San Gabriel River Debris Collection

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The ProblemBallona Creek November 2001
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Problem Definition

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

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Solution ???

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

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San Gabriel River Debris ControlDesign
Considerations
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Design Considerations

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

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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

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Location
Coyote / Fullerton / Coyote North
SGR / Coyote Creek
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San Gabriel River
Downstream Sites
Coyote Creek
90 of debris load at mouth LACDPW nets in
place
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11/08/02
02/12/03
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Debris Collection Methods

• Floating Systems
• Nets
• Trash Rack
• CDS Stilling Basin

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Floating System with Skirt
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Floating Cage
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Fixed / Floating Net
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Removal

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

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Manual Removal
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Crane with Scoop
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San Gabriel River Debris ControlDebris Net
Design
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Proposed Design - A
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Proposed Design - B
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Proposed Design C D
Direction of Flow
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San Gabriel River Debris ControlPhysical Model
Testing
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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

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Facility

• Canadian Hydraulics Laboratory, Ottawa
• High Volume Flume

Web Address www.chc.nrc.ca
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Modeling Approach

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

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Model Construction
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Model Instrumentation
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Existing Net
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Existing Net
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Existing Net
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Existing Net
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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 (gt50) of neutrally buoyant
  material lost under net

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Cylinder Net
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Cylinder Net
Large head-drop across net Back of buoys dry Held
in place by tension, not buoyancy
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Cylinder Net
Flow over top of net Floating debris lost
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Tetrahedral Net
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Tetrahedral Net
Held up in water column due to hydrodynamic shape
rather than buoyancy
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Tetrahedral Net
Balance of tension in 3 cables Get it wrong and
net tips over floating debris lost
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Tetrahedral Net
Balance of tensions adjusted
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Tetrahedral Net
Change the flow
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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 (gt50) of neutrally buoyant
  material lost under net
• At high flow, loses floating material but retains
  captured neutrally buoyant material

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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

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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

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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)

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San Gabriel River Debris ControlPostscript