International Dredging Review

International Dredging Review

J.F. Brennan Co. Inc. a full service marine contractor is the supervising contractor for the remediation of a five-mile stretch of Wisconsin’s Lower Fox River known as Little Lake Buttes des Morts.

Located just north (downstream) of Lake Winnebago the project area is designated Operable Unit No. 1 the first of five such units in the river remediation.

The Lower Fox which flows from south to north includes 39 miles of river between Lake Winnebago and Green Bay. The river was contaminated by PCB’s in effluent from seven paper mills in the 1950’s and 1960’s prior to passage of legislation in the 1970’s that restricted discharge of contaminants into the nation’s rivers and lakes.

In total the remediation of the Lower Fox River is one of the largest sub-aqueous cleanup projects in the United States. Brennan will remove 150000 cubic yards of contaminated sediment in this their third year in Operable Unit No. 1. It is estimated that it will take 25 to 30 years to complete dredging the entire 39 miles of contaminated riverbed.

Little Lake Butte des Morts involves discharges made by two paper mills during operations to recycle carbonless paper and there are seven mills involved along the river as a whole. Negotiations concerning other contaminated stretches are still in progress.

Prior to the dredging work large scale sampling and engineering work were carried out to model the entire lake bottom surface to a level that would remove all PCBs greater than one part per million in concentration. Different dewatering systems were reviewed and analyzed to determine how the dredged material could be dewatered and removed from the area in the most efficient and cost effective manner. Removal operations began in 2004 and have continued in 2005 and 2006.

Brennan is using two specially-designed and automated Moray dredges that can dig to a four-inch accuracy in less than 20 inches of water pumping PCB-contaminated material to a custom processing system.

Dredging Little Lake Buttes des Morts

There are numerous dredging and dewatering challenges on this project. A high level of accuracy is required to minimize the removal and processing of non-contaminated sediment. J.F. Brennan Co. Inc. refers to the high precision removal as “surgical dredging” since the modeled bottom is an irregular surface. Many of the areas slated for removal are in extremely shallow water and have large amounts of organic material. The operation must be managed to maximize the dewatering of sediment.

Brennan turned to two long time suppliers — Dredging Supply Company (DSC) and Kruse Controls – to provide equipment. Because of DSC’s ability to customize dredging solutions Brennan deals exclusively with them for specialized projects.

For this job DSC built two eight-inch swinging articulating ladder dredges. At Brennan’s request they engineered articulated ladders which bend two feet behind the cutter to allow the cutter to enter the material at a small angle when dredging in shallow water. The articulating portion of the swinging ladder has improved accuracy without compromising production and the modified hull design has allowed the dredges to operate in water less than 20 inches deep.

DSC supplied leveling sensors for the ladder section as well as automated features that allow for the ladder to self-level. A larger hull was required to accommodate the added weight of the ladder and to provide adequate flotation and stability. Because the Moray dredge is a swinging ladder dredge the center of gravity of is constantly changing based on ladder depth ladder swing arcs and the position of the articulating ladder end section.

Other features DSC provided were a mass-flow production monitoring system Viscous cutter generator for AC power lever room modified to accommodate GPS and computer systems root knife cutter screen discharge knife valve clay knife and a night lighting package.

The flow and density meters allow Brennan to manage the amount of slurry being sent to the dewatering system and to maximize solids in the system when new material face levels allow.

Equally important to the success of the project has been the system that Kruse Controls has designed. Kruse Controls mathematically modeled the dredge ladders and employed the model in the field. The articulated ladders require real time measurement of angles at the trunion and the knuckle two feet from the cutter as well as the ladder swing angle and pitch and roll of the vessel. Inclinometers and swing sensors calibrate the dredge ladders to within ½ inch.

All sensory data is combined with an RTK GPS system and visually displayed by Dredgepack in the dredge lever room.
The Kruse Controls system also includes a remote networking system that allows Brennan to trend dredge parameters such as line velocity and density. This system also allows Kruse Controls and Hypack to perform remote real-time troubleshooting from their offices on the East Coast through a dedicated IP address.

Dewatering System

Sediment dewatering for this project also presents a challenge. Brennan has a long history in dewatering and views dredging and dewatering as a related intricate system. Whatever the dredge does or does not do affects dewatering. On this project the dredge operators understand the importance of gallon management and operate their equipment to minimize the amount of free water being sent to the dewatering system.

This system includes a specialized thickener unit polymer injection unit and Geotube geotextile dewatering tubes. The thickener unit is designed by Clearwater Industries of Milwaukee Wisconsin and removes all material 1/8 inch and larger prior to thickening the remaining material. Because there is little sandy material in the dredge prism the sand is not removed but is processed and sent to the Geotubes along with the other sediment. The thickener system was designed to handle and pump this mixture. The thickener unit removes from 60 percent to 90 percent of the free water increasing the slurry density to the tubes by over 100 percent.

The dewatering pad is a five-acre lined area where the Geotubes are eventually stacked four high. The tubes are 60 feet in circumference in four lengths ranging from 175 feet to 205 feet. Filled tubes stand six feet high.
The dewatering system also contains two polymer injection points. Ciba Specialty Chemicals has designed a system that allows for dosing of polymer to align itself with the flow and density of the slurry. The system contains two real-time loops: one prior to the thickener and one prior to slurry injection into the tubes. This system is incredibly reliable and greatly minimizes the amount of polymer needed to accomplish dewatering objectives. The polymers used are FC2224 and FC2305.

The final stage is the passive dewatering of the filled tubes which are allowed to sit undisturbed for 40 to 75 days prior to load-out. The PCB’s are attached to the sediment which is trucked to a landfill when dry.

The water is pumped to a treatment plant then returned to the river with undetectable levels of solids.

All dredging and dewatering projects are a balance between production goals and dewatering goals. The passive dewatering of the geotextile tubes is a balance between these goals where production of the dredges is not compromised by the tubes and solids are maximized as time allows.


The success of an intricate project such as this one depends on teamwork. J.F. Brennan Co. Inc. relies on all of its team members and blends its own expertise with the vendors’ expertise to develop solutions that give clients the best value. Brennan chooses the people and companies that it works with based on their ability to make a solution work and the success at the Little Lake Buttes des Morts project bears out this philosophy.