Philip Miles, P. Eng. (Senior Geotechnical Engineer, Acres International Limited, Niagara Falls, Ontario, Canada)
Donald Marr, P. Eng. (Manager, Engineering and Environment, Atlas Specialty Steels, Welland, Ontario, Canada)
A dredging demonstration was carried out in the Welland River to test a sediment removal technology being considered for a full scale cleanup. The demonstration involved the controlled removal of 127 m3 of industrial mill scale and contaminated sediment using a modified Mud Cat MC-915 ENV dredge. Contaminants consisted of several metals, phosphorus and oil and grease which exceeded provincial sediment quality guidelines. Dredge modifications focused on minimizing the resuspension of contaminated sediment while maximizing the solids content of the dredgeate and included a special auger head and boom assembly, a new traversing winch assembly, new dredge sponsons, and conversion capability for the dredge to accept standard Mud Cat components. Dredging parameters were monitored and recorded using dredge mounted instrumentation and a data logging system.
In the late 1980s, investigations in the Welland River in the City of Welland, Ontario, Canada identified a 1.25-km stretch of the river, which contained a significant volume of industrial contamination and contaminated river sediments resulting from past discharges by Atlas Specialty Steels (Atlas), other local industry and the city.
In the late 1990, a proposal was accepted by Environment Canada’s Great lakes Cleanup Fund for partial funding of a small dredging demonstration under the Contaminated Sediment Removal Technology Demonstration Program (CSRTDP). The demonstration project was the first carried out under CSRTDP. A second successful proposal was accepted by the Cleanup Fund for partial funding of a treatment demonstration under the Contaminated Sediment Treatment Technology Program (COSTTEP). The dredging and treatment demonstrations were carried out concurrently.
Site Description and Contaminant Characterization
The dredging site was located on the lower reach of the Welland River.
The width of the Welland River varies from approximately 40 to 60 m. The maximum depth of water is approximately 4 m. Historical average river flow ranges from approximately 14.2 m3/s in June/July to 24.6 m3/s in March, corresponding to current velocities of 0.15 to 0.26 m/s. Due to downstream flow controls, the river undergoes apparent flow reversals on a daily basis.
Past industrial discharges through the McMaster Avenue and another nearby downstream outfall have resulted in two accumulations of reef-type deposits of oily, black, fine to coarse granular, metallic industrial mill scale, totaling approximately 5000 m3. The maximum thickness of the industrial deposits is approximately 2.5 m.
Approximately 25 000 m3 of clay and silt river sediments have also been variably impacted by the contaminated discharges.
The mill scale and the contaminated sediments contain concentrations of several metals, including copper, chromium, iron, lead, manganese, nickel and zinc, as well as phosphorus and oil and grease which exceed the Ontario Ministry of the Environment and Energy (OMOEE) sediment quality guidelines (OMOE, 1991).
Description of Sediment Removal Technology
The main components of the sediment removal technology were the dredge and the piping system used to convey the contaminated slurry to the NFP, the dredge instrumentation, and the silt curtain which was installed at the dredge site.
The selected equipment was expected to meet the following design requirements:
- minimize sediment resuspension in the water column to protect downstream water quality
- remove contaminants without excessive removal of clean sediment
- handle excavation of cohesive clayey silt sediment
- function under existing river flow and site conditions
- maximize solids concentrations in the dredged material
- operate at reasonable production rates
- be compatible with a continuous flow, high volume treatment technology
- contain and transport dredged material between the dredging site and the treatment site.
Modified Mud Cat Dredge
The Mud Cat dredge, manufactured by Mud Cat International (Mud Cat) of Baltimore, MD, USA, was selected as the preferred dredging technology for the demonstration. It is built to operate in shallow marine environments and features an effective sediment removal system consisting of a boom-mounted horizontal auger and a centrifugal slurry pump. The dredge was relatively easily modified to include innovative components designed to satisfy the environmental demands of the project.
The Mud Cat MC-915 model was selected as the basic dredge for the demonstration. The class of material to be dredged and the depth of operation were within the capabilities of the MC model and in addition, with its slurry pump located between the hulls, it offered more scope for required boom and auger modifications. The specially prepared MC-915 ENV (ENV=Environmental) dredge incorporated modified components which were initially designed and fabricated by Mud Cat for the demonstration. It retained many features of the standard MC-915 dredge including working capacity, engine, drive, pump, hydraulic system, electrical system and propulsion. The dredge underwent preliminary wet performance testing at a facility in Baltimore. Subsequent modifications were made by the Atlas in Welland.
The following modifications were incorporated into the dredge.
- Special MC-915 ENV auger head and boom assembly:
- one auger head with hydraulic forward tilt and manual transverse tilt capability
- one dual-convergence, variable-pitch, multi-flight auger
- full rear shroud behind auger
- Removable vibrating front shroud including:
- removable front screens
- top mounts for vibrating motors
- Special MC-920 type truss boom assembly including suction hose
- Hydraulic equipment package including:
- auger head tilt indicator
- two variable control hydraulic vibrators
- auger reducer and motor
- boom winch system
- New depth gauge scale
- Assorted connecting hardware for system
- Complete double wrap traversing winch assembly
- Two new trunnions with pins
- Two new sponsons to support boom assembly
- Conversion capability to accept standard MC-915 components as required.
The piping system to convey the dredged material to the treatment facility consisted of a floating section of 200-mm diameter flexible butyl rubber hose and rigid polyethylene (PE) pipe connecting the dredge to a slurry sampling station located on shore, 1500 m of land-based fused-jointed PE pipe and a booster pump.
In order to assess the project and the dredge performance, an instrumentation/data logging package was added to the dredge. Analog displays provided the dredge operator with real-time indicators of dredging performance. The instrumentation included:
- one nucleonic densitometer with spool piece
- one electromagnetic flowmeter
- one dredge head vibration sensor
- one dredge head turbidity sensor
- analog displays for slurry velocity, slurry density, vibration, and production rate
- one dredge cab-mounted data logger with 4 input channels complete with IBM PC compatible support software, cable and NEMA 4 enclosure
- connecting hardware and cables to link the data logger to sensors and a remote portable computer
Silt Curtain Structure
A commercially available silt curtain was selected for the project and was modified by the manufacturer to meet specific demonstration requirements. It consisted of an impermeable polyester-reinforced vinyl fabric which extended the full depth of the water column. Segmented foam flotation members were fabricated into the full length of the top edge.
Demonstration of Technology
The demonstration involved the removal and treatment of approximately 127 m3 of industrial mill scale and contaminated sediment from within the silt curtain. Dredging was carried out in a downstream to upstream direction only. The bank of the river was not disturbed; however, dredging along the sloping river bottom was carried out.
The successful completion of the project involved the coordination of a variety of activities which enabled both the dredging technology and the treatment technology to be demonstrated concurrently.
The dredging program was controlled by Operational and Performance Standards which were issued by Environment Canada (Environment Canada, 1991) as criteria for evaluating the dredging technology. It considered three categories, namely sediment removal, transport and pretreatment. The main concerns under sediment removal are the containment of resuspended contaminants and removal efficiency. No dredging was carried out until the enclosing silt curtain had been installed.
The modified MC-915 ENV dredge was first tested in uncontaminated sediment within the silt curtain just upstream of the McMaster Avenue outfall, proving that the technology could be used with little environmental impact. Dredging in contaminated sediment commenced in late October. Five to eight test runs per day were conducted and evaluated over the next 12 days.
The operating procedures required that daily activities be coordinated to optimize both dredging time and data acquisition. Each test run also required synchronization with the sediment treatment plant and the booster station operation, with regard to start and stop of dredging and flow rate control.
Dredging started in the ‘original’ mode and during the course of the dredging, numerous planned modifications were made to the dredge and the operating procedures to allow evaluation of the dredge in terms of slurry production yields and turbidity. The impact of each modification on sediment resuspension inside and outside the silt curtain was monitored as part of the water quality monitoring program. These modifications included:
- removal of the shroud screens (‘screen off’ mode) to reduce ‘ploughing’ of the sediment in front of the dredge head. The screens did not allow passage of weeds and cohesive sediments
- welding small steel bars across the suction intake to minimize the entry of debris
- installation of a check valve in the pipeline at the dredge discharge to minimize backflow
- relocation of the shroud vibrators in an effort to impart a more horizontal action to the head.
Midway through the demonstration the modified auger was replaced with the standard toothed auger and near the end of the demonstration the auger shroud was removed (‘shroud off’ mode’).
In the early stages of dredging, all operating parameters for any given test run were kept as nearly constant as possible. As dredging progressed, a more flexible control of the dredging equipment by the dredge operator was adopted (‘Variable Q’ mode) allowing him to change parameters such as engine speed, advance rate and depth of cut during a test, in order to maximize sediment removal. This operating procedure resulted in less standby time for the dredge and an increase in the number of test runs per day in the latter part of the demonstration program.
The Mud Cat technology was successful in removing contaminated sediment from the riverbed and transporting the sediment to the treatment site and was well suited to the site conditions, even though slurry density and percent solids were less than had been anticipated. The field modifications made to the dredging equipment during the project allowed assessment of its performance and specifically determination of the impact of the modifications on sediment resuspension and on dredge productivity. The conclusions from the dredge evaluation are summarized as follows.
- The vacuum suction of the dredge played a major part in minimizing resuspended solids. No sustained plumes of resuspended material propagating away from the dredge head were observed. High turbidity levels during ‘pump off’ conditions are attributed to movement of the dredge between runs and to backwashing through the pipeline.
- Dredging in the ‘original’ mode resulted in an overall average turbidity of 18.5 FTU (Formazin turbidity units) at the dredge head compared to background turbidity of 5 FTU. Dredging in the ‘shroud off’ mode with the standard auger resulted in an overall average turbidity of 17.6 FTU. Operating in the more flexible ‘Variable Q’ mode resulted in longer periods of dredging and less frequent plugging of the pipeline while still maintaining low turbidity levels at the dredge head (overall average 13.8 FTU). Operating in the ‘screen off’ mode resulted in the lowest turbidity levels at the dredge head (overall average 5.4 FTU) but also resulted in frequent blockage of the intake or dredge pump.
- The intake screen on the dredge head limited the movement of sediment, especially the cohesive clayey silt, to the auger.
- The overall average percent solids (by weight), including mill scale and river sediment, in the pumped slurry was low (2.1%, excluding rinsing) and varied considerably during the demonstration due to the structure of the dredging program (with frequent starts, stops and flushing of the pipeline) and the generally cautious approach to the dredging to minimize environmental concerns. Also, the dredge head did not tilt transversely, as designed, and dredging across the sloping river bottom did not allow even entry of sediment into the dredge head.
- Some of the field modifications to the dredging equipment had a significant affect on the slurry solids production rate. The removal of the intake screen increased the overall average percent solids to 3.7% while the removal of the dredge head shroud resulted in the highest overall average percent solids of 4.4%. Peaks averaging 22% solids in the ‘shroud off’ mode are indicative of the maximum achievable production rate.
- The shroud-mounted vibrators did not have a significant positive affect on dredge performance.
- The mill scale was dredged at a higher percent solids that the river sediments. Based on the evaluated data, 10% solids (by weight) is identified as a conservative estimate of the average percent solids achievable during full-scale dredging in mill scale using the ‘shroud off’ (or comparable) mode. A conservative estimate of 5% solids has been identified as achievable for the river sediments. Dredging across the river, instead of parallel to it, should also yield a more efficient sediment removal.
- TSS concentrations at a distance of 10 m away from the dredge were well below the Environment Canada criteria of 25 mg/L at a distance of 25 m. The maximum TSS concentration measured a distance of 10 m away from the dredge was 21 mg/L.
- The Gheen couplings on the flexible section of the pipeline were not sufficient to eliminate leakage of slurry without the addition of Victaulic clamps.
- The instrumentation and data logger installed on the dredge provided reliable data for real time monitoring of the dredging operation. The cab-mounted displays of slurry density and flow allowed the dredge operator to optimize the sediment removal process.
- The silt curtain performed well with regard to the containment of river sediment that became resuspended during the dredging demonstration. No evidence of a downstream impact of the dredging was measured.
The authors wish to acknowledge the participation of Environment Canada, the OMOEE, the Niagara River Remedial Action Plan Public Advisory Committee and the local Welland River Cleanup Committee. Partial funding by Environment Canada and the OMOEE is also acknowledged.
Ontario Ministry of the Environment, 1991. The Provincial Sediment Quality Guidelines (Draft). Water Resources Branch, Toronto.
Environment Canada, 1991. Operational and Performance Standards. Environmental Protection, Ontario
by Carol Ancheta