PIANC Dredging 2012 Conference Castro Cove Sediment Remediation Project, Richmond, California Ian Austin PhD PE - URS Corporation Paul Ryan – Chevron Environmental Management Company Mark Sutton – Dixon Marine Services Why of Interest? Technically challenging mudflat environment Specialty equipment designed to place “no-net-fill” 20-acre cap Consolidation of hydraulically placed beneficial re-use mud layer Shear stress modeling prior to sheet pile removal Castro Cove Sediment Remediation Project Site Location • Embayment of San Pablo Bay in north San Francisco Bay • Adjacent to active refinery • Mudflats at +2 to +3 feet MLLW • Shear strengths; 50, 150 psf at 1, 2 foot depths Castro Cove Sediment Remediation Project AOC • Effluent discharged into Cove from early 1900s until 1987 • ERA: 20-acre AOC based on PAHs, mercury, benthic risk • 2007: Dredged 97,000 cy sediment, placed upland • Approximately 6-inch thick layer of residual contamination remained in AOC after dredging Castro Cove Sediment Remediation Project Engineered Cap Design • Cap designed using lab tests and chemical flux modeling, expert peer review (Danny Reible) • Sequential Batch Leach, Thin Column Leach, and cap-loading tests used to develop partition coefficients and sorption capacities • Design includes; • “No-net-fill” requirement, < 97,000 cy • 1.5 acres of Reactive Core Mats (RCM), • 6-inch+ sand layer for chemical isolation and containment, • 18-inch+ clean Bay mud layer to restore biological viability for benthic communities Castro Cove Sediment Remediation Project Sand Placement Challenge • Performed “in-the-wet” with 3 to 5 feet of water inside 2,700 foot sheet pile enclosure • Adjacent to an operating refinery • Place a nominal 6-inch layer of sand over 20- acres (45,000 cy maximum) • Could not disturb or re-suspension residual contamination • RWQCB Order only allowed discharge of water inside sheet-pile after sand placement and WQ confirmation testing: closed loop system • Required finesse, rather than bulk production Castro Cove Sediment Remediation Project Sand Placement thr Specialized Equipment Design: • Worked with DMS on hydrodynamic design of spreader • Hydraulic placement of sand pumped 5,000 feet, needed to reduce velocities from 14 fps to 2 –> 3 fps • Used computer model (HEC-RAS) to design spreader plate, flare width and slope • Modular construction based on two 10-ft by 40-ft pontoons Sand Placement Automated sand slurry system used to produce accurate slurry concentrations: spreader position only variable Castro Cove Sediment Remediation Project Sand Layer Confirmation: • Sand barge delivery volumes • Bi-weekly bathymetric surveys using purpose- built, shallow-water, 5-beam “sweep” system • Sand core collection in clear PVC tubes Castro Cove Sediment Remediation Project Mud Layer Placement Challenge: • Two weeks prior to start-up, required to change from hydraulic to mechanical dredging due to post-permit (DF&G) change in position regarding impacts to long-fin smelt • Beneficial reuse of 37,500 cy of dredge sediment from Richmond Rod & Gun Club Yacht Harbor dredging • Beneficial reuse of 13,000 cy from Point San Pablo Yacht Harbor; 8,000 feet of pipe Castro Cove Sediment Remediation Project Mechanical Hydraulic Dredge Castro Cove Sediment Remediation Project AOC Confirmation Cross Sections Total sand and sediment volume: 95,500 cy Castro Cove Sediment Remediation Project Clay Layer Consolidation Challenge: • Dewatered AOC to promote consolidation of mud slurry • Extended consolidation time from one month to four months based on shear strengths • Measured in-situ shear strength in surrounding mudflats • Measured strength of mud in AOC along diagonal transect – field shear vane measurements Castro Cove Sediment Remediation Project Shear Vane Transect Castro Cove Sediment Remediation Project MIKE 21 Shear stress modeling of sheet pile removal Modeled tide-specific sequencing options for reintroducing the tide First: 300-ft pushed down to create 1-foot high weir -> minimize initial tidal prism Second: sheet removal with two barge cranes along north face Castro Cove Sediment Remediation Project Conclusions • Cooperation between regulatory agency, owner, engineer, contractor needed to complete the project (from 1997 to 2012) • Integration of science, engineering, construction needed to meet technical challenges (material delivery, mud consolidation, sheet pile removal) .