & infiltrate WQv for entire site Approx. cost saving vs. conventional drainage & galleries = $ 95,000 Site is graded to direct runoff to one of the bioretention systems NO STRUCTURAL DRAINAGE8/24/2010 Trinkaus Engineering, LLC
  54. 54. THE END8/24/2010 Trinkaus Engineering, LLC