Environmental Dredging – Roles of Environmental and Standard Clamshell Dredge Buckets at Money Point in Chesapeake, VA

摘要

Background. The highly industrialized Elizabeth River in southeastern Virginia is identified as a toxic region of concern by the multi-state Chesapeake Bay program. Historic wood-treatment manufacturing practices resulted in extensive polycyclic aromatic hydrocarbons (PAH) contamination in tidal river sediment at “Money Point” located in the Southern Branch of the Elizabeth River. Approach. A community stakeholder group led by the nonprofit Elizabeth River Project, funded through a federally approved in-lieu fee mitigation program, and managed by the nonprofit Living River Restoration Trust completed Phase 2 sediment rehabilitation efforts at Money Point in Chesapeake, Virginia, during the summer of 2011. The selected hybrid remedy in the Phase 2 area combines mechanical dredging of 15,000 cubic yards of marine sediment followed by restoration sand placement to isolate generated and undisturbed dredge residuals. Marine sediment within the dredge prism was composed of competent glacial clay and silt covered with one to three feet of unconsolidated organic silt. A pre-construction side-scan survey did not detect excessive debris in the Phase 2 area however historic literature identified former dock and pier structures centered over the dredge work area. To minimize the moisture content in the dredged material and control the associated dredge material dewatering and stabilization cost, the project dredge plan called for the combined use of both environmental and standard clamshell dredge buckets. Results. The environmental clamshell dredge bucket was found to be most effective in areas where debris-free, soft, unconsolidated sediment was encountered. Dredging activity confirmed the accuracy of the historic literature regarding the presence of the former dock and pier structures and a large amount of broken piles and dimensional lumber debris was encountered in those areas. The standard clamshell dredge bucket proved to be more efficient at removing debris and debris-containing sediment. The standard clamshell bucket was also more efficient at removing competent clay and silt sediment, resulting in higher dredging production rates and lower moisture content.