TARGETED DREDGING OF NEAR-SHORE DEBRIS AND SEDIMENT, UPPER TRENTON CHANNEL, DETROIT RIVER: A CASE STUDY

摘要

Under regulatory orders, dredging was conducted to remove up to 30.000 cubic yards (cy) (22,900 m~3) of contaminated sediments and debris along approximately 1,200 ft (366 m) of the west shoreline of the Upper Trenton Channel, Detroit River in Riverview Michigan. The objective was removal of overlying materials to the surface of a regional clay layer prior to installation of a sheet pile groundwater barrier along the shoreline for closure of an historical upland waste disposal area. Water depths ranged up to 27 ft (8.2 m) and current velocities up to 2.4 feet per second (0.73 meters per second [m/s]). The project is presented as a case study with focus on project definition to satisfy regulatory requirements, design and performance of the turbidity control system, equipment selection and performance in debris and fine-grained materials, the contractors' approaches to satisfying performance-based specifications, dredged material handling, and review of project completion metrics.This project area experiences periodic high river velocities, a major design issue for turbidity controls. This design challenge was met using a sheet pile flow deflection wall coupled with a single heavy fabric silt curtain with piling and cable support, and overlapping sections. Velocity measurements and hydrodynamic modeling were used for design, to test alternative configurations and potential impacts to an adjacent bridge pier. A dual criteria turbidity action level was requested in the permit and compliance monitoring utilized telemetric data collection, visual time-trend monitoring and alarm notification of exceedences. Dredging employed shore-based long reach excavators and barge mounted cranes operating clam shells and a Cable Ann environmental clamshell. During dredging, shoreline bank slopes were field-modified from design for stability and the sheet pile wall installation immediately followed dredging. Dredging was performed in fall 2006 and contingency planning had to consider potential for freeze-out.Debris separation from sediments was a regulatory requirement. Debris had significant impacts on production rates and equipment durability. Field modification to dredge cut reporting was implemented to deal with repeat damage to position sensors in removing large debris. In meeting these challenges, dredging was completed in the November 2006 - January 2007 period. This presentation will be a useful case study review to future projects conducted in similar settings and with similar requirements.