Detroit Riverwalk Sediment Remediation

Significant Project Features

• Mechanically placed reactive capping material across 67,500 SF in the Detroit River AOC
• Successfully completed reactive cap installation that isolated PAH contaminates from the aquatic environment and eliminates potential exposure
• Achieved over 7,200 safe workhours without a lost-time incident
• EPA Region 5 GLNPO Project

Project Gallery

History & Location Details

Project Description

Site Preparation

Sevenson prepared USEPA-approved work plans; obtained all required permits; and submitted a Local Notice to Mariners form to the U.S. Coast Guard upon receipt of the contract award. Crews mobilized company-owned equipment needed to complete the scope of work. Site preparation included installation of staging and stockpiling areas; installation of erosion controls; installation of decontamination; and construction of temporary offices, loadout areas, and the upland support area.

Turbidity Controls

Turbidity management and sediment resuspension were critical components of this project. Sevenson installed turbidity control measures and utilized best management practices (BMPs) to minimize resuspension and migration of suspended sediment during debris removal and capping operations. Crews installed 1,750 LF of Siltdam Type 3 permeable 12-in. float, high flow 400 GPM adjustable curtains, and installed 1,750 LF of 50-in. oil boom, as an additional protective measure, around the entire work area. Daily inspections were performed to monitor  for the presence of oil sheens, turbidity plumes; and to ensure there were no gaps, tears or breaches in the curtain. Two turbidity sensors, upstream and downstream buoys, were deployed to monitor the turbidity in real time. Turbidity data was continually collected every 10 minutes and evaluated with respect to project thresholds.

Debris Removal

Before full cap placement, Sevenson removed superficial debris (metal, wood) from the river that may have interfered with cap placement. Crews utilized a PC400 barge-mounted excavator with various buckets (conventional bucket with thumb attachment, clamshell and grapple attachment) to remove specific debris. The excavator was GPS-equipped to target precise areas. Debris was placed into 100 CY hopper scows and transported to the staging area for subsequent dewatering and disposal characterization. All debris was transported off-site for disposal. Debris dewatering and decontamination water was pumped through a geotextile bag contained in the dewatering box to capture suspended solids prior to being pumped to and stored in an onsite frac tank. All collected water was transported off-site for disposal.

Capping

Prior to full-scale placement, Sevenson completed placement in a pilot test area to demonstrate the means and methods of the chemical isolation layer, as well as allow for QA collection of isolation layer samples. The results indicated an average of 3% activated carbon was achieved and approved for implementation throughout capping. To achieve this, 34G stone and AquaGate+6% were blended based upon weight calculations to develop the correct volume rations. As a conservative measure, Sevenson increased the percentage above 3% (approximately 5-6%) to ensure the 3% was achieved. Capping was performed in two operations. The first operation took place from the water using a PC400LF long-reach barge mounted excavator. The second operation took place from the riverbank using a PC400LF excavator and placed cap material along the shoreline out to approximately 35 ft. Capping layers were placed in uniform layers and excavator operators implemented environmental BMPs including controlled lift and swing speeds during material placement to limit resuspension of sediment. AquaGate in-situ treatment was used for the 1-ft. chemical isolation layer area and placed by releasing the material near the water surface. The armor layer was placed using submerged methods near the riverbed to minimize potential mud waves and sediment disturbance. Armor layer materials were placed in 12-in. lifts starting at the toe of the slope and progressing upslope toward the cap layers. Sevenson placed capping materials across 67,500SF. Hydrographic surveys were performed by a MI-licensed surveyor and were completed prior to debris removal, during, and after capping to document final conditions.

Site Restoration

Restoration of the shoreline and upland areas included stabilization of disturbed areas, removal of all construction site features, replacement of erosion controls including silt fencing, as deemed necessary, inspection and repair of all durable covers, and installation of gravel to restore the work area to pre-construction conditions. Post-construction samples were collected and analyzed. Sevenson disassembled the turbidity curtain and removed the steel piles.