Los Angeles Basin Contaminated Sediments Task Force

Summary of Aquatic Disposal and Dredge Operations
Committee Meeting
April 10, 2000

1. Meeting time and location: April 6, 2000 at the Port of Long Beach from 1 pm to 3 pm.
2. Attendees: Doland Cheung (USACE), Tom Johnson, Rick Cameron (POLB), Mike Lyons (RWQCB), Dean Smith (LA County), Mitzy Taggert (Heal the Bay), Todd Le, Mike Rose, Kathryn Curtis (POLA).
3. At the April meeting, Mr. Marc Stearns from Manson Construction Co. spoke to the subcommittee on Best Management Practices (BMPs) from a contractor’s perspective.

Notes:

Turbidity

• Function of grain size

• At Slip 2, 90% of settlement occurs within 20-30 mins., remaining 10% in 2-3 days.

• Cutterhead dredge produces lowest turbidity and is relatively localized.

• Best to minimize turbidity by getting in and out of project as fast as possible.
• Slowing down the operation is not useful in reducing turbidity.
• Tidal effects
  • Dredging in ebb vs. flood tide largely irrelevant.
  • If required, would prefer velocity spec vs. tidal spec for contracting purposes.

Equipment

• Clarification on terminology between closed-buckets and enviro-buckets.
  • Closed-bucket: clamshell bucket with plates that seal the top when closed, but has drainage holes to allow water to overflow (Manson’s version is a cable-arm bucket).
  • Enviro-bucket: clamshell bucket that is completely sealed when closed and does not allow overflow.
    • Low volume
    • Suitable for soft material only (<25 blow count mat’l)
• Any bucket still generates turbidity due to surface tension and vacuum/vortex created during raising of bucket in the water column.
  • Silt Curtains
  • Effectiveness is highly dependent on ocean currents.
  • Currents greater than 1-1/2 knots are problematic and lead to a catch-22 situation.
    • In low currents, turbidity is localized so is a silt curtain even necessary?
    • In high currents, turbidity spreads, but silt curtains are very difficult to maintain properly, thus less effective.
  • High winds can lift large curtains out of the water (like a sail).
  • Sinkage problems due to excessive growth.
  • Effective only in the top few feet where higher current velocities reside.
  • At Pier E, turbidity stayed localized due to low currents, thus the silt curtain was mostly a drill (Manson’s opinion).

Contracts/Permits

• Clear communication is key.
• Clearly identify goals and intent of restrictions.
• Allow changes to silt curtain specs to accommodate varying site conditions during construction.
• No-overflow (flowback or backflow) requirement is most difficult to deal with.
• Completeness of site investigation can lower dredging costs.

4. The next scheduled meeting of the Aquatic Subcommittee will be on April 18, 2000 from 10 am to 12 pm at the Corps of Engineers office in downtown Los Angeles.

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