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The Problem Ballona Creek November 2001

The Problem Ballona Creek November 2001. Problem Definition. Floating and Positively Buoyant Debris Washing up on Seal Beach Impacts Visual Amenity Water Quality Costs Collection Removal Disposal. Solution ???. Source of Debris at Seal Beach is primarily the San Gabriel River Watershed

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The Problem Ballona Creek November 2001

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  1. The ProblemBallona Creek November 2001

  2. Problem Definition • Floating and Positively Buoyant Debris Washing up on Seal Beach • Impacts • Visual Amenity • Water Quality • Costs • Collection • Removal • Disposal

  3. Solution ??? • Source of Debris at Seal Beach is primarily the San Gabriel River Watershed • Install Debris Collection Device in the River

  4. San Gabriel River Debris ControlDesign Considerations

  5. Design Considerations • Location • Debris Characteristics • Quantity of Debris • Method to “Catch” Debris • Storm Event Flows (1y, 2y, 5y) • Hydraulic Impact • Maintenance • Safety

  6. Location • Concrete channel for ease of operation • easier to remove trash • no navigation issues • Downstream: San Gabriel River, Coyote Creek confluence • Upstream: Fullerton Creek, Coyote Creek, Coyote Creek North Fork confluence

  7. Location Coyote / Fullerton / Coyote North SGR / Coyote Creek

  8. San Gabriel River Downstream Sites Coyote Creek ~ 90% of debris load at mouth LACDPW nets in place

  9. 11/08/02 02/12/03

  10. Debris Collection Methods • Floating Systems • Nets • Trash Rack • CDS Stilling Basin

  11. Floating System with Skirt

  12. Floating Cage

  13. Fixed / Floating Net

  14. Removal • Manually with rakes or pitchforks • Crane and scoop with water in channel • Front-end loader in dry channel • Automatic conveyor system • Eductor Truck

  15. Manual Removal

  16. Crane with Scoop

  17. San Gabriel River Debris ControlDebris Net Design

  18. Proposed Design - A

  19. Proposed Design - B

  20. Proposed Design – C & D Direction of Flow

  21. San Gabriel River Debris ControlPhysical Model Testing

  22. Design Testing • Static Analysis – insufficient • Learning from Existing Nets • Physical Model Testing • try a variety of designs • vary the flow rate • look at dynamic response to debris load • optimize the design

  23. Facility • Canadian Hydraulics Laboratory, Ottawa • High Volume Flume Web Address www.chc.nrc.ca

  24. Modeling Approach • Construction of Channel in Flume • Construction of Model Net Designs • Selection of Debris Material • floating, positively / neutrally / negatively buoyant

  25. Model Construction

  26. Model Instrumentation

  27. Existing Net

  28. Existing Net

  29. Existing Net

  30. Existing Net

  31. Existing Net - Summary • Hydrodynamic Performance • OK at low flows • buoys and net submerged at medium to higher flows • net held up in water column by tension in cables rather than buoyancy of floats • Debris Capture • OK at low flows, captures floating material • can capture positively buoyant material • High proportion (>50%) of neutrally buoyant material lost under net

  32. Cylinder Net

  33. Cylinder Net Large head-drop across net Back of buoys dry Held in place by tension, not buoyancy

  34. Cylinder Net Flow over top of net Floating debris lost

  35. Tetrahedral Net

  36. Tetrahedral Net Held up in water column due to hydrodynamic shape rather than buoyancy

  37. Tetrahedral Net Balance of tension in 3 cables Get it wrong and net tips over floating debris lost

  38. Tetrahedral Net Balance of tensions adjusted

  39. Tetrahedral Net Change the flow…

  40. Tetrahedral Net - Summary • Hydrodynamic Performance • OK at low & med flows • At medium/high flows, moderate head-drop across net & back of buoys dry • appropriate tension in tow cable and hydrodynamic shape of buoys controls position in water column • Reducing area of net below the waterline lead to improved net performance at higher flows • Can be difficult to “balance” tensions in three cables • Debris Capture • OK at low & med flows, captures floating material • can capture slightly positively buoyant material • High proportion (>50%) of neutrally buoyant material lost under net • At high flow, loses floating material but retains captured neutrally buoyant material

  41. Test D - Higher Flow • Continues to be effective at higher flows • Positively buoyant material in net assists in keeping it afloat • Buoys act as spreaders • More buoyancy and/or hydrodynamic lift would assist keeping net above water level

  42. Cylindrical Net - Summary • Increasing downstream scoop very effective for capture of positively buoyant material • As previously, tension in cables controls position of Buoy in water column (rather than buoyancy). However, increased buoyancy or hydrodynamic lift would improve performance at higher flows • Hinged “gate” assists in keeping net at correct level in water column • Easier to “balance” tensions in two cables (compared with tetrahedral system) • “Loose” net design may present maintenance difficulties

  43. Summary of Findings • Dynamic Analysis Required • Dynamic stability changes with flow and load • KISS • Simple designs are better • Catch 22 • More effective the net at capture of positively and neutrally buoyant material - greater head-drop across net (ie upstream impact to hydraulics)

  44. San Gabriel River Debris ControlPostscript

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